Phase transition lowering in dynamically compressed silicon

Abstract

Silicon, being one of the most abundant elements in nature, attracts wide-ranging scientific and technological interest. Specifically, in its elemental form, crystals of remarkable purity can be produced. One may assume that this would lead to Si being well understood, and indeed, this is the case for many ambient properties, as well as for higher pressure behaviour under quasi-static loading. However, despite many decades of study, a detailed understanding of the response of silicon to rapid compression such as that experienced under shock impact-remains elusive. Here, we combine a novel Free Electron Laser (FEL) based X-ray diffraction geometry with laser-driven compression to elucidate the importance of shear generated during shock compression on the occurrence of phase transitions. We observe the lowering of the hydrostatic phase boundary in elemental silicon, an ideal model system for investigating high-strength materials, analogous to planetary constituents. Moreover, we unambiguously determine the onset of melting above 14 GPa, previously ascribed to a solid-solid phase transition, undetectable in the now conventional shocked diffraction geometry; transitions to the liquid state are expected to be ubiquitous in all systems at sufficiently high pressures and temperatures.

Structural transformations including melting and recrystallization during shock compression and release of germanium up to 45 GPa

“…Similar to shocked Ge, the high-pressure XRD patterns for shocked Si exhibited significant preferred orientation up to ~30 GPa before losing texture prior to complete melting near ~33 GPa. The high degree of texture observed in shocked single crystal Si from 26-30 GPa [15,18] indicates that the equilibrium melt boundary was not reached, contradicting a recent claim of equilibrium shock melting of Si over a surprisingly wide stress range,14-27 GPa [40]. Whereas only the cd phase was observed upon release of shocked Ge, both cd and β-Sn phases were observed upon release of shocked Si.…”

Anharmonic and Anomalous Trends in the High-Pressure Phase Diagram of Silicon

“…Having obtained these discrete melting points for all phases up to 4 TPa, we fit them with the Kechin equation [76], a modified form of the Simon-Glatzel equation [77] with parameters a i (i = 1,2,3) and (P 0 ,T 0 ), to all the (T m ,P m ) melting points in the cd and the other phases separately, to take into account the opposite slopes of the melting curve. The values of these constants for the negative slope branch of the melt curve is T 0 = 92.37 K, P 0 = 21.31 GPa, a1 = 2.27 GPa, a2 = 1.983, and a3= 0.013; whereas the values for the remainder of the melt curve is T 0 = 10458. observations [46,78]. One interesting observation is that Cmce did not show up as a thermodynamically stable phase at T=0 K in our PBE-GGA calculations, although previous LDA calculations [79] have shown Cmce to be stable for a narrow range of pressure along the cold curve.…”

Quasi-hydrostatic equation of state of silicon up to 1 megabar at ambient temperature

“…Under uniaxial compression, the reported phase transition has been shown to occur at 12 GPa along the [100] and at lower pressure along the [111] direction via electrical resistance measurement 11 . Under shock compression, instead, the transformation is observed in the 6–14 GPa pressure range 12–15 .…”