Knowledge Bank

The spectrum of brucite, $Mg(OH)2$, has been studied in the region from $400cm-1$ to $11000cm-1$. In addition to careful reinvestigation of the spectra in the previously studied regions, the present research includes polarized reflectivity measurements in the highly opaque lattice region ($350cm-1$ to $1000cm-1$), and around $3700cm-1$ to determine the optical constants and to decide about the strongest band in this highly absorbing region, extensive low temperature (at liquid nitrogen point) spectra in the near infrared region, detailed powder spectra of chemically prepared $Mg(OH)2$ and $Mg(OD)2$, and transmission measurements in the second overtone region. The Raman spectrum of the crystal has been reported fur the first lime. A factor group analysis has been earned out obtaining the symmetry classification and selection rules for the normal modes. An assignment has been attempted for the internal and external fundamentals and for the OH overtones. The possibility of explaining the complex spectra, in the near infrared region as combinations of the stretching and librational modes has also been explored The model proposed by Hexter and Dows, which assumes a simple harmonic librational potential energy, is found to be completely inadequate. Two modifications have been proposed an interaction term involving the two non-equivalent OH ions in the unit cell of the crystal, and a potential energy expression dependent upon 3-fold and 6-fold symmetry to incorporate the special features of the brucite space group. The results of perturbation calculations showed only a few features in common with the observed spectra. It appears that for a complete explanation one has to assume additional combinations with lattice modes which become active in combination.""