Structural, electronic, and magnetic characteristics of Np2Co17

Abstract

A previously unknown neptunium-transition-metal binary compound Np${}_{2}$Co${}_{17}$ has been synthesized and characterized by means of powder x-ray diffraction, ${}^{237}$Np M\"ossbauer spectroscopy, superconducting-quantum-interference-device magnetometry, and x-ray magnetic circular dichroism (XMCD). The compound crystallizes in a Th${}_{2}$Ni${}_{17}$-type hexagonal structure with room-temperature lattice parameters $a=8.3107(1)$ $\AA{}$ and $c=8.1058(1)$ \AA{}. Magnetization curves indicate the occurrence of ferromagnetic order below ${T}_{C}>350$ K. M\"ossbauer spectra suggest a Np${}^{3+}$ oxidation state and give an ordered moment of ${\ensuremath{\mu}}_{\mathrm{Np}}=1.57(4)$ ${\ensuremath{\mu}}_{B}$ and ${\ensuremath{\mu}}_{\mathrm{Np}}=1.63(4)$ ${\ensuremath{\mu}}_{B}$ for the Np atoms located, respectively, at the $2b$ and $2d$ crystallographic positions of the $P{6}_{3}/mmc$ space group. Combining these values with a sum-rule analysis of the XMCD spectra measured at the neptunium ${M}_{4,5}$ absorption edges, one obtains the spin and orbital contributions to the site-averaged Np moment [${\ensuremath{\mu}}_{S}=\ensuremath{-}1.88(9)$ ${\ensuremath{\mu}}_{B}$, ${\ensuremath{\mu}}_{L}=3.48(9)$ ${\ensuremath{\mu}}_{B}$]. The ratio between the expectation value of the magnetic-dipole moment and the spin magnetic moment (${m}_{\mathrm{md}}/{\ensuremath{\mu}}_{S}=+1.36$) 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 ${\ensuremath{\mu}}_{\mathrm{Co}}\ensuremath{\simeq}1.6$ ${\ensuremath{\mu}}_{B}$. 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.