Structure of polymorphous modifications of double sodium and indium phosphate

Polymorphous modifications of beta- and alpha-Na3In(PO4)(2) were synthesized and found to be related to the beta-K2SO4 structure type. They crystallize in a monoclinic system (space groups P2(1)/c and P2(1)/m, respectively), with unit cell parameters a = 7.127(1) Angstrom, b = 18.220(1) Angstrom, c = 8.616(1) Angstrom, beta = 143.36(1)degrees for beta-Na3In(PO4)(2); and a = 8.6335(1) Angstrom, b = 5.4550(1) Angstrom, c = 7.0482(1) Angstrom, beta = 90.295(1)degrees for alpha-Na3In(PO4)(2). Their crystal structures were determined by Rietveld analysis [R-wp = 7.72, R-I = 3.02 (beta-); R-wp = 6.59, R-I = 3.06 (alpha-)]. Indium cations occupy the X position while sodium cations occupy the X and Y positions of the beta-K2SO4-type structure. The PO43- tetrahedron rotation leads to a decrease in the coordination number of the Y position. (C) 2000 Academic Press.