Glasslike excitations in chemically disordered crystals: Alkali-earth lanthanum fluoride mixed crystals

Through measurements of internal friction and speed of sound at low temperatures we have followed the evolution of glasslike low-energy excitations in chemically disordered crystalline CaF2 and SrF2 containing increasing concentrations of LaF3. Although small concentrations do exhibit relaxational processes at low temperatures, low-energy excitations that can be well described with the tunneling model are observed only when the LaF3 concentration exceeds approximately 20 mol %. This result is shown to agree with the earlier results on BaF2 containing LaF3. In the mixed host (BaF2)(0.5)(SrF2)(0.5), on the other hand, even small LaF3 concentrations lead to excitations that are well described with the tunneling model. These results confirm the earlier findings by Watson that random stresses are essential for the creation of glasslike excitation in solids. [S0163-1829(99)10425-9]