Acta Crystallographica Section B

Structural Science

Volume 58, Part 5 (October 2002)


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Acta Cryst. (2002). B58, 743-749    [doi:10.1107/S0108768102013782]

New phosphate langbeinites, K2MTi(PO4)3 (M = Er, Yb or Y), and an alternative description of the langbeinite framework

S. T. Norberg

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 [M5X6O39] 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).

Online 24 September 2002

hkldisplay filedownload file

Structure factor file (CIF format) (47.8 kbytes)
Contains datablock KErTP

hkldisplay filedownload file

Structure factor file (CIF format) (47.8 kbytes)
Contains datablock KYbTP

hkldisplay filedownload file

Structure factor file (CIF format) (48.0 kbytes)
Contains datablock KYTP

hkldisplay filedownload file

Structure factor file (CIF format) (65.9 kbytes)
Contains datablock KCrTP

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