SELF-RADIATION DAMAGE AND SiO4 POLYMERIZATION IN CRYSTALS OF ZIRCON FROM NUCLEAR MAGNETIC RESONANCE AND MOLECULAR DYNAMICS SIMULATION DATA

UDC 549.514.81 : 544.228 : 004.94

https://doi.org/10.15407./mineraljournal.39.04.058

А.Е. Grechanovsky 1, A.B. Brik 1, N.N. Eremin 2, V.V. Radchuk 3, A.M. Kalinichenko 1, L.I. Kanunikova 1

1 M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine

34, Acad. Palladin Ave., 03142, Kyiv, Ukraine

E-mail: grechanovsky@gmail.com

2 Lomonosov Moscow State University

1, Leninsky Gory, GSP-1, 119992, Moscow, Russia

Е-mail: neremin@geol.msu.ru

3 Institute of Telecommunications and Global Information Space of the NAS of Ukraine

13, Chokolovsky Blvd., 03186, Kyiv, Ukraine

Е-mail: valentyn.radchuk@gmail.com

SELF-RADIATION DAMAGE AND SiO4 POLYMERIZATION IN CRYSTALS OF ZIRCON FROM NUCLEAR MAGNETIC RESONANCE AND MOLECULAR DYNAMICS SIMULATION DATA

Language: Russian

Mineralogical journal 2017, 39 (4): 58-66

Abstract: Zircon is considered as a promising matrix for the utilization of nuclear fuel and weapons-grade plutonium. However, the alpha decay of radioactive elements causes the destruction of the crystal structure and transition of zircon to the metamict state. Investigations of metamict zircons by the method of nuclear magnetic resonance (NMR) have established a number of characteristic features of the structure of metamict zircons. Despite a large amount of research, the ideas about the mechanisms of radiation damage of zircon structure remain controversial. Computer simulation methods are used to obtain additional data on the processes of zircon radiation damage. One of the most powerful computational methods is that of computer molecular dynamics (MD). The aim of this work was to study the local structure of partially metamict zircons using the methods of MD, simulation and NMR. To obtain information on the local structure of zircon, spectra of zircon samples from Yastrubtsi ore occurence (the Ukrainian Shield) with different metamict degrees were obtained by the method of high-resolution nuclear magnetic resonance (MAS NMR) on the 29Si nucleus. It is established that the phase of amorphous SiO2 is practically absent in these zircons. Results show that mainly Q1–2 and Q3 polymerization takes place in zircon. The MAS NMR results are in good agreement with the data of X-ray phase analysis. The MD method consists in calculating the paths of motion of all atoms in the system based on Newton’s second law. In addition to the Coulomb interaction of all electrostatic charges, the interatomic potentials take into account short-range interaction between atoms. In the zircon structure, a supercell of 15 × 15 × 17 unit cells (91800 atoms) was chosen. To model the overlap of the cascades, 5 % of the Zr atoms were replaced by Th atoms in the center of the cell. The potentials for interatomic interactions reproduce structural parameters with an accuracy of up to 1.5 %. The MD method was used to simulate the overlapping of more than 120 cascades of displacements that occur during moving of recoil nuclei (analogues of Th atoms) with energy of 7.5 keV. The results show that in zircon throughout the entire volume there are regions with polymerization indices Q1 and Q2. The areas with polymerization index Q3 are present as separate islands. There is also an insignificant amount of silica (amorphous SiO2) phase. The models of zircon amorphization (models of "direct amorphization", "single overlapping" and "double overlapping") are considered. The results obtained by us using the MD method show that the most reliable model is the "direct amorphization" model.

Keywords: radiation mineralogy, radiation resistance of minerals, molecular dynamics method, nuclear magnetic resonance, computer simulation of structure, zircon.

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