  The coupling between antiferrodistortion (AFD) and ferroelectric (FE)
polarization, universal for all tilted perovskite multiferroics, is known to
strongly correlate with domain wall functionalities in the materials. The
intrinsic mechanisms of domain wall phenomena, especially AFD-FE
coupling-induced phenomena at the domain walls, have continued to intrigue the
scientific and technological communities because of the need to develop the
future nano-scale electronic devices. Over the past years, theoretical studies
often show controversial results, owing to the fact that they are neither
sufficiently nor directly corroborated with experimental evidences. In this
work, the AFD-FE coupling at uncharged 180{\deg} and 71{\deg} domain walls in
BiFeO3 films are investigated by means of aberration-corrected scanning
transmission electron microscopy with high resolution (HR-STEM) and
rationalized by phenomenological Landau-Ginsburg-Devonshire (LGD) theory. We
reveal a peculiar morphology at the AFD-FE walls, including kinks, meandering,
triangle-like regions with opposite oxygen displacements and curvature near the
interface. The LGD theory confirms that the tilt gradient energy induces these
unusual morphology and the features would change delicately with different
kinds of domain walls. Moreover, the 180{\deg} AFD-FE walls are proved to be
conductive with an unexpected reduction of Fe-O-Fe bond angle, which is
distinct from theoretical predictions. By exploring AFD-FE coupling at domain
walls and its induced functionalities, we provide exciting evidences into the
links between structural distortions and its electronic properties, which
benefit a lot for fundamental understanding for domain wall functionalities as
well as functional manipulations for novel nano-devices.
