  This study reports atomic-scale characterization of structural defects in
Yb2Ti2O7, a pyrochlore oxide whose subtle magnetic interactions is prone to
small perturbations. Due to discrepancies in the reported magnetic ground
states, it has become a pressing issue to determine the nature of defects in
this system. In the present study, we use atomic resolution scanning
transmission electron microscopy techniques to identify the type of defects in
the ytterbium titanate single crystals grown by the conventional optical
floating zone (FZ) method. In addition to the known point defects of
substitution Yb on Ti B-sites, extended defects such as dissociated
superdislocations and anti-phase boundaries were discovered for the first time
in this material. Such defects were prevalently observed in the FZ grown single
crystals (of a darker color), in contrast to the stoichiometric white
polycrystalline powders or high quality colorless single crystals grown by the
traveling solvent floating zone (TSFZ) technique. The lattice strains from
these extended defects result in distortions of Yb-tetrahedron. A change of Ti
valance was not detected at the defects. Our findings provide new insights into
understanding the nature of defects that are of great importance for the
physical property studies of geometrically frustrated compounds. Furthermore,
this work sheds light on the complicated core structure of superdislocations
that have large Burgers vectors in oxides with complex unit cells.
