Abstract
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 = 43 GPa. For Np2Co17, fits to the Birch-Murnaghan and Vinet equations of state give values of the isothermal bulk modulus and its pressure derivative of B-0 = 286 GPa and B-0' = 3, revealing that this Np compound is a highly incompressible solid with stiffness comparable to that of superhard covalently bonded materials. For the Lu2T17 (T = Co, Ni) compounds, the measured bulk modulus changes from B-0 = 137 GPa for T = Co to B-0 = 257 GPa for T = Ni. 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 B-0 is predicted to assume values intermediate between those measured for Lu2Ni17 and Np2Co17.
Highlights
Np2Co17 is a transuranic analog of the widely investigated Sm2Co17 high-performance permanent magnet [1,2]
The unit cell is composed of 38 atoms with the Np atoms residing at 2b and 2d special positions, and the Co atoms at the 4f, 6g, 12k, and 12j sites
Mossbauer spectra suggest a Np3+ oxidation state and give an ordered moment of μNp = 1.57(4) μB and μNp = 1.63(4) μB for the Np atoms located, respectively, at the 2b and 2d crystallographic positions. Combining these values with a sum rule analysis of the XMCD spectra measured at the neptunium M4,5 absorption edges, the spin and orbital contributions to the site-averaged Np moment (μS = −1.88(9) μB, μL = 3.48(9) μB ) have been determined
Summary
Np2Co17 is a transuranic analog of the widely investigated Sm2Co17 high-performance permanent magnet [1,2]. Mossbauer spectra suggest a Np3+ oxidation state and give an ordered moment of μNp = 1.57(4) μB and μNp = 1.63(4) μB for the Np atoms located, respectively, at the 2b and 2d crystallographic positions Combining these values with a sum rule analysis of the XMCD spectra measured at the neptunium M4,5 absorption edges, the spin and orbital contributions to the site-averaged Np moment (μS = −1.88(9) μB , μL = 3.48(9) μB ) have been determined. Using the third-order Birch-Murnaghan EOS [7,8,9] or the analytic approximation to the Vinet EOS [10] to fit the data, we have found that the bulk modulus B0 increases from 137 GPa in Lu2Co17 to 286 GPa in Np2Co17 The latter value is among the highest reported for an intermetallic compound [11].
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