Epitaxial stannate pyrochlore thin films: Limitations of cation stoichiometry and electron doping

Abstract

We have studied the growth of epitaxial films of stannate pyrochlores with a general formula A2Sn2O7 (A = La and Y) and find that it is possible to incorporate ∼25% excess of the A-site constituent; in contrast, any tin excess is expelled. We unravel the defect chemistry, allowing for the incorporation of excess A-site species and the mechanism behind the tin expulsion. An A-site surplus is manifested by a shift in the film diffraction peaks, and the expulsion of tin is apparent from the surface morphology of the film. In an attempt to increase La2Sn2O7 conductivity through n-type doping, substantial quantities of tin have been substituted by antimony while maintaining good film quality. The sample remained insulating as explained by first-principles computations, showing that both the oxygen vacancy and antimony-on-tin substitutional defects are deep. Similar conclusions are drawn on Y2Sn2O7. An alternative n-type dopant, fluorine on oxygen, is shallow according to computations and more likely to lead to electrical conductivity. The bandgaps of stoichiometric La2Sn2O7 and Y2Sn2O7 films were determined by spectroscopic ellipsometry to be 4.2 eV and 4.48 eV, respectively.

Is Ba3In2O6 a high-Tc superconductor?

“…where E tot [X q ] and E bulk tot are the total energies of the defective supercell (for a given defect X in the charge state q) and the bulk energy, respectively. The third term represents the energy needed to exchange atoms with thermodynamic reservoirs, where n i indicates the number of atoms of the species i removed or added to create the defect, and µ i are the corresponding chemical potentials [46]. We computed the phase diagram of Ba-In-O with different possible dopants at 0 K at the same level of theory and determined the limits of the allowed chemical potential of these species.…”