STABILITY OF NONSTOICHIOMETRIC LAO1-XF1- X-RAY-DIFFRACTION INVESTIGATION(2X PHASES )

The X-ray diffraction patterns of the polycrystalline LaO1-xF1+2x (0.0 5 less than or equal to x less than or equal to 0.3) phases were measu red at room temperature and analyzed by the Rietveld profile refinemen t method. All compounds crystallize in the tetragonal PbFCl-type struc ture with P4/nmm as the space group. No distortion to lower orthorhomb ic symmetry was observed. The tetragonal lattice parameters a and c in crease with increasing excess of fluoride. The La-oxygen (La-fluorine) distances first decrease (increase) but then these trends are reverse d. The Global Instability Index calculated from the bond valence model increases towards the higher fluoride content of the compounds indica ting decreasing stability.