Knowledge Bank

The infrared spectra of single crystals of $NaNO3,CaCO3,CaMg(CO3)2$ have been investigated at about $10\circ K$ with polarized light in the spectral region between $700cm-1$ and $2300cm-1$. Besides the strong absorption bands corresponding to the inner vibrations of the $NO3-$ or $CO3-$ molecules, a great number of weaker bands were observed which are attributed to combinations of these inner vibrations with external lattice modes. The observed structure of these bands is attributed to transitions with wavevectors ending at the center and at the boundary of the Brillouin-zone. The observed transitions are compared with selection rules derived for certain points in the Brillouinzone and conclusions about the dispersion of the vibrational branches in these crystals are discussed.