Order-disorder phase transition in NH4AlF4

In this study we calculate the specific heat C-VI for NH4AlF4 due to the nearest-neighbor interactions between the NH4+ tetrahedra using an Ising model superimposed on an Einstein and/or Debye model. The specific beat C-VI calculated using a power-law formula is in good agreement with the observed C-P 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