Model for the ordered phases in KCN and NaCN

We present, for KCN and NaCN in their electrically disordered and ordered low-temperature phases, a microscopic model using interatomic forces of the Born-Mayer type. We calculate with our model the atomic positions, the normal modes, and the elastic constants of KCN and NaCN in their ordered phase. %'e also calculate the atomic positions in the disordered phase. We adjust the parameters of our model to fit available experimental data. We then use this model to investigate other possible ordered structures of KCN and NaCN. Under the restriction that the order-disorder phase transition be continuous, we find that there are theoretically only six such structures. Using our model, we calculate the atomic positions as well as the energy per molecule for each of these structures. Our model predicts correctly which of these structures is most energetically favorable and should be experimentally observed.