REDOX RESPONSE MODEL FOR PARTLY SUBSTITUTED CUPRATES

Inhomogeneous substitutions (e.g. clustering and preferential site occ upations) in compounds such as YBa2(Cu1-xMx)3Oy with M = Fe, Co, Ni, e tc. are an unavoidable response to the changing redox environments in these O intercalation compounds. We collect here experimental evidence for various types of inhomogeneous distributions predicted on a therm odynamic model which considers the preferential ''desorption'' of O fr om various local environments. The parameter organizing the redox envi ronment is DELTAH (incremental enthalpy of oxygen desorption per mol O2). Theory suggests a sequence of discernible states represented by t he acronym CLUSTER. With decreasing DELTAH, indications for several o f these states are obtained from Mossbauer and other experiments. Thes e states are characteristically shifted in DELTAH for different M. Al so, further substitutions according to (YX)(BaZ)2(CuM)3Oy with X = Ca, Z = Sr, La can change the relevant DELTAH for these states, allowing prediction of their preparation conditions. Aspects of the complex el ectronic phase diagram as a function of redox preparation are discusse d.