Order-Disorder Transition in NH4Cl. I. Phenomenological Theory

Abstract

A phenomenological theory is presented to explain the thermodynamic behavior of ammonium chloride near the first-order phase transition at 242 °K. The theory is similar to that of Garland and Renard for N${\mathrm{H}}_4$Cl and other theories for magnetic systems, but is cast in a form convenient for analysis of experimental data according to various scaling laws. The lattice system gives rise to nonsingular terms in the Helmholtz free energy of the total system. The order-disorder system gives rise to a singular term in the free energy. The coupling of the two subsystems then produces the possibility of the first-order transition.