FAR-INFRARED SPECTROSCOPY OF THE K1-X(NH4)(X)I ORIENTATIONAL GLASS

Far-infrared reflectivity and transmission measurements of ammonium an d 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 ph onon was observed. Additional weaker absorption assigned to longitudin al and transverse acoustic phonons from the Brillouin zone boundary ap peared below 80 cm(-1) at low temperatures. This effect appears not on ly for samples with x = 0.43 and 0.59 where orientational-glass phases (g(1), g(2)) and the phase epsilon with medium range antiferroelectri c order appear at low temperatures, respectively, but also in the spec tra 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 all ows the activation of acoustic phonons from the whole Brillouin zone e ven at room temperature. Published by Elsevier Science B.V.