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

TL;DR

The study investigates how type-I and type-VIII sodium borosilicide clathrates respond to high pressure, revealing an isostructural volume collapse near $13$ GPa in type-I borosilicates, driven by pressure-induced migration of Si atoms from the Si(6c) site, consistent with theoretical predictions. In contrast, type-VIII borosilicides display conventional elastic compression up to at least $20$ GPa and maintain the same crystal symmetry. The metallic character of the type-VIII phase is supported by Raman and reflectance measurements, while the bulk moduli of boron-containing clathrates indicate stiffening with boron incorporation in type-I frameworks. Overall, the work elucidates an atomistic mechanism for a pressure-induced, isostructural transition in borosilicide clathrates and demonstrates the potential to tune their mechanical and electronic properties under extreme conditions.

Abstract

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.

Figures (11)

• Figure 1: a) Crystal structure of type-I Na$_8$B$_x$Si$_{46-x}$ clathrate. b) Evolution of the in situ XRD diffraction signal (between 7.1 and 8.4°) through high-pressure high temperature conditions (large volume press experiment, 1100 K – 3.5 GPa). c) XRD powder pattern (between 35 and 40°) of the sample synthetized at 3.5 GPa and 1150 K, the intensities of the computed phases are calculated from Na$_8$B$_1$Si$_{45}$ to Na$_8$B$_4$Si$_{42}$ fixed stoichiometry with boron atom inside the 16i and 24k Wyckoff sites. d) Crystal structure of type-VIII Na$_8$B$_x$Si$_{46-x}$ clathrate. e) Part of type-VIII Na$_8$B$_{4.1}$Si$_{41.9}$ single-crystal XRD detector image with some indexed reflections. f) Raman spectrum at ambient condition of type-I borosilicide powder sample and Na$_8$B$_{4.1}$Si$_{41.9}$ type-VIII single-crystal clathrate.
• Figure 2: a) The P-V experimental data and their corresponding Vinet EoS fitting along with the picture of Na$_8$B$_{4.8}$Si$_{41.2}$ and Na$_8$B$_{3.7}$Si$_{42.3}$ samples inside the DAC. The black curve corresponds to the boron-free type-I Na$_8$Si$_{46}$ clathrate. The blue curve corresponds to the type-VIII Na$_8$B$_{4.1(1)}$Si$_{41.9(1)}$ clathrate. All the other samples correspond to the Na$_8$B$_x$Si$_{46-x}$ type-I clathrates ranging from orange to purple according to their initial lattice parameter. The initial V$_0$ volume of the borosilicides type-I clathrates synthetized at 3.5 GPa and 1150 K are unknown and, thus, cannot be fixed during the equations of state fitting. The pressure and volume/atom error bars are included in the width of markers. For type-I borosilicide clathrates, above 13 GPa, the solid lines serve as guides to highlight the volume collapse. b) Waterfall plotting of Na$_8$B$_{3.7}$Si$_{42.3}$ type-I raw data (222), (320) and (322) XRD triplet. c) Waterfall plotting of Na$_8$B$_{4.2}$Si$_{41.8}$ type-VIII raw data (222) and (321) XRD doublet.
• Figure 3: a) Site-occupancy factors for the 6d and 16i Wyckoff sites with a schematic representation of Si diffusion into Na$_8$B$_{4.8}$Si$_{41.2}$ sample and b) induced microstrain calculated from peaks enlargement.
• Figure 4: SEM image with AsB detector of the polished sample synthetized at 4 GPa and 1500 K.
• Figure 5: XRD powder pattern (between 80 and 105°) of the sample synthetized at 3.5 GPa and 1150 K, the intensities of the computed phases are calculated from Na$_8$B$_1$Si$_{45}$ to Na$_8$B$_4$Si$_{42}$ fixed stoichiometry with boron atom inside the 16i and 24k Wyckoff sites. The intense unlabeled peaks correspond to silicon.