Thermal conductivity of rare earth fluoride crystals

Abstract

High-temperature thermal properties of three neighboring rare-earth scandates DyScO 3 , TbScO 3 and GdScO 3 were compared to La 0.29 Sr 0.71 Al 0.65 Ta 0.35 O 3 (LSAT) and sapphire. To calculate thermal conductivity, heat capacity and thermal diffusivity were measured by differential scanning calorimetry and laser flash technique, respectively. DyScO 3 and TbScO 3 showed an untypical rise in the thermal conductivity above 900 K, while for GdScO 3 , LSAT and sapphire the expected decrease at elevated temperatures could be observed. These results lead to the proposal of a new type of heat transport by migrating ions.

Quasi-transverse ultrasound absorption due to point defects and anharmonic scattering processes in cubic crystals with positive and negative anisotropies of the second-order elastic moduli

“…In accordance with the prevailing concepts [1][2][3][4][5][6][8][9][10][11], the main mechanism of the transverse phonon relaxation in normal three-phonon scattering processes is the Landau-Rumer mechanism [14] when the merging of the transverse and the longitudinal phonon produces a longitudinal phonon (T + L → L). The expression for the relaxation rate of phonons moving in an arbitrary direction relative to the crystallographic axes of crystals was obtained for this mechanism in terms of the anisotropic continuum model in [20].…”

“…where g is the isotopic disorder factor, M = M i − M, M i is the mass of the i th isotope, M = i C i M i is the average mass of the isotope composition, C i is the concentration of the i th isotope, while σ 1 , σ 2 and σ 3 characterize the contributions from the change of the unit cell mass, the force constants and the lattice deformation to the scattering cross-section of phonons by impurities (for more information see [5,[24][25][26]). Expression (2) can be written in the form…”

“…A problem encountered in studies of the ultrasound absorption [1][2][3][4], the phonon transport [5,6] and the thermal emf of the electron-phonon drag [7] in semiconductor and dielectric crystals is the correct consideration of the effect of the cubic anisotropy on the spectrum, the oscillation mode polarization and the phonon relaxation rates. When the temperatures are sufficiently low and the inequality ω qλ τ λ ph (q, T ) 1 is fulfilled (τ λ ph (q, T ) = 1/ν λ ph (q, T ), ν λ ph (q, T ) being the phonon relaxation rate and ω qλ the frequency of phonons with wavevector q and polarization λ, T is the temperature), the dominant contribution to the volume absorption of sound waves is due to the scattering by defects, including isotopic scattering, and normal processes of phonon-phonon scattering (see, e.g., [1,4]).…”

“…We shall restrict ourselves to their consideration in this paper. The isotropic medium approximation [4][5][6][8][9][10][11], which is commonly used to estimate the probability of various scattering processes, is inadequate for germanium, silicon, diamond and other semiconductor crystals having cubic symmetry and a considerable anisotropy both of the second-and third-order elasticity moduli. The anisotropic continuum model provides a convenient solution to these problems.…”