Luminescence in Anion-Deficient Hafnia Nanotubes.

Hafnia-based nanostructures and other high-k dielectrics are promising wide-gap materials for developing new opto- and nanoelectronic devices. They possess a unique combination of physical and chemical properties, such as insensitivity to electrical and optical degradation, radiation damage stability, a high specific surface area, and an increased concentration of the appropriate active electron-hole centers. The present paper aims to investigate the structural, optical, and luminescent properties of anodized non-stoichiometric HfO2 nanotubes. As-grown amorphous hafnia nanotubes and nanotubes annealed at 700 °C with a monoclinic crystal lattice served as samples. It has been shown that the bandgap Eg for direct allowed transitions amounts to 5.65 ± 0.05 eV for amorphous and 5.51 ± 0.05 eV for monoclinic nanotubes. For the first time, we have studied the features of intrinsic cathodoluminescence and photoluminescence in the obtained nanotubular HfO2 structures with an atomic deficiency in the anion sublattice at temperatures of 10 and 300 K. A broad emission band with a maximum of 2.3-2.4 eV has been revealed. We have also conducted an analysis of the kinetic dependencies of the observed photoluminescence for synthesized HfO2 samples in the millisecond range at room temperature. It showed that there are several types of optically active capture and emission centers based on vacancy states in the O3f and O4f positions with different coordination numbers and a varied number of localized charge carriers (V0, V-, and V2-). The uncovered regularities can be used to optimize the functional characteristics of developed-surface luminescent media based on nanotubular and nanoporous modifications of hafnia.

Computational prediction of new magnetic materials.

The discovery of new magnetic materials is a big challenge in the field of modern materials science. We report the development of a new extension of the evolutionary algorithm USPEX, enabling the search for half-metals (materials that are metallic only in one spin channel) and hard magnetic materials. First, we enabled the simultaneous optimization of stoichiometries, crystal structures, and magnetic structures of stable phases. Second, we developed a new fitness function for half-metallic materials that can be used for predicting half-metals through an evolutionary algorithm. We used this extended technique to predict new, potentially hard magnets and rediscover known half-metals. In total, we report five promising hard magnets with high energy product (|BH|MAX), anisotropy field (Ha), and magnetic hardness (κ) and a few half-metal phases in the Cr-O system. A comparison of our predictions with experimental results, including the synthesis of a newly predicted antiferromagnetic material (WMnB2), shows the robustness of our technique.

Impact of oxygen content on preferred localization of p- and n-type carriers in La0.5Sr0.5Fe1-xMnxO3-δ.

The oxygen content in La0.5Sr0.5Fe1-xMnxO3-δ, measured by coulometric titration in a wide range of oxygen partial pressure at various temperatures, was used for defect chemistry analysis. The obtained data were well approximated by a model assuming defect formation in La0.5Sr0.5Fe1-xMnxO3-δvia Fe3+ and Mn3+ oxidation reactions and charge disproportionation on Fe3+ and Mn3+ ions. The partial molar enthalpy and entropy of oxygen in La0.5Sr0.5Fe1-xMnxO3-δ obtained by statistical thermodynamic calculations were found to be in satisfactory agreement with those obtained using the Gibbs-Helmholtz equations, thus further confirming the adequacy of the model. The impact of manganese substitution on defect equilibrium in La0.5Sr0.5Fe1-xMnxO3-δ was shown to be attributed to a lower enthalpy of Mn3+ oxidation reaction (vs. for the oxidation of Fe3+) and the charge disproportionation reaction on Mn3+ (vs. for that on Fe3+). The former makes Mn4+ ions more resistant to reduction than Fe4+. The latter favors the presence of Mn2+, Mn3+, and Mn4+ ions in oxides in comparable concentrations. The distribution of charge carriers over iron and manganese ions was determined as a function of oxygen content in La0.5Sr0.5Fe1-xMnxO3-δ.

Synthesis of Lithium Phosphorus Oxynitride (LiPON) Thin Films by Li3PO4 Anodic Evaporation in Nitrogen Plasma of a Low-Pressure Arc Discharge.

Thin amorphous films of LiPON solid electrolyte were prepared by anodic evaporation of lithium orthophosphate Li3PO4 in an arc discharge with a self-heating hollow cathode at a nitrogen pressure of 1 Pa. Distribution of the arc current between two electrodes having an anode potential provided independent control of the evaporation rate of Li3PO4 and the density of nitrogen plasma. Stabilization of the evaporation rate was achieved using a crucible with multi-aperture cover having floating potential. The existence of a threshold value of discharge current (40 A) has been established, which, upon reaching ionic conductivity over 10-8 S/cm, appears in the films. Probe diagnostics of discharge plasma were carried out. It has been shown that heating the films during deposition by plasma radiation to a temperature of 200 °C is not an impediment to achieving high ionic conductivity of the films. Dense uniform films of LiPON thickness 1 µm with ionic conductivity up to 1 × 10-6 S/cm at a deposition rate of 4 nm/min are obtained.

Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder X-ray Diffraction, and Density Functional Theory Calculations.

The structures of several fluoroscandate compounds are presented here using a characterization approach combining powder X-ray diffraction and solid-state NMR. The structure of K5Sc3F14 was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF4, Li3ScF6, KSc2F7, and Na3ScF6 compounds were studied in detail from solid-state 19F and 45Sc NMR experiments. The 45Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The 19F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the 19F and 45Sc NMR parameters were carried out using plane-wave basis sets and periodic boundary conditions (CASTEP), and the results were compared with the experimental data. A good agreement between the calculated shielding constants and experimental chemical shifts was obtained. This demonstrates the good potential of computational methods in spectroscopic assignments of solid-state 45Sc NMR spectroscopy.