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The titanomagnetites (Fe 2- x Ti x O 4 , x ⩽ 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe 2 TiO 4 were deposited by molecular beam epitaxy (MBE) on MgO(001) at 250-375 °C. The homogeneous incorporation of Ti, Fe valence state, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe 3 O 4 and rutile TiO 2 . Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe 3+ , Fe 2+ , Fe 0 ). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe 2+ . However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe 2+ was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe 2+ was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the low temperature deposition conditions imposed by use of MgO substrates limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized Fe 2 TiO 4 by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on thermally stable substrates., (© 2021 IOP Publishing Ltd.)

Recent investigations on spinel CoMn 2 O 4 have shown its potential for applications in water splitting and fuel cell technologies as it exhibits strong catalytic behavior through oxygen reduction reactivity. To further understand this material, we report for the first time the synthesis of single-crystalline Co 1+x Mn 2-x O 4 thin films using molecular beam epitaxy. By varying sample composition, we establish links between cation stoichiometry and material properties using in-situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, x-ray absorption spectroscopy, and spectroscopic ellipsometry. Our results indicate that excess Co ions occupy tetrahedral interstitial sites at lower excess Co stoichiometries, and become substitutional for octahedrally-coordinated Mn at higher Co levels. We compare these results with density functional theory models of stoichiometric CoMn 2 O 4 to understand how the Jahn-Teller distortion and hybridization in Mn-O bonds impact the ability to hole dope the material with excess Co. The findings provide important insights into CoMn 2 O 4 and related spinel oxides that are promising candidates for inexpensive oxygen reduction reaction catalysts.

Band gap engineering in SrTiO3 and related titanate perovskites has long been explored due to the intriguing properties of the materials for photocatalysis and photovoltaic applications. A popular approach in the materials chemistry community is to substitutionally dope aliovalent transition metal ions onto the B site in the lattice to alter the valence band. However, in such a scheme there is limited control over the dopant valence, and compensating defects often form. Here we demonstrate a novel technique to controllably synthesize Fe(2+)- and Fe(3+)-doped SrTiO3 thin films without formation of compensating defects by co-doping with La(3+) ions on the A site. We stabilize Fe(2+)-doped films by doping with two La ions for every Fe dopant, and find that the Fe ions exhibit a low-spin electronic configuration, producing optical transitions in the near infrared regime and degenerate doping. The novel electronic states observed here offer a new avenue for band gap engineering in perovskites for photocatalytic and photovoltaic applications.

We have investigated the intrinsic properties of SrCrO3 epitaxial thin films synthesized by molecular beam epitaxy. We find compelling evidence that SrCrO3 is a correlated metal. X-ray photoemission valence band and O K-edge x-ray absorption spectra indicate a strongly hybridized Cr3d-O2p state crossing the Fermi level, leading to metallic behavior. Comparison between valence band spectra near the Fermi level and the densities of states calculated using density functional theory (DFT) suggests the presence of coherent and incoherent states and points to strong electron correlation effects. The magnetic susceptibility can be described by Pauli paramagnetism at temperatures above 100 K, but reveals antiferromagnetic behavior at lower temperatures, possibly resulting from orbital ordering.

The structural properties of phase-pure epitaxial (Fe1-xCrx)2O3 thin films deposited on α-Al2O3(0 0 0 1) substrates by oxygen-plasma-assisted molecular beam epitaxy are investigated across the composition range using x-ray photoelectron spectroscopy, high-resolution x-ray diffraction, scanning transmission electron microscopy and electron energy loss spectroscopy, and non-Rutherford resonant elastic scattering measurements. The films possess a columnar grain structure with uniform mixing of cations on the nanometer scale. Fe-rich films are relaxed and appear to be slightly oxygen-rich, while Cr-rich films remain partially strained to the Al2O3 substrate and are found to be oxygen deficient. A model is proposed to explain the oxygen stoichiometry results based on the energetics of oxygen defect formation and rate of oxygen diffusion in the corundum lattice, and the dependence on the cation composition. Deliberately introducing residual compressive biaxial strain into (Fe1-xCrx)2O3 thin films (x = 0, 0.41, 0.52) by employing a Cr2O3 buffer layer is shown to narrow the optical bandgap, from 1.80(1) eV for relaxed (Fe0.47Cr0.53)2O3 to 1.77(1) eV for partially strained (Fe0.48Cr0.52)2O3. The relationships which are elucidated between epitaxial film structure and optical properties can be applied to bandgap optimization in the (Fe,Cr)2O3 system.

To develop a model system containing regularly spaced misfit dislocations for studies of the radiation resistance of nanoscale defects, epitaxial thin films of Cr, Mo, and Cr(x)Mo(1-x) alloys were deposited on MgO(001) by molecular beam epitaxy. Film compositions were chosen to vary the lattice mismatch with MgO. The film structure was investigated by x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and scanning transmission electron microscopy (STEM). Epitaxial films with reasonably high crystalline quality and abrupt interfaces were achieved at a relatively low deposition temperature, as confirmed by STEM. However, it was found by XRD and RBS in the channeling geometry that increasing the Mo content of the CrMo alloy films degraded the crystalline quality, despite the improved lattice match with MgO. XRD rocking curve data indicated that regions of different crystalline order may be present within the films with higher Mo content. This is tentatively ascribed to spinodal decomposition into Cr-rich and Mo-rich regions, as predicted by the Cr(x)Mo(1-x) phase diagram.