# From diamond to BC8 to simple cubic and back: kinetic pathways to post-diamond carbon phases from metadynamics

## TL;DR

This study uses advanced simulations to reveal the atomistic mechanisms and pathways for transforming diamond into post-diamond carbon phases like BC8 and simple cubic under extreme pressures, guiding future experiments.

## Contribution

It introduces a novel metadynamics approach with machine-learned potentials to uncover transformation pathways and identifies conditions favorable for synthesizing specific post-diamond phases.

## Key findings

- Nucleation of liquid droplets initiates phase transformation.
- BC8 phase is favored near 1.8 TPa and 3500-5000 K.
- A new metastable P222 structure was discovered.

## Abstract

The experimental observation of elusive post-diamond carbon phases at extreme pressures remains a major challenge in high-pressure science. Using metadynamics with coordination-number-based collective variables and SNAP machine-learned interatomic potential, we uncover atomistic mechanisms governing the transformation of cubic and hexagonal diamond into post-diamond phases above 1.5 TPa. The transition initiates via homogeneous nucleation of nanoscale liquid droplets, which rapidly crystallize into either BC8 (below 1.8 TPa) or simple cubic phases (above 2.1 TPa), once the liquid nucleus surpasses a critical size. Favorable conditions for synthesizing BC8 are identified near 1.8 TPa and 3500--5000 K. Decompression pathways from simple cubic and BC8 phases were also simulated to study possible experimental recovery of post-diamond carbon allotropes at ambient conditions. We also find a new metastable low-enthalpy structure with four-coordinated carbon atoms and space group P222. Our insights provide a theoretical foundation for experimental discovery of ultra-dense carbon phases under extreme conditions.