X-ray study of phase transition in betaine phosphite

The phase transition of betaine phosphite crystal associated with the ordering Of PO3 radicals at phase I to II transition has been studied by X-ray scattering: The observations are concerned on (1) the temperature dependence of lattice constants, (2) the temperature dependence of the order parameter Of PO3 which followed the relation eta proportional to (T-c - T)(beta), beta = 0.33(1), and (3) the anisotropic distribution of critical diffuse scattering in the reciprocal space above T-c; elongated along the c-direction, shortened along the V-direction. The critical X-ray diffuse scattering has been analyzed on an Ising model to obtain effective interaction between PO3 ions. The results are summarized as follows. (1) The correlation lengths along three principal directions of anisotropic scattering distribution was estimated. (2) The temperature dependence of the correlation length follows nearly the mean-field behavior, xi proportional to (T - T-c)(-nu), nu = 0.5.