A POTASSIUM TANTALUM(V) TETRASILICATE KTASI2O7

The crystal structure of a new potassium tantalum(V) cyclotetrasilicat e, KTaSi2O7 (K2Ta2Si4O14), has been determined by single crystal X-ray diffraction. KTaSi2O7 crystallizes in the tetragonal system, space gr oup P4/mbm with a = 8.735(2) and c = 7.999(2) Angstrom, V = 610.3(2) A ngstrom(3), Z = 4, d(X) = 4.225 g/cm(3), lambda (MoK alpha) = 0.71073 Angstrom, mu = 190.7 cm(-1), F' (000) = 704, T = 293 K, R = 0.0421, R( W) = 0.0789 for 451 unique reflections (R(int) = 0.0227) with F-0(2) > 0. The framework structure of KTaSi2O7 is built up from chains of cor ner-sharing TaO6 octahedra running parallel to the fourfold axes and l inked together by four-membered Si4O12 Single units. AC impedance meas urements on KTaSi2O7 indicate low ionic conductivities, 5.6 x 10(-9) a nd 3.0 x 10(-7) Omega . cm(-1) at 333 and 546 degrees C, respectively. An activation energy of 0.83 +/- 0.02 eV is observed. (C) 1996 Academ ic Press, Inc.