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 [Phys. Rev. Lett. 75, 1965 (1995)] that random stresses are essential for the creation of glasslike excitation in solids.