By the local density approximation with on-site Coulomb repulsion $U(\mathrm{LDA}+U)$ method with spin-orbit coupling $(\mathrm{LDA}+U+\mathrm{SO})$ the magnetic state and electronic structure have been investigated for plutonium in $\delta$ and $\alpha$ phases and for the Pu compounds PuN, $\mathrm{Pu}\mathrm{Co}{\mathrm{Ga}}_5$, $\mathrm{Pu}{\mathrm{Rh}}_2$, $\mathrm{Pu}{\mathrm{Si}}_2$, PuTe, and PuSb. In agreement with experiment we found for metallic plutonium in both phases a nonmagnetic ground state with Pu ions in $f^6$ 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 $f^{5∕2}$ and $f^{7∕2}$ subbands even in the LDA calculation. The Fermi level is in a pseudogap between them, so that the $f^{5∕2}$ 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 $J_H=0.48\mathrm{eV}$ 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 $f^6$ configuration with nonzero magnetic moments was found in $\mathrm{Pu}\mathrm{Co}{\mathrm{Ga}}_5$, $\mathrm{Pu}{\mathrm{Si}}_2$, and PuTe, while PuN, $\mathrm{Pu}{\mathrm{Rh}}_2$, and PuSb have the $f^5$ 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.

• Received 28 December 2004

DOI:https://doi.org/10.1103/PhysRevB.72.024458