Two Ta 2 O 5 ‐Rich Alkali‐Metal Lanthanide(III) Oxotantalates(V): K 2 LaTa 5 O 15 and NaTm 2 Ta 15 O 41

Alkali‐metal containing lanthanide oxotantalates(V) with high Ta 2 O 5 content can be obtained using surfacially oxygen‐contaminated tantalum containers for acid‐base reactions with lanthanide oxides and alkali‐metal halides as fluxes. Following this way, the two title compounds K 2 LaTa 5 O 15 and NaTm 2 Ta 15 O 41 were prepared as colorless single crystals. K 2 LaTa 5 O 15 crystallizes in the tetragonal space group P 4/ mbm (no. 127) with a = 1256.02(9), c = 392.14(3) pm and c / a = 0.312 ( Z = 2), NaTm 2 Ta 15 O 41 orthorhombically (space group: Pbam , no. 55) with a = 1261.43(7), b = 3080.3(2) and c = 386.05(2) pm ( Z = 2). The quite similar a and c axes for both compounds suggest analogies in their structures and there are indeed common motifs within the oxotantalate(V) networks erected from corner‐sharing [TaO 6 ] 7– octahedra. Even more interesting is the fact that in NaTm 2 Ta 15 O 41 an increased coordination number (CN = 7) for some Ta 5+ cations can be found just like in the binary Ta 2 O 5 .