YF[SeO3]-type rare-earth metal(III) fluoride oxoselenates(IV) MF[SeO3] (M = Y, Ho-Lu) crystallize monoclinically in space group P21/c (no. 14) with Z = 4. Obeying the lanthanoid contraction, the lattice parameters decrease successively from a = 657.65(7), b = 689.71(7), c = 717.28(7) pm and β = 99.036(5)° for YF[SeO3] at 298 K to a = 648.39(6), b = 681.28(7), c = 705.81(7) pm and β = 98.657(5)° for LuF[SeO3] at 100 K (LT-LuF[SeO3], LT ≡ Low Temperature). The M3+ cations are occupying the general site 4e. Contrary to the triclinic structure of RT-LuF[SeO3] (RT ≡ Room Temperature; CN(Lu3+) = 7) the higher coordination number eight is achieved for the M3+ cations in all YF[SeO3]-type compounds. This results in [MO6F2]11− polyhedra (d(M-O) = 228 - 243/225 - 239 plus 263/258 pm, d(M-F) = 219/216 pm, M = Y/Lu), which are connected via common O・ ・ ・O edges to form infinite chains 1∞{[MOe 4/2Ot 2/1Ft 2/1]7−} (e ≡ edge-sharing, t ≡ terminal) running along [010]. Neighboring chains share common O3・ ・ ・O3 and O3・ ・ ・F edges generating 2∞{[M(O3)3/3(O2)2/2(O1)1/1F2/2]4−} sheets parallel to the (100) plane. Finally, these 2 ∞{[MO3F]4−} sheets are interconnected by Se4+ cations, which appear in isolated ψ1-tetrahedral [SeO3]2− anions (d(Se-O) = 167 - 175 pm). For the synthesis of the YF[SeO3]-type rare-earth metal(III) fluoride oxoselenates( IV) MF[SeO3] (M = Y, Ho-Lu), the rare-earth metal sesquioxides (M2O3) and trifluorides (MF3), and selenium dioxide (SeO2) in molar ratios of 1 : 1 : 3 with the fluxing agent CsBr were reacted within five days at 700 - 750 °C in evacuated graphitized silica ampoules.
Low-temperature specific heat study of antiferromagnetic transition in ternary rare-earth metal silicide R5Ir4Si10 (R=Tb, Dy, Ho, Er)