Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

Highly Efficient Wide Bandgap Perovskite Solar Cells With Tunneling Junction by Self-Assembled 2D Dielectric Layer

Reducing non-radiative recombination and addressing band alignment mismatches at interfaces remain major challenges in achieving high-performance wide-bandgap perovskite solar cells. This study proposes the self-organization of a thin two-dimensional (2D) perovskite BA2PbBr4 layer beneath a wide-bandgap three-dimensional (3D) perovskite Cs0.17FA0.83Pb(I0.6Br0.4)3, forming a 2D/3D bilayer structure on a tin oxide (SnO2) layer. This process is driven by interactions between the oxygen vacancies on the SnO2 surface and hydrogen atoms of the n-butylammonium cation, aiding the self-assembly of the BA2PbBr4 2D layer. The 2D perovskite acts as a tunneling layer between SnO2 and the 3D perovskite, neutralizing the energy level mismatch and reducing non-radiative recombination. This results in high power conversion efficiencies of 21.54% and 19.16% for wide-bandgap perovskite solar cells with bandgaps of 1.7 and 1.8 eV, with open-circuit voltages over 1.3 V under 1-Sun illumination. Furthermore, an impressive efficiency of over 43% is achieved under indoor conditions, specifically under 200 lux white light-emitting diode light, yielding an output voltage exceeding 1 V. The device also demonstrates enhanced stability, lasting up to 1,200 hours.

Critical Role of Nonrigid Unit and Spiral Acoustical Modes in Designing Colossal Negative Thermal Expansion

Exploring the relationship between thermal expansion and structural complexity is a challenging topic in the study of modern materials where volume stability is required. This work reports a new family of negative thermal expansion (NTE) materials, AM(CN)4 with A = Li and Na and M = B, Al, Ga, and In. Here, the compounds of LiB(CN)4 and NaB(CN)4 were only synthesized; others were purely computationally studied. A critical role of nonrigid vibrational modes and spiral acoustical modes has been identified in NaB(CN)4. This understanding has been exploited to design the colossal NTE materials of NaM(CN)4 (M = Al, Ga, In). A joint study involving synchrotron X-ray diffraction, Raman spectroscopy, and first-principles calculations has been conducted to investigate the thermal expansion mechanism. It has been found that the A atoms can either increase the symmetry of the crystal structure, inducing stronger NTE, or lower the crystal symmetry, thus resulting in positive thermal expansion. Conversely, the M-site atoms do not affect the crystal structure. However, as the radius of the M atoms increases, the ionic nature of the C-M bonds strengthens and the CN vibrations become more flexible, thereby enhancing the NTE behavior. This study provides new insights to aid in the discovery and design of novel NTE materials and the control of thermal expansion.

Biphenyl tetracarboxylic acid-based metal-organic frameworks: a case of topology-dependent thermal expansion

The large inherent flexibility and highly modular nature of metal-organic frameworks (MOFs) make them ideal candidates for the study of negative thermal expansion (NTE). Among diverse organic ligands, the biphenyl unit, which can unrestrictedly rotate along its C-C single bond, can largely enhance the structural flexibility. Herein, we explored the thermal expansion behaviors of four indium biphenyl tetracarboxylates (BPTCs). Owing to the different dihedral angles of BPTC ligands and coordination mode of In3+, they show distinct topologies: InOF-1 (nti), InOF-2 (unc), InOF-12 (pts) and InOF-13 (nou). Intriguingly, it is found that the thermal expansion is highly dependent on the specific topology. The MOFs featuring mononuclear nodes show normal positive thermal expansion (PTE), and the magnitudes of coefficients follow the trend of InOF-2 < InOF-12 < InOF-13, inversely related to averaged molecular volumes. In contrast, the InOF-1, composed of a 1D chain of corner-shared InO6 octahedrons, shows pronounced NTE. Detailed high-resolution synchrotron powder X-ray diffraction and lattice dynamic analyses shed light on the fact that NTE in the InOF-1 is a synergy effect of the spring-like distortion of the inorganic 1D helical chain and twisting of the BPTC ligands. The present work shows how the topological arrangement of building blocks governs the thermal expansion behaviors.

Exploring negative thermal expansion materials with bulk framework structures and their relevant scaling relationships through multi-step machine learning

Discovering new negative thermal expansion (NTE) materials is a great challenge in experiment. Meanwhile, the machine learning (ML) method can be another approach to explore NTE materials using the existing material databases. Herein, we adopt the multi-step ML method with efficient data augmentation and cross-validation to identify around 1000 materials, including oxides, fluorides, and cyanides, with bulk framework structures as new potential NTE candidate materials from ICSD and other databases. Their corresponding coefficients of negative thermal expansion (CNTE) and temperature ranges are also well predicted. Among them, about 57 materials are predicted to have an NTE probability of 100%. Some predicted NTE materials were tested by the first-principles calculations with quasi-harmonic approximation (QHA), which indicates that the ML results are in good agreement with the first principles calculation results. Based on the comprehensive analysis of the existing and predicted NTE materials, we established three universal relationships of CNTE with an average electronegativity, porosity, and temperature range. From these, we also identified some important critical values characterizing the NTE property, which can serve as an important criterion for designing new NTE materials.

Local Structure and Dynamics in MPt(CN)6 Prussian Blue Analogues

We use a combination of X-ray pair distribution function (PDF) measurements, lattice dynamical calculations, and ab initio density functional theory (DFT) calculations to study the local structure and dynamics in various MPt(CN)6 Prussian blue analogues. In order to link directly the local distortions captured by the PDF with the lattice dynamics of this family, we develop and apply a new "interaction-space" PDF refinement approach. This approach yields effective harmonic force constants, from which the (experiment-derived) low-energy phonon dispersion relations can be approximated. Calculation of the corresponding Grüneisen parameters allows us to identify the key modes responsible for negative thermal expansion (NTE) as arising from correlated tilts of coordination octahedra. We compare our results against the phonon dispersion relations determined using DFT calculations, which identify the same NTE mechanism.

Negative Thermal Expansion Materials as Anti-Thermal-Quenching Phosphor Matrixes: Status, Opportunities, and Challenges

Inorganic phosphor materials often face the common phenomenon of luminescence thermal quenching (TQ), which deteriorates their device performance and consequently limits their applicability for broad applications. Thus, exploring thermally stable and even anti-TQ phosphors is viable to meet the urgent requirements of lighting technology and many other luminescence-based applications. One of the emerging approaches devoted to solving the TQ issue of phosphors, especially at elevated temperatures, is to employ negative thermal expansion (NTE) properties occurring in some unique inorganic materials. The present Review focuses on the progress of exploring NTE-based inorganic phosphor materials that have demonstrated unusual negative TQ with enhancing upconversion and downshift luminescence upon elevating temperature. We have also provided a brief history of exploring NTE phosphors for thermally stable and enhanced emission along with the investigation methods and proposed mechanisms of these unusual phenomena. To summarize, we have further discussed some opportunities and challenges that NTE materials face as host matrixes for anti-TQ phosphors. Overall, the aim of this Review is to stimulate the exploration of new NTE-based inorganic phosphors, the correlation of their fundamental structural changes with varying temperature, and the investigation of their potential for broad applications.

The role of local non-tetragonal polar displacements in the temperature- and pressure-induced phase transitions in PbTiO3-BiMeO3 ferroelectrics

In situ high-pressure/high-temperature Raman-scattering analyses on PbTiO3 , 0.92PbTiO3 - 0.08Bi(Zn0.5 Ti0.5 )O3 and 0.83PbTiO3 - 0.17Bi(Mg0.5 Ti0.5 )O3 single crystals reveal an intensity transfer between the fine-structure components of the A1 (TO) soft mode. The enhancement of the lowest-energy subpeak, which stems from intrinsic local non-tetragonal polar distortions, along with the suppression of the tetragonal A1 (1TO) fundamental mode with increasing pressure and temperature indicates the key role of the local polarization fluctuations in transformation processes and emphasizes the significance of the order-disorder phenomena in both the pressure- and temperature-induced phase transitions of pure PbTiO3 and its solid solutions with complex perovskites. Moreover, the temperature and pressure evolution of the fraction of the local non-tetragonal polar distortions is highly sensitive to the type of B-site substituent.

Radical Cation Salts of Hetera-Buckybowls: Polar Crystals, Negative Thermal Expansion and Phase Transition

Radical cation salts of π-conjugated polycycles are rich in physical properties. Herein, two kinds of hetera-buckybowls, ethoxy-substituted trithiasumanene (3SEt) and triselenasumanene (3SeEt), are synthesized as electron donors. Galvanostatic oxidation of them affords radical cation salts (3SEt)5 (TTFMPB)3 , (3SeEt)5 (TTFMPB)3 , (3SEt)4 PMA, and (3SeEt)4 PMA, where PMA is Keggin-type phosphomolybdate and TTFMPB is tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate. In these salts, 3SEt/3SeEt are partially charged and show distinct conformation change with the site charge and counter anions. In TTFMPB salts, (TTFMPB)- forms hexagonal channels that accommodate the packing columns of 3SEt/3SeEt. In particular, (3SEt)5 (TTFMPB)3 adopts the R3c space group and is a polar crystal with the columns of 3SEt all in the up-bowl direction. The PMA salts of 3SEt/3SeEt are polar crystals (C2 space group) with 3SEt/3SeEt being planar and forming columnar stacks. (3SeEt)4 PMA shows a structural modulation below 200 K, namely, negative thermal expansion (NTE) of the unit cell volume and enlargement of the intermolecular distances between neighboring 3SeEt molecules. The four salts are semiconductors with an activation energy of 0.18-0.38 eV. The conductivity of (3SeEt)4 PMA shows a reversible transition upon cooling and heating, in accordance to the NTE structural modulation. This work paves the way toward conducting materials based on hetera-buckybowls.

Thermally Induced Lattice-Defective Oxygen Breathing in Perovskite-Structure Stannates with High-Contrast Reversible Thermochromism

Inorganic thermochromic materials exhibit a tunable color gamut and a wide chromatic temperature range, indicating their potential for intelligent adaptive applications in thermal warning, temperature indication, thermal regulation, and interactive light-to-thermal energy conversion. However, most metal-oxide-based thermochromic materials show weak chromaticity adaption with the change of temperature, which needs further understanding of the microscopic principle to clarify the potential route to improve the contrast and identifiability for fabricating better thermochromic materials. Using perovskite-structure (AMO3) alkaline earth metal stannate (Ba1-xSrxSnO3, 0.0 ≤ x ≤ 1.0) as a model system, this paper reports for the first time the mechanism of the properties of thermally induced defect-enhanced charge transfer-type (CTT) thermochromic materials and the strategy for regulating their thermochromic properties by A-site cations. BaSnO3 exhibits continuously reversible thermochromic properties with high contrast from weak light yellow (b* = 11) to strong bright yellow (b* = 58) between room temperature and 550 °C. In-situ high-temperature X-ray diffraction (in-situ XRD), in-situ UV-vis absorption spectroscopy (in-situ UV-vis), thermogravimetric (TG), and electron paramagnetic resonance (EPR) spectra indicate that this excellent thermochromic phenomenon is attributed to the weakening of Sn-O bond hybridization at high temperatures, as well as the formation of a large number of oxygen vacancies at the top of the valence band, and the enhanced charge transfer resulting from the generation of impurity levels in the Sn2+ 5s2 intermediate. Replacing Ba2+ by Sr2+ in Ba1-xSrxSnO3 successfully tuned the thermochromic properties, which is attributed to the Sr2+ doping level-directed oxygen defect concentration and deoxygenation rate. The demonstrated defect-enhanced charge transfer behavior promotes a feasible route for lattice oxygen-mediated thermochromic materials and provides a fundamental relationship between thermally induced defects and colorimetry.

Anisotropic Thermal Expansion of Structurally Related Lanthanide-Mercury(II) Cyanide Coordination Polymers

Three sets of related lanthanide-mercury(II) cyanide coordination polymers were synthesized by the reaction of LnCl3·xH2O (Ln = Ce, Nd, Eu, Gd, Tb, Dy, Ho, Tm, Yb, and Lu) with Hg(CN)2 and structurally characterized. [Ce(OH2)5][Hg(CN)2Cl]3·2H2O is a 3-D material with sheet-based architecture; its thermal expansion behavior shows uniaxial negative thermal expansion (-18.3(8), 39(2), and 68.3(16) ppm K-1 along the a, b, and c axes, respectively). This anisotropic thermal behavior is postulated to be driven elastically by weak Hg···Cl interactions: large area expansion of the sheets causes negative thermal expansion in the perpendicular direction. Using lanthanides heavier than Ce yielded 2-D sheet-based compounds with the formula [Ln(OH2)x]2[Hg(CN)2]5Cl6·2H2O (Ln = Nd and Eu, x = 7; Ln = Gd, Tb, Dy, Ho, Tm, Yb, and Lu, x = 6). Although there was also evidence for elastic behavior within these materials, both showed uniaxial zero thermal expansion (Ln = Nd: 27.9(17), 22.4(10), and 0.6(12) ppm K-1 along the I, II, and III principal axes, respectively; Ln = Tb: 39.6(12), 1.1(17), and 36.1(7) ppm K-1 along the a, b, and c axes, respectively). Despite their similar structural architecture, this zero thermal expansion was found to occur in different directions─within the plane of the 2-D sheets for [Nd(OH2)7]2[Hg(CN)2]5Cl6·2H2O but in the direction perpendicular to the 2-D sheets for [Tb(OH2)6]2[Hg(CN)2]5Cl6·2H2O. Overall, this system of compounds reveals the delicate relationship between coordination polymer structure and thermal expansion.

Electronic origin of negative thermal expansion in samarium hexaboride revealed by X-ray diffraction and total scattering

Samarium hexaboride, SmB6, is a negative thermal expansion (NTE) material whose structure is similar to other known NTE materials such as the family of Prussian blues. In the Prussian blues, NTE is due to a phonon mechanism, but we recently showed from DFT calculations that this is unlikely in SmB6 (Li et al., Phys. Chem. Chem. Phys. 2023, 25, 10749). We now report experimental X-ray diffraction and pair distribution function analysis of this material in the temperature range 20-300 K. The interatomic distances shown by both methods are consistent with the NTE instead arising from an electronic effect, by which the samarium atoms lose electrons and thus have a smaller ionic radius as the temperature increases.

Thermal expansion anisotropy of Fe23Mo16 and Fe7Mo6 μ-phases predicted using first-principles calculations

The intermetallic μ-phase, which precipitates in steels and superalloys, can noticeably soften the mechanical properties of their matrix. Despite the importance of developing superalloys and steels, the thermodynamic properties and directions of thermal expansion of the μ-phase are still poorly studied. In this work, the thermal expansion paths, elastic, thermal and thermodynamic properties of the Fe23Mo16 and Fe7Mo6 μ-phases are studied using the first-principles-based quasi-harmonic Debye-Grüneisen approach. A method that avoids differentiation in many variables is used. The free energies consisting of the electronic, vibrational and magnetic energy contributions, calculated along different paths of thermal expansions, were compared among themselves. A path with the least free energy was chosen as the trajectory of thermal expansion. Negative thermal expansion of the Fe7Mo6 compound was predicted, while Fe23Mo16 exhibits conventional thermal expansion. The thermal expansions of both these compounds are not isotropic. The elastic constants, moduli, heat capacities, Curie and Debye temperatures were predicted. The obtained results satisfactorily agree with the available experimental data. Physical factors affecting the stability of Fe23Mo16 and Fe7Mo6 have been studied. This study presents an essential feature of thermal expansion of the μ-phase of the Fe-Mo system, which can provide an insight into future developments.

Antiferroelectricity-Induced Negative Thermal Expansion in Double Perovskite Pb2 CoMoO6

Materials with negative thermal expansion (NTE) attract significant research attention owing to their unique physical properties and promising applications. Although ferroelectric phase transitions leading to NTE are widely investigated, information on antiferroelectricity-induced NTE remains limited. In this study, single-crystal and polycrystalline Pb2 CoMoO6 samples are prepared at high pressure and temperature conditions. The compound crystallizes into an antiferroelectric Pnma orthorhombic double perovskite structure at room temperature owing to the opposite displacements dominated by Pb2+ ions. With increasing temperature to 400 K, a structural phase transition to cubic Fm-3m paraelectric phase occurs, accompanied by a sharp volume contraction of 0.41%. This is the first report of an antiferroelectric-to-paraelectric transition-induced NTE in Pb2 CoMoO6 . Moreover, the compound also exhibits remarkable NTE with an average volumetric coefficient of thermal expansion αV = -1.33 × 10-5 K-1 in a wide temperature range of 30-420 K. The as-prepared Pb2 CoMoO6 thus serves as a prototype material system for studying antiferroelectricity-induced NTE.

Unusual thermo-mechanical properties of the Janus Mo2ScC2OH MXene monolayer

In this study, we investigated the structural, electronic, mechanical, dynamical, and thermal properties of the Janus Mo2ScC2OH MXene monolayer using ab initio calculations based on density functional theory. The results showed that Janus Mo2ScC2OH is metallic and mechanically stable. We found that Mo2ScC2OH has high Poisson's ratio. Phonon dispersion calculations revealed that there are no imaginary frequencies, suggesting that the Janus Mo2ScC2OH monolayer is dynamically stable. Debye temperature, Grüneisen parameters, thermodynamic properties, and lattice thermal conductivity have also been calculated. The results revealed that Mo2ScC2OH has high negative Grüneisen parameters, indicating that it could be a negative thermal expansion material. Additionally, we noted that the Janus Mo2ScC2OH monolayer exhibits a relatively low lattice thermal conductivity, with a notable contribution from the ZA mode.

Storage and Diffusion of Carbon Dioxide in the Metal Organic Framework MOF-5─A Semi-empirical Molecular Dynamics Study

Metal-organic frameworks (MOFs) have attracted increasing attention due to their high porosity for exceptional gas storage applications. MOF-5 belongs to the family of isoreticular MOFs (IRMOFs) and consists of Zn4O6+ clusters linked by 1,4-benzenedicarboxylate. Due to the large number of atoms in the unit cell, molecular dynamics simulation based on density functional theory has proved to be too demanding, while force field models are often inadequate to model complex host-guest interactions. To overcome this limitation, an alternative semi-empirical approach using a set of approximations and extensive parametrization of interactions called density functional tight binding (DFTB) was applied in this work to study CO2 in the MOF-5 host. Calculations of pristine MOF-5 yield very good agreement with experimental data in terms of X-ray diffraction patterns as well as mechanical properties, such as the negative thermal expansion coefficient and the bulk modulus. In addition, different loadings of CO2 were introduced, and the associated self-diffusion coefficients and activation energies were investigated. The results show very good agreement with those of other experimental and theoretical investigations. This study provides detailed insights into the capability of semi-empirical DFTB-based molecular dynamics simulations of these challenging guest@host systems. Based on the comparison of the guest-guest pair distributions observed inside the MOF host and the corresponding gas-phase reference, a liquid-like structure of CO2 can be deduced upon storage in the host material.

Giant uniaxial negative thermal expansion in FeZr2 alloy over a wide temperature range

Negative thermal expansion (NTE) alloys possess great practical merit as thermal offsets for positive thermal expansion due to its metallic properties. However, achieving a large NTE with a wide temperature range remains a great challenge. Herein, a metallic framework-like material FeZr₂ is found to exhibit a giant uniaxial (1D) NTE with a wide temperature range (93-1078 K, [Formula: see text]). Such uniaxial NTE is the strongest in all metal-based NTE materials. The direct experimental evidence and DFT calculations reveal that the origin of giant NTE is the couple with phonons, flexible framework-like structure, and soft bonds. Interestingly, the present metallic FeZr₂ excites giant 1D NTE mainly driven by high-frequency optical branches. It is unlike the NTE in traditional framework materials, which are generally dominated by low energy acoustic branches. In the present study, a giant uniaxial NTE alloy is reported, and the complex mechanism has been revealed. It is of great significance for understanding the nature of thermal expansion and guiding the regulation of thermal expansion.

Phonon mechanism for the negative thermal expansion of zirconium tungstate, ZrW2O8

Negative thermal expansion (NTE) in ZrW₂O₈ was investigated using a flexibility analysis of ab initio phonons. It was shown that no previously proposed mechanism adequately describes the atomic-scale origin of NTE in this material. Instead it was found that the NTE in ZrW₂O₈ is driven, not by a single mechanism, but by wide bands of phonons that resemble vibrations of near-rigid WO₄ units and Zr-O bonds at low frequency, with deformation of O-W-O and O-Zr-O bond angles steadily increasing with increasing NTE-phonon frequency. It is asserted that this phenomenon is likely to provide a more accurate explanation for NTE in many complex systems not yet studied.

Influence of the physical properties of the layered oxyselenides Bi2YO4Cu2Se2 through Ni doping

We have synthesized a series of Ni-doped layered oxyselenides Bi2YO4Cu2-xNixSe2 (0 ≤ x ≤ 0.4). The crystal structure and physical properties were studied through X-ray diffraction, and electric and thermo transport measurements. We also performed DFT calculations to study the electric structure of the designed Bi2YO4Ni2Se2, which is similar to that of KNi2Se2.

Colossal negative thermal expansion in a cucurbit[8]uril-enabled uranyl-organic polythreading framework via thermally induced relaxation

It is an ongoing goal to achieve the effective regulation of the thermal expansion properties of materials. In this work, we propose a method for incorporating host-guest complexation into a framework structure and construct a flexible cucurbit[8]uril uranyl-organic polythreading framework, U₃(bcbpy)₃(CB8). U₃(bcbpy)₃(CB8) can undergo huge negative thermal expansion (NTE) and has a large volumetric coefficient of -962.9 × 10^-6 K^-1 within the temperature range of 260 K to 300 K. Crystallographic snapshots of the polythreading framework at various temperatures reveal that, different from the intrinsic transverse vibrations of the subunits of metal-organic frameworks (MOFs) that experience NTE via a well-known hinging model, the remarkable NTE effect observed here is the result of a newly-proposed thermally induced relaxation process. During this process, an extreme spring-like contraction of the flexible CB8-based pseudorotaxane units, with an onset temperature of ∼260 K, follows a period of cumulative expansion. More interestingly, compared with MOFs that commonly have relatively strong coordination bonds, due to the difference in the structural flexibility and adaptivity of the weakly bonded U₃(bcbpy)₃(CB8) polythreading framework, U₃(bcbpy)₃(CB8) shows unique time-dependent structural dynamics related to the relaxation process, the first time this has been reported in NTE materials. This work provides a feasible pathway for exploring new NTE mechanisms by using tailored supramolecular host-guest complexes with high structural flexibility and has promise for the design of new kinds of functional metal-organic materials with controllable thermal responsive behaviour.

Stress-Induced Inversion of Linear Dichroism by 4,4'-Bipyridine Rotation in a Superelastic Organic Single Crystal

The molecular crystals that manifest unusual mechanical properties have attracted growing attention. Herein, we prepared an organic single crystal that shows bidirectional superelastic transformation in response to shear stress. Single-crystal X-ray diffractions revealed this crystal-twinning related shape change is owed to a stress-controlled 90° rotation of 4,4'-bipyridine around the hydrogen bonds of a chiral organic trimer. As a consequence of the 90° shift in the aromatic plane, an interconversion of crystallographic a-, b-axes (a→b' and b→a') was detected. The molecular rotations changed the anisotropic absorption of linearly polarized light. Therefore, a stress-induced inversion of linear dichroism spectra was demonstrated for the first time. This study reveals the superior mechanical flexibilities of single crystals can be realized by harnessing the molecular rotations and this superelastic crystal may find applications in optical switching and communications.

Metamaterial with Tunable Positive and Negative Hygrothermal Expansion Inspired by a Four-Fold Symmetrical Islamic Motif

A metamaterial with controllable positive and negative thermal and hygroscopic expansions is investigated herein by inspiration from a range of Islamic geometric patterns. Constructing from eight pairs of pin-jointed Y-elements, each unit cell manifests eight rhombi that are arranged circumferentially, thereby manifesting four axes of symmetry. By attachment of bimaterial spiral springs of contrasting expansion coefficients to the far arms of the paired Y-elements, a change in the environment’s thermal or hygroscopic condition alters the offset angle of the paired Y-elements such that the unit cell of the metamaterial ranges from the eight-pointed star to the regular octagon. The effective coefficient of thermal expansion (CTE) and the coefficient of moisture expansion (CME) of this metamaterial were developed for small and large changes in environmental fluctuations using infinitesimal and finite models, respectively. Generated data indicates that the sign and magnitude of the effective thermal and hygroscopic expansion coefficients can be controlled by geometrical descriptors of the bimaterial spiral spring—such as its coil number and the ratio of its mean radius to its thickness—as well as the properties of the bimaterial’s layers such as their expansion coefficients, Young’s moduli and, in the case of effective hygroscopic expansion, their relative absorptivity. The obtained results suggest that the proposed metamaterial can be designed to perform as highly sensitive thermal and/or moisture sensors, as well as other functional materials or devices that take advantage of environmental changes as stimuli.

A Poling-Free Supramolecular Crown Ether Compound with Large Piezoelectricity

Supramolecular host-guest ferroelectrics based on solution processing are highly desirable because they are generally created with intrinsic piezoelectricity/ferroelectricity and do not need further poling. Poly(vinylidene fluoride) (PVDF) in the electric-active beta phase after stretching/annealing still shows no piezoelectric response unless poled. Although many supramolecular host-guest ferroelectrics have been discovered, their piezoelectricity is relatively small. Based on H/F substitution, we reported a supramolecular host-guest compound [(CF₃-C₆H₄-NH₃)(18-crown-6)][TFSA] (CF₃-C₆H₄-NH₃ = 4-trifluoromethylanilinium, TFSA = bis(trifluoromethanesulfonyl)ammonium) with a remarkable piezoelectric response of 42 pC/N under no poling condition. The introduction of F atoms increases phase transition temperature, polar axes, second harmonic generation (SHG) intensity, and piezoelectric coefficient d₃₃. To our knowledge, such a large piezoelectric performance of [(CF₃-C₆H₄-NH₃)(18-crown-6)][TFSA] makes its d₃₃, piezoelectric voltage coefficient g₃₃, and mechanical quality factor Q_m the highest among the reported supramolecular host-guest ferroelectric compounds and even larger than the values of PVDF. This work provides inspiration for optimizing piezoelectricity on molecular materials.

Significant Zero Thermal Expansion Via Enhanced Magnetoelastic Coupling in Kagome Magnets

Zero-thermal-expansion (ZTE) alloys, as dimensionally stable materials, are urgently required in many fields, particularly in highly advanced modern industries. In this study, high-performance ZTE with a negligible coefficient of thermal expansion a_v as small as 2.4 ppm K^-1 in a broad temperature range of 85-245 K is discovered in Hf_0.85 Ta_0.15 Fe₂ C_0.01 magnet. It is demonstrated that the addition of trace interstitial atom C into Ta-substituted Hf_0.85 Ta_0.15 Fe₂ exhibits significant capability to tune the normal positive thermal expansion into high-performance ZTE via enhanced magnetoelastic coupling in stabilized ferromagnetic structure. Moreover, direct observation of the magnetic transition between ferromagnetic and triangular antiferromagnetic states via Lorentz transmission electron microscopy, along with corresponding theoretical calculations, further uncovers the manipulation mechanism of ZTE and negative thermal expansion. A convenient and effective method to optimize thermal expansion and achieve ZTE with interstitial C addition may result in broadened applications based on the strong correlation between the magnetic properties and crystal structure.

Sign change in c-axis thermal expansion constant and lattice collapse by Ni substitution in transition-metal zirconide superconductor Co1-xNixZr2

Recently, c-axis negative thermal expansion (NTE) was observed in a CoZr₂ superconductor and related transition-metal zirconides. Here, we investigated the structural, electronic, and superconducting properties of Co_1-xNi_xZr₂ to achieve systematic control of c-axis NTE and switching from NTE to positive thermal expansion (PTE) by Ni substitution. At x ≤ 0.3, c-axis NTE was observed, and the thermal expansion constant α_c approached zero with increasing x. At x = 0.4-0.6, c-axis thermal expansion close to zero thermal expansion (ZTE) was observed, and PTE appeared for x ≥ 0.7. On the superconducting properties, we observed bulk superconductivity for x ≤ 0.6, and bulk nature of superconductivity is suppressed by Ni heavy doping (x ≥ 0.7). For x ≤ 0.6, the evolution of the electronic density of states well explains the change in the superconducting transition temperature (T_c), which suggests conventional phonon-mediated superconductivity in the system. By analyzing the c/a ratio, we observed a possible collapsed transition in the tetragonal lattice at around x = 0.6-0.8. The lattice collapse would be the cause of the suppression of superconductivity in Ni-rich Co_1-xNi_xZr₂ and the switching from NTE to PTE.

Physical Mechanism and Chemical Trends in the Thermal Expansion of Inorganic Halide Perovskites

The considerable thermal expansion of halide perovskites is one of the challenges to device stability, yet the physical origin and modulation strategy remain unclear. Herein, we report first-principles calculations of the thermal properties of halide perovskites at 300 K using oxides as a reference. We found that the large thermal expansion of halide perovskites can mainly be attributed to their low bulk modulus and volumetric heat capacity because of the soft crystal lattice, whereas composition-dependent anharmonicity emerges as the most important factor in determining thermal expansion with the same structure. We discovered that thermal expansion of halide perovskites can be decreased by weakening the B-X bond to promote the octahedral anharmonicity. We further proposed an effective thermal expansion coefficient descriptor of halide perovskites with a Pearson correlation coefficient of nearly -80%. Our findings provide insights into the underlying mechanisms and chemical trends in the thermal expansion behavior of halide perovskites.

Polymorphs with Remarkably Distinct Physical and/or Chemical Properties

Polymorphism in crystals is known since 1822 and the credit goes to Mitscherlich who realized the existence of different crystal structures of the same compound while working with some arsenate and phosphate salts. Later on, this phenomenon was observed also in organic crystals. With the advent of different technologies, especially the easy availability of single crystal XRD instruments, polymorphism in crystals has become a common phenomenon. Almost 37 % of compounds (single component) are polymorphic to date. As the energies of the different polymorphic forms are very close to each other, small changes in crystallization conditions might lead to different polymorphic structures. As a result, sometimes it is difficult to control polymorphism. For this reason, it is considered to be a nuisance to crystal engineering. It has been realized that the property of a material depends not only on the molecular structure but also on its crystal structure. Therefore, it is not only of interest to academia but also has widespread applications in the materials science as well as pharmaceutical industries. In this review, we have discussed polymorphism which causes significant changes in materials properties in different fields of solid-state science, such as electrical, magnetic, SHG, thermal expansion, mechanical, luminescence, color, and pharmaceutical. Therefore, this review will interest researchers from supramolecular chemistry, materials science as well as medicinal chemistry.

Reduced thermal expansion by surface-mounted nanoparticles in a pillared-layered metal-organic framework

Control of thermal expansion (TE) is important to improve material longevity in applications with repeated temperature changes or fluctuations. The TE behavior of metal-organic frameworks (MOFs) is increasingly well understood, while the impact of surface-mounted nanoparticles (NPs) on the TE properties of MOFs remains unexplored despite large promises of NP@MOF composites in catalysis and adsorbate diffusion control. Here we study the influence of surface-mounted platinum nanoparticles on the TE properties of Pt@MOF (Pt@Zn₂(DP-bdc)₂dabco; DP-bdc^2-=2,5-dipropoxy-1,4-benzenedicarboxylate, dabco=1,4-diazabicyclo[2.2.2]octane). We show that TE is largely retained at low platinum loadings, while high loading results in significantly reduced TE at higher temperatures compared to the pure MOF. These findings support the chemical intuition that surface-mounted particles restrict deformation of the MOF support and suggest that composite materials exhibit superior TE properties thereby excluding thermal stress as limiting factor for their potential application in temperature swing processes or catalysis.

Tailoring Negative Thermal Expansion via Tunable Induced Strain in La-Fe-Si-Based Multifunctional Material

Zero thermal expansion (ZTE) composites are typically designed by combining positive thermal expansion (PTE) with negative thermal expansion (NTE) materials acting as compensators and have many diverse applications, including in high-precision instrumentation and biomedical devices. La(Fe_1-x,Si_x)13-based compounds display several remarkable properties, such as giant magnetocaloric effect and very large NTE at room temperature. Both are linked via strong magnetovolume coupling, which leads to sharp magnetic and volume changes occurring simultaneously across first-order phase transitions; the abrupt nature of these changes makes them unsuitable as thermal expansion compensators. To make these materials more useful practically, the mechanisms controlling the temperature over which this transition occurs and the magnitude of contraction need to be controlled. In this work, ball-milling was used to decrease particles and crystallite sizes and increase the strain in LaFe_11.9Mn_0.27Si_1.29H_x alloys. Such size and strain tuning effectively broadened the temperature over which this transition occurs. The material's NTE operational temperature window was expanded, and its peak was suppressed by up to 85%. This work demonstrates that induced strain is the key mechanism controlling these materials' phase transitions. This allows the optimization of their thermal expansion toward room-temperature ZTE applications.

Thermal Expansion of 3C-SiC Obtained from In-Situ X-ray Diffraction at High Temperature and First-Principal Calculations

In situ X-ray crystallography powder diffraction studies on beta silicon carbide (3C-SiC) in the temperature range 25-800 °C at the maximum peak (111) are reported. At 25 °C, it was found that the lattice parameter is 4.596 Å, and coefficient thermal expansion (CTE) is 2.4 ×10-6/°C. The coefficient of thermal expansion along a-direction was established to follow a second order polynomial relationship with temperature (α11=-1.423×10-12T2+4.973×10-9T+2.269×10-6). CASTEP codes were utilized to calculate the phonon frequency of 3C-SiC at various pressures using density function theory. Using the Gruneisen formalism, the computational coefficient of thermal expansion was found to be 2.2 ×10-6/°C. The novelty of this work lies in the adoption of two-step thermal expansion determination for 3C-SiC using both experimental and computational techniques.

Diverse Thermal Expansion Behaviors in Ferromagnetic Cr1-δTe with NiAs-Type, Defective Structures

Negative thermal expansion (NTE) and zero thermal expansion (ZTE) properties are of great significance for the long-life stable operation of precision equipment. However, there are still existing challenges in finding new materials that exhibit NTE or ZTE over a wide temperature range. Here, we report negative, zero, and positive thermal expansion in NiAs-type, defective Cr_1-δTe, containing three compounds: hexagonal CrTe, monoclinic Cr₃Te₄, and trigonal Cr₅Te₈. CrTe shows the NTE behavior from 280 to 340 K with the volume coefficient of thermal expansion α_V = -27.6 × 10^-6 K^-1. Cr₃Te₄ shows the ZTE behavior over a wide temperature range of 180-320 K (α_V = 0.16 × 10^-6 K^-1). And Cr₅Te₈ holds the PTE behavior over the whole temperature range (α_V = 38.5 × 10^-6 K^-1). All of the samples show obvious anisotropic thermal expansion on heating. Combined with the magnetic measurements, it can be confirmed that the NTE and ZTE properties in ferromagnetic Cr_1-δTe originate from the magnetovolume effect (MVE). Such NiAs-type, defective compounds with similar compositions but different structures provide a new perspective for tuning the NTE properties of materials and searching for new materials with ZTE over a wide temperature range.

Negative Thermal Expansion in Fluoroapatite Pb5(VO4)3F Enhanced by the Steric Effect of Pb2

Negative thermal expansion (NTE) is an unusual thermophysical phenomenon and has gained attention as a way of controlling thermal expansion. Here, we report a substantial NTE in fluoroapatite Pb₅(VO₄)₃F in a limited temperature range. The dilatometric study revealed volume shrinkage below 150 K, giving a linear thermal expansion coefficient of α_L = -44 ppm/K in the temperature range from 140 to 120 K upon heating. The NTE behavior is associated with a structural transition from the hexagonal (P6₃/m) phase to the monoclinic (P2₁/b) phase. Such a structural transition has been found in other apatite-type compounds, but the magnitude of the volume change in Pb₅(VO₄)₃F is remarkable. Our structural analysis revealed that the structural transition is classified as an antiferroelectric-to-paraelectric transition and the volume change during the transition is enhanced by the steric effect of 6s² lone-pair electrons of Pb²⁺.

A Seawater-Corrosion-Resistant and Isotropic Zero Thermal Expansion (Zr,Ta)(Fe,Co)2 Alloy

Zero thermal expansion (ZTE) alloys as dimensionally stable materials are usually challenged by harsh environmental erosion, since ZTE and corrosion resistance are generally mutually exclusive. Here, a high-performance alloy, Zr_0.8 Ta_0.2 Fe_1.7 Co_0.3 , is reported, that shows isotropic ZTE behavior (α_l  = 0.21(2) × 10^-6 K^-1 ) in a wide temperature range of 5-360 K, high corrosion resistance in a seawater-like solution compared with classic Invar and stainless Invar, and excellent cyclic thermal and structural stabilities. Such stabilities are attributed to the cubic symmetry, the controllable magnetic order, and the spontaneously formed passive film with Ta and Zr chemical modifications. The results are evidenced by X-ray/neutron diffraction, microscopy, spectroscopy, and electrochemistry investigations. Such multiple stabilities have the potential to broaden the robust applications of ZTE alloys, especially in marine services.

Single-Crystal to Single-Crystal Transformation of a Spin-Crossover Hybrid Perovskite via Thermal-Induced Cyanide Linkage Isomerization

Linkage isomers involving changes in the bonding mode of ambidentate ligands have potential applications in data storage, molecular machines, and motors. However, the observation of the cyanide-linkage-isomerism-induced spin change (CLIISC) effect characterized by single-crystal X-ray diffraction remains a considerable challenge. Meanwhile, the high-spin and low-spin states can be reversibly switched in spin-crossover (SCO) compounds, which provide the potential for applications to data storage, switches, and sensors. Here, a new perovskite-type SCO framework (PPN)[Fe{Ag(CN)₂}₃] (PPN⁺ = bis(trisphenylphosphine)iminium cation) is synthesized, which displays the unprecedented aging and temperature dependences of hysteretic multistep SCO behaviors near room temperature. Moreover, the thermal-induced cyanide linkage isomerization from Fe^II-N≡C-Ag^I to Fe^II-C≡N-Ag^I is revealed by single-crystal X-ray diffraction, Raman, and Mössbauer spectra, which is associated with a transition from the mixed spin state to the low-spin state and a dramatic volume shrinkage. Considering the wide use of cyanogen in magnetic systems, the association of CLIISC and SCO opens a new dimension to modulate the spin state and realize a colossal negative thermal expansion.

Low temperature magnetic behavior and thermal expansion anomaly of cubic CeTiO3

Lanthanum-based LnBO₃ perovskite oxides have demonstrated fascinating magnetic properties and spin–lattice coupling. In this work, we report an unusual thermal expansion anomaly coupled with the magnetic ordering in the cubic CeTiO₃ with the vacancy of Ce ions. The magnetic behaviors and lattice thermal expansion at low temperature were systematically investigated using the temperature dependence of the magnetization measurements and low temperature X-ray powder diffraction. It is clearly revealed that there are two magnetic transitions in the cubic CeTiO₃ from 5 to 350 K: one is a magnetic ordering–disordering transition at 300 K and the other one might be a change of the magnetic component near 32 K. Both the magnetization and hysteresis change correspondingly upon cooling. Intriguingly, a lattice thermal expansion anomaly is found below the magnetic ordering temperature, which indicates a strong coupling of spin and lattice, i.e., a magnetovolume effect (MVE). Our findings provide the possibility of adjusting thermal expansion behavior and magnetic properties by introducing a vacancy of Ln atoms in lanthanum-based perovskite oxides.

An unusual thermal expansion anomaly with magnetic ordering in cubic CeTiO₃ was found. A magnetic ordering–disordering transition at 300 K and a change of the magnetic component near 32 K were noted. A magnetovolume effect was found below the magnetic ordering temperature.

Understanding Large Negative Thermal Expansion of NdFe(CN)6 through the Electronic Structure and Lattice Dynamics

A large negative thermal expansion (NTE) (α_v = -4.1 × 10^-5 K^-1, 100-525 K) has been discovered in NdFe(CN)₆. Here, the synchrotron X-ray diffraction and lattice dynamics calculations using the density functional theory were conducted to understand the NTE in NdFe(CN)₆. The information obtained on the bond nature of the Nd-N≡C-Fe linkage and on the atomic thermal vibrations suggests that the transverse vibrations of the -N≡C- group, in particular from N atoms, produced the NTE in NdFe(CN)₆. This is corroborated by the calculated Grüneisen parameters, which confirm the relationship between NTE and CN atomic vibrations. The results provide a helpful contribution toward the realization of new materials with negative or controllable thermal expansion.

Transformation of Thermal Expansion from Large Volume Contraction to Nonlinear Strong Negative Thermal Expansion in PbTiO3-Bi(Co1-xFex)O3 Perovskites

Controlling negative thermal expansion (NTE) is an important topic in the study of NTE materials. Generally, a large magnitude of NTE with a wide NTE operation temperature window is preferable for applications of NTE materials, as a stronger NTE can be used to tailor the coefficient of thermal expansion (CTE) of materials with positive thermal expansion by forming composites more efficiently. However, controlling the NTE in single-phase materials is still a significant challenge. In present study, we proposed a promising method to control the thermal expansion from large volume contraction in a limited temperature widow (x = 0, ΔV = -4.8%, 675-700 °C) to a nonlinear strong NTE over a wider temperature range (x = 0.8, α̅V = -6.12 × 10-5/°C, RT to 600 °C) by means of adjusting the proportion of cations with different ferroelectric activities in 0.5PbTiO3-0.5Bi(Co1-xFex)O3 ferroelectrics. The obtained NTE was stronger than many of the currently available NTE materials, and the operation window of NTE was also in an extended temperature range. The unusual transformation is well explained by the spontaneous volume ferroelectrostriction effect, which was evidenced by joint experimental and theoretical studies. The present work not only may pave the way for controllable large NTE in PbTiO3-based ferroelectrics but also could be extended to magnetic NTE materials, whose NTE is coupled with magnetism.

A series of dynamic single crystals of [MII(en)3]SO4 (M = Ni, Mn, and Cd) shows tunable dielectric properties and anisotropic thermal expansion

A series of dynamic single crystals with a chemical formula of [M^II(en)₃]SO₄ (en = ethylene and M^II = Ni^II, Mn^II, and Cd^II) was synthesized. As the temperature decreases, these materials exhibit dielectric switching in the vicinity of the phase transition point accompanied by anisotropic thermal expansion in the cell parameters as a consequence of the order-disorder structural change of SO2-4 in a cavity surrounded by five [M^II(en)₃]²⁺ complex cations. Because the variation of metal centers with different ionic radii changes the shape of the complex cation, which affects the distribution of hydrogen-bond interactions around the SO2-4, the dynamic motion of SO2-4 is substantially tuned. Correspondingly, the dielectric properties and anisotropic thermal expansion of materials were largely shifted, especially in the single crystals of [Mn^II(en)₃]SO₄, whose structural change is distinctly different from the crystals of Ni(II) and Cd(II). The detailed structural mechanism accounting for the different physical properties of these materials was discussed.

A Combination of Calcination and the Spark Plasma Sintering Method in Multiferroic Ceramic Composite Technology: Effects of Process Temperature and Dwell Time

This study reports a combined technological process that includes synthesis by the calcination powder route and sintering by the Spark Plasma Sintering (SPS) method for multiferroic ceramic composites in order to find the optimal sintering conditions. The effects of temperature on the SPS process and dwell time on the microstructure and dielectric properties of the PF composites were discussed. Research has shown that using the SPS method in the technological process of the multiferroic composites favors the correct densification of powders and allows for obtaining a fine-grained microstructure with good properties and electrophysical parameters in the composite material. The optimal set of parameters and properties is demonstrated by the sample obtained at the temperature of 900 °C for 3 min, i.e., resistivity (6.4 × 10⁸ Ωm), values of the dielectric loss factor (0.016), permittivity at room temperature (753) and permittivity at the phase transition temperature (3290). Moreover, due to the high homogeneity of the microstructure, the strength of the material against electric breakdown increases (when examining the ferroelectric hysteresis loop, the application of a high electric field (3—3.5 kV/mm) is also possible at higher temperatures). In the case of the composite material tested, both the lower and higher temperatures as well as the shorter and longer dwell times (compared to the optimal SPS process conditions) did not contribute to the improvement of the microstructure or the set of usable parameters of the composite materials. The strength of the ceramic samples against electric breakdown has also diminished, while the phenomenon of leakage current increased.

Chemical Diversity for Tailoring Negative Thermal Expansion

Negative thermal expansion (NTE), referring to the lattice contraction upon heating, has been an attractive topic of solid-state chemistry and functional materials. The response of a lattice to the temperature field is deeply rooted in its structural features and is inseparable from the physical properties. For the past 30 years, great efforts have been made to search for NTE compounds and control NTE performance. The demands of different applications give rise to the prominent development of new NTE systems covering multifarious chemical substances and many preparation routes. Even so, the intelligent design of NTE structures and efficient tailoring for lattice thermal expansion are still challenging. However, the diverse chemical routes to synthesize target compounds with featured structures provide a large number of strategies to achieve the desirable NTE behaviors with related properties. The chemical diversity is reflected in the wide regulating scale, flexible ways of introduction, and abundant structure-function insights. It inspires the rapid growth of new functional NTE compounds and understanding of the physical origins. In this review, we provide a systematic overview of the recent progress of chemical diversity in the tailoring of NTE. The efficient control of lattice and deep structural deciphering are carefully discussed. This comprehensive summary and perspective for chemical diversity are helpful to promote the creation of functional zero-thermal-expansion (ZTE) compounds and the practical utilization of NTE.

Realization of Negative Thermal Expansion in Lead-Free Bi0.5K0.5VO3 by the Suppression of Tetragonality

Bi_1/2K_1/2VO₃ is a lead-free PbTiO₃-type compound with a tetragonality (c/a = 1.054) comparable to that of typical ferroelectric PbTiO₃ (c/a = 1.064) with negative thermal expansion (NTE) during the tetragonal-to-cubic phase transition; therefore, Bi_1/2K_1/2VO₃ is a potential lead-free NTE material if its metastable perovskite structure can be maintained at high temperatures. In the present experiment, electron doping in Bi_1/2K_1/2VO₃ was conducted through substituting K⁺ with La³⁺ to suppress the tetragonality and achieve NTE. La substitution successfully suppressed the tetragonality of Bi_1/2K_1/2VO₃ and also improved its thermal stability. Moreover, both composition- and temperature-induced tetragonal-to-cubic phase transitions occurred. In particular, a large volume shrinkage with a large negative thermal coefficient of expansion (CTE) was obtained for Bi_0.5K_0.46La_0.04VO₃ during the tetragonal-to-cubic phase transition (ΔV = -0.66%). Hence, this study extends the NTE family and also sheds light on the exploration of lead-free piezoelectric materials with controllable thermal expansion.

Efficient and thermally stable broadband near-infrared emission from near zero thermal expansion AlP3O9:Cr3+ phosphors

A near zero thermal expansion NIR phosphor, AlP3O9:Cr3+, with high emission efficiency and excellent thermal stability is synthesized. A compact NIR pc-LED is fabricated and has a promising application in advanced nondestructive analysis technology.

Bi0.33Zr2(PO4)3, a negative thermal expansion material with a Nasicon-type structure

A new negative thermal expansion ceramic material, Bi0.33Zr2(PO4)3, with a sodium zirconium phosphate structure, has been investigated and the results are presented.

Differences in thermal expansion and motion ability for herringbone and face-to-face π-stacked solids

A series of aromatic organic molecules functionalized with different halogen atoms (I/ Br), motion-capable groups (olefin, azo or imine) and molecular length were designed and synthesized. The molecules self-assemble in the solid state through halogen bonding and exhibit molecular packing sustained by either herringbone or face-to-face π-stacking, two common motifs in organic semiconductor molecules. Interestingly, dynamic pedal motion is only achieved in solids with herringbone packing. On average, solids with herringbone packing exhibit larger thermal expansion within the halogen-bonded sheets due to motion occurrence and molecular twisting, whereas molecules with face-to-face π-stacking do not undergo motion or twisting. Thermal expansion along the π-stacked direction is surprisingly similar, but slightly larger for the face-to-face π-stacked solids due to larger changes in π-stacking distances with temperature changes. The results speak to the importance of crystal packing and intermolecular interaction strength when designing aromatic-based solids for organic electronics applications.

Molecular tiltation and supramolecular interactions induced uniaxial NTE and biaxial PTE in bis-imidazole-based co-crystals

Uniaxial NTE and biaxial PTE has been observed in bis-imidazole-based co-crystals induced by molecular tiltation and supramolecular interactions.