J. Holsa et al., CHARACTERIZATION OF THE NONSTOICHIOMETRY IN LANTHANUM OXYFLUORIDE BY FT-IR ABSORPTION, RAMAN-SCATTERING, X-RAY-POWDER DIFFRACTION AND THERMAL-ANALYSIS, Polyhedron, 16(19), 1997, pp. 3421-3427
The FT-IR absorption, FT-Raman scattering, X-ray powder diffraction (X
PD), and thermogravimetry were used to explore the non-stoichiometry i
n LaOF. The TGA-DTA analyses between 30 and 1500 degrees C showed that
the LaO1-xF1+2x phases yielded the stoichiometric LaOF as an intermed
iate product. The temperature of formation of the LaOF and La2O3 incre
ased 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 sub
sequently by the bond valence calculations. All LaO1-xF1+2x phases pos
sess the tetragonal PbFCl-type structure (space group: P4/nmm; Z = 2)
while the stoichiometric LaOF has the hexagonal SmSI-type structure (s
pace 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 i
ncreasing excess fluoride. The GII value for each LaO1-xF1+2x exceeded
the limit (ca 0.2) for a possible breakdown of the tetragonal structu
re indicating the inherent instability of these phases. The room tempe
rature FTIR and FT-Raman spectra between 100 and 500 cm(-1) and 50 and
1000 cm(-1), respectively, were investigated using the factor group a
nalysis to get more information of the local structure in LaOF. All fo
ur IR modes (2A(2u)+ 2E(u)) for the both forms and five of the six Ram
an modes (3A(1g)+ 3E(g) and A(1g)+ 2B(1g)+ 3E(g)) for the hexagonal an
d the tetragonal form, respectively, were observed. The Raman spectra
of the tetragonal LaOCl and all LaO1-xF1+2x were found remarkable simi
lar. (C) 1997 Elsevier Science Ltd.