COMB Potential Simulating Metal Uranium and Hyper- stoichiometric - - PowerPoint PPT Presentation

COMB Potential Simulating Metal Uranium and Hyper- stoichiometric UO2+x

Yangzhong Li, Simon R. Phillpot Department of Materials Science and Engineering University of Florida

Structure of α-U

α-U lattice Exp COMB DFT a (Å) 2.85 2.78 2.74 b (Å) 5.87 6.15 5.86 c (Å) 4.96 4.80 4.96 y 0.102 0.092 0.099 V (Å3) 82.99 80.95 79.64

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α-U EC Exp COMB C11 214.7 164.7 C12 46.5 94.0 C13 21.8 61.3 C22 198.6 204.5 C23 107.6 20.3 C33 267.1 261.8 C44 124.5 135.1 C55 73.4 62.7 C66 74.3 54.5 B 113.1 109.0 G 84.34 73.6 bcc-U Exp DFT COMB a0 (Å) 3.47 3.43 3.51 E – E(α) 0.28 0.45 B (GPa) 113 136 135

MD Simulation – Lattice Parameter

• COMB captures the anisotropic variation of unit

cell lattice

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2.5 3 3.5 4 4.5 5 5.5 6 6.5 100 200 300 400 500 600 700 800 900 1000 1100 1200

Lattice Parameter (Å) Temperature (K)

α-β Exp a Exp b Exp c Exp 3√V β-γ MD a MD b MD c MD 3√V

UO2 Static Property

DFT COMB Exp a0 (Å) 5.41 5.47 5.47 C11 387 386 389 C12 113 119 119 C44 71 60 60 B 204 211 209 G 93 90 84 Uranium Charge 2.5 0.9

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(eV) DFT COMB Exp OI

• 2.45
• 1.16

OV 6.35 9.12 UI 7.33 5.84 UV 3.84 15.72 O FP 3.90 7.96 3-4.6 U FP 11.17 21.56 8.5-9.6 Schottky 14.03 33.9 6-7

UO2 MD Simulation

• Structural stability
• Solid: 0-3200K
• O melting: 3200-4200K
• Lattice melting: >4200K

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10 20 30 40 50 60 70 80 90 100 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 Cp*10^5 (eV/K) T (K)

COMB Exp O melting 3200K 2670K Lattice melting 4200K 3100K CTE×106 10 10-30 ΔH fusion (kJ/mol) 51 70 ± 4 ΔV/V fusion 7.6% 8.0%

UO2+x Model

• U sublattice remains in fcc structure
• Original cubic O lattice rearrange to

accommodate new O interstitial

• Most O remains in cubic structure
• Some O cube become cuboctahedron (COT)
• Willis 2:2:2 model

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Willis 2:2:2 Model

• A model of conjecture based on diffraction data
• 2 O vacancy; 2 O’ displaced by 1Å; 2 O’’ displaced by 1Å
• Further developed to explain the U4O9 structure; the 2:2:2

chain

• The 2:2:2 chain model contradict with later exp data
• The Willis 2:2:2 model is proved unstable by DFT
• The 2:2:2 chain structure in COMB changes from 10

coordinated to 8 for U

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COT Structure

• DFT shows a COT oxygen within a fcc U is a

stable structure

• COT with an extra O at the center is more stable

than a one without

• COMB shows that U4O9 becomes UO2+x at

1400K; matches exp result

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Method Structure U–O rU (Å) rO (Å) UOU (deg) OUO (deg) Exp 2.79 2.79 2.93 127.9 141.5 DFT U4O9-o 2.76 2.76 2.90 127.9 141.6 U4O9-v 2.92 2.92 2.82 139.7 129.8 COMB U4O9-o 2.26 2.65 3.09 112.6 151.5 U4O9-v 2.37 3.15 2.81 141.6 116.1

Center O in COT

• A center O stabilize the COT structure (top row)
• Otherwise COT collapses to form cube (down row)

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0K 300K 600K 1000K 2000K

UO2.25 Structure

• Pristine UO2 unit cell + one O at cubic center
• O displaced from cubic center in 10ps
• O forming fragments of COT
• COT centers on U but not O atom!
• COT-building process might never completed in

COMB MD since it requires highly cooperating motion of O

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A small portion of UO2.25 showing the formation process of a COT structure at 40ps

Conclusion

• COMB U potential is able to capture the order

and sign of lattice variation at different direction

• COMB UO2 potential is able to generate

consistent results with DFT and experiment

• UO2 lattice, elastic, defect, dynamic and fusion

parameters are of good quality

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DFT & Exp COMB Willis 2:2:2 unit and chain 2:2:2 unit not stable 2:2:2 chain is wrong 2:2:2 unit & chain disassemble COT, U4O9 O-centerd COT matches exp result better U4O9 with O center in MD is more stable than without UO2.25 Becomes U4O9 at this O/U ratio Form COT fragment, but centered by U