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

TL;DR

This study investigates the high-pressure behavior of sodium borosilicide clathrates, revealing an isostructural phase transition in type-I but not in type-VIII, supported by experimental and spectroscopic evidence.

Contribution

First experimental validation of a pressure-induced phase transition in sodium borosilicide clathrates, highlighting differences between type-I and type-VIII structures.

Findings

Isostructural phase transition at 13 GPa in type-I clathrates

Silicon atom diffusion causes the phase transition

Type-VIII exhibits conventional elastic compression

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.