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.