DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Density Functional Theory study of defects in zirconolite Jack Mulroue

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal

Density Functional Theory study of defects in zirconolite

Jack Mulroue

University College London

11/13 April 2011Huddersfield


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 2008http://www.synrocansto.com/CaseStudies/UKPlutonium/CaseStudies/UKGlassCeramicAdv.html


• 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.


• VASP– Periodic plain wave DFT code

• PBE exchange and correlation• Γ point sampling of the Brillouin zone• PAW pseudopotentials• 176 ion cell


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

Miseki et al. - Chemistry Letters 38, 2009Rossell – 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.


Excess of TiCaZr0.875Ti2.125O7

• 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.


Excess of ZrCaZr1.125Ti1.875O7

• 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.


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.


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).


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.


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.


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.


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.


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.


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.


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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DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Density Functional Theory study of defects in zirconolite Jack Mulroue