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Structural Phase Transition in CeMnSi under Pressure and Comparative Structural Properties of $R$MnSi ($R$ = La, Ce, Pr, Nd)

Yukihiro Kawamura, Sae Nishiyama, Jun-ichi Hayashi, Keiki Takeda, Chihiro Sekine, Hiroshi Tanida

TL;DR

This work reveals that CeMnSi uniquely undergoes a pressure-induced structural transition at $P_s\sim5.7$ GPa, unlike LaMnSi, PrMnSi, and NdMnSi up to ~10 GPa. The transition is accompanied by an anisotropic contraction, notably a rapid decrease in the lattice-parameter ratio $c/a$, and an anomalously low bulk modulus $B_0$, signaling Ce valence instability and enhanced $c$–$f$ hybridization. The high-pressure phase is best described as monoclinic $P2_1/m$, inferred from the appearance of forbidden reflections and peak splittings in powder XRD, though single-crystal XRD is needed for definitive structure determination. Overall, the results emphasize strong lattice–electronic coupling in Ce-based $RTX$ systems and show a distinct pressure response for CeMnSi compared with its lanthanide analogues.

Abstract

Powder X-ray diffraction experiments under pressure up to $\sim$10 GPa were performed on tetragonal CeFeSi-type $R$MnSi ($R$ = La, Ce, Pr, Nd). A structural phase transition was observed in CeMnSi at a critical pressure of $P_{\rm s}$ $\sim$ 5.7 GPa. In contrast, LaMnSi, PrMnSi, and NdMnSi do not exhibit any structural transitions within the same pressure range. The lattice parameter ratio $c/a$ of CeMnSi decreases rapidly as pressure approaches $P_{\rm s}$, whereas the $c/a$ ratios of the other $R$MnSi increase monotonically with pressure. CeMnSi also shows a relatively small bulk modulus: $B_0$ $\sim$ 41.4(4) GPa in the 0--2 GPa range and $B_0$ $\sim$ 32.8(2) GPa in the 4--5 GPa range, suggesting valence instability under pressure. The structural transition in CeMnSi is attributed to the pressure-induced decrease in $c/a$ and its low bulk modulus. Above $P_{\rm s}$, the X-ray diffraction pattern indicates a transition to a monoclinic structure with space group No. 11, $P2_1/m$. These findings highlight the unique pressure response of CeMnSi and provide insight into the coupling between lattice and electronic degrees of freedom in Ce-based intermetallic systems.

Structural Phase Transition in CeMnSi under Pressure and Comparative Structural Properties of $R$MnSi ($R$ = La, Ce, Pr, Nd)

TL;DR

This work reveals that CeMnSi uniquely undergoes a pressure-induced structural transition at GPa, unlike LaMnSi, PrMnSi, and NdMnSi up to ~10 GPa. The transition is accompanied by an anisotropic contraction, notably a rapid decrease in the lattice-parameter ratio , and an anomalously low bulk modulus , signaling Ce valence instability and enhanced – hybridization. The high-pressure phase is best described as monoclinic , inferred from the appearance of forbidden reflections and peak splittings in powder XRD, though single-crystal XRD is needed for definitive structure determination. Overall, the results emphasize strong lattice–electronic coupling in Ce-based systems and show a distinct pressure response for CeMnSi compared with its lanthanide analogues.

Abstract

Powder X-ray diffraction experiments under pressure up to 10 GPa were performed on tetragonal CeFeSi-type MnSi ( = La, Ce, Pr, Nd). A structural phase transition was observed in CeMnSi at a critical pressure of 5.7 GPa. In contrast, LaMnSi, PrMnSi, and NdMnSi do not exhibit any structural transitions within the same pressure range. The lattice parameter ratio of CeMnSi decreases rapidly as pressure approaches , whereas the ratios of the other MnSi increase monotonically with pressure. CeMnSi also shows a relatively small bulk modulus: 41.4(4) GPa in the 0--2 GPa range and 32.8(2) GPa in the 4--5 GPa range, suggesting valence instability under pressure. The structural transition in CeMnSi is attributed to the pressure-induced decrease in and its low bulk modulus. Above , the X-ray diffraction pattern indicates a transition to a monoclinic structure with space group No. 11, . These findings highlight the unique pressure response of CeMnSi and provide insight into the coupling between lattice and electronic degrees of freedom in Ce-based intermetallic systems.

Paper Structure

This paper contains 10 sections, 1 equation, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Experiment
  3. Results
  4. Structural Transition in CeMnSi
  5. Pressure dependence of lattice parameters
  6. Discussion
  7. Pressure and chemical pressure dependence of $c/a$
  8. $B_0$ of $R$MnSi ($R$ = La, Ce, Pr, Nd)
  9. Crystal structure above $P_{\rm s}$ of CeMnSi
  10. Summary

Figures (8)

  • Figure 1: (Color online) Powder XRD patterns of CeMnSi under pressure at 300 K, along with the calculated pattern based on the space group $P4/nmm$. Each XRD pattern is vertically offset for clarity. Arrows indicate characteristic peaks associated with the structural phase transition.
  • Figure 2: (Color online) (a1)--(d1) Pressure dependent 2$\theta$ values (filled symbols, left axis) and FWHM (open symbols, right axis) for the diffraction index 220, 200, 111, and 100 below 5.7 GPa, and for the corresponding peaks above 5.7 GPa. For the peak near 2$\theta$$\sim$ 14$^\circ$, observed above 5.7 GPa, two distinct peaks are assumed, as the profile clearly indicates the presence of multiple components. (a2)--(d2) Enlarged views of the XRD powder patterns of CeMnSi at 300 K under pressures of 6.1 GPa (top), 5.7 GPa (middle), and 4.8 GPa (bottom).
  • Figure 3: (Color online) XRD powder patterns of (a) LaMnSi, (b) PrMnSi, and (c) NdMnSi under pressure at 300 K, along with calculated patterns based on the $P4/nmm$ space group.
  • Figure 4: (Color online) Pressure-dependent lattice parameters: (a) lattice constant $a$, (b) lattice constant $c$, (c) their ratio $c/a$, and (d) unit-cell volume $V$ for LaMnSi (squares), CeMnSi (circles), PrMnSi (triangles), and NdMnSi (inversed triangles) at 300 K.
  • Figure 5: (Color online) (a) Volume ($V$) dependence of the $c/a$ ratio in $R$MnSi ($R$ = La, Ce, Pr, Nd) under pressure (open symbols). For comparison, the $V$ dependence of $c/a$ at ambient pressure for $R$MnSi ($R$ = La, Ce, Pr, Nd, Sm, Gd, Y) reported by H. Kido et al. (filled triangles) Kido and by H. Tanida et al., (filled circles) Tani2023 is also shown. (b) Volume ($V$) dependence of the $c/a$ ratio in $R$CoSi ($R$ = La, Ce, Pr) under pressure Kawa2022 (open symbols), along with ambient pressure data for $R$CoSi ($R$ = La, Ce, Pr, Nd, Sm, Gd, Tb) reported by O.I. Bodak et al. (filled circles) Bodak1970 and R. Welter et al., (filled triangles) Welter1994J. Dashed lines are guides for the eye.
  • ...and 3 more figures