The Department of Materials Science and Engineering welcomed Masahiro Yoshimura of the Promotion Center for Global Materials Research and National Cheng Kung University in Taiwan for the MSE Seminar Series. Yoshumura presented a seminar titled “Formation of Un-Common Valences and Defects in Perovskite Lattice via Revisiting Madelung Energy and Site Potential.” Below is the abstract:
Many functional oxides would take Perovskite structures (ABO3). The major reason should be understood by the fact that Perovskite lattices would have larger Madelung lattice energies than those of other lattices like NaCl, fluoite, corundum, spinel, garnet.,etc. Since the ionization potential [loss] can be compensated by high lattice site potential [gain], high valence ion can be stabilized in a lattice site with high lattice-site potential. For example, Ce4+, Pr4+, and Tb4+ can be stabilized in a Fluorite lattice, and more stabilized in the B site of Perovskite ABO3 lattice. Un-common high-valence ions like Co3+, Ni3+, Fe4+ in the B-sites of the Perovskite ABO3 lattices can be explained similarly. On the other hand, those high-valence ions cannot be stabilized in the A site of Perovskites nor various A2O3, like A-type,C-type,Mn2O3,or corundum, lattices thus they take rather common valence states in those cases. The Madelung Lattice energy (U) is Enthalpy at 0 K,thus it consists of ca. 90% of total free energy(G) of the compound. The entropy term by Temperature and PO2 can contribute < 10% of U at 300K. Therefore it might be rather minor effects for those internal lattice potential energy. Point defects may form in lattice sites having small lattice site potentials. In large potential sites may collapse the pint defects to form edge-sharing(shear) structure(s). It has been clearly demonstrated under summarizing the data.