Localized Symmetry Breaking for Tuning Thermal Expansion in ScF3 Nanoscale Frameworks

Atomic Layer Deposition of ScF3 and ScxAlyFz Thin Films

In this paper, we present an ALD process for ScF3 using Sc(thd)3 and NH4F as precursors. This is the first material made by ALD that has a negative thermal expansion over a wide-temperature range. Crystalline films were obtained at the deposition temperatures of 250-375 °C, with a growth per cycle (GPC) increasing along the deposition temperature from 0.16 to 0.23 Å. Saturation of the GPC with respect to precursor pulses and purges was studied at 300 °C. Saturation was achieved with Sc(thd)3, whereas soft saturation was achieved with NH4F. The thickness of the films grows linearly with the number of applied ALD cycles. The F/Sc ratio is 2.9:3.1 as measured by ToF-ERDA. The main impurity is hydrogen with a maximum content of 3.0 at %. Also carbon and oxygen impurities were found in the films with maximum contents of 0.5 and 1.6 at %. The ScF3 process was also combined with an ALD AlF3 process to deposit ScxAlyFz films. In the AlF3 process, AlCl3 and NH4F were used as precursors. It was possible to modify the thermal expansion properties of ScF3 by Al3+ addition. The ScF3 films shrink upon annealing, whereas the ScxAlyFz films show thermal expansion, as measured with HTXRD. The thermal expansion becomes more pronounced as the Al content in the film is increased.

Equation of state predictions for ScF3 and CaZrF6 with neural network-driven molecular dynamics

In silico property prediction based on density functional theory (DFT) is increasingly performed for crystalline materials. Whether quantitative agreement with experiment can be achieved with current methods is often an unresolved question, and may require detailed examination of physical effects such as electron correlation, reciprocal space sampling, phonon anharmonicity, and nuclear quantum effects (NQE), among others. In this work, we attempt first-principles equation of state prediction for the crystalline materials ScF3 and CaZrF6, which are known to exhibit negative thermal expansion (NTE) over a broad temperature range. We develop neural network (NN) potentials for both ScF3 and CaZrF6 trained to extensive DFT data, and conduct direct molecular dynamics prediction of the equation(s) of state over a broad temperature/pressure range. The NN potentials serve as surrogates of the DFT Hamiltonian with enhanced computational efficiency allowing for simulations with larger supercells and inclusion of NQE utilizing path integral approaches. The conclusion of the study is mixed: while some equation of state behavior is predicted in semiquantitative agreement with experiment, the pressure-induced softening phenomenon observed for ScF3 is not captured in our simulations. We show that NQE have a moderate effect on NTE at low temperature but does not significantly contribute to equation of state predictions at increasing temperature. Overall, while the NN potentials are valuable for property prediction of these NTE (and related) materials, we infer that a higher level of electron correlation, beyond the generalized gradient approximation density functional employed here, is necessary for achieving quantitative agreement with experiment.

Enhanced Thermoelectric Performance and Low Thermal Conductivity in Cu2GeTe3 with Identified Localized Symmetry Breakdown

Highly efficient and eco-friendly thermoelectric generators rely on low-cost and nontoxic semiconductors with high symmetry and ultralow lattice thermal conductivity κ_L. We report the rational synthesis of the novel cubic (Ag, Se)-doped Cu₂GeTe₃ semiconductors. A localized symmetry breakdown (LSB) was found in the composition of Cu_1.9Ag_0.1GeTe_1.5Se_1.5 (i.e., CAGTS15) with an ultralow κ_L of 0.37 W/mK at 723 K, the lowest value outperforming all Cu₂GeCh₃ (Ch = S, Se, and Te). A joint investigation of synchrotron X-ray techniques identifies the LSB embedded into the cubic CAGTS15 host matrix. This LSB is an Ångström-scale orthorhombic symmetry unit, characteristic of multiple bond lengths, large anisotropic atomic displacements, and distinct local chemical coordination of anions. Computational results highlight that such an unusual orthorhombic symmetry demonstrates low-frequency phonon modes, which become softer and more predominant with increasing temperatures. This unconventional LSB promotes bond complexity and phonon scattering, highly beneficial for extraordinarily low lattice thermal conductivity.

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.

Hidden Local Symmetry Breaking in a Kagome-Lattice Magnetic Weyl Semimetal

Exploring the relationship between intriguing physical properties and structural complexity is a central topic in studying modern functional materials. Co₃Sn₂S₂, a newly discovered kagome-lattice magnetic Weyl semimetal, has triggered intense interest owing to the intimate coupling between topological semimetallic states and peculiar magnetic properties. However, the origins of the magnetic phase separation and spin glass state below T_C in this ordered compound are two unresolved yet important puzzles in understanding its magnetism. Here, we report the discovery of local symmetry breaking surprisingly co-emerges with the onset of ferromagnetic order in Co₃Sn₂S₂, by a combined use of neutron total scattering and half-polarized neutron diffraction. An anisotropic distortion of the cobalt kagome lattice at the atomic/nano level is also found, with distinct distortion directions among the two Co1 and four Co2 atoms. The mismatch of local and average symmetries occurs below T_C, indicating that Co₃Sn₂S₂ evolves to an intrinsically lattice disordered system when the ferromagnetic order is established. The local symmetry breaking with intrinsic lattice disorder provides new understanding of the puzzling magnetic properties. Our density functional theory (DFT) calculation indicates that the local symmetry breaking is expected to reorient local ferromagnetic moments, unveiling the existence of the ferromagnetic instability associated with the lattice instability. Furthermore, DFT calculation unveils that the local symmetry breaking could affect the Weyl property by breaking the mirror plane. Our findings highlight the fundamentally important role that the local symmetry breaking plays in advancing our understanding on the magnetic and topological properties in Co₃Sn₂S₂, which may draw attention to explore the overlooked local symmetry breaking in Co₃Sn₂S₂, its derivatives and more broadly in other topological Dirac/Weyl semimetals and kagome-lattice magnets.

Multiple valence bands convergence and strong phonon scattering lead to high thermoelectric performance in p-type PbSe

Thermoelectric generators enable the conversion of waste heat to electricity, which is an effective way to alleviate the global energy crisis. However, the inefficiency of thermoelectric materials is the main obstacle for realizing their widespread applications and thus developing materials with high thermoelectric performance is urgent. Here we show that multiple valence bands and strong phonon scattering can be realized simultaneously in p-type PbSe through the incorporation of AgInSe₂. The multiple valleys enable large weighted mobility, indicating enhanced electrical properties. Abundant nano-scale precipitates and dislocations result in strong phonon scattering and thus ultralow lattice thermal conductivity. Consequently, we achieve an exceptional ZT of ~ 1.9 at 873 K in p-type PbSe. This work demonstrates that a combination of band manipulation and microstructure engineering can be realized by tuning the composition, which is expected to be a general strategy for improving the thermoelectric performance in bulk materials.

Chemical pressure in functional materials

Chemical pressure, a strange but familiar concept, is a lattice internal force caused by lattice strain with chemical modifications and arouses great interest due to its diversity and efficiency to synthesize new compounds and tune functional materials. Different from physical pressure loaded by an external force that is positive, chemical pressure can be either positive or negative (contract a lattice or expand it), often through flexible and mild chemical synthesis strategies, which are particularly important as a degree of freedom to manipulate material behaviors. In this tutorial review, we summarize the features of chemical pressure as a methodology and demonstrate its role in synthesizing and discovering some typical magnetically, electrically, and thermally responsive functional materials. The measure of chemical pressure using experimental lattice strain and elastic modulus was proposed, which can be used for quantitative descriptions of the correlation between lattice distortion and properties. From a lattice strain point of view, we classify chemical pressure into different categories: (i) chemical substitution, (ii) chemical intercalation/de-intercalation, (iii) size effect, and (iv) interface constraint, etc. Chemical pressure affects chemical bonding and rationalizes the crystal structure by modifying the electronic structure of solids, regulating the lattice symmetry, local structure, phonon structure effects etc., emerging as a general and effective method for synthesizing new compounds and tuning functional materials.

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.

Inflection in size-dependence of thermally enhanced up-conversion luminescence of UCNPs

The investigation on the size-dependence of UCL thermal behavior of UCNPs with the size down to 3 nm reveals an inflection in the size-dependence of the thermo-enhanced luminescence of UCNPs.

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.

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.

Role of "Dumbbell" Pairs of Fe in Spin Alignments and Negative Thermal Expansion of Lu2Fe17-Based Intermetallic Compounds

Knowledge of negative thermal expansion (NTE) is an interesting issue in the field of materials science and engineering. It has been proposed that the unique dumbbell pairs of Fe (dumbbells) are highly entangled in the NTE behaviors of R₂Fe₁₇ (R = rare earth) compounds but still remain controversial. Here, a facile method is employed to explore the role of dumbbells in spin alignments and NTE by the nonstoichiometric design of Lu_2-xFe₁₇ compounds. The powder synchrotron X-ray diffraction, magnetometry, and neutron powder diffraction investigations indicate that a decrease of the Lu content can enhance the dumbbell concentration and motivate an incommensurate magnetic structure simultaneously. However, increasing the dumbbell concentration makes little difference in the amplitude of the ordered magnetic moments of Fe sublattices, which reveals an equivalent NTE behavior for Lu_2-xFe₁₇ compounds. This work gives insight into the role that dumbbells played in spin alignments and NTE for Lu₂Fe₁₇-based compounds, correcting the previously proposed conjecture and probably conducive to adjusting the related magnetic performances of R₂Fe₁₇ compounds in the future.

Evidence of the enhanced negative thermal expansion in (1 − x)PbTiO3-xBi(Zn2/3Ta1/3)O3

Enhanced polarization displacement in (1 − x)PbTiO₃-xBi(Zn_2/3Ta_1/3)O₃ solutions has been reported.

Perovskite-related ReO3-type structures

Materials with the perovskite ABX3 structure play a major role across materials chemistry and physics as a consequence of their ubiquity and wide range of useful properties. ReO3-type structures can be described as ABX3 perovskites in which the A-cation site is unoccupied, giving rise to the general composition BX3, where B is typically a cation and X is a bridging anion. The chemical diversity of such structures is extensive, ranging from simple oxides and fluorides, such as WO3 and AlF3, to complex structures in which the bridging anion is polyatomic, such as in the Prussian blue-related cyanides Fe(CN)3 and CoPt(CN)6. The same ReO3-type structure is found in metal-organic frameworks, for example, ln (im)3(im = imidazolate) and the well-known MOF-5 structure, where the B-site cation is polyatomic. The extended 3D connectivity and openness of this structure type leads to compounds with interesting and often unusual properties. Notable among these properties are negative thermal expansion (for example, ScF3), photocatalysis (for example, CoSn(OH)6), thermoelectricity (for example, CoAs3) and superconductivity in a phase that is controversially described as SH3 with a doubly interpenetrating ReO3 structure. We present an account of this exciting family of materials and discuss future opportunities in the area.

Negative Thermal Expansion in Nanosolids

Nanosolids usually exhibit a variety of peculiar physical features due to the size effect. The unique surface electronic states and coordination structures of nanosolids make them particularly important as promising functional materials. After several decades of research effort on the preparation processes and formation mechanisms of nanomaterials, the attention of nanoscience has been shifted to their functionalization and utilization. In the development of nanodevices, the thermal expansion matching between nanosized components is becoming increasingly important for the selection of units and design of nanodevices. In nanosolids, particularities of bonding features and coordination environments lead to size-dependent thermal expansion behavior that is significantly different from the behavior of their bulk counterparts. Thus, size tuning becomes one of the most efficient techniques in tailoring lattice thermal expansion. Unlike the traditional tailoring methods like chemical doping, the modification of chemical bonds and lattice vibration modes mainly contributing to the abnormal thermal expansion of nanosolids can be realized by adjustment of local coordination on the surface and surface/interface lattice strain. With the introduction of the nanosizing effect, the functional properties of nanosolids can be thoroughly remolded, which provides a huge space for functional applications of negative thermal expansion (NTE) nanosolids. However, understanding the origin of novel thermal expansion in nanosolids remains a challenging issue because of the lack of knowledge of precise atomic arrangements at both long-range and local structure levels. In this Account, by virtue of various advanced characterization techniques, we provide a comprehensive understanding at the atomic level of the abnormal thermal expansion behaviors in nanosized PbTiO₃-based compounds, oxides, fluorides, and bimetallic alloys. Our results demonstrate that nanoscale structural features can be used to alter the spontaneous polarization, surficial/interfacial coordination, local lattice symmetry, and elemental distribution, resulting in the crossover of thermal expansion from the bulk and the generation of zero thermal expansion (ZTE). Furthermore, structural peculiarities in nanosolids, e.g., the lack of long-range coherence, abnormal surficial/interfacial bonding, lattice imperfection, and distribution of local phases, open the door for local-scale manipulations of the physical properties of electronic structure and lattice vibration during adjustment of thermal expansion. For the development of nanodevices with high thermostability, atomic-level information on the nanostructure thermal evolution provides a guideline for intelligent designs of the functional components and matrix. Understanding of the structural transformation in nanosolids will help future exploration of functional nanomaterials based on short-range atomistic design and optimization.

Negative Thermal Expansion in (Hf,Ti)Fe2 Induced by the Ferromagnetic and Antiferromagnetic Phase Coexistence

Negative thermal expansion (NTE) is an intriguing physical phenomenon that can be used in the applications of thermal expansion adjustment of materials. In this study, we report a NTE compound of (Hf,Ti)Fe₂, while both end members of HfFe₂ and TiFe₂ show positive thermal expansion. The results reveal that phase coexistence is detected in the whole NTE zone, in which one phase is ferromagnetic (FM), while the other is antiferromagnetic (AFM). With increasing temperature, the FM phase is gradually transformed to the AFM one. The NTE phenomenon occurs in the present (Hf,Ti)Fe₂ because of the fact that the unit cell volume of the AFM phase is smaller than that of the FM phase, and the mass fraction of the AFM phase increases with increasing temperature. The construction of phase coexistence can be a method to achieve NTE materials in future studies.

Negative thermal expansion in cubic FeFe(CN)6 Prussian blue analogues

A new isotropic negative thermal expansion compound of FeFe(CN)₆ has been found, in which the transverse vibrations of N atoms dominate in its NTE behavior.