ON TANTALATES AND NIOBATES RICH IN CATIONS - ON NA5TAO5 AND NA5NBO5

Colourless, transparent single crystals of Na5TaO5 [annealed mixtures of Na2O, Li2O, and Ta2O5, Na:Li:Ta = 6.6:1.1:1, Ni-cylinder, 1000-degrees-C, 75 d] as well as Na5NbO5 [annealed mixtures of Na2O, Li2O, and Nb2O5, Na:Li:Nb = 6.6:1.1:1, Ni-cylinder, 1000-degrees-C, 75 d] have been prepared. Single crystal data show that both isotypic oxides represent a deformed variant of the NaCl-type of structure [Na5TaO5: 1154 from 1250 I(o)(hkl), four-cycle diffractometer Philips PW 1100, omega-2-theta scan, Ag-K-alphaBAR, R = 4.88%, space group C2/c with a = 629.3(1) pm, b = 1025.4(2) pm, c = 1004.6(2) pm, beta-106.80(2)-degrees, Z = 4 and Na5NbO5: 998 from 1247 I(o)(hkl), four-cycle diffractometer Philips PW 1100, omega-2-theta scan, Ag-K-alphaBAR, R = 8.58% and R(W) = 7.67%, space group C2/c with a = 629.1(1) pm, b = 1024.4(2) pm, c = 1004.2(2) pm, beta = 106.80(2)-degrees, Z = 4]. The Madelung Part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, the latter derived from Mean Effective Fictive Ionic Radii, MEFIR, as well as Charge Distribution, CHARDI, are calculated.