An infrared investigation of the otavite--magnesite solid solution

The local mixing and ordering behavior of the otavite-magnesite solid solution has been characterized using infrared powder absorption spectroscopy in the spectral region 50-2000 [cm.sup.-1] at room temperature. Lattice modes due to Cd translations (75-200 [cm.sup.-1]) and Mg translations (200-350 [cm.sup.-1]) were both observed in the IR spectra for samples of intermediate composition. Peak positions of Cd translations remain constant as a function of composition, whereas those of Mg translations decrease in wavenumber with increasing otavite content. This suggests that the otavite-magnesite solid solution displays a combination of one-mode and two-mode behavior, most likely due to the large difference in atomic weight between magnesium and cadmium. Vibrational bands relating to the C[O.sup.2-.sub.3] groups in the spectral region 600-900 [cm.sup.-1] vary linearly as a function of composition and are not sensitive to the degree of order. The vibrational band at ~1400-1450 [cm.sup.-1] also varies linearly with composition, but R[bar.3] ordered samples show a marked increase in frequency compared to their R[bar.3]c counterparts. Positive deviations from linearity are observed for the effective line width determined using the autocorrelation method (expressed in terms of [DELTA]corr values) for both lattice modes and bending vibrations of the C[O.sup.2-.sub.3] molecular groups, whereas a linear variation as a function of composition of the [DELTA]corr values of the C[O.sup.2-.sub.3] stretching mode is observed for the 800 [degrees]C solid solution. Only in the region 600-900 [cm.sup.-1] is an effect of order observed as a reduction in [DELTA]corr values with respect to the sample with R[bar.3]c symmetry. The difference in [DELTA]corr values between the ordered and disordered samples was used to determine the local order parameter, q, which is consistent with a tricritical order-disorder transition. Keywords: IR spectroscopy, otavite-magnesite solid solution, autocorrelation, phase transition, carbonates.