Optic Axis Rotation and Bertin Surface Deformation 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 modifications of the Bertin surfaces induced by either the distortion of crystallographic cells, the applied plane stress, or cell misalignment with respect to the specimen faces. In both cases, the distortions of the Bertin surfaces result in the reshaping of the interference pattern observed by conoscopy. We provide, for different observation directions of the crystals, analytical relations that allow for the evaluation of the optic plane and the optical indicatrix rotation with or without stress. By the means of these relations, interference image reshaping allows us to detect, provided that some conditions hold, the crystallographic axes’ rotation. This work is a theoretical study aiming to evaluate the optic axes and crystallographic cell orientation by means of conoscopic observations.