  The ammonium halides present an interesting system for study in view of their
polymorphism and the possible internal rotation of the ammonium ion. The static
properties of the mixed ionic crystal NH$_4$Cl$_{1-x}$Br$_x$ have been recently
investigated, using three-body potential model (TDPM) by the application of
Vegard's law. Here, by using a simple theoretical model, we estimate the bulk
modulus of their ternary alloys NH$_4$Cl$_{1-x}$Br$_x$, in terms of the bulk
modulus of the end members alone. The calculated values are comparable to those
deduced from the three-body potential model (TDPM) by the application of
Vegard's law.
