Refinement of the crystal structure of rhombohedral KU2(PO4)3 as a representative of orthophosphates with the NaZr2(PO4)3 structure

The crystal structure of the high-temperature β modification of synthetic orthophosphate KU2(PO4)3 was refined from powder X-ray diffraction data by the Rietveld method: sp. gr. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$R\bar 3c$$ \end{document}, the unit-cell parameters a= 9.113(1) Å and c= 24.997(1) Å. The isotropic refinement converged to Rwp = 6.15, RB = 2.14, RF = 3.52, and S = 0.42. It was confirmed that β-KU2(PO4)3 belongs to the structure type of sodium zirconium phosphate containing an actinide atom in a sixfold (octahedral) coordination formed by oxygen atoms, which is unusual for orthophosphates. The principal interatomic distances and bond angles in the structure are reported.