Local vibrations and negative thermal expansion in ZrW2O8

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.

Electrochemical and Structural Investigation of ReO3

The structural evolution with alkali ion insertion, and the subsequent thermal evolution of the alkali-ion inserted ReO₃ electrodes are shown by employing in situ and ex situ synchrotron X-ray diffraction (XRD). During Na and K insertion, there is a combination of intercalation into ReO₃ and a two-phase reaction. Interestingly in the case of Li insertion, a more complex evolution is noted, which suggests a conversion reaction takes place at deep discharge (insertion). Following these ion insertion studies, extracted electrodes at various states of discharge (kinetically determined) were examined with variable temperature XRD. The thermal evolution of the A_x ReO₃ phases, where A=Li, Na, or K, are significantly modified from the parent ReO₃ thermal evolution. This shows the impact of alkali-ion insertion on the thermal properties of ReO₃ .

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.

Tailoring Thermal Expansion of (LiFe)0.5xCu2-xP2O7 via Codoping LiFe Diatoms in Cu2P2O7 Oxide

Tailoring the thermal expansion coefficient of negative thermal expansion (NTE) materials to achieve near-zero thermal expansion has attracted great attention recently. Here, LiFe diatoms are adopted to substitute Cu in Cu₂P₂O₇ oxide to design Li-O-P and Fe-O-P linkages, with the stronger bond strength of Li-O and Fe-O compared to Cu-O and hence lowering the bond strength of P-O. With increasing the diatomic LiFe in (LiFe)_0.5xCu_2-xP₂O₇, new Raman bands corresponding to LiFeP₂O₇ appear and the NTE coefficient decreases gradually to near-zero thermal expansion at x = 1 (α_v = -0.90 × 10^-6 °C^-1, -100 to 55 °C). Comparing (LiFe)_0.5CuP₂O₇ with Cu₂P₂O₇ and LiFeP₂O₇, the average bond length of P-O increases while the bond angle of P-O-P decreases, and this is verified by some weakened vibrational energies of terminal PO₃ and P-O-P, resulting in the obvious red shift of Raman bands. Ceramic (LiFe)_0.5CuP₂O₇ presents a lower difference in grain size and a higher relative density than Cu₂P₂O₇ and LiFeP₂O₇.

Intrinsic volumetric negative thermal expansion in the "rigid" calcium squarate

The calcium squarate with a rigid framework is found to exhibit volumetric negative thermal expansion (NTE) with the coefficient -9.51(5) × 10^-6 K^-1 and uniaxial zero thermal expansion (ZTE, -0.14(4) × 10^-6 K^-1) over a wide temperature. Detailed comparison of the long-range and local structure sheds light on the fact that the anomalous thermal expansion originates from the transverse vibration of the bridging squarate ligand, although it has been tightly bonded by five calcium ions. We believe that this study can provide a deep insight into the origin of NTE and the structural flexibility of metal organic frameworks (MOFs).

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.

Achieving High Performances of Ultra-Low Thermal Expansion and High Thermal Conductivity in 0.5PbTiO3-0.5(Bi0.9La0.1)FeO3@Cu Core-Shell Composite

Achieving high performances of ultra-low thermal expansion (ULTE) and high thermal conductivity remains challenging, due to the strong phonon/electron-lattice coupling in ULTE materials. In this study, the challenge has been solved via the construction of the core-shell structure in 0.5PbTiO₃-0.5(Bi_0.9La_0.1)FeO₃@Cu composites by the electroless plating, which can simultaneously combine the advantages of the negative thermal expansion material of 0.5PbTiO₃-0.5(Bi_0.9La_0.1)FeO₃ in controlling thermal expansion, and copper metal in high thermal conductivity. By changing the volume fraction of copper, the coefficient of thermal expansion of composites can be adjusted continuously from positive to negative. In particular, a ULTE (ΔT = 400 K) has been achieved in the composite of 35 vol % Cu. Intriguingly, a 3D thermal conductive network copper structure is formed for thermal conducting, which can double the thermal conductivity of the 35 vol % Cu composite from the methods by the traditional mixing (32 W·m^-1·K^-1) up to the core-shell structure (60 W·m^-1·K^-1). The present work not only provides a composite material with excellent comprehensive properties but also proposes a general chemical method to resolve the problem of low thermal conductivity in most ULTE materials.

Intrinsic Isotropic Near-Zero Thermal Expansion in Zn4B6O12X (X = O, S, Se)

Zero thermal expansion (ZTE) materials, keeping size constant as temperature varies, are valuable for resisting the deterioration of the performance from environmental temperature fluctuation, but they are rarely discovered due to the counterintuitive temperature-size effect. Herein, we demonstrate that a family of borates with sodalite cage structure, Zn₄B₆O₁₂X (X = O, S, Se), exhibits intrinsic isotropic near-ZTE behaviors from 5 to 300 K. The very low thermal expansion is mainly owing to the coupling rotation of [BO₄] rigid groups constrained by the bonds between Zn and cage-edged O atoms, while the central atoms in the cage have a negligible contribution. Our study has significant implications on the understanding of the ZTE mechanism and exploration of new ZTE 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.

Structure and Negative Thermal Expansion in Zr0.3Sc1.7Mo2.7V0.3O12

A₂M₃O₁₂-based materials have received considerable attention owing to their wide range of negative thermal expansion (NTE) and chemical flexibility toward novel materials design. However, the structure and NTE mechanism remain challenging. Here, Zr⁴⁺ and V⁵⁺ are used as a unit to compensatorily replace Sc³⁺ and Mo⁶⁺ in Sc₂Mo₃O₁₂ to tune its thermal expansion. Its crystal structure, phase transition, NTE property, and corresponding mechanisms are studied by high-resolution synchrotron X-ray diffraction, powder X-ray diffraction, ultralow-frequency Raman spectroscopy, and density functional theory calculations. The results show that Zr_0.3Sc_1.7Mo_2.7V_0.3O₁₂ adopts an orthorhombic (Pbcn) structure at room temperature, with V atoms occupying the position of Mo1 atoms and Zr atoms occupying the position of Sc atoms, and transforms to monoclinic (P2₁/a) structure at ∼133 K (45 K lower than that of Sc₂Mo₃O₁₂). It exhibits excellent NTE in a broader range. Most of the phonon modes below 350 cm^-1 have negative Grüneisen parameters, of which the lowest and next-lowest frequency (38.5 and 45.8 cm^-1) optical phonon modes arising from the translational vibrations of the Sc/Zr and Mo/V atoms in the plane of the nonlinear linkage Sc/Zr-O-Mo/V have the largest and next-largest negative Grüneisen parameters and positive total anharmonicity, and contribute most to the NTE.

Molecular dynamics simulation of zirconium tungstate amorphization and the amorphous-crystalline interface

Classical molecular dynamics (MD) simulations were performed to provide a conceptual understanding of the amorphous-crystalline interface for a candidate negative thermal expansion (NTE) material, ZrW₂O₈. Simulations of pressure-induced amorphization at 300 K indicate that an amorphous phase forms at pressures of 10 GPa and greater, and this phase persists when the pressure is subsequently decreased to 1 bar. However, the crystalline phase is recovered when the slightly distorted 5 GPa phase is relaxed to 1 bar. Simulations were also performed on a two-phase model consisting of the high-pressure amorphous phase in direct contact with the crystalline phase. Upon equilibration at 300 K and 1 bar, the crystalline phase remains unchanged beyond a thin layer of disrupted structure at the crystalline-amorphous interface. Differences in local atomic structure at the interface are quantified from the simulation trajectories.

Uniaxial thermal expansion behaviors and ionic conduction in a layered (NH4)2V3O8

The zero/negative thermal expansion (ZTE/NTE), which is an intriguing physical property of solids, has been observed in a few families of materials. ZTE materials possess practical applications in specific circumstances such as space-related applications, engineering structures and precision instrument. Generally, NTE materials are used as additives to form a composite of the ZTE material with positive thermal expansion material. It is still a tremendous challenge to design new families of ZTE/NTE materials. Herein, we presented a temperature-dependent single crystal structure analysis in 110-300 K for a layered (NH4)2V3O8, which crystallizes in a tetragonal space group P4bm and comprises mixed valence [V3O82-]∞ monolayers and NH4+ residual in the interlayer spaces. Along the c-axis, (NH4)2V3O8 demonstrated uniaxial expansion behaviors, i.e., ZTE with αc = -1.10 × 10-6 K-1 in 110-170 K and NTE with αc = -16.25 × 10-6 K-1 in 170-220 K. Along the a-axis, (NH4)2V3O8 exhibited ZTE with αa = + 2.06 × 10-6 K-1 in 240-300 K. The mechanisms of ZTE and NTE were explored using structural analysis. The conduction of NH4+ ions in the interlayer space was studied, indicating that the conductivity rapidly rises with the expansion of interlayer space at temperatures of >293 K. This study discloses that layered vanadates are promising ZTE/NTE materials.

Substitutions of Zr4+/V5+ for Y3+/Mo6+ in Y2Mo3O12 for Less Hygroscopicity and Low Thermal Expansion Properties

In this investigation, Zr_xY_2-xV_xMo_3-xO₁₂ (0 ≤ x ≤ 1.4) is developed and the effects of the substitutions of Zr⁴⁺/V⁵⁺ for Y³⁺/Mo⁶⁺ in Y₂Mo₃O₁₂ on the hygroscopicity and thermal expansion property are investigated. For the smaller substitution content (x ≤ 0.5), their crystal structures remain orthorhombic, while there is crystal water still in the lattice. The linear coefficients of thermal expansions (CTEs), for x = 0.1, 0.3, 0.5, and 0.7, are about -4.30 × 10^-6, -0.97 × 10^-6, 0.85 × 10^-6, and 0.77 × 10^-6 K^-1, respectively, from 476 to 773 K, which means that the linear CTE could be changed linearly with the substitution content of Zr⁴⁺/V⁵⁺ for Y³⁺/Mo⁶⁺ in Y₂Mo₃O₁₂. As long as the substitution content reaches x = 1.3/1.4, almost no hygroscopicity and low thermal expansion from room temperature are obtained and are discussed in relation to the crystal structure and microstructure.

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.

Negative Thermal Expansion Near the Precipice of Structural Stability in Open Perovskites

Negative thermal expansion (NTE) describes the anomalous propensity of materials to shrink when heated. Since its discovery, the NTE effect has been found in a wide variety of materials with an array of magnetic, electronic and structural properties. In some cases, the NTE originates from phase competition arising from the electronic or magnetic degrees of freedom but we here focus on a particular class of NTE which originates from intrinsic dynamical origins related to the lattice degrees of freedom, a property we term structural negative thermal expansion (SNTE). Here we review some select cases of NTE which strictly arise from anharmonic phonon dynamics, with a focus on open perovskite lattices. We find that NTE is often present close in proximity to competing structural phases, with structural phase transition lines terminating near T=0 K yielding the most prominent displays of the SNTE effect. We further provide a theoretical model to make precise the proposed relationship among the signature behavior of SNTE, the proximity of these systems to structural quantum phase transitions and the effects of phase fluctuations near these unique regions of the structural phase diagram. The effects of compositional disorder on NTE and structural phase stability in perovskites are discussed.

First-Principles Structural, Mechanical, and Thermodynamic Calculations of the Negative Thermal Expansion Compound Zr2(WO4)(PO4)2

The negative thermal expansion (NTE) material Zr₂(WO₄)(PO₄)₂ has been investigated for the first time within the framework of the density functional perturbation theory (DFPT). The structural, mechanical, and thermodynamic properties of this material have been predicted using the Perdew, Burke and Ernzerhof for solid (PBEsol) exchange-correlation functional, which showed superior accuracy over standard functionals in previous computational studies of the NTE material α-ZrW₂O₈. The bulk modulus calculated for Zr₂(WO₄)(PO₄)₂ using the Vinet equation of state at room temperature is K ₀ = 63.6 GPa, which is in close agreement with the experimental estimate of 61.3(8) at T = 296 K. The computed mean linear coefficient of thermal expansion is -3.1 × 10^-6 K^-1 in the temperature range ∼0-70 K, in line with the X-ray diffraction measurements. The mean Grüneisen parameter controlling the thermal expansion of Zr₂(WO₄)(PO₄)₂ is negative below 205 K, with a minimum of -2.1 at 10 K. The calculated standard molar heat capacity and entropy are C _P ⁰ = 287.6 and S ⁰ = 321.9 J·mol^-1·K^-1, respectively. The results reported in this study demonstrate the accuracy of DFPT/PBEsol for assessing or predicting the relationship between structural and thermomechanical properties of NTE materials.

Extended X-Ray Absorption Fine Structure of ZrW2O8: Theory vs. Experiment

Extended x-ray absorption fine structure (EXAFS) is well-suited for investigations of structure and disorder of complex materials. Recently, experimental measurements and analysis of EXAFS have been carried out to elucidate the mechanisms responsible for the negative thermal expansion (NTE) in zirconium tungstate (ZrW₂O₈). In contrast to previous work suggesting that transverse O-displacements are largely responsible, the EXAFS analysis suggested that correlated rotations and translations of octahedra and tetrahedra within the structure are a major source. In an effort to resolve this controversy, we have carried out ab initio calculations of the structure, lattice vibrations, and EXAFS of ZrW₂O₈ based on real-space multiple-scattering calculations using the FEFF9 code and auxiliary calculations of structure and Debye-Waller factors. We find that the theoretical simulations are consistent with observed EXAFS, and show that both of the above mechanisms contribute to the dynamical structure of ZrW₂O₈.

Low-Frequency Phonon Driven Negative Thermal Expansion in Cubic GaFe(CN)6 Prussian Blue Analogues

The understanding of the negative thermal expansion (NTE) mechanism is vital not only for the development of new NTE compounds but also for effectively controlling thermal expansion. Here, we report an interesting isotropic NTE property in cubic GaFe(CN)₆ Prussian blue analogues (α _l = -3.95 × 10^-6 K^-1, 100-475 K), which is a new example to understand the complex NTE mechanism. A combined study of synchrotron X-ray diffraction, X-ray total scattering, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations shows that the NTE of GaFe(CN)₆ originates from the low-frequency phonons (< ∼100 cm^-1), which are directly related to the transverse vibrations of the atomic -Ga-N≡C-Fe- chains. Both the Ga-N and Fe-C chemical bonds are much softer to bend than to stretch. The direct evidence that transverse vibrational contribution to the NTE of GaFe(CN)₆ is dominated by N, instead of C atoms, is illustrated. It is interesting to find that the polyhedra of GaFe(CN)₆ are not rigid, which is a starting assumption in some models describing the NTE properties of other systems. The NTE mechanism can be vividly described by the "guitar-string" effect, which would be the common feature for the NTE property of many open-framework functional materials, such as Prussian blue analogues, oxides, cyanides, metal-organic frameworks, and zeolites.

Structural, vibrational and thermal expansion properties of Sc2W4O15

A novel oxide material with the formula of Sc2W4O15 and orthorhombic symmetry is synthesized by solid state reactions and its structure, composition, vibrational properties and thermal expansion are investigated and identified by temperature-dependent X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray photoelectron spectrometry (XPS) and dilatometry. It is shown that the oxide material with an orthorhombic symmetry shows a similar structure to that of Sc2W3O12, but with W partially occupying the position of Sc, leading to not only the corner-sharing ScO6-WO4 connections but also the corner-sharing WO6-WO4 connections. Raman spectroscopic studies show that compared to Sc2W3O12, the FWHMs of most Raman modes in Sc2W4O15 increase due to the occupation of W6+ in the Sc3+ position. Besides, the W-O bonds in Sc2W4O15 are slightly harder than those in Sc2W3O12. An intrinsic thermal contraction in a wide range of temperatures (93-572 K) is demonstrated, which is attributed to the librational and translational vibrations of the corner-sharing polyhedra as well as the transverse vibrations of the bridging O atoms in the Sc-O-W and W-O-W linkages.

Efficient Calculation of the Negative Thermal Expansion in ZrW2O8

We present a study of the origin of the negative thermal expansion (NTE) on ZrW₂O₈ by combining an efficient approach for computing the dynamical matrix with the Lanczos algorithm for generating the phonon density of states in the quasi-harmonic approximation. The simulations show that the NTE arises primarily from the motion of the O-sublattice, and in particular, from the transverse motion of the O atoms in the W–O and W–O–Zr bonds. In the low frequency range these combine to keep the WO₄ tetrahedra rigid and induce internal distortions in the ZrO₆ octahedra. The force constants associated with these distortions become stronger with expansion, resulting in negative Grüneisen parameters and NTE from the low frequency modes that dominate the positive contributions from the high frequency modes. This leads us to propose an anharmonic, two-frequency Einstein model that quantitatively captures the NTE behavior.

Negative Hydration Expansion in ZrW_{2}O_{8}: Microscopic Mechanism, Spaghetti Dynamics, and Negative Thermal Expansion

We use a combination of x-ray diffraction, total scattering, and quantum mechanical calculations to determine the mechanism responsible for hydration-driven contraction in ZrW_{2}O_{8}. The inclusion of H_{2}O molecules within the ZrW_{2}O_{8} network drives the concerted formation of new W─O bonds to give one-dimensional (─W─O─)_{n} strings. The topology of the ZrW_{2}O_{8} network is such that there is no unique choice for the string trajectories: the same local changes in coordination can propagate with a large number of different periodicities. Consequently, ZrW_{2}O_{8}·H_{2}O is heavily disordered, with each configuration of strings forming a dense aperiodic "spaghetti." This new connectivity contracts the unit cell via large shifts in the Zr and W atom positions. Fluctuations of the undistorted parent structure towards this spaghetti phase emerge as the key negative thermal expansion (NTE) phonon modes in ZrW_{2}O_{8} itself. The large relative density of NTE phonon modes in ZrW_{2}O_{8} actually reflects the degeneracy of volume-contracting spaghetti excitations, itself a function of the particular topology of this remarkable material.

Transverse vibration driven large uniaxial negative and zero thermal expansion in boron bridged REPt3B framework materials

In this work anomalous uniaxial thermal expansion behaviour at low temperatures along the c-direction of the tetragonal phase of different members of the antiperovskite REPt3B (RE = Sm, Gd-Tm) compounds is reported. Negative or zero thermal expansion (NTE/ZTE) behaviour in these compounds arises due to the transverse vibration of boron atoms in the linear Pt-B-Pt linkage. The coefficient of thermal expansion along the c-axis tends to become more negative in annealed compounds in comparison to those estimated for as-cast samples. While the as-cast TmPt3B and HoPt3B exhibit essentially ZTE behaviour, the NTE coefficient of annealed GdPt3B (∼-28 ppm K-1) is found to be even larger than that of the well known framework material ZrW2O8 (∼-9 ppm K-1) reported in the literature.

Near-Zero Thermal Expansion and Phase Transitions in HfMg1-x Zn x Mo3O12

The effects of Zn²⁺ incorporation on the phase formation, thermal expansion, phase transition, and vibrational properties of HfMg_1-x Zn _x Mo₃O₁₂ are investigated by XRD, dilatometry, and Raman spectroscopy. The results show that (i) single phase formation is only possible for x ≤ 0.5, otherwise, additional phases of HfMo₂O₈ and ZnMoO₄ appear; (ii) The phase transition temperature from monoclinic to orthorhombic structure of the single phase HfMg_1-x Zn _x Mo₃O₁₂ can be well-tailored, which increases with the content of Zn²⁺; (iii) The incorporation of Zn²⁺ leads to an pronounced reduction in the positive expansion of the b-axis and an enhanced negative thermal expansion (NTE) in the c-axes, leading to a near-zero thermal expansion (ZTE) property with lower anisotropy over a wide temperature range; (iv) Replacement of Mg²⁺ by Zn²⁺ weakens the Mo-O bonds as revealed by obvious red shifts of all the Mo-O stretching modes with increasing the content of Zn²⁺ and improves the sintering performance of the samples which is observed by SEM. The mechanisms of the negative and near-ZTE are discussed.

Avoiding the invasion of H2O into Y2Mo3O12 by coating with C3N4 to improve negative thermal expansion properties

The hygroscopicity of Y₂Mo₃O₁₂ has serious influences on its mechanic and negative thermal expansion (NTE) properties. The reported partial ion substitution for Y³⁺ in Y₂Mo₃O₁₂ could reduce the hygroscopicity, however, the expected NTE properties disappear disappointedly. In this investigation, it is found that avoiding the invasion of crystal water and extending the NTE temperature range of Y₂Mo₃O₁₂ to room temperature could be realized together by heating with CO(NH₂)₂. The X-ray diffraction patterns, infrared absorption spectra, scanning electron microscopy images and X-ray photoelectron spectra suggest the formation of small hydrophobic molecules (C₃N₄, C, etc.) coated on the surface, which could clog the microchannels in Y₂Mo₃O₁₂ to avoid the invasion of crystal water. The investigation paves a way to improve the NTE properties by neglecting the influences of water molecules on the stretching vibrations of MoO₄ tetrahedra and the transverse vibrations of bridge oxygen atoms.

Effect of atomic vibrations in XANES: polarization-dependent damping of the fine structure at the Cu K-edge of (creat)2CuCl4

Polarization-dependent damping of the fine structure in the Cu K-edge spectrum of creatinium tetrachlorocuprate [(creat)₂CuCl₄] in the X-ray absorption near-edge structure (XANES) region is shown to be due to atomic vibrations. These vibrations can be separated into two groups, depending on whether the respective atoms belong to the same molecular block; individual molecular blocks can be treated as semi-rigid entities while the mutual positions of these blocks are subject to large mean relative displacements. The effect of vibrations can be efficiently included in XANES calculations by using the same formula as for static systems but with a modified free-electron propagator which accounts for fluctuations in interatomic distances.

Negative Poisson's Ratio in Modern Functional Materials

Materials with negative Poisson's ratio attract considerable attention due to their underlying intriguing physical properties and numerous promising applications, particularly in stringent environments such as aerospace and defense areas, because of their unconventional mechanical enhancements. Recent progress in materials with a negative Poisson's ratio are reviewed here, with the current state of research regarding both theory and experiment. The inter-relationship between the underlying structure and a negative Poisson's ratio is discussed in functional materials, including macroscopic bulk, low-dimensional nanoscale particles, films, sheets, or tubes. The coexistence and correlations with other negative indexes (such as negative compressibility and negative thermal expansion) are also addressed. Finally, open questions and future research opportunities are proposed for functional materials with negative Poisson's ratios.

Negative thermal expansion and associated anomalous physical properties: review of the lattice dynamics theoretical foundation

Negative thermal expansion (NTE) is the phenomenon in which materials shrink rather than expand on heating. Although NTE had been previously observed in a few simple materials at low temperature, it was the realisation in 1996 that some materials have NTE over very wide ranges of temperature that kick-started current interest in this phenomenon. Now, nearly two decades later, a number of families of ceramic NTE materials have been identified. Increasingly quantitative studies focus on the mechanism of NTE, through techniques such as high-pressure diffraction, local structure probes, inelastic neutron scattering and atomistic simulation. In this paper we review our understanding of vibrational mechanisms of NTE for a range of materials. We identify a number of different cases, some of which involve a small number of phonons that can be described as involving rotations of rigid polyhedral groups of atoms, others where there are large bands of phonons involved, and some where the transverse acoustic modes provide the main contribution to NTE. In a few cases the elasticity of NTE materials has been studied under pressure, identifying an elastic softening under pressure. We propose that this property, called pressure-induced softening, is closely linked to NTE, which we can demonstrate using a simple model to describe NTE materials. There has also been recent interest in the role of intrinsic anharmonic interactions on NTE, particularly guided by calculations of the potential energy wells for relevant phonons. We review these effects, and show how anhamonicity affects the response of the properties of NTE materials to pressure.

Negative thermal expansion and broad band photoluminescence in a novel material of ZrScMo2VO12

In this paper, we present a novel material with the formula of ZrScMo₂VO₁₂ for the first time. It was demonstrated that this material exhibits not only excellent negative thermal expansion (NTE) property over a wide temperature range (at least from 150 to 823 K), but also very intense photoluminescence covering the entire visible region. Structure analysis shows that ZrScMo₂VO₁₂ has an orthorhombic structure with the space group Pbcn (No. 60) at room temperature. A phase transition from monoclinic to orthorhombic structure between 70 and 90 K is also revealed. The intense white light emission is tentatively attributed to the n- and p-type like co-doping effect which creates not only the donor- and acceptor-like states in the band gap, but also donor-acceptor pairs and even bound exciton complexes. The excellent NTE property integrated with the intense white-light emission implies a potential application of this material in light emitting diode and other photoelectric devices.

Reprobing the mechanism of negative thermal expansion in siliceous faujasite

Combined Rietveld refinement and pair distribution function analysis of total neutron scattering data unveils the finer details of the negative thermal expansion mechanism of siliceous faujasite.

On EXAFS Debye-Waller factor and recent advances

The effects of structural and vibrational disorder on the EXAFS signals are parameterized in terms of the Debye Waller (DW) factor. Here the vibrational contribution is addressed, which for most systems can be singled out by studying the temperature dependence of the EXAFS DW factor, which corresponds to a good accuracy to the parallel mean square relative displacement (MSRD) around the inter-atomic equilibrium distance. By comparing the first-shell EXAFS thermal expansion with the crystallographic thermal expansion one can evaluate the perpendicular MSRD. The results of recent measurements on copper and on several tetrahedral semiconductors are here critically compared and some properties of the MSRDs are discussed, such as the dependence of correlation, force constants and vibrational anisotropy on crystal structure and bond ionicity as well as the relative merits of the correlated Debye and Einstein models. The anharmonic contribution to the parallel MSRD of CdTe has been evaluated and a quasi-harmonic analysis has been attempted, leading to an estimation of the bond Grüneisen parameter.

Recent advances in the application of total scattering methods to functional materials

In this Feature Article, we provide a description of some of the most recent results obtained in the field of materials science by means of total scattering methods and PDF analysis.