Magnetic state and electronic structure of theδandαphases of metallic Pu and its compounds

By the local density approximation with on-site Coulomb repulsion U0.2em0ex(LDA+U) method with spin-orbit coupling (LDA+U+SO) the magnetic state and electronic structure have been investigated for plutonium in δ and α phases and for the Pu compounds PuN, PuCoGa₅, PuRh₂, PuSi₂, PuTe, and PuSb. In agreement with experiment we found for metallic plutonium in both phases a nonmagnetic ground state with Pu ions in f⁶ configuration with zero values of spin, orbital, and total moments. This result is determined by a strong spin-orbit coupling in the $5f$ shell. It leads to the clear splitting of $5f$ states into f5∕2 and f7∕2 subbands even in the LDA calculation. The Fermi level is in a pseudogap between them, so that the f5∕2 subshell is already almost completely filled with six electrons before Coulomb correlation effects are taken into account. The competition between spin-orbit coupling and the exchange (Hund) interaction (favoring a magnetic ground state) in the $5f$ shell is so delicately balanced that a small increase (less than 15%) of the exchange interaction parameter value from JH=0.480.3em0exeV obtained in the constrained LDA calculation would result in a magnetic ground state with nonzero spin and orbital moment values. For the Pu compounds investigated in the present work, a predominantly f⁶ configuration with nonzero magnetic moments was found in PuCoGa₅, PuSi₂, and PuTe, while PuN, PuRh₂, and PuSb have the f⁵ configuration with sizable magnetic moment values. Whereas the pure $jj$ coupling scheme was found to be valid for metallic plutonium, an intermediate coupling scheme is needed to describe the $5f$ shell in Pu compounds. The results of our calculations show that the exchange interaction term in the Hamiltonian should be treated in a general matrix form for Pu and its compounds.