Transcript RAJ Mansfield talk 140410

Modelling mixed metal
fluorides for optical
applications
Robert A Jackson
Lennard-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)
Plan of talk
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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:
inverted perovskite structure
• 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).
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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!
http://www.gemstone.org/gem-by-gem/english/amethyst.html
*’As much iron as would fit on the head of a pin can colour one cubic foot of quartz’
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Blue John: CaF2 with Fcentres
• 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 Tbdoped 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
a/Å
exp
calc
% diff
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
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
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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
5D
4

5D
4
 7F5
7F
4
Exp. /cm-1
Calc. /cm-1
17181
18037
18116
19900
17724
18041
19111
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
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