Optic Axes rotation and Bertin surfaces deformations in Lead Tungstate (PWO) and other Tetragonal crystals by stress and misalignment of crystallographic cells: a theoretical study

For tetragonal Lead Tungstate (PWO) and other Tetragonal crystals we study the modifications of the Bertin surfaces induced by either the distortion of crystallographic cells, induced by applied plane stress, or the cell misalignment with respect to the specimen faces. In both cases the distorsions of the Bertin surfaces result in a reshaping of the interference pattern observed by conoscopy. We provide, for different observation directions of the crystals, the analytical relations which allow for the evaluation of the optic plane, optical indicatrix rotation with or without stress and, by the means of these relations, the interference images reshaping allows to detect, provided some conditions hold, the crystallographic axes rotation. The scope of this work is a theoretical study aiming the evaluation of the Optic axes and crystallographic cell orientation by means of conoscopic observations