Ce3Cl5[SiO4] and Ce3Cl6[PO4]: A chloride-rich chloride silicate of cerium as compared to the phosphate

By reacting CeCl3 with CeO2, cerium and SiO2, or P2O5, respectively, in mol ar ratios of 5:3:1:3 or 8:3:1:2, respectively, in sealed evacuated silica t ubes (7 d, 850 degreesC) colorless, rod-shaped single crystals of Ce3Cl5[Si O4] (orthorhombic, Pnma; a = 1619.7(2), b = 415.26(4), 1423.6(1) pm; Z = 4) and Ce3Cl6[PO4] (hexagonal, P6(3)/m; a = 1246.36(9), c = 406.93(4) pm; Z = 2) are obtained as products insensitive to air and water. Excess cerium tr ichloride as flux promotes crystal growth and can be rinsed off again with water after the reaction. The crystal structures are determined by discrete [SiO4](4-) or [PO4](3-) tetrahedra as isolated units. Both, the chloride s ilicate Ce3Cl5[SiO4] and the chloride phosphate Ce3Cl6[PO4]. exhibit struct ural similarities to CeCl3 (UCl3 type), when four or three Cl- anions are e ach substituted formally by one [SiO4](4-) Or [PO4](3-) unit, respectively, in the tripled formula (Ce3Cl9). The coordination number for Ce3+ is thus raised from nine in CeCl3 to ten in Ce3Cl5[SiO4] and Ce3Cl6[PO4], along wit h a drastic reduction of the molar volume with the transition from Ce3Cl9 ( V-m = 186.17 cm(3)/mol) to Ce3Cl5[SiO4] (V-m = 144.15 cm(3)/mol) and Ce3Cl6 [PO4] (V-m = 164.84 cm(3)/mol). The polyhedra of coordination around Ce3+ c an be described as quadruple-capped trigonal prisms, which in addition to s even Cl- anions each also show another three oxygen atoms of two ortho-sili cate or orthophosphate tetrahedra, respectively.