STRUCTURE OF GAMMA-LI3ASO4 BY HIGH-TEMPERATURE POWDER NEUTRON-DIFFRACTION

The structure of gamma-Li3AsO4 has been refined by Rietveld analysis of high resolution powder neutron diffraction data collected at 770 and 850-degrees-C. The structure is related to that of gamma-Li3PO4, being a distorted hexagonal close-packed arrangement of oxide ions with half the tetrahedral sites filled by cations. Arsenic occupies the same sites as phosphorus in gamma-Li3PO4. Li+ ions show positional disorder; Li(1) ions are split into central and off-center positions within their tetrahedral sites; Li(2) ions are distributed over pairs of face sharing tetrahedral sites at 850-degrees-C while occupying only one site at 770-degrees-C. The powder neutron data show anisotropic broadening of hkl peaks with h = 2n + 1. The broadening has been accounted for using a modified Rietveld code. The broadened peaks correspond to those reflections that are not common to the related low temperature beta-phase and are associated with a doubling of the a-axis during the beta-gamma transition. The origin of the broadening is the small size of the gamma-phase domains in the a-direction; adjacent domains are probably connected by antiphase boundaries. (C) 1994 Academic Press, Inc.