Far-infrared spectroscopy of the K1-x(NH4)xI orientational glass

Far-infrared reflectivity and transmission measurements of ammonium and potassium iodide (K1-x(NH4)(x)I) mixed crystals with x = 0.08, 0.43 and 0.59 were performed and quantitatively evaluated for temperatures between 300 and 5 K. Classical two-mode behaviour of the reststrahl phonon was observed. Additional weaker absorption assigned to longitudinal and transverse acoustic phonons from the Brillouin zone boundary appeared below 80 cm(-1) at low temperatures. This effect appears not only for samples with x = 0.43 and 0.59 where orientational-glass phases (g(1), g(2)) and the phase epsilon with medium range antiferroelectric order appear at low temperatures, respectively, but also in the spectra of x = 0.08 sample where no phase-transition occurs. In all cases, the NH4+ ions break the long-range order of the KI lattice, which allows the activation of acoustic phonons from the whole Brillouin zone even at room temperature. Published by Elsevier Science B.V.