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It seems that the remarkable properties of the high temperaturesuperconducting oxides, especially the Insulator-Metal Transition (IMT) and theMetal-Superconductor Transition (MST) both originate from the competition(interplay) between ionic versus in-plane covalence nature of bonds in thesematerials. As a result of this competition, the microscopic order parameter,that is firmly identified to be the local field estimated from the ionicpolarization at the sub-unit level (one half of the unit cell), shows a strongtemperature as well as chemical doping dependence. While the out-of-planeionicity is responsible for the interlayer charge transfer (electrons) thatreduces it leading to IMT, the in-plane covalency is responsible for thein-plane intersite transfer of charge (holes) that increases the ionicity andleads to MST. This interplay of charge transfer, driven by the out-of-planeionicity and the in-plane covalency, leads at the critical temperature Tc to alocal field avalanche (ionic polarization catastrophe at the sub-unit celllevel) that triggers an out-of-plane correlation between the local polarizationfields at the unit cell and drives the compound into the superconducting state.The asymmetry of the free charge carrier density breaks locally the inversionsymmetry of the order parameter leading to an unconventional pairing mechanismcharacterized by the CPT transformation where C is the charge conjugationsymmetry, P is the parity symmetry and T is the time reversal symmetry. It isestablished that the out-of-plane ionicity versus in-plane covalency is anecessary condition for the occurrence of both IMT and MST. The underlinedmicroscopic mechanism provides a route to a possible unified description ofhigh-Tc superconductivity.

Out-of-plane Ionicity versus In-plane Covalency Interplay in High-Tc Superconducting Oxides