Diffuse X-Ray Scattering Study of Lead Magnesium Niobate Single Crystals

Single crystals of lead magnesium niobate, PbMg_1/3Nb_2/3O₃, were studied with x-ray scattering measurements at temperatures between 10 and 350 K. The diffuse scattering around odd-integer Bragg reflections ( H+K+L = odd) has a significant anisotropic component with ledges along 〈110〉 directions. We show that the transverse optic (ferro distortion) modes whose polarizations are along 〈11̅ 0〉 directions are soft and responsible for the observed diffuse scattering. Upon cooling, these modes undergo a freezing process with q-dependent freezing temperatures. Below the lowest freezing temperature ( T≲190K) the diffuse scattering follows 1/q² law, indicating an elastically distorted, static structure.