Role of strain field on the rotational tunnelling spectra of NH4+ ions in (NH4)xK1−xI

It is known that the rotational tunnelling spectra of NH4+ ions in (NH4)xK1−xI show of significant changes with increases in the NH4 ion concentration. This Letter reports the results of high-resolution neutron inelastic measurements of (NH4)0.02K0.98I, (NH4)0.02(K0.9Rb0.1)0.98I and (NH4)0.1Rb0.9I, and shows unequivocally that the observed changes in the tunnelling spectra of NH4+ ions in (NH4)xK1−xI with x arise because of increase in both the NH4+–NH4+ interaction and in the strain field in the lattice. The effect of changes in the strain field on the tunnelling spectra are qualitatively different from those arising from a change in NH4+–NH4+ interaction.