On the role of U/ThO8 polyhedral distortions in controlling the high-pressure zircon&#8594;reidite type transition in UxTh1-xO4
Authors: Mondal Sudip Kumar 
Published: March 2020

&lt;p&gt;&lt;strong&gt;On the role of U/ThO&lt;sub&gt;8 &lt;/sub&gt;polyhedral distortions in controlling the high-pressure zircon&amp;#8594;reidite type transition in U&lt;sub&gt;x&lt;/sub&gt;Th&lt;sub&gt;1-x&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Sudip Kumar Mondal&lt;sup&gt;1&lt;strong&gt;,&lt;/strong&gt;2&lt;/sup&gt;, Pratik Kr Das&lt;sup&gt;2&lt;strong&gt;,&lt;/strong&gt;3&lt;/sup&gt;, Nibir Mandal&lt;sup&gt;2&lt;/sup&gt; and Ashok Arya&lt;sup&gt;4&lt;/sup&gt;&lt;br&gt;1 Department of Physics, Jadavpur University, Kolkata 700032, India&lt;br&gt;2 Faculty of Science, High Pressure and Temperature Laboratory, Jadavpur University, Kolkata 700032, India&lt;br&gt;3 The Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, N-0315, Norway&lt;br&gt;4 Material Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Coffinite (USiO&lt;sub&gt;4&lt;/sub&gt;) and thorite (ThSiO&lt;sub&gt;4&lt;/sub&gt;) are conspicuous radiogenic silicates in the geonomy. They form U&lt;sub&gt;1&lt;/sub&gt;&lt;sub&gt;-&lt;/sub&gt;&lt;sub&gt;x&lt;/sub&gt;Th&lt;sub&gt;x&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt; (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&lt;sub&gt;1&lt;/sub&gt;&lt;sub&gt;-&lt;/sub&gt;&lt;sub&gt;x&lt;/sub&gt;Th&lt;sub&gt;x&lt;/sub&gt;SiO&lt;sub&gt;4 &lt;/sub&gt;solid solution, and investigated their mechanical behaviour. Our ab-initio calculations revealed a unique interconnection of phase transition pressure (p&lt;sub&gt;t&lt;/sub&gt;) 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&lt;sub&gt;t&lt;/sub&gt;&amp;#8217;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/ThO&lt;sub&gt;8&lt;/sub&gt;-triangular dodecahedra (snub-disphenoids). We have parameterized two new factors: &amp;#948; (longitudinal distortions) and &amp;#963;&lt;sup&gt;2&lt;/sup&gt; (angular distortions) to quantify the polyhedral distortions. A detailed analysis of the snub-disphenoidal distortions demonstrates that the difference in angular distortion of UO&lt;sub&gt;8&lt;/sub&gt; and ThO&lt;sub&gt;8&lt;/sub&gt; polyhedra (i.e. &amp;#963;&lt;sub&gt;U&lt;/sub&gt;&lt;sup&gt;2 &lt;/sup&gt;and &amp;#963;&lt;sub&gt;Th&lt;/sub&gt;&lt;sup&gt;2&lt;/sup&gt;) between zircon- and&amp;#160; 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&lt;sub&gt;0.5&lt;/sub&gt;Th&lt;sub&gt;0.5&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt;. Our result is also substantiated by the minimum compressibility observed for the zircon-type U&lt;sub&gt;0.5&lt;/sub&gt;Th&lt;sub&gt;0.5&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt;. It is worthwhile to note that the distortions parameters, &amp;#948; and &amp;#963;&lt;sup&gt;2&lt;/sup&gt; are defined without any attribute to external parameters. They are also independent to the elements occupying the polyhedra. Thus, we propose that these parameters: &amp;#948; and &amp;#963;&lt;sup&gt;2&lt;/sup&gt; can also be used to calculate the distortions of similar AB&lt;sub&gt;8&lt;/sub&gt;-type snub-disphenoids observed in zircon-, reidite-, fergusonite- and wolframite-type mineral phases.&lt;/p&gt;