First-Principles Calculations on the Crystal/Electronic Structure and Phase Stability of H‑Doped SrFeO₂

It has been recently reported that H-doping of SrFeO₂ induces an insulator-to-metal transition but generates only a small amount of carrier electrons. To investigate the crystal structure of H-doped SrFeO₂ (SFOH) and the origin of its peculiar transport properties, we performed DFT-based first-principles calculations. Through structural sampling and total energy calculations, we showed that the doped hydrogen atoms exist in hydride form (H^–). Incorporation of the hydride drastically changed the valence state or the d-band configuration of the Fe ions adjacent to the doped hydride, resulting in a metallic density of states in specific hydride configurations. Thermodynamic analysis revealed that the formation of an insulating phase with O-site hydride was energetically preferable, but metallic phases with O-site hydride or interstitial hydride could also be present at reaction temperature, suggesting that SFOH is a mixture of an insulating matrix and metallic domains. This two-phase model accounted for the observed low carrier density as well as the metallic transport properties

Ferrimagnetism and Ferroelectricity in Cr-Substituted GaFeO₃ Epitaxial Films

GaFeO₃-type iron oxides are promising multiferroic materials due to the coexistence of a large spontaneous magnetization and polarization near room temperature. However, magnetic substitution, which is a general method to control multiferroic properties, is difficult due to instability of the substituted GaFeO₃. In this study, Ga_0.5Cr_0.5FeO₃ epitaxial thin films are successfully fabricated through epitaxial stabilization. These films exhibit in-plane ferrimagnetism and out-of-plane ferroelectricity simultaneously. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements of the Ga_0.5Cr_0.5FeO₃ film reveal that the oxidation states of the Fe and Cr ions are trivalent. In addition, some Fe ions are located at tetrahedral Ga1 sites. Compared to the GaFeO₃ film, the Ga_0.5Cr_0.5FeO₃ film shows a higher magnetic phase transition temperature (240 K), weaker saturation magnetization at 5 K, and a unique temperature dependence of the magnetization behavior. The effects of Cr substitution on the magnetic properties are strongly affected by the sites of the Fe³⁺ (3d⁵) and Cr³⁺ (3d³) ions. Furthermore, room-temperature ferroelectricity in the GaFeO₃ and Ga_0.5Cr_0.5FeO₃ films was demonstrated. Interestingly, the change in the ferroelectric parameters via Cr substitution is very small, which disagrees with the previously proposed polarization switching mechanism. Our findings are key to understanding the genuine polarization switching mechanism of the multiferroic GaFeO₃ system

Reversible Changes in Resistance of Perovskite Nickelate NdNiO₃ Thin Films Induced by Fluorine Substitution

Perovskite nickel oxides are of fundamental as well as technological interest because they show large resistance modulation associated with phase transition as a function of the temperature and chemical composition. Here, the effects of fluorine doping in perovskite nickelate NdNiO₃ epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiO_3–<i>x</i>F_<i>x</i> films is controlled with precision by varying the reaction time. The fully fluorinated film (<i>x</i> ≈ 1) is highly insulating and has a bandgap of 2.1 eV, in contrast to NdNiO₃, which exhibits metallic transport properties. Hard X-ray photoelectron and soft X-ray absorption spectroscopies reveal the suppression of the density of states at the Fermi level as well as the reduction of nickel ions (valence state changes from +3 to +2) after fluorination, suggesting that the strong Coulombic repulsion in the Ni 3d orbitals associated with the fluorine substitution drives the metal-to-insulator transition. In addition, the resistivity of the fluorinated films recovers to the original value for NdNiO₃ after annealing in an oxygen atmosphere. By application of the reversible fluorination process to transition-metal oxides, the search for resistance-switching materials could be accelerated