As part of a series of studies on energy transfer processes between optical centers in condensed matter, an analysis is made of the problem of a crystalline system made up by a majority of regular optical centers and small amounts of defective optical centers. Within the conceptual framework of the crystal model, analytic equations are derived that allow a direct connection between the microscopic study of the interaction and kinetic type equations to describe the relaxation dynamics from excited states. As an example, Ln(3+) ions in elpasolite type lattices are used, showing the biexponential character of the decay curves. These results can be used in the study of relaxation from excited states in crystals with heterogeneities due to crystallographic defects, trace impurities, or slightly distorted optical centers.