Long- and short-range order in stuffed titanate pyrochlores

Abstract We report a structural study of the stuffed pyrochlore series Ln 2 (Ti 2− x Ln x )O 7− 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, Ho 2 TiO 5 and Yb 2 TiO 5 , reveal that small domains (∼50 A 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.