Crystal dynamics of KCN and NaCN in the disordered cubic phase

Abstract

The lattice dynamics of KCN and NaCN in their disordered cubic phase has been studied by coherent inelastic neutron scattering and infrared reflection measurements. Acoustic phonon branches for cyanide single crystals were measured in the [001] direction, with more limited measurements taken in the [110] direction. The one-phonon scattering generally appeared as weak peaks superimposed on a large background of non-one phonon scattering, particularly as measurements were extended to higher energy. No peaks clearly assignable to optical phonons were observed in the neutron measurements but the infrared reflection spectra for polycrystalline pellets were analyzed to derive TO and LO phonon energies (at q=0) of 17.0±0.6 and 28.5±2.5 meV. These measurements (and a comparison with the rather easily measured acoustic and optical phonons in a KBr crystal of similar volume) indicate that phonons do not exist as well-characterized excitations in the cyanides at higher frequencies and finite wave vectors. The unusual character of these results is attributed to the dynamical disorder of the cyanide ions in the fcc crystal and is compared with recent phonon measurements in the analogous fcc phase of ND4I. The [001] transverse acoustic branches of both KCN and NaCN show unusually low frequencies below q=0.5, which are consistent with recent elastic constant results and with the very large Debye–Waller factors derived from previous neutron diffraction results, and further suggest that the phase transitions in the cyanides are ’’driven’’ by a soft shear mode.