Insight into the Fergusonite–Scheelite Phase Transition of ABO₄‑Type Oxides by Density Functional Theory: A Case Study of the Subtleties of the Ground State of BiVO₄

BiVO4 (BVO) is an important photocatalytic and ferroelastic material. It has been extensively studied using density functional theory (DFT). However, on optimization, at a commonly employed level of theory using the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional, the monoclinic scheelite (ms-BVO) structure transforms into a higher-symmetry tetragonal scheelite (ts-BVO) phase spontaneously, which has also been confirmed by other groups. Such a transformation is highly unusual, as one would expect the transition to a lower symmetry structure to be modeled well at this level of theory, as is the case with, for example, the perovskite BaTiO3, and hints at a subtle interplay between structural and electronic properties. In this work, we demonstrate that this phase transition nevertheless can be described accurately with DFT but only using a hybrid density functional with ∼60% Hartree–Fock (HF) exchange. We find a soft phonon mode in ts-BVO, which corresponds to the phase transition from ts-BVO to ms-BVO associated with a double-well potential characterizing this phase transition, implying that the transition is of the second order. We find two key factors that can explain this surprising behavior. First, the polarizability of the Bi3+ ion, with an on-site contribution from the hybridization of its 6s and 6p states, is notably underestimated by DFT. Moreover, the effective radius of the Bi3+ ion proves to be too large. With the 60% HF exchange hybrid functional, the description of the polarizability of Bi3+ does not improve but the radii of the Bi3+ ions approach more realistic values. The polarizability of the O and V ions are reasonably described already by PBE. To gain further insight into the problem, we investigated the structural stability of other ABO4 oxides, including ScVO4, LaNbO4, YTaO4, and CaWO4, and related materials. Some of them have similar behavior to BVO, whose ground-state monoclinic structure proves to be unstable using commonly employed DFT approaches. In particular, for ScVO4, we found that the scheelite tetragonal and fergusonite monoclinic structures cannot be distinguished using the PBEsol functional. But the fergusonite monoclinic structure becomes stable using the hybrid functionals with high fractions of HF exchange, which points to the crucial role of the accurate ionic size reproduction by the method of choice as the on-site Sc3+ polarizability is too low to have a significant effect. Our findings would be of high interest for the study of other problematic materials with subtle size and polarization properties, especially ABO4 oxides that undergo similar phase transitions

BaBi₂O₆: A Promising n‑Type Thermoelectric Oxide with the PbSb₂O₆ Crystal Structure

Thermoelectric materials offer the possibility of enhanced energy efficiency due to waste heat scavenging. Based on their high-temperature stability and ease of synthesis, efficient oxide-based thermoelectrics remain a tantalizing research goal; however, their current performance is significantly lower than the industry standards such as Bi2Te3 and PbTe. Among the oxide thermoelectrics studied thus far, the development of n-type thermoelectric oxides has fallen behind that of p-type oxides, primarily due to limitations on the overall dimensionless figure of merit, or ZT, by large lattice thermal conductivities. In this article, we propose a simple strategy based on chemical intuition to discover enhanced n-type oxide thermoelectrics. Using state-of-the-art calculations, we demonstrate that the PbSb2O6-structured BaBi2O6 represents a novel structural motif for thermoelectric materials, with a predicted ZT of 0.17–0.19. We then suggest two methods to enhance the ZT up to 0.22, on par with the current best earth-abundant n-type thermoelectric at around 600 K, SrTiO3, which has been much more heavily researched. Our analysis of the factors that govern the electronic and phononic scattering in this system provides a blueprint for optimizing ZT beyond the perfect crystal approximation

Limits to doping of wide band gap semiconductors

The role of defects in materials is one of the long-standing issues in solid-state chemistry and physics. On one hand, intrinsic ionic disorder involving stoichiometric amounts of lattice vacancies and interstitials is known to form in highly ionic crystals. There is a substantial literature on defect formation and the phenomenological limits of doping in this class of materials; in particular, involving the application of predictive quantum mechanical electronic structure computations. Most wide band gap materials conduct only electrons and few conduct holes, but rarely are both modes of conduction accessible in a single chemical system. The energies of electrons and holes are taken from the vertical ionization potentials and electron affinities; polaronic trapping of carriers is excluded. While the focus here is defect energetics, the atomic and electronic structures have been carefully examined in all cases to ensure physical results were obtained.</p

Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs₃Bi₂Br₉

We report a study on the optical properties of the layered polymorph of vacancy-ordered triple perovskite Cs₃Bi₂Br₉. The electronic structure, determined from density functional theory calculations, shows the top of the valence band and bottom of the conduction band minima are, unusually, dominated by Bi s and p states, respectively. This produces a sharp exciton peak in the absorption spectra with a binding energy that was approximated to be 940 meV, which is substantially stronger than values found in other halide perovskites and, instead, more closely reflects values seen in alkali halide crystals. This large binding energy is indicative of a strongly localized character and results in a highly structured emission at room temperature as the exciton couples to vibrations in the lattice

Enhanced Photocatalytic and Antibacterial Ability of Cu-Doped Anatase TiO₂ Thin Films: Theory and Experiment

Multifunctional thin films which can display both photocatalytic and antibacterial activity are of great interest industrially. Here, for the first time, we have used aerosol-assisted chemical vapor deposition to deposit highly photoactive thin films of Cu-doped anatase TiO2 on glass substrates. The films displayed much enhanced photocatalytic activity relative to pure anatase and showed excellent antibacterial (vs Staphylococcus aureus and Escherichia coli) ability. Using a combination of transient absorption spectroscopy, photoluminescence measurements, and hybrid density functional theory calculations, we have gained nanoscopic insights into the improved properties of the Cu-doped TiO2 films. Our analysis has highlighted that the interactions between substitutional and interstitial Cu in the anatase lattice can explain the extended exciton lifetimes observed in the doped samples and the enhanced UV photoactivities observed

Limits to doping of wide band gap semiconductors

The role of defects in materials is one of the long-standing issues in solid-state chemistry and physics. On one hand, intrinsic ionic disorder involving stoichiometric amounts of lattice vacancies and interstitials is known to form in highly ionic crystals. There is a substantial literature on defect formation and the phenomenological limits of doping in this class of materials; in particular, involving the application of predictive quantum mechanical electronic structure computations. Most wide band gap materials conduct only electrons and few conduct holes, but rarely are both modes of conduction accessible in a single chemical system. The energies of electrons and holes are taken from the vertical ionization potentials and electron affinities; polaronic trapping of carriers is excluded. While the focus here is defect energetics, the atomic and electronic structures have been carefully examined in all cases to ensure physical results were obtained.</p

Limits to Doping of Wide Band Gap Semiconductors

Limits to Doping of Wide Band Gap Semiconductor

Single Step Solution Processed GaAs Thin Films from GaMe₃ and ^<i>t</i>BuAsH₂ under Ambient Pressure

This article reports on the possibility of low-cost GaAs formed under ambient pressure via a single step solution processed route from only readily available precursors, ^<i>t</i>BuAsH₂ and GaMe₃. The thin films of GaAs on glass substrates were found to have good crystallinity with crystallites as large as 150 nm and low contamination with experimental results matching well with theoretical density of states calculations. These results open up a route to efficient and cost-effective scale up of GaAs thin films with high material properties for widespread industrial use. Confirmation of film quality was determined using XRD, Raman, EDX mapping, SEM, HRTEM, XPS, and SIMS