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Complex transition-metal oxides offer an interesting class of materials for use as oxygen reduction reaction (ORR) catalysts. A recent study proposed a volcano-shape relationship between ORR activi...

A-site-layer-ordered double perovskite YBaCo2O6 with unusually high valence Co3.5+ was found to be an unusual ferromagnetic metal with strong electron correlation. The compound was obtained by topochemically oxidizing the A-site-layer-ordered YBaCo2O5, which contains nominal Co2.5+, in ozone at a low temperature. YBaCo2O6 crystallizes as a fully oxygenated A-site-layer-ordered double perovskite with a tetragonal structure at room temperature and shows an orthorhombic distortion below approximately 140 K. This anisotropic structural distortion induces ferromagnetic ordering of the mixed-valence Co spins with intermediate spin states, suggesting strong spin–lattice coupling in this system. In the ferromagnetic state, the up-spin band mainly consisting of hybridized Co 3d and O 2p crosses the Fermi level, giving the metallic behavior, regardless of the strong electron correlation.

The perovskite Ca0.5Bi0.5FeO3 containing unusually high valent Fe3.5+ undergoes sequentially charge disproportionation (CD) of the Fe centres and intersite charge transfer (CT) between Bi and Fe. From structural, magnetic, and transport property characterization, we found that substitution of Fe with Co occurs isoelectronically to form Ca0.5Bi0.5 3+(Fe1−xCox)3.5+O3 and destabilizes the CD state. This results in materials exhibiting only intermetallic charge transfer behaviour in the region 0.01 < x < 0.67. The CT transitions for these materials only involves Fe3.5+ whereas Co remains in the 3.5+ oxidation state at all temperatures. The doped Co3.5+ ions give Pauli-paramagnetic like conducting behavior. The Co-doping effect is very different from that observed in CaFe1−xCoxO3. The charge transition behavior of Fe3.5+ in the present system are also in contrast to those in Ca0.5Bi0.5FeO3 and Sr0.5Bi0.5FeO3

Cation ordering in the 1322 perovskite CaCu3Fe2Nb2O12 has been investigated by synchrotron X-ray diffraction structure analysis, high-resolution electron microscopy, and magnetic measurements. All of the samples have full 1:3 ordering of Ca and Cu cations at the perovskite A-sites, but the Fe and Nb cations at the B sites show different degrees of ordering depending on the synthesis conditions. A slow-cooled sample has almost complete rock-salt-type 1:1 cation ordering at the B sites, but a rapidly cooled sample has no long-range B-cation arrangement. The ordered compound shows a large saturated magnetization (5.7 μB/formula unit (fu)) below 170 K due to the ferrimagnetism of the A′-site Cu2+ and B-site Fe3+ spins, whereas the B-site disordered form has a small magnetization (

The intersite charge transfer at room temperature was found to occur by applying pressure in the A-site-ordered double-perovskite-structure oxide LaCu3Fe4O12. The pressure-induced charge transfer b...

A series of A-site-ordered perovskites ACu3Sn4O12 with A = Ca2+, Sr2+, and Pb2+ were prepared under high-temperature and high-pressure conditions. They have Cu2+ ions with S = 1/2 spins at the A site in the ABO3 perovskite structure and have nonmagnetic Sn4+ ions at the B sites. An increase in the size of the divalent A-site ions from Ca to Pb increases the cubic lattice constant by only 0.6%. The cubic lattice with rigid CuO4 units thus hardly expands and the different sizes of the A-site ions are accommodated by change of the SnO6 octahedra. The difference in the size of the A-site ions has little effect on the ferromagnetic transition temperatures, because the magnetic properties of these perovskites are determined primarily by the direct ferromagnetic interaction between Cu2+ ions at the A site.

Crystallographic features and stability of the tetragonal phase of PbVO3 were investigated under ambient and high pressure and compared with those of PbTiO3. PbVO3 is isotypic with PbTiO3 [at 300 K, space group P4mm; a = 3.803 91(5) A and c = 4.676 80(8) A, Z = 1]. Tetragonality (c/a) of PbVO3 was the largest among the PbTiO3-type compounds and increased monotonically with increasing temperature from 12 to 570 K without transition to cubic phase. The large tetragonality and the atomic position determined from synchrotron X-ray powder diffraction data suggest a large ferroelectric polarization above 100 μC/cm2. Above 570 K in air, PbVO3 was oxidized to Pb2V2O7. Application of high pressure prevented the oxidation and thus a tetragonal-to-cubic phase transition accompanied by an insulator-to-metal phase transition was observed from about 2 GPa at room temperature.

A new cobalt oxide, SrCo6O11, was synthesized by reacting γ-NaxCoO2 with SrCl2 under high pressure. This material crystallizes in the R-type hexagonal ferrite structure containing Co3O8 Kagome layers separated by two kinds of triangularly arranged Co−O pillars. One of the pillars, CoO5 bipyramids, produces interesting magnetism such as plateaus in magnetization.