Systematic Charge Distribution Changes in Bi- and Pb-3d Transition Metal Perovskite Oxides
Laboratory for Materials and Structures, Tokyo Institute of Technology, 226-8503, Yokohama, Japan
Bi and Pb are main group elements, but these have charge degree of freedom depending on the 6s2 and 6s0 electronic configurations. These are therefore called valence skippers. Since 6s states of these elements are close to the d level of transition metal and oxygen 2p level, BiMO3 and PbMO3 (M: 3d transition metals) exhibit systematic valence distribution changes. From left to right in the periodic table, BiCrO3 to BiCoO3 are all Bi3+M3+O3. However, BiNiO3 has an unusual Bi3+0.5Bi5+0.5Ni2+O3 valence state. An intermetallic charge transfer between Bi5+ and Ni2+ takes place under pressure leading to the Bi3+Ni3+O3 high-pressure phase. BiNiO3 decomposes on heating at 500 K, but La substitution for Bi or Fe substitution for Ni destabilizes the Bi charge disproportionation and (Bi,La)3+(Ni,Fe)3+O3 appears on heating at an ambient pressure. Because of the contract of Ni-O bond owing to the oxidation of Ni2+ to Ni3+, negative thermal expansion, shrinkage of volume on heating, is observed. Similar charge distribution change is observed three times in PbMO3. PbVO3 is Pb2+V4+O3 like Pb2+Ti4+O3, but PbCrO3 is found to be Pb2+0.5Pb4+0.5Cr3+O3. PbCoO3 has turned out to be Pb2+Pb4+3Co2+2Co3+2O12. PbNiO3 has a valence distribution of Pb4+Ni2+O3. Namely, PbMO3 changes from Pb2+M4+O3 to Pb2+0.5Pb4+0.5Cr3+O3 (average valence state of Pb3+M3+O3) to Pb2+0.25Pb4+0.75Co2+0.5Co3+0.5O3 (Pb3.5+Co2.5+O3) and to Pb4+M2+O3 according to the order in the periodic table and the depth of d level.
Brief CV of Prof. AZUMA MASAKI:
Masaki Azuma is currently a full professor at Laboratory for Materials and Structures, Tokyo Institute of Technology and is chair of Tokyo Tech World Research Hub Initiative. He obtained his Ph. D degree in 1995 from Kyoto University. His research interests are experimental solid state chemistry and physics on low-dimensional and frustrated magnets, multiferroics, lead-free piezoelectrics and negative thermal expansion materials.