A model for ramp compression from ab initio calculations

Ramp compression has emerged as a novel experimental technique to probe matter at extreme pressures, while keeping temperatures lower than shock compression. The compression is thought to be quasi-isentropic, which is ideal to probe the convective zones of planetary interiors. However, a complete theoretical framework to model ramp compression for crystalline materials, such as the Rankine-Hugoniot equations for shock compression, is still lacking. In this work, we present a model of ramp compression that is based on thermodynamics and ab initio calculations where we approximate ramp loading as a series of compression and relaxation steps, which involve both uniaxial and isotropic compression. We apply our model to diamond and we compare with the stress-density data reported from experiments for ramp-compressed diamond [1-3]. We find good agreement between our model and a multiple-shock Hugoniot scheme [4], as well as with a recently proposed strength model for diamond based on plastic work [5].

References:

[1] D. Bradley et al., PRL 102, 075503 (2009).

[2] R. Smith et al., Nature 511, 330 (2014).

[3] A. Lazicki et al., Nature 589, 532 (2021).

[4] F. Gonzalez-Cataldo et al., PRB 104, 134104 (2021).

[5] D.C. Swift et al., PRB 105, 014109 (2022).