X-ray scattering study of soft-optic-mode freezing in lead magnesium niobate single crystals Original Research Article

An elementary scattering equation is derived to show that diffuse X-ray scattering intensity near a Bragg reflection can be attributed either entirely to optic modes or to acoustic modes in simple cases. It is used to ascertain that the observed diffuse scattering near odd-integer reflections originates from 〈110〉 transverse optic modes. Detailed temperature-dependent measurements reveal that, unlike usual phase transition, freezing of the optic mode is dispersive. That is, the freezing temperature increases with q. At temperatures lower than ∼173 K, the modes of all length scale freeze to form a statically distorted structure.