A-Site and B-Site Charge Orderings in an s–d Level Controlled Perovskite Oxide PbCoO3

Ambient pressure synthesis and structure and magnetic properties of a new A- and B-site ordered multinary quadruple perovskite

Quadruple perovskites with high magnetic transition temperatures are an interesting class of compounds but are synthesized typically under high pressure. Ambient pressure synthesis of new multinary quadruple perovskites having a high global instability index (GII) and transition temperature can be interesting for future exploration of high-TC oxides. A new A- and B-site ordered multinary quadruple perovskite, LaCu3Fe2RuSbO12, is synthesized by conventional solid-state reactions at ambient pressure. Rietveld structure refinement revealed that the compound crystallizes in the Pn3̄ space group with a lattice parameter of 7.4556(4) Å. The compound showed complete 1 : 3 ordering of La and Cu at the A-site and 1 : 1 rock-salt ordering of Fe with Ru/Sb at the B-site. The compound is also probed with scanning and transmission electron microscopy and XPS to investigate the chemical composition, microstructure, lattice and oxidation states of the elements. Magnetic studies revealed antiferromagnetic (AFM) correlations with magnetic ordering transitions at ∼170 and 40 K. Furthermore, the M-H hysteretic behavior at 100 and 5 K indicated ferrimagnetism due to short-range AFM interactions among Fe3+(3d5) and Ru4+(4d4) spins involving Cu2+(↑)-Fe3+(↓)-Ru4+(↑) triads. The specific heat data reaffirmed the magnetic signatures while electrical transport showed semiconducting behavior with variable range hopping. The details of synthesis and structural and compositional studies along with the magnetic and electrical transport properties of LaCu3Fe2RuSbO12 are reported in this paper.

Tuning the Chiral Growth of Plasmonic Bipyramids via the Wavelength and Polarization of Light

Circularly polarized light (CPL) is a versatile tool to prepare chiral nanostructures, but the mechanism for inducing enantioselectivity is not well understood. This work shows that the energy and polarization of visible photons can initiate photodeposition at different sites on plasmonic nanocrystals. Here, CPL on achiral gold bipyramids (AuBPs) creates hot holes that oxidatively deposit PbO2 asymmetrically. We show for the first time that the location of PbO2 photodeposition and hence optical dissymmetry depends on the CPL wavelength. Specifically, 488 and 532 nm CPL induce PbO2 growth in the middle of AuBPs, whereas 660 nm CPL induces PbO2 growth at the tips. Our observations show that wavelength-dependent plasmonic field distributions are more important than surface lightning rod effects in localizing plasmon-mediated photochemistry. The largest optical dissymmetry occurs at excitation wavelengths between the transverse and longitudinal resonances of the AuBPs because higher-order modes are required to induce chiral electric fields.

Comparative Study on the Magnetic and Transport Properties of B-Site Ordered and Disordered CaCu3Fe2Os2O12

The B-site Fe/Os ordered and disordered quadruple perovskite oxides CaCu₃Fe₂Os₂O₁₂ were synthesized under different high-pressure and high-temperature conditions. The B-site ordered CaCu₃Fe₂Os₂O₁₂ is a system with a very high ferrimagnetic ordering temperature of 580 K having the Cu²⁺(↑)Fe³⁺(↑)Os⁵⁺(↓) charge and spin arrangement. In comparison, the highly disordered CaCu₃Fe₂Os₂O₁₂ has a reduced magnetic transition temperature of about 350 K. The Cu²⁺Fe³⁺Os⁵⁺ charge combination remains the same without any sign of changes in the valence state of the constituent ions. Although the average net moments of each sublattice are reduced, the average ferrimagnetic spin arrangement is unaltered. The robustness of the basic magnetic properties of CaCu₃Fe₂Os₂O₁₂ against site disorder may be taken as an indication of the tendency to maintain the short-range order of the atomic constituents.

Negative Thermal Expansion in Fluoroapatite Pb5(VO4)3F Enhanced by the Steric Effect of Pb2

Negative thermal expansion (NTE) is an unusual thermophysical phenomenon and has gained attention as a way of controlling thermal expansion. Here, we report a substantial NTE in fluoroapatite Pb₅(VO₄)₃F in a limited temperature range. The dilatometric study revealed volume shrinkage below 150 K, giving a linear thermal expansion coefficient of α_L = -44 ppm/K in the temperature range from 140 to 120 K upon heating. The NTE behavior is associated with a structural transition from the hexagonal (P6₃/m) phase to the monoclinic (P2₁/b) phase. Such a structural transition has been found in other apatite-type compounds, but the magnitude of the volume change in Pb₅(VO₄)₃F is remarkable. Our structural analysis revealed that the structural transition is classified as an antiferroelectric-to-paraelectric transition and the volume change during the transition is enhanced by the steric effect of 6s² lone-pair electrons of Pb²⁺.

Emergent physical properties of perovskite-type oxides prepared under high pressure

The perovskite ABO₃ family demonstrates a wide variety of structural evolutions and physical properties and is arguably the most important family of complex oxides. Chemical substitutions of the A- and/or B-site and modulation of oxygen content can effectively regulate their electronic behaviors and multifunctional performances. In general, the BO₆ octahedron represents the main unit controlling the electronic and magnetic properties while the A-site ion is often not involved. However, a series of unconventional perovskite materials have been recently synthesized under high pressure, such as the s-d level controlled Pb-based perovskite family and quadruple perovskite oxides containing transition metal ions at the A-site. In these compounds, the intersite A-B correlations play an important role in electronic behaviors and further induce many emergent physical properties.

Intermetallic Charge Transfer in V-Substituted PbCrO3

PbCrO₃ features an unusual charge distribution Pb_0.5²⁺Pb_0.5⁴⁺Cr³⁺O₃ with Pb charge disproportionation at ambient pressure. A charge transfer between Pb and Cr is induced by the application of pressure resulting in Pb²⁺Cr⁴⁺O₃ charge distribution and a large volume collapse. Here, structural and charge distribution changes in PbCr_1-xV_xO₃ are investigated. Despite a cubic crystal structure in 0 ≤ x ≤ 0.60, discontinuous reduction in the unit cell volume was observed between x = 0.35 and 0.40. Hard X-ray photoemission spectroscopy confirmed the change in Pb charge state from the coexisting Pb²⁺ and Pb⁴⁺ at x = 0.35 to single Pb²⁺ at x = 0.40. This indicates that V substitution stabilizes the high pressure cubic Pb²⁺Cr⁴⁺O₃-type phase. With further increase in the V substitution, the PbVO₃-type polar tetragonal phase appeared at x = 0.80.

Observation of novel charge ordering and spin reorientation in perovskite oxide PbFeO3

PbMO₃ (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO₃ remain unclear. Herein, we reveal that PbFeO₃ crystallizes into an unusual 2a_p × 6a_p × 2a_p orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb²⁺_0.5Pb⁴⁺_0.5FeO₃ lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb²⁺ and Pb⁴⁺) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe³⁺ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

Stabilized Charge, Spin, and Orbital Ordering by the 6s2 Lone Pair in Bi0.5Pb0.5MnO3

Bi and Pb ions with charge degree of freedom depending on 6s² and 6s⁰ electronic configurations were combined with the Mn ion in a perovskite oxide. Comprehensive theoretical and experimental investigations revealed the Bi³⁺_0.5Pb²⁺_0.5Mn³⁺_0.5Mn⁴⁺_0.5O₃ charge ordered state with CE-type spin and dz² orbital orderings as observed in La_0.5Ca_0.5MnO₃, Nd_0.5Sr_0.5MnO₃, and Bi_0.5Sr_0.5MnO₃. The charge and orbital orderings were preserved above 500 K owing to the stereochemical activity of Bi³⁺ and Pb²⁺ ions which stabilized the structural distortion.

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.

Pressure-induced semiconductor-to-metal phase transition of a charge-ordered indium halide perovskite

Phase transitions in halide perovskites triggered by external stimuli generate significantly different material properties, providing a great opportunity for broad applications. Here, we demonstrate an In-based, charge-ordered (In⁺/In³⁺) inorganic halide perovskite with the composition of Cs₂In(I)In(III)Cl₆ in which a pressure-driven semiconductor-to-metal phase transition exists. The single crystals, synthesized via a solid-state reaction method, crystallize in a distorted perovskite structure with space group I4/m with a = 17.2604(12) Å, c = 11.0113(16) Å if both the strong reflections and superstructures are considered. The supercell was further confirmed by rotation electron diffraction measurement. The pressure-induced semiconductor-to-metal phase transition was demonstrated by high-pressure Raman and absorbance spectroscopies and was consistent with theoretical modeling. This type of charge-ordered inorganic halide perovskite with a pressure-induced semiconductor-to-metal phase transition may inspire a range of potential applications.

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, Ce_1/2Cu₃Ti₄O₁₂, 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.

Rise of A-site columnar-ordered A2A'A''B4O12 quadruple perovskites with intrinsic triple order

A-site-ordered AA'₃B₄O₁₂ quadruple perovskites (with twelve-fold coordinated A and square-planar coordinated A' sites) were discovered in 1967. Since then, there have been considerable research efforts to synthesize and characterize new members of such perovskites. These efforts have led to the discoveries of many interesting physical and chemical properties, such as inter-site charge transfer and disproportionation, giant dielectric constant, multiferroic properties, reentrant structural transitions and high catalytic activity. The first member of A-site columnar-ordered A₂A'A''B₄O₁₂ quadruple perovskites (with ten-fold coordinated A, square-planar coordinated A' and tetrahedrally coordinated A'' sites), CaFeTi₂O₆, was discovered in 1995, and for 19 years it was the only representative of this family. In the last few years, A₂A'A''B₄O₁₂ perovskites have experienced rapid growth. Herein, we present a brief overview of the recent developments in this field and highlight an under-investigated status and great potential of A₂A'A''B₄O₁₂, which can be prepared mainly at high pressure and high temperature. The presence of the A'' site gives an additional degree of freedom in designing such perovskites. The A₂A'A''B₄O₁₂ perovskites are discussed in comparison with well-known AA'₃B₄O₁₂ perovskites.

Charge and orbital orders and structural instability in high-pressure quadruple perovskite CeCuMn6O12

We prepared a quadruple perovskite CeCuMn6O12 under high-pressure and high-temperature conditions at 6 GPa and about 1670 K and investigated its structural, magnetic and transport properties. CeCuMn6O12 crystallizes in space group Im-3 above T CO  =  297 K; below this temperature, it adopts space group R-3 with the 1:3 (Mn4+:Mn3+) charge and orbital orders. Unusual compressed Mn3+O6 octahedra are realized in CeCuMn6O12 similar to CaMn7O12 with the  -Q 3 Jahn-Teller distortion mode. Below about 90 K, structural instability takes place with phase separation and the appearance of competing phases; and below 70 K, two R-3 phases coexist. CeCuMn6O12 exhibits a ferromagnetic-like transition below T C  =  140 K, and it is a semiconductor with the magnetoresistance reaching about  -40% at 140 K and 70 kOe. We argued that the valence of Ce is  +3 in CeCuMn6O12 with the Ce3+([Formula: see text])([Formula: see text])O12 charge distribution in the charge-ordered R-3 phase and Ce3+([Formula: see text])([Formula: see text])O12 in the charge-disordered Im-3 phase.

Systematic charge distribution changes in Bi- and Pb-3d transition metal perovskites

Charge distribution changes in Bi- and Pb-3d transition metal perovskite type oxides were examined. The change in the depth of the d level of the transition metal causes the intermetallic charge transfer.

High-Pressure Synthesis of the Cobalt Pyrochlore Oxide Pb2Co2O7 with Large Cation Mixed Occupancy

The novel A₂B₂O₇-type compound Pb₂Co₂O₇ was synthesized at 8 GPa and 1673 K. Synchrotron X-ray diffraction shows a cubic pyrochlore structure with space group Fd3̅m. Rietveld structural analysis reveals a large cation mixed occupancy at both A and B sites by about 40%, the greatest value found in the pyrochlore family. In combination with the X-ray absorption spectroscopy results, the specific chemical composition and charge states are determined to be (Co_0.6Pb_0.4)³⁺₂(Pb_0.6Co_0.4)⁴⁺₂O₇, in which both the A-site Co³⁺ and the B-site Co⁴⁺ are low-spin. Due to the tetrahedral geometric frustration effects as well as the random Co⁴⁺ and Pb⁴⁺ distribution at the B site, spin glassy behavior is well observed following the conventional critical slowing down feature in Pb₂Co₂O₇.

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.