Computer modelling of materials: from nuclear fuels to nuclear clocks Rob Jackson 24 November 2010

A wide range of materials … From: To: 2

Plan of talk Why do computer modelling of materials? Types of problem What techniques do we use? Examples: – Nuclear fuels – Optical materials – Geological materials – Materials for nuclear clock development 3 Keele Research Seminar, 24 November 2010

Role of Computational Chemistry, and where Materials Modelling ‘sits’ Computational Chemistry Fundamental calculations to: Predict parameters often unavailable from experiment. Elucidate ‘mechanistic’ information. Materials Modelling can: Calculate material structures and properties. Help explain/rationalise experimental data. Predict material structures and properties. 4 Keele Research Seminar, 24 November 2010

Types of problem Modelling the structures of nuclear fuels (UO 2, PuO 2, MOX) Modelling optical materials (YLiF 4, BaMgF 4 ) – Predicting the location of dopant ions – Calculating and predicting optical transitions Modelling geological materials (e.g. zircon, ZrSiO 4 and related materials ) – USiO 4 → PbSiO 4 Nuclear clocks: 229 Th doping in LiCaAlF 6 5 Keele Research Seminar, 24 November 2010

Techniques The main technique employed is atomistic modelling. The material structure (lattice parameters, ion positions) is provided, and interactions between ions are defined by interionic potentials: – These are simple mathematical functions that represent the important interactions between atoms: Van der Waals forces Electron repulsion A well-known example is the Lennard-Jones potential. 6 Keele Research Seminar, 24 November 2010

Sir John Lennard-Jones ( ) Sir John Lennard-Jones was a mathematical physicist who became the first professor of theoretical chemistry in the UK, in Cambridge, where he worked from He was born John Jones; Lennard was his wife’s surname. In 1953 he was appointed 2 nd Principal of the University College of North Staffordshire, which later became Keele University. 7 Keele Research Seminar, 24 November 2010

The Lennard-Jones potential Lennard-Jones developed his potential in 1931, 22 years before coming to Keele: V = Ar -12 – Cr -6 The first term represents electron repulsion, and the second van der Waals attraction. A potential is thus defined for the interaction between each pair of atoms. How the parameters are obtained could fill another seminar! 8 Keele Research Seminar, 24 November 2010

More on techniques used The basis of atomistic simulation is energy minimisation: structures are calculated corresponding to an energy minimum and properties are calculated for that structure. We are interested in defects; they destroy the periodicity of the unit cell, and need special treatment, and a method called the Mott- Littleton approximation is used. 9 Keele Research Seminar, 24 November 2010

Mott-Littleton approximation Region I Ions are strongly perturbed by the defect and are relaxed explicitly with respect to their Cartesian coordinates. Region II Ions are weakly perturbed and therefore their displacements, with the associated energy of relaxation, can be approximated. Region IIa Defect Region I © Mark Read (AWE) 10 Keele Research Seminar, 24 November 2010

Modelling nuclear fuels Motivation: understanding the effect of the fission process on the structure and properties of UO 2, PuO 2 and other actinide oxides. This work forms part of a collaboration with AWE, and is the basis of Scott Walker’s PhD project. In addition, Gemma Turner (3 rd year project student) is modelling MOX (mixed oxide fuel, UO 2 /PuO 2 ). 11 Keele Research Seminar, 24 November 2010

Why Study Uranium Dioxide? Understanding Corrosion [1]R. J. Pearce, I. Whittle, D. A. Hilton, The Oxidation of Uranium in Carbon Dioxide and Carbon Monoxide (A Review), J. Nucl. Mater. 33 (1969) [2]J. R. Petherbridge, T. B. Scott, J. Glascott, C. Younes, G. C. Allen, I. Findlay, Characterisation of the surface over-layer of welded uranium by FIB, SIMS and Auger electron spectroscopy, J. Alloys Compd. 476 (1-2) (2009) 543–549. [3]R. M. Harker, The influence of oxide thickness on the early stages of the massive uranium-hydrogen reaction, J. Alloys Compd. 426 (1-2) (2006) 106–117. [1] Understanding factors limiting or inducing uranium corrosion is of interest to a variety of industrial activities. [2] Extreme affinity of pure uranium for oxygen is well documented. At least 16 oxides are known between UO 2 and UO 3 and are the principal products of uranium metal corrosion. Once formed as a layer on the surface of metallic uranium, the oxides act as a passive barrier to further corrosion. [2,3] Thus it is the generally accepted view that the reactivity of uranium towards various gases is primarily affected by the properties of its native oxide layer. For example, in the case of uranium–hydrogen systems, the surface oxide layer prevents rapid concentration of hydrogen at the metal surface and, as a result, provides a limiting influence on the onset of the gas–solid reaction that forms pyrophoric uranium hydride (UH 3 ). [3] 12

Simulation of Uranium Dioxide Simulation of the bulk lattice → 13 Keele Research Seminar, 24 November 2010

Experimental Data for Empirical Fitting S. A. Barrett, A. J. Jacobson, B. C. Tofield, B. E. F. Fender, The Preparation and Structure of Barium Uranium Oxide BaUO 3+x, Acta Cryst. 38 (Nov) (1982) 2775–2781. Elastic Constants / GPa ReferenceC 11 C 12 C 44 Dolling et al. [1]401 ± 9108 ± 2067 ± 6 Wachtman et al. [2]396 ± ± ± 0.17 Fritz [3]389.3 ± ± ± 0.3 Dielectric Constants / GPa Reference Static  0 High Frequency  ∞ Dolling et al. [1]245.3 [1] G. Dolling, R. A. Cowley, A. D. B.Woods, Crystal Dynamics of Uranium Dioxide, Canad. J. Phys. 43 (8) (1965) 1397–1413. [2] J. B. Wachtman, M. L. Wheat, H. J. Anderson, J. L. Bates, Elastic Constants of Single Crystal UO 2 at 25°C, J. Nucl. Mater. 16 (1) (1965) 39–41. [3] I. J. Fritz, Elastic Properties of UO 2 at High-Pressure, J. Appl. Phys. 47 (10) (1976) 4353–

How good is the fit? Comparison of Model with Experiment ParameterCalc.Obs. %% ParameterCalc.Obs. %% Lattice Constant [Å] C 11 [GPa] U 4+ – U 4+ Separation [Å] C 12 [GPa] U 4+ – O 2- Separation [Å] C 44 [GPa] O 2- – O 2- Separation [Å] Bulk Modulus [GPa] Static Dielectric Constant High Frequency Dielectric Constant See: M S D Read, R A Jackson, Journal of Nuclear Materials, 406 (2010) 293– Keele Research Seminar, 24 November 2010

Some results from UO 2 modelling Formation energies for defects (vacancies, dopants) in the structure can be obtained. Location of dopant ions in the structure, and atoms formed from fission processes (e.g. Xe) can be predicted. Surface energies can be calculated and crystal morphology predicted: 16 Keele Research Seminar, 24 November 2010

Surface Simulations Morphology 111 If UO 2 crystallites attain thermodynamic equilibrium, the morphology will be dominated by the (111) surfaces, forming an octahedron 17 Keele Research Seminar, 24 November 2010

Modelling optical materials Motivation: we are interested in helping to develop new materials for optical applications, including solid state lasers and scintillators for detection of ionising radiation. Interesting (and useful) optical properties can be added to metal fluorides and metal oxides by doping, usually with lanthanide elements. This topic is the theme of Tom Littleford’s PhD project (also funded by AWE). 18 Keele Research Seminar, 24 November 2010

Blue John: CaF 2 with F-centres The picture shows a sample of Blue John, CaF 2 coloured by the presence of F-centres (electrons trapped at vacant F - sites in the crystal). There is a Blue John mine at Castleton in Derbyshire. 19 Keele Research Seminar, 24 November 2010

Amethyst: SiO 2 with Fe 3+ impurities The picture shows a sample of amethyst, which is quartz, SiO 2 doped with Fe 3+ ions from Fe 2 O 3. The value of the quartz is drastically increased by the presence of a relative small number* of Fe 3+ ions! *’As much iron as would fit on the head of a pin can colour one cubic foot of quartz’ 20 Keele Research Seminar, 24 November 2010

More on amethyst The colour is due to the Fe 3+ ions occupying the Si 4+ sites, so a charged [FeO 4 ] 4- centre results. The amount of iron present is very small, about 40 parts per million! Brazilian amethyst, value $94.50 (June 2007) 21 Keele Research Seminar, 24 November 2010

Doping for technological applications For most applications, doping with rare earth cations is carried out: The rare earth elements are chosen because of their emission wavelengths as dopants (in the  m range). 22 Keele Research Seminar, 24 November 2010

KYF 4 Hexagonal (P3 1 ) a = b = Å c = Å KY 3 F 10 Cubic (Fm3m) a = b = c = Å K 2 YF 5 Orthorhombic (Pna2 1 ) a = Å b = Å c = Å Host Materials: mixed metal fluorides Plus K 3 YF 6 Monoclinic (P21/n) 23 Keele Research Seminar, 24 November 2010 E M Maddock, PhD thesis (2010)

Structural modelling studies of KYF materials (E M Maddock, PhD thesis 2010) A common set of interatomic potentials was fitted to all 4 materials, giving reasonable agreement with structures to within a few % (better than 1% for KYF 4 shown below). KYF 4 Exp (Å)Calc (Å)% Diff a=b c Keele Research Seminar, 24 November 2010

Solution energies for RE doping Solution energies give the total energy needed for doping to take place. Potentially 2 sites are available, Y and K. Solution energies were calculated for doping at the Y 3+ site (and the K + site with various forms of charge compensation). As expected the lowest energy site is the Y 3+ site (no charge compensation needed). 25 Keele Research Seminar, 24 November 2010

Crystal morphology and RE doping We are interested in the answers to 2 questions here: 1.What is the crystal morphology of the pure materials and how is it affected by doping? 2.Do the dopants have a tendency to segregate to the crystal surface? In both cases there are implications for the use of doped materials in devices. 26 Keele Research Seminar, 24 November 2010

Morphology: Wulff plots Wulff plots can be constructed to give morphologies based on surface energies, & also issues like low indices & interplanar spacing. An example is shown for KY 3 F 10 : Miller indexE surface /Jm Keele Research Seminar, 24 November 2010

Surface segregation of dopants If a material is doped, it is important to know if the dopant ion remains in the bulk or moves to the surface. The segregation energy (E seg ) of a dopant is defined as the difference between the energy to substitute it at the surface and in the bulk: E seg = E (dopant, surface) – E (dopant, bulk) A negative value of the segregation energy indicates that there will be a tendency for surface segregation to occur for a particular dopant. 28 Keele Research Seminar, 24 November 2010

Segregation energies to dominant surfaces in KY 3 F 10 / eV dopant La Ce Pr Nd Sm Eu Gd dopant Tb Dy Ho Er Tm Yb Lu Keele Research Seminar, 24 November 2010

Modelling optical properties As well as understanding what happens to the structure and morphology, we are interested in trying to predict the optical transitions of dopant ions. This can be done in 2 ways: – Crystal field calculations – Quantum mechanics (embedded clusters) Some results from crystal field calculations will be shown: Keele Research Seminar, 24 November

Keele Seminar - 13 June 2007 (KPA)31 Calculation of energy levels for LaF 3 : Ce 3+ [11] R A Buchanan, H E Rast, H H Caspers, J. Chem. Phys (1966) Poor agreement for low energy transitions Much better agreement (within 10% or better) for higher energy transitions R A Jackson, M E G Valerio, J B Amaral, M A Couto dos Santos and E M Maddock Phys. Stat. Sol. (c) 4(3) (2007) Energy levels in cm -1

Modelling geological materials: Zircon and coffinite Zircon readily accommodates U at the Zr site, and the fully substituted compound, USiO 4, is the mineral coffinite. Starting with zircon and progressively substituting U at the Zr site allows the structure of coffinite to be predicted, and the result can be compared with the experimental structure: 32 Keele Research Seminar, 24 November 2010

Coffinite The structure is predicted to better than -2% Structures for the full range of solid solutions can be calculated. Predicted coffinite structure Exp (Å)Calc (Å)% a=b c Black, interstitial coffinite cementing a sub-angular quartzose sandstone. Schumacher Coll. (Temple Mountain, San Rafael District (San Rafael Swell), Emery Co., Utah, USA) 33

Coffinite and radioactive decay U decays radioactively, eventually to Pb. Due to the long t 1/2 of U, the oldest samples of coffinite found have around 3% Pb. The structure of the end member, PbSiO 4, can be predicted, as can the full Pb-U solid solution. PbSiO 4 Exp (Å)Calc (Å) % a=b? c? Attempted synthesis of PbSiO 4 (Keelite) is in progress! Older samples of coffinite are being searched for. 34 Keele Research Seminar, 24 November 2010

Development of nuclear clocks 229 Th 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! They also have advantages of improved stability over existing atomic clocks. 35 Keele Research Seminar, 24 November 2010

Practical considerations The 229 Th nucleus needs to be embedded in a VUV-transparent crystal for use in devices. Metal fluorides, e.g. LiCaAlF 6 /LiSrAlF 6 have been identified as being suitable. A modelling study was therefore carried out, to find where the Th ions substitute in the lattice.* * Details in ‘Computer modelling of thorium doping in LiCaAlF 6 and LiSrAlF 6 : application to the development of solid state optical frequency devices’ by Jackson et al, Journal of Physics: Condensed Matter 21 (2009) Keele Research Seminar, 24 November 2010

Results of modelling study and planned experimental study The modelling showed that the Th 4+ ions preferentially substitute at the Ca 2+ site, with charge compensation by F - interstitial ions. Crystal growth experiments are in progress, but hindered by the difficulties of growing fluoride systems, plus the cost (and location) of 229 Th ($50k/mg!). This is a collaboration with UCLA and LANL. 37 Keele Research Seminar, 24 November 2010

Bi III 2 Zr IV 2 O 7 Bi III 2 Ti IV 2 O 7 Bi III 2 Hf IV 2 O 7 Pyrochlores and Defect Fluorite Materials (with Richard Darton) Defect Fluorite Pyrochlore

Bi 2 Zr 2 O 7 : where modelling can help Can Bi 2 Zr 2 O 7 exist as a pyrochlore phase ? Can we predict intermediate structures ? Can the structure be doped with +2, +3 and +4 cations ? e.g. SrTiZr 2 O 7 (Doping will change structure and therefore properties) e.g. dielectrics, nuclear waste storage materials Can we predict new materials ? Keele Research Seminar, 24 November Bi III 2 Zr IV 2 O 7 Bi III 2 Ti IV 2 O 7 Bi III 2 Hf IV 2 O 7

Plan for pyrochlores project Model Bi 2 Ti 2 O 7 (known structure, but …). Substitute Zr for Ti and calculate the energy minimised structure. Compare with the structure synthesised by Luke Daniels (predict powder pattern and compare with experimental pattern). We can then model intermediate structures and doped materials. Keele Research Seminar, 24 November

Summary The technique of materials modelling has been introduced and set in the overall context of computational chemistry. Some current examples have been considered, both of complete and ongoing projects. I have (hopefully) given you an idea of the scope of the technique, and what can be achieved. Keele Research Seminar, 24 November

Acknowledgements Keele University Centre for the Environmental, Physical and Mathematical Sciences (iEPSAM) 42 Keele Research Seminar, 24 November 2010 Liz Maddock, Tom Littleford, Scott Walker, Michael Montenari, Richard Darton (Keele) Mark Read, Dave Plant (AWE) Mário Valerio, Jomar Amaral, Marcos Rezende (UFS) Eric Hudson (UCLA)