Ce3Cl5[SiO4] und Ce3Cl6[PO4]: Ein chloridreiches Chloridsilicat und ‐phosphat des Cers im Vergleich

Ce₃Cl₅SiO₄] and Ce₃Cl₆PO₄]: A Chloride‐Rich Chloride Silicate of Cerium as Compared to the Phosphate By reacting CeCl₃ with CeO₂, cerium and SiO₂, or P₂O₅, respectively, in molar ratios of 5 : 3 : 1 : 3 or 8 : 3 : 1 : 2, respectively, in sealed evacuated silica tubes (7 d, 850 °C) colorless, rod‐shaped single crystals of Ce₃Cl₅SiO₄] (orthorhombic, Pnma; a = 1619.7(2), b = 415.26(4), 1423.6(1) pm; Z = 4) and Ce₃Cl₆PO₄] (hexagonal, P6₃/m; a = 1246.36(9), c = 406.93(4) pm; Z = 2) are obtained as products insensitive to air and water. Excess cerium trichloride 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 silicate Ce₃Cl₅SiO₄] and the chloride phosphate Ce₃Cl₆PO₄], exhibit structural similarities to CeCl₃ (UCl₃ type), when four or three Cl^– anions are each 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 Ce₃Cl₅SiO₄] and Ce₃Cl₆PO₄], along with a drastic reduction of the molar volume with the transition from Ce₃Cl₉ (V_m = 186.17 cm³/mol) to Ce₃Cl₅SiO₄] (V_m = 144.15 cm³/mol) and Ce₃Cl₆PO₄] (V_m = 164.84 cm³/mol). The polyhedra of coordination around Ce³⁺ can be described as quadruple‐capped trigonal prisms, which in addition to seven Cl^– anions each also show another three oxygen atoms of two ortho‐silicate or ortho‐phosphate tetrahedra, respectively.