Extinction corrections for a highly perfect crystal (SrTiO₃)

Accurate neutron-diffraction data from a highly perfect crystal of SrTiO₃ have been used to assess the extinction model of Becker & Coppens [Acta Cryst. (1974), A30, 129-147, 148-153; Acta Cryst. (1975), A31, 417--425] in refining reliable thermal parameters from data very strongly affected by extinction. The model incorporates approximations not evidently reasonable in this case - namely, the (usual) kinematical approximation and the mosaic-block description of crystal microstructure. However, it is shown that a careful choice of parameterization within the overall motel (e.g. between a Gaussian and a Lorentzian function for the distribution of mosaic-block orientations) can yield a uniformly good description of the extinction per se - except for reflections extinguished by more than about 90% on intensity. The refined thermal parameters are in remarkably good agreement with values obtained independently from lattice-dynamical calculations.