  Monte Carlo methods are used to study the phase transition in ammonium
chloride from the orientationally ordered $\delta$ phase to the orientationally
disordered $\gamma$ 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.
