Superstructure ordering in lanthanum-doped lead magnesium niobate

Superstructure reflections of lead magnesium niobate (PMN), Pb(Mg1/3Nb2/3)O -3, were measured with synchrotron radiation x-ray diffraction. Our data co nclusively demonstrate that the (h+1/2,k+1/2,l+1/2)-type reflections are th e result of chemical ordering between the Mg and Nb ions in domains of 50 A ngstrom. Structure factor analysis of these peaks reveals an oxygen displac ement within the ordered regions of 0.044(3) Angstrom along a < 100 > direc tion towards the Nb ion. Single crystals of La-doped PMN (La-PMN) exhibit m uch larger chemically ordered regions (up to 900 Angstrom for 10% La-PMN). Structural analysis of the La-PMN suggests that as the ordered region incre ases in size, the ordering changes from complete Nb:Mg ordering and approac hes Nb:(Mg2/3Nb1/3) ordering. Although anticipated from electrostatic energ y arguments, our conclusions are based on a clear trend in the bond length of the mixed site as a function of La doping. (C) 2000 American Institute o f Physics. [S0021-8979(00)01302-5].