An explanation of the critical epr linewidth transitions in hexammine nickel halides

Abstract

It is shown that the point-charge model used by Bates and Stevens (1969) to explain the main features of the epr linewidth transitions and low-temperature entropy anomalies in hexammine nickel halides also predicts critical behaviour at temperatures Tc which depend on the halide. The critical behaviour arises from ordering of NH3 tetrahedra due to nearest-neighbour electrostatic interactions between point charges assigned to the protons of neighbouring (Ni(NH3)6)2+ clusters. The minimum free energy of the crystal is computed as a function of temperature and the critical behaviour is clearly demonstrated.