Seminário de Matéria Condensada - 09/08/2018, 11:00, Sala 528 (Torre Nova)

Some ABX3 perovskites exhibit different local environments (DLE) for the same B atoms in the lattice, an effect referred to as disproportionation, distinguishing such compounds from common perovskites that have single local environments (SLE). The basic phenomenology associated with such disproportionation involves the absence of B-atom charge ordering, the creation of different B-X bond length for different local environments, the appearance of metal to insulator transition, and the formation of ligand holes. We point out that this broad phenomenology is common to several ABX3 compounds such as BaBiO3, CsTlF3, CsAuCl3, SmNiO3 and CaFeO3. We show that underlying much of this phenomenology is the ‘self-regulating response’, whereby in strongly bonded metal-ligand systems with high lying ligand orbitals, the system protects itself from creating highly charged cations by transferring ligand electrons to the metal, thus preserving a nearly constant metal charge, while creating B-ligand bond alternation and ligand-like conduction band (‘ligand hole’ states). Using as a guide the lowering of the total energy in DLE relative to SLE, we show that density functional calculations describe this phenomenology across the whole chemical bonding range without resort to special correlation effects, naturally developing bond alternation, gaping of the metallic SLE state, and absence of charge ordering with ligand hole formation.