  We investigate effects of interactions between substitutional defects on the
properties of defect crystals at low temperatures, where defect motion is
governed by quantum effects. Both, thermal and dynamical properties are
considered. The influence of interactions on defect motion is described via a
collective effect. Our treatment is semiclassical in the sense that we analyze
collective effects in a classical setting, and analyze the influence on
quantized defect motion only thereafter. Our theory describes a crossover to
glassy behavior at sufficiently high defect concentration. Our approach is
meant to be general. For the sake of definiteness, we evaluate most of our
results with parameters appropriate for Li-doped KCl crystals.
