Characterization of the non-stoichiometry in lanthanum oxyfluoride by FT-IR absorption, Raman scattering, X-ray powder diffraction and thermal analysis

The FT-IR absorption, FT-Raman scattering, X-ray powder diffraction (XPD), and thermogravimetry were used to explore the non-stoichiometry in LaOF. The TGA-DTA analyses between 30 and 1500 degrees C showed that the LaO1-xF1+2x phases yielded the stoichiometric LaOF as an intermediate product. The temperature of formation of the LaOF and La2O3 increased with increasing excess of fluoride. The room temperature XPD data in 6.5 less than or equal to 2 theta less than or equal to 121 degrees range were analyzed by the Rietveld profile refinement method and subsequently by the bond valence calculations. All LaO1-xF1+2x phases possess the tetragonal PbFCl-type structure (space group: P4/nmm; Z = 2) while the stoichiometric LaOF has the hexagonal SmSI-type structure (space group: R (3) over bar m; Z = 6). The unit cell parameters a and c as well as the global instability index (GII) values increased with increasing excess fluoride. The GII value for each LaO1-xF1+2x exceeded the limit (ca 0.2) for a possible breakdown of the tetragonal structure indicating the inherent instability of these phases. The room temperature FTIR and FT-Raman spectra between 100 and 500 cm(-1) and 50 and 1000 cm(-1), respectively, were investigated using the factor group analysis to get more information of the local structure in LaOF. All four IR modes (2A(2u)+ 2E(u)) for the both forms and five of the six Raman modes (3A(1g)+ 3E(g) and A(1g)+ 2B(1g)+ 3E(g)) for the hexagonal and the tetragonal form, respectively, were observed. The Raman spectra of the tetragonal LaOCl and all LaO1-xF1+2x were found remarkable similar. (C) 1997 Elsevier Science Ltd.