Abstract
A 4:1 (volume ratio) methanol–ethanol (ME) mixture and silicone oil are two of the most widely used liquid pressure-transmitting media (PTM) in high-pressure studies. Their hydrostatic limits have been extensively studied using various methods; however, the evolution of the atomic structures associated with their emerging nonhydrostaticity remains unclear. Here, we monitor their structures as functions of pressure up to ∼30 GPa at room temperature using in situ high-pressure synchrotron x-ray diffraction (XRD), optical micro-Raman spectroscopy, and ruby fluorescence spectroscopy in a diamond anvil cell. No crystallization is observed for either PTM. The pressure dependence of the principal diffraction peak position and width indicates the existence of a glass transition in the 4:1 ME mixture at ∼12 GPa and in the silicone oil at ∼3 GPa, beyond which a pressure gradient emerges and grows quickly with pressure. There may be another liquid-to-liquid transition in the 4:1 ME mixture at ∼5 GPa and two more glass-to-glass transitions in the silicone oil at ∼10 GPa and ∼16 GPa. By contrast, Raman signals only show peak weakening and broadening for typical structural disordering, and Raman spectroscopy seems to be less sensitive than XRD in catching these structural transitions related to hydrostaticity variations in both PTM. These results uncover rich pressure-induced transitions in the two PTM and clarify their effects on hydrostaticity with direct structural evidence. The high-pressure XRD and Raman data on the two PTM obtained in this work could also be helpful in distinguishing between signals from samples and those from PTM in future high-pressure experiments.
Highlights
Pressure is one of the most important fundamental thermodynamic parameters dictating the state of matter
From a comprehensive study combining ruby fluorescence spectroscopy, in situ high-pressure synchrotron x-ray diffraction (XRD), and optical micro-Raman spectroscopy in diamond anvil cells (DACs), atomic structural information associated with the development of hydrostaticity has been obtained for the two most commonly used liquid pressure-transmitting media (PTM) for high-pressure experiments, namely, a 4:1 ME mixture and silicone oil
A pressure-induced glass transition, which accounts for the emergence of nonhydrostaticity, occurs at ∼11 to 12 GPa in the 4:1 ME mixture and ∼3 GPa in the silicone oil, and below these pressures both PTM can be considered to be ideally hydrostatic
Summary
Pressure is one of the most important fundamental thermodynamic parameters dictating the state of matter. All gases and liquids will inevitably solidify through phase transitions at critical high pressures, which causes degradation of hydrostaticity and the presence of differential stress components (pressure gradient) beyond so-called hydrostatic limit pressures.6–8 Inert gases, such as He and Ne, are considered the best PTM owing to their stable chemical properties and low x-ray scattering background signals. By measuring the specific heat capacity and thermal conductivity, Sandberg and Sundqvist claimed that silicone oil undergoes a glass transition at very low pressure of ∼1 GPa. Tateiwa and Haga used the ruby fluorescence emission line width under high pressure and low temperature and demonstrated that hydrostaticity in silicone oil is worse than that in the 4:1 ME mixture at pressures above ∼4 GPa. The hydrostaticity limits in both liquid PTM are conjectured to be associated with solidification caused by pressure-induced glass transitions or crystallization, but this has yet to be confirmed by direct evidence.. XRD patterns with careful background subtraction and Raman spectra of these two PTM reveal rich structural transitions, which are correlated well with the development of hydrostaticity
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
