Theory of piezo-optic birefringence in mixed crystals of equimolar concentration with NaCl structure

Following the theoretical approach of Bansigir & Iyengar [Acta Cryst. (1961), 14, 670-674], an expression for P₁₁ - P₁₂ is developed for a mixed crystal of KC1-KBr with equimolar concentration. It was assumed that among the six nearest neighbours of the K⁺ ion, three are Cl^- and the other three Br^-, and that each K⁺ is accompanied by Cl^- and Br^- on either side in all three principal directions. The expression P₁₁ - P₁₂ is used to evaluate the polarizabilities, reversal wavelength, ratio and absolute values of strain-optical constants P₁₁ and P₁₂.