Paul D. Bristowe

The synthesis, structure and electronic properties of a lead-free hybrid inorganic–organic double perovskite (MA)2KBiCl6 (MA = methylammonium)

In a search for lead-free materials that could be used as alternatives to the hybrid perovskites, (MA)PbX3, in photovoltaic applications, we have discovered a hybrid double perovskite, (MA)2KBiCl6, which shows strong similarities to the lead analogues. Spectroscopic measurements and nanoindentation studies are combined with density functional calculations to reveal the properties of this interesting system.

ON THE STRUCTURAL UNIT GRAIN-BOUNDARY DISLOCATION MODEL FOR GRAIN-BOUNDARY STRUCTURE

Abstract The applicability of the structural unit/grain boundary dislocation (GBD) model to the atomistic structures of [001] and [011] tilt boundary series recently calculated by Sutton, Vitek and Wang is studied in detail. It is found that a hierarchy of structural unit/GBD descriptions exists which may be used to describe such boundaries. Each description corresponds to a different choice of “delimiting boundary” structural units which are mixed together to make up all boundaries in the series. In all cases the structural units exhibit systematic distortions as the tilt angle changes. As the number of delimiting boundaries in a series is increased, and the range of structures is “quantized” on a finer scale, the distortions of the structural units decreases, but the amount of information regarding the GBD structure is decreased. It is pointed out that different members of the hierarchy of descriptions can be used advantageously for different purposes.

Exploring the properties of lead-free hybrid double perovskites using a combined computational-experimental approach

Density functional theory screening of the hybrid double perovskites (MA)2BIBiX6 (BI = K, Cu, Ag, Tl; X = Cl, Br, I) shows that systems with band gaps similar to those of the MAPbX3 lead compounds can be expected for BI = Cu, Ag, Tl. Motivated by these findings, (MA)2TlBiBr6, isoelectronic with MAPbBr3, was synthesised and found to have a band gap of ∼2.0 eV.

DETERMINATION OF VACANCY MECHANISM FOR GRAIN-BOUNDARY SELF-DIFFUSION BY COMPUTER-SIMULATION

It is currently well established that the fast self-diffusion which occurs along grain boundaries (GBs) in metals must occur by a point defect exchange mechanism. For example, it is known that rapid GB diffusion can transport a net current of atoms along GBs during both sintering and diffusional creep, and that the two species in a binary substitutional alloy diffuse at different rates in GBs. However, it has not been possible to establish firmly whether the defect mechanism involves the exchange of atoms with vacancy or interstitial point defects. It has been suspected that the vacancy exchange mechanism must apply but it has been difficult to prove this hypothesis because of a lack of detailed information at the atomistic level. The results are presented of an effort to establish the GB self-diffusion mechanism in a bcc iron ..sigma.. = 5 (36.9/sup 0/) (001) (310) tilt boundary using the combined methods of computer molecular statics and molecular dynamics simulation.

Synthesis, crystal structure, and properties of a perovskite-related bismuth phase, (NH4)3Bi2I9

Organic-inorganic halide perovskites, especially methylammonium lead halide, have recently led to remarkable advances in photovoltaic devices. However, due to environmental and stability concerns around the use of lead, research into lead-free perovskite structures has been attracting increasing attention. In this study, a layered perovskite-like architecture, (NH4)3Bi2I9, is prepared from solution and the structure solved by single crystal X-ray diffraction. The band gap, which is estimated to be 2.04 eV using UV-visible spectroscopy, is lower than that of CH3NH3PbBr3. The energy-minimized structure obtained from first principles calculations is in excellent agreement with the X-ray results and establishes the locations of the hydrogen atoms. The calculations also point to a significant lone pair effect on the bismuth ion. Single crystal and powder conductivity measurements are performed to examine the potential application of (NH4)3Bi2I9 as an alternative to the lead containing perovskites.

Ab initio study of the oxygen vacancy in SrTiO3

The electrical properties of SrTiO3 are strongly dependent on the oxygen vacancy concentration. We have studied the atomic and electronic properties of a single oxygen vacancy using a local spin density approximation-plane-wave pseudopotential method. The total energies, relaxed geometries, valence charge and spin densities, and densities of electron states are calculated for different charge states of the vacancy. The doubly positively charged state is found to be the most stable. With some charge states, a spin-polarized defect state is observed. Furthermore, we have evaluated the formation energy of the vacancy in the OO→VO + ½O2 process.

Ab initio and classical simulations of defects in SrTiO3

Abstract We have performed a computational study of point defects in strontium titanate (SrTiO3) and also examined the antiferrodistortive (AFD) phase transition that occurs in this perovskite material. The simulations include atomic structure optimisations using both empirical and local density approximation (LDA)–plane-wave pseudopotential (PWPP) methods. We report results regarding the relaxed atomic geometries, electron densities of states and Mulliken charges. The results indicate that the oxygen vacancy may have a destabilising influence on the cubic structure.

The atomic structure of twin boundaries in rutile

Abstract The equilibrium atomic structure of the (101) and (301) twin boundaries in rutile (TiO2) has been computed using an ionic shell model and compared with high resolution electron microscopy images of the same boundaries using image simulation. The lowest-energy (101) twin boundary structure is characterized by an in-plane translation of 1/2〈111〉 which conserves the mirror symmetry of the metal sublattice but which imposes a displacement on the oxygen sublattice. The lowest-energy (301) twin-boundary structure involves no in-plane translation and is characterized by mirror symmetry of both the metal and the oxygen sublattices. The computed mirror symmetry of the metal sublattice for both twin boundaries is in agreement with the electron microscopy observations. A small distortion present in the simulated images is discussed in terms of the interatomic potential used in the calculations.

The role of dipole moment in determining the nonlinear optical behavior of materials: ab initio studies on quaternary molybdenum tellurite crystals

The linear and nonlinear optical (NLO) properties of a series of quaternary molybdenum tellurite crystals are studied using a first-principles computational approach and an empirical dipole model. The calculated second harmonic generation (SHG) coefficients are consistent with the experimental values and the calculations show that their magnitude is independent of the dipole moment. Using a flexible dipole model based on the concept of bond-valence it is shown that the size of the NLO effects is determined by the compliance with the dipole moment in response to external perturbation rather than the intrinsic dipole moment of the structure. The study revises the common belief that SHG effects depend on the dipole moment inherent to the unperturbed structure and will help guide the search for new NLO crystals including those that are not necessarily polar.

Computer simulation study of the structure of vacancies in grain boundaries

The structure of vacancies in grain boundaries has been investigated by computer molecular statics employing pairwise potentials. In order to gain an impression of the vacancy structures which may occur generally, a number of variables was investigated, including metal type, boundary type, degree of lattice coincidence, and choice of boundary site. In all cases the vacancies remained as distinguishable point defects in the relatively irregular boundary structures. However, it was found that the vacancy often induced relatively large atomic displacements in the core of the boundary. These displacements often occurred only in the direct vicinity of the vacancy, but in certain cases they were widely distributed in the boundary, sometimes at surprisingly large distances. In certain cases the displacements included a large inward relaxation of one, or more, of the atoms  neighboring the vacancy, and the initial vacant site became effectively ″split″. These results were classified and discussed in relation to t...