Density Functional
Theory study of
defects in zirconolite
Jack Mulroue
University College London
11/13 April 2011
Huddersfield
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Background for zirconoilite
• Zirconolite is the
proposed ceramic
encapsulent for
geological disposal.
• Zirconolite is able to
accommodate Pu and U
as well as Hf and Gd into
its crystal lattice.
• Actinide bearing phase in
SYNROC, glass ceramics
and single phase waste
forms.
Geological disposal options for HLW and spent fuel - NDA 2008
http://www.synrocansto.com/CaseStudies/UKPlutonium/CaseStudies/UKGlassCeramicAdv.html
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• CaZrTi2O7
• Monoclinic crystal
structure (α≠ 90°,
β=γ= 90°)
• Structure comprises
of 4 and 6
coordinated Ti layers
separated by layers of
alternating Ca and Zr
ions.
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• VASP
– Periodic plain wave DFT code
•
•
•
•
PBE exchange and correlation
Γ point sampling of the Brillouin zone
PAW pseudopotentials
176 ion cell
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Calculated bulk properties
Band gap of 2.8 eV
(3.6 eV experimental)
Lattice parameters
Lattice
parameter
DFT
(Å)
Experimental
(Å)
%
Error
a
12.09
12.45
2.89
b
14.14
14.55
2.82
c
11.08
11.39
2.72
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Miseki et al. - Chemistry Letters 38, 2009
Rossell – Nature 283, 1980
Effect of stoichiometry
• Zirconolite is able to exist in a range of
stoichiometries.
• CaZrxTi3-xO7 where 0.80 < x < 1.37
• Experiments observe 5 fold coordinated Ti
polyhedra instead of 4 fold coordinated Ti
found in the perfect structure.
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Excess of Ti
CaZr0.875Ti2.125O7
• The substituted Ti is 5
fold coordinated in the
Zr site.
• Has no effect on the
coordination of the
four 4 fold
coordinated Ti
polyhedra.
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Excess of Zr
CaZr1.125Ti1.875O7
• The two 4 fold
coordinated Ti polyhedra
in the same row as the
substituted Zr become 5
coordinated.
• Therefore to remove all 4
fold coordinated Ti
polyhedra, with x = 1.25,
the two Zr must be in
different Ti layers.
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Point defects in zirconolite
• The point defects have been studied to
gain a fundamental understanding into
defect behaviour, to allow for more
accurate predictions on the long term
durability of the waste form.
• Each defect has been studied in the
various charge states, from 0 to formal
charge.
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Oxygen vacancies
• 4 of the 7 oxygen
environments have been
studied.
• The 2 excess electrons
generated from the neutral
oxygen vacancy localised on 2
Ti ions.
• The singly charged vacancy
localises the excess electron
on the Ti ion it was coordinated
too.
• Similar behaviour is observed
in anatase (TiO2).
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Titanium vacancies
• The defect structure has a
significant dependence on the
chemical environment of the Ti.
• Vacancy in the 6 fold
coordinated chains, results in
the formation of an O2 molecule
in certain charge states and a 5
coordinated Ti in others.
• A vacancy of the other 6
coordinated Ti, results in the 4
coordinated Ti becoming 6
coordinated.
• The vacant 4 coordinated Ti
resulted in the creation different
numbers of 5 coordinated Ti in
different charge states.
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Ca and Zr vacancies
• The Ca vacancy results in
distortion around the vacancy
site.
• The neutral Zr vacancy results
in the formation of an O2
molecule.
• The singly charged vacancy
results in the creation of 5
coordinated at the expense of
a 6 coordinated Ti.
• The higher charge states
results in distortion around the
vacancy site.
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Locating the interstitials
• The interstitial sites were studied using random structure
analysis, based on the AIRSS method.
• The 88 ion unit cell was optimised using soft oxygen
pseudopotentials, with the cell parameters fixed to the
experimental lattice vectors.
• The interstitial ion was then added with randomly
assigned coordinates and all the ions were allowed to
relax.
• This was carried out 100 times for each interstitial.
• The lowest energy structure was then placed inside the
1x2x1 supercell to obtain the interstitial structure.
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Oxygen interstitials
• The oxygen interstitial can only
exist in two charge states, 0
and -1.
• The neutral interstitial forms an
oxygen dumb-bell with a lattice
oxygen at high interstitial
concentrations. However at
lower concentrations the
interstitial causes two 7
coordinated Ti and a 5
coordinated Ti.
• The singly charged interstitial
produces a 5 coordinated Ti
polyhedron caused by
displacement of two lattice
oxygen ions on the same
polyhedron.
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Ca and Ti interstitials
• The cation interstitials
were found only to
exist in the +2 charge
state.
• Both Ca and Ti
interstitials reside
within the <010>
channels which run
through the crystal.
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Zr interstitials
• The most stable
interstitial configuration
was the Zr ion
substituting for a Ti lattice
ion.
• Bond length increase in
the polyhedron due to
substitution.
• This configuration was
0.27 eV more stable than
the Zr remaining as the
interstitial located in the
<010> channel.
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Conclusions
• The presents of non-stoichiometry and
vacancies in the experimental samples causes
the difference in coordination of the Ti
polyhedron.
• O2 molecules could form at vacancy sites within
the lattice.
• The substitution caused by the Zr interstitial may
have a significant effect on the recovery of the
lattice from an irradiation event.
• Require more fundamental experimental work to
allow accuracy of our models to assessed.
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