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Our density functional study reveals a peculiar relation between thenormalized Th content i.e. Th/(U+Th) and the hydrostatic pressure enforcingzircon to reidite type transition in uranothorite solid solution. We found thatthe transition pressure exhibits a minimum when Th/(U+Th) = 0.50 i.e. when theunit cell contains equal amount of U and Th. Any change in normalized Thcontent, as we move towards the U and Th end-members of the series, shows asignificant jump in zircon-reidite transition pressure. In order to explainthis behavior, we proposed a novel method of quantifying the distortions ofU/Th-coordination polyhedra which manifest themselves as a triangulardodecahedron or snub-disphenoid. We have theoretically defined two parameters:one related to the two geometrically different U/Th-O bond lengths, having afixed ratio between them for regular triangular dodecahedra irrespective of thevolume and the other with three kinds of O-U-O and O-Th-O bond angles withmultiplicities of 2, 4 and 12, respectively. The bond angle distortion for bothU and Th-polyhedra were found to show a maximum in zircon phase and a minimumin reidite phase corresponding to Th/(U+Th) = 0.5. Since, the difference inangular distortion is minimum in case of Th/(U+Th) = 0.5, a lesser hydrostaticpressure is capable of triggering the transition, compared to the other membersof this series. The bond length distortion is also characterized by a maximumin the reidite phase at the equal concentration of U and Th. Our finding isalso complemented by the minimum compressibility observed in the zircon phasewhen the atomic percentage of U and Th are equal.

journal of physics. condensed matter

A novel approach to quantify the structural distortions of U/Th  snub-disphenoids and their role in zircon to reidite type phase transitions  of uranothorite