Two New Phosphate Langbeinites, Rb2YbTi(PO4)3 and Rb2Yb0.32Ti1.68 (PO4)3, Investigated at 293 and 150 K.

The rubidium ytterbium titanium phosphates Rb2YbTi(PO4)(3), ( I), and Rb(2)Yb(0.3)2Ti(1.68)(PO4)(3), (II), have been structurally characterized from X-ray data collected at both 293 and 150 K. Compound ( II) is blue owing to the presence of mixed-valence titanium (41% Ti3+ + 59% Ti4+). Both (I) and (II) belong to the langbeinite structure type, with mixed Yb/Ti populations in the two crystallographically independent octahedral sites ( of symmetry 3). Ytterbium favours one of these sites, where about two-thirds of the Yb atoms are found. The O-atom displacement parameters are large in both compounds at both temperatures.