A neutron diffraction study of the order-disorder phase transition in sodium nitrate

The temperature dependence of the critical scattering above T-c and the order parameter below T-c have been measured for sodium nitrate (NaNO3) using time-of Eight neutron diffraction. Sodium nitrate undergoes a structural phase transition from a low-temperature rhombohedral R (3) over bar c structure to a high-temperature R (3) over bar m structure which is characterized by the disappearance of superlattice reflections at positions corresponding to the Z point of the high-temperature Brillouin zone. In the critical region below T-c = 548.74 +/- 0.47 K the order parameter displays a crossover behaviour. For temperatures T < T-L approximate to 543 K the temperature dependence of the order parameter can be described by a power law in the effective reduced temperature t* = \T-c* - T\/T-c* with beta = 0.22 +/- 0.02 and an effective critical temperature T-c* = 551.02 +/- 0.54 K. However, for temperatures T-L < T < T-c the temperature dependence of the order parameter follows a power law in the reduced temperature t = \T-c - T\/T-c with beta = 0.41 +/- 0.02. At temperatures T > T-c the critical scattering takes the usual form of a Lorentzian lineshape, and the correlation length and susceptibility can be described by power-law equations in the reduced temperature with the exponents nu = 0.65+/-0.05 and gamma = 1.27+/-0.04 respectively. There is a second Lorentzian lineshape in the diffraction pattern above T-c which is much wider than the Lorentzian describing the critical scattering and is essentially independent of temperature.