Natural birefringence in alkali halide single crystals

The birefringence induced by spatial dispersion (BISD) of NaCl, KCl, RbCl, NaBr, KBr, RbBr, CsBr, NaI, KI, RbI, and CsI single crystals has been measured at room temperature near their fundamental absorption edge. The sign of the birefringence is negative ($n_{11¯0}$$n_{001}$) for chlorides and iodides. For bromides no BISD has been obtained. By fitting the experimental results with a microscopic model one obtains the warping parameter (${\gamma }_2$-${\gamma }_3$) of these compounds. ${\gamma }_2$-${\gamma }_3$ estimated from the available band calculation for the alkali halides does not agree with the abnormally high values measured for iodides and chlorides.