Electrical conductivities of YBa 2M3O y ( M: Fe, Co) system

Abstract

YBa 2 M 3O 9− δ ( M: Fe, Co) systems with composition similar to the YBa 2Cu 3O 7 superconductor were synthesized. Two types of Y 1.2Ba 3Fe 3O z (orthorhombic, a = b = 4.06 Å, c = 4.04 Å) and YBa 2Fe 3O 9−δ (tetragonal, a = 3.94 Å, b = 3.92 Å, c = 3.90 Å) with perovskite-type structure were obtained for the iron oxide system, while YBa 2Co 3O 9−δ perovskite (cubic, a = 4.14 Å) was obtained for the cobalt oxide system. The conductivities and the Seebeck coefficients were measured in the temperature range from 650°C to room temperature. The carrier densities and the mobilities were calculated by using a small polaron hopping model. All the samples were p-type semiconductors and showed hopping conduction. The mobility of holes was much larger for YBa 2Co 3O 9−δ than for YBa 2Fe 3O 9−δ, although the unit cell is larger for the former oxide than for the latter. This is explained through the larger overlapping integral of the σ∗ hopping band formed in YBa 2Co 3O 9−δ than the π∗ hopping band formed in YBa 2Fe 3O 9−δ.