Molecular-dynamics Study of Phase Transitions in Alkali Thiocyanates

An account is presented of our studies of the order-disorder phase transitions in KN3, RbN3, CsN3, KSCN, RbSCN and CsSCN. These are based on parameter-free inter-ionic potentials based on the Gordon-Kim modified electron gas formalism extended to molecular ions.With these potentials we performed static structural relaxations and supercell molecular dynamics and predict with reasonable accuracy the temperatures for the onset of the transitions. In particular we address the question of how the N-3(-) and SCN- ions reorient to yield the transitions.It was found that in case of azides we observed hindered rotations of the anions about all three crystallographic axes, in all three systems. However, in the thiocyanates only CsSCN yielded hindered rotations about all three axes. In KSCN and RbSCN the order-disorder transition appears rather to involve large amplitude librations of the SCN- ions, primarily about the c axis.