Thin Film Synthesis and Structural Characterization of a New Kinetically Preferred Polymorph in the RE2Ti2O7 (RE = La-Y) Family

A new polymorph was found for all RE2Ti2O7 (RE = La-Y) compounds during their growth as epitaxial films on SrTiO3(110) substrates. Detailed structural characterization using X-ray and electron diffraction revealed that the new polymorph adopts an orthorhombic crystal structure with the space group Ammm and unit cell parameters (for La) of a = 7.89, b = 5.54, c = 13.45 angstrom; the refined lattice parameters decreased monotonically with decreasing size of the RE cation. Though these parameters are similar to those of the known monoclinic (110)-layered perovskite polymorph, the structures are not related by simple distortive transformations. While both of these polymorphs can be formed as epitaxial films on SrTiO3(110) substrates at T similar to 900 degrees C, the new polymorph is favored in conditions where diffusion is more limited between pulses. This implies that the new polymorph is kinetically favored during nucleation over the (110)-layered perovskite phase, in agreement with the smaller electroneutral unit of the new polymorph.