Superconductivity and the Chemical Bond

Abstract

Experimental evidence suggests the operation of electron rules based on bond ordering of doped radical CuO bonds (BO) and lattice pressure (LP) in cuprate superconductivity. Ideas are developed for understanding the crystal chemical principles involved in these BO. They are then extended to materials on the basis of other electronegative elements. For cuprates, a quantitative T _c formalism is based on radical bond or hole concentration in the CuO₂ plane (h_p) per O in the extended planar system including the apical O (radical formalism or RF). The deleterious effect of the apical O is ascribed to bond resonance with the CuO₂ plane. A range in the critical hole concentration exists for optimal T _c depending on LP. Accordingly the small Sr analogs tend to be limited at smaller hole concentrations h _p = 1/6, while the Ba analogs can optimize at 1/3. Selected experimental evidence is used to construct plausible BO patterns. Superconductivity is related to strings of local pairs of trijugate half bonds. Electronic freezing can occur in domain walls of alternating single holes, e.g., in La₂CuO₄ type materials at h _p = 1/8. Predictions for this BO model have been corroborated by details in the transistor doping of CaCuO₂ and C ₆₀ as a universal sequence of T _c development with hole doping becomes apparent for both cases. Hallmarks include linear portions up to an optimal BO (RF slopes), which extrapolate to the origin, and a “sharp” peak with another linear region thereafter. They further include T _c onset at a critical BO, where an adverse BO is cooperatively transformed to a beneficial one with a slope related to RF by rational numbers. Tendencies to electronic freezing [e.g., h _p = 1/8 in CaCuO₂ and an analogous one in C ₆₀] occur near the optimal BO because of increasing LP. Some of these features occur at identical numbers of atoms per hole for both systems and others are related numerically. T _c–hole slopes can supply hints for the mechanisms of BO transformations. The universality of this behavior suggests a common origin for all high T _c materials in pairs of doped covalent bonds of radical character and a “musical” BO phenomenology with respect to levels and limits.