Single crystals of lead magnesium niobate, ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3$, were studied with x-ray scattering measurements at temperatures between 10 and 350 K. The diffuse scattering around odd-integer Bragg reflections ( $H+K+L=\mathrm{odd}$) has a significant anisotropic component with ledges along $〈110〉$ directions. We show that the transverse optic (ferro distortion) modes whose polarizations are along $〈1\overline{1}0〉$ 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 ( $T\lesssim 190\mathrm{K}$) the diffuse scattering follows ${1/q}^2$ law, indicating an elastically distorted, static structure.

• Received 25 April 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.3950