  We present a synchrotron grazing incidence x-ray diffraction analysis of the
domain structure and polar symmetry of highly strained BiFeO3 thin films grown
on LaAlO3 substrate. We revealed the existence of periodic elastic nanodomains
in the pure tetragonal-like BFO ultrathin films down to a thickness of 6 nm. A
unique shear strain accommodation mechanism is disclosed. We further
demonstrated that the periodicity of the nanodomains increases with film
thickness but deviates from the classical Kittel's square root law in ultrathin
thickness regime (6 - 30 nm). Temperature-dependent experiments also reveal the
disappearance of periodic modulation above 90C due to a MC-MA structural phase
transition.
