Anisotropy of thermal properties of pure and doped LiNbO₃ single crystals

Abstract
Lithium niobate crystals are widely used in modern acousto-electronic and optoelectronic devices. Physical properties of the lithium niobate can be modified by doping crystals during process of their growth, among others thermal properties of lithium niobate can be changed by impurities incorporated into the crystal. The thermal diffusivities of investigated samples were determined by modified Ångström's method. Measurements were carried out for pure crystal and samples doped with following elements: Cu (0.05 %at.), Fe (0.1 %at.), Dy (0.1 at%, 1.0 %at., and 1.7 %at.), Er (0.1 %at.), and Er + Yb (1.0 %at. Er + 0.1 %at. Yb, 1.0 %at. Er + 0.5 %at. Yb, 1.0 %at. Er + 1.0 %at. Yb). Because of the anisotropy of single crystals the thermal diffusivity measurements were performed in different crystallographic directions. Obtained results show that the thermal diffusivity in [001] direction (c-axis) is higher than in directions in (001) plane. Impurities cause drop of the thermal diffusivity, which in the case of samples doped with Dy, Er and Yb can reach 40% of the value for pure crystal. The velocity of longitudinal acoustic waves and the density were additionally determined. It allowed an analysis of the influence of impurities on mean free path of phonons.