Off-centering of rare-earth ion in (Ba,R)(Ti,Mg)O3 (R = Gd, Dy)

To investigate the off-centering behavior of rare-earth ions that were partially substituted for A site ions in ABO3-type crystals, accurate crystal structure analyses of (Ba,R)(Ti,Mg)O3 (R = Gd, Dy) were performed by high-energy synchrotron radiation X-ray powder diffraction analysis and the maximum entropy method (MEM)/Rietveld method. The R ions were observed at the six equivalent off-center sites in the 〈100〉 directions from the corner of a unit cell in the cubic phase. In the tetragonal phase, the R ions occupied one off-center site along the [001] direction. The R off-centering can be attributed to the covalent nature of the bond between the R and O ions in (Ba,R)(Ti,Mg)O3 crystals.