Third-order elastic constants of cubic sodium cyanide and potassium cyanide

The complete third-order elasticity tensors of cubic NaCN and KCN have been determined from stress-shifted resonance frequencies of thick plates at 293 K. The two cyanides exhibit quite similar nonlinear elastic properties which, however, differ strongly from those of comparable alkali halides such as NaCl and KCl. The 'longitudinal' constants c₁₁₁ are ca five times smaller and the 'transverse' constants c₁₁₂ and c₁₁₃ ca eight times larger than those in NaCl and KCl. The Cauchy relations are strongly violated, as is already known for the second-order elastic constants. The critical shear resistance c₄₄, which is connected with the phase transitions observed at 283 K (NaCN) and 168 K (KCN), exhibits a considerable dependence on hydrostatic pressure only in the case of NaCN: c₄₄ drops rapidly with increasing pressure. The absolute change of c₄₄ per hydrostatic pressure unit, however, is comparable to that in alkali halides. The nonlinear elastic behaviour of NaCN and KCN is qualitatively interpreted by interactions of volume-conserving type (as existing in fluids).