The investigation of the real structure and mechanical stresses in berlinite and langasite crystals

The structural perfection of the berlinite (AlPO/sub 4/) and the langasite (La/sub 3/Ga/sub 5/SiO/sub 14/) crystals have been investigated with the help of X-ray topography. The inclusions, dislocations and different types of twins have been observed. The influence of the uniaxial stress along the crystallographic X and Y directions on the change of the light polarization plane rotation in these crystals have been studied. The appearance of two types of the circular birefringence: the screw and axial types have been found. The method of the separation of these effects have been developed, that have allowed to increase the sensitivity of the method of control of the internal stresses in crystals.