  Defects in oxides usually exhibit exotic properties that may be associated
with the local lattice dynamics. Here, at atomic spatial resolution, we
directly measure phonon modes of an antiphase boundary (APB) in SrTiO3
freestanding membrane and correlate them with the picometer-level structural
distortion. We find that the SrTiO3 APB introduces new defect phonon modes that
are absent in bulk SrTiO3. These modes are highly sensitive to the subtle
structure distortion, i.e., the SrTiO3 APB generates the local electric dipoles
forming an antiferroelectric configuration, which significantly softens the
transverse optical (TO) and longitudinal optical (LO) modes at {\Gamma} point.
Correlating the local phonons with the subtle structural distortion, our
findings provide valuable insights into understanding the defect properties in
complex oxides and essential information for their applications such as
thermoelectric devices.
