| AVS 62nd International Symposium & Exhibition | |
| Actinides and Rare Earths Focus Topic | Wednesday Sessions |
| Session AC+AS+MI-WeM |
| Session: | Magnetism, Complexity and Superconductivity in the Actinides and Rare Earths |
| Presenter: | Itzhak Halevy, Nuclear Research Center Negev, Israel |
| Authors: | I. Halevy, Nuclear Research Center Negev, Israel A. Hen, Institute for Transuranium Elements (ITU), Germany I. Orion, Ben Gurion University, Israel E. Colineau, ITU, Germany R. Eloirdi, ITU, Germany J.C. Griveau, ITU, Germany F. Wilhelm, ESRF, France A. Rogalev, ESRF, France N. Magnani, ITU, Germany A.B. Shick, ITU, Germany R. Caciuffo, ITU, Germany |
| Correspondent: | Click to Email |
A previously unknown neptunium-transition-metal binary compound Np2Co17 has been synthesized and
characterized by means of powder x-ray diffraction, 237Np Mössbauer spectroscopy, superconducting-quantum-interference-device magnetometry, and x-ray magnetic circular dichroism. The compound crystallizes
in a Th2Ni17-type hexagonal structure with room-temperature lattice parameters a= 8.3107Å and c=
8.1058Å. Magnetization curves indicate the occurrence of ferromagnetic order below TC>350 K. Mössbauer
spectra suggest a Np3+ oxidation state and give an ordered moment of μNp=1.57μBand μNp=1.63μB
for the Np atoms located, respectively, at the 2b and 2d crystallographic positions of the P63/mmc space group.
Combining these values with a sum-rule analysis of the XMCD spectra measured at the neptunium M4,5 absorption
edges, one obtains the spin and orbital contributions to the site-averaged Np moment [μS=−1.88μB,
μ L =3.48μB]. The ratio between the expectation value of the magnetic-dipole moment and the spin magnetic
moment (mmd/μS=1.36) is positive as predicted for localized 5f electrons and lies between the values
calculated in intermediate-coupling (IC) and jj approximations. The expectation value of the angular part of
the spin-orbit-interaction operator is in excellent agreement with the IC estimate. The ordered moment averaged
over the four inequivalent Co sites, as obtained from the saturation value of the magnetization, is μCo~1.6μB.
The experimental results are discussed against the predictions of first-principles electronic-structure calculations
based on the spin-polarized local-spin-density approximation plus the Hubbard interaction. The structural behavior of Np2Co17 is investigated by means of high pressure diamond-anvil compression measurements and is compared with that of the isostructural compounds Lu2Co17 and Lu2Ni17. The Th2Ni17-type hexagonal crystal structure is preserved with no measurable discontinuous volume collapses up to the highest achieved pressure, p=43GPa. For Np2Co17, fits to the Birch-Murnaghan and Vinet equations of state give values of the isothermal bulk modulus and
its pressure derivative of B0=286GPa and B0=3, revealing that this Np compound is a highly incompressible solid with stiffness comparable to that of superhard covalently bonded materials. The isothermal equation of state for the studied compounds are in excellent agreement with the results of ab initio fully-relativistic, full potential local spin-density functional calculations. Theoretical estimates of the bulk modulus are given also for Np2Ni17, for which B0 is predicted to assume values intermediate between those measured for Lu2Ni17 and Np2Co17.