Localisation des ions mobiles Tl⁺ dans le conducteur ionique TlNb₂O₅F de type pyrochlore

Abstract

The non-stoichiometric phase TlxNbOz+xFl_ x (0.5 < x < 1) has a cubic pyrochlore structure (Fd3m) and its TI + ionic conductivity increases notably with x. For x = 0.5 [a = 10.506 (3) A, Z = 8] standard treatment allowed the location of the heavy ions, T1 +, and a structure refinement was carried out which led to a final R value of 0.069 (R w = 0-054) for 121 independent reflexions collected on an automated three-circle diffractometer. The thallous ions are found on two sets of 32(e) positions, displaced respectively by 0.69 and 0.64 A away from the empty ideal 8(b) positions. Observed F o values cannot be explained on the basis of an anharmonical treatment of TI + movement. The properties of the motion of the ions are related both to the high polarizability of T1 + ions and to the large number of sites offered to the cations by the NbO2+ xFi-_x network. conduisent b, des valeurs tr+s +lev+es du facteur d'agitation thermique isotrope du thallium comme le montre le Tableau 1.