The temperature dependence of the resistivity and Seebeck coefficient for the two p-type systems ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$, 0≤δ≤0.09, and ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$, 0≤x≤0.3, are reported and interpreted in the context of overall phase diagrams. Above room temperature, the ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ system tends to lose oxygen at 1 atm ${\mathrm{O}}_2$; superconductive samples exhibit a first-order loss of oxygen above 500 K to revert to the antiferromagnetic phase. Below a transition temperature ${\mathit{T}}_{\mathit{s}}$≊300 K, compositions with 0<δ<0.05 undergo phase segregation to an antiferromagnetic and a superconductive phase; the superconductive phase appears to undergo a further dynamic segregation into hole-rich and hole-poor domains in the interval ${\mathit{T}}_{\mathit{c}}$<T<${\mathit{T}}_{\mathrm{\rho }}$≊100 K. In the system ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$, the holes move diffusively, with a Δ${\mathit{H}}_{\mathit{m}}$=0, above ${\mathit{T}}_{\mathit{l}}$≊300 K for the compositions 0<x≤0.21; the system undergoes a transition from a p-type two-dimensional conductor to an n-type three-dimensional conductor in the interval 0.22≤x<0.35.

Compositions with 0<x≤0.12 are metastable in the range ${\mathit{T}}_{\mathit{c}}$<T<${\mathit{T}}_{\mathit{l}}$ where the holes continue to move diffusively, but charge fluctuations appear in the range ${\mathit{T}}_{\mathit{c}}$<T<${\mathit{T}}_{\mathrm{\rho }}$≤150 K. Compositions with 0.15≤x≤0.2 appear to undergo a transition from a polaronic gas to a polaronic (Luttinger) liquid on cooling through ${\mathit{T}}_{\mathit{l}}$; superconductive pairs are condensed from the homogeneous polaronic-liquid normal state at ${\mathit{T}}_{\mathit{c}}$. The origin of the unusual electron-lattice interactions in the normal state of the superconductive compositions is attributed to a coexistence of ionic and covalent bonding at a transition from more ionic to covalent Cu:3${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$-O:2${\mathit{p}}_{\mathrm{\sigma }}$ bonding in the ${\mathrm{CuO}}_2$ sheets; through the transition the orbital hybridization and Hubbard U parameter vary sensitively with both the Cu-O bond length and the formal local oxidation state at a Cu atom.

• Received 29 September 1992

DOI:https://doi.org/10.1103/PhysRevB.47.5275