  We present an aberration corrected scanning transmission electron microscopy
(ac-STEM) analysis of the perovskite (LaFeO3) and pyrochlore (Yb2Ti2O7 and
Pr2Zr2O7) oxides and demonstrate that both the shape and contrast of visible
atomic columns in annular dark-field (ADF) images are sensitive to the presence
of nearby atoms of low atomic number (e.g. oxygen). We show that point defects
(e.g. oxygen vacancies), which are invisible - or difficult to observe due to
limited sensitivity - in X-ray and neutron diffraction measurements, are the
origin of the complex magnetic ground state of pyrochlore oxides. In addition,
we present, for the first time, a method by which light atoms can be resolved
in quantitative ADF-STEM images. Using this method, we resolved oxygen atoms in
perovskite and pyrochlore oxides.
