Pr 5 F[SiO 4 ] 2 [SeO 3 ] 3 : Another Complex Fluoride Oxosilicate Oxoselenate(IV)

During attempts to synthesize lanthanoid(III) fluoride oxoselenates(IV) with the simple composition M F[SeO 3 ], not only Pr 3 F[SeO 3 ] 4 , but also Pr 5 F[SiO 4 ] 2 [SeO 3 ] 3 appeared as pale green crystalline by‐products in the case of praseodymium. Pr 5 F[SiO 4 ] 2 [SeO 3 ] 3 crystallizes triclinically in space group P $\bar{1}$ (no. 2) with a = 701.14(5), b = 982.68(7), c = 1286.79(9) pm, α = 70.552(3), β = 76.904(3), γ = 69.417(3)° and Z = 2. The five crystallographically different Pr 3+ cations on the general positions 2 i show coordination numbers of eight and nine. [(Pr1)O 8 ] 13– and [(Pr2)O 8 ] 13– polyhedra are connected to $\bar{1}$ {[(Pr1, 2) 2 O 12 ] 18– } chains along the [100] direction. [(Pr3)O 7 F] 12– , [(Pr4)O 8 F] 14– and [(Pr4)O 8 F] 14– polyhedra generate [F(Pr3, 4, 5) 3 O 19 ] 30– units about their central F – anion in triangular Pr 3+ coordination. These units form $\bar{1}$ {[F(Pr3, 4, 5) 3 O 16 ] 24– } strands, again running parallel to [100]. Their alternating connection with the $\bar{1}$ {[(Pr1, 2) 2 O 12 ] 18– } chains results in $\bar{1}$ {[Pr 5 O 20 F] 26– } sheets parallel to the (001) plane. Like in the already known related compound Er 3 F[SiO 4 ][SeO 3 ] 2 , a three‐dimensional network $\bar{1}$ {[Pr 5 O 17 F] 20– } is achieved without the contribution of both the tetravalent silicon and selenium components. However, two Si 4+ and three Se 4+ cations forming tetrahedral [SiO 4 ] 4– and ψ 1 ‐tetrahedral [SeO 3 ] 2– units with all O 2– anions guarantee the charge balance. The formation of Pr 5 F[SiO 4 ] 2 [SeO 3 ] 3 was observed when praseodymium sesquioxide (Pr 2 O 3 : in‐situ produced from Pr and Pr 6 O 11 in a molar ratio of 3 / 11 : 4 / 11 ),praseodymium trifluoride (PrF 3 ) and selenium dioxide (SeO 2 ) in 1:1:3 molar ratios were reacted with CsBr as fluxing agent for five days at 750 °C in evacuated fused silica (SiO 2 ) ampoules.