GLASS-LIKE PROPERTIES OBSERVED IN LOW-FREQUENCY RAMAN-SCATTERING OF MIXED FLUORITE CRYSTALS

The low-frequency Raman spectra of (CaF2)(1-x)(LaF3)(x) mixed crystals in the concentration range from 0.05 less than or equal to x less tha n or equal to 0.4 have been measured as a function of temperature from 4.2 K to 295 K. Concentration dependent quasi-elastic Raman scatterin g is found for frequencies below 10 cm(-1) for A(lg) + E(g) scattering but not for T-2g symmetry scattering. The temperature dependence of R aman intensity suggests that this effect is dominated by scattering fr om a distribution of two-level systems between 4.2 K and 20 K. The low est impurity-induced vibrational Boson scattering peak at 82 cm(-1) wh ich shifts to higher frequency with increasing dopant concentration, a lso only appears in A(lg) + E(g) symmetry, indicating that the two dif ferent kinds of glass-like excitations in these mixed crystals may hav e a common origin.