Temperature dependence of the moduli of elasticity of NaClO₃ and NaBrO₃ single crystals

The temperature dependences of the moduli of elasticity c_ik(T) of single crystals of sodium chlorate NaClO₃ and sodium bromate NaBrO₃ in the temperature range 77-300 K were investigated experimentally with the help of the resonance method. It is shown that these compounds are elastically anisotropic dielectrics with elastic anisotropy factor A<1 and ratios of the Young moduli and shear moduli of the form E₁₀₀>E₁₁₀>E₁₁₁ and G₁₀₀<G₁₁₀<G₁₁₁. The experimental data on c_ik(T) were interpreted using a lattice model taking into account the central and noncentral interactions of ions in two coordination spheres. The force constants of these interactions are presented. It is shown that the model chosen describes well the temperature dependences c_ik(T), if both the noncentral interaction of the ions and the contribution of the vibrational part of the free energy to the moduli of elasticity are taken into account.