Structure of Polymorphous Modifications of Double Sodium and Indium Phosphate

Polymorphous modifications of β- and α-Na3In(PO4)2 were synthesized and found to be related to the β-K2SO4 structure type. They crystallize in a monoclinic system (space groups P21/c and P21/m, respectively), with unit cell parameters a=7.127(1) Å, b=18.220(1) Å, c=8.616(1) Å, β=143.36(1)° for β-Na3In(PO4)2; and a=8.6335(1) Å, b=5.4550(1) Å, c=7.0482(1) Å, β=90.295(1)° for α-Na3In(PO4)2. Their crystal structures were determined by Rietveld analysis [Rwp=7.72, RI=3.02 (β-); Rwp=6.59, RI=3.06 (α-)]. Indium cations occupy the X position while sodium cations occupy the X and Y positions of the β-K2SO4-type structure. The PO3−4 tetrahedron rotation leads to a decrease in the coordination number of the Y position.