FAR-INFRARED REFLECTIVITY SPECTROSCOPY OF POTASSIUM AMMONIUM DIHYDROGENPHOSPHATE MIXED-CRYSTALS

The temperature dependence of the far-infrared reflectivity spectra of the Potassium ammonium dihydrogenpliosphate mixed crystal system, K1-x(NH4)xH2PO4, is reponed for three selected ammonium concentrations, x = 0.01, 0.3 and 0.97, over a wide temperature range, 7-250 K. A phase transition is observed in mixed crystals for x = 0.01 and 0.97. The occurrence of a glassy state in the mixed crystal for x = 0.3 is shown by the spectroscopic study: no ferro- or antiferroelectric order appears upon cooling to liquid helium temperature, and the onset of freezing at about 120 K is manifested by the low-frequency dielectric dispersion observed below 10 cm-1.