Mixed-Valent Ce Disrupts Magnetic Ordering in CeFe$_2$Ga$_8$

TL;DR

This work investigates how mixed valence on Ce$^{3+}$/Ce$^{4+}$ on a single site affects magnetism in the quasi-one-dimensional intermetallic CeFe2Ga8. Using a comprehensive set of techniques—PXRD, XPS, Mössbauer spectroscopy, magnetometry, resistivity, and heat capacity—the authors show that the Fe sublattice is nonmagnetic and Ce carries the magnetic moment, but the Ce moments form short-range correlations without establishing long-range order down to the lowest temperatures studied. The observed ZFC–FC splitting, reduced effective moments, and AC-susceptibility peaks indicate short-range Ce-based interactions rather than spin-glass behavior or Kondo-driven heavy fermion physics, with the mixed Ce valence acting to disrupt magnetic ordering. Overall, the study provides a detailed phase-space map of how mixed valency can suppress magnetic order in Q1D Ce-based intermetallics and offers a framework for analyzing short-range spin interactions in related materials.

Abstract

Mixed valency in intermetallics with lanthanide cations is well established as a pathway to unusual charge transport, complex magnetism, and superconductivity. In this work, we report a comprehensive study of the structural, magnetic, electronic, and thermal properties of the mixed valent compound CeFe$_2$Ga$_8$. Powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopy (XPS) characterize CeFe$_2$Ga$_8$ as a quasi-one-dimensional (Q1D) compound with mixed-valent Ce$^{3+}$ and Ce$^{4+}$ on a single crystallographic site. $^{57}$Fe Mössbauer spectroscopy indicates that the Fe sublattice is nonmagnetic, in direct contrast with recent reports of this compound. Low-temperature electrical resistivity and heat capacity measurements show no evidence of magnetic ordering, and a modest Sommerfeld coefficient ($γ$) of 22.7 mJ/mol$\cdot$K$^2$ make extensive Kondo hybridization unlikely. DC and AC magnetic susceptibility data suggest short-range magnetic order at $\sim$5.2 and 7.6 K with no frequency dependence, ruling out canonical spin-glass behavior in this compound. Additionally, the magnetic susceptibility data does not contain any broad features that are typically associated with an intermediate valence state in Ce, suggesting either high-temperature valence fluctuation or a different mechanism of mixed valency. This work demonstrates that mixed-valent Ce inhibits magnetic ordering in CeFe$_2$Ga$_8$ and provides a broader picture for how to analyze short-range spin interactions in Q1D intermetallics.

Figures (7)

• Figure 1: The unit cell of CeFe$_2$Ga$_8$ (a) which crystallizes in orthorhombic symmetry in the Pbam space group. Rietveld refinement of the powder X-ray diffraction (PXRD) pattern (b) confirms the CeFe$_2$Al$_8$ structure type. The inset shows a typical single crystal with a 1 mm scale bar.
• Figure 2: X-ray photoelectron spectroscopy (XPS) of Ce 3$d$ core levels in CeFe$_2$Ga$_8$. The fitted spectrum shows Ce$^{3+}$ components including 3d$_{5/2}$ peaks and 3d$_{3/2}$ peaks, along with Ce$^{4+}$ 3d$_{5/2}$ and 3d$_{3/2}$ peaks. The data indicate mixed-valent Ce in a 7:3 ratio between Ce$^{3+}$ and Ce$^{4+}$.
• Figure 3: $^{57}$Fe Mössbauer spectroscopy data for CeFe$_2$Ga$_8$ showing absorption spectra at different temperatures (left) and temperature dependence of the hyperfine parameters (right): Isomer Shift(IS), Quadrupole Splitting(QS), Line Width(LW).
• Figure 4: Electrical transport properties of CeFe$_2$Ga$_8$: (a) Resistivity ($\rho$) vs temperature with inset showing the low-temperature region. (b) Variable-temperature magnetoresistance.
• Figure 5: Heat capacity measurements of CeFe$_2$Ga$_8$. (a) $C_p$. The inset shows the low-temperature data with applied magnetic field at 0, 1 and 3 T. (b) $C_p/T$ vs $T^2$ analysis from 10 - 25 K yielding a Sommerfeld coefficient ($\gamma$) of 34.2 mJ/mol$\cdot$K$^2$ and a Debye temperature ($\theta_D$) of 248 K.