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

Abstract

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 A; 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 ∼ 900 °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 pol...