Prediction of new iodine-containing apatites using machine learning and density functional theory
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rtificial Intelligence Research Letter
Prediction of new iodine-containing apatites using machine learning and density functional theory Timothy Q. Hartnett* and Mukil V. Ayyasamy*, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USA Prasanna V. Balachandran , Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USA; Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904, USA Address all correspondence to Prasanna V. Balachandran at [email protected] (Received 20 February 2019; accepted 26 July 2019)
Abstract The authors develop a computational approach that integrates machine learning (ML) and density functional theory (DFT) with experimental data to predict formable and thermodynamically stable iodine-containing apatites. This is an important problem because radioactive iodine is toxic and capturing it in solid waste forms have implications in remediation treatments. The authors train ML models using 336 compositions and screen 54 iodine-containing compounds in apatite stoichiometry. ML models predict 18 as formable and 24 as nonformable in the apatite structure; 12 compounds were identified to be uncertain. DFT convex hull predicted two to be thermodynamically stable, one as metastable, and nine as unstable.
Introduction Long-term storage and safe disposal of toxic wastes (nuclear, radioactive, or otherwise) is a pressing environmental concern that must be addressed for a sustainable future. Glasses and crystalline ceramic materials represent some of the common inorganic host materials that have been explored and have shown promise for the immobilization of waste materials.[1,2] One of the waste forms that is generated from nuclear power plants is the toxic and radioactive iodine (I-129)[3], and there is a growing priority to capture radioactive iodine, because unattended I-129 entering the ecosystem is a severe concern due to its long half-life, high solubility in water, and potential human ingestion.[4] More recently, materials that form an “apatite” crystal structure type have been considered as a potential host lattice for immobilizing the I-129 isotope.[3,5] Compounds with the apatite crystal structure type have a general formula A10(BO4)6X2, where the A-site is occupied by larger monovalent (Na+, K+, etc.), divalent (Ca2+, Sr2+, Ba2+, Pb2+, etc.), or trivalent (La3+, Y3+, Ce3+, Sm3+, etc.) cations, the B-site is occupied by smaller metals and metalloids (P5+, As5+, V5+, Si4+, etc.), and the X-site is filled by halide, hydroxide, or oxide anions (F−, Cl−, Br−, I−, OH−, O2−, etc.). The aristotype or the parent crystal structure of an apatite has a hexagonal symmetry in the space group, P63/m (# 176), with a one-dimensional channel parallel to the c-axis. Crystallographically, there are two distinct A-sites, AI and
* These two authors contributed equally to this work.
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AII, occupying the Wyckoff sites 4f and 6h, respectively. The crystal structure is made up
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