DIFFUSE-X-RAY SCATTERING AND SHORT-RANGE ORDER IN THALLIUM ANTIMONYL GERMANATE, TLSBOGEO4

The potassium titanyl phosphate (KTP) analogue material thallium antim onyl germanate has been studied at 295 K using diffuse X-ray scatterin g techniques. It has been shown that previously reported 'diffuse arcs ' of scattering are due to decomposition products (mainly Tl2Ge2O5) ne ar the surface of the crystals. In addition the experiments revealed t he presence of continuous diffuse scattering. A preliminary analysis o f this scattering shows that it originates predominantly from the diso rder of the thallium positions between alternative sites, Tl and Tl', as suggested by the average structure determination. Monte Carlo simul ation has been used to model the diffuse scattering distribution and h as revealed strong short-range correlations between the positions of T l in neighbouring sites. The overall effect of these correlations resu lts in a tendency for the Tl/Tl' occupancy to alternate from cell to c ell along the b crystal direction, but to be in the same sense from ce ll to cell along the c crystal direction. Experimental constraints hav e so far limited any exploration of the variation along a.