On the born relation for crystals with diamond and sphalerite structure

The experimental data on the elastic constants C_ij for crystals with diamond and sphalerite structure at T=293 K were used to check the Born relation Δ4C₁₁(C₁₁?C₄₄)/(C₁₁+C₁₂)²=1. The relation was shown to be satisfied with a low accuracy for diamond due to a large scatter in the C_ij experimental values and with accuracies of 8.3, 7.6, 1.6, and 1.0% for Si, SiC, Ge, and α-Sn, respectively. For II–VI, III–V, and I–VII compounds with sphalerite structure, Λ was found to systematically deviate from unity toward lower values, and it was shown that the quantity (1-Λ) can be used to estimate the bond ionicity in these crystals. The effect of anharmonicity on the Λ values for Ge, Si, GaAs, InAs, and ZnSe was estimated; this effect was found to be insignificant.