On the role of U/ThO8 polyhedral distortions in controlling the high-pressure zircon→reidite type transition in UxTh1-xO4

On the role of U/ThO₈ polyhedral distortions in controlling the high-pressure zircon→reidite type transition in U_xTh_1-xO₄

 

Sudip Kumar Mondal^1,2, Pratik Kr Das^2,3, Nibir Mandal² and Ashok Arya⁴
1 Department of Physics, Jadavpur University, Kolkata 700032, India
2 Faculty of Science, High Pressure and Temperature Laboratory, Jadavpur University, Kolkata 700032, India
3 The Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, N-0315, Norway
4 Material Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India

 

Coffinite (USiO₄) and thorite (ThSiO₄) are conspicuous radiogenic silicates in the geonomy. They form U_1-xTh_xSiO₄ (uranothorite) solid solutions in zircon-type phase. Investigating the phase-evolution of these minerals is of utmost significance in realizing their applicability in the front-as well as at the back-end of nuclear industries and also from geological perspective, such as geochronology. We carried out a systematic study of zircon- to reidite-type (tetragonal I41/amd to I41/a) structural transitions of U_1-xTh_xSiO₄ solid solution, and investigated their mechanical behaviour. Our ab-initio calculations revealed a unique interconnection of phase transition pressure (p_t) with the change in U-Th concentration in the solid solution. The transition pressure is found to be minimum (6.82 GPa) for x = 0.5 whereas for the endmembers coffinite and thorite p_t’s are 8.52 and 8.68 GPa, respectively. We developed a novel method to estimate the longitudinal and angular distortions of the highly irregular U/ThO₈-triangular dodecahedra (snub-disphenoids). We have parameterized two new factors: δ (longitudinal distortions) and σ² (angular distortions) to quantify the polyhedral distortions. A detailed analysis of the snub-disphenoidal distortions demonstrates that the difference in angular distortion of UO₈ and ThO₈ polyhedra (i.e. σ_U² and σ_Th²) between zircon- and  reidite-type phases becomes minimum when U and Th percentage are equal, leading to the structural phase transition at the minimum hydrostatic pressure for the unique chemical composition: U_0.5Th_0.5SiO₄. Our result is also substantiated by the minimum compressibility observed for the zircon-type U_0.5Th_0.5SiO₄. It is worthwhile to note that the distortions parameters, δ and σ² are defined without any attribute to external parameters. They are also independent to the elements occupying the polyhedra. Thus, we propose that these parameters: δ and σ² can also be used to calculate the distortions of similar AB₈-type snub-disphenoids observed in zircon-, reidite-, fergusonite- and wolframite-type mineral phases.