Discovering Large Isotropic Negative Thermal Expansion in Framework Compound AgB(CN)4 via the Concept of Average Atomic Volume

Mixed anion control of enhanced negative thermal expansion in the oxysulfide of PbTiO3

The rare physical property of negative thermal expansion (NTE) is intriguing because materials with a large NTE over a wide temperature range can serve as high-performance thermal expansion compensators. However, the applications of NTE are hindered by the fact that most of the available NTE materials show small magnitudes of NTE, and/or NTE occurs only in a narrow temperature range. Herein, for the first time, we investigated the effect of anion substitution instead of general Pb/Ti-site substitutions on the thermal expansion properties of a typical ferroelectric NTE material, PbTiO3. Intriguingly, the substitution of S for O in PbTiO3 further increases the tetragonality of PbTiO3. Consequently, an unusually enhanced NTE with an average volumetric coefficient of thermal expansion of V = -2.50 × 10-5 K-1 was achieved over a wide temperature range (300-790 K), which is in contrast to that of pristine PbTiO3 (V = -1.99 × 10-5 K-1, RT-763 K). The intensified NTE is attributed to the enhanced hybridization between Pb/Ti and O/S atoms by the substitution of S, as evidenced by our theoretical investigations. We therefore demonstrate a new technique for introducing mixed anions to achieve a large NTE over a wide temperature range in PbTiO3-based ferroelectrics.

Significant Enhancement of Negative Thermal Expansion Under Low Pressure in Cu2P2O7

Much effort is made to achieve the negative thermal expansion (NTE) control, but rare methods reached the improvement of intrinsic NTE. In the present work, a significantly enhanced NTE is realized in Cu2P2O7 by applying low pressure. Especially, the volumetric coefficient of thermal expansion (CTE) of Cu2P2O7 reached to -50.0 × 10-6 K-1 (150-325K) under 0.25 GPa, which is increased by 47.5% compared to its NTE in a similar temperature range under atmosphere pressure. This character enables a more effective manifestation of the thermal compensation role of Cu2P2O7 in composites. The enhanced NTE mechanisms are analyzed by high pressure synchrotron X-ray diffraction, neutron diffraction at variable temperature and pressure, as well as density functional theory (DFT) calculations. The results show that applied pressure accelerates the contraction of the distance between adjacent CuO layers and CuO columns. Meanwhile, the low-frequency phonon contribution to NTE in α-Cu2P2O7 is improved. This work is meaningful for the exploration of methods to enhance NTE and the practical application of NTE materials.

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.

Regulating the thermal expansion of a [FePt(CN)4] layer by axial coordination and dimensional reduction

Thermal expansion regulation by chemical decoration at a molecular level is of great technological value for materials science. Herein, we show that the spin crossover active compound Fe(pyz)Pt(CN)4 (pyz = pyrazine) shows a rare 2D negative thermal expansion (NTE) in the ab-plane. By introducing axial coordination iodine ions or reducing the framework dimension from 3D to 2D, the NTE behavior can be effectively switched to positive thermal expansion (PTE) or even zero thermal expansion (ZTE). Moreover, it is found that different spin states of Fe2+ also influence the magnitude of NTE. Compared with the low-spin (LS) sate, the high-spin (HS) state tends to enhance the magnitude of NTE. Combined in situ structural and Raman spectral analyses revealed that the NTE mainly originates from the transverse vibration of a bridging cyano group and the tailorable thermal expansion is closely related to the state of the Fe-CN-Pt linkage. The present study shows how the rational regulation of the building unit and framework dimensions can effectively control thermal expansion behaviors. This insight can serve as guidance for designing and synthesizing novel NTE materials.

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.

BaCoO2 with Tetrahedral Cobalt Coordination: The Missing Element to Understand Energy Storage and Conversion Applications in BaCoO3-δ-Based Materials

Barium-cobaltate-based perovskite (BaCoO3-δ) and barium-cobaltate-based nanocomposites have been intensively studied in energy storage and conversion devices mainly due to flexible oxygen stoichiometry and tunable nonprecious transition metal oxidation states. Although a rich and complex family of structural polymorphs has already been reported for these perovskites in the literature, the potential structural evolution that may occur during the oxygen reduction reaction and the oxygen evolution reaction has not been investigated so far. In this study, we synthesized and characterized the lowest Co-oxidation state possible in the compound, BaCoO2, which exhibits a quartz-derived, trigonal structure with a helicoidally corner-sharing, CoO4-tetrahedral-framework as already proposed by Spitsbergen et al. Oxygen can reversibly be inserted in such a crystal structure to form BaCoO3-δ, i.e., with 0 ≤ δ ≤ 1, based on the results of an in situ coupled thermogravimetric - neutron diffraction study and which presents therefore giant oxygen capacity storage due to the extreme tunability of the electronic configuration of the cobalt cations which defines the fundamental origins of the materials performance. The reversible conversion of BaCoO2 to BaCoO3-δ associated with a similar electronic conductivity above 900 K permits to clarify the high potential of BaCoO3-δ-based energy storage and conversion devices.

Giant Negative Thermal Expansion in Ultralight NaB(CN)4

Negative thermal expansion (NTE) is crucial for controlling the thermomechanical properties of functional materials, albeit being relatively rare. This study reports a giant NTE (αV ∼-9.2 ⋅ 10-5  K-1 , 100-200 K; αV ∼-3.7 ⋅ 10-5  K-1 , 200-650 K) observed in NaB(CN)4 , showcasing interesting ultralight properties. A comprehensive investigation involving synchrotron X-ray diffraction, Raman spectroscopy, and first-principles calculations has been conducted to explore the thermal expansion mechanism. The findings indicate that the low-frequency phonon modes play a primary role in NTE, and non-rigid vibration modes with most negative Grüneisen parameters are the key contributing factor to the giant NTE observed in NaB(CN)4 . This work presents a new material with giant NTE and ultralight mass density, providing insights for the understanding and design of novel NTE materials.

Mechanism of Negative Thermal Expansion in Monoclinic Cu2P2O7 from First Principles

Negative thermal expansion (NTE) materials generally have high-symmetry space groups, large average atomic volumes, and corner-sharing octahedral and tetrahedral coordination structures. By contrast, monoclinic α-Cu2P2O7, which has a small average atomic volume and edge-sharing structure, has been reported to exhibit NTE, the detailed mechanism of which is unclear. In this study, we investigate the A2B2O7 polymorphs and analyze the NTE behavior of α-Cu2P2O7 using first-principles lattice-dynamics calculations. From the polymorphism investigation in 20 A2B2O7 compounds using 6 representative crystal structures, small A and B cationic radii are found to stabilize the α-Cu2P2O7-type structure. We then analyze the NTE behavior of α-Cu2P2O7 using quasi-harmonic approximation. Our calculated thermal expansion coefficients and anisotropic atomic displacement parameters were in good agreement with those of the experimental reports at low temperatures. From the mode-Grüneisen parameter distribution plotted over the entire first-Brillouin zone, we found that the phonon contributing most significantly to NTE emerges not into the special points but between them. In this phonon mode, the O connecting two PO4 tetrahedra rotates, and the Cu and O vibrate perpendicular to the bottom of the CuO5 pyramidal unit, which folds the ac lattice plane. This vibration behavior can explain the experimentally reported anisotropic NTE behavior of α-Cu2P2O7. Our results demonstrate that the most negative mode-Grüneisen parameter contributing to NTE behavior is not always located on high-symmetry special points, indicating the importance of lattice vibration analyses for the entire first-Brillouin zone.

The formation energy, phase transition, and negative thermal expansion of Fe2-xScxW3O12

Tungstates with a molecular formula A2W3O12 exhibits a wider negative thermal expansion (NTE) temperature range than molybdates but are challenging to synthesize, especially when A = Fe or Cr with metastable structures. To enhance the structural stability of Fe2W3O12, Sc with lower electronegativity is adopted to substitute Fe according to Fe2-xScxW3O12, considering the thermodynamic stability of Sc2W3O12. It is shown that the solid solutions can be easily synthesized and the phase transition temperature (PTT) can be tuned to well below room temperature (RT). Theoretical calculations and experimental results show that the formation energy decreases and the W-O bond in Fe-O-W gradually strengthens as the substitution of Sc in Fe2-xScxW3O12 increases, indicating an increase in structural stability. NTE is enhanced after phase transition with an increase in the Sc content. The reduction in PTT and the enhancement in NTE properties of Fe2W3O12 could result in a decrease in the effective electronegativity of the Fe-site elements, resulting in a low formation energy and strengthened W-O bond in Fe-O-W, which corresponds to a more stable structure.

Janus-Type Mixed-Valent Copper-Cyanido Honeycomb Layers

The synthesis of Janus-type layers, which possess front and back sides that consist of different structures, remains a major challenge in the field of two-dimensional materials. In this study, two Janus-type layered coordination polymers, namely, CuII(NEtH2)(NMe2H·H2O)CuI(CN)3 (1) and CuII(NMe2H)(NMe2H·H2O)CuI(CN)3 (2), were synthesized via a simple one-pot procedure using copper(II) nitrate and sodium cyanido in mixed solutions of dimethylamine and ethylamine. Uniquely, 1 and 2 were composed of cyanido-bridged neutral layers and exhibited a CuICuII mixed-valent state. Meanwhile, using a solution of pure dimethylamine for the synthesis yielded the monovalent three-dimensional framework (NMe2H2)[CuI2(CN)3] (3). Results indicated that the simultaneous use of two mixed amines gave rise to the controlled reduction of CuII ions during the reaction. In addition, each face of the layers was coordinated by different amines on the axial positions of the CuII sites, resulting in anisotropic Janus layers. Furthermore, the thermal expansion behavior of 2 was investigated, demonstrating that the neutral [CuICuII(CN)3] layer was relatively rigid compared with the analogous anionic [CuI2(CN)3]- layer.

Influence of A/B Element Substitution on Negative Thermal Expansion in AB(CN)6 (A = Al, Ga, In; B = Co, Fe, Mn, Cr, V, Ti): A Density Functional Theory Study

Negative thermal expansion as an abnormal physical behavior of materials has promising applications in a high sophisticated equipment field, but the materials are rare. Here, we use the first-principles calculations based on density functional theory combined with the recently developed average atomic volume (AAV = V/N, where V is unit cell volume and N is the number of atoms in the unit) rule to predict the large isotropic negative thermal expansion materials of Prussian blue analogues AB(CN)6 (A = Al, Ga, In; B = Co, Fe, Mn, Cr, V, Ti) in a wide temperature range. Our results clearly show that the coefficient of negative thermal expansion has a near-linear relationship with the average atomic volume of the systems and is also influenced by the element substitution at the A or B site. Lattice dynamic simulations indicate that the main contribution to the negative thermal expansion comes from the low-frequency transverse vibration of the (B)-C≡N-(A) groups, especially the transverse vibration of the N atoms. Thus, the element substitution at the A site (binding to N) can tune the negative thermal expansion behavior of the systems more effectively than that at the B site (binding to C), indicating the different roles of bonds on the negative thermal expansion. Our present work not only expands the kinds of isotropic materials but also gives some insights into the relationship between the average atomic volume and negative thermal expansion.

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.

Switch of Thermal Expansions Triggered by Itinerant Electrons in Isostructural Metal Trifluorides

Manageable thermal expansion (MTE) of metal trifluorides can be achieved by introducing local structure distortion (LSD) in the negative thermal expansion ScF₃. However, an open issue is why isostructural TiF₃, free of LSD, exhibits positive thermal expansion. Herein, a combined analysis of synchrotron X-ray diffraction, X-ray pair distribution function, and rigorous first-principles calculations was performed to reveal the important role of itinerant electrons in mediating soft phonons and lattice dynamics. Metallic TiF₃ demonstrates itinerant electrons and a suppressed Grüneisen parameter γ ≈ -20, while insulating ScF₃ absence of itinerant electrons has a considerable γ ≈ -120. With increasing electron doping concentrations in ScF₃, soft phonons become hardened and the γ is repressed significantly, identical to TiF₃. The presented results update the thermal expansion transition mechanism in framework structure analogues and provide a practical approach to obtaining MTE without inducing sizable structure distortion.

Tuning Negative Thermal Expansion in AlPO4-17 by Insertion of Guest Molecules

The very strong negative thermal expansion in the porous aluminophosphate AlPO₄-17 with a hexagonal erionite structure was tuned by the insertion of oxygen molecules at high pressure. The structure of the oxygen-filled material was determined in situ at high pressure by synchrotron, single-crystal X-ray diffraction. The thermal expansion of this material was measured precisely at 0.38 GPa by synchrotron X-ray powder diffraction. Whereas the overall volume thermal expansion only exhibits a small change with respect to empty AlPO₄-17 at ambient pressure, the expansion along the a direction decreases almost to zero and the expansion along c increases by a factor of 7. Such highly anisotropic thermal expansion properties are of great interest for mechanical and optical applications as in two directions the dimensions of the material are extremely stable, whereas a very strong linear negative thermal expansion of -2.2 × 10^-5 K^-1 is observed in the perpendicular direction. Guest insertion is thus a very powerful tool for tuning the thermal expansion properties of porous materials.

Insight into the Relationship between Negative Thermal Expansion and Structure Flexibility: The Case of Zn(CN)2-Type Compounds

High structure flexibility can lead to large negative thermal expansion (NTE), but the reason is not clear. In this work, first-principles calculations have been carried out to investigate the relationship between NTE and structure flexibility in Zn(CN)₂-type compounds. Smaller bulk modulus corresponds to larger compressibility, thus making the crystal structure more flexible and more suitable for NTE. It indicated that the ionic nature of the bond and the bond length jointly affect the structural flexibility and then act on the transverse vibration of C and N atoms. The results of lattice dynamic suggested that higher structural flexibility promotes a greater number of low-frequency optical modes with negative Grüneisen parameters, resulting in a larger NTE. This work also gives us new insight into the design of NTE materials.

Pressure enhanced negative thermal expansion in 2H CuScO2 from first-principles calculations

Finding materials with negative thermal expansion (NTE) property is challenging. Tuning NTE is of fundamental and technological importance. Pressure enhanced negative thermal expansion behavior in 2H CuScO₂ is found and expounded using density functional theory (DFT) and quasi-harmonious approximation (QHA). The frequencies of low energy modes and Grüneisen parameters decrease under pressure, but the bulk modulus increases with pressure. The transverse vibration of Cu atoms becomes stronger under pressure and the materials undergo thermal softening. These factors including thermal softening, pressure induced decrease of Grüneisen parameters and pressure induced strengthening of transverse vibration of Cu atoms all contribute to the enhanced negative thermal expansion property in 2H CuScO₂ in view of the thermodynamic relationship , Grüneisen's theory of thermal expansion and the mechanism of NTE, respectively.

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.

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.

Anomalous Thermal Expansion in Ta2WO8 Oxide Semiconductor over a Wide Temperature Range

Expansion of material is one of the major impediments in the high precision instrument and engineering field. Low/zero thermal expansion compounds have drawn great attention because of their important scientific significance and enormous application value. However, the realization of low thermal expansion over a wide temperature range is still scarce. In this study, a low thermal expansion over a wide temperature range has been observed in the Ta₂WO₈ oxide semiconductor. It is a balance effect of the negative thermal expansion of the a axis and the positive thermal expansion of the b axis and the c axis to achieve low thermal expansion behavior. The results of the means of variable temperature X-ray diffraction and variable pressure Raman spectroscopy analysis indicated that the transverse vibration of bridging oxygen atoms is the driving force, which is corresponding to the low-frequency lattice modes with a negative Grüneisen parameter. The present study provides one wide band gap semiconductor Ta₂WO₈ with anomalous thermal expansion behavior.

Size and crystal symmetry breaking effects on negative thermal expansion in ScF3 nanostructures

Nowadays, one of the most typical and important potential applications of negative thermal expansion (NTE) materials is to prepare zero thermal expansion or controllable coefficient thermal expansion materials by compounding them with positive thermal expansion materials. The research on NTE properties at the nanoscales is the basis and premise for the realization of high-quality composites. Here, using first-principles calculations, we take a typical open framework material ScF₃ as an example to study a new NTE mechanism at the nanoscale, which involves edge and size effects, as well as crystal symmetry breaking. By analyzing the vibrational modes in ultrathin ScF₃ films, three effects contributing to the NTE properties are identified, namely, the acoustic mode (ZA mode) induced by surface truncation, the enhanced rotations of ScF₆ octahedra in the surface layer and the suppressed rotations of ScF₆ octahedra in the inner layer due to crystal symmetry breaking. With increasing thickness, the effect of the ZA mode vibration gradually weakens, while the rotations of the ScF₆ octahedra in the surface and inner layers are enhanced. Ultimately, the approximately mutual compensation of these three effects makes the NTE coefficients of different thicknesses almost unchanged. Finally, we simply generalize our conclusions to zero dimensional nanoparticles. This work reveals a new NTE mechanism in low-dimensional open framework materials, which serves as a guide in designing NTE materials at the nanoscale.

Understanding Negative Thermal Expansion of Zn2GeO4 through Local Structure and Vibrational Dynamics

Zn₂GeO₄ is a multifunctional material whose intrinsic thermal expansion properties below ambient temperature have not been explored until now. Herein, the thermal expansion of Zn₂GeO₄ is investigated by synchrotron X-ray diffraction, with the finding that Zn₂GeO₄ exhibits very low negative (α_v = -2.02 × 10^-6 K^-1, 100-300 K) and positive (α_v = +2.54 × 10^-6 K^-1, 300-475 K) thermal expansion below and above room temperature, respectively. A combined study of neutron powder diffraction and extended X-ray absorption fine structure spectroscopy shows that the negative thermal expansion (NTE) of Zn₂GeO₄ originates from the transverse vibrations of O atoms in the four- and six-membered rings with ZnO₄-GeO₄ tetrahedra. In addition, the results of temperature- and pressure-dependent Raman spectra identify the low-frequency phonon modes (50-150 cm^-1) with negative Grüneisen parameters softening upon pressuring and stiffening upon heating during the lattice contraction, thus contributing to the NTE. This study not only reports the interesting thermal expansion behavior of Zn₂GeO₄ but also provides further insights into the NTE mechanism of novel structures.

Magnetic structure and uniaxial negative thermal expansion in antiferromagnetic CrSb

Negative thermal expansion (NTE) has been found in a growing number of ferromagnetic and ferrimagnetic materials; however, it remains a challenge to discover antiferromagnetic (AFM) NTE materials. Here, we report the uniaxial NTE properties of AFM intermetallic CrSb systematically, and reveal its uniaxial NTE mechanism for the first time. The present AFM intermetallic CrSb shows uniaxial NTE at high temperature and over a broad temperature window (αa = -6.55 × 10-6 K-1, 360-600 K). The direct experimental evidence of neutron powder diffraction reveals that NTE is induced by the AFM ordering of the Cr atom. The present study demonstrates that due to the transition from an AFM ordered structure to a paramagnetic disordered configuration, the negative contribution to the thermal expansion from the magnetovolume effect overwhelms the positive contribution from anharmonic phonon vibration. This study is of interest to find antiferromagnetic NTE materials.

Zero Thermal Expansion in Ta2Mo2O11 by Compensation Effects

Although zero thermal expansion (ZTE) materials have broad application prospects for high precision engineering, they are rare. Here, a new ZTE material, Ta₂Mo₂O₁₁ (α_l = 0.37 × 10^-6 K^-1, 200-600 K), is reported. A joint study of high-resolution synchrotron X-ray diffraction, temperature- and pressure-dependent Raman spectroscopy, and first-principles calculations was performed to investigate the structure and dynamics of Ta₂Mo₂O₁₁ with the aim of understanding its ZTE mechanism. Ta₂Mo₂O₁₁ displays a layered structure, stacking along the [001] direction. Analysis of the phonon modes indicates that positive and negative contributions to thermal expansion are balanced, and a shrinkage occurs along the layers, while the interlayer distance expands with increasing temperature, thus giving rise to the ZTE behavior of Ta₂Mo₂O₁₁. The present study provides a promising ZTE material and new insights into the mechanisms of thermal expansion.

Effect of H2O Molecules on Thermal Expansion of TiCo(CN)6

Understanding the role of guest molecules in the lattice void of open-framework structures is vital for tailoring thermal expansion. Here, we take a new negative thermal expansion (NTE) compound, TiCo(CN)₆, as a case study from the local structure perspective to investigate the effect of H₂O molecules on thermal expansion. The in situ synchrotron X-ray diffraction results showed that the as-prepared TiCo(CN)₆·2H₂O has near-zero thermal expansion behavior (100-300 K), while TiCo(CN)₆ without water in the lattice void exhibits a linear NTE (α_l = -4.05 × 10^-6 K^-1, 100-475 K). Combined with the results of extended X-ray absorption fine structure, it was found that the intercalation of H₂O molecules has the clear effect of inhibiting transverse thermal vibrations of Ti-N bonds, while the effect on the Co-C bonds is negligible. The present work displays the inhibition mechanism of H₂O molecules on thermal expansion of TiCo(CN)₆, which also provides insight into the thermal expansion control of other NTE compounds with open-framework structures.

Negative and zero thermal expansion in α-(Cu2-xZnx)V2O7 solid solutions

Negative or zero thermal expansion (NTE or ZTE) of materials is intriguing for controllable thermal expansion. We report a series of orthorhombic α-Cu2-xZnxV2O7 (x = 0, 0.1, 0.2), in which the volumetric coefficients of thermal expansion are successfully tuned from -10.19 × 10-6 K-1 to -1.58 × 10-6 K-1 in the temperature range of 100-475 K by increasing the content of Zn2+. It has been revealed that the transverse vibrations of oxygen bonded with vanadium are dominant in the contraction of the bc plane, leading to the overall volume NTE in α-Cu2V2O7. The introduction of Zn2+ densifies the crystal structure, which is presumed to suppress the space of transverse vibrations and results in the ZTE in α-Cu1.8Zn0.2V2O7. This work presents an effective method to realize ZTE in anisotropic framework systems.

Effect of bond on negative thermal expansion of Prussian blue analogues MCo(CN)6(MFe, Ti and Sc): a first-principles study

Negative thermal expansion (NTE) is an abnormal physical behavior that has promising applications for high precision thermal control. Since Prussian blue analogues have the two central linking atoms of -C≡N-, they have large structure flexibility and are suitable to explore new NTE materials. However, understanding the nature of structure flexibility from the point of view of chemical bonding is important and urgent. Here, we adopt for the first time first-principles calculations to predict that the cubic TiCo(CN)6and ScCo(CN)6have NTE behavior. The calculated results for M in MCo(CN)6(M = Fe, Ti and Co) indicated that the Sc-N bond is the strongest, but with the weakest direction dependence among the M-N bonds in the three systems. The lattice dynamics calculations results revealed that the low-frequency phonon vibration modes for NTE in MCo(CN)6have much stronger relationship with the M-N bond feature. The present work reveals the important role of the related bond in the NTE open-framework materials.

Hydrate formation and its effects on the thermal expansion properties of HfMgW3O12

HfMgW₃O₁₂ is a representative material with negative thermal expansion in the ABM₃O₁₂ (A = Zr, Hf; B = Mg, Mn, Zn, M = W, Mo) family. Herein we report a novel feature of hydration in HfMgW₃O₁₂ and its effect on the thermal expansion and its structures which have not been determined previously. It is found that hydrate formation in HfMgW₃O₁₂ occurs under ambient or moisture conditions and restrain the low energy librational and translational and even high energy bending and stretching motions of the polyhedra. The coefficient of thermal expansion could be tailored from negative to zero and positive depending on the hydration levels. The unhydrated HfMgW₃O₁₂ adopts an orthorhombic structure with space group Pna2₁ (33) without phase transition at least from 80 K to 573 K, but pressure-induced structure transition and amorphization are found to occur at about 0.19 Gpa and above 3.93 GPa, respectively.

Controlling thermal expansion within mixed cocrystals by tuning molecular motion capability

Thermal expansion behavior is tuned by incorporating motion-capable or -incapable molecules into organic solids.

Controlling thermal expansion (TE) behaviors of organic materials is challenging because several mechanisms can govern TE, such as noncovalent interaction strength and structural motions. Here, we report the first demonstration of tuning TE within organic solids by using a mixed cocrystal approach. The mixed cocrystals contain three unique molecules, two of which are present in variable ratios. These two molecules either lack or exhibit the ability to undergo molecular motion in the solid state. Incorporation of higher ratios of motion-capable molecules results in larger, positive TE along the motion direction. Addition of a motion-incapable molecule affords solids that undergo less TE. Fine-tuned TE behavior was attained by systematically controlling the ratio of motion-capable and -incapable molecules in each solid.