Über Oxotantalate der Lanthanide des Formeltyps MTaO 4 (M = La – Nd, Sm – Lu)

About Lanthanide Oxotantalates with the Formula MTaO 4 (M = La – Nd, Sm – Lu) Besides being a by‐product of solid state syntheses in tantalum ampoules the lanthanide(III) oxotantalates of the formula MTaO 4 can be easily prepared by sintering lanthanide sesquioxide M 2 O 3 and tantalum(V) oxide Ta 2 O 5 with sodium chloride as flux. Under these conditions two structure types emerge depending upon the M 3+ cationic radius. For M = La – Pr the MTaO 4 ‐type tantalates crystallize in the space group P2 1 /c with lattice constants of a = 762(±1), b = 553(±4), c = 777(±4) pm, β = 101(±1)° and four formula units per unit cell. With M = Nd, Sm – Lu, the monoclinic cell dimensions (space group P2/c) shrink to the lattice constants like a = 516(±9), b = 551(±9), c = 534(±9) pm, β = 96.5(±0.3)° and there are only two formula units present. Both structures show a coordination sphere of eight oxygen atoms for the lanthanide trications shaped as distorted square antiprism for the structure with the larger lanthanides (in the following referred to as A‐type ) and as trigonal dodecahedron for the structure with the smaller ones (called as B‐type in the following). The coordination environment about the Ta 5+ cations can be described as a slightly distorted octahedron (CN = 6) for the A‐type structure of MTaO 4 and a heavily distorted one (CN = 6) for the B‐type. The difference between the two types results from the interconnection of these [TaO 6 ] 7− octahedra. Whereas they are connected via four vertices to form corrugated layers according to $^{2}_{\infty}\rm[TaO^{v}_{4/2}O^{t}_{2/2}]^{3-}$ parallel the bc ‐plane in the A‐type, the octahedra of the B‐type MTaO 4 structure share edges to built up zig‐zag chains $^{1}_{\infty}\rm[TaO^{e}_{4/2}O^{t}_{2/2}]^{3-}$ along the c axis.