Combinatorial substrate epitaxy (CSE) is used to investigate polymorph competition between metastable scrutinyite (α-PbO2) structured (s-) and stable rutile structured (r-) SnO2 during local epitaxial growth across orientation space on polycrystalline columbite (c-) CoNb2O6 substrates. Growth occurs in a grain-over-grain fashion, where individual grains of c-CoNb2O6 support the growth of individual grains of SnO2. Both metastable s-SnO2 and stable r-SnO2 crystals are observed, each growing on specific ranges of substrate orientations and each having a single specific orientation relationship (OR) with substrate grains on which it grew. s-SnO2 adopts the unit-cell over unit-cell OR that can be expressed as the alignment of primary eutactic planes: (100)s*[001]s*∥(100)c*[001]c* (where the * indicates the use of Pcnb setting). s-SnO2 grains grow on a slight majority of orientations and specifically on orientations inclined from the (010) pole of c*-CoNb2O6. r-SnO2 adopts an OR that can be expressed as the alignment of a secondary set of eutactic planes: (101)r[010]r∥(010)c*[001]c*. r-SnO2 grows only on orientations close to the (010) pole of c*-CoNb2O6. The collected set of observations is discussed and rationalized with respect to the combination of misfit strain energies and cation packing interfacial energy penalties. We conclude that CSE should allow for the rational and computationally guided development of new materials adopting scrutinyite, rutile, and related structures.

Two of three levels of hierarchical ferroelastic domains of γ-WO3 ceramics are shown to be piezo-responsive (polar) for surfaces of any general orientation. Importantly, the surface activity for the photocathodic reduction of Ag+ and photoanodic oxidation of Pb2+/Mn2+ ions is correlated to those same two levels of ferroelastic domains for all orientations.

•The phase and orientation of PLD TiN/TaN layers were determined by substrate, T, and thickness.•TaN formed epitaxial (001)-oriented hexagonal films at high T and > 2.5 nm from the interface of TiN(111).•(111)-oriented all cubic superlattices (SLs) were slightly harder than similar (100)-SLs.•At high temperatures, (100)-SLs were prone to interdiffusion while (111)-SLs were not.•A secondary hardness enhancement of > 40 GPa occurred for (111)-SLs with hexagonal TaN and 10 nm < Λ < 20 nm.Monolithic and superlattice films composed of TaN and TiN layers were synthesized using pulsed laser deposition on a variety of substrates, and were characterized using X-ray diffraction, transmission electron microscopy, and nanoindentation. Films deposited on square-symmetry substrate surfaces, e.g., (100)-oriented MgO, TiN, or TaN, were epitaxial (100)-oriented rock-salt (rs-)polymorphs at both 750 °C and 950 °C. On hexagonal-symmetry substrate surfaces, e.g., Al2O3(001), both TiN and TaN were epitaxial (111)-oriented rs-polymorphs at 750 °C, as were TiN films deposited at 950 °C. Similar TaN films deposited at 950 °C, however, were (001)-oriented hexagonal (h-)TaN on Al2O3(001) and > 2.5 nm away from rs-TiN(111) buffer layers, including in superlattices with thick TaN layers. Superlattices (all with a 3:7 TiN:TaN ratio, > 500 nm thick, which took > 13 h to complete) deposited at 750 °C behaved as expected, exhibiting clear superlattice peaks and a maximum hardness when the superlattice period was between 5 and 10 nm. No superlattice peaks were observed in the (100)-oriented isostructural rs-superlattices deposited at 950 °C, indicating that a significant degree of interdiffusion occurred, but the amount was orientation dependent, as (111)-oriented isostructural rs-layers were similar at the two temperatures. Non-isostructural superlattices deposited at 950 °C that had (111)-oriented rs-layers and (001) h-TaN layers also exhibited little interdiffusion and, importantly, had a secondary hardness maximum near a chemical repeat period of 15 nm, rendering it the hardest superlattice for these longer chemical repeat periods. These composite results indicate that multi-functionality could be built into polycrystalline coatings because the phases, chemical stabilities, and hardness properties of TiN/TaN superlattices can be strongly dependent on local orientation and chemical periodicity.

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.

•Local orientations were determined for Fe2O3 films on polycrystalline SrTiO3.•Hematite grew epitaxially over all orientation space of the perovskite substrate.•> 90% of film grains had a single orientation relationship with the substrate.•The preferred epitaxial orientation aligns the eutactic planes and directions.•Eutaxial growth is revealed as a general phenomenon using high-throughput methods.The grain-by-grain orientation relationships between an Fe2O3 film, grown using pulsed laser deposition, and a polycrystalline SrTiO3 substrate were determined using electron backscatter diffraction. This high-throughput investigation, we call combinatorial substrate epitaxy, enables the characterization of film growth on all grain orientations in a single experiment, allowing the determination of the preferred epitaxial orientation (PEO) of this non-isostructural film/substrate pair. Heavily-twinned rhombohedral α-Fe2O3 (hematite) grew epitaxially over the entire orientation space of the cubic perovskite substrate. Over 500 local orientation relationships (ORs) were investigated and more than 90% of these ORs, regardless of the interface plane normal, could be described using a single epitaxial OR  : 0001101¯0Fe2O311111¯0SrTiO3. This OR aligns the eutactic (nearly close-packed) planes and directions between these dissimilar crystal structures. Importantly, the growth of Fe2O3 on a single crystalline (100)-SrTiO3 results in several different orientation relationships. These results suggest that growth on high Miller-index (low-symmetry) surfaces provides more general information about the PEO than growth on low Miller-index (high-symmetry) surfaces. The epitaxial film growth on high Miller-index surfaces and the overwhelming observation of the eutaxial OR support the hypothesis that a very small number of simple crystallographic descriptors guide epitaxial film growth over all of orientation space, even for non-isostructural film/substrate pairs.

Substrate- and thickness-related effects on the oxygen surface exchange of La0.7Sr0.3MnO3 (LSM) thin films were investigated to understand better cathode reactivity in solid oxide fuel cells. Epitaxial (100)-oriented LSM films were fabricated on (100)-SrTiO3 and (110)-NdGaO3 substrates and were characterized using electrical conductivity relaxation. A strong substrate effect on the chemical surface exchange coefficient (kchem) was observed, with a higher kchem found for films on SrTiO3 than those on NdGaO3. Two distinct activation energies (Ea) were observed for kchem, which were assigned to two parallel exchange processes; the relative contributions from each depended on the substrate, film thickness, and temperature. For films coherently strained to the substrates, kchem values differed by almost an order of magnitude, whereas Ea was ∼1.5 (± 0.1) eV on both substrates. For relaxed films, kchem values differed only by a factor of 2, and Ea was ∼0.75 (± 0.1) eV on both substrates. We discuss the strain effect relative to the native surface exchange and the thickness effect relative to the extended defect populations in the films. The outcome of this study sheds light on how microstructural features affect surface chemistry in modified cathodes.Keywords: (La,Sr)MnO3; dislocations; solid oxide fuel cells; strain; surface exchange; thin films;

The oxygen surface exchange of La0.7Sr0.3MnO3 (LSM) thin films was investigated using the electrical conductivity relaxation (ECR) method. Epitaxial (100)-, (110)-, and (111)-oriented LSM films were fabricated on corresponding SrTiO3 (STO) substrates using pulsed laser deposition. The LSM films had well-controlled surface qualities, exhibited bulk-like steady-state electrical properties, and exhibited surface dominated responses in ECR. The chemical surface exchange coefficients (kchem) were determined and varied from ≈ 1 × 10− 6 to 65 × 10− 6 cm/s, depending on temperature and orientation, with activation energies of between 0.8 and 1.2 eV. At 800 °C, a four fold variation is observed in the kchem values, with (110)/(100) being the highest/lowest, explained well by the high activation energy for (110), ≈ 1.16 eV, and the low energy for (111) and (100), ≈0.83 eV.Research highlights► Smooth, flat (100), (110), and (111) oriented La0.7Sr0.3MnO3 films were fabricated with PLD. ► All films had surface dominated responses in electrical conductivity relaxation measurements. ► From 600 to 800 °C, chemical exchange coefficients varied from Kchem ≈ 1 to 65 × 10− 6 cm/s. ► The activation energy of Kchem varied: E(110) ≈ 1.16 eV, and E(111) and E(100) ≈ 0.83 eV. ► At 800 °C, a 4-fold variation in Kchem was observed, with (110) the highest and (100) the lowest.

The influence of substrate temperature, process gas, deposition pressure, and substrate type on the phase selection, orientation/epitaxy, and growth morphology of thin films in the SrNbOy (y≈3.0 or 3.5) family was investigated. Pulsed laser deposited films (from a Sr2Nb2O7 target) obtained in both oxygen and nitrogen atmospheres upon various substrates were characterized with X-ray diffraction, energy dispersive spectroscopy, atomic force microscopy, and transmission electron microscopy. In oxygen atmospheres, films adopted the (110)-layered perovskite structure of the target. Higher temperatures, lower pressures of oxygen, and use of (110)-oriented SrTiO3 substrates lead to highly crystalline, epitaxial films of Sr2Nb2O7. The use of nitrogen atmospheres resulted in cubic perovskite SrNbO3 formation: epitaxial, textured, or polycrystalline films were obtained depending on the substrate; no nitrogen incorporation could be observed on the anion sublattice. On SrTiO3, the cubic perovskite films followed a cube-on-cube epitaxy and planar defects were observed to occur on the (110) perovskite planes.Phase selection of SrNbOy films is influenced primarily by the process gas (in the 1–100 mTorr pressure range). In oxygen, films adopt the Sr2Nb2O7 (110)-layered perovskite structure (left). In nitrogen, films adopt the cubic perovskite SrNbO3 structure (right). The films’ microstructure depends, however, on the substrate type and temperature.

Two of three levels of hierarchical ferroelastic domains of γ-WO3 ceramics are shown to be piezo-responsive (polar) for surfaces of any general orientation. Importantly, the surface activity for the photocathodic reduction of Ag+ and photoanodic oxidation of Pb2+/Mn2+ ions is correlated to those same two levels of ferroelastic domains for all orientations.