CRYSTAL-STRUCTURE OF NAALAS2O7

We present a structural analysis of NaAlAS2O7, M(r) = 311.81, monoclinic, P2(1)/c, a 6.9114(7), b = 8.1345(12), c = 9.5446(10) angstrom, beta = 107.51(1)-degrees, V = 511.74 angstrom3, Z = 4, d. = 4.02, d(cal) = 4.05 g/cm3, MoKalpha, lambda = 0.7107 angstrom, mu = 140.3 cm-1, F(000) = 146, R = 0.039, R(w) = 0.038 for all the 1023 independent observed reflections. The structure of NaAtAS2O7 is compared to that of the M(I)M(III)P2O7 form II diphosphates. The two structures are composed of the same building units, M(III)O6 octahedra and X2O7 (X = As or P) groups. The main difference is that the parts played by the X2O7 groups and the M(III)O6 octahedra, as also the stacking of the polyhedra, are exchanged in the two structures. The change is shown to be likely due to the length of the As-0-As bridge and makes it possible to account for the unexpected contraction of the a parameter in NaAlAs2O7 in spite of its being the lengthening direction of the X-O-X bridge. (C) 1994 Academic Press, Inc.