X-ray scattering study of high temperature phase transition of betaine phosphate

The I-II phase transition of betaine phosphate crystal associated with the ordering Of PO4 groups 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 PO4 which followed the relation eta proportional to (T-c - T)(beta), beta = 0.42(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 b*-direction. The critical X-ray diffuse scattering has been analyzed. The correlation lengths along three principal directions of anisotropic scattering distribution were estimated. The temperature dependence of the correlation length and staggered susceptibility were discussed.