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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 ( T C ). The central peak in the x ( z z ) y geometry is strongly dependent on temperature. Its intensity and width show an anomaly in temperature dependence near T C . The anomalous part of the intensity is well expressed by a function of temperature ( T C - 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 NO - 3 ions which becomes active with increasing temperature.