Valence Transitions in Negative Thermal Expansion Material SrCu3Fe4O12

High-Pressure Synthesis of Semiconducting PbCu3Mn4O12 with Near-Room-Temperature Ferrimagnetic Order

A transition-metal oxide of PbCu3Mn4O12 was prepared at 1523 K and 10 GPa. An A-site-ordered quadruple perovskite structure with the space group Im3̅ is assigned for this compound. Based on bond-valence-sum calculations and X-ray absorption spectroscopy, the charge combination is determined to be PbCu32+Mn44+O12. Due to Cu2+(↑)-Mn4+(↓) antiferromagnetic coupling, a near-room-temperature ferrimagnetic phase transition is observed at approximately 287 K. PbCu3Mn4O12 exhibits a semiconducting electric transport property with the energy band gap Eg ≈ 0.2 eV. In addition, considerable low-field magnetoresistance effects are observed at lower temperatures. This study provides an intrinsic near-room-temperature ferrimagnetic semiconductor that exhibits potential applications in next-generation spintronic devices.

Robust Crystal Phase Separation with Distinct Charge, Orbital, and Spin Orders in AgMn7O12

An AA'3B4O12-type A-site-ordered quadruple perovskite oxide AgMn7O12 was prepared by high-pressure and high-temperature methods. At room temperature, the compound crystallizes into a cubic Im3̅ symmetry with a charge distribution of AgMn33+Mn43.5+O12. With the temperature decreasing to TCO,OO ≈ 180 K, the compound undergoes a structural phase transition toward a monoclinic C2/m symmetry, giving rise to a B-site charge- and orbital-ordered AgMn33+Mn23+Mn24+O12 phase. Moreover, this charge-/orbital-ordered main phase coexists with the initial cubic AgMn33+Mn43.5+O12 phase in the wide temperature range we measured. The charge-/orbital-ordered phase shows two antiferromagnetic phase transitions near 125 and 90 K, respectively. Short-range ferromagnetic correlations are found to occur for the initial B-site mixed cubic phase around 35 K. Because of the robust phase separation, considerable magnetoresistance effects are observed below TCO,OO in AgMn7O12.

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.

High-Pressure Synthesis of a B-site Co2+/Mn4+ Disordered Quadruple Perovskite LaMn3Co2Mn2O12

A new oxide, LaMn₃Co₂Mn₂O₁₂, was synthesized under high-pressure (7 GPa) and high-temperature (1423 K) conditions. The compound crystallizes in an AA'₃B₄O₁₂-type quadruple perovskite structure with space group Im3̅. The Rietveld structural analysis combined with soft X-ray absorption spectroscopy reveals the charge combination to be LaMn³⁺₃Co²⁺₂Mn⁴⁺₂O₁₂, where the La³⁺ and Mn³⁺ are 1:3 ordered respectively at the A and A' sites, whereas the Co²⁺ and Mn⁴⁺ are disorderly distributed at the B site. This is in sharp contrast to R₂Co²⁺Mn⁴⁺O₆ (R = La and rare earth) double perovskites, in which the Co²⁺ and Mn⁴⁺ charge states are always orderly distributed with a rocksalt-type fashion, giving rise to a long-range magnetic ordering. As a result, LaMn₃Co₂Mn₂O₁₂ displays spin glassy magnetic properties due to the random Co²⁺ and Mn⁴⁺ distribution, as demonstrated by dc and ac magnetic susceptibility as well as specific heat measurements. Possible factors that affect the B-site degree of order in perovskite structures are discussed.

A Sequential Electron Doping for Quadruple Perovskite Oxides ACu3Co4O12 (A = Ca, Y, Ce)

A novel quadruple perovskite oxide CeCu₃Co₄O₁₂ has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K. Rietveld refinement of the synchrotron X-ray powder diffraction pattern reveals that this oxide crystallizes in a cubic quadruple perovskite structure with the 1:3-type ordering of Ce and Cu ions at the A-site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the ACu₃Co₄O₁₂ series (A = Ca, Y, Ce) from Ca²⁺Cu³⁺₃Co^3.25+₄O₁₂ to Y³⁺Cu³⁺₃Co³⁺₄O₁₂ to Ce⁴⁺Cu^2.67+₃Co³⁺₄O₁₂, where the electrons are doped in the order from B-site (Co^3.25+ → Co³⁺) to A'-site (Cu³⁺ → Cu^2.67+). This electron-doping sequence is in stark contrast to the typical B-site electron doping for simple ABO₃-type perovskite and quadruple perovskites CaCu₃B₄O₁₂ (B = V, Cr, Mn), further differing from the monotonical A'-site electron doping for Na_1-xLa_xMn₃Ti₄O₁₂ and A'- and B-site electron doping for AMn₃V₄O₁₂ (A = Na, Ca, La). The differences in the electron-doping sequences are interpreted by rigid-band models, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.

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

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

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

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

Novel catalytic properties of quadruple perovskites

Quadruple perovskite oxides AA'₃B₄O₁₂ demonstrate a rich variety of structural and electronic properties. A large number of constituent elements for A/A'/B-site cations can be introduced using the ultra-high-pressure synthesis method. Development of novel functional materials consisting of earth-abundant elements plays a crucial role in current materials science. In this paper, functional properties, especially oxygen reaction catalysis, for quadruple perovskite oxides CaCu₃Fe₄O₁₂ and AMn₇O₁₂ (A = Ca, La) composed of earth-abundant elements are reviewed.

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.

Synthesis, Structure, and Rigid Unit Mode-like Anisotropic Thermal Expansion of BaIr2In9

This Article reports the synthesis of large single crystals of BaIr2In9 using In flux and their characterization by variable-temperature single-crystal and synchrotron powder X-ray diffraction, resistivity, and magnetization measurements. The title compound adopts the BaFe2Al9-type structure in the space group P6/mmm with room temperature unit cell parameters a = 8.8548(6) Å and c = 4.2696(4) Å. BaIr2In9 exhibits anisotropic thermal expansion behavior with linear expansion along the c axis more than 3 times larger than expansion in the ab plane between 90 and 400 K. This anisotropic expansion originates from a rigid unit mode-like mechanism similar to the mechanism of zero and negative thermal expansion observed in many anomalous thermal expansion materials such as ZrW2O8 and ScF3.

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.