Preparation and Structure of New Orthophosphates Isostructural with the Mineral Langbeinite: A2R1.5Ta0.5(PO4)3 (A = K, Rb; R = Ga, Gd, Dy, Ho, Er, Yb)

— New orthophosphates with the general formula A 2 R 1.5 Ta 0.5 (PO 4 ) 3 (where A = K or Rb and R = Ga, Gd, Dy, Ho, Er, or Yb) have been prepared by solid-state reactions and characterized by IR spectroscopy and X-ray diffraction. The results demonstrate that they crystallize in the structure of the mineral langbeinite (sp. gr. P 2 1 3) and that their cubic cell parameter increases with increasing lanthanide ionic radius. The crystal structure of K 2 Dy 1.5 Ta 0.5 (PO 4 ) 3 has been refined using neutron powder diffraction data. The potassium and dysprosium cations in this phosphate have been shown to be distributed at random over framework sites. The synthesized compounds are of interest as crystalline single-phase mineral-like matrices for prolonged isolation of dangerous radionuclides from the biosphere.