Determination of a centrosymmetric crystal structure using experimentally determined phases with the direct method

摘要

[[abstract]]The relation Sp = SL*SR between the signs of the triplet phase products Sp, the multiple diffraction line profiles SL and the crystal lattice rotations SR is applied to the structure determination of a centrosymmetric Cs10Ga6Se14 crystal [space group C2/m, a = 18.233 (7), b = 12.889 (5), c = 9.668 (3) Å, = 108.2°, z = 2]. This structure could not be solved by the ordinary MULTAN program, including MUL TAN-80. From the profile analysis of the multiple diffraction pattern of 311 using a four-circle single-crystal diffractometer with Cu K radiation, 12 phase relations among closely linked reflections are determined. Four possible sets of 14 reflection phases are deduced. Using these four sets as the starting phases for MULTAN-74, 225 correct phases are developed. The final E map shows all the atomic positions of Cs, Ga and Se. The structure refinement gives R = 0.047, Rw = 0.053 [1907 reflections with I > 2(I)]. Details about the procedure are given.