Structural, electronic and magnetic properties of V2O5−x: An ab initio study

Electronic and thermodynamic properties of native point defects in V2O5: a first-principles study

The formation of native point defects in semiconductors and their behaviors play a crucial role in material properties. Although the native defects of V2O5 include vacancies, self-interstitials, and antisites, only oxygen vacancies have been extensively explored. In this work, we carried out first-principles calculations to systematically study the properties of possible native defects in V2O5. The electronic structure and the formation energy of each defect were calculated using the DFT+U method. Defect concentrations were estimated using a statistical model with a constraint of charge neutrality. We found that the vanadyl vacancy is a shallow acceptor that could supply holes to the system. However, the intrinsic p-type doping in V2O5 hardly occurred because the vanadyl vacancy could be readily compensated by the more stable donor, i.e., the oxygen vacancy and oxygen interstitial, instead of holes. The oxygen vacancy is the most dominant defect under oxygen-deficient conditions. However, under extreme O-rich conditions, a deep donor of oxygen interstitial becomes the major defect species. The dominant oxygen vacancy under synthesized conditions plays an important role in determining the electronic conductivity of V2O5. It induces the formation of compensating electron polarons. The polarons are trapped at V centers close to the vacancy site with the effective escaping barriers of around 0.6 eV. Such barriers are higher than that of the isolated polaron hopping (0.2 eV). The estimated polaron mobilities obtained from kinetic Monte Carlo simulations confirmed that oxygen vacancies act as polaron-trapping sites, which diminishes the polaron mobility by 4 orders of magnitude. Nevertheless, when the sample is synthesized at elevated temperatures, a number of thermally activated polarons in samples are quite high due to the high concentrations of oxygen vacancies. These polarons can contribute as charge carriers of intrinsic n-type semiconducting V2O5.

Fabrication of Cu1.5Mn1.5O4 Nanoparticles Using One Step Self-Assembling Route to Enhance Energy Consumption

The preparation of copper manganite (hopcalite, Cu1.5Mn1.5O4), as a single phase, was achieved by using a sustainable method of green synthesis. This method is based on the replacement of the conventional “brute force” ceramic preparation by the recent “soft force” green synthesis via the egg white assisted one-step method. In other words, we present a facile and rapid methodology to prepare the nanocrystalline Cu1.5Mn1.5O4 spinel as a single phase, compared to our previous work using ceramic and glycine-assisted combustion methods. The as-synthesized copper manganite was characterized using X-ray diffraction (XRD), Fourier-transform infrared (FTIR), energy-dispersive spectroscopy (EDS), and scanning electron microscope (SEM). We used a vibrating sample magnetometer to determine the magnetic properties of the prepared sample (VSM). XRD, FTIR, SEM, EDS and transmittance electron micrograph (TEM) resulted in synthesis of a successful cubic spinel Cu1.5Mn1.5O4 system with a sponge crystal structure. The particles of the prepared materials are polycrystalline in their nature and the sizes ranged between 50 and 100 nm. The magnetic measurement demonstrated that the generated nanostructure has been found to exhibit ferromagnetism at room temperature with an optimum saturation magnetization value (0.2944 emu/g).

Phase identification of vanadium oxide thin films prepared by atomic layer deposition using X-ray absorption spectroscopy

The chemical and local structures of vanadium oxide (VO_x) thin films prepared by atomic layer deposition (ALD) were investigated by soft X-ray absorption spectroscopy. It is shown that the as-deposited film was a mixture of VO₂ and V₂O₅ in disordered form, while the chemistry changed significantly after heat treatment, subject to the different gas environment. Forming gas (95% N₂ + 5% H₂) annealing resulted in a VO₂ composition, consisting mostly of the VO₂ (B) phase with small amount of the VO₂ (M) phase, whereas O₂ annealing resulted in the V₂O₅ phase. An X-ray circular magnetic dichroism study further revealed the absence of ferromagnetic ordering, confirming the absence of oxygen vacancies despite the reduction of V ions in VO₂ (V⁴⁺) with respect to the precursor used in the ALD (V⁵⁺). This implies that the prevalence of VO₂ in the ALD films cannot be attributed to a simple oxygen-deficiency-related reduction scheme but should be explained by the metastability of the local VO₂ structures.

X-ray absorption spectroscopy reveals the local structures of atomic-layer-deposited vanadium oxide films subject to heat treatments.

Electronic and Thermoelectric Properties of V2O5, MgV2O5, and CaV2O5

Developing new thermoelectric materials with high performance can broaden the thermoelectric family and is the key to fulfill extreme condition applications. In this work, we proposed two new high-temperature thermoelectric materials—MgV2O5 and CaV2O5—which are derived from the interface engineered V2O5. The electronic and thermoelectric properties of V2O5, MgV2O5, and CaV2O5 were calculated based on first principles and Boltzmann semi-classical transport equations. It was found that although V2O5 possessed a large Seebeck coefficient, its large band gap strongly limited the electrical conductivity, hence hindering it from being good thermoelectric material. With the intercalation of Mg and Ca atoms into the van der Waals interfaces of V2O5, i.e., forming MgV2O5 and CaV2O5, the electronic band gaps could be dramatically reduced down to below 0.1 eV, which is beneficial for electrical conductivity. In MgV2O5 and CaV2O5, the Seebeck coefficient was not largely affected compared to V2O5. Consequently, the thermoelectric figure of merit was expected to be improved noticeably. Moreover, the intercalation of Mg and Ca atoms into the V2O5 van der Waals interfaces enhanced the anisotropic transport and thus provided a possible way for further engineering of their thermoelectric performance by nanostructuring. Our work provided theoretical guidelines for the improvement of thermoelectric performance in layered oxide materials.

Thermal activation of charge carriers in ionic and electronic semiconductor β-AgIVVO3 and β-AgIVVO3@VV1.6VIV0.4O4.8 composite xerogels

Silver vanadium oxide (SVO) and Silver Vanadium Oxide/Vanadium Oxide (SVO@VO) composite hydrogels are formed from the self-entanglement of β-AgVO₃ nanoribbons and slightly reduced vanadium oxide (VO) (V^V_1.6V^IV_0.4O_4.8) nanoribbons; respectively. Starting from randomly distributed nanoribbons within hydrogels, and after a controlled drying process, a homogeneous xerogel system containing tuneable SVO : VO ratios from 1 : 0 to 1 : 1 can be obtained. The precise nanoribbons compositional control of these composite system can serve as a tool to tune the electrical properties of the xerogels, as it has been demonstrated in this work by impedance spectroscopy (IS) experiments. Indeed, depending on the composition and temperature conditions, composite xerogels can behave as electronic, protonic or high temperature ionic conductors. In addition, the electric and protonic conductivity of the composite xerogels can be enhanced (until a critical irreversible point), through the temperature triggered charge carrier creation. As concluded from thermogravimetry, IR, UV-Vis and EPR spectroscopy studies, besides the SVO : VO ratio, the thermal induced oxidation/reduction of V⁵⁺ to V⁴⁺, and thermally triggered release of strongly bonded water molecules at the nanoribbon surface are the two key variables that control the electric and ionic conduction processes within the SVO and composite SVO/VO xerogels.

Assembly of SVO and slightly reduced VO nanoribbons in inorganic hydrogels enables the formation of proton conductor and electron conductor xerogels depending on the SVO/VO ratio. Thermal charge carrier activation results in enhanced conductivity.

Anomalous lattice behavior of vanadium pentaoxide (V2O5): X-ray diffraction, inelastic neutron scattering and ab initio lattice dynamics

We present structural and dynamical studies of layered vanadium pentaoxide (V₂O₅). The temperature dependent X-ray diffraction measurements reveal highly anisotropic and anomalous thermal expansion from 12 K to 853 K. The results do not show any evidence of structural phase transition or decomposition of α-V₂O₅, contrary to the previous transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) experiments. The inelastic neutron scattering measurements performed up to 673 K corroborate the result of our X-ray diffraction measurements. The analysis of the experimental data is carried out using ab initio lattice dynamics calculations. The important role of van der Waals dispersion and Hubbard interactions in the structure and dynamics is revealed through ab initio calculations. The calculated anisotropic thermal expansion behavior agrees well with temperature dependent X-ray diffraction. The mechanism of anisotropic thermal expansion and anisotropic linear compressibility is discussed in terms of calculated anisotropy in the Grüneisen parameters and elastic coefficients. The calculated Gibbs free energy in various phases of V₂O₅ is used to understand the high pressure and temperature phase diagram of the compound.