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Microscopic investigation of enhanced Pauli paramagnetism in metallic Pu$_2$C$_3$

R. Yamamoto, M. S. Cook, A. R. Altenhof, P. Sherpa, S. Park, J. D. Thompson, H. E. Mason, D. C. Arellano, D. V. Prada, P. H. Tobash, F. Ronning, E. D. Bauer, N. Harrison, W. A. Phelan, A. P. Dioguardi, M. Hirata

TL;DR

Pu2C3 is a narrow-5f-band, weakly correlated metal with enhanced Pauli paramagnetism. The study combines XRD, 13C NMR, magnetic susceptibility, specific heat, and DFT band-structure calculations to characterize its structure and electronic properties. Key findings include a global cubic lattice with two distinct carbon environments, a Sommerfeld coefficient γ = $45$ mJ mol$^{-1}$ Pu$^{-1}$ K$^{-2}$, a Wilson ratio RW ≈ 1.3, and metallic Korringa behavior with modest ferromagnetic spin fluctuations; DFT reveals a narrow 5f band near E_F, consistent with itinerant 5f electrons. Together, these results establish Pu2C3 as a prime reference for itinerant actinide physics and illuminate the localization–delocalization competition in plutonium compounds.

Abstract

A combined study of the structural and electronic properties of polycrystalline Pu$_2$C$_3$ is reported based on x-ray diffraction, specific heat, magnetic susceptibility, ${}^{13}$C nuclear magnetic resonance (NMR), and band structure calculations. X-ray diffraction reveals a global noncentrosymmetric cubic lattice, with a nearest-neighbor C--C bond length of $r = 1.38$ Å. ${}^{13}$C NMR measurements indicate that the global cubic symmetry is locally broken, revealing two unique carbon environments. Magnetic susceptibility suggests enhanced Pauli paramagnetism, and specific heat reveals a moderately large electronic Sommerfeld coefficient $γ= 45$ mJ mol$_{\mathrm{Pu}}^{-1}$ K$^{-2}$, with a Wilson ratio $R_W \approx 1.3$ further indicating moderate correlations. ${}^{13}$C nuclear spin-lattice relaxation rate ($1/T_1$) and Knight shift ($K$) measurements find metallic Korringa behavior (i.e., $T_1TK^2=$ const.) with modest ferromagnetic spin fluctuations at low temperature. Taken together, the data point to a delocalized nature of a narrow 5$f$-electron band with weak electronic correlations. Density functional theory band-structure calculations confirm the appearance of such narrow 5$f$ bands near the Fermi level. Our data provide prime evidence for a plutonium-based metallic system with weak electronic correlations, which sheds new light on the understanding of complex paramagnetism in actinide-based metallic compounds.

Microscopic investigation of enhanced Pauli paramagnetism in metallic Pu$_2$C$_3$

TL;DR

Pu2C3 is a narrow-5f-band, weakly correlated metal with enhanced Pauli paramagnetism. The study combines XRD, 13C NMR, magnetic susceptibility, specific heat, and DFT band-structure calculations to characterize its structure and electronic properties. Key findings include a global cubic lattice with two distinct carbon environments, a Sommerfeld coefficient γ = mJ mol Pu K, a Wilson ratio RW ≈ 1.3, and metallic Korringa behavior with modest ferromagnetic spin fluctuations; DFT reveals a narrow 5f band near E_F, consistent with itinerant 5f electrons. Together, these results establish Pu2C3 as a prime reference for itinerant actinide physics and illuminate the localization–delocalization competition in plutonium compounds.

Abstract

A combined study of the structural and electronic properties of polycrystalline PuC is reported based on x-ray diffraction, specific heat, magnetic susceptibility, C nuclear magnetic resonance (NMR), and band structure calculations. X-ray diffraction reveals a global noncentrosymmetric cubic lattice, with a nearest-neighbor C--C bond length of Å. C NMR measurements indicate that the global cubic symmetry is locally broken, revealing two unique carbon environments. Magnetic susceptibility suggests enhanced Pauli paramagnetism, and specific heat reveals a moderately large electronic Sommerfeld coefficient mJ mol K, with a Wilson ratio further indicating moderate correlations. C nuclear spin-lattice relaxation rate () and Knight shift () measurements find metallic Korringa behavior (i.e., const.) with modest ferromagnetic spin fluctuations at low temperature. Taken together, the data point to a delocalized nature of a narrow 5-electron band with weak electronic correlations. Density functional theory band-structure calculations confirm the appearance of such narrow 5 bands near the Fermi level. Our data provide prime evidence for a plutonium-based metallic system with weak electronic correlations, which sheds new light on the understanding of complex paramagnetism in actinide-based metallic compounds.

Paper Structure

This paper contains 12 sections, 1 equation, 9 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experimental and theoretical details
  3. Synthesis and characterization
  4. Magnetic susceptibility and heat capacity measurements
  5. NMR measurements
  6. Band-structure calculations
  7. Results and Discussion
  8. Structural analysis
  9. Specific heat and magnetic susceptibility
  10. NMR shift and spin-lattice relaxation rate
  11. Band structure calculation
  12. Conclusions

Figures (9)

  • Figure 1: Crystallographic structure of Pu$_2$C$_3$. Pu atoms are denoted by large blue spheres and C atoms by small pink spheres. The local environment of a nearest neighbor C--C pair is shown on the left, and the full unit cell (gray box) is shown on the right.
  • Figure 2: ${}^{13}$C NMR spectra of Pu$_2$C$_3$ powder at selected temperatures at 2.0 T.
  • Figure 3: ${}^{13}$C NMR spectra of Pu$_2$C$_3$ powder measured at different magnetic fields at 150 K. Experimental spectra are shown in black and simulated curves with SIMPSON in blue. The ${}^{13}$C frequency scale is shown as an offset relative to the site-averaged ${}^{13}$C isotropic shift $\overline{\delta_{\mathrm{iso}}}$ (center of gravity frequency of the spectrum). Spectra recorded between 1.2 and 5.5 T were with a "Fresh" sample and spectra between 6.0 and 12.0 T were with an "Aged" sample. The broad signal seen in the negative frequency end at 6.0 and 7.0 T originates from aged sample with some disorder effects. Note that for the data above 6.0 T, very short last delays were used in the pulse sequence to minimize the signal from the "Aged" sample that has a much longer $T_1$ relative to that of the "Fresh" sample (see the SI for details of disorder effects).
  • Figure 4: Magnetic field dependence of ${}^{13}$C NMR parameters of Pu$_{2}$C$_{3}$ powder. Spectra were fitted with two Lorentzian forms to deduce the Lorentzian splitting (a) and linewidth (b). Lower field data (triangles; below 5.7 T) were taken at 150 K in the "fresh" sample, whereas higher field data (circles; above 6 T) were collected a few month later in an aged sample at 35 K. Solid lines are guide to the eyes. Linewidth data at 150 K were multiplied by a constant factor of 1.225 to account for the temperature dependence and match the low temperature data (see the SI). Error bars indicate estimated systematic uncertainties arising from differing pulse sequence parameters.
  • Figure 5: Specific heat and magnetic susceptibility of polycrystalline Pu$_2$C$_3$. (a) Temperature dependence of specific heat $C_p/T$ at zero field. The inset shows the low-temperature (6.3 $< T <$ 20 K) magnification plotted as $C_p/T$ against $T^{2}$. The solid line is the fit to the function $C_p/T = \gamma + \beta T^{2}$. (b) Temperature dependence of magnetic susceptibility $\chi$ at 2.0 T (black squares) and 5.7 T (red circles). The inset shows magnetization $M$ as a function of magnetic field $B~(=\mu_{0}H)$ at elevated temperatures from 5 to 100 K.
  • ...and 4 more figures