Hungru Chen

A new potential model for barium titanate and its implications for rare-earth doping

We present a new set of interatomic potentials for modelling the BaTiO3 perovskite system. The potential model is fitted using multiple parameters to a range of experimental and ab initio data including the cohesive energy and lattice parameters of BaTiO3, BaO and rutile TiO2. This procedure provides internal consistency to the potential model for studying the energetics of the defect chemistry of BaTiO3. This is tested by examining rare-earth cation doping in BaTiO3 and considering all five possible compensation schemes. Our simulations are in agreement with experiment and predict small rare-earth cations to dope exclusively on the Ti site; medium sized rare-earth cations to dope on both the Ti and Ba sites and large rare-earth cation doping exclusively on the Ba-site. For Ba-site substitution the simulations predict electron compensation to be energetically unfavourable compared to the formation of Ti vacancies.

Effects of cationic substitution on structural defects in layered cathode materials LiNiO2

The electrochemical properties of layered rock salt cathode materials are strongly influenced by defects. The three most common defects in LiNiO2-based compounds, namely extra Ni, Li–Ni anti-site and oxygen vacancy defects have been investigated. The calculated defect formation energies are very low in LiNiO2, consistent with the difficulty in synthesizing stoichiometric defect-free LiNiO2. A systematic study is conducted to examine the effect of Co, Mn and Al substitution on defect formation. It is shown that the presence of Ni2+ in the Li layer can be rationalized using ideas of superexchange interactions. In addition, a correlation between oxygen vacancy formation energy and oxygen charge is noted. This explains the better thermal stability obtained by early transition metal or Al substitutions.

Crystal structure, defect chemistry and oxygen ion transport of the ferroelectric perovskite, Na0.5Bi0.5TiO3: insights from first-principles calculations

Recent experimental studies have shown how A-site nonstoichiometry in Na0.5Bi0.5TiO3 (NBT) can dramatically alter its electrical properties and conduction mechanisms. In nominal NBT and Bi-deficient NBi0.49T, electrical conductivity primarily comes from oxygen ion conduction and not electronic conduction. This is contrary to the behaviour of traditional titanates and could potentially give NBT a role in the design of new intermediate-temperature solid oxide fuel cells (SOFCs). In this study, we use density functional theory (DFT) with the Hubbard U correction to investigate the much debated local structure and defect chemistry of NBT, with the primary focus on oxygen vacancy formation and oxygen ion transport. We confirm significant cation and oxygen displacement in both the Cc and R3c structures. Small ordering energies confirm an essentially random distribution of A-site ions. Oxygen vacancies are shown to preferentially form in the vicinity of Bi ions, confirming that weak Bi–O bonds do indeed promote oxygen ion migration. Nudged elastic band (NEB) calculations predict oxygen migration energies that are in excellent agreement with experiment. Oxygen ion migration to Bi-rich chemical environments is generally shown to be unfavourable, while migration to Na-rich chemical environments produces lower migration energies. Our calculations confirm many of the assumptions recently reported in experimental studies and provide a far greater understanding into the unique and complex defect chemistry of NBT than is currently available.

First principles study of intrinsic point defects in hexagonal barium titanate

Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states, and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both mono- and di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of va...

First-Principles Calculations of Oxygen Vacancy Formation and Metallic Behavior at a β-MnO2 Grain Boundary

Nanostructured MnO2 is renowned for its excellent energy storage capability and high catalytic activity. While the electronic and structural properties of MnO2 surfaces have received significant attention, the properties of the grain boundaries (GBs) and their contribution to the electrochemical performance of the material remains unknown. Through density functional theory (DFT) calculations, the structure and electronic properties of the β-MnO2 Σ 5(210)/[001] GB are studied. Our calculations show this low energy GB has a significantly reduced band gap compared to the pristine material and that the formation of oxygen vacancies produces spin-polarized states that further reduce the band gap. Calculated formation energies of oxygen vacancy defects and Mn reduction at the GB core are all lower than the equivalent bulk value and in some cases lower than values recently calculated for β-MnO2 surfaces. Oxygen vacancy formation is also shown to produce a metallic behavior at the GB with defect charge distributed over a number of oxygen and manganese sites. The low energies of oxygen defect formation and the potential creation of conductive GB pathways are likely to be important to the electrochemical performance of β-MnO2.

Protonic defects in yttria stabilized zirconia: incorporation, trapping and migration

Both classical and quantum mechanical simulation techniques have been applied to investigate the incorporation, migration and potential binding of protonic defects in bulk yttria-stabilised zirconia (YSZ). The calculated redox reaction energies are found to be high, although the reduction energies are lower than those of bulk cubic ZrO2 and are shown to decrease further with increasing Y content. The hydration energies for YSZ are also lower than the values calculated for bulk ZrO2 and are found to be lowest when the oxygen ion is in close proximity to at least one Y ion. Strong binding (proton trapping) energies are observed between the protons and additional acceptor dopants including Sc, Yb and Gd. These energies are found to vary significantly depending on local configuration and again are generally lower than the values for ZrO2. Density functional theory (DFT) calculations are used to determine energy barriers for proton transfers via neighbouring oxygen ions (Grötthuss-type mechanism). Energy barriers of 0.32-0.42 eV are obtained for the pathways with the closest O-O interatomic distances and are found to be very comparable to well-established proton conducting materials.

Artificial layered perovskite oxides A(B0.5B′0.5)O3 as potential solar energy conversion materials

Perovskite oxides with a d0 electronic configuration are promising photocatalysts and exhibit high electron mobilities. However, their band gaps are too large for efficient solar energy conversion. On the other hand, transition metal cations with partially filled dn electronic configurations give rise to visible light absorption. In this study, by using hybrid density functional theory calculations, it is demonstrated that the virtues of the two categories of materials can be combined in perovskite oxide A(B0.5B′0.5)O3 with a layered B-site ordering along the [001] direction. The electronic structures of the four selected perovskite oxide compounds, La(Ti0.5Ni0.5)O3, La(Ti0.5Zn0.5)O3, Sr(Nb0.5Cr0.5)O3, and Sr(Nb0.5Fe0.5)O3 are calculated and discussed.