modelling mixed metal fluorides for optical applications robert a jackson lennard-jones laboratories...

Modelling mixed metal fluorides for optical

applications

Robert A JacksonLennard-Jones Laboratories

School of Physical and Geographical Sciences Keele University, Keele, Staffs ST5 5BG, UK

Thanks especially to:Elizabeth Maddock, Thomas Littleford (Keele)Mark Read (AWE), Dave Plant (AWE/.../Keele)

Mario Valerio, Jomar Amaral, Marcos Rezende (UFS)Sonia Baldochi (IPEN)

Eric Hudson (UCLA), David deMille (Yale)

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RSC SSCG conference, April 2010

Plan of talk

• What materials are involved?

• What is the motivation?

• Methodologies employed.

• Case studies.

• Future work.

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What materials?

• Mainly mixed metal fluorides and oxides• They do not have to have complex structures –

e.g. BaLiF3:

• For optical applications, doping is usually necessary. Rare earth (RE) ions are typically used, as their emission wavelengths are suitable for optical applications (in the m range).

inverted perovskite structure

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How important is doping to enhance optical properties?

• The picture shows a sample of amethyst, which is quartz, SiO2 doped with Fe3+ ions from Fe2O3.

• The value of the quartz is drastically increased by the presence of a relative small number* of Fe3+ ions!

*’As much iron as would fit on the head of a pin can colour one cubic foot of quartz’

http://www.gemstone.org/gem-by-gem/english/amethyst.html

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Blue John: CaF2 with F-centres

• The picture shows a sample of Blue John, CaF2 coloured by the presence of F-centres (electrons trapped at vacant F- sites in the crystal).

• Blue John is mined in a relatively few locations, including Castleton in Derbyshire.

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Optical Materials: motivation

• We are interested in understanding the behaviour and properties of materials with applications in a range of devices:

• Solid state lasers, where the laser frequency can be ‘tuned’ by changing the dopant.

• Scintillator devices for detecting electromagnetic or particle radiation.

• Nonlinear optical devices, frequency doublers and optical waveguides.

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Methodology

• The calculations are carried out in 2 stages:

1. Standard energy minimisation/Mott-Littleton calculations to establish location of dopants and charge compensation mechanisms, involving calculation of solution energies.*

2. Crystal field (or QM) calculations to access electronic properties and optical transitions.

* Some new developments will be mentioned later.

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Case study 1: Nd- and Tb-doped BaY2F8*

• BaY2F8, when doped with RE ions, in this case Nd3+ and Tb3+, has applications as a scintillator for radiation detection.

• This material has been the focus of a joint experimental and modelling study.

• Modelling can (i) predict location of dopant ions, and (ii) predict optical properties.

* Based on: ‘Structural and optical properties of Nd- and Tb-doped BaY2F8’ by Valerio et al, Optical Materials 30 (2007) 184–187

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Sequence of the modelling study

1. Derivation of an interatomic potential for BaY2F8, and for the RE-lattice interactions.

2. Calculation of intrinsic defect properties of the material to allow prediction of intrinsic disorder.

3. Calculation of solution energies, used to predict the location of the RE dopants.

4. Calculation of optical properties using crystal field methods.

* Details in: ‘Computer modelling of BaY2F8: defect structure, rare earth doping and optical behaviour’ by Amaral et al, Applied Physics B 81 (2005) 841- 846

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Potential fitting and solution energy calculations

M3+ doping at the Y3+ site in BaY2F8

MF3 + YY → MY + YF3

Esol= -Elatt(MF3)+ E(MY)+ Elatt(YF3)

Calculated values for Nd, Tb are:

0.64 eV, 0.32 eV

exp calc % diff

a/Å 6.98 6.96 -0.44

b/Å 10.52 10.67 1.42

c/Å 4.26 4.20 -1.61

/ 99.7 98.4 -1.31

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Crystal field calculations

• The RE ions are predicted to substitute at the Y sites, and relaxed coordinates of the RE ion and the nearest neighbour F ions are used as input for a crystal field calculation.

• Crystal field parameters Bkq are calculated,

which can then be used in two ways – (i) assignment of transitions in measured optical spectra, and (ii) direct calculation of predicted transitions.

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How good is the method?

• In the OM paper, measured and calculated transitions were compared, and a typical agreement of between 3-5% was observed:

transition Exp. /cm-1 Calc. /cm-1

5D4 7F4

17181 17724

18037 18041

5D4 7F5

18116 19111

19900 19364

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Summary of case study and other applications

• The method described has been shown to be able to calculate optical transitions for RE dopant ions in BaY2F8, and reasonable agreement has been obtained with experimental data, implying that it can be used predictively.

• It has been applied to several other fluoride and oxide materials, including LiNbO3.

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Alternative approaches

• In future we intend to used embedded QM methods (e.g. ChemShell) to model these systems in more detail.

• The overall aim is to be able to tailor combinations of host crystal and dopant for given optical applications.

• This work forms part of our new collaboration with AWE.

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Case study 2: Th in LiCaAlF6/LiSrAlF6

• 229Th is being investigated for use in ‘nuclear clocks’; its first nuclear excited state is (unusually) only ~ 8 eV above the ground state, and can be probed by VUV radiation.

• Nuclear clocks promise up to 6 orders of magnitude improvement in precision over next generation atomic clocks!

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Case Study 2:practical considerations

• The 229Th nucleus needs to be embedded in a VUV-transparent crystal for use in devices.

• Metal fluorides, e.g. LiCaAlF6/LiSrAlF6 have been identified as being suitable.

• A modelling study was therefore carried out.*

* Details in ‘Computer modelling of thorium doping in LiCaAlF6 and LiSrAlF6: application to the development of solid state optical frequency devices’ by Jackson et al, Journal of Physics: Condensed Matter 21 (2009) 325403

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Modelling Th in LiCaAlF6/LiSrAlF6 – (i)

• In previous work potentials were fitted to the host lattices, and defect properties obtained, including the location of RE dopants (more of a challenge than in BaY2F8!)*

• The challenge was to determine the optimal location of a Th4+ ion in the material.

• Charge compensation will be needed wherever substitution occurs, and resulting defects might affect optical properties.

* See ‘Computer modelling of defect structure and rare earth doping in LiCaAlF6 and LiSrAlF6’ by Amaral, Plant, et al, J. Phys.: Condensed Matter 15 (2003) 2523–2533

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Modelling Th in LiCaAlF6/LiSrAlF6 – (ii)

• Having fitted a Th4+ - F- potential to the ThF4 structure, solution energies were calculated for doping at the Li (+1), Ca/Sr (+2) and Al (+3) sites, with a range of charge compensation mechanisms.

• The lowest energy scheme was found to correspond to location at a Ca2+/Sr2+ site with charge compensation by F- interstitials.

• Crystal growth studies are in progress, but delayed by scarcity/cost* of 229Th, and politics!

* $50k/mg

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Future work (i): concentration dependent solution energies

• In modelling the doping of materials, we make extensive use of the concept of solution energies to determine location of dopants, charge compensation mechanisms etc.

• We are developing new methods which enable us to calculate solution energies as a function of dopant concentration.

• These should overcome the major problem with predictions based on solution energies, which are currently limited to isolated defects.

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Concentration dependent solution energies (i)

• The basis of the technique is to model, directly, the process for preparing the doped materials:

• e.g. producing doped BaAl2O4:

0.5x M2O3 + BaO + (1 - 0.5x) Al2O3 BaAl2-xMxO4

• We calculate the solution energy of the process by calculating the energy of the reaction directly.

• The left hand side is straightforward; for the right hand side we assume (for solution at the Al site):

E [BaAl2-xMxO4]= (1–0.5x) Elatt(BaAl2O4) + x E(MAl)

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Concentration dependent solution energies (ii)

• The result of these calculations is that we can obtain solution energies as a function of dopant concentration, up to the limit of non-interacting defects.

• The method is still being developed, but results are promising, and publications have been submitted!

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Future work (ii): reappraisal of modelling of UO2

• We are looking at UO2 again, 23 years after our last paper on this subject*!

• The focus will be on modelling hydrogen gas incorporation and diffusion.

• A summary of the literature has been carried out with a view to deciding which potential to use, etc.

* 'The Calculation of Basic Defect Parameters in UO2’ by Jackson et al, Phil. Mag. A, 53, 27-50 (1986)

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Acknowledgements

Keele University Centre for the Environmental, Physical and Mathematical Sciences

modelling mixed metal fluorides for optical applications robert a jackson lennard-jones laboratories...

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