Athermal Directions in KGd(WO4)2 Crystal

All orientations of the phase normal, at which the phase of a light wave propagating through a crystal is independent of its temperature (athermal phase directions), have been numerically found for a KGd(WO4)2 crystal. There are two sets of such directions for each isonormal wave. Their intersection with the main planes of the optical indicatrix of the crystal yields four athermal directions in the planes orthogonal to the axes with the minimum and intermediate refractive indices and two directions in the plane orthogonal to the axis with the maximum refractive index.