Optical Indices of Refraction of WO3

A modified Chaulnes' method using polarized light has been developed to measure the indices of refraction of W${\mathrm{O}}_3$. By this method, the three principal indices of refraction of a crystal were obtained by observations in the direction of only one optic principal axis. For W${\mathrm{O}}_3$, the mean value of the indices of refraction and the maximum value of birefringence at room temperature were found to be 2.5 and 0.42, respectively. The crystals were positive below 740°C and negative above this temperature. Distinct anomalies in the indices of refraction were observed at 330°C and 740°C. The results are compared with those for some other perovskite-type ferroelectrics and antiferroelectrics.