Monte Carlo Studies of the Orientational Order-Disorder Phase Transition in Solid Ammonium Chloride
Loading...
Date
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Description
Monte Carlo methods are used to study the phase transition in ammonium chloride from the orientationally ordered $δ$ phase to the orientationally disordered $γ$ phase. An effective pair potential is used to model the interaction between ions. Thermodynamic properties are computed in the canonical and isothermal-isobaric ensembles. Each ammonium ion is treated as a rigidly rotating body and the lattice is fixed in the low-temperature CsCl geometry. A simple extension of the Metropolis Monte Carlo method is used to overcome quasiergodicity in the rotational sampling. In the constant-$NVT$ calculations the lattice is held rigid; in the constant-$NpT$ calculations the lattice parameter is allowed to fluctuate. In both ensembles the order parameter rapidly falls to zero in the range (200 - 250)K, suggesting that the model disorders at a temperature in fair agreement with the experimental disordering temperature (243K). Peaks in the heat capacity and thermal expansivity curves are also found in the same temperature range.
submitted to Journal of Chemical Physics
submitted to Journal of Chemical Physics