Single-crystal neutron diffraction study of beta-Cs-3(HSO4)(2)[H2-x(SxP(1-x))O-4] (x similar or equal to 0.5) at 15 K

The structure of beta-Cs-3(HSO4)(2)[H2-x(SxP1-x)O-4] has been examined by s ingle-crystal neutron diffraction at 15 K. The compound crystallizes in spa ce group C2/c and contains four formula units in the unit cell, with lattic e parameters a = 19.769(9), b = 7.685(2), c = 8.858 (3) Angstrom and beta = 100.60 (4)degrees. Refinement of P, S and H site occupancies indicated tha t the value of x (in the stoichiometry) is 0.500 (6). This, together with t he unit-cell volume of 1322.8(14)Angstrom(3), implies a density of 3.463 Mg m(-3). The structure contains zigzag rows of XO4 anions, where X = P or S, that alternate, in a checkerboard fashion, with zigzag rows of Cs cations. Moreover, there is one proton site, H(3), with an occupancy of 0.25 and on e X-atom site, X(1), that is occupied by 0.5 P and 0.5 S. These features ar e in general agreement with a previous X-ray structure determination carrie d out at 298 K. In contrast to the X-ray study, however, it was found that two different structural models adequately fit the diffraction data. In the first model, the proton vacancies and the P atoms were assumed to be rando mly distributed over the H(3) and X(1) sites, respectively, and to have no impact on the local structure. In the second model, several atoms were assi gned split occupancies over two neighboring sites, to reflect the presence or absence of a proton vacancy, and the presence of P or S on the X(1) site . Refinement assuming the first model, in which anisotropic displacement pa rameters for 12 of 14 atom sites in the asymmetric unit were employed, yiel ded residuals wR(F-2) = 0.084 and wR(F) = 0.038. For the second model, in w hich anisotropic displacement parameters were utilized for only the five at oms that were not split relative to the first model, the residuals were wR( F-2) = 0.081 and wR(F) = 0.036.