Positron angular correlation studies of phase transitions in ammonium halides

The ammonium halides present an interesting system for positron angular correlation study in view of their polymorphism and the internal dynamics of ammonium ion. Cohesive energy calculations [1] have shown that ammonium ions are predominantly ionic and the hydrogen bonding contribution to cohesive energy is less than 3%. It is shown here that the positron angular correlation is very sensitive to the nature of the chemical bond of the hydrogen ion with the lattice. In the ordered phase, at a considerably lower temperature below the transition, the ammonium ion assumes a rigid configuration displaying the full effect of covalent bond. The angular correlation broadens at this phase. At higher phases the effect of covalency is washed out due to torsional oscillations and the angular correlation curve becomes narrower. But in the phase I modification of NH4I, the width of angular distribution becomes larger indicating the creation of cation vacancies due to the relative instability of the ammonium ion in this lattice.