Phase boundary between Na-Si clathrates of structures I and II at high pressures and high temperatures

TL;DR

This study investigates the high-pressure and high-temperature phase boundary between two Na-Si clathrate structures, revealing stability conditions and transformation pathways relevant for designing superhard, metallic silicon-based materials.

Contribution

It provides the first in situ and ex situ experimental analysis of Na-Si clathrate formation and stability under high pressure and temperature conditions, including a tentative phase diagram.

Findings

Na24+xSi136 (structure II) is stable at lower temperatures than Na8Si46 (structure I) up to 6 GPa.

Clathrate transitions occur below silicon melting temperatures.

Crystallization products are highly sensitive to sodium concentration.

Abstract

Understanding the covalent clathrate formation is a crucial point for the design of new superhard materials with intrinsic coupling of superhardness and metallic conductivity. Silicon clathrates have the archetype structures that can serve an existant model compounds for superhard clathrate frameworks "Si-B", "Si-C", "B-C" and "C" with intercalated atoms (e.g. alkali metals or even halogenes) that can assure the metalic properties. Here we report the in situ and ex situ studies of high-pressure formation and stability of clathrates Na8Si46 (structure I) and Na24+xSi136 (structure II). Experiments have been performed using standard Paris-Edinburgh cells (opposite anvils) up to 6 GPa and 1500 K. We have established that chemical interactions in Na-Si system and transition between two structures of clathrates occur at temperatures below silicon melting. The strong sensitivity of crystallization products to the sodium concentration have been observed. A tentative diagram of clathrate transformations has been proposed. At least up to ~6 GPa, Na24+xSi136 (structure II) is stable at lower temperatures as compared to Na8Si46 (structure I).