HTSCs with apical oxygen replaced by halogens

The fundamental structural unity of familiar oxide HTSCs and their oxyhalide counterparts, with apical oxygen ions replaced by halogen ions, is demonstrated and discussed. As a consequence of this unity, major physical phenomena are expected to be common for both classes, such as doping effects, concentration-driven insulator—metal transitions, the unusual pair superconductivity, and the temperature dependence anomalies in the critical magnetic field, linear expansion coefficient, and heat capacity jumps. New experimental results are discussed, accounted for by the exciton mechanism of superconductivity with the local nature of excitations taken into account.