A Central Peak in the Order-Disorder Phase Transition of Sodium Nitrate

A central peak in the low-frequency Raman spectra of sodium nitrate has been observed in a temperature range from room temperature to above the phase transition point (Tc). The central peak in the x(zz)y geometry is strongly dependent on temperature. Its intensity and width show an anomaly in temperature dependence near Tc. The anomalous part of the intensity is well expressed by a function of temperature (Tc — T)1/2 with a critical index of the order parameter. The origin of the central peak is assigned to an intrinsic one, that is, a motion of NO3- ions which becomes active with increasing temperature.