Copper oxide superconductors: A distinguishable thermodynamic state

The temperature dependence of the resistivity and Seebeck coefficient for the two p-type systems La₂{CuO}_{4+{{δ}}}, 0≤δ≤0.09, and{La}_{2{{-}}x}Srₓ{CuO}₄, 0≤x≤0.3, are reported and interpreted in the context of overall phase diagrams. Above room temperature, the{La}₂CuO_4+δ system tends to lose oxygen at 1 atm O₂; superconductive samples exhibit a first-order loss of oxygen above 500 K to revert to the antiferromagnetic phase. Below a transition temperature Tₛ{}300 K, compositions with 00.05 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 TcTT_ρ{}100 K. In the system La_2-x{Sr}ₓCuO₄, the holes move diffusively, with a {Δ}Hₘ=0, above Tₗ{}300 K for the compositions 0x{≤}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{≤}x0.35.Compositions with 0x{≤}0.12 are metastable in the range TcTTₗ where the holes continue to move diffusively, but charge fluctuations appear in the range TcTT_ρ{≤}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 Tₗ; superconductive pairs are condensed from the homogeneous polaronic-liquid normal state at Tc. 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_σ 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.