Raman and infrared studies of rotational-translational modes in stress-aligned ferroelastic KCN and NaCN

KCN and NaCN crystals, when cooled from the pseudocubic disordered phase into the low-temperature phase of parallel C${\mathrm{N}}^{-}$ order and orthorhombic symmetry, break up into a multidomain structure. The resultant multiple scattering of light prevents any measurements of polarized Raman or ir spectra, so that the assignment of bands to particular librational and/or translational modes has been highly speculative and controversial in previous works. By application of uniaxial stress in the ferroelastic phase, we have succeeded in aligning a large fraction of the orthorhombic domains and in reducing the multiple scattering of light. This has enabled us, for the first time, to measure polarized spectra of the librational-translational modes, the C${\mathrm{N}}^{-}$ stretching and combinations of both, using Raman and ir techniques. From experiments with various stress and light geometries, the symmetry properties of all observed Raman and ir translation are determined. This supplies a solid experimental basis for the assignment of these bands to normal modes of the crystal and for the analysis of the librational-translational coupling of these modes.