Negative thermal expansion and antiferromagnetism in the actinide oxypnictide NpFeAsO

T. Klimczuk, H. C. Walker, R. Springell, A. B. Shick, A. H. Hill, P. Gaczyński, K. Gofryk, S. A. J. Kimber, C. Ritter, E. Colineau, J.-C. Griveau, D. Bouëxière, R. Eloirdi, R. J. Cava, and R. Caciuffo

Phys. Rev. B 85, 174506 – Published 9 May 2012

Abstract

A neptunium analog of the LaFeAsO tetragonal layered compound has been synthesized and characterized by a variety of experimental techniques. The occurrence of long-range magnetic order below a critical temperature $T_{N}$ $=$ 57 K is suggested by anomalies in the temperature-dependent magnetic susceptibility, electrical resistivity, Hall coefficient, and specific-heat curves. Below $T_{N}$ , powder neutron diffraction measurements reveal an antiferromagnetic structure of the Np sublattice, with an ordered magnetic moment of 1.70 $\pm$ 0.07 $μ_{B}$ aligned along the crystallographic $c$ axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3 $μ_{B}$ for the ordered magnetic moment on the iron. High-resolution x-ray powder diffraction measurements exclude the occurrence of lattice transformations down to 5 K, in sharp contrast to the observation of a tetragonal-to-orthorhombic distortion in the rare-earth analogs, which has been associated with the stabilization of a spin-density wave on the iron sublattice. Instead, a significant expansion of the NpFeAsO lattice parameters is observed with decreasing temperature below $T_{N}$ , corresponding to a relative volume change of about 0.2 $%$ and to an Invar behavior between 5 and 20 K. First-principles electronic structure calculations based on the local spin density plus Coulomb interaction and the local density plus Hubbard-I approximations provide results in good agreement with the experimental findings.

Received 5 April 2012

DOI:https://doi.org/10.1103/PhysRevB.85.174506

Authors & Affiliations

T. Klimczuk^1,2,*, H. C. Walker^3,4, R. Springell^5,6, A. B. Shick^1,7, A. H. Hill^3,†, P. Gaczyński¹, K. Gofryk⁸, S. A. J. Kimber³, C. Ritter⁹, E. Colineau¹, J.-C. Griveau¹, D. Bouëxière¹, R. Eloirdi¹, R. J. Cava¹⁰, and R. Caciuffo¹

¹Institute for Transuranium Elements, JRC, European Commission, Postfach 2340, 76125 Karlsruhe, Germany
²Faculty of Applied Physics and Mathematics, Gdansk University of Technology, Narutowicza 11/12, 80-952 Gdansk, Poland
³European Synchrotron Radiation Facility, 6 rue Jules Horowitz, Boîte Postale 220, 38043 Grenoble Cedex 9, France
⁴Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany
⁵London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
⁶Royal Commission for the Exhibition of 1851 Research Fellow, Interface Analysis Centre, University of Bristol, Bristol BS2 8BS, United Kingdom
⁷Institute of Physics, ASCR, Na Slovance 2, 18221 Prague 8, Czech Republic
⁸Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
⁹Institute Laue-Langevin, 6 rue Jules Horowitz, Boîte Postale 156, 38042 Grenoble Cedex 9, France
¹⁰Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA

^*tomasz.klimczuk@ec.europa.eu
^†Present address: Johnson Matthey Technology Centre, Sonning Common, United Kingdom.

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Vol. 85, Iss. 17 — 1 May 2012

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Images

Figure 1
Fit (solid blue line) of the refined structural model (Rietveld method) to the high-resolution room-temperature x-ray powder diffraction data (red circles) for NpFeAsO, confirming the tetragonal $P 4 / n m m$ ZrCuSiAs-type structure. The data were collected on the ID31 (ESRF) high-resolution x-ray powder diffractometer with a wavelength of $λ = 0.35$ Å. Tick marks are shown for two refined phases: NpFeAsO (top, blue) and an $8 %$ NpO $_{2}$ impurity (bottom, green). Insets (a) and (b) make a comparison of data for NpFeAsO and SmFeAsO,[7] where only SmFeAsO shows the splitting of the room-temperature (red line) tetragonal ( $P 4 / n m m$ ) (111) Bragg reflection into the orthorhombic ( $C m m a$ ) (201) and (021) reflections at 5 K (blue line). Insets (c) and (d) show the variation in the $a$ and $c$ lattice parameters and of the unit cell volume as a function of temperature, as extracted from the x-ray powder diffraction measurements, revealing a marked negative thermal expansion below $T ≃ 60$ K in NpFeAsO. The red line through the volume data is a fit to the second order of the Grüneisen approximation for the zero-pressure equation of state, as discussed in the text.Reuse & Permissions
Figure 2
Bulk physical properties of NpFeAsO. (a) The magnetic susceptibility $χ = Δ M / Δ H$ data (solid circles) and $d (χ T) / d T$ (solid line). The inset shows a Curie-Weiss fit to the inverse susceptibility from which an effective paramagnetic moment of $2.78 \pm 0.06 μ_{B}$ and a Curie-Weiss temperature of $- 128 \pm 2$ K are obtained. (b) The electrical resistivity measured in $H = 0$ T. The inset highlights the magnetically ordered region, showing two fits to the data, indicating that the low-temperature resistivity does not vary simply as $T^{2}$ (red line); instead, an improved fit is obtained after including an antiferromagnetic gap (blue line). (c) The Hall coefficient as a function of temperature. The inset shows the least-squares fit described in the text. (d) The heat capacity, including an inset showing a fit to the low-temperature data.Reuse & Permissions
Figure 3
Variable-temperature neutron diffraction surface across $T_{N}$ , showing the appearance of the structurally forbidden (100) reflection and the increase in intensity of the (101) and (111) reflections due to magnetic ordering.Reuse & Permissions
Figure 4
Plot of the Rietveld fit to the difference of the 90 and 30 K neutron powder diffraction data sets using a $P 4 / n^{'} m^{'} m^{'}$ magnetic model. Portions of the difference spectra where strong nuclear reflections led, due to thermal expansion, to strong up-down features were excluded from the fit. The right inset shows the constant presence of background features at all temperatures in the midmomentum transfer region. The left inset shows the proposed antiferromagnetic structure.Reuse & Permissions
Figure 5
Electronic structure calculations for NpFeAsO: the total DOS (per unit cell), and atom, spin and orbital resolved DOS as a result of the FLL $+$ LSDA $+$ U $=$ 3 eV calculations.Reuse & Permissions
Figure 6
(left) Band structure for NpFeAsO and (right) Fermi surface for NpFeAsO, calculated using the FLL $LSDA + U$ $=$ 3 eV calculations.Reuse & Permissions

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