Eric Cockayne

Oxygen vacancies in N doped anatase TiO2: Experiment and first-principles calculations

We have determined the electronic and atomic structure of N doped TiO2 using a combination of hard x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Our results reveal that N doping of TiO2 leads to the formation of oxygen vacancies and the combination of both N impurity and oxygen vacancies accounts for the observed visible light catalytic behavior of N doped TiO2.

Lattice dynamics of BaTiO3, PbTiO3, and PbZrO3: A comparative first-principles study

The full phonon dispersion relations of lead titanate and lead zirconate in the cubic perovskite structure are computed using first-principles variational density-functional perturbation theory, with ab initio pseudopotentials and a plane-wave basis set. Comparison with the results previously obtained for barium titanate shows that the change of a single constituent (Ba to Pb, Ti to Zr) has profound effects on the character and dispersion of unstable modes, with significant implications for the nature of the phase transitions and the dielectric and piezoelectric responses of the compounds. Examination of the interatomic force constants in real space, obtained by a transformation which correctly treats the long-range dipolar contribution, shows that most are strikingly similar, while it is the differences in a few key interactions which produce the observed changes in the phonon dispersions. These trends suggest the possibility of the transferability of force constants to predict the lattice dynamics of perovskite solid solutions.

Total-energy-based prediction of a quasicrystal structure

Quasicrystals are metal alloys whose noncrystallographic symmetries challenge traditional methods of structure determination. We employ quantum-based total-energy calculations to predict the structure of a decagonal quasicrystal from first-principles considerations. Our Monte Carlo simulations take as input the knowledge that a decagonal phase occurs in Al-Ni-Co near a given composition and use a limited amount of experimental structural data. The resulting structure obeys a nearly deterministic decoration of tiles on a hierarchy of length scales related by powers of t, the golden mean.

Lattice dynamics inPbMg1∕3Nb2∕3O3

Lattice dynamics for five ordered PMN supercells were calculated from first principles by the frozen phonon method. Maximal symmetries of all supercells are reduced by structural instabilities. Lattice modes corresponding to these instabilities, equilibrium ionic positions, and infrared reflectivity spectra were computed for all supercells. Results are compared with our experimental data for a chemically disordered PMN single crystal.

First-principles-based simulations of relaxor ferroelectrics

The phenomenology of Pb(B,B′)O3 perovskite-based relaxor ferroelectrics (RFE) is reviewed, with emphasis on the relationship between chemical short-range order and the formation of polar nanoregions in the temperature range between the “freezing” temperature, T f, and the Burns temperature, T B. Results are presented for first-principles-based effective Hamiltonian simulations of (PSN), and simulations that were done with empirically modified variants of the PSN Hamiltonian. Arbitrarily increasing the magnitudes of local electric fields, caused by an increase in chemical disorder, broadens the dielectric peak, and reduces the ferroelectric (FE) transition temperature; and sufficiently strong local fields suppress the transition. Similar, but more dramatically glassy results are obtained by using the PSN dielectric model with a distribution of local fields that is appropriate for (PMN). The results of these simulations, and reviewed experimental data, strongly support the view that within the range , polar nanoregions are essentially the same as chemically ordered regions. In PSN a ferroelectric phase transition occurs, but in PMN, a combination of experimental and computational results indicate that pinning from local fields is strong enough to suppress the transition and glassy freezing is observed.

Origin of diffuse scattering in relaxor ferroelectrics

High-pressure and variable temperature single-crystal synchrotron x-ray measurements combined with first principles based molecular-dynamics simulations were used to study diffuse scattering in the relaxor ferroelectric system

Structure and phason energetics of Al-Co decagonal phases

{\text{PbSc}}_{1/2}{\text{Nb}}_{1/2}{\text{O}}_{3}

Comparative dielectric response in CaTiO3 and CaAl1/2Nb1/2O3 from first principles

. Constant temperature experiments show a pressure-induced transition to the relaxor phase, in which butterfly- and rod-shaped diffuse scattering occurs around the {h00} and {hh0} Bragg spots. Simulations qualitatively reproduce the observed diffuse scattering features as well as their pressure-temperature behavior and show that they arise from polarization correlations between chemically ordered regions, which in previous simulations were shown to behave as polar nanoregions. Simulations also exhibit radial diffuse scattering [elongated toward and away from

Ferroelectric Phase Transitions in Nano-Scale Chemically Ordered PbSc 0.5 Nb 0.5 O 3 Using a First-Principles Model Hamiltonian

\mathbf{Q}=(000)

Effect of ionic substitutions on the structure and dielectric properties of hafnia: A first principles study

] that persists even in the paraelectric phase; consistent with previous neutron experiments on