Multiple cascade radiation damage simulations of pyrochlore

ABSTRACT

We report molecular dynamics simulations of multiple radiation damage cascades in the pyrochlores Gd₂Ti₂O₇ and Gd₂Zr₂O₇ and the solid solution Gd₂(Zr_xTi_1–x)₂O₇ (x = 0.25, 0.50, 0.75). Using a simulation cell of 360,448 atoms, for each compound 2200 decay events are simulated over a total time of 10 ns, with each recoiling uranium atom (primary-knock-on atom) assigned initial kinetic energy of 5 keV. The structures generated are analysed using Steinhardt local order parameters. There is a large increase in volume for the Ti pyrochlore associated with a transition to an amorphous structure which resembles the melt while preserving the local environment of the Ti. The calculated dose for amorphisation is 20 eV atom⁻¹ which compares well with experiment overlap of cascade and damage accumulation drives the amorphisation suppressessing the healing mechanisms. The behaviour of the zirconate is different – the substantial anion disorder produced by each recoil event is followed by healing and reversion to the parent pyrochlore. In the solid solution the onset of amorphisation is delayed to later times on increasing the Zr concentration and overall swelling reduced. Our simulations highlight the importance of ion mobility, associated with the weaker Zr–O bonds, in healing.

KEYWORDS:

• Pyrochlore
• multiple cascades
• radiation damage
• molecular dynamics
• healing

Acknowledgements

Support from EPSRC is gratefully acknowledged (grants EP/H013814/1, EP/H012230/1 and EP/H012990/1). AA thanks EPSRC for a project studentship. All the cascade runs were carried out using the computational facilities of the Advanced Computing Research Centre, University of Bristol – http://www.bris.ac.uk/acrc/.

Data availability statement

Data are available at the University of Bristol data repository (https://data.bris.ac.uk/data) at https://doi.org/10.5523/bris.1aognlcuc4yjp2ly9mysyvgkje

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 The experimental dosage of the order of 10¹⁹ α-particles (with energy of ∼10 keV per α-particle) per gram of Gd₂Ti₂O₇ (RMM 522) translates to an amorphisation dose of 0.5×10²⁶ eV per mol of Gd₂Ti₂O₇. The simulation dosage for amorphisation of Gd₂Ti₂O₇ is calculated as the product of the PKA energy by the number of events leading to 90% amorphisation, divided by the total number of atoms in the MD cell. This results in 20 eV per atom or 1.4×10²⁶ eV per mol of Gd₂Ti₂O₇.

Additional information

Funding

Support from Engineering and Physical Sciences Research Council (EPSRC) is gratefully acknowledged [grant numbers EP/H013814/1, EP/H012230/1 and EP/H012990/1). AA thanks EPSRC for a project studentship.