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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 Gruneisen parameter. The present study provides one wide band gap semiconductor Ta2WO8 with anomalous thermal expansion behavior.

Oxygen vacancies have a profound effect on the electric, magnetic, and optical properties of transition-metal oxides, but their effect on thermal expansion properties has rarely been studied. Herei...

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 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, T...

Perovskite Er1-xCaxMnO3 (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5) was synthesized using a solid-state method. Thermal expansion behavior was tested using a thermal dilatometer and high-temperature X-ray diffraction (XRD). The experimental results indicated the doping contents of Ca (x) in the Er1-xCaxMnO3 have a dramatic effect on their thermal expansion behavior. The samples of Er1-xCaxMnO3 (x = 0.1,0.2 and 0.25) exhibit positive thermal expansion (PTE) characteristics while Er0.7Ca0.3MnO3 (x = 0.3) exhibits a negative thermal expansion (NTE) property with a thermal expansion coefficient of −3.1 × 10−6 K−1 in room temperature (RT) −750 K. In addition, Er0.6Ca0.4MnO3 (x = 0.4) exhibits NTE properties only at RT–500 K, and Er0.5Ca0.5MnO3 (x = 0.5) exhibits PTE properties at RT–750 K. The thermal shrinkage mechanism is the Jahn–Teller effect of the Mn3+ ions and the double exchange of Mn3+–O–Mn4+ in Er0.7Ca0.3MnO3. This phenomenon causes Mn–O octahedral distortion and oxygen vacancy, causing Er0.7Ca0.3MnO3 to become anisotropic. This feature results in the elastic deformation of Er0.7Ca0.3MnO3 during heating, which consumes the void and displays NTE at macro level.

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.

CuSCN, as a new type of inorganic hole-transporting semiconductor with a wide bandgap (>3.4 eV), is attracting much attention in the fabrication of perovskite solar cells. In this article, by using first-principles density functional theory (DFT) and the quasi-harmonic approximation (QHA) approach, we have studied lattice dynamics and abnormal thermal expansion of the system, including α- and β-CuSCN phases. The influence of the abnormal thermal expansion of the lattice on the electronic structure, especially on the bandgap of the system, was explored and discussed. We found that due to the shearing modes and the three acoustic modes along the direction of the c-axis, the α- and β-CuSCN show a negative thermal expansion (NTE) in the direction of the c-axis. The torsion modes of the Cu–N–C–S atomic chains in the α-CuSCN may lead to an NTE in the directions of the a, b-axes of the α-phase. As a result, our theoretical results demonstrated that the α-CuSCN exhibits an anisotropic bulk NTE. While the β-CuSCN displays a strong uniaxial negative thermal expansion in the direction of the c-axis, in the directions of the a, b-axes, it exhibits positive thermal expansion. Our DFT calculations also predicted that the α-CuSCN has a direct bandgap, which increases slightly with increasing temperature. However, the β-CuSCN has an indirect bandgap at low temperature, which converts to a direct bandgap near the temperature of 375 K due to the strong positive expansion in the ab plane of the phase. Our work revealed the mechanisms of the abnormal thermal expansion of the two phases and a strong coupling between the anisotropic thermal expansion and the electronic structures of the system.

The control of thermal expansion is attractive but remains challenging for the studies of negative thermal expansion materials. Here, we report a continuously controllable thermal expansion from negative to positive by means of adjusting the magnetic moment of Fe in Er2(Fe,Co)14B intermetallic compounds. Interestingly, a zero thermal expansion (ZTE) has been achieved in a wide temperature range (ΔT = 355 K), which is larger than most existing ZTE alloys. The spontaneous magnetostriction (ωS) reveals a negative contribution arising from the magneto-volume effect to the controllable thermal expansion. The present study could be suitable for the design of controllable thermal expansion of other alloys related to the magneto-volume effect.

Negative thermal expansion (NTE) behavior is an interesting physical phenomenon, but the number of NTE materials is limited. In this study, a new NTE compound has been found, FeFe(CN)6 Prussian blue analogue, where the average linear coefficient of thermal expansion (αl) is −4.260 × 10−6 K−1 between 100 and 450 K. The NTE properties and local vibration dynamics have been investigated by joint experiments of synchrotron X-ray diffraction, X-ray pair distribution function, and extended X-ray absorption fine structure spectroscopy. It has been observed that the Fe–C/Fe–N bonds expand with increasing temperature, while the unit cell shrinks in FeFe(CN)6. The vibration directions of both Fe–C and Fe–N prefer to be perpendicular to the linkage of Fe–CN–Fe rather than being parallel. More pieces of evidence indicate that the transverse vibrations of N atoms dominate the NTE behavior of FeFe(CN)6. The present results prove directly that the transverse thermal vibrations of C and N atoms are crucial for the occurrence of the NTE of Prussian blue analogues.

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 investigat...

Exploring new abnormal thermal expansion materials is important to understand the nature of thermal expansion. Metal–organic framework (MOF) with unique structure flexibility is an ideal material to study the thermal expansion. This work adopts the high-resolution variable-temperature powder x-ray diffraction to investigate the structure and intrinsic thermal expansion in Sr-MOF ([Sr(DMPhH2IDC)2] n ). It has the unique honeycomb structure with one-dimensional (1D) channels along the c-axis direction, the a–b plane displays layer structure. The thermal expansion behavior has strong relationship with the structure, ZTE appears in the a–b plane and large PTE along the c-axis direction. The possible mechanism is that the a/b layers have enough space for the transverse thermal vibration of polydentate ligands, while along the c-axis direction is not. This work not only reports one interesting zero thermal expansion material, but also provides new understanding for thermal expansion mechanism from the perspective of the structural model.

We report a new type of near-zero thermal expansion material β-CuZnV2O7 in a large temperature range from 173 K to 673 K. It belongs to a monoclinic structure (C2/c space group) in the whole temperature range. No structural phase transition is observed at atmospheric pressure based on the x-ray diffraction and Raman experiment. The high-pressure Raman experiment demonstrates that two structural phase transitions exist at 0.94 GPa and 6.53 GPa, respectively. The mechanism of negative thermal expansion in β-CuZnV2O7 is interpreted by the variations of the angles between atoms intuitively and the phonon anharmonicity intrinsically resorting to the negative Grüneisen parameter.

Knowledge of controllable thermal expansion is a fundamental issue in the field of materials science and engineering. Direct blocking of the thermal expansions in positive thermal expansion materials is a challenging but fascinating task. Here we report a near zero thermal expansion (ZTE) of SnO2 achieved from twin crystal nanowires, which is highly correlated to the twin boundaries. Local structural evolutions followed by pair distribution function revealed a remarkable thermal local distortion along the twin boundary. Lattice dynamics investigated by Raman scattering evidenced the hardening of phonon frequency induced by the twin crystal compressing, giving rise to the ZTE of SnO2 nanowires. Further DFT calculation of Gruneisen parameters confirms the key role of compressive stress on ZTE. Our results provide an insight into the thermal expansion behavior regarding to twin crystal boundaries, which could be beneficial to the applications.

Negative thermal expansion (NTE) is one of intriguing properties for solid compounds. Here we report structure and tunable negative thermal expansion in the (Hf1-xNbx)Fe2 magnetic alloys. The NTE mechanism has been investigated by a combined analysis of neutron powder diffraction, synchrotron X-ray diffraction, and macroscopic magnetic measurements. Direct experimental evidence shows that magnetovolume effect is the root of NTE of ferromagnetic phase, while the paramagnetic phase responds to the general positive thermal expansion (PTE). Especially, by adjusting the amount of Nb chemical substitution in (Hf1-xNbx)Fe2, the coexistence of NTE ferromagnetic and PTE paramagnetic phases can tune the large NTE (x = 0.05, αv = -23.13 × 10-6 K-1, 323~398 K) to the relatively low thermal expansion (x = 0.15, αv = -8.28×10-6 K-1, 173~323 K). The present study reveals the direct link between NTE and magnetic structure, and shows that thermal expansion could be tuned by the coexistence of magnetic and paramagnetic phases.

Negative thermal expansion (NTE) as an interesting physical behavior is promising for thermal expansion control engineering. β-CuSCN consists of linear chain units with NTE along the c-axis. The NTE mechanism of β-CuSCN is investigated by variable temperature x-ray diffraction, temperature- and pressure-dependent Raman spectra, and first-principles calculations. It is found that the quasi rigid unit modes associated with the rotations of S–C≡N–Cu chains driven by Cu and S antiphase transverse vibrations and longitudinal acoustic and transverse acoustic modes involving the collective motions of atoms have large negative Gruneisen parameters, contributing significantly to the NTE of c-axis. Translational and librational motions of C≡N units, in which C and N atoms vibrate in the same and opposite directions have much smaller negative Gruneisen parameters, contribute only a minor part to the NTE, which is different from the known NTE mechanism of cyanides and Prussian blue analogous.

An orthorhombic AlMoVO7 was synthesized by solid-state method. Its phase, expansion property, and Raman spectra were analyzed in detail. Analysis showed that AlMoVO7 has near-zero expansion in 173–673 K. With increasing temperature, the librational and translational vibrations of [AlO6] and [MoO4] caused the Al-O-Mo chain bending. The thermal expansion of AlMoVO7 unit cell has an anisotropic and near-zero expansion. The thermal expansion performance of AlMoVO7 block depends on its unit cell structure and microstructure. Therefore, near-zero expansion in 173–673 K is shown.

A novel negative thermal expansion material of Zn2V1·7P0·3O7 is prepared using a solid-state method. Its phase, microstructure, thermal expansion property, and Raman spectra are analyzed in detail. Experimental results show that Zn2V1·7P0·3O7 exhibits excellent negative expansion characteristics with an expansion coefficient of −4.47 × 10−6 K−1 in a wide temperature range of RT-673 K. The underlying mechanism of the negative thermal expansion of Zn2V1·7P0·3O7 is as follows. The distorted [ZnO5] trigonal bipyramides and [V2O7] structural units with staggered Cs symmetry (part of which V is replaced by P) in Zn2V1·7P0·3O7 are connected by sharing O2 and O3 atoms to form a chain, such that the sample has a layered staggered structure. The lateral vibration of the O3 atoms and the coupling twist and rotation effect of [ZnO5] and [VO4] polyhedrons intensify, causing angles between Zn–O3(long)-V and Zn–O3(short)-V to decrease gradually. The gap between the inner layers of unit cell is gradually reduced, and the volume of Zn2V1·7P0·3O7 shrinks slowly.

Negative thermal expansion (NTE) of materials is an intriguing phenomenon challenging the concept of traditional lattice dynamics and of importance for a variety of applications. Herein, we investigate the mechanism of NTE in the phenacite structure Zn2GeO4 using first-principles calculations with quasi-harmonic approximation (QHA). It is shown that in contrast to the NTE driven by the low energy optical-phonon modes in most open-framework oxides, the NTE in Zn2GeO4 is dominated by the lowest-frequency acoustic phonon modes which possess large negative Gruneisen parameters. Due to the three coordination nature of oxygen in the phenacite structure, the lateral vibration of oxygen is largely inhibited. The Raman and infrared spectra are measured to provide further proof for the rationality of the theoretical calculation. It is predicted and experimentally demonstrated that elemental doping has a negligible effect on the thermal expansion property in the structure because it is dictated by the long wavelength acoustic phonons which should be insensitive to local defects. This work provides an understanding on the NTE in the phenacite structure and may prompt the search for new NTE materials with similar structures.

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)6 and ScCo(CN)6 have 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)6 have 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.

The multiple sintering compounds of perovskite Sm0.85Zn0.15MnO3, which are denoted as S1, S2, S3, S4, and S5, have been synthesized by a solid-state method. The analysis of their crystal structure and microstructure were conducted by X-ray diffractometer and scanning electron microscopy, respectively. The average particle size and porosity were counted by microscopic image analysis. The thermal-expansion behavior of the compounds was studied by a dilatometer and high-temperature XRD. Experimental results indicated that the once sintering sample (S1) of Sm0.85Zn0.15MnO3 possesses the negative thermal expansion (NTE) properties with a coefficient of thermal expansion of −8.40 × 10−6 K−1 in the range of 519–1023 K. By contrast, the thermal expansion behavior for the multiple sintering samples (S2–S5) is not same as that of S1. The porosity of the samples dramatically affects their thermal expansion behavior. The samples S1 and S2 exhibit the NTE characteristics as their porosity was larger than 9.5%, but the samples S3, S4, and S5 exhibit positive thermal expansion characteristics, as their porosity is less than 9.5%. In addition, anisotropy of thermal expansion plays a crucial role in the NTE mechanism. The mechanism underlying the NTE of Sm0.85Zn0.15MnO3 (S1) involves the coupling effect of the high porosity and the microstructural NTE effect caused by crystal grains with anisotropic thermal expansion and intercrystalline pores.

β-Cu2V2O7 is prepared by using the solid-state method. We discuss that β-Cu2V2O7 exhibits large negative thermal expansion property over a wide temperature range and no phase change occurs. The coefficient of thermal expansion is −20.2 × 10−6 K−1 in temperature range of RT–673 K. The change of cell parameters is obtained from High-temperature XRD. Its phase, microstructure, thermal expansion property, and Raman spectra are analyzed in detail. The underlying mechanism of the negative thermal expansion of β-Cu2V2O7 involves the coupling effect of the tetrahedron caused by the lateral vibration of the bridge oxygen atom and tensile effect of the tetrahedron. The partial collapse caused by the loss of oxygen atoms is also crucial in this mechanism.

Negative thermal expansion (NTE), whereby lattices contract upon heating, is of considerable interest for its wide applications in many fields. Molecular materials have been widely investigated as catalysts, sensors, etc., which usually endure temperature vibration. NTE can become a substantial means for controlling the coefficients of thermal expansion. Molecular materials possess plentiful structures and can be easily decorated, making them ideal platforms for thermal expansion modification. In this feature article, we provide an overview of the recent developments in utilizing NTE in molecular materials and summarize some mechanisms leading to NTE. The discussion of NTE in molecular materials concerns many factors, including transverse vibration, geometric flexibility, host-guest interactions, spin crossover, molecular packing rearrangement and molecular conformational changes.

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.

The achievement of controlling thermal expansion is important for open-framework structures. The present work proposes a feasible way to adjust the coefficient of thermal expansion continuously from negative to positive via inserting guest Na+ ions or H2O molecules into a GaFe(CN)6 framework. The guest ions or molecules have an intense dampening effect on the transverse vibrations of CN atoms in the −Ga–N≡C–Fe– linkage, especially for the N atoms. This study demonstrates that electrochemical or redox intercalation of guest ions will be an effective way to tune thermal expansion in those negative thermal expansion open-framework materials induced by low-frequency phonons.

The control of thermal expansion of solid compounds is intriguing but remains challenge. In this study, we investigate the effect of guests on the thermal expansion of open-framework structures. It is interesting to observe that the presence of guest ions (K+) and molecules (H2O) can largely switch thermal expansion of YFe(CN)6 from negative (αv = -33.67*10-6 K-1) to positive (αv = +42.72*10-6 K-1), a range which covers the thermal expansion of most inorganic compounds. The mechanism of such giant switching of thermal expansion is revealed via joint studies of synchrotron X-ray diffraction, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations. The presence of guest ions or molecules plays a critical damping effect on transverse vibrations, thus inhibiting the negative thermal expansion. The present study demonstrates an effective method to control the thermal expansion in open-framework materials by adjusting the presence of guests.

Thermal expansion properties of solids are of fundamental interest and control of thermal expansion is important for practical applications but can be difficult to achieve. Many framework-type materials show negative thermal expansion when internal cages are empty but positive thermal expansion when additional atoms or molecules fill internal voids present. Here we show that redox intercalation offers an effective method to control thermal expansion from positive to zero to negative by insertion of Li ions into the simple negative thermal expansion framework material ScF3, doped with 10% Fe to enable reduction. The small concentration of intercalated Li ions has a strong influence through steric hindrance of transverse fluoride ion vibrations, which directly controls the thermal expansion. Redox intercalation of guest ions is thus likely to be a general and effective method for controlling thermal expansion in the many known framework materials with phonon-driven negative thermal expansion., The positive thermal expansion exhibited by most materials at increased temperatures is a severe issue for many high precision applications. Here, Xing and co-workers show that redox intercalation of Li ions into a ScF3 framework offers effective control of the thermal expansion for this simple material.

The efficiency of gas sensors varies enormously from fundamental study to practical application. This big gap comes mainly from the complex and unpredictable effect of atmospheric environment, especially in humidity. Here, the cross-sensitivity to humidity of a SnO2 sensor from local structural and lattice evolutions is studied. The sensing response of ethanol is found to be efficiently activated by adsorbing trace of water but inhibited as humidity increases. By X-ray diffraction, pair distribution function of synchrotron and ab initio calculations, the independent effect of water and ethanol on lattice and local structure are clearly revealed, which elucidate the intricate sensing reactions. The formation of hydrogen bonds and repulsion of ethoxides play key roles in the structural distortions, and also in adsorption energies that are critical to the sensitive behavior. The results show the sensor performance coupled with local structural evolution, which provides a new insight into the controversial effects of humidity on SnO2 sensors.