Far-Infrared and Near-Millimeter Spectroscopy of the Rb 0.5 (NH 4 ) 0.5 H 2 PO 4 Dipolar Glass

Abstract

The dielectric response of the Rb0.5(NH4)0.5H2PO4 dipolar glass was determined in the spectral range 8-650 cm-1 and temperature range 10-300 K using BWO monochromatic transmission spectroscopy (8-18 cm-1) and Fourier transform reflection spectroscopy (20-650 cm-1). Above the glass transition temperature Tg approximately=100 K the low-frequency dynamics (below about 100 cm-1) is similar to that of an averaged pure crystal. The observed one- and two-mode behaviour in the phonon region confirms the earlier Raman results. Below Tg a broad background absorption in addition to phonon peaks remain present down to the lowest temperatures especially for the E perpendicular to c polarization. The authors suggest that this is caused by breaking of the quasi-momentum conservation selection rule. The appearance of new forbidden modes below Tg gives evidence of polar microregions. The low-frequency dispersion below Tg is caused by different collective proton-hopping process from the higher-frequency soft-mode-type hopping, probably because of specific diffusion of Takagi defects.