One dimensional hindered rotor model for order-disorder phase transitions in ammonium halide crystals

The hamiltonian for the disordered phase (O h 1 ) of the ammonium halide crystals is derived on the basis of a point charge model which also takes into account the repulsion between nearest neighbour ions (NH 4 + - X - ). This hamiltonian is reduced to that for interacting one dimensional hindered rotors, where rotation of NH 4 + ions is restricted to that around the z axis. Order-disorder phase transitions in pure ammonium halide crystals (NH 4 Cl, NH 4 Br and NH 4 I) and NH 4 Br x Cl 1-x mixed crystals are discussed in terms of the dynamic susceptibilities calculated from the model hamiltonian. The present model seems to reproduce the general behaviour of the phase diagrams. It also provides a plausible explanation for the mechanisms of the phase transitions. By considering the effect of pressure in the model hamiltonian, the possibility of occurrence of new ordered phases at high pressure is discussed within the framework of the present model.