Control of Bond-Strain-Induced Electronic Phase Transitions in Iron Perovskites

Large Perpendicular Magnetic Anisotropy Induced by an Intersite Charge Transfer in Strained EuVO2H Films

Perovskite oxides ABO3 continue to be a major focus in materials science. Of particular interest is the interplay between A and B cations as exemplified by intersite charge transfer (ICT), which causes novel phenomena including negative thermal expansion and metal-insulator transition. However, the ICT properties were achieved and optimized by cationic substitution or ordering. Here we demonstrate an anionic approach to induce ICT using an oxyhydride perovskite, EuVO2H, which has alternating layers of EuH and VO2. A bulk EuVO2H behaves as a ferromagnetic insulator with a relatively high transition temperature (TC) of 10 K. However, the application of external pressure to the EuIIVIIIO2H bulk or compressive strain from the substrate in the thin films induces ICT from the EuIIH layer to the VIIIO2 layer due to the extended empty V dxy orbital. The ICT phenomenon causes the VO2 layer to become conductive, leading to an increase in TC that is dependent on the number of carriers in the dxy orbitals (up to a factor of 4 for 10 nm thin films). In addition, a large perpendicular magnetic anisotropy appears with the ICT for the films of <100 nm, which is unprecedented in materials with orbital-free Eu2+, opening new perspectives for applications. The present results provide opportunities for the acquisition of novel functions by alternating transition metal/rare earth layers with heteroanions.

Revealing the Role of Hydrogen in Electron-Doping Mottronics for Strongly Correlated Vanadium Dioxide

Hydrogen-associated electron-doping Mottronics for d-band correlated oxides (e.g., VO₂) opens up a new paradigm to regulate the electronic functionality via directly manipulating the orbital configuration and occupancy. Nevertheless, the role of hydrogen in the Mottronic transition of VO₂ is yet unclear because opposite orbital reconfigurations toward either the metallic or highly insulating states were both reported. Herein, we demonstrate the root cause for such hydrogen-induced multiple electronic phase transitions by ¹H quantification using nuclear reaction analysis. A low hydrogenation temperature is demonstrated to be vital in achieving a large hydrogen concentration (n_H ≈ 10²² cm^-3) that further enhances the t_2g orbital occupancy to trigger electron localizations. In contrast, elevating the hydrogenation temperatures surprisingly reduces n_H to ∼10²¹ cm^-3 but forms more stable metallic H_0.06VO₂. This leads to the recognition of a weaker hydrogen interaction that triggers electron localization within VO₂ via Mottronically enhancing the orbital occupancies.

Os Doping Suppressed Cu-Fe Charge Transfer and Induced Structural and Magnetic Phase Transitions in LaCu3Fe4-xOsxO12 (x = 1 and 2)

B-site Os-doped quadruple perovskite oxides LaCu₃Fe_4-xOs_xO₁₂ (x = 1 and 2) were prepared under high-pressure and high-temperature conditions. Although parent compound LaCu₃Fe₄O₁₂ experiences Cu-Fe intermetallic charge transfer that changes the Cu³⁺/Fe³⁺ charge combination to Cu²⁺/Fe^3.75+ at 393 K, in the Os-doped samples, the Cu and Fe charge states are found to be constant 2+ and 3+, respectively, indicating the complete suppression of charge transfer. Correspondingly, Os⁶⁺ and mixed Os^4.5+ valence states are determined by X-ray absorption spectroscopy for x = 1 and x = 2 compositions, respectively. The x = 1 sample crystallizes in an Fe/Os disordered structure with the Im3̅ space group. It experiences a spin-glass transition around 480 K. With further Os substitution up to x = 2, the crystal symmetry changes to Pn3̅, where Fe and Os are orderly distributed in a rocksalt-type fashion at the B site. Moreover, this composition shows a long-range Cu²⁺(↑)Fe³⁺(↑)Os^4.5+(↓) ferrimagnetic ordering near 520 K. This work provides a rare example for 5d substitution-suppressed intermetallic charge transfer as well as induced structural and magnetic phase transitions with high spin ordering temperature.

A combinatory ferroelectric compound bridging simple ABO3 and A-site-ordered quadruple perovskite

The simple ABO₃ and A-site-ordered AA′₃B₄O₁₂ perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg₃Ti₄O₁₂, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg²⁺ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg₃Ti₄O₁₂. Thus PbHg₃Ti₄O₁₂ appears to be a combinatory link from simple ABO₃ perovskites to A-site-ordered AA′₃Ti₄O₁₂ perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′₃B₄O₁₂-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.

There are few reports of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here, the authors find one with phase transition from a high-temperature centrosymmetric paraelectric phase to a low-temperature non-centrosymmetric ferroelectric phase in a high pressure synthesized compound.

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.

A robust thermal-energy-storage property associated with electronic phase transitions for quadruple perovskite oxides

The quadruple perovskite oxides RCu₃Fe₄O₁₂ (R: rare-earth metals) exhibit large latent-heat capacities (25 J g^-1 at maximum) with variable transition temperatures between 254 and 368 K, whereas their transition entropies are almost completely retained. This finding proposes an effective way to design robust thermal-energy-storage materials with various operating temperatures.

Sequential Spin State Transition and Intermetallic Charge Transfer in PbCoO3

Spin state transitions and intermetallic charge transfers can essentially change material structural and physical properties while excluding external chemical doping. However, these two effects have rarely been found to occur sequentially in a specific material. In this article, we show the realization of these two phenomena in a perovskite oxide PbCoO₃ with a simple ABO₃ composition under high pressure. PbCoO₃ possesses a peculiar A- and B-site ordered charge distribution Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ with insulating behavior at ambient conditions. The high spin Co²⁺ gradually changes to low spin with increasing pressure up to about 15 GPa, leading to an anomalous increase of resistance magnitude. Between 15 and 30 GPa, the intermetallic charge transfer occurs between Pb⁴⁺ and Co²⁺ cations. The accumulated charge-transfer effect triggers a metal-insulator transition as well as a first-order structural phase transition toward a Tetra.-I phase at the onset of ∼20 GPa near room temperature. On further compression over 30 GPa, the charge transfer completes, giving rise to another first-order structural transformation toward a Tetra.-II phase and the reentrant electrical insulating behavior.

Group-theoretical analysis of 1:3 A-site-ordered perovskite formation

The quadruple perovskites AA'3B4X12 are characterized by an extremely wide variety of intriguing physical properties, which makes them attractive candidates for various applications. Using group-theoretical analysis, possible 1:3 A-site-ordered low-symmetry phases have been found. They can be formed from a parent Pm{\bar 3}m perovskite structure (archetype) as a result of real or hypothetical (virtual) phase transitions due to different structural mechanisms (orderings and displacements of atoms, tilts of octahedra). For each type of low-symmetry phase, the full set of order parameters (proper and improper order parameters), the calculated structure, including the space group, the primitive cell multiplication, splitting of the Wyckoff positions and the structural formula were determined. All ordered phases were classified according to the irreducible representations of the space group of the parent phase (archetype) and systematized according to the types of structural mechanisms responsible for their formation. Special attention is paid to the structural mechanisms of formation of the low-symmetry phase of the compounds known from experimental data, such as: CaCu3Ti4O12, CaCu3Ga2Sn2O12, CaMn3Mn4O12, Ce1/2Cu3Ti4O12, LaMn3Mn4O12, BiMn3Mn4O12 and others. For the first time, the phenomenon of variability in the choice of the proper order parameters, which allows one to obtain the same structure by different group-theoretical paths, is established. This phenomenon emphasizes the fundamental importance of considering the full set of order parameters in describing phase transitions. Possible transition paths from the archetype with space group Pm{\bar 3}m to all 1:3 A-site-ordered perovskites are illustrated using the Bärnighausen tree formalism. These results may be used to identify new phases and interpret experimental results, determine the structural mechanisms responsible for the formation of low-symmetry phases as well as to understand the structural genesis of the perovskite-like phases. The obtained non-model group-theoretical results in combination with crystal chemical data and first-principles calculations may be a starting point for the design of new functional materials with a perovskite structure.

Complementary evaluation of structure stability of perovskite oxides using bond-valence and density-functional-theory calculations

Estimation of structure stability is an essential issue in materials design and synthesis. Global instability index (GII) based on bond-valence method is applied as a simple indication, while density functional theory calculation is adopted for accurate evaluation of formation energy. We compare the GII and total energy of typical ABO₃-type perovskite oxides and rationalize their relationship, proposing that the criteria for empirically unstable structures (GII > 0.2 valence unit) correspond to the difference in total energy of 50-200 meV per formula unit.

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.

On the energy scale involved in the metal to insulator transition of quadruple perovskite EuCu3Fe4O12: infrared spectroscopy and ab-initio calculations

Optical measurements were carried out by infrared spectroscopy on AA′₃B₄O₁₂ A-site ordered quadruple perovskite EuCu₃Fe₄O₁₂ (microscopic sample) as function of temperature. At 240 K (=T_MI), EuCu₃Fe₄O₁₂ undergoes a very abrupt metal to insulator transition, a paramagnetic to antiferromagnetic transition and an isostructural transformation with an abrupt large volume expansion. Above T_MI, optical conductivity reveals a bad metal behavior and below T_MI, an insulating phase with an optical gap of 125 meV is observed. As temperature is decreased, a large and abrupt spectral weight transfer toward an energy scale larger than 1 eV is detected. Concurrently, electronic structure calculations for both high and low temperature phases were compared to the optical conductivity results giving a precise pattern of the transition. Density of states and computed optical conductivity analysis identified Cu_3dxy, Fe_3d and O_2p orbitals as principal actors of the spectral weight transfer. The present work constitutes a first step to shed light on EuCu₃Fe₄O₁₂ electronic properties with optical measurements and ab-initio calculations.

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.

First-principles study of negative thermal expansion mechanism in A-site-ordered perovskite SrCu3Fe4O12

We investigate electrical and magnetic properties of an A-site-ordered perovskite SrCu₃Fe₄O₁₂ and clarify its negative thermal expansion mechanism.