AWE Presentation: 29 April 2013

R A JacksonSchool of Physical & Geographical Sciences

Keele Universityr.a.jackson@keele.ac.uk

http://www.keele.ac.uk/chemistry/staff/rjackson/

Plan of presentation

• Tom Littleford’s project: summary and highlights:– Materials studied– Methodology– Results

• Publications and conference presentations from both projects.

Computer modelling of optical materials for laser applications: rare earth doping

in YLiF4 and BaMgF4

• The project was concerned with the development of new materials for optical applications (e.g. solid state lasers).

• Two mixed metal fluorides were modelled, both of which have potential optical applications when doped with rare earth ions.

Research aims

• To determine the optimum location of rare earth dopants in these materials by modelling the energetics of the solution process.

• To calculate the morphology of the undoped materials and to see how this is affected by the presence of dopants.

More about the materials

The materials used need to be transparent in the laser range and to easily accept optical doping cations.

YLiF4 (YLF) BaMgF4 (BMF)

Summary of computational methods used - 1

• Interactions between ions are described by potentials, empirically fitted to structures and properties, giving the following expression for the lattice energy:

• The potential parameters A, and C are specified for each interaction.

Summary of computational methods used - 2

• Structures and properties are determined by energy minimisation:– Predict the structure corresponding to an energy

minimum.– Calculate lattice properties (elastic constants,

dielectric constants, phonon frequencies etc.) at this structure.

• Reproduction of experimental values gives confidence in further use of the potential.

Summary of computational methods used - 3

• Defects in materials are modelled by the Mott-Littleton approximation, which places the defect at the centre of a region of the lattice where interactions are considered explicitly, surrounded by regions where approximations are applied.

• Details of the method are on the next slide:

Lattice defects: Mott-Littleton approximation*

The Mott-Littleton approximation is a two region approach:

Region IIons are strongly perturbed by the defect and are relaxed explicitly. This region typically contains ions ~600 ions.

Region IIaIons are weakly perturbed and therefore relaxation energy is approximated. This region typically contains ~1300 ions.

* N F Mott, M J Littleton: Trans. Faraday Soc. , 1938, 4, 485

Substitution and solution energies

• Substitution energies are the energies involved in substituting an ion into the material, but they do not take into account all the energetic terms involved in the solution process.

• Solution energies include all these terms, so they can be used to determine where the ion will substitute, and what form of charge compensation will occur (if it is needed).

Summary of computational methods used - 4

• Surfaces are of interest in this project because if surface energies can be calculated, morphologies can be predicted.

• For device construction, morphology is important:– If a material is doped with a particular ion, does

this result in a change in its morphology?• Details of the method are on the next slide:

Surface

Surface Simulation

The method uses the same interatomic potentials as before but we create a surface region and an bulk region.

Ions in region I are allowed to fully relax after the cutting.

Ions in region II are fixed at their equilibrium bulk coordinates.

Cuts are performed for all low Miller Indices. The energy of the stable surfaces are then calculated.

Block I

Region I

Block I

Region II

Block II

Region I

Block II

Region II

Unit Cell

Regions I and II are shown on the next slide.

Region I and II in surface simulations

Prediction of morphology from surface energies

Low surface energy stable surface

surface appears in morphology

The procedure is to calculate all low index surface energies.

The method then allows the affect of dopants on morphology to be predicted, since their presence will affect surface energies.

Diagram is based on a Wulff construction, which relates morphology to surface energy.

Results highlights: bulk properties*

• The structures and properties of the ‘perfect’ materials have been modelled successfully.

• For YLiF4, rare earth (lanthanide) dopants substitute preferentially at the Y3+ site as expected.

• For BaMgF4, charge compensation is needed (3+ ions at 2+ sites), and there are a variety of possibilities involving either Ba or Mg vacancies.

* R A Jackson, T E Littleford, G E Newby, D F Plant, ‘Computer modelling of rare earth doping in BMF & YLF’, IOP Conf. Series: Mat. Sci. & Eng. 15 012048 (2010)

YLF: perfect and defective morphology calculations*

For YLF, the morphology has been calculated for the perfect crystal and in the presence of La3+ doping. The main difference is the replacement of the (111) surface by the (110) surface.

* T E Littleford, R A Jackson, M S D Read, ‘An atomistic simulation study of the effects of dopants on the morphology of YLiF4’, Phys. Stat. Sol. C 10 (2) 156-159 (2013)

BMF perfect morphology calculations*

• Defect morphology calculations on BMF are still in progress, but the perfect morphology is given.

* T E Littleford, R A Jackson, M S D Read, ‘Modelling rare-earth doped BaMgF4: a potential laser material’, Phys. Stat. Sol. C 10 (2) 153-155 (2013)

Key: front face (001), top face (010), upper side face (110)

Summary

• Rare-earth doping in YLF and BMF has been modelled:– Location of dopants (and charge compensation schemes)

has been established– Perfect lattice morphologies have been calculated– Dopant morphologies have been obtained for YLF and are

being obtained for BMF• Future work includes use of ab initio methods to

calculate optical transition energies – software dependent!

• AWE funding is gratefully acknowledged.

Publications from the projects

R A Jackson, T E Littleford, G E Newby, D F Plant, ‘Computer modelling of rare earth doping in BMF and YLF’, IOP Conf. Series: Materials Science and Engineering 15 012048 (2010)

T E Littleford, R A Jackson, M S D Read, ‘An atomistic surface simulation study predicting morphologies and segregation in yttrium lithium fluoride’, Surface Science 606 1550–1555 (2012)

T E Littleford, R A Jackson, M S D Read, ‘Modelling rare-earth doped BaMgF4: a potential laser material’, Phys. Stat. Sol. C 10 (2) 153-155 (2013)

T E Littleford, R A Jackson, M S D Read, ‘An atomistic simulation study of the effects of dopants on the morphology of YLiF4’, Phys. Stat. Sol. C 10 (2) 156-159 (2013)

S R Walker, R A Jackson, M S D Read, ‘Atomistic modelling of actinide oxides for nuclear fuel applications’, Phys. Stat. Sol. C 10 (2) 197-201 (2013)

Major conference presentations

• Tom LittlefordEURODIM2010, Pécs, Hungary, July 2010ICDIM2012, Santa Fe, USA, June 2012

• Scott WalkerMMSNF, Aix en Provence, France, Sept 2011ICDIM2012, Santa Fe, USA, June 2012

(Plus RSC Solid State Group meetings)