Maged F. Bekheet

Orthorhombic In2O3: A Metastable Polymorph of Indium Sesquioxide

The way is open for the physical and chemical characterization and single-crystal growth of the orthorhombic o′-In2O3 polymorph. Orthorhombic In2O3 is synthesized from rhombohedral corundum-type rh-In2O3 under moderately high-pressure and high-temperature conditions (8–9 GPa, 600–1100 °C) followed by recovery to ambient pressure and temperature. The crystal-structure data at ambient conditions confirm unambiguously the Rh2O3(II)-type structure.

In situ high pressure high temperature experiments in multi-anvil assemblies with bixbyite-type In2 O3 and synthesis of corundum-type and orthorhombic In2 O3 polymorphs

Our in situ high pressure high temperature experiments in multi-anvil assemblies unambiguously evidence the stability of bixbyite-type c-In2O3 at 6 GPa from room temperature to ca. 600°C. At 5.5 GPa and ca. 1100°C, c-In2O3 reacts with free carbon from the amorphous Si‒B‒C‒N capsule being reduced to metallic indium. The material recovered from the ex situ multi-anvil experiment at 6 GPa and 1100°C using the Mo capsule is inhomogeneous, thereby its phase composition depends on the specimen position from the furnace midline that in turn is characterized by the inhomogeneous temperatures. In the midpoint of the furnace, at the highest temperature point, c-In2O3 completely transforms into a corundum-type rh-In2O3 polymorph that is recovered under ambient conditions, as confirmed by X-ray powder and electron diffraction and Raman spectroscopy. Transmission electron microscopic characterization indicates the growth of single crystals of corundum-type rh-In2O3 with an average crystal size of ∼3 μm in the specimen part away from the furnace midline. The automated electron diffraction tomography analysis and X-ray powder-diffraction point out at the possible formation of orthorhombic In2O3 polymorphs.

Electronic, optical and photocatalytic properties of three-layer perovskite Dion–Jacobson phase CsBa2M3O10 (M = Ta, Nb): a DFT study

Structure, electronic and optical properties have been studied to consider the photocatalytic activity of the three-layer perovskite Dion–Jacobson phase CsBa2M3O10 (M = Ta, Nb) using the density functional theory (DFT) method with the plane-wave pseudo-potential in the frame of the GGA proposed by Perdew, Burke, and Ernzerhof (PBE). CsBa2Ta3O10 and CsBa2Nb3O10 are indirect band gap semiconductors with different types of M–O bonds, from pure covalent to pure ionic. According to the symmetry breaking of MO6 octahedra from the Oh point group to centrosymmetric (D4h) and non-centrosymmetric (C4v) point groups, the generation of local internal fields, the promotion of electron–hole pairs separation in the very initial process of photo-oxidation and the improvement in photocatalytic activity are suggested. Site and angular projected densities of state for CsBa2M3O10 show that the valence and conduction bands are made mainly of O and M orbitals, therefore the presence of a dipole moment at the MO6 octahedra will play an important role in separating electrons and holes. According to the optical properties simulation, the origin of interband transitions for CsBa2Ta3O10 and CsBa2Nb3O10 are from O-p and M-d orbitals. CsBa2Ta3O10 and CsBa2Nb3O10 are UV-light-responsive photocatalysts with plasmon energies of 13.16 and 12.07 eV, respectively, which can possibly oxidize/decompose organic pollutants and generate hydrogen from water splitting.

Low temperature synthesis of nanocrystalline MnIn2O4 spinel

Manganese indate, MnIn(2)O(4), with magnetic Mn(2+) ions on A-sites was synthesized by coprecipitation method followed by the calcination of the resulting precursor in air at 500 °C.

Effective masses, electronic and optical properties of (111)-layered B-site deficient hexagonal perovskite Ba5M4O15 (M = Ta, Nb): a DFT study using HSE06

Equilibrium lattice parameters, electronic structures and optical properties of (111)-layered B-site deficient hexagonal perovskite Ba5M4O15 (M = Ta, Nb) were studied by first-principles computations on the basis of density functional theory using the norm-conserving-type pseudo-potential technique and screened nonlocal exchange-correlation functional HSE06 as defined by Heyd, Scuseria, and Ernzerhof. The calculated band dispersions showed that Ba5Ta4O15 and Ba5Nb4O15 are indirect band gap materials (A → G) with band gaps of 3.81 and 3.56 eV, respectively. The effective masses of photogenerated electrons and holes for Ba5Ta4O15 and Ba5Nb4O15 were evaluated in two principal directions at the G (Gamma) point. The Ta–O and Nb–O bonds in the MO6 octahedral environments have polar covalent nature due to the p–d hybridization between O-2p and Ta-5d or Nb-4d orbitals. Since the valence and conduction bands of Ba5Ta4O15 and Ba5Nb4O15 mainly consist of O-2p and Ta-5d or Nb-4d states, changes in the structure of the MO6 octahedral units can be effective for the band gap energy and consequently photocatalytic activity of Ba5Ta4O15 and Ba5Nb4O15. The optical analysis revealed that the main peak of the imaginary part of the complex dielectric function of Ba5Ta4O15 and Ba5Nb4O15 corresponds to the interband electronic transition from O-2p to Ta-5d or Nb-4d. Also, anisotropies in the effective masses of photogenerated charge carriers and static dielectric tensors of Ba5Ta4O15 and Ba5Nb4O15 in an arbitrary crystallographic direction are presented. High photocatalytic activity of Ba5Ta4O15 and Ba5Nb4O15 for hydrogen generation from water splitting and photodegradation of organic pollutants and/or dye molecules under UV light is related to the light effective masses of photogenerated charge carriers. For the efficient solar-energy conversion, the electronic band structures, such as band-edge position and band gap, of Ba5Ta4O15 and Ba5Nb4O15 can be tuned by doping.

Structural investigations of La0.6Sr0.4FeO3−δ under reducing conditions: kinetic and thermodynamic limitations for phase transformations and iron exsolution phenomena

The crystal structure changes and iron exsolution behavior of a series of oxygen-deficient lanthanum strontium ferrite (La0.6Sr0.4FeO3−δ, LSF) samples under various inert and reducing conditions up to a maximum temperature of 873 K have been investigated to understand the role of oxygen and iron deficiencies in both processes. Iron exsolution occurs in reductive environments at higher temperatures, leading to the formation of Fe rods or particles at the surface. Utilizing multiple ex situ and in situ methods (in situ X-ray diffraction (XRD), in situ thermogravimetric analysis (TGA), and scanning X-ray absorption near-edge spectroscopy (XANES)), the thermodynamic and kinetic limitations are accordingly assessed. Prior to the iron exsolution, the perovskite undergoes a nonlinear shift of the diffraction peaks to smaller 2θ angles, which can be attributed to a rhombohedral-to-cubic (Rc to Pmm) structural transition. In reducing atmospheres, the cubic structure is stabilized upon cooling to room temperature, whereas the transition is suppressed under oxidizing conditions. This suggests that an accumulation of oxygen vacancies in the lattice stabilize the cubic phase. The exsolution itself is shown to exhibit a diffusion-limited Avrami-like behavior, where the transport of iron to the Fe-depleted surface-near region is the rate-limiting step.

Scaled-up solvothermal synthesis of nanosized metastable indium oxyhydroxide (InOOH) and corundum-type rhombohedral indium oxide (rh-In2O3)

Abstract Phase pure metastable indium oxyhydroxide (InOOH) with crystallite size in the range ca. 2–7 nm is synthesized by a nonaqueous solvothermal synthesis route in ethanol. The influence of synthesis parameters such as temperature, basicity (pH), synthesis time, and water content is carefully addressed. T-pH maps summarize the impact of synthesis temperature and pH and reveal that phase pure InOOH is obtained in water-free solutions at mild temperatures (150–180°C) in highly basic conditions (pH>12). Subsequent calcination of InOOH at 375–700°C in ambient air atmosphere results in metastable nanoscaled rhombohedral indium oxide (rh-In2O3). The synthesis protocol for phase pure nanocrystalline InOOH material was successfully upscaled allowing for obtaining ca. 3 g of phase-pure InOOH with a yield of ca. 78%. The upscaled InOOH and rh-In2O3 batches are now available for a detailed in-situ characterization of the mechanism of decomposition of InOOH to rh-In2O3 to c-In2O3 as well as for the characterization of the functional properties of InOOH and rh-In2O3 materials.