Nuclear quadrupole resonance and nuclear magnetic resonance studies of K2PtCl6 type mixed crystals

Phase transitions from cubic to lower symmetry phases in antifluorite (K2PtCl6) structures have been ascribed to lattice mode-softening. We study the effect of impurities on lattice vibrations and transition temperatures in such structures. For mixed rhenates we find a lowering of the frequency of the lattice vibrations with increasing impurity content and a corresponding increase in transition temperature. For mixed stannates the n.q.r. signal disappears at high temperature, perhaps due to reorientational motion of the stannate anion.