La4Te7O20: A new ternary rare-earth metal(III) oxotellurate(IV) with isolated [TeO3]2- and [Te3O8]4- anions

The new rare-earth metal(III) oxotellurate(IV) La4Te7O20 was obtained as a single-crystalline by-product of the La2Te3O9 synthesis through a solid-state reaction of the corresponding binary oxides La2O3 and TeO2. The colorless, lath-shaped crystals of La4Te7O20 crystallize in the monoclinic space group C2/c with the lattice parameters a = 2740.63(16) pm, b = 580.92(3) pm, c = 1326.71(8) pm and beta = 106.849(3)degrees for Z = 4. The structure consists of two crystallographically distinct lanthanum-oxygen polyhedra with the coordination numbers of seven plus one and eight (d(La3+-O2-) = 241 - 262 pm plus 310 pm). These lanthanum-oxygen polyhedra are attached to one psi(1)(eq)-trigonal [TeO4](4-) bipyramid (d(Te4+-O2-) = 186 - 214 pm, 2 x each) and three different psi(1)-[TeO3](2-) tetrahedra (d(Te4+-O2-) = 184 - 189 pm) with twice the frequency of occurrence. So the central [TeO4](4-) core is linked with two [TeO3](2-) groups via common oxygen vertices to form an isolated [Te3O8](4) - anion, leading to the structural formula La-4[Te3O8][TeO3](4). The presence of all these different oxotellurate(IV) units is confirmed by a single-crystal Raman spectrum.