Structural, Vibrational, Elastic and Topological Properties of PaN Under Pressure

Electronic, structural, vibrational and elastic properties of PaN have been studied both at ambient and high pressures, using first principles methods with several commonly used parameterizations of the exchange-correlation energy. The generalized gradient approximation (GGA) reproduces the ground state properties satisfactorily. Under pressure PaN is found to undergo a structural transition from NaCl to the R-3m structure near 58 GPa. The high pressure behavior of the acoustic phonon branch along the (1,0,0) and (1,1,0) directions, and the C44 elastic constant are anomalous, which signals the structural transition. With GGA exchange-correlation, a topological transition in the charge density occurs near the structural transition which may be regarded as a quantum phase transition, where the order parameter obeys a mean field scaling law. However, the topological transition is absent when other exchange-correlation functionals are invoked (local density approximation (LDA) and hybrid functional). Therefore, this constitutes an example of GGA and LDA leading to qualitatively different predictions, and it is of great interest to examine experimentally whether this topological transition occurs.

The Electronic Correlation Strength of Pu

An electronic quantity, the correlation strength, is defined as a necessary step for understanding the properties and trends in strongly correlated electronic materials. As a test case, this is applied to the different phases of elemental Pu. Within the GW approximation we have surprisingly found a universal scaling relationship, where the f-electron bandwidth reduction due to correlation effects is shown to depend only on the local density approximation bandwidth and is otherwise independent of crystal structure and lattice constant.

Electronic structure and ionicity of actinide oxides from first principles calculations

The ground state electronic structures of the actinide oxides AO, A2O3 and AO2 (A=U, Np, Pu, Am, Cm, Bk, Cf) are determined from first-principles calculations, using the self-interaction corrected local spin-density (SIC-LSD) approximation. Emphasis is put on the degree of f-electron localization, which for AO2 and A2O3 is found to follow the stoichiometry, namely corresponding to A(4+) ions in the dioxide and A(3+) ions in the sesquioxides. In contrast, the A(2+) ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidation and reduction of the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onwards. Our study reveals a strong link between preferred oxidation number and degree of localization which is confirmed by comparing to the ground state configurations of the corresponding lanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compounds will form where the calculated ground state valency agrees with the nominal valency expected from a simple charge counting.

GW correlation effects on plutonium quasiparticle energies: changes in crystal-field splitting

We present results for the electronic structure of plutonium by using a recently developed quasiparticle self-consistent $GW$ method (qsgw). We consider a paramagnetic solution without spin-orbit interaction as a function of volume for the face-centered cubic (fcc) unit cell. We span unit-cell volumes ranging from 10% greater than the equilibrium volume of the $delta$ phase to 90 % of the equivalent for the $alpha$ phase of Pu. The self-consistent $GW$ quasiparticle energies are compared to those obtained within the Local Density Approximation (LDA). The goal of the calculations is to understand systematic trends in the effects of electronic correlations on the quasiparticle energy bands of Pu as a function of the localization of the $f$ orbitals. We show that correlation effects narrow the $f$ bands in two significantly different ways. Besides the expected narrowing of individual $f$ bands (flatter dispersion), we find that an even more significant effect on the $f$ bands is a decrease in the crystal-field splitting of the different bands.

Groundstate electronic structure of actinide carbides and nitrides

The self-interaction corrected (SIC) local spin-density approximation (LSD) is used to investigate the groundstate valency configuration of the actinide ions in the actinide mono-carbides, AC (A = U, Np, Pu, Am, Cm), and the actinide mono-nitrides, AN. The electronic structure is characterized by a gradually increasing degree of f-electron localization from U to Cm, with the tendency towards localization being slightly stronger in the (more ionic) nitrides compared to the (more covalent) carbides. The itinerant band-picture is found to be adequate for UC and acceptable for UN, whilst a more complex manifold of competing localized and delocalized f-electron configurations underlies the groundstates of NpC, PuC, AmC, NpN, and PuN. The fully localized 5f-electron configuration is realized in CmC (f7), CmN (f7), and AmN (f6). The observed sudden increase in lattice parameter from PuN to AmN is found to be related to the localization transition. The calculated valence electron densities of states are in good agreement with photoemission data.