A powder diffraction study of M I BaIn 2 (PO 4 ) 3 (M I =Na, K, Cs) with a langbeinite-type structure

Three phosphates, M I BaIn 2 (PO 4 ) 3 with M I =Na, K, Cs, isostructural to the langbeinite structure, have been studied from powder diffraction data collected with monochromatic radiation obtained from a conventional X-ray source. Precise powder data are reported, as well as cell parameters, i.e., a =10.026 08(9) Å, a =10.121 57(13) Å and a =10.226 94(9) Å for M I =Na, K and Cs, respectively. A Rietveld refinement has been carried out (space group P 2 1 3), with final R F factors, 0.061, 0.041 and 0.027, and R wp factors, 0.196, 0.142 and 0.129, for M I =Na, K and Cs, respectively. There are two octahedrally coordinated In 3+ ions in the asymmetric unit and the final refinements suggest disorder on the two sites of the M I /Ba sublattice.