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 Lambda = 4C(11)(C-11 - C-44)/(C-11 + C-12)(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 alpha-Sn, respectively. For II-VI, III-V, and I-VII compounds with sphalerite structure, Lambda was found to systematically deviate from unity toward lower values, and it was shown that the quantity (1 - Lambda) can be used to estimate the bond ionicity in these crystals. The effect of anharmonicity on the Lambda values for Ge, Si, GaAs, InAs, and ZnSe was estimated; this effect was found to be insignificant. (C) 2003 MAIK "Nauka/Interperiodica".