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

Abstract

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 β ​= ​106.849(3)° 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 ψeq1-trigonal [TeO4]4– bipyramid (d(Te4+–O2–) ​= ​186 – 214 pm, ​2× each) and three different ψ1-[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 La4[Te3O8][TeO3]4. The presence of all these different oxotellurate(IV) units is confirmed by a single-crystal Raman spectrum.