Model of Ramp Compression of Diamond from Ab Initio Simulations

TL;DR

This paper develops an ab initio simulation model to understand temperature and stress conditions during ramp compression of diamond, revealing that uniaxial stress persists at high compression and that heating alone cannot explain experimental data.

Contribution

The study introduces a new ab initio simulation framework for ramp compression, accounting for uniaxial stress and heating effects in diamond and carbon phases.

Findings

Heating during ramp compression can raise temperature beyond isentropic predictions.

Uniaxial stress remains significant at high compression levels.

Heating alone does not fully explain experimental equation of state measurements.

Abstract

Ramp compression experiments characterize high-pressure states of matter at temperatures well below those present in shock compression. However, because temperature is typically not directly measured during ramp compression, it is uncertain how much heating occurs under these shock-free conditions. Here, we performed a series of ab initio simulations on carbon in order to match the density-stress measurements of Smith et al. [Smith, et al., Nature 511, 330 (2014)]. We considered isotropically as well as uniaxially compressed solid carbon in the diamond and BC8 phases, with and without defects, as well as liquid carbon. Our idealized model ascribes heating during ramp compression to an initially uniaxially compressed cell transforming isochorically into an isotropically (hydrostatic equivalent) compressed state having lower internal energy, hence higher temperature so as to conserve energy. Multiple such heating events can occur during a single ramp experiment, leading to higher temperatures than with isentropic compression. Comparison with experiments shows that heating alone does not explain the equation of state measurements on diamond, instead implying that a significant uniaxial stress component remains present at high compression. The temperature predictions of our ramp compression model remain to be verified with future laboratory measurements.