Elastic constants of ammonium fluoroberyllate by thermal diffuse scattering of X-rays

All the nine elastic constants of orthorhombic ammonium fluoroberyllate [(NH₄)₂.BeF₄] have been determined at room temperature (305 K) from the measurement of the intensities of thermal diffuse scattering of X-rays using a single-crystal diffractometer. Monochromatic Mo Kα radiation obtained by reflection from a curved quartz crystal was used. The intensity of diffusely scattered X-rays has been corrected for divergence, polarization, skew correction, absorption, second-order thermal diffuse scattering and general scattering. Isodiffusion contours have been drawn around the 400 node. The values of elastic constants in units of 10¹⁰ Nm^-2 are C₁₁ = 3.82 (30), C₂₂ = 3.56 (28), C₃₃ = 2.45 (19), C₄₄ = 0.96 (05), C₅₅ = 1.01 (05), C₆₆ = 0.79 (04), C₁₂ = 1.78 (18), C₁₃ = 1.52 (15), C₂₃ = 1.41 (14). An estimate of the Debye temperature has been made by calculating the mean sound velocity in the crystal from these elastic-constant values.