COPPER-OXIDE SUPERCONDUCTORS - A DISTINGUISHABLE THERMODYNAMIC STATE

The temperature dependence of the resistivity and Seebeck coefficient for the two p-type systems La2CuO4+delta, 0 less-than-or-equal-to delta less-than-or-equal-to 0.09, and La2-xSrxCuO4, 0 less-than-or-equal-to X less-than-or-equal-to 0.3, are reported and interpreted in the context of overall phase diagrams. Above room temperature, the La2CuO4+delta system tends to lose oxygen at 1 atm 02; superconductive samples exhibit a first-order loss of oxygen above 500 K to revert to the antiferromagnetic phase. Below a transition temperature T(s) almost-equal-to 300 K, compositions with 0 < delta < 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 T(c) < T< T(rho) almost-equal-to 100 K. In the system La2-xSrxCuO4, the holes move diffusively, with a DELTAH(m) = 0, above T(l) almost-equal-to 300 K for the compositions 0 < x less-than-or-equal-to 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 less-than-or-equal-to x < 0.35. Compositions with 0 < x less-than-or-equal-to 0. 12 are metastable in the range T(c) < T < T(l) where the holes continue to move diffusively, but charge fluctuations appear in the range T(c) < T < T(rho) less-than-or-equal-to 150 K. Compositions with 0. 15 less-than-or-equal-to x less-than-or-equal-to 0. 2 appear to undergo a transition from a polaronic gas to a polaronic (Luttinger) liquid on cooling through T(l); superconductive pairs are condensed from the homogeneous polaronic-liquid normal state at T(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:3dx2-y2-O:2p(sigma) bonding in the CuO2 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.