A Study of PMN Crystal Structure Below The Phase Transition Temperature

X-ray diffraction studies of PMN crystal at 103, 183, 203 K show that the disorder in shifts of Pb atoms below T _m develops successively with lowering temperature: from the preferentially uniaxial Pb displacements to equal disordered ones along the axes X, Y, Z. In PMN, in the absence of an external electric field the corresponding rhombohedral ferroelectric phase does not appear in the volume of a whole crystal. The relaxation rotations of oxygen octahedra BO₆ in the vicinity of T _m may be the cause for the appearance of superstructure.