Temperature-induced valence transition and associated lattice collapse in samarium fulleride

Assembly of a trapped valent CeIII/IV-TCNQ complex through metal-ligand redox cooperativity

Complex (Cp3CeIV)2(TCNQ)(CeIIICp3)2 (1) was prepared by reducing neutral TCNQ0 (tetracyanoquinodimethane) with Cp3Ce(THF). Two types of cerium centres with a dianionic TCNQ2- moiety are present in 1, wherein each of the four cyano-groups are bound by a cation. Formation of this trapped-valent organocerium compound 1 is facilitated by metal-ligand redox cooperativity. Characterization of 1 was carried out using structural-, magnetometry-, and IR-spectroscopic analyses. Photophysical studies on this compound reveal CeIII luminescence, and opens up avenues for promising multifunctional, mixed-valent lanthanide materials.

Tunable valence tautomerism in lanthanide-organic alloys

The inimitable electronic structures of the lanthanide (Ln) ions are key to advanced materials and technologies involving these elements. The trivalent ions are ubiquitous and are used much more widely than the divalent and tetravalent analogues, which possess vastly different optical and magnetic properties. Hence, alteration of the valence electron count by external stimuli can lead to dramatic changes in materials properties. Compounds exhibiting a temperature-induced complete Ln(III) ⇄ Ln(II) switch, referred to as a valence tautomeric (VT) transition, are rare. Here we present an abrupt and hysteretic VT transition in a lanthanide-based coordination polymer, SmI2(pyrazine)3, driven by the interconversion of Sm(II)-pyrazine(0) and Sm(III)-pyrazine(·-) redox pairs. Alloying SmI2(pyrazine)3 with Yb(II) yields isomorphous Sm1-xYbxI2(pyrazine)3 solid solutions with VT transition critical temperatures ranging widely from 200 K to ∼50 K at ambient pressure. These findings demonstrate a simple strategy to realize thermally switchable magnetic materials with chemically tunable transition temperatures.

Realization of Organocerium-Based Fullerene Molecular Materials Showing Mott Insulator-Type Behavior

Electron-rich organocerium complexes (C5Me4H)3Ce and [(C5Me5)2Ce(ortho-oxa)], with redox potentials E1/2 = -0.82 V and E1/2 = -0.86 V versus Fc/Fc+, respectively, were reacted with fullerene (C60) in different stoichiometries to obtain molecular materials. Structurally characterized cocrystals: [(C5Me4H)3Ce]2·C60 (1) and [(C5Me5)2Ce(ortho-oxa)]3·C60 (2) of C60 with cerium-based, molecular rare earth precursors are reported for the first time. The extent of charge transfer in 1 and 2 was evaluated using a series of physical measurements: FT-IR, Raman, solid-state UV-vis-NIR spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, and magnetic susceptibility measurements. The physical measurements indicate that 1 and 2 comprise the cerium(III) oxidation state, with formally neutral C60 as a cocrystal in both cases. Pressure-dependent periodic density functional theory calculations were performed to study the electronic structure of 1. Inclusion of a Hubbard-U parameter removes Ce f states from the Fermi level, opens up a band gap, and stabilizes FM/AFM magnetic solutions that are isoenergetic because of the large distances between the Ce(III) cations. The electronic structure of this strongly correlated Mott insulator-type system is reminiscent of the well-studied Ce2O3.

Electronic origin of negative thermal expansion in samarium hexaboride revealed by X-ray diffraction and total scattering

Samarium hexaboride, SmB6, is a negative thermal expansion (NTE) material whose structure is similar to other known NTE materials such as the family of Prussian blues. In the Prussian blues, NTE is due to a phonon mechanism, but we recently showed from DFT calculations that this is unlikely in SmB6 (Li et al., Phys. Chem. Chem. Phys. 2023, 25, 10749). We now report experimental X-ray diffraction and pair distribution function analysis of this material in the temperature range 20-300 K. The interatomic distances shown by both methods are consistent with the NTE instead arising from an electronic effect, by which the samarium atoms lose electrons and thus have a smaller ionic radius as the temperature increases.

Strong enhancement of magnetic ordering temperature and structural/valence transitions in EuPd3S4 under high pressure

We present a comprehensive study of the inhomogeneous mixed-valence compound, EuPd3S4, by electrical transport, X-ray diffraction, time-domain 151Eu synchrotron Mössbauer spectroscopy, and X-ray absorption spectroscopy measurements under high pressure. Electrical transport measurements show that the antiferromagnetic ordering temperature, TN, increases rapidly from 2.8 K at ambient pressure to 23.5 K at ~19 GPa and plateaus between ~19 and ~29 GPa after which no anomaly associated with TN is detected. A pressure-induced first-order structural transition from cubic to tetragonal is observed, with a rather broad coexistence region (~20 GPa to ~30 GPa) that corresponds to the TN plateau. Mössbauer spectroscopy measurements show a clear valence transition from approximately 50:50 Eu2+:Eu3+ to fully Eu3+ at ~28 GPa, consistent with the vanishing of the magnetic order at the same pressure. X-ray absorption data show a transition to a fully trivalent state at a similar pressure. Our results show that pressure first greatly enhances TN, most likely via enhanced hybridization between the Eu 4f states and the conduction band, and then, second, causes a structural phase transition that coincides with the conversion of the europium to a fully trivalent state.

Crystal growth and negative thermal expansion properties of a Y2Mo4O15 single crystal

A bulk-size single crystal of Y₂Mo₄O₁₅ with 20 × 11 × 8 mm³ was successfully grown by the top-seed solution growth (TSSG) method. The full-width at half maximum of (100) and (010) crystal faces is 37 and 27 arcsec, respectively. The thermal conductivity coefficients κ ₁₁, κ ₂₂, κ ₃₃, and κ ₁₃ are determined to be 1.519, 2.097, 0.445, and 0.997 W m^-1 K^-1, respectively. It is worth noting that the Y₂Mo₄O₁₅ crystal shows significant anisotropy thermal expansion properties, which exhibits a negative thermal expansion along the b-axis (α ₂₂ = -5.11 × 10^-6 K^-1). The crystal structure analysis shows that the shrinking of Mo-O bond lengths along the b-axis with the increasing temperature would be the main origin of the negative thermal expansion properties for Y₂Mo₄O₁₅ crystal, which does not comply with the current mechanism.

Novel Valence Transition in Elemental Metal Europium around 80 GPa

Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.

Commensurate Stacking Phase Transitions in an Intercalated Transition Metal Dichalcogenide

Intercalation and stacking-order modulation are two active ways in manipulating the interlayer interaction of transition metal dichalcogenides (TMDCs), which lead to a variety of emergent phases and allow for engineering material properties. Herein, the growth of Pb-intercalated TMDCs-Pb(Ta_1+x Se₂ )₂ , the first 124-phase, is reported. Pb(Ta_1+x Se₂ )₂ exhibits a unique two-step first-order structural phase transition at around 230 K. The transitions are solely associated with the stacking degree of freedom, evolving from a high-temperature (high-T) phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low-temperature (low-T) phase again with R3msymmetry, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the transitions on warming. At low-T, bulk superconductivity with T_c  ≈ 1.8 K is observed. The underlying physics of the structural phase transitions are discussed from first-principle calculations. The symmetry analysis reveals topological nodal lines in the band structure. The results demonstrate the possibility of realizing higher-order metal-intercalated phases of TMDCs and advance the knowledge of polymorphic transitions, and may inspire stacking-order engineering in TMDCs and beyond.

Mechanisms for Pressure-Induced Isostructural Phase Transitions in EuO

We study pressure-induced isostructural electronic phase transitions in the prototypical mixed valence and strongly correlated material EuO using the global-hybrid density functional theory. The simultaneous presence in the valence of highly localized d- and f-type bands and itinerant s- and p-type states, as well as the half-filled f-type orbital shell with seven unpaired electrons on each Eu atom, have made the description of the electronic features of this system a challenge. The electronic band structure, density of states, and atomic oxidation states of EuO are analyzed in the 0-50 GPa pressure range. An insulator-to-metal transition at about 12 GPa of pressure was identified. The second isostructural transition at approximately 30-35 GPa, previously believed to be driven by an oxidation from Eu(II) to Eu(III), is shown instead to be associated with a change in the occupation of the Eu d orbitals, as can be determined from the analysis of the corresponding atomic orbital populations. The Eu d band is confined by the surrounding oxygens and split by the crystal field, which results in orbitals of e_{g} symmetry (i.e., d_{x^{2}-y^{2}} and d_{2z^{2}-x^{2}-y^{2}}, pointing along the Eu-O direction) being abruptly depopulated at the transition as a means to alleviate electron-electron repulsion in the highly compressed structures.

Synthesis and structural analysis of nonstoichiometric ternary fulleride K 1.5 Ba 0.25 CsC 60

The existence of cation-vacancy sites in fullerides might lead to long-range ordering and generate a new vacancy-ordered superstructure. The purpose of this work is to search whether or not long-range ordering of vacant tetrahedral sites, namely superstructure emerges in nonstoichiometric K 1.5 Ba 0.25 CsC 60 fulleride. Therefore, K 1.5 Ba 0.25 CsC 60 with cation-vacancy sites is synthesized using a precursor method to avoid inadequate stoichiometry control and formation of impurity phases within the target composition. For this purpose, first, phase-pure K 6 C 60 , Ba 6 C 60 and Cs 6 C 60 precursors are synthesized. Stoichiometric quantities of these precursors are used for further reaction with C 60 to afford K 1.5 Ba 0.25 CsC 60 . Rietveld analysis of the high-resolution synchrotron X-ray powder diffraction data of the precursors and K 1.5 Ba 0.25 CsC 60 confirms that K 6 C 60 , Ba 6 C 60 and Cs 6 C 60 are single-phase and they crystallize in a body-centered-cubic structure ( Im 3) as reported in the literature. The analysis also shows that K 1.5 Ba 0.25 CsC 60 phase can be perfectly modeled using a face-centered cubic structure. No new peaks appear which could have implied the appearance of a superstructure. This suggests that there is no long-range ordered arrangement of vacant tetrahedral sites in K 1.5 Ba 0.25 CsC 60 .

Bi-Material Negative Thermal Expansion Inverted Trapezoid Lattice based on A Composite Rod

Negative thermal expansion (NTE) lattices are widely used in aerospace engineering where the structures experience large temperature variation. However, the available range of NTE of the current lattices is quite narrow, which severely limits their engineering application. In this paper, we report an inverted trapezoid lattice (ITL) with large NTE. The NTE of the ITL is 2.6 times that of a typical triangular lattice with the same height and hypotenuse angle. Theoretically, with a pin-jointed assumption, the ITL can improve the NTE by order of magnitude if the length ratio of the composite rod is changed. In the presented ITL, a composite rod is utilized as the base of the ITL. The composite rod has large inner NTE. The inverted trapezoid structure converts the inner NTE to the vertical direction contraction and obtains an extra NTE. Finite element simulations and experimental verification by interferometric measurement were conducted to verify the large thermal expansion of the ITL.

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.

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.

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.

Anisotropic Near-Zero Thermal Expansion in REAg xGa4- x ( RE = La-Nd, Sm, Eu, and Yb) Induced by Structural Reorganization

In this work, we have discovered the anisotropic near-zero thermal expansion (NZTE) behavior in a family of compounds REAg _xGa_{4- x} ( RE = La-Nd, Sm, Eu, and Yb). The compounds adopt the CeAl₂Ga₂ structure type and were obtained as single crystals in high yield by metal flux growth technique using gallium as active flux. Temperature-dependent single crystal X-ray diffraction suggests that all the compounds exhibit near zero thermal expansion along c direction in the temperature range of 100-450 K. Temperature-dependent X-ray absorption near-edge spectroscopic study confirmed ZTE behavior is due to the geometrical features associated within the crystal structure. The anisotropic NZTE behavior was further established by anisotropic magnetic measurements on selected single crystals. The atomic displacement parameters, apparent bond lengths, bond angles, and structural distortion with respect to the temperature reveal that geometric features associated with the structural distortion cause the anisotropic NZTE along c-direction. The preliminary magnetic studies suggest all the compounds are paramagnetic at room temperature except LaAgGa₃. Electrical resistivity study reveals that compounds from this series are metallic in nature.

Switching from positive to negative axial thermal expansion in two organic crystalline compounds with similar packing

Switching from positive to negative axial thermal expansion in pure organic materials is reported for the first time. This rare phenomenon has been rationalized based on the packing of molecules in crystal structures and transverse thermal vibrations of atoms in the molecule. Unique packing of the molecules in the crystal structure contributes to the restricted movement of molecules along the c axis. Subsequently, contraction of molecular dimensions with increasing temperature, due to transverse vibrations of some atoms, assists with the switch from Positive Thermal Expansion (PTE) to Negative Thermal Expansion (NTE).

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.

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.

Evidence for negative thermal expansion in the superconducting precursor phase SmFeAsO

The fluorine-doped rare-earth iron oxypnictide series SmFeAsO1-x F x (0 [Formula: see text] 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies (Margadonna et al 2009 Phys. Rev. B. 79 014503), the parent compound SmFeAsO exhibits a tetragonal-to-orthorhombic structural distortion at [Formula: see text]  =  130 K which is rapidly suppressed by [Formula: see text] 0.10 deep within the superconducting dome. The change in unit cell symmetry is followed by a previously unreported magnetoelastic distortion at 120 K. The temperature dependence of the thermal expansion coefficient [Formula: see text] reveals a rich phase diagram for SmFeAsO: (i) a global minimum at 125 K corresponds to the opening of a spin-density wave instability as measured by pump-probe femtosecond spectroscopy (Mertelj et al 2010 Phys. Rev. B 81 224504) whilst (ii) a global maximum at 110 K corresponds to magnetic ordering of the Sm and Fe sublattices as measured by magnetic x-ray scattering (Nandi et al 2011 Phys. Rev. B 84 055419). At much lower temperatures than [Formula: see text], SmFeAsO exhibits a significant negative thermal expansion on the order of  -40 ppm · K-1 in contrast to the behaviour of other rare-earth oxypnictides such as PrFeAsO (Kimber et al 2008 Phys. Rev. B 78 140503) and the actinide oxypnictide NpFeAsO (Klimczuk et al 2012 Phys. Rev. B 85 174506) where the onset of [Formula: see text] 0 only appears in the vicinity of magnetic ordering. Correlating this feature with the temperature and doping dependence of the resistivity and the unit cell parameters, we interpret the negative thermal expansion as being indicative of the possible condensation of itinerant electrons accompanying the opening of a SDW gap, consistent with transport measurements (Tropeano et al 2009 Supercond. Sci. Technol. 22 034004).

Negative Thermal Expansion over a Wide Temperature Range in Fe-Doped MnNiGe Composites

Fe-doped MnNiGe alloys were successfully synthesized by solid-state reaction. Giant negative thermal expansion (NTE) behaviors with the coefficients of thermal expansion (CTE) of −285.23 × 10⁻⁶ K⁻¹ (192–305 K) and −1167.09 × 10⁻⁶ K⁻¹ (246–305 K) have been obtained in Mn_0.90Fe_0.10NiGe and MnNi_0.90Fe_0.10Ge, respectively. Furthermore, these materials were combined with Cu in order to control the NTE properties. The results indicate that the absolute value of CTE gradually decreases with increasing Cu contents. In Mn_0.92Fe_0.08NiGe/x%Cu, the CTE gradually changes from −64.92 × 10⁻⁶ K⁻¹ (125–274 K) to −4.73 × 10⁻⁶ K⁻¹ (173–229 K) with increasing value of x from 15 to 70. The magnetic measurements reveal that the NTE behaviors in this work are strongly correlated with the process of the magnetic phase transition and the introduction of Fe atoms could also change the spiral anti-ferromagnetic (s-AFM) state into ferromagnetic (FM) state at low temperature. Our study launches a new candidate for controlling thermal expansion properties of metal matrix materials which could have potential application in variable temperature environment.

Area negative thermal expansion in a mixed metal mixed organic MOF: “elevator-platform” mechanism induced by O–H⋯O hydrogen bonding

Rare area negative thermal expansion of a new mixed metal mixed organic MOF has been described using an “elevator-platform” analogy induced by O–H⋯O hydrogen bonding.

Uniaxial negative thermal expansion induced by moiety twisting in an organic crystal

Anomalous thermal expansion of a new diyn-diol molecule was studied by means of variable-temperature single-crystal X-ray diffraction. Analysis of the unit cell axes as a function of temperature shows that the material experiences uniaxial negative thermal expansion. Packing analysis of the crystal structures reveals twisting of the cyclopentyl moiety relative to the diyne spine with increasing temperature.

Giant thermal expansion and α-precipitation pathways in Ti-alloys

Ti-alloys represent the principal structural materials in both aerospace development and metallic biomaterials. Key to optimizing their mechanical and functional behaviour is in-depth know-how of their phases and the complex interplay of diffusive vs. displacive phase transformations to permit the tailoring of intricate microstructures across a wide spectrum of configurations. Here, we report on structural changes and phase transformations of Ti–Nb alloys during heating by in situ synchrotron diffraction. These materials exhibit anisotropic thermal expansion yielding some of the largest linear expansion coefficients (+ 163.9×10⁻⁶ to −95.1×10⁻⁶ °C⁻¹) ever reported. Moreover, we describe two pathways leading to the precipitation of the α-phase mediated by diffusion-based orthorhombic structures, α″_lean and α″_iso. Via coupling the lattice parameters to composition both phases evolve into α through rejection of Nb. These findings have the potential to promote new microstructural design approaches for Ti–Nb alloys and β-stabilized Ti-alloys in general.

Complex phase transformations in β-stabilised titanium alloys can dramatically change their α and β microstructures, providing tailorability for aerospace or biomaterial applications. Here the authors show that Ti-Nb alloys exhibit giant thermal expansions and identify two new pathways that lead to α phase formation.

Isotropic Negative Thermal Expansion Metamaterials

Negative thermal expansion materials are important and desirable in science and engineering applications. However, natural materials with isotropic negative thermal expansion are rare and usually unsatisfied in performance. Here, we propose a novel method to achieve two- and three-dimensional negative thermal expansion metamaterials via antichiral structures. The two-dimensional metamaterial is constructed with unit cells that combine bimaterial strips and antichiral structures, while the three-dimensional metamaterial is fabricated by a multimaterial 3D printing process. Both experimental and simulation results display isotropic negative thermal expansion property of the samples. The effective coefficient of negative thermal expansion of the proposed models is demonstrated to be dependent on the difference between the thermal expansion coefficient of the component materials, as well as on the circular node radius and the ligament length in the antichiral structures. The measured value of the linear negative thermal expansion coefficient of the three-dimensional sample is among the largest achieved in experiments to date. Our findings provide an easy and practical approach to obtaining materials with tunable negative thermal expansion on any scale.

Inverse Charge Transfer in the Quadruple Perovskite CaCu3Fe4O12

Structural and spectroscopic analyses revealed that the quadruple perovskite CaCu3Fe4O12 undergoes an "inverse" electron charge transfer in which valence electrons move from B-site Fe to A'-site Cu ions (∼3Cu(∼2.4+) + 4Fe(∼3.65+) → ∼3Cu(∼2.2+) + 4Fe(∼3.8+)) simultaneously with a charge disproportionation transition (4Fe(∼3.8+) → ∼2.4Fe(3+) + ∼1.6Fe(5+)), on cooling below 210 K. The direction of the charge transfer for CaCu3Fe4O12 is opposite to those reported for other perovskite oxides such as BiNiO3 and ACu3Fe4O12 (A = Sr(2+) or the large trivalent rare-earth metal ions), in which the electrons move from A/A'-site to B-site ions. This finding sheds a light on a new aspect in intermetallic phenomena for complex transition metal compounds.

Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

Negative thermal expansion (NTE) is an intriguing physical property of solids, which is a consequence of a complex interplay among the lattice, phonons, and electrons. Interestingly, a large number of NTE materials have been found in various types of functional materials. In the last two decades good progress has been achieved to discover new phenomena and mechanisms of NTE. In the present review article, NTE is reviewed in functional materials of ferroelectrics, magnetics, multiferroics, superconductors, temperature-induced electron configuration change and so on. Zero thermal expansion (ZTE) of functional materials is emphasized due to the importance for practical applications. The NTE functional materials present a general physical picture to reveal a strong coupling role between physical properties and NTE. There is a general nature of NTE for both ferroelectrics and magnetics, in which NTE is determined by either ferroelectric order or magnetic one. In NTE functional materials, a multi-way to control thermal expansion can be established through the coupling roles of ferroelectricity-NTE, magnetism-NTE, change of electron configuration-NTE, open-framework-NTE, and so on. Chemical modification has been proved to be an effective method to control thermal expansion. Finally, challenges and questions are discussed for the development of NTE materials. There remains a challenge to discover a "perfect" NTE material for each specific application for chemists. The future studies on NTE functional materials will definitely promote the development of NTE materials.

Cation deficiency effect on negative thermal expansion of ferroelectric PbTiO3

This article describes a strong correlation between the spontaneous polarization and NTE features in 8% Pb²⁺ deficient PT (P₉₂T), 2% Ti⁴⁺ deficient PT (PT₉₈) and pure PbTiO₃(PT) samples, which meets the mechanism of spontaneous volume ferroelectrostriction.

Structure induced Yb valence changes in the solid solution Yb(x)Ca(1-x)C2

The solid solution Yb(x)Ca(1-x)C2 (0 ≤ x ≤ 1) was synthesized by reaction of the elements at 1323 K. The crystal structures within this solid solution, as elucidated from synchrotron powder diffraction data, depend on x and exhibit some interesting features that point to a structure dependent valence state of Yb. Compounds with x ≥ 0.75 crystallize in the tetragonal CaC2 type structure (I4/mmm, Z = 2) and obey Vegard's law; for x ≤ 0.75 the monoclinic ThC2 type structure (C2/c, Z = 4) is found, which coexists with the monoclinic CaC2-III type structure (C2/m, Z = 4) for x ≤ 0.25. The monoclinic modifications show a strong deviation from Vegard's law. Their unit cell volumes are remarkably larger than expected for a typical Vegard system. HERFD-XANES spectroscopic investigations reveal that different Yb valence states are responsible for the observed volume anomalies. While all tetragonal compounds contain mixed-valent Yb with ∼75% Yb(3+) (similar to pure YbC2), all monoclinic modifications contain exclusively Yb(2+). Therefore, Yb(x)Ca(1-x)C2 is a very rare example of a Yb containing compound showing a strong structure dependence of the Yb valence state. Moreover, temperature dependent synchrotron powder diffraction, neutron TOF powder diffraction, and HERFD-XANES spectroscopy experiments reveal significant Yb valence changes in some compounds of the Yb(x)Ca(1-x)C2 series that are induced by temperature dependent phase transitions. Transitions from the tetragonal CaC2 type structure to the monoclinic ThC2 or the cubic CaC2-IV type structure (Fm3m, Z = 4) are accompanied by drastic changes of the mean Yb valence from ∼2.70 to 2.0 in compounds with x = 0.75 and x = 0.91. Finally, the determination of lattice strain arising inside the modifications with ordered dumbbells (ThC2 and CaC2 type structures) by DSC measurements corroborated our results concerning the close relationship between crystal structure and Yb valence in the solid solution Yb(x)Ca(1-x)C2.

Reentrant valence transition in EuO at high pressures: beyond the bond-valence model

The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at ≈45  GPa is accompanied by a reentrant Eu valence transition into a lower valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.

Negative thermal expansion materials: technological key for control of thermal expansion

Most materials expand upon heating. However, although rare, some materials contract upon heating. Such negative thermal expansion (NTE) materials have enormous industrial merit because they can control the thermal expansion of materials. Recent progress in materials research enables us to obtain materials exhibiting negative coefficients of linear thermal expansion over -30 ppm K-1. Such giant NTE is opening a new phase of control of thermal expansion in composites. Specifically examining practical aspects, this review briefly summarizes materials and mechanisms of NTE as well as composites containing NTE materials, based mainly on activities of the last decade.

Colossal negative thermal expansion in BiNiO3 induced by intermetallic charge transfer

The unusual property of negative thermal expansion is of fundamental interest and may be used to fabricate composites with zero or other controlled thermal expansion values. Here we report that colossal negative thermal expansion (defined as linear expansion <-10(-4) K(-1) over a temperature range ~100 K) is accessible in perovskite oxides showing charge-transfer transitions. BiNiO(3) shows a 2.6% volume reduction under pressure due to a Bi/Ni charge transfer that is shifted to ambient pressure through lanthanum substitution for Bi. Changing proportions of coexisting low- and high-temperature phases leads to smooth volume shrinkage on heating. The crystallographic linear expansion coefficient for Bi(0.95)La(0.05)NiO(3) is -137×10(-6) K(-1) and a value of -82×10(-6) K(-1) is observed between 320 and 380 K from a dilatometric measurement on a ceramic pellet. Colossal negative thermal expansion materials operating at ambient conditions may also be accessible through metal-insulator transitions driven by other phenomena such as ferroelectric orders.

Raman spectroscopic study of the rare-earth fullerides Eu6-xSrxC60

We present Raman spectroscopic studies of the isostructural and isoelectronic Eu(6-x)Sr(x)C(60) (x = 0, 3, 5, 6) and Ba(6)C(60) compounds. The Raman spectra of the Eu-based fullerides show dramatic changes compared to the pure alkaline-earth systems, including significant broadening, splitting and frequency shifts of the fivefold degenerate H(g) intramolecular modes of C(60). Moreover, the A(g)(2) mode exhibits an even larger downshift and a remarkable broadening. These findings are consistent with distortions of the C(60) molecular cages and a considerable electron-phonon coupling strength-strongly enhanced in the Eu containing systems-originating from the strong orbital hybridization between the metal atom and the C(60) molecule.