CRYSTAL-STRUCTURE OF NAGATE2O6 - ASPECTS OF TENOM POLYHEDRAL POLYMERIZATION IN A LAYER TELLURITE

The crystal structure of hydrothermally synthesized NaGaTe2O6 [space g roup Pcab (No. 61); a=7.8402(4), b=10.3265(5), c=13.3696(7) Angstrom; Z=8] was determined from single-crystal X-ray data and refined to R=0. 031 (for 2335 unique reflections with F-0>3 sigma (F-0) and 2 Theta le ss than or equal to 70 degrees). The atomic arrangement can formally b e described as a layer structure dominated by a simple AB stacking seq uence of corrugated Te(1)(n)Te(2)(n)O-6n and NanGanO6n layers along th e c-axis. The NanGanO6n layers are built up from GaO6 octahedra and Na O, polyhedra which are connected to both Ga2O10 dimers and infinite [N aO5](n) chains parallel to [100]. The tetravalent tellurium atoms exhi bit typical intermediate [3 + 2] and [3 + 1] coordination for Te(1) an d Te(2) respectively: the one-sided three-fold coordination (Te-O: 1.8 4 Angstrom to 1.92 Angstrom) is extended by additional Te-O bonds (2.5 6 Angstrom to 2.69 Angstrom). Based on simple bond-valence considerati ons, a relationship between the variable Te4+Ox-coordination, the degr ee of polymerization, and the influence of individual polyhedron types linked to the TexOy unit is pointed out and discussed for NaGeTe2O6. (C) 1998 Elsevier Science S.A.