A NEUTRON-DIFFRACTION STUDY OF THE ORDER-DISORDER PHASE-TRANSITION INSODIUM-NITRATE

The temperature dependence of the critical scattering above T-c and th e order parameter below T-c have been measured for sodium nitrate (NaN O3) 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 structur e which is characterized by the disappearance of superlattice reflecti ons 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 t he order parameter displays a crossover behaviour. For temperatures T < T-L approximate to 543 K the temperature dependence of the order par ameter can be described by a power law in the effective reduced temper ature 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 temperatur es T-L < T < T-c the temperature dependence of the order parameter fol lows a power law in the reduced temperature t = \T-c - T\/T-c with bet a = 0.41 +/- 0.02. At temperatures T > T-c the critical scattering tak es the usual form of a Lorentzian lineshape, and the correlation lengt h and susceptibility can be described by power-law equations in the re duced temperature with the exponents nu = 0.65+/-0.05 and gamma = 1.27 +/-0.04 respectively. There is a second Lorentzian lineshape in the di ffraction pattern above T-c which is much wider than the Lorentzian de scribing the critical scattering and is essentially independent of tem perature.