James A. Dawson

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.

The application of a new potential model to the rare-earth doping of SrTiO3 and CaTiO3

We have performed a computational study on the rare-earth (RE) doping of the perovskite structured materials; SrTiO3 and CaTiO3. The calculations have been completed using new Sr–O and Ca–O potentials in combination with a recently developed set of interatomic potentials, previously fitted and tested on polymorphs of BaTiO3. Particular attention has been given to the energetic and structural consequences of rare-earth doping via the five major dopant incorporation schemes. For SrTiO3, large RE ions dope at the Sr-site via a Sr vacancy mechanism, whereas smaller RE ions prefer doping via self-compensation due to the size of the ions being approximately half way between the size of the larger Sr-site and smaller Ti-site. Our simulations show that for CaTiO3, large to mid-sized RE ions (La to Eu) energetically favour Ca-site doping with Ca vacancy charge compensation and smaller ions dope via self-compensation. On comparison with previous calculations for BaTiO3, our results show the effect of the A-site size decrease from Ba to Ca on the favoured incorporation mechanism. The results for both materials are in good agreement with experiment. An overall assessment of the RE-doping in this perovskite series (ATiO3, where A = Ba, Sr or Ca) is given.

An atomistic study into the defect chemistry of hexagonal barium titanate

Using a recently established BaTiO3 potential model specifically designed for the calculation of defect energetics, atomistic simulations have been carried out on the intrinsic defect chemistry and Rare Earth (RE3+) doping of hexagonal barium titanate (h-BaTiO3). Five charge compensation schemes have been considered as well as potential cluster binding energies. The results show that ion size arguments are obeyed. In the dilute concentration limit, large RE3+ cations dope at the Ba-site via a titanium vacancy mechanism and mid sized RE3+ cations dope at the Ba and Ti sites simultaneously via a self compensation mechanism. In contrast, small RE3+ cations dope exclusively on the Ti-site via an oxygen vacancy compensation scheme. Comparisons between the hexagonal and cubic phases of BaTiO3 have also been drawn. It is suggested that Ba-site doping is less favorable and that Ti-site doping is considerably more favorable in h-BaTiO3 and that different defect configurations have a significant effect on the bindi...

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.