  A microscopic model for the diluted spin-mixed compounds
(R_xY_(1-x))_2BaNiO_5 (R=magnetic rare-earth) is studied using Quantum Monte
Carlo (QMC). The ordering temperature is shown to be a universal function of
the impurity concentration x and the intrinsic Ni-chain correlation length. An
effective model for the critical modes is derived. The possibility of a quantum
critical point driven by the rare earth-concentration and the existence of a
Griffiths phase in the high dilution limit is investigated. Several possible
experimental approaches to verify the results are put forward.
