  The crystal structure of BiFeO3/BaxSr1-xTiO3 (BFO/BST) heterostructures with
x = 0.2, 0.6 and 0.8, grown on single-crystal MgO (001) substrate was
investigated by x-ray diffraction and Raman spectroscopy in order to determine
the influence of mismatch-induced strains and spontaneous polarization in BST
buffer layers on BFO layers. The lattice parameter of the BFO layers was shown
to decrease with increasing concentration of Ba ions, despite the increasing
in-plain lattice parameters of tetragonal unit cells of BST layers. The
rhombohedral angle of the crystal structure of BFO layers demonstrates an
increase towards the ideal cubic perovskite structure with the appearance of
the built-in electric field, induced by the spontaneous polarization in buffer
layers. This result provides a remarkable tool for the control of polarization
in BFO layers and other ferroelectric films in general, by changing the
built-in electric field from ferroelectric buffer layer without changing a
single crystal substrate.
