ELSEVIER Physica B 244 (1998) 172 179 Far-infrared spectroscopy of the K I-x(NH4)xI orientational glass S. Kamba a'*, J. Petzelt a, J. Pokorny ~, A. Loidl b ~lnstitute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Prague 8, 180 40, Czech Republic b Universitiit Augsburg, Universitiitstrasse 1, D-86159 Augsburg, Germany Abstract Far-infrared reflectivity and transmission measurements of ammonium and potassium iodide (K l_x(NH4)xI) 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 80cm-1 at low temperatures. This effect appears not only for samples with x = 0.43 and 0.59 where orientational-glass phases (gx, g2) and the phase ~ 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 NH~ 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. PACS: 63.20 - e; 68.35Rh; 78.30j Keywords: Orientational glass; Far-infrared spectroscopy; Phase transitions; (NH 4, K)I 1. Introduction In ideally ordered quasi-harmonic crystals, in- frared (IR) and Raman spectroscopies yield basi- cally the first-order active phonons from the Bril- louin zone (BZ) centre and the number of active modes can be predicted from the knowledge of the site symmetry of all atoms in the unit cell by a stan- dard factor-group analysis (see, e.g. Ref. [1]). In the case of orientational and dipolar glasses, the local symmetry is broken and a relaxation of the selec- tion rules is usually observed and more peaks appear in the spectra than predicted from factor- *Corresponding author. Fax: 00420 2 82 12 27; e-mail: kamba@fzu.cz. 0921-4526/98/$19.00 Published by Elsevier Science B.V. PII S092 1-4526(97)0048 3-3 group analysis because the phonons from the whole BZ can be activated in the spectra reflecting a weighted phonon density of state (PDOS). Appar- ent higher-mode damping and also stronger temperature dependence of mode parameters (eigenfrequency, damping, spectral strength) can be expected. Until now we have studied these effects on the well-known dipolar glass system Rbl-x(NH4)xHzPO4 and on its deuterated analog [2, 3]. In this paper, we report on new far-IR (FIR) results obtained on KI_x(NH4)xI orientational glass. FIR properties of pure and doped potassium iodide (KI) were studied many times already long ago, in connection with the investigation of one- or two-mode behaviour in mixed crystals (e.g.,