Combinatorial substrate epitaxy: a new approach to growth of complex metastable compounds

A high-throughput processing-characterization method, called combinatorial substrate epitaxy (CSE), was developed that enables the investigation of epitaxial stabilization of metastable compositions in complex structures. To demonstrate the approach, we fabricated RE2Ti2O7 (RE = Dy, Gd, Sm, La) in a polymorphic structure for which RE = Dy, Gd, and Sm are metastable and Dy2Ti2O7 has not been previously observed. Dense sintered pellets of Sr2Nb2O7, which adopts the 110-layered perovskite (LP) structure, were prepared as substrates, polished flat, and characterized locally using electron backscatter diffraction (EBSD). Thin films of RE2Ti2O7 were deposited using pulsed laser deposition and were then characterized with EBSD. The EBSD patterns from all film-substrate pairs matched in a grain-by-grain fashion, which demonstrates that the films are in local epitaxial registry with the Sr2Nb2O7 grains over a wide spread of crystallographic orientations for the substrate surface. Furthermore, the EBSD patterns demonstrate that all RE2Ti2O7 films, whether stable or metastable in the bulk, adopt the 110-LP structure. Transmission electron microscopy was used to investigate more closely the metastable Sm2Ti2O7 films. The film-substrate interfaces are atomically smooth with relaxed epitaxial registry, indicating that the microcrystalline substrates can be treated as local single-crystal substrates and the metastable films are stable against back-transformation on strain relaxation. Electron diffraction patterns for Sm2Ti2O7 films are consistent with the monoclinic 110-LP unit cells. This work demonstrates that CSE allows for the growth of new materials that are thermodynamically and kinetically difficult to realize otherwise.