Raman scattering study of phase transitions in Cs1-x(NH4)(x)H2AsO4 mixed crystals

Abstract

Temperature dependence of Raman scattering spectra in the cesium-ammonium dihydrogen arsenate (CADA-x, x=0.20) mixed crystal was observed to study anomalies associated with the dipole glass freezing phenomena in the system. A(1) symmetry internal vibrations of arsenate ions were found to deviate from the normal temperature dependence of the third and fourth order anharmonic phonon-phonon interactions at around T(f)similar or equal to 80 K, which was ascribed to the onset of the short range random potential fixing of protons. The B-2 symmetry soft mode derived from the harmonic oscillator coupled mode analysis was found to be a relaxational type characterized by omega(2)/gamma(proportional to)(T-T-o) with T(o)similar or equal to 28.8 K, where omega(2)/gamma corresponds to the relaxation bandwidth of the soft mode. The large difference between the two anomaly temperatures was attributed to the enhanced effective piezoelectric constant associated with the electrostrictive contributions from the random local cluster polarizations. [S0163-1829(99)01217-5].