Na-3[Ti2P2O10F]: A new oxyfluorinated titanium phosphate with an ionic conductive property

Titanium phosphate, Na-3[Ti2P2O10F]center dot xH(2)O (1), has been synthesized under hydrothermal conditions and structurally characterized using powder X-ray diffraction, neutron diffraction, and selected area electron diffraction (SAED). 1 crystallizes in the tetragonal space group I4/mmm with a = 6.4207(1) A and c = 10.6762(2) A and can be described by the stacking of a square-net sheet consisting of alternative linkage of TiFO5 octahedra and PO4 tetrahedra. The TiFO5 octahedron is highly distorted, with a short "titanyl" Ti-O bond and a long Ti-F bond. The "titanyl" oxygen is almost doubly bonded to Ti; the square-net sheets are linked only by sharing common F atoms, which lead to a rather opened framework containing 2D channels in the ab plane. The counter cations Na+ and water molecules reside in the channels. At high temperature, the adsorbed water molecules escape from the channels, leading to an anhydrous phase Na-3[Ti2P2O10F] (2) with exactly the same structural framework as that of 1. 2 is stable at high temperature in an Ar atmosphere, but decomposes to other known titanium phosphates and oxide at about 650 degrees C in air. The Na ions in the compound are exchangeable. In addition, 2 exhibits a Na conductivity comparable to that of the NASICON type titanium phosphates (1.0 x 10(-4) Omega(-1) cm(-1) at 200 degrees C).