Gradient-Elastic Tensor of Alkali Halide Crystals

Abstract

Origin of electric field gradient due to strains in alkali halide crystals is unravelled from the experimental determination of gradient-elastic tensors. Tensors for ²³Na, ³⁵Cl, ⁸¹Br and ¹²⁷I in a series of crystals are determined from the quadrupolar broadening of resonance lines due to solute ions, using lattice distortion derived from lattice parameter changes.
Results are interpreted by the theory which takes account of the contribution from valence electrons, besides point charge arrays, and gives an expression of a gradient-elastic tensor in terms of an anti-shielding factor, overlap integrals and a degree of covalency.
An excellent agreement is found between the theoretical and experimental tensor values for ²³Na in NaCl, for which both theory and experiment are least ambiguous. For other crystals, a degree of covalency is tentatively estimated, with reasonable results.