X-ray topographic study of DKDP single crystals grown from solution on KDP seeds

Abstract

Defect structures in single crystals of deuterated KH 2PO 4 (DKDP) grown on seeds cut from crystals of KH 2PO 4 (KDP) have been studied by X-ray diffraction topography. Three types of KDP seeds were used: (a) plate (011); (b) top pyramid {011}; (c) plate (001). In DKDP crystals grown on KDP plates (011), elastic stresses due to a difference in lattice parameter a between DKDP and KDP have been observed. The plane (011) in DKDP is misoriented by an angle of several minutes or arc relative to this plane in KDP so that the DKDP crystal/KDP seed interface is a tilt-angle boundary. Numerous solution inclusions at the interface give rise to a very high dislocation density in the DKDP crystals. Similar phenomena have been also observed in DKDP crystals grown on KDP top pyramid seeds, except that the misorientation varies both from one growth sector to other and within the same sector. As a result, these DKDP crystals have a block structure. The perfection of DKDP crystals grown on KDP plates (001) is rather high and like that of KDP crystals grown on KDP seeds of this type. It is suggested that “soft” growth conditions in this case are due to the occlusion, during the regeneration of growth faces {011}, of a macroscopic solution volume which minimizes direct contact between the growing DKDP crystal and the KDP seed plate.