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Preferential site ordering alters the magnetic structure of Sm$_3$Ru$_4$Sn$_{13-x}$Ge$_x$ ($x = 0$-2)

Jacob W. Fritsky, Hui-Fei Zhai, Yifeng Zhao, Aryan Rauniyar, Antia S. Botana, Jason F. Khoury

TL;DR

This work investigates site-selective Ge alloying in Sm$_3$Ru$_4$Sn$_{13-x}$Ge$_x$ to tune magnetic order in a filled-skutterudite framework. Through Sn-flux synthesis, single-crystal XRD, XPS, magnetometry, heat capacity, and DFT, Ge preferentially occupies the 2a tetrel site (60.2% for $x=1$, 100% for $x=2$), inducing chemical pressure that alters bond lengths and suppresses $T_N$ from $7.3$ K to $4.1$ K while broadening the AFM transition. DFT reveals an increased DOS at the Fermi level due to Ge $p$ states, linking DOS changes to potential modifications in $J$–$J$ coupling and spin frustration. The results establish a design principle for site-ordered quantum materials in the Ln$_3$M$_4$X$_{13}$ family, where controlled site occupancy can tailor magnetic phases without changing the Sm$^{3+}$ $J$-state, with implications for tuning quantum states in intermetallics.

Abstract

An important aspect of materials research is the ability to tune different physical properties through controlled alloying. The Ln$_3$M$_4$X$_{13}$ (Ln = Lanthanide, M = Transition Metal, X = Tetrel) filled skutterudite family is of interest due to the tunability of its constituent components and their effects on physical properties, such as superconductivity and complex magnetism. In this work, Sm$_3$Ru$_4$Sn$_{13-x}$Ge$_x$ (x = 0 -- 2) was synthesized via excess Sn-flux and characterized using powder and single-crystal X-ray diffraction, magnetometry, X-ray photoelectron spectroscopy, and heat capacity. Sm$_3$Ru$_4$Sn$_{13}$ and its Ge-solid-solution members crystallize in the Pm-3n space group, which has two unique Wyckoff positions for the tetrel (X) site. In the solid solution members, Ge shows preferential occupancy for one of the two Wyckoff sites, reaching $\sim$60$\%$ and 100$\%$ occupancy when x = 1 and 2, respectively. Magnetometry and heat capacity measurements of Sm$_3$Ru$_4$Sn$_{13}$ indicated antiferromagnetic ordering at $T_N$ = 7.3 K. However, Sm$_3$Ru$_4$Sn$_{12}$Ge and Sm$_3$Ru$_4$Sn$_{11}$Ge$_2$ showed notably lower-temperature antiferromagnetic phase transitions with substantial peak-broadening at $T_N$ = 5.5 K and 4.1 K, respectively. These data suggest that alloying Ge into Sm$_3$Ru$_4$Sn$_{13}$ causes magnetic frustration within the structure, likely attributable to a change in the density of states from additional Ge $p$ states at the Fermi level. This work demonstrates that preferentially alloying Ge in Sm$_3$Ru$_4$Sn$_{13-x}$Ge$_x$ allows for more precise tunability of its magnetic structure, elucidating design principles for different quantum phases in intermetallic materials.

Preferential site ordering alters the magnetic structure of Sm$_3$Ru$_4$Sn$_{13-x}$Ge$_x$ ($x = 0$-2)

TL;DR

This work investigates site-selective Ge alloying in SmRuSnGe to tune magnetic order in a filled-skutterudite framework. Through Sn-flux synthesis, single-crystal XRD, XPS, magnetometry, heat capacity, and DFT, Ge preferentially occupies the 2a tetrel site (60.2% for , 100% for ), inducing chemical pressure that alters bond lengths and suppresses from K to K while broadening the AFM transition. DFT reveals an increased DOS at the Fermi level due to Ge states, linking DOS changes to potential modifications in – coupling and spin frustration. The results establish a design principle for site-ordered quantum materials in the LnMX family, where controlled site occupancy can tailor magnetic phases without changing the Sm -state, with implications for tuning quantum states in intermetallics.

Abstract

An important aspect of materials research is the ability to tune different physical properties through controlled alloying. The LnMX (Ln = Lanthanide, M = Transition Metal, X = Tetrel) filled skutterudite family is of interest due to the tunability of its constituent components and their effects on physical properties, such as superconductivity and complex magnetism. In this work, SmRuSnGe (x = 0 -- 2) was synthesized via excess Sn-flux and characterized using powder and single-crystal X-ray diffraction, magnetometry, X-ray photoelectron spectroscopy, and heat capacity. SmRuSn and its Ge-solid-solution members crystallize in the Pm-3n space group, which has two unique Wyckoff positions for the tetrel (X) site. In the solid solution members, Ge shows preferential occupancy for one of the two Wyckoff sites, reaching 60 and 100 occupancy when x = 1 and 2, respectively. Magnetometry and heat capacity measurements of SmRuSn indicated antiferromagnetic ordering at = 7.3 K. However, SmRuSnGe and SmRuSnGe showed notably lower-temperature antiferromagnetic phase transitions with substantial peak-broadening at = 5.5 K and 4.1 K, respectively. These data suggest that alloying Ge into SmRuSn causes magnetic frustration within the structure, likely attributable to a change in the density of states from additional Ge states at the Fermi level. This work demonstrates that preferentially alloying Ge in SmRuSnGe allows for more precise tunability of its magnetic structure, elucidating design principles for different quantum phases in intermetallic materials.

Paper Structure

This paper contains 16 sections, 2 equations, 6 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Experimental Section
  3. Reagents
  4. Synthesis
  5. Single Crystal X-ray Diffraction
  6. X-ray Photoelectron Spectroscopy (XPS)
  7. Density Functional Theory Calculations
  8. Magnetometry
  9. Heat Capacity
  10. Results and Discussion
  11. Synthesis and Structure
  12. XPS
  13. Magnetometry
  14. Heat Capacity
  15. Density Functional Theory Calculations
  16. ...and 1 more sections

Figures (6)

  • Figure 1: The unit cell of A) Sm$_3$Ru$_4$Sn$_{13}$, B) Sm$_3$Ru$_4$Sn$_{12}$Ge, and C) Sm$_3$Ru$_4$Sn$_{11}$Ge$_2$. All three compounds crystallize in the Pm$\bar{3}$n space group. Ge preferentially occupies the 2a Wyckoff site, filling 60.2% and 100% of the site for x = 1 and 2, respectively. D) Coordination environment and $J$-state of the Sm$^{3+}$ ion within the filled skutterudite. Distorting the point group symmetry does not change the electron filling of the $J$-state in Sm$^{3+}$. E) Microscope image of mm-scale as-grown single crystal of Sm$_3$Ru$_4$Sn$_{11}$Ge$_2$.
  • Figure 2: A) X-ray photoelectron spectra (XPS) of Sm$_3$Ru$_4$Sn$_{13}$ for Sn 3d and B) Comparison of normalized Sn 3d peaks in Sm$_3$Ru$_4$Sn$_{13-x}$Ge$_x$. Increasing the concentration of Ge smoothly changes the contribution from Sn 3d$_{5/2}$ to Sn 3d$_{3/2}$.
  • Figure 3: The field-cooled (FC) magnetic susceptibility of A) Sm$_3$Ru$_4$Sn$_{13}$ from 2 -- 300 K. An antiferromagnetic (AFM) transition is present at $\sim$11.1 K. The values from the fitted Curie-Weiss data are $\theta_{CW}$ = -122.02 K and $\mu_{eff}$ = 1.00 $\mu_{B}$. B) A comparison of Sm$_3$Ru$_4$Sn$_{13-x}$Ge$_x$. Each compound shows an AFM transition at $\sim$10.6 K and $\sim$9.5 K for x = 1 and 2, respectively. The $\theta_{CW}$ decreased to -33.35 K and -42.39 K for x = 1 and 2, respectively (Figure S5). C) Magnetic moment ($\mu_{B}$) versus applied magnetic field of Sm$_3$Ru$_4$Sn$_{13-x}$Ge$_x$ from 0 T to 9 T.
  • Figure 4: Heat capacity for Sm$_3$Ru$_4$Sn$_{13-x}$Ge$_x$. A) The Néel temperature (at 0 T) transitions from T$_N$ = 7.3 K to 4.1 K as the concentration of Ge increases. Peak broadening is also present, which suggests Ge incorporation causes magnetic frustration. B) Sm$_3$Ru$_4$Sn$_{13}$ exhibits field-insensitive behavior up to 7 T. C) The fittings for C$_{ph}$ and C$_{mag}$ using a two-mode Debye fit on Sm$_3$Ru$_4$Sn$_{13}$. The Debye temperatures are approximately 313 K and 143 K, and the sum of the oscillator terms is $\sim$20.4, in good agreement with the stoichiometry of Sm$_3$Ru$_4$Sn$_{13}$. D) The magnetic entropy (S$_{mag}$) for all three compounds. Each compound approaches Rln(2) (5.76 J mol$^{-1}$ K$^{-1}$), indicating $J$ = $\frac{1}{2}$, consistent with the predicted value for Sm$^{3+}$.
  • Figure 5: Electronic band structure and density of states (DOS) for (A) Sm$_3$Ru$_4$Sn$_{13}$ and (B) Sm$_3$Ru$_4$Sn$_{11}$Ge$_2$. The right panels in each figure correspond to the orbital-resolved density of states. (C) The total density states of Sm$_3$Ru$_4$Sn$_{13}$ and Sm$_3$Ru$_4$Sn$_{11}$Ge$_2$ compounds. The Fermi energy has been set to zero.
  • ...and 1 more figures