Colossal Volume Contraction in Strong Polar Perovskites of Pb(Ti,V)O3

Noncollinear Electric Dipoles in a Polar Chiral Phase of CsSnBr3 Perovskite

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two phases of the 3D perovskite, CsSnBr3, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K < T < 85 K, space group P21/m) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I (T < 77 K, space group P21) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and pronounced electrostriction and negative thermal expansion along the polar axis (Q22 ≈ 1.1 m4 C-2; αb = -7.8 × 10-5 K-1) through the onset of polarization. The structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and 81Br nuclear quadrupole resonance spectroscopies. Preliminary attempts to suppress unintentional hole doping to allow for ferroelectric switching are described. Together, the polar symmetry, small band gap, large spin-orbit splitting of Sn 5p orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr3 or its neighboring solid solutions as candidates for bulk Rashba effects.

Antiferroelectricity-Induced Negative Thermal Expansion in Double Perovskite Pb2 CoMoO6

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

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

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

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

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

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.

Zero Thermal Expansion and Strong Covalent Binding of VB2 Compound

Zero thermal expansion (ZTE) is an intriguing phenomenon by virtue of its peculiar lack of expansion and contraction with temperature. The achievement of ZTE in a metallic material is a desired but challenging task. Here we report the ZTE behavior of a single-phase metallic VB₂ compound, stacking with the V and B atomic layers along the c direction (α_V = 2.18 × 10^-6 K^-1, 5-150 K). Neutron powder diffraction demonstrates that the ZTE behavior is entangled in the direct blocking of the lattice expansion along all crystallographic directions with temperature. X-ray photoelectron spectroscopy and density functional theory calculations indicate that strong covalent binding adheres the nearest-neighbor B-B and V-B pairs, which is proposed to control the ZTE within both the basal plane and the c direction. An intimate correlation is revealed between the covalent binding and the lattice parameters. Our work indicates the opportunity to design metallic ZTE with strong chemical binding in the future.

Solid Solutions between PbVO3 and BiCoO3

(1 - x)PbVO₃-xBiCoO₃ solid solutions with 0 ≤ x ≤ 1 were prepared at a high pressure of 5-6 GPa and a high temperature of 1223-1473 K. They adopt a polar tetragonal P4mm structure for the 0 ≤ x ≤ 0.3 and 0.75 ≤ x ≤ 1 ranges with giant tetragonal distortions and a cubic Pm3̅m structure for the 0.4 ≤ x ≤ 0.7 range. High-temperature structural studies with synchrotron X-ray powder diffraction showed that polarization, calculated by the point-charge model, and the tetragonal distortion remained nearly constant in the x = 0.8 sample from 295 K up to the decomposition temperature of about 700 K. Magnetic and differential scanning calorimetry measurements showed that the Néel temperature, T_N, nearly linearly decreased from 470 K for x = 1 to 250 K for x = 0.75 (with T_N = 395 K for x = 0.9 and T_N = 295 K for x = 0.8). Long-range magnetic ordering also takes place at T_N = 44 K for x = 0. All other samples with 0.1 ≤ x ≤ 0.7 demonstrated spin-glass-like magnetic properties and notably reduced Weiss temperatures. Effective magnetic moments estimated for the x = 0.6, 0.65, and 0.7 cubic samples gave evidence that cobalt is present in the +2 and +3 oxidation states, and Co³⁺ cations take the low-spin state.

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.

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.

Polar layered coordination polymers incorporating triazacyclononane-triphosphonate metalloligands

Reactions of metalloligands M^III(notpH₃) (M = Fe, Co and notpH₆ = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid)) with Zn(OAc)₂ under hydrothermal conditions resulted in new metal phosphonates Zn₂Fe(notp)Cl(H₂O) (1) and ZnCo(notpH)(H₂O)·2H₂O (2). They crystallize in polar space groups P6₃ (for 1) and Pca2₁ (for 2), respectively, and exhibit layer structures in which the inorganic layers are separated by the organic groups of the notp ligands. However, the layer topologies of the two compounds are quite different. In 1, the layer contains 6-membered rings composed of one {FeN₃O₃} octahedron, one {Zn1O₃Cl}, one {Zn2O₄} and three {PO₃C} tetrahedra via corner-sharing connections, while in 2, the layer contains 10-membered rings composed of two {CoO₃N₃} octahedra, three {ZnO₄} and five {PO₃C} tetrahedra via vertex-sharing connections. Dielectric measurements on single crystals of 2 confirmed the presence of high dielectric anisotropy. Proton conductivity measurements revealed that the proton conduction is more favourable in 2 due to the presence of a continuous hydrogen bond network in this compound.

Unprecedented lattice volume expansion on doping stereochemically active Pb2+ into uniaxially strained structure of CaBa1-xPbxZn2Ga2O7

Lone pair cations like Pb²⁺ are extensively utilized to modify and tune physical properties, such as nonlinear optical property and ferroelectricity, of some specific structures owing to their preference to adopt a local distorted coordination environment. Here we report that the incorporation of Pb²⁺ into the polar “114”-type structure of CaBaZn₂Ga₂O₇ leads to an unexpected cell volume expansion of CaBa_1-xPb_xZn₂Ga₂O₇ (0 ≤ x ≤ 1), which is a unique structural phenomenon in solid state chemistry. Structure refinements against neutron diffraction and total scattering data and theoretical calculations demonstrate that the unusual evolution of the unit cell for CaBa_1-xPb_xZn₂Ga₂O₇ is due to the combination of the high stereochemical activity of Pb²⁺ with the extremely strained [Zn₂Ga₂O₇]⁴⁻ framework along the c-axis. The unprecedented cell volume expansion of the CaBa_1−xPb_xZn₂Ga₂O₇ solid solution in fact is a macroscopic performance of the release of uniaxial strain along c-axis when Ba²⁺ is replaced with smaller Pb²⁺.

Lone pair cations can impart interesting features in some structures, such as noncentrosymmetry. Here the authors show unexpected cell volume expansion in a polar “114”-type oxide upon replacing Ba²⁺ with a smaller Pb²⁺, and attribute it to high stereochemical activity of Pb²⁺ with the strained framework.

Negative thermal expansion and the role of hybridization in perovskite-type PbTiO3-Bi(Cu0.5Ti0.5)O3

PbTiO₃-BiMeO₃ ferroelectrics have attracted much attention due to not only their extremely large polarization and piezoelectricity but also their controllable thermal expansion.

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.

Robust Giant Tetragonal Distortion Coupled with High-Spin Co3+ in Electron-Doped BiCoO3

BiCoO₃ is a PbTiO₃ type of perovskite oxide with a giant tetragonal distortion (c/a = 1.27) that shows a pressure-induced transition from tetragonal to orthorhombic phases accompanied by a large volume shrinkage at 3 GPa. In this study, we carried out electron doping of BiCoO₃ by substituting Ti⁴⁺ for Co³⁺ in order to destabilize the tetragonal phase and observe a giant negative thermal expansion (NTE) at ambient pressure. BiCo_1-xTi_xO₃ (x = 0, 0.1, 0.2, and 0.25) was successfully obtained by using high-pressure synthesis. However, the c/a ratio of the tetragonal phase was almost constant against x (≤0.2), and NTE was not observed at any x, suggesting that the tetragonal distortion coupled with high-spin Co³⁺ is robust against electron doping. In x = 0.25, a metastable orthorhombic phase was obtained by the high-pressure synthetic process, while it partially transformed into a tetragonal phase after annealing at 600 K. The stability of the giant tetragonal phase is strongly connected with the spin state of Co³⁺.

Giant Magnetoelectric Coupling in Multiferroic PbTi1-x V x O3 from Density Functional Calculations

Giant magnetoelectric coupling is a very rare phenomenon that has gained much attention in the past few decades due to fundamental interest as well as practical applications. Here, we have successfully achieved giant magnetoelectric coupling in PbTi1-x V x O3 (x = 0-1) using a series of generalized gradient-corrected GGA (generalized gradient approximation), including on-site Coulomb repulsion (U)-corrected spin-polarized calculations based on accurate density functional theory. Our total energy calculations show that PbTi1-x V x O3 stabilizes in C-type antiferromagnetic ground state for x > 0.123. With the substitution of V into PbTiO3, the tetragonal distortion is highly enhanced accompanied by a linear increase in polarization. In addition, our band structure analysis shows that for lower x values, the tendency to form two-dimensional magnetism of PbTi1-x V x O3 decreases. The orbital magnetic polarization was calculated with self-consistent field method by including orbital polarization correction in the calculation as well as from the computed X-ray magnetic dichroism spectra. A nonmagnetic metallic ground state is observed for the paraelectric phase for V concentration (x) = 1 competing with a volume change of 10% showing a large magnetovolume effect. Our orbital-projected density of states as well as orbital ordering analysis suggest that the orbital ordering plays a major role in the magnetic-to-nonmagnetic transition when going from ferroelectric to paraelectric phase. The calculated magnetic anisotropic energy shows that the direction [110] is the easy axis of magnetization for x = 1 composition. The partial polarization analysis shows that the Ti/V-O hybridization majorly contributes to the total electrical polarization. The present study adds a new series of compounds to the magnetoelectric family with rarely existing giant coupling between electric- and magnetic-order parameters. These results show that such kind of materials can be used for novel practical applications where one can change the magnetic properties drastically (magnetic to nonmagnetic, as shown here) with external electric field and vice versa.

Negative Thermal Expansion of Ni-Doped MnCoGe at Room-Temperature Magnetic Tuning

Compounds that exhibit the unique behavior of negative thermal expansion (NTE)-the physical property of contraction of the lattice parameters on warming-can be applied widely in modern technologies. Consequently, the search for and design of an NTE material with operational and controllable qualities at room temperature are important topics in both physics and materials science. In this work, we demonstrate a new route to achieve magnetic manipulation of a giant NTE in (Mn_0.95Ni_0.05)CoGe via strong magnetostructural (MS) coupling around room temperature (∼275 to ∼345 K). The MS coupling is realized through the weak bonding between the nonmagnetic CoGe-network and the magnetic Mn-sublattice. Application of a magnetic field changes the NTE in (Mn_0.95Ni_0.05)CoGe significantly: in particular, a change of Δ L/ L along the a axis of absolute value 15290(60) × 10^-6-equivalent to a -31% reduction in NTE-is obtained at 295 K in response to a magnetic field of 8 T.

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.

Adjustable Magnetic Phase Transition Inducing Unusual Zero Thermal Expansion in Cubic RCo2-Based Intermetallic Compounds (R = Rare Earth)

Metallic materials that exhibit negligible thermal expansion or zero thermal expansion (ZTE) have great merit for practical applications, but these materials are rare and their thermal expansions are difficult to control. Here, we successfully tailored the thermal expansion behaviors from strongly but abruptly negative to zero over wide temperature ranges in a series of (Gd,R)(Co,Fe)₂ (R = Dy, Ho, Er) intermetallic compounds by tuning the composition to bring the first-order magnetic phase transition to second-order. Interestingly, an unusual isotropic ZTE property with a coefficient of thermal expansion of α _l = 0.16(0) × 10^-6 K^-1 was achieved in cubic Gd_0.25Dy_0.75Co_1.93Fe_0.07 (GDCF) in the temperature range of 10-275 K. The short-wavelength neutron powder diffraction, synchrotron X-ray diffraction, and magnetic measurement studies evidence that this ZTE behavior was ascribed to the rare-earth-moment-dominated spontaneous volume magnetostriction, which can be controlled by an adjustable magnetic phase transition. The present work extends the scope of the ZTE family and provides an effective approach to exploring ZTE materials, such as by adjusting the magnetism or ferroelectricity-related phase transition in the family of functional materials.

Giant isotropic negative thermal expansion in Y-doped samarium monosulfides by intra-atomic charge transfer

Stimulated by strong demand for thermal expansion control from advanced modern industries, various giant negative thermal expansion (NTE) materials have been developed during the last decade. Nevertheless, most such materials exhibit anisotropic thermal expansion in the crystal lattice. Therefore, strains and cracks induced during repeated thermal cycling degrade their performance as thermal-expansion compensators. Here we achieved giant isotropic NTE with volume change exceeding 3%, up to 4.1%, via control of the electronic configuration in Sm atoms of SmS, (4 f)⁶ or (4 f)⁵(5d)¹, by partial replacement of Sm with Y. Contrary to NTE originating from cooperative phenomena such as magnetism, the present NTE attributable to the intra-atomic phenomenon avoids the size effect of NTE and therefore provides us with fine-grained thermal-expansion compensators, which are strongly desired to control thermal expansion of microregions such as underfill of a three-dimensional integrated circuit. Volume control of lanthanide monosulfides via tuning of the 4 f electronic configuration presents avenues for novel mechanical functions of a material, such as a volume-change driven actuator by an electrical field, which has a different drive principle from those of conventional strain-driven actuators such as piezostrictive or magnetostrictive materials.

A case of multifunctional intermetallic compounds: negative thermal expansion coupling with magnetocaloric effect in (Gd,Ho)(Co,Fe)2

The negative thermal expansion (NTE) and magnetocaloric effect (MCE) are highly correlated in (Gd,Ho)(Co,Fe)₂, both of which are governed by the rare earth moment, whereas the MCE is further reinforced through an intermediate contribution from NTE.

Enhanced tetragonality and large negative thermal expansion in a new Pb/Bi-based perovskite ferroelectric of (1 − x)PbTiO3–xBi(Zn1/2V1/2)O3

With the introduction of Bi(Zn_1/2V_1/2)O₃, both tetragonality and negative thermal expansion of PbTiO₃ have been enhanced.

Large Negative Thermal Expansion Induced by Synergistic Effects of Ferroelectrostriction and Spin Crossover in PbTiO3-Based Perovskites

The discovery of unusual negative thermal expansion (NTE) provides the opportunity to control the common but much desired property of thermal expansion, which is valuable not only in scientific interests but also in practical applications. However, most of the available NTE materials are limited to a narrow temperature range, and the NTE effect is generally weakened by various modifications. Here, we report an enhanced NTE effect that occurs over a wide temperature range α ‾ V = - 5.24 × 10 - 5 ∘ C - 1 , 25 - 575 ∘ C , and this NTE effect is accompanied by an abnormal enhanced tetragonality, a large spontaneous polarization, and a G-type antiferromagnetic ordering in the present perovskite-type ferroelectric of (1-x)PbTiO3-xBiCoO3. Specifically, for the composition of 0.5PbTiO3-0.5BiCoO3, an extensive volumetric contraction of ~4.8 % has been observed near the Curie temperature of 700 °C, which represents the highest level in PbTiO3-based ferroelectrics. According to our experimental and theoretical results, the large NTE originates from a synergistic effect of the ferroelectrostriction and spin crossover of cobalt on the crystal lattice. The actual NTE mechanism is contrasted with previous functional NTE materials, in which the NTE is simply coupled with one ordering such as electronic, magnetic, or ferroelectric ordering. The present study sheds light on the understanding of NTE mechanisms, and it attests that NTE could be simultaneously coupled with different orderings, which will pave a new way toward the design of large NTE materials.

Tunable Thermal Expansion from Negative, Zero, to Positive in Cubic Prussian Blue Analogues of GaFe(CN)6

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.

Colossal Negative Thermal Expansion in Electron-Doped PbVO3 Perovskites

Colossal negative thermal expansion (NTE) with a volume contraction of about 8 %, the largest value reported so far for NTE materials, was observed in an electron-doped giant tetragonal perovskite compound Pb_1-x Bi_x VO₃ (x=0.2 and 0.3). A polar tetragonal (P4mm) to non-polar cubic structural transition took place upon heating. The coefficient of thermal expansion (CTE) and the working temperature could be tuned by changing the Bi content, and La substitution decreased the transition temperature to room temperature. Pb_0.76 La_0.04 Bi_0.20 VO₃ exhibited a unit cell volume contraction of 6.7 % from 200 K to 420 K. Interestingly, further gigantic NTE of about 8.5 % was observed in a dilametric measurement of a Pb_0.76 La_0.04 Bi_0.20 VO₃ polycrystalline sample. The pronounced NTE in the sintered body should be attributed to an anisotropic lattice parameter change.

Progress of Research in Negative Thermal Expansion Materials: Paradigm Shift in the Control of Thermal Expansion

To meet strong demands for the control of thermal expansion necessary because of the advanced development of industrial technology, widely various giant negative thermal expansion (NTE) materials have been developed during the last decade. Discovery of large isotropic NTE in ZrW₂O₈ has greatly advanced research on NTE deriving from its characteristic crystal structure, which is now classified as conventional NTE. Materials classified in this category have increased rapidly. In addition to development of conventional NTE materials, remarkable progress has been made in phase-transition-type NTE materials using a phase transition accompanied by volume contraction upon heating. These giant NTE materials have brought a paradigm shift in the control of thermal expansion. This report classifies and reviews mechanisms and materials of NTE to suggest means of improving their functionality and of developing new materials. A subsequent summary presents some recent activities related to how these giant NTE materials are used as practical thermal expansion compensators, with some examples of composites containing these NTE materials.

Temperature controlled formation of polar copper phosphonates showing large dielectric anisotropy and a dehydration-induced switch from ferromagnetic to antiferromagnetic interactions

The reaction of copper(ii) chloride with 2-carboxyphenylphosphonic acid (2-cppH3) under hydrothermal conditions at 100 °C results in the formation of a polar compound, Cu3(2-cpp)2(H2O)5 (1), which shows large dielectric anisotropy. A similar reaction at 140 °C results in a centrosymmetric phase, Cu3(2-cpp)2(H2O)2 (2). Compound 1 can convert into 2 upon increasing the reaction temperature under hydrothermal conditions, and further to Cu3(2-cpp)2 (3) upon thermal treatment at 240 °C. This process is accompanied by a switch from ferromagnetic in 1 to antiferromagnetic interactions in 2 and 3.

Large spontaneous polarization in polar perovskites of PbTiO3–Bi(Zn1/2Ti1/2)O3

With the substitution of Bi(Zn_0.5Ti_0.5)O₃, a continuously enhanced spontaneous polarization up to 112 μC cm⁻² has been observed in (1 − x)PbTiO₃–xBi(Zn_0.5Ti_0.5)O₃.