X-RAY AND NEUTRON-DIFFRACTION STUDIES OF THE DIFFUSE PHASE-TRANSITION IN PBMG1/3NB2/3O3 CERAMICS

X-ray and neutron diffraction patterns of lead magnesium niobate (PMN) were recorded between 800 and 5 K. The evolution of the cubic cell parameter and of the thermal expansion coefficient was analyzed and structural refinements were carried out by the Rietveld method. In the cubic phase of space group Pm3m the atoms exhibit important shifts from their special positions. A disordered structural model leading to a discrepancy factor Rwp = 7.21% is proposed. The nucleation of polar microregions begins at 600 K; it is revealed on the X-ray and neutron diffraction patterns only by some line tails which increase when the temperature is lowered. A structural refinement at 5 K leads to a Rwp factor of about 11%. Therefore this model has to be improved in order to take into account the existence of polar microregions with very small correlation lengths. © 1991.