Measurement of compressibility in polycrystalline MgO using the reflectivity method

Abstract

A polycrystalline sample of MgO with a density of 3.5803 g/cm 3 was obtained; on this the infrared reflection band and the elastic constants were measured. Velocities of propagation at 53-Mc/s waves were used to obtain the adiabatic elastic constants at 25°C. The adiabatic elastic moduli, based upon the measured density, are: Bulk modulus, K s , 1723.7 kbars; shear modulus, G S , 1274.4 kbars; and Poisson ratio, σ, 0.203. Using the expansivity data of Ganesan, the isothermal bulk modulus, was calculated to be k T = 1698 kbars. The elastic constants of the poly-crystalline sample agree with the averaged single-crystal elastic constants. The reflection band was measured and fitted to a single oscillator dispersion curve to obtain: the ‘reststrahlen’ lattice wavelength, λ 0 = 25.0 μ; the dielectric constant, ϵ 0 = 9.8, and the index of refraction, n = 1.783. The reflection band of the polycrystalline sample agrees with the measurements made on a single-crystal of MgO. The isothermal value of the bulk modulus computed from λ 0 by the Szigeti equation is 1640 kbars, and agrees within 4 per cent of the measured value of B T . Comparison of the value of B T from the Szigeti equation with the measured value of B T is also made for SiC and ZnO. It is concluded that the Szigeti equation holds for covalent compounds.