Which oxidation state of uranium and thorium as point defects in xenotime is favorable?

Abstract

Relativistic study of xenotime, YPO4, containing atoms thorium and uranium as point defects is performed in the framework of cluster model with using the compound-tunable embedding potential (CTEP) method proposed by us recently [1]. The Y-(PO4)6-Y'22-O'104 cluster for xenotime is considered, in which central part, [Y-(PO4)6] −15 , is the main cluster, whereas outermost 22 atoms of yttrium and 104 atoms of oxygen are treated as its environment and compose electron-free CTEP with the total charge of +15. The P and O atoms of the orthophosphate groups nearest to the central Y atom are treated at all-electron level. The central Y, its substitutes, Th and U, together with environmental Y atoms are described within different versions of the generalized relativistic pseudopotential method [2]. Correctness of our cluster and CTEP models, constructed in the paper, is justified by comparing the Y-O and P-O bond lengths with corresponding periodic structure values of the YPO4 crystal, both experimental and theoretical.Using this cluster model, chemical properties of solitary point defects, X = U, Th, in xenotime are analyzed. It has been shown that the oxidation state +3 is energetically more profitable than +4 not only for thorium but for uranium as well (∆E ≈ 5 eV) despite the notably higher ionic radius of U +3 compared to Y +3 , whereas ionic radii of U +4 and Y +3 are close. This leads to notable local deformation of crystal geometry around the U +3 impurity in xenotime and contradicts to widespread opinion about favorite oxidation state of uranium in such kind of minerals [3].

Compound-tunable embedding potential method and its application to ytterbium fluoride crystals YbF$_2$ and YbF$_3$

“…Compound-tunable embedding potential method developed by us in [1,2] to describe electronic structure of fragments which may contain defects with f −elements is applied to crystals containing periodically arranged lanthanide atoms. The YbF 2 and YbF 3 crystals are considered such that 4f shell is excluded from both the crystal and cluster stages of generating the CTEP to make calculations with crystal code stable.…”

“…For simulation of the crystal structure, the basis set overflow arising in presence of diffuse orbitals can be significant, since relatively small atomic basis sets are used for such studies to avoid their linear dependence. In turn, they can be well corrected in cluster calculations with compound-tunable embedding potential (CTEP) [1,2]. The electronic properties of the Yb atom in YbF 2 and YbF 3 crystals are studied using the following cluster model.…”

“…Such a fragment with embedding potential is usually called the "embedded cluster" or "cluster, embedded in a crystal". This work is devoted to theoretical modeling of ytterbium di-and tri-fluorides using compound-tunable embedding potential method described in work [1,2].…”

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Compound-tunable embedding potential (CTEP) method developed in [1,2] to describe electronic structure of fragments in materials is applied to crystals containing periodically arranged lanthanide atoms, which can have open 4f shell. We consider YbF2 and YbF3 as examples such that 4f shell is excluded from both the crystal and cluster stages of generating the CTEP.

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