[en] The effect of temperature and concentration on the lattice parameters and amplitude-weighted phonon density of states in mixed salts of ammonium-potassium halides is investigated by neutron powder diffraction and incoherent inelastic neutron scattering. In the disordered α-phase (NaCl type) ammonium ions exhibit a fast stochastic reorientation at phonon frequency rates down to ca. 80 K. At 10 K, the incoherent inelastic neutron scattering spectra display four distinct ammonium excitations: two (resonant) modes below and two (localized) above the Debye cut-off energy of potassium halides. High-frequency localized modes correspond to translational and librational vibrations of NH₄ ions. These modes are typical for the ordered phases of ammonium halides. The effect of ammonium concentration on localized and resonant modes is studied for the K_1-x(NH₄)_xI mixed salts. The harmonic excitations of ammonium in a hypothetical low-temperature α-phase of NH₄I are approximated to ca. 30, 95, 155 and 250 cm^-1. In a real low-temperature ordered γ-phase of NH₄I, translational ammonium vibrations are observed at ca. 140-160 cm^-1 and librational vibrations at ca.300 cm^-1