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

Abstract

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.