Ab initio calculations of the phase-transition mechanism in potassium thiocyanate

We present the density-functional theory calculations of the microscopic mechanism of the order-disorder phase transition in potassium thiocyanate. The original method of calculations indicates that all molecules from the unit cell are taking part in the reorientation process. The leading phase-transition mechanism should involve both pure torsions of SCN dumbbells around the c axis and around the axis in the (a,b) plane. With increasing amplitude of rotations, torsional vibrations around the c axis become energetically favorable. The calculations also yield a considerable coupling between the reorientation of SCN molecules and the lattice strains. The present results explain previous contradictory experimental data concerning the mechanism of structural phase transition in thiocyanates.