Superstructure ordering in lanthanum-doped lead magnesium niobate

Superstructure reflections of lead magnesium niobate (PMN), Pb(Mg1/3Nb2/3)O3, were measured with synchrotron radiation x-ray diffraction. Our data conclusively demonstrate that the (h+12,k+12,l+12)-type reflections are the result of chemical ordering between the Mg and Nb ions in domains of 50 Å. Structure factor analysis of these peaks reveals an oxygen displacement within the ordered regions of 0.044(3) Å along a 〈100〉 direction towards the Nb ion. Single crystals of La-doped PMN (La–PMN) exhibit much larger chemically ordered regions (up to 900 Å for 10% La–PMN). Structural analysis of the La–PMN suggests that as the ordered region increases in size, the ordering changes from complete Nb:Mg ordering and approaches Nb:(Mg2/3Nb1/3) ordering. Although anticipated from electrostatic energy arguments, our conclusions are based on a clear trend in the bond length of the mixed site as a function of La doping.