LOCAL MODEL FOR CHARGE DELOCALIZATION IN PARTLY SUBSTITUTED CUPRATE SUPERCONDUCTORS

Partial substitutions according to YBa2(Cu(1-x)M(x))(3)O-y with M = Ni etc. in connection with various redox sequencings allow for complex s trategies in obtaining novel materials. We analyze redox related struc tural 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 b ehavior such as n type superconductivity at low y and semiconductivity 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. I n this case (1)3 and (1)2 coordinations dictate the spatial behavior o f (1)4 which leads to charge localization. We show that this correspon ds to a Mott type transition which has a general implication in degrad ing superconducting properties on further reoxygenation.