Diffuse X-ray Scattering and Short-Range Order in Thallium Antimonyl Germanate, TlSbOGeO₄

The potassium titanyl phosphate (KTP) analogue material thallium antimonyl germanate has been studied at 295 K using diffuse X-ray scattering techniques. It has been shown that previously reported `diffuse arcs' of scattering are due to decomposition products (mainly Tl₂Ge₂O₅) near the surface of the crystals. In addition the experiments revealed the presence of continuous diffuse scattering. A preliminary analysis of this scattering shows that it originates predominantly from the disorder of the thallium positions between alternative sites, Tl and Tl', as suggested by the average structure determination. Monte Carlo simulation has been used to model the diffuse scattering distribution and has revealed strong short-range correlations between the positions of Tl in neighbouring sites. The overall effect of these correlations results in a tendency for the Tl/Tl' occupancy to alternate from cell to cell along the b crystal direction, but to be in the same sense from cell to cell along the c crystal direction. Experimental constraints have so far limited any exploration of the variation along a.