Modern strategies for safe handling of high level waste (HLW) and their long-term disposal in deep geological formations include the immobilization of radionuclides in the form of mineral-like matrices. The...

Illinois 601 15. USC M S . received 14th M a y 1970Ahtract. Starting from bare-ion pseudopotentials, net crystal potentials are evalutiied in direct space by (a) the usual linear screening technique and (b) the Thomas-Fermi method. For the metals considered (Na and AI), the potentials are quite similar, crossing outside the cores and never differing by more than 0.02 a.u. in the case of Na nor by more than 0.2 a.u. in the case of Al. The similarity of the results based on (a) and (b) suggest the use of the pseudopotential concept in conjunction with the rather general method (b) under conditions when (a) is questionable, that is whenever the zeroth order uniform electron gas solution is a poor physical approximation to the actual system under study (specific examples being molecules, insulating solids and interfaces).Methods (a) and (bj both yield shallower wells than those most often used as input information in band structure calculations and obtained by superposing atomic data.

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].

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