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  xperiments.

Xiehang Chen, Hongbo Lou, Zhidan Zeng, Benyuan Cheng, Xin Zhang, Ye Liu, Dazhe Xu, Ke Yang, and Qiaoshi Zeng, “Structural transitions of 4:1 methanol–ethanol mixture and silicone oil under high pressure” , Matter and Radiation at Extremes 6, 038402 (2021) abstract

Pressure dependence of hydrostaticity inside the sample chamber of a DAC with a 4:1 ME mixture as the PTM. The hydrostaticity was characterized by the different pressures (open symbols, left vertical axis) obtained from three ruby balls at
different locations inside the sample chamber and their standard deviation (solid symbols, right vertical axis). The arrow marks the onset pressure of nonhydrostaticity. The inset shows an image of the 4:1 ME mixture at ∼9 GPa along with the
three ruby balls inside the DAC.