CaO and SrO single crystal elastic constants and their pressure derivatives

Abstract

Values for the elastic stiffness constants of CaO and SrO have been obtained at room temperature using the ultrasonic pulse-echo technique. Results are, in units of 10 12 dynes/cm 2, B C C ′ CaO 1 ⋅ 14 ± 0 ⋅ 09 0⋅ 81 ± 0 ⋅ 02 0⋅ 82 ± 0⋅ 02 SrO 0 ⋅ 88 ± 0 ⋅ 07 0⋅ 56 ± 0 ⋅ 02 0⋅ 64 ± 0⋅ 02 Where the notation B = ( C 11 + 2 C 12)/3, C = C 44, C′ = ( C 11 − C 12)/2 has been used. In addition the pressure derivatives of these constants were measured with the results d B /d P d C /d P d C ′ /d P CaO 6 ⋅ 0 ± 1 ⋅ 3 0⋅ 6 ± 0 ⋅ 1 3⋅ 4 ± 0⋅ 8 SrO 6 ⋅ 0 ± 0 ⋅ 7 −0⋅ 2 ± 0 ⋅ 1 4⋅ 0 ± 0⋅ 2. These results are combined with similar data for MgO and a detailed comparison is made between the elastic properties of the series MgO-CaO-SrO and the analogous alkali halide series NaF-KF-RbF. A simple (repulsion between nearest neighbors only) Born model for the cohesive energy is tested in both series of compounds and found to be less satisfactory for the oxides. The effects of modifying this model by addition of next nearest neighbor repulsion and by varying the ionic charge are considered: neither approach significantly improves theory-experiment agreement but the varying of the charge leads to the interesting effect of making the elastic properties and the Born model parameters of the oxides correlate more closely with those of the fluorides.