Brillouin scattering studies of orientationally disordered sodium cyanide and potassium cyanide single crystals

Abstract

Brillouin spectra of acoustic phonons in the orientationally disordered phase in KCN and NaCN have been studied. The phonon frequencies have been determined as a function of temperature. The frequency of the transverse acoustic phonon propagating along (110) and polarized along (001) corresponds well with mean field theory for the phase transition for the two cyanide crystals. The elastic constants C₁₁, C₁₂,, and C₄₄ have been determined as a function of temperature in the cubic disordered phase. The Cauchy relation for cubic crystals is found to be invalid in KCN and NaCN. The two types of phase transitions in KCN and NaCN are discussed in terms of the vanishing shear modulii C₄₄ and (C₁₁−C₁₂)/2, each of which induces lattice instability when it becomes small. The integrated intensities of the shear modes in KCN and NaCN were also measured as a function of temperature. The intensity enhancement upon approaching the phase transition temperature has been found to be due to the decrease of C₄₄. The relative values of the elasto‐optical constant P₄₄ in KCN and NaCN have also been determined as a function of temperature.