  Perovskite oxides form an eclectic class of materials owing to their
structural flexibility in accommodating cations of different sizes and
valences. They host well known point and planar defects, but so far no line
defect has been identified other than dislocations. Using analytical scanning
transmission electron microscopy (STEM) and ab initio calculations we have
detected and characterized the atomic and electronic structures of a novel line
defect in NdTiO3 perovskite. It appears in STEM images as a perovskite cell
rotated by 45 degrees. It consists of self-organized Ti-O vacancy lines
replaced by Nd columns surrounding a central Ti-O octahedral chain containing
Ti4+ ions, as opposed to Ti3+ in the host. The distinct Ti valence in this line
defect introduces the possibility of engineering exotic conducting properties
in a single preferred direction and tailoring novel desirable functionalities
in this Mott insulator.
