Isostructural phase transition and equation of state of type-I and type-VIII metallic sodium borosilicide clathrates

Electronic properties of silicon-based clathrates can be tuned by boron incorporation into the silicon cage network. Sodium borosilicides clathrate outstands with uncommon stoichiometry and expected metallic properties, in contrast to other alkali metal semiconductive Zintl borosilicides. In this study, we report an experimental investigation of the high-pressure behavior of type-I and type-VIII sodium borosilicide clathrates. An isostructural phase transition, marked by an abrupt volume collapse at 13 GPa, is observed exclusively in type-I sodium borosilicide clathrates. This transition is attributed to the pressure-induced diffusion of silicon atoms from the Si(6c) site. This mechanism provides the first experimental validation of a transition predicted theoretically for this class of materials. Isostructural phase transitions were only observed in type-I borosilicide. In contrast, the type-VIII borosilicide phase exhibits conventional elastic compression. The metallic character was established using reflectance spectroscopy over a wide energy range, in good agreement with crystallographic data on the boron content.

💡 Research Summary

This work presents a comprehensive high‑pressure investigation of sodium borosilicide clathrates that adopt either the type‑I (Pm3̅n) or type‑VIII (I4̅3m) framework. Polycrystalline type‑I Na₈BₓSi₄₆₋ₓ (x ≈ 2.9–3.8 at %) was synthesized at 3.5 GPa and 1150 K, while single‑crystal type‑VIII Na₈B₄·₁Si₄₁·₉ was obtained at 4 GPa and 1500 K. Structural characterization by powder and single‑crystal X‑ray diffraction, Raman, FTIR and broadband reflectance confirmed metallic behavior for both phases and identified the specific Wyckoff sites occupied by boron (16i and 24k in type‑I; 8c in type‑VIII).

In situ synchrotron XRD measurements were performed in diamond‑anvil cells up to 22 GPa at 300 K, using gold, silicon, neon and argon as pressure calibrants to ensure accurate pressure determination. Type‑I clathrates displayed an abrupt, ~3 % volume collapse at 13 GPa without any change in space‑group symmetry, indicating an isostructural phase transition. Rietveld refinements revealed a pronounced decrease in the occupancy of the Si(6c) site above 13 GPa, while the Si(16i) site remained fully occupied. This suggests pressure‑induced diffusion of silicon atoms from the 6c position, creating vacancies that effectively increase the local boron concentration and stiffen the covalent framework, thereby driving the volume collapse. The transition is fully reversible upon decompression and is accompanied by increased microstrain, as evidenced by peak broadening.

Conversely, the type‑VIII clathrate showed conventional elastic compression throughout the investigated pressure range, following a second‑order Vinet equation of state with bulk modulus B₀ ≈ 90 GPa and pressure derivative B₀′ ≈ 3.8. No discontinuities or symmetry changes were observed, reflecting the greater structural rigidity of the asymmetric 20+3 polyhedral network.

Bulk modulus values for the various compositions were extracted: the boron‑free Na₈Si₄₆ exhibits B₀ ≈ 64.7 GPa (B₀′ ≈ 6.8), while boron‑doped type‑I phases have B₀ ranging from 86 to 99 GPa with B₀′ between 1.5 and 3.7, indicating that boron incorporation significantly stiffens the lattice.

The metallic character of both clathrates was corroborated by reflectance spectra that resemble those of gold or aluminum, with no transmission observed across the near‑ and mid‑infrared ranges. Raman spectra of the type‑VIII phase display a characteristic Fano line shape and an anti‑resonance dip near 512 cm⁻¹, further confirming metallicity.

Overall, the study provides the first experimental validation of a pressure‑induced isostructural transition in boron‑doped silicon clathrates, linked to silicon vacancy formation at the 6c site. It also delivers the first equation‑of‑state parameters for type‑VIII sodium borosilicide clathrates. These findings expand the understanding of how chemical substitution and high‑pressure processing can tune the structural and electronic properties of clathrate materials, offering new pathways for designing metallic or semimetallic frameworks for thermoelectric, superconducting, or energy‑storage applications.