Long- and short-range order in stuffed titanate pyrochlores

We report a structural study of the stuffed pyrochlore series Ln2(Ti2−xLnx)O7−x/2 (Ln=Ho, Yb; 0⩽x⩽0.67). Electron microscopy and Rietveld refinements of neutron powder diffraction data for the x=0.67 end members, Ho2TiO5 and Yb2TiO5, reveal that small domains (∼50 Å or less) exist where the Ln and Ti/Ln sublattices are pyrochlore like, while the average structure is fluorite like. Both the Ho and Yb stuffed pyrochlore series for 0.1⩽x⩽0.5 are shown to be a composite of long- and short-range-ordered pyrochlore phases. The relative fraction of long-range vs. short-range pyrochlore order decreases with increasing Ln doping. An additional complex structural modulation of the pyrochlore structure is observed in electron diffraction and high-resolution electron microscopy images.