Fast interfacial charge transfer in α-Fe2O3-δCδ/FeVO4-x+δCx-δ@C bulk heterojunctions with controllable phase content

Vibrational spectra and optical properties of Fe1-xCrxVO4 solid solutions: With a group theory analysis

The present manuscript reports vibrational spectra and optical studies of polycrystalline Fe_1-xCr_xVO₄ solid solutions through FT-IR spectroscopy augmented with a group theory (G.T.) analysis and UV-Visible DRS spectroscopy. Full set of IR and Raman modes are determined by G. T. for various crystal symmetries in FeVO₄-CrVO₄ solid solutions where Triclinic, Monoclinic and Orthorhombic structures evolve with increasing Cr concentration. Experimentally obtained vibrational modes support the structural phase transitions and confirm formation of continuous solid solutions in Fe_1-xCr_xVO₄. The Diffuse Reflectance Spectra (DRS) of Fe_1-xCr_xVO₄ depicts the electronic structure and different optical transitions due to absorption of photon energy. The d-d transitions are manifested for all compounds in terms of crystal field stabilization energy (CFSE) caused by distorted lattice sites. The band gap energy of Fe_1-xCr_xVO₄ is calculated using Tauc formula. It shows a red shift initially within triclinic structure then blue shift with the increase of Cr concentration. Urbach energy (E_u) tails in the spectra show the electronic structural disorder in Fe_1-xCr_xVO₄ due to impurity energy levels of Cr ions within band gap region. It is observed that E_u decreases with the doping concentration due to the increase in crystal symmetry corresponding to the structural phase transitions in Fe_1-xCr_xVO₄.

Stability of FeVO4 under Pressure: An X-ray Diffraction and First-Principles Study

The high-pressure behavior of the crystalline structure FeVO₄ has been studied by means of X-ray diffraction using a diamond-anvil cell and first-principles calculations. The experiments were carried out up to a pressure of 12.3 GPa, until now the highest pressure reached to study an FeVO₄ compound. High-pressure X-ray diffraction measurements show that the triclinic P1̅ (FeVO₄-I) phase remains stable up to ≈3 GPa; then a first-order phase transition to a new monoclinic polymorph of FeVO₄ (FeVO₄-II') with space group C2/ m is observed, having an α-MnMoO₄-type structure. A second first-order phase transition is observed around 5 GPa toward the monoclinic ( P2/ c) wolframite-type FeVO₄-IV structure, which is stable up to 12.3 GPa in coexistence with FeVO₄-II'. The unit cell volume reductions for the first and second phase transitions are Δ V = -8.5% and -13.1%. It was observed that phase transitions are irreversible and both high-pressure phases remain stable once the pressure is released. Calculations were performed at the level of the generalized gradient approximation plus Hubbard correction (GGA+ U) and with the hybrid Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functional in order to have a good representation of the pressure behavior of FeVO₄. We found that theoretical results follow the pressure evolution of structural parameters of FeVO₄, in good agreement with the experimental results. Also, we analyze FeVO₄-II (orthorhombic Cmcm, CrVO₄-type structure) and -III (orthorhombic Pbcn, α-PbO₂-type structure) phases and compare our results with the literature. Going beyond the experimental results, we study some possible post-wolframite phases reported for other compounds and we found a phase transition for FeVO₄-IV to raspite (monoclinic P2₁/ c) type structure (FeVO₄-V) at 36 GPa (Δ V = -8.1%) and a further phase transition to the AgMnO₄-type (monoclinic P2₁/ c) structure (FeVO₄-VI) at 66.5 GPa (Δ V = -3.7%), similar to the phase transition sequence reported for InVO₄.