New phosphate langbeinites, K2 MTi(PO4)3 (M = Er, Yb or Y), and an alternative description of the langbeinite framework

Abstract

Three new potassium rare-earth/titanium phosphate structures, K2ErTi(PO4)3 (KErTP), K2YbTi(PO4)3 (KYbTP) and K2YTi(PO4)3 (KYTP), are presented, all of which are characterized by single-crystal X-ray diffraction studies. In addition, a fourth structure, K2CrTi(PO4)3 (KCrTP), has been reinvestigated. All structures are isostructural to the langbeinite-type structure and result from changes made to the growth constituents in high-temperature flux-growth experiments intended to give structurally modified potassium titanyl phosphate (KTP). The two crystallographically independent octahedra sites (site symmetry 3) have a mixed Ti/M (M = Er, Yb, Y or Cr) population, although the rare-earth metals favour one site while chromium favours the other. An alternative approach for the description of the channels and cation cages in langbeinite and related structures is given using [M 5 X 6O39] units. The framework of langbeinite is compared with that of nasicon using these alternative building units. All of the investigated structures crystallize in space group P213 with Z = 4; a = 10.1053 (2) Å, R = 0.023 (KErTP); a = 10.0939 (8) Å, R = 0.022 (KYbTP); a = 10.1318 (6) Å, R = 0.047 (KYTP); a = 9.8001 (2) Å, R = 0.016 (KCrTP).