Measurements of the Seebeck coefficient α(T) and the resistivity R(T) of polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ quenched from different temperatures ${\mathit{T}}_{\mathit{q}}$ and of polycrystalline (${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$)(${\mathrm{Ba}}_{2\mathrm{-}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$)${\mathrm{Cu}}_3$${\mathrm{O}}_{6.96\pm 0.01}$ were made to distinguish the contributions to α(T) from the Cu(1)O chains and the ${\mathrm{CuO}}_2$ sheets. The ${\mathrm{CuO}}_2$ sheets give a positive contribution to α(T) that is composed of a weakly temperature-dependent part ${\mathrm{\alpha }}_0$ that decreases sensitively with increasing oxidation and an enhancement part δα that gives a broad maximum to α(T) at a ${\mathit{T}}_{\max }$≊140 °C. These two features together have been shown to distinguish the superconductive compositions from the parent and overdoped compositions in other copper-oxide supeconductors. In fully oxidized ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.96}$, the contribution to α(T) from the ${\mathrm{CuO}}_2$ sheets is obscured by a dominant, competitive negative contribution to α(T) from the chains that has a similar, but negative, δα enhancement.

• Received 6 September 1994

DOI:https://doi.org/10.1103/PhysRevB.51.3250