Laser Raman spectra of mixed crystals of [(NH4)1−x Kx]2 SO4

Abstract

The Raman spectra of mixed crystals of [(NH4)1−x K x ]2 SO4 in the region 50–3400 cm−1 at 293 K and below 223 K have been reported. At room temperature 293 K, as the concentration of K+ ion increases in the crystal up to 50%, the frequencies of the totally symmetric vibrations of SO 4 2− and NH 4 + ions increase and thereafter the frequency of SO 4 2− vibration decreases and attains the value in K2SO4. This change in frequency up to 50% of potassium concentration is due to the breaking of hydrogen bonds of the type N-H...O. The behaviour of Raman intensities of A g (v 1) mode of SO 4 2− for various concentrations (x=0, 0·03, 0·11, 0·5, 0·85) suggest that the phase transition changes from first order type to one of second order. The phase transition in mixed crystals of [(NH4)1−x K x ]2 SO4 can be a cooperative phenomenon arising from a coupling between (NH4)+ ions through hydrogen bonds with the distorted SO 4 2− ions in the low temperature phase.