Transition metals (TMs) exhibit charge degree of freedom, resulting in interesting properties, such as charge ordering related to metal-insulator transitions, high-temperature superconductivity, colossal magnetoresistance, and high thermopower. Metal ions with half-integer valence tend to split into two spatially ordered integer valence ions. To realize a half-integer valence state and charge ordering in the B site of a perovskite ABO3, two or more elements with different valences need to be mixed in the A site.
A group of researchers, Prof. Masaki Azuma from Tokyo Institute of Technology, and Dr.Yuki Sakai at the Kanagawa Academy of Science and Technology and colleagues, have reported an unusual half-integer average charge system Pb3.5+M2.5+O3 with charge ordering in the A and B sites of a perovskite PbCoO3. Furthermore, the charge orderings in these sites were stabilized by tuning the energy levels of the Pb 6s and TM 3d orbitals. Bond valence sum calculations revealed a valence distribution of Pb2+Pb4+3Co2+2Co3+2O12, with Pb and Co exhibiting charge ordering despite the chemical composition of PbCoO3. As expected, the average oxidation state was Pb3.5+Co2.5+O3, with half-integer valences in both A and B sites of the perovskite structure stabilized by the balanced Pb 6s and Co 3d levels. The valence distribution of PbMO3 was controlled by tuning the depth of the d level of M. The complex valence distribution is expected to change on perturbations, e.g., pressure and chemical modification. For instance, when the Co charge ordering is melted, Pb2+0.25Pb4+0.75Co2.5+O3 is formed, Pb2+0.5Pb4+0.5Co3+O3 is first formed by the intermetallic charge transfer between Pb and Co and then possibly Pb2+Co4+O3 under pressure.
In the future, the application of the strategy of realizing mixed valence states in the A and B sites of perovskite compounds via the tuning of the energy difference between Pb 6s and transition metal 3d orbitals will be reported for other systems with valence-skipping elements, e.g., Au, Tl, and Sb.