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Atomistic Simulation @ Cambridge

Department of Materials Science & Metallurgy

Oxide Materials for Electronic Applications

D. Muñoz Ramo and P.D. Bristowe

Sponsors: European Commission (FP7)

  • Oxide materials towards a matured post-silicon electronics era (ORAMA).
  • Density functional calculations of band stuctures, doping mechanism, extended and point defects. Transport, dielectric and optical properties of crystalline or amorphous TCO and ASO materials.

Research Highlight

Density functional theory study of stoichiometric and nonstoichiometric ZnO grain boundaries

W. Korner, P. D. Bristowe and C. Elsasser, Phys. Rev. B 84, 045305 (2011)

We present a density–functional theory (DFT) analysis of stoichiometric and non-stoichiometric ZnO tilt grain boundaries (GBs) that reveals under which conditions such extrinsically undoped GBs may become electrically active. In the case of ZnO the self-interaction correction (SIC) scheme used allows a more accurate description of the formation energies as well as the electronic levels than the local density approximation (LDA). The results obtained with the SIC scheme deviate in some crucial ways from the LDA results of recent years. First, stoichiometric and non-stoichiometric ZnO GBs can show occupied deep levels whenever oxygen atoms are under-coordinated. Second, ZnO GBs with an oxygen excess at the boundary plane can exhibit unoccupied deep levels which may account for an experimentally observed weak varistor effect found in undoped polycrystalline ZnO.

View along [0001] of Σ = 7 (1230)  38.2° tilt grain boundary in stoichiometric ZnO. Two supercells are shown. The grain boundaries are located along the vertical dotted lines.

Nonlinear Optical Crystals

D. Muñoz Ramo, Z. Lin and P.D. Bristowe

Sponsors: Royal Society

  • Density functional calculations on damage resistant mid-IR nonlinear optical crystals.
  • Joint International Project. Collaborator with the Technical Institute of Physics and Chemistry, Beijing, China

Research Highlight

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

X. Jiang, S. Zhao, Z. Lin, J. Luo, P. D. Bristowe, X. Guan and C. Chen, Journal of Materials Chemistry C, 2. pp. 530-537 (2014)

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.

The relationship between the flexibility index and the SHG coefficient for various NLO crystals.

High Temperature Electrolyser/Fuel Cell Systems

C. Zhang, B. Yildiz, B. Yu and P.D. Bristowe

Sponsors: Low Carbon Energy University Alliance (LCEUA)

  • Electrode stability and degradation of HTEFC materials
  • Collaboration between Tsinghua University, MIT and University of Cambridge

Research Highlight

First principles calculations of oxygen vacancy formation in barium-strontium-cobalt-ferrite

C. Zhang and P. D. Bristowe, RSC Advances, 3. pp. 12267-12274 (2013)

Perovskite-type barium-strontium-cobalt-ferrite (BSCF) is a potentially significant material in the development of electrodes for solid oxide fuel cells and electrolysis cells, primarily because of its large oxygen vacancy concentration and mobility. Using density functional theory (DFT) with the DFT+U approach, we perform first principles calculations of oxygen vacancy formation in bulk BSCF with the composition Ba0.5Sr0.5Co0.75Fe0.25O3-δ, in which a range of oxygen deficiencies (0.125 ≤ δ ≤ 0.875) is investigated. Contrary to previous DFT studies for δ = 0.125, DFT+U predicts that the non-stoichiometric structure is more stable than its stoichiometric counterpart with δ = 0, and that this originates from a significantly different atomic and electronic structure following the introduction of the Hubbard termin the calculations. As more vacancies are created there is no tendency for ordering and the total vacancy formation energy becomes positive and increases as the oxygen deficiency increases. Using ab initio thermodynamics, the hat of formation of BSCF as a function of oxygen partial pressure and temperature is determined and the results are in broad agreement with available stoichiometry measurements.

DFT-optimized local structure surrounding an oxygen vacancy in BSCF, represented by a dashed circle, between two Co ions (light blue). Ions labeled nn are indicative of the nearest neighbors to the vacancy.

Grain Boundary Structure and Kinetics

I. Toda-Caraballo and P. D. Bristowe

  • Molecular dynamics of grain boundary free energies, migration mechanisms and mobilities
  • Grain boundary stiffness as a function of orientation and inclination
  • Dislocations, facets and steps in asymmetric tilt boundaries

Research Highlight

A molecular dynamics study of grain boundary free energies, migration mechanisms and mobilities in a bcc Fe-20Cr alloy

I. Toda-Caraballo, P.D. Bristowe and C. Capdevila, Acta Materialia, 60. pp. 1116-1128 (2012)

Curvature driven migration of a series of <110> tilt grain boundaries in a bcc Fe-20Cr alloy is simulated using molecular dynamics to investigate the relationship between atomic migration mechanism and mobility at medium to high temperatures. The boundaries studied include low-angle boundaries (LAGBs), high-angle boundaries (HAGBs) and singular boundaries such as the coherent twin. The steady state boundary shape and curvature are compared to a simple analytical model which incorporates the dependence of absolute mobility and free energy on boundary inclination. The comparison indicates that the 109.5° (112) Σ3 coherent twin boundary will have relatively low energy but high mobility. This result is attributed to a particularly effective repeated shuffle mechanism which occurs on the twinning plane. Two other migration mechanisms are observed, one involving the motion of <111> glissile dislocations in LAGBs and the other involving uncorrelated atomic shuffles in HAGBs sometimes associated with interfacial steps.

Snapshots of the 11.5°<110> (771) Σ99 tilt boundary and its complementary counterpart (168.5° <110> (1114) Σ99) as they migrate downwards and reduce the areas of the half-loops. Grain boundary dislocations are highlighted in red.