Point defects and their clusters in single crystals, free from grain boundaries and having a low density of dislocations, were studied by means of high-resolution diffuse X-ray scattering. Differences in the defects in colourless crystals (type A) and crystals having a yellow tinge (type B), which were grown using different raw materials, were investigated. Specimens with diffracting surfaces along (111), (112) and (100) planes were studied. A multi-crystal X-ray diffractometer, using a well-collimated and highly monochromated beam set in (+,-,-,+) configuration. The diffraction curves of all of the samples were quite narrow, with half-widths ranging from 7 to 11arcsec. These were close to the theoretically expected values, when instrumental broadening was taken into account. The observed distribution of the diffuse X-ray scattering intensity showed that not all of the point defects were isolated, and that a significant fraction agglomerated into clusters. Experimental data on the diffuse X-ray scattering intensity were analysed by using a phenomenological model for a small concentration of dislocation loops, within which point defects were loosely clustered; with weak interactions between them. From this analysis, the cluster radius, cluster volume, number of point defects within a cluster and the relative concentrations of point-defect clusters among samples were estimated. It was observed that cluster sizes did not vary from sample to sample. However, the concentration of clusters was about twice as high in coloured samples as compared with colourless ones from the same boule. Annealing of the crystals (1273K) produced an increase, in point-defect clusters, by a factor of about 200. This was accompanied by a reduction in the volume of clusters by a factor of about 0.14.

Study of Point Defects in As-Grown and Annealed Bismuth Germanate Single Crystals. G.Bhagavannarayana, A.Choubey, Y.V.Shubin, K.Lal: Journal of Applied Crystallography, 2005, 38[3], 448-54