Effects of substituting calcium for yttrium on the properties of YBa2Cu3O7−δ

Abstract

We report measurements of the room-temperature lattice parameters, of the resistivity (ρ) for temperatures up to 300 K, and of the absolute thermoelectric power up to 400 K, of single-phase sintered ${\mathrm{Y}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{Ca}}_{\mathit{y}}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, for y=0–0.3 and for a range of values of δ. For pure ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ we confirm earlier reports that an optimal δ exists, for which ${\mathit{T}}_{\mathit{c}}$ is maximal (${\mathit{T}}_{\mathit{c}\mathit{m}}$), and that the thermoelectric power changes sign in the vicinity of this concentration. Similar behavior, though with a lower ${\mathit{T}}_{\mathit{c}\mathit{m}}$ is observed in samples with y=0.1. For y=0.2 and 0.3, the maximum of ${\mathit{T}}_{\mathit{c}}$(δ) flattens off into a plateau. The enhancement of the magnitude of the negative thermoelectric power by Ca doping is much less than expected. This is probably due to the fact that most of the extra charge introduced by the Ca is screened, mainly by the cointroduction of oxygen vacancies.