NaPr6Cl12[SiO4][PO4] and La6.333Cl12[SiO4][PO4]: Two chloride silicate phosphates of rare-earth metals with cation-filled channel structures

The lanthanum chloride silicate phosphate La6.333Cl12[SiO4][PO4] (a=1249.68(9)a=1249.68(9), c=413.56(3) pmc=413.56(3) pm) and the sodium praseodymium chloride silicate phosphate NaPr6Cl12[SiO4][PO4] (a=1247.87(9)a=1247.87(9), c=401.13(2) pmc=401.13(2) pm) both crystallize hexagonally with the space group P63/mP63/m and one formula unit per unit cell. In an attempt to synthesize chloride derivatized lanthanide oxophosphates using Pr6O11, Pr, and P2O5 with an excess of NaCl as flux, pale green lath-shaped single crystals of NaPr6Cl12[SiO4][PO4] were obtained from silica vessels. Also lath-shaped, but colourless single crystals of La6.333Cl12[SiO4][PO4] emerged from the reaction of La[PO4] with a fivefold excess of LaCl3 again carried out in evacuated, torch-sealed silica ampoules. Obviously, the container material SiO2 did participate in both cases, most probably activated by the chloride fluxes or traces of moisture. The lanthanide trications (Pr3+ and (La2)3+) are situated at the Wyckoff position 6h showing a coordination number of ten (7 Cl and 3 O atoms) with a coordination polyhedron of tetracapped trigonal prismatic shape. The sodium cations in NaPr6Cl12[SiO4][PO4] as well as the second type of lanthanum cations (La1)3+ in La6.333Cl12[SiO4][PO4] are located in the channel running along [001] which is built up by chloride anions exclusively. While the Na+ cations occupy the Wyckoff positions 2a   (CN=3+6CN=3+6) and 2b   (CN=6CN=6) with a ratio of 13:23, the (La1)3+ cations reside exclusively in 2a   (CN=9CN=9) with an occupancy of 13. The [SiO4]4−/[PO4]3− tetrahedra seem to be arranged in strands along [13231] and [23131] forming apical vertex-shared trigonal bipyramids (≡ face-shared tetrahedra) in which of course only one of the two oxygen tetrahedra can be centred by a P5+ or a Si4+ cation statistically.

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