Oxygen Sublattice Research Articles

Band gap engineering and transport properties of Ba2In2O5: effect of fluorine doping and hydration

Two types of fluorine doped barium indate solid solutions were prepared by a solid state method: Ba2-0.5xIn2O5-xFx (x = 0, 0.1, 0.2) and Ba2In2O5-0.5yFy (y = 0, 0.1, 0.25). Good agreement between the theoretical and experimental values of the pycnometric densities for the studied solid solutions confirms these two distinctly different models of solid solution formation. According to analysis of the XRD data introduction of fluorine ions into the oxygen sublattice of barium indate leads to a decrease in the a lattice parameter and unit cell volume for both types of solid solutions. The effect of fluorine doping and hydration on the band gap of barium indate was studied by means of diffuse reflectance spectroscopy. The estimated value of the band gap width Eg for undoped Ba2In2O5 is 2.94 eV and is in good agreement with literature data. Introduction of fluorine results in a slight increase of Eg and emergence of an additional absorption band in the region of 2.56-2.65 eV, near the fundamental absorption edge, which can be attributed to the -defects appearing upon fluorine doping. The increase of Eg with fluorine introduction correlates well with the decrease of the electronic transport numbers and can be explained by two competing effects: (1) lowering of the top of the valence band due to replacement of O2- ions by F-; and (2) changes in the local structure, i.e., lattice contraction and tilting of the InO(F)x framework. Hydration of the barium indate also leads to an increase of Eg, which was attributed to structural transformation from orthorhombic symmetry to tetragonal. However, with an increase in the fluorine concentration such changes of Eg become less pronounced because of the decrease of the hydration degree due to formation of -defects.

Фазовые переходы в гафнате свинца в условиях высокого давления

Phase transitions in lead hafnate (PbHfO3) at high temperature and high pressure were studied by single-crystal diffraction and diffuse scattering of synchrotron radiation, using heated diamond anvil cell. Measurements were carried out in the temperature range 182 ° С <T <316 ° C and the pressure range 1.6 GPa <P <2.6 GPa. Four areas, characterized by different patterns of diffraction, have been identified on temperature increase, corresponding to antiferroelectric phase, a set of modulated phases, and two phases with distortions of the oxygen sublattice. In the lower-temperature phase of the latter two, a temperature-dependent diffuse scattering maximum is observed. It corresponds to a non-zero wave vector, and indicates the possible nature of the transition to the modulated phase as a transition due to condensation of an incommensurate soft mode.

Thin-film stabilization of LiNbO3-type ZnSnO3 and MgSnO3 by molecular-beam epitaxy

In polar crystals, cooperative ionic displacement produces a macroscopic spontaneous polarization. Among such polar materials, LiNbO3-type wide bandgap oxides are particularly appealing because they offer useful ferroelectric properties and also potentially lead to multiferroic materials. Using molecular-beam epitaxy, we investigated the thin-film growth of high-pressure phase LiNbO3-type ZnSnO3 and discovered a polar oxide candidate, MgSnO3. We found that LiNbO3-type substrates play an essential role in the crystallization of these compounds, though corundum-type Al2O3 substrates also have the identical crystallographic arrangement of oxygen sublattice. Optical transmittance and electrical transport measurements revealed their potential as a transparent conducting oxide. Establishment of a thin-film synthetic route would be the basis for exploration of functional polar oxides and research on conduction at ferroelectric interfaces and domain walls.

Atomic Modeling and Electronic Structure of Mixed Ionic–Electronic Conductor SrTi1–xFexO3–x/2+δ Considered as a Mixture of SrTiO3 and Sr2Fe2O5

As mixed ionic–electronic conductors (MIECs), ABO3 perovskite oxides and derivative compounds are candidates for applications such as solid oxide fuel and electrolysis cells. Understanding the atomic configuration and electronic structure of MIECs is important because they form the basis of ionic and electronic conductivities; but it is challenging because the materials tend to be non-dilute systems with substantial partial occupancies of the perovskite sublattices. In this work, we present a computational model for Fe-substituted SrTiO3 (STF, SrTi1–xFexO3–x/2+δ), a representative perovskite-derivative MIEC, by considering it as a mixture of perovskite SrTiO3 (x = 0, δ = 0) and brownmillerite Sr2Fe2O5 (x = 1, δ = 0). Our model accounts for disorder in the form of Ti and Fe species on the perovskite B-site sublattice and of O atoms and vacancies VO on the oxygen sublattice. The defect chemistry and electronic structure of STF across the full composition range 0 ≤ x ≤ 1 and for small |δ| is addressed within...

First-Principles Calculations of the Electronic Structure of Imperfect Crystals in the Coherent Potential Approximation

The results of electronic structure calculations of imperfect crystals—nonstoichiometric metal oxides (TiO2 – x, TiO1 – x, Al2O3 – x) are reviewed. All calculations were performed within the density functional theory in the coherent potential approximation with vacancies randomly distributed in metal and oxygen sublattices. It is found that the deviations from stoichiometric composition are accompanied by appearance of vacancy induced electronic states inside the band gap of initial insulating oxides.

Electrical Properties of Fluoro-Substituted Perovskites Ba2 − 0.5xCaNbO5.5 −xFx

The effect of F--doping on the transport properties of perovskite-type complex oxides based on barium—calcium niobate Ba2CaNbO5.5 is analyzed. It is found that, regardless of the mechanism of introducing fluoride ions into the oxygen sublattice, the О2– and Н+ conductivities increase in the range of low fluoride concentrations due to an increase in the mobility of current carriers.

The influence of fluorine doping on transport properties in the novel proton conductors Ba4In2Zr2O11-0.5xFx with perovskite structure

The effect of fluorine doping on the transport properties of perovskite-related complex oxides based on oxygen-ion conductor Ba4In2Zr2O11 is analyzed. The effect of fluorine doping and air humidity on the cell volume is established. The introduction of fluorine into the oxygen sublattice leads to the decreasing in the cell volume. The hydration of samples leads to the lattice expansion for both undoped and doped samples in comparison with unhydrated samples. The amount of water uptake decreases with the increase in the fluorine concentration. It was shown that fluorine doping allows to increase the proton conductivity in the region of low fluorine concentrations. The determining factor is not the change in the concentration of the current carrier, but the increase in its mobility.

On the Mechanism of Thermal Expansion of Orthorhombically Modified Copper Pyrovanadate

A set of X-ray diffraction data on the crystal-lattice transformations of the α-Cu2V2O7 phase demonstrates that, in regions with an anomalous negative and near-zero volumetric thermal expansion, both vanadium‒oxygen and copper‒oxygen sublattices are maximally deformed. The near-zero and negative volumetric thermal expansion of the α-Cu2V2O7 unit cell arises from the degeneration of zigzag copper‒oxygen columns at invariable distances between Cu–O sublattice layers.

Energetics of defect production in fluorite-structured CeO2 induced by highly ionizing radiation

Understanding the energetics of radiation-induced defects is critical to the development of next-generation materials for nuclear and other energy systems and for aerospace applications. However, it remains a challenge to experimentally characterize defect morphologies and energies, especially in regard to anion defects in irradiated oxides. Here, using a combination of advanced structural and calorimetric characterization techniques, we show that the energetic response of defects in ${\mathrm{CeO}}_{2}$ is strongly coupled to atomic disordering on the oxygen sublattice induced by energetic heavy ions. Fitting of calorimetric data yields an estimate of stored energy in the form of defects and microstrain. These results provide a means to calculate the efficiency of structural destabilization and aid in a better understanding of defect formation and annealing mechanisms in fluorite-structured materials subject to extreme conditions.

Structure evolution of h.c.p./c.c.p. metal oxide interfaces in solid-state reactions.

The structure of crystalline interfaces plays an important role in solid-state reactions. The Al2O3/MgAl2O4/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl2O4 layers have been grown between Al2O3 and MgO, and the atomic structure of Al2O3/MgAl2O4 interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (h.c.p.) stacking in Al2O3 to cubic close-packed (c.c.p.) stacking in MgAl2O4. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the Al2O3/MgAl2O4 interface into Al2O3 is accomplished by the glide of partial dislocations accompanied by the exchange of Al3+ and Mg2+ cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAl2O4 grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent Σ3 grain boundaries form. The newly grown MgAl2O4 grains compete with each other, leading to a growth selection and successive coarsening of the MgAl2O4 grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar h.c.p./c.c.p. transition.

Electronic Structure of Aluminum Oxide with Oxygen Vacancies

Results of numerical calculations of the electronic structure of nonstoichiometric aluminum oxide with a concentration of oxygen vacancies of 6% have been presented. The calculations have been performed within the scope of the density-functional theory of the coherent-potential approximation with a disordered location of vacancies. It has been established that the presence of oxygen vacancies leads to the appearance of a peak in the density of states inside the energy gap and additional electronic states at the bottom of the conduction band, which gives a decrease in the energy gap to 2 eV. The simulation of the aluminum oxide of composition Al2[O0.98]3 $${\text{O}}_{{{\text{0}}{\text{.06}}}}^{{{\text{int}}\,{\text{erstitial}}}}$$ with vacancies in the oxygen sublattice and oxygen atoms in interstices leads to a semiconducting character of the energy spectrum with a band gap of ~1 eV, which is formed between the p states of the impurity interstitial oxygen atoms and the s states of the vacancies.

Direct Visualization and Image Simulations of Oxygen Sublattice Occupancy in Thin Cuprate Films

Direct Visualization and Image Simulations of Oxygen Sublattice Occupancy in Thin Cuprate Films

Density functional theory-based ab initio molecular dynamics simulation of ionic conduction in N-/F-doped ZrO2 under epitaxial strain

In this study, we investigated oxide ion conduction in N-/F-doped ZrO2 systems under tensile epitaxial strain by ab initio molecular dynamics (MD) simulations. In our previous study, we discussed the effects of lattice strain, oxygen vacancies, and cation dopants on oxide ion conduction in Y2O3-stabilized ZrO2/SrTiO3 heterostructures. In the present study, we demonstrate that at a certain oxygen vacancy concentration, N-/F-doping can enhance the oxide ion conductivity of ZrO2 systems. We discuss the effects of N-/F-doping from the point of view of structural changes in the oxygen sublattice of the systems. We found that anion doping caused flipping of the oxygen sublattice structure, which was enhanced with increasing anion concentration. This flipping motion played an important role in enhancing the oxide ion conductivity of ZrO2.

Pressure-Induced Sublattice Disordering in SnO_{2}: Invasive Selective Percolation.

SnO_{2} powders and single crystal have been studied under high pressure using Raman spectroscopy and abinitio simulations. The pressure-induced changes are shown to drastically depend on the form of the samples. The single crystal exhibits phase transitions as reported in the literature, whereas powder samples show a disordering of the oxygen sublattice in the first steps of compression. This behavior is proposed to be related to the defect density, an interpretation supported by abinitio simulations. The link between the defect density and an amorphouslike Raman signal is discussed in terms of the invasive percolation of the anionic sublattice. The resistance of the cationic sublattice to the disorder propagation is discussed in terms of cation close packing. This result on SnO_{2} may be extended to other systems and questions a "traditional" crystallographic description based on polyhedra packing, as a decoupling between both sublattices is observed.

Catalytic effect of nanostructured CeO2 coating on the electrochemical performance of Li(Li,Ni,Mn,Co)O2

CeO2 coatings (1.0, 3.5 and 5.0 mass%) have been obtained at the surface of 150–200 nm Li[Li0.13Ni0.2Mn0.47Co0.2]O2 (LLNMC) particles. According to TEM and STEM-EDX data, at 1% CeO2, the coating consists of 20–60 nm spherical ceria particles, while at 5%, their size is increased to 150–200 nm. The reversible electrochemical capacity of the coated samples is larger than for bare LLNMC, especially at higher discharge rates. The maximum capacity is observed for the 1% CeO2-coated sample (235 mAh g−1 at U = 2.0–4.8 V). It is essential that the extra capacity of CeO2-coated samples is appeared since the first cycles and, hence, could not be attributed to the protection of LLNMC from the electrochemical degradation by CeO2. The cyclic voltammetry curves of the 1% coated sample demonstrate a considerable effect at U = 4.2–4.6 V that could be associated with redox processes in the oxygen sublattice of LLNMC. Taking into account a significant activation of the oxygen species at the surface of ceria nanoparticles, the observed capacity increase of LLNMC could be affiliated with the catalytic effect of CeO2 on the oxidation of lattice oxygen in LLNMC rather than the protecting effect of ceria particles.

Evidence of random distribution of carbon impurities in oxygen sites of zinc oxide

A density functional study of substitutional carbon impurities in ZnO-host matrix with 6.25 at. % was performed applying both the supercell method and the coherent potential approximation. The first of these methods assumes the spatially ordered arrangement of carbon impurities in oxygen sites, whereas the second one simulates the spatially disordered impurity substitution of oxygen sublattice. In conjunction with a theoretical part, the X-ray photoelectron spectra of pure and C-doped zinc oxide were measured. The mapping of the valence band of the carbon-loaded ZnO allows establishing the appearance of impurity C 2p electronic states on the valence band top. The experimental energy position of these impurity C 2p states is theoretically reproducing essentially better by the assumption about random distribution of carbon impurities in oxygen sublattice.

Local Structure Adaptations and Oxide Ionic Conductivity in the Type III Stability Region of (1 – x)Bi2O3·xNb2O5

Starting from a previously published stoichiometric model for the commensurate Type III phase in the (1 – x)Bi2O3·xNb2O5 system, Bi94Nb32O221 (x = 0.254), we have developed a crystal-chemical model of this phase across its solid–solution range 0.20 ≤ x ≤ 0.26. After using annular dark-field scanning transmission electron microscopy to identify the metal sites that support nonstoichiometry, we show that the maximum possible range of that nonstoichiometry is 0.198 ≤ x ≤ 0.262, perfectly consistent with the experimental result. Intersite cation defects on these sites provide some local coordinative flexibility with respect to the surrounding oxygen sublattice, but not enough to create continuous fluorite-like channels like those found in the high-temperature incommensurate Type II phase. This explains the reduced oxide-ionic conductivity of Type III compared to Type II at all temperatures and compositions, regardless of which phase is thermodynamically stable under those conditions. The solid–solution model ...

Mictomagnetic State in an EuBaCo2–xO5.5–δ Single Crystal

The magnetic properties of an EuBaCo1.9O5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo2O5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 A, b = 7.833 A, and c = 7.551 A. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below TC = 242 K. At T < 300 K, all three spin states of the Co3+ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co3+ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co3+ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo1.9O5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo1.9O5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.

Synthesis of MnNb2O6 with hierarchical structure as a novel electrode material for high-performance supercapacitors

Columbite compound MnNb2O6, consisted the layers with slightly distorted hexagonal-closed-packed oxygen sublattice, has abundant reaction sites whereby the interlayered ions can occur faradic reaction to enhance the charge storage capability. Here, we first report three-dimensional nanostructure of MnNb2O6 as electrode materials for electrochemical pseudocapacitor synthesized through hydrothermal method. The crystal morphologies of MnNb2O6 show an evolution with the hydrothermal time, from primary spindle-like particles grown for 12 h and undergrown star-like crystals for 18 h, to hierarchical star-like crystals for 1 day and overgrown crystals for 3 days. MNO-1d sample synthesized for 1 day possesses ordered six-leaf nanostructure composed of hierarchical nano-sheets (thickness ∼30 nm). To explore its potential applications for supercapacitor, MnNb2O6 sample as electrode material is characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge measurements. MNO-1d demonstrates an outstanding specific capacitance of 350 F g−1 and rate capability with a high capacitance retention of ∼90% of its initial capacitance after 10000 cycles at a high current density of 10 A g−1.

Is Pauling’s Hypothesis Violated in the Hexagonal Modification of Ice?

The effect of long-range dipole interaction on the fulfilment of Pauling’s hypothesis concerning the degeneracy of proton configurations satisfying the ice rules is studied for the case of hexagonal ice. It is shown that the ice rules imply a significant reduction of the contributions to the ground state energy from the next-nearest neighbors. The ground state energy of proton subsystem as a function of quasimomentum ceases to change if more than a hundred primitive unit cells in each direction are taken into account. The final amplitude of the energy changes are about 70 K. The analysis reveals the proton ordering in hexagonal ice with the simple unit cell of the oxygen sublattice is impossible. However, that does not exclude proton ordering with a more complex cell caused by distortion of the hexagonal oxygen lattice.