HoF[SeO 3 ] und Ho 3 F[SeO 3 ] 4 : Zwei fluorid‐derivatisierte Oxoselenate(IV) des Holmiums

HoF[SeO 3 ] and Ho 3 F[SeO 3 ] 4 : Two Fluoride‐ Derivatized Holmium Oxoselenates(IV) Three different structure types of rare‐earth metal(III) fluoride oxoselenates(IV) with the formula M F[SeO 3 ] ( M = Y, La, Lu) exist so far, whereas for the intermediate lanthanoids M 3 F[SeO 3 ] 4 ‐type compounds are known with the elements gadolinium and dysprosium. Holmium is now the first lanthanoid realizing both formula types of fluoride oxoselenates(IV), HoF[SeO 3 ] as well as Ho 3 F[SeO 3 ] 4 . HoF[SeO 3 ] crystallizes isotypically to YF[SeO 3 ] in the monoclinic system with space group P 2 1 / c ( a = 657.84(6), b = 688.73(7), c = 718.25(7) pm, β = 99.213(5)° and Z = 4). Ho 3+ at the general site 4 e is surrounded by six oxide and two fluoride anions forming [HoO 6 F 2 ] 11− polyhedra ( d (Ho–O) = 229−244 plus 261 pm, d (Ho–F) = 219 and 221 pm, CN(Ho 3+ ) = 8). Like in YF[SeO 3 ] the $^{2}_{\infty}\rm \{[HoO_{3}F]^{4-}\}$ sheets resulting from condensation of these polyhedra are interconnected by Se 4+ cations showing a ψ 1 ‐tetrahedral oxygen coordination as discrete [SeO 3 ] 2− anions ( d (Se–O) = 167 – 175 pm), but by imaginary removal of the latter only $^{1}_{\infty}\rm \{[HoF^{v}_{2/2}]^{2+}\}$ zigzag chains running parallel to the [001] direction remain. Ho 3 F[SeO 3 ] 4 crystallizes in the non ‐centrosymmetric hexagonal space group P 6 3 mc ( a = 1030.24(3), c = 680.37(2) pm and Z = 2). Three [HoO 8 F] 14− polyhedra ( d (Ho–O) = 232–254 pm and d (Ho–F) = 233 pm, CN(Ho 3+ ) = 9) at a time are connected via common O2···F edges and four attached ψ 1 ‐tetrahedral [SeO 3 ] 2− anions ( d (Se–O) = 169–170 pm) to complete the basic building block {FHo 3 [(Se1)(O1) 3 ][(Se2)(O2)(O3) 2 ] 3 }. These entities with F − ‐centered (Ho 3+ ) 3 triangles represent a single formula unit and share triangular (O1,O3,O3) faces with six other blocks in [001] direction. Large hexagonal channels are remaining in between which harbor the “lone‐pair” electrons of the isolated [(Se2)(O2)(O3) 2 ] 2− anions. Due to the free position of z / c in space group P 6 3 mc , z / c = 1 / 4 was chosen for the unique fluoride anion on site 2 b (site symmetry: 3 m .) in order to unify the descriptions for all M 3 F[SeO 3 ] 4 ‐type compounds eventually. The syntheses of HoF[SeO 3 ] and Ho 3 F[SeO 3 ] 4 succeeded by reacting holmium sesquioxide (Ho 2 O 3 ), holmium trifluoride (HoF 3 ) and selenium dioxide (SeO 2 ) in molar ratios of 1 : 1 : 3 and 4 : 1 : 12, respectively, with CsBr as fluxing agent within five days at 750 °C in evacuated graphitized fused‐silica ampoules.