OXOMETALLATES OF A NEW TYPE - ON BA3NANBO6 AND BA3NATAO6

For the first time in form of colourless, transparent single crystals of Ba3NaNbO6 [annealed mixtures of BaO, Na2O and Nb2O5, Ba:Na:Nb = 3.3:1.1:1, Ni-cylinder, 1100-degrees-C, 3d] as well as Ba3NaTaO6 [annealed mixtures of BaO, Na2O and Ta2O5, Ba:Na:Ta=3.3:1.1:1, Ni-cylinder, 1100-degrees-C, 3d] have been prepared. The crystal structure was solved by fourcycle-diffractometer data [Ba3NaNbO6: Mo-KalphaBAR, 356 out 356 I0 (hkl), space group R3cBAR with a = 1026.6(1) pm, c = 1195.3(2) pm (Guinier-Simon powder data), Z = 6, R = 2.4%, R(W) = 2.0% and Ba3NaTaO6: Ag-KalphaBAR, 498 out of 498 I0 (hkl), space group R3cBAR with a = 1 027.6(1) pm, c = 1 196.0(2) pm (Guinier-Simon powder data), Z = 6, R = 4.9%, R(W) = 4.4%], parameters see text. The Ba3M part of structure (M = Nb, Ta) corresponds to a slightly (hexagonal) deformed Nb3Al arrangement with Na inserted along [001] between adjacent M(V), which are nearly perfectly octahedrally surrounded by 60. The structural relations are deduced by Schlegel Diagrams. The Madelung Part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, the latter derived from Mean Effective Ionic Radii, MEFIR, as well as Charge Distribution, CHARDI, are calculated.