Origin of superstructures in (double) perovskite thin films

摘要

We have investigated the origin of superstructure peaks as observed by X-ray diffraction of multi-ferroic Bi(Fe_(0.5)Cr_(0.5))O_3 thin films grown by pulsed laser deposition on single crystal SrTiO_3 substrates. The photon energy dependence of the contrast between the atomic scattering factors of Fe and Cr is used to rule out a chemically ordered double perovskite Bi_2FeCrO_6 (BFCO). Structural calculations suggest that the experimentally observed superstructure occurs due to unequal cation displacements along the pseudo-cubic direction that mimic the unit cell of the chemically ordered compound. This result helps to clarify discrepancies in the correlations of structural and magnetic order reported for Bi_2FeCrO_6. The observation of a superstructure in itself is not a sufficient proof of chemical order in double perovskites.