QUASI-ELASTIC LIGHT SCATTERING AND DYNAMIC PARAMETERS OF THE INTERNAL MOTION IN SUPERIONIC CRYSTALS OF LNF_3 (Ln = La, Ce)

摘要

Low-frequency light scattering in superionic crystals of the LaF_3 structural type was studied. The laser light scattering spectra of LnF_3 single crystals at 90° scattering geometry were obtained over a wide temperature range. The scattering intensity increased sharply in the temperature range in which the mobility of the fluoride ions increased. The experimental scattering curves were represented as the sum of three central Lorentz contours. The "narrow" Lorentzian had width Γ_1 = 0.8-4.2 cm~(-1); the "medium" Lorentzian, Γ_2 = 4.2-27 cm~(-1); and the "wide" Lorentzian, Γ_3 = 42-96 cm~(-1). These corresponded to relaxation times τ_1 ≈ (0.13-0.67) × 10~(-11) s, τ_2 ≈ (0.20-1.27) × 10~(-12) s, and τ_3 ≈ (0.56-1.27) × 10~(-13) s that were associated with different types of displacement of fluoride ions into interstices. The activation energy ΔE_d obtained from the temperature dependence of the Lorentz contour width was found to be many times less than the energy E_d determining the dynamic parameters of ion motion in the LnF_3 lattice.