Two-Dimensional Orientational Order-Disorder Phase Transition in NH 4 AlF 4

Abstract

In NH 4 AlF 4 the NH 4 + molecules form a layered structure where the interlayer distance, 6.367 Å, is about twice as large as the intralayer nearest neighbours distance, 3.597 Å. There is an orientational order-disorder transition at T c = 156.9 - 0.1 K, explained by the Ising model, with first and second neighbours and a weak interlayer coupling. Monte Carlo simulations are presented based on this model.