The rare earth metal(III) oxotellurates(IV) RE 2 Te 3 O 9 in the B -type structure with RE = Y and Sm–Er

Abstract The rare earth metal(III) oxotellurates(IV) RE 2 Te 3 O 9 ( RE = Y, Sm–Tb, Ho and Er) could be synthesized through solid-state reactions at temperatures near 850 °C from mixtures of the oxides RE 2 O 3 and TeO 2 in a 1 : 3 M ratio with cesium bromide (CsBr) as fluxing agent. They crystallize in the monoclinic space group P 2 1 / c , thus being isostructural to the already known B -type Dy 2 Te 3 O 9 with unit cells and molar volumes decreasing monotonously from Sm 2 Te 3 O 9 ( a = 1389.81(9) pm, b = 545.14(3) pm, c = 2304.97(14) pm, β = 98.942(3)°) to Er 2 Te 3 O 9 ( a = 1365.71(9) pm, b = 535.48(3) pm, c = 2261.04(14) pm, β = 99.053(3)°) according to the lanthanoid contraction. The crystal structure contains four crystallographically independent RE 3+ cations, which are hepta- or octacoordinated by oxygen atoms. Three types of rare earth metal-oxygen polyhedra [( RE 1– RE 3)O 8 ] 13− form two types of zigzag chains by edge-sharing running along [010], which are further connected with each other via common edges to layers spreading out parallel to (100). The remaining [( RE 4)O 7 ] 11− polyhedron is attached to a layer above and below via common edges generating corrugated R E 4 O 17 22 − ∞ 2 ${}_{\infty }{}^{2}\left\{{\left[{RE}_{4}{\mathrm{O}}_{17}\right]}^{22-}\right\}$ sheets parallel to the (100) plane. For the oxotellurate(IV) partial structure, the six crystallographically different Te 4+ cations in ψ 1 -tetrahedral [TeO 3 ] 2– units are linked through strong secondary Te 4+ ···O 2− contacts to form screw-like Te 5 O 15 10 − ∞ 1 ${}_{\infty }{}^{1}\left\{{\left[{\text{Te}}_{5}{\mathrm{O}}_{15}\right]}^{10-}\right\}$ strands propagating along the [010] direction with 2 1 screw axes running through their centres. The Raman spectra of all B -type RE 2 Te 3 O 9 members are reported and discussed, and the orientation of the lone pairs at the Te 4+ cations is defined.