Electronic structure and magnetic state of transuranium metals under pressure

Abstract

Electronic structure of bcc Np, fcc Pu, Am, and Cm pure metals under pressure has been investigated employing the LDA+U method with spin-orbit coupling (LDA+U +SO). Magnetic state of the actinide ions was analyzed in both LS and jj coupling schemes to reveal the applicability of corresponding coupling bases. It was demonstrated that whereas Pu and Am are well described within the jj coupling scheme, Np and Cm can be described appropriately neither in {mσ}, nor in {jmj } basis, due to intermediate coupling scheme realizing in these metals that requires some finer treatment. The LDA+U +SO results for the considered transuranium metals reveal bands broadening and gradual 5f electron delocalization under pressure. 71.27.+a, Prominent structural transition from δ-to α-phase of plutonium gives substantial total volume contraction [1]. In americium and curium the similar transitions were also found under high pressure or increasing temperature [2,3]. These volume collapses are usually related to the drastic electron delocalization. In curium, additional magnetic stabilization of intermediate phases was also found [3,4].In many strongly correlated materials Coulomb and Hund interactions are dominant, whereas spin-orbit (SO) coupling is comparably smaller and usually can be treated within various perturbative techniques. In the case of 5f electronic shell of actinide metals all these three terms in Hamiltonian are of comparable strength. That results in a very sensitive (non)magnetic ground state, but also in coupling schemes varying from usual LS (Russel-Saunders) one. An intermediate or jj coupling schemes were found to be more appropriate for 5f electrons in transuranium metals [5,6,7,8].In actinide elements spin-orbit coupling is stronger than exchange Hund interaction, and hence the jj coupling scheme can be valid with a well defined total moment J, but in this case spin S and orbital L moments are not well defined. Then the basis of eigenfunctions of total moment operator {jm j } is the best choice, since the matrix of spin-orbit coupling operator and occupation matrix are diagonal in this basis. However the exchange interaction (spin-polarization) term in the Hamiltonian is not diagonal.In some 5f elements intermediate coupling scheme is realized, then the occupation matrix has nondiagonal elements in both {mσ} and {jm j } orbital bases. Therefore, both terms in the Hamiltonian: spin-orbit coupling and exchange interaction, should be taken in a general nondiagonal matrix form.Numerous band methods and approximations have been applied to describe magnetic and spectral properties of transuranium metals, see [5] for review. Nonmagnetic ground state of pure plutonium metal observed experimentally [9] was reproduced in the electronic structure calculations in the LDA+U +SO calculations [10] (local density approximation supplemented with the Hubbard U -correction and spin-orbit coupling). In these calculations the exchange interaction was found to be the reason of artificial antiferromagnetic ordering in various LSDA+U...

A thermodynamic model of thermal end elastic properties of curium

“…In addition, we account for the available experimental data on the thermal expansion and bulk modulus of Cm. The coefficient of electronic heat capacity c = 3.3 mJ/(mol K 2 ) has been evaluated by taking the data [4] on the electronic structure of Cm, according to which the total density of states at the Fermi energy of Cm equals 1.4 1/(eV atom). It should be noted that the electronic densities of states of Cm are rather flat near the Fermi energy [4], so the coefficient of electronic heat capacity of Cm can be assumed to be almost temperatureindependent up to the highest temperature considered in the present study.…”

“…In the paper [3], proceeding from the comparison of the thermophysical and thermochemical properties of the lanthanide and actinide series, it has been pointed to the possible identity of the entropies and heat capacities of curium and gadolinium. However, according to the results of ab initio calculations [4,5] the electronic structures of these two metals are different and thus the influence of the electronic subsystem on the heat capacity of curium and gadolinium cannot be the same. Moreover, one has to expect considerable difference in the values of lattice anharmonicity parameters and of related thermal and elastic properties of these metals.…”

Electrical resistivity of pure transuranium metals under pressure

“…(5) and calculation of the s-electrons coherent potential with Eq. (4) and the ER investigation within the many-band conductivity model, the partial densities of states of actinides metals from ab initio LDA+U+SO calculations are used [26]. Numerical simulations provides a usual way to estimate the effective mass of the conductivity electrons m à j and the modification of ''accepting'' DOSes with temperature, and one can try to estimate the absolute values of ER of actinides also.…”

“…That allows the possibility to apply simple calculation schemes for numerical investigations of temperature dependencies of ER, because the results, obtained for cubic phases of metals under investigation, are close to the corresponding real systems. Hence, it seems more appropriate to use in the theoretical analysis of the ER of the actinides metals their Density of States (DOS), but calculated for cubic phases for fitted volumes [26] as a starting points of numerical investigation of ER vs. temperature and pressure.…”

“…The set of the CPA equations for the manyband conductivity model, accounting for strong electron-phonon scattering and the possibility of intro-and intra-band transitions of scattering electrons was derived and applied for the ER of fccPu, Am, and Cm and bcc-Np with numerical investigations using DOSes of pure metals calculated within the LDA+U+SO method [26]. Using ab initio obtained parameters, in terms of the proposed conductivity model, the nature of high-resistivity states in actinides will be discussed accounting for the initial metals DOS modifications as a result of temperature smearing-out and its renormalization due to inter-band s ?…”