New evidence for rhombohedral symmetry in the relaxer ferroelectric Pb(Mg1/3Nb2/3)O-3

Neutron diffraction data have been collected from powders of lean magnesium niobate (PMN) at 300 and 10 K, following their synthesis in an optimised t wo-step reaction. Subsequent Rietveld refinement indicates that macroscopic rhombohedral symmetry is obtained at both temperatures. Moreover, the exis tence of a polar structure at room temperature has been independently confi rmed by second harmonic generation measurements. The derived crystal struct ures indicate R3m and R3c symmetry at 300 and 10 K, respectively. The signi ficance of these results for an understanding of the mechanism of relaxor f erroelectricity in PMN is discussed. (C) 1999 Elsevier Science Limited. All rights reserved.