In situ structural investigation of Fe-S-Si immiscible liquid system and evolution of Fe-S bond properties with pressure

Fe-S-Si immiscibility has been investigated using in situ X-ray methods at high pressure and high temperature. An in situ X-ray diffraction study of immiscible liquids for P similar to 5 GPa and T/Tm similar to 1.1 has been performed, showing differences in structural properties between S-rich and Si-rich coexisting liquid phases. Moreover, the respective role of S and Si on Fe alloys has been quantitatively investigated in Fe-X liquids (X = S, Si) with 20% wt of light elements. The transition from immiscible to miscible textures has been observed in the ternary mixture by in situ X-ray radiography for the Fe-18wt% S-8.5wt% Si (Fe-28.8at% Si-11.9at% S) sample composition between 12 and 16 GPa. Closure of the miscibility gap occurs in the same pressure range as the Fe-S eutectic liquid evolves toward a compact structure. By working within the framework of the hard sphere (HS) fluid commonly used in liquid state theory, we show that the equation of state (EOS) for additive HS reproduces very well the compressibility of Fe-Si alloys measured in the 0 5 GPa pressure range, whereas that of Fe-S alloys behave quite differently, with a high degree of covalency. However it is also stressed that at higher pressures (P > 15 GPa) liquid Fe-S adopts a structure close to that exhibited by Fe-Si alloys, a feature which suggests that the compressibility of the two alloys should behave similarly at very high pressure due to transition of S behaviour from covalent to interstitial. Hence for Fe-S liquid under core conditions, we conclude that the sound velocity in Fe-S alloy is compatible with PREM model.