Localized modes of ammonium ion in KCl lattice

Following the standard Green’s functions matrix partitioning technique, the force constant changes needed to explain the translational (6.8 THz) and torsional (10.1 THz) modes occurring in the KCl:NH ₄ ⁺ system are calculated. Three different defect site symmetries are considered for the ammonium ion impurity. These are (i)O _h, in which the ammonium ion is a free rotor, (ii)T _d, in which it is a hindered rotor and (iii)C _4v , in which it rotates freely about a N - H ... Cl axis and librates around the other two crystallographic axes.O _h defect symmetry explains only the translational mode, while in the other two symmetries both the modes are explained with reasonable changes in the force constants. It is also shown that the same set of force constant changes explains the local modes in the deuterated sample as well.