Order-disorder phase transition in NH4AlF4

Abstract

In this study we calculate the specific heat CVI for NH4AlF4 due to the nearest-neighbor interactions between the NH+4 tetrahedra using an Ising model superimposed on an Einstein and/or Debye model. The specific heat CVI calculated using a power-law formula is in good agreement with the observed CP for the NH4AlF4 system. This is an indication that NH4AlF4 undergoes a weak first-order or a nearly second-order phase transition as predicted by our model.