Density Functional Theory Calculations On Magnetic Properties Of Actinide Compounds
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Density Functional Theory Calculations On Magnetic Properties Of Actinide Compounds Eugene Heifets and Denis Gryaznov Institute for Solid State Physics, University of Latvia, Kengaraga 8, LV-1063,Riga, Latvia. ABSTRACT We have performed a detailed analysis of the magnetic (collinear and noncollinear) order and atomic and electron structures of UO2, PuO2 and UN on the basis of density functional theory with the Hubbard electron correlation correction (DFT+U). We have shown that the 3-k magnetic structure of UO2 is stabilized for the Hubbard parameter value of U=4.6 eV (while J=0.5 eV) when Dudarev’s formalism is used. UO2 keeps cubic shape in this structure. Two O atoms nearest to each U atom in direction of its magnetic moment move toward this U atom. Neither UN nor PuO2 shows the energetical preference for the rhombohedral distortion, in contrast to UO2, and, thus, no complex 3-k magnetic structure in these materials. Both materials have the AFM tetragonal structure at reasonable choice of parameters U and J. INTRODUCTION Actinide compounds continue to attract a great interest for both materials scientists and nuclear engineers. Their properties combine a strong electron correlation and relativistic effects of 5f valence electrons. In this paper, we study collinear and non-collinear magnetic structures of three basic actinide materials UO2, PuO2 and UN. All these materials have face-centred cubic (f.c.c.) actinide sub-lattice: the two oxides have fluorite structure and UN rock-salt structure. Experiments suggest that at low temperatures UN is anti-ferromagnetic1 with a collinear magnetic order, where U magnetic moments alternate along the direction, while UO2 is anti-ferromagnetic (AFM) with the so-called noncollinear 3-k ordering of U magnetic moments. All recent theoretical considerations2-5 employing the DFT+U technique or hybrid exchangecorrelation functional suggested the 1-k (collinear) AFM order for insulating PuO2, while experiment suggests that PuO2 is diamagnetic6. Thus, it is important to compare magnetic orders and accompanying lattice distortions for three considered compounds (UO2, UN, and PuO2) using the same method. Ignoring the lattice distortions may lead to a wrong electronic structure and significant errors in the defect energetics7. As it was already mentioned, these materials reveal the same f.c.c. structure in the actinide sublattice. Therefore, similar structure of exchange interactions could be expected. To the best of our knowledge, the only successful first-principles modeling of the noncollinear magnetic ordering in UO2 was published by Laskowski et. al.8. This study employed the DFT+U technique within the local spin density approximation (LSDA)9 and all-electron linearized augmented plane wave plus local orbitals method (L/APW+lo)10. In these computations, the 3-k structure appears to be more stable, if the double counting correction accurately includes spin-polarization of the electron density8. Also, no significant lattice distortions were found in early experimental studies of UO2. O
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