Brillouin Scattering Studies of the Antiferrodistortive Phase Transition in N_a NO₃

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The longitudinal acoustic mode propagating along the three-fold z axis in NaNO₃ has been studied by Brillouin scattering in the temperature range between 23.22°C and 299.55°C.The frequency shift and the spectral width of the Brillouin peakshow anomalous behavior near the transitiontemperature T_c=276.5°C.The relaxation time of the antiferrodistortive mode of NO₃ ionsis calculated from these anomalies through the use ofDebye type susceptibility.The temperature dependence of the relaxationtime has been determined below T_c to be τ_f/(T_c-T)whereτ_fis(6.0 ± 0.7)× 10^-1sec· deg.