Anomalous isotope effect on Raman linewidths in KH₂AsO₄ and KD₂AsO₄ crystals

Experiment on Raman scattering in KH₂AsO₄ and KD₂AsO₄ single crystals in the temperature range of 50–300 K has shown an anomalous isotope effect on the linewidths of a few A₁ modes in the 200–900 cm⁻¹ region. The widths in the deuterated crystal are narrowed by a factor of two to four depending on the line and temperature. The behaviour of the proton relaxation time has been obtained as a function of temperature and found to be very similar to those obtained by Lowendes et. al. from the spectral feature of the soft mode. A drastic decrease in the linewidths by a factor of three occurs at the ferroelectric phase transition.