This project has been focused on the microscopic study of polymorphic phase transitions and hysteresis cycles in alkali halides observed experimentally. In [B37] the Perturbed Ion (PI) method has been used for a number of alkali halides to analyse the Gibbs free energy surfaces in the framework of the Buerger's mechanism (under zero temperature). It was found that the transition path depends strongly on pressure and shows large and asymmetric Gibbs barriers from both sides of the transition. Then a simple master equation kinetic model has been proposed which enabled us to understand many of the empirical facts around the phenomenon of hysteresis.