Unveiling the Energetic and Structural Properties of Pu Doped Zircon Through Electrochemical Equilibrium Diagram from DFT+U Calculations

Zircon (ZrSiO4) mineral is a sustainable and promising material to store of radioactive waste that has received extensive attention by material, geochemical and environmental scientists. Although the incorporation of actinide elements in zircon lattices has been experimentally studied, bare fundamental work are carried out to systemically assess the structural and chemical stabilities of Pu doped zircon. The primary aim to unveil the Pu immobilization mechanism and assess the stability of PuxZr1-xSiO4 is carried out by calculating the formation energies, electron and hole affinities, and electronic levels of Pu doped zircon based on density functional theory. Our results reveal under mu = mu(O-poor) condition Pu-Si(4+), Pu-Zr(1+) and Pu-Zr(0) are respectively energetically favorable to form with increasing the electronic chemical potential. Besides, Pu-Zr(4+) is energetically favorable in an n-type environment under all these three conditions (i.e., mu = mu(O-poor), mu = mu(Pu/Zr), mu = mu(Pu/Si)). In addition, Pu doping will induce local structural distortion. Intriguingly however, self-repairing the symmetry of [ZrO8] polyhedra is first observed via the structural distortion in Pu-Zr(4+) configuration, which in turn could enhance the structural stability of PuxZr1-xSiO4. Ab initio molecular dynamic simulations demonstrate the configurations with negative formation energies are thermal stable at 500 K. The charge density difference and charge transfer are investigated to describe the chemical bonding nature. It is demonstrated Pu(5f)-O(2p) hybridization is more profound for interstitial Pu. Moreover, the bonding character of surrounding Zr atoms along [010] direction is almost identical to the pristine one, while it is distinctly changed towards [100] and [001] directions, showing remarkable anisotropy of PuxZr1-xSiO4. Oppositely, the ionicity in Pu-O bond is mainly featured when Zr or Si sites are substituted by Pu atoms which becomes stronger with increasing the hole doping process. (C) 2020 Elsevier B.V. All rights reserved.