Local model for charge delocalization in partly substituted cuprate superconductors

Partial substitutions according to YBa2(Cu1 − xMx)3Oy with M = Ni etc. in connection with various redox sequencings allow for complex strategies in obtaining novel materials. We analyze redox related structural data on selected model compounds with special emphasis on a new local structural approach, derive aspects of their electronic behavior, and gain an expanded understanding of the charge balance function of Cu(1). We elaborate on the conditions for antitypes to conventional behavior such as n type superconductivity at low y and semi-conductivity at high y. We show that a localization catastrophe can occur when the population of (1)4 configurations (Cu(1) coordinated by 4 O) is low. In this case (1)3 and (1)2 coordinations dictate the spatial behavior of (1)4 which leads to charge localization. We show that this corresponds to a Mott type transition which has a general implication in degrading superconducting properties on further reoxygenation.