DISLOCATIONS AND SUBGRAIN BOUNDARIES IN HIGHLY MAGNESIUM-DOPED LITHIUM-NIOBATE CRYSTALS

The extension and distribution of dislocations and subgrain boundaries in highly magnesium-doped lithium niobate crystals at different stage s of growth have been investigated using chemical etching and optical microscopy. The relations between dislocation densities, subgrain boun daries and optical quality of the crystals have been also studied. It was found that there is a core with relatively high dislocation densit y in the central region of the crystal shoulder. In the shouldering st age, the dislocations gathering in the core extend to its surrounding regions and the dislocation density tends to be homogeneous. Tailing i ncreases the dislocation density in the bottom part of the crystal and causes inhomogeneous distribution of dislocations in that part. Subgr ain boundaries are apt to form in high dislocation density regions, an d neighbouring multiple subgrain boundaries tend to reform a more stab le single subgrain boundary. The extinction ratios and conoscope image s of crystals are worst in the dislocation gathering regions around th e ends of subgrain boundaries, and dislocations are the basic cause of poor optical quality of crystals.