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.

中文翻译：

---

高压下4：1甲醇-乙醇混合物和硅油的结构转变

4：1（体积比）的甲醇-乙醇（ME）混合物和硅油是高压研究中使用最广泛的两种液体压力传输介质（PTM）。他们的静水极限已经用各种方法进行了广泛的研究。然而，与它们新兴的非静水性相关的原子结构的演化仍不清楚。在这里，我们使用原位监测其结构，该结构在室温下可承受高达30 GPa的压力金刚石砧室中的高压同步加速器X射线衍射（XRD），光学显微拉曼光谱和红宝石荧光光谱。两种PTM均未观察到结晶。主要衍射峰位置和宽度的压力相关性表明，在约12 GPa的4：1 ME混合物和在约3 GPa的硅油中存在玻璃化转变，超过此压力梯度会出现并随压力快速增长。4：1 ME混合物在〜5 GPa处可能还有另一个液-液转变，而在硅油中在〜10 GPa和〜16 GPa处又有两个玻璃-玻璃化转变。相比之下，拉曼信号仅显示出典型结构紊乱的峰值减弱和扩展，拉曼光谱法在捕获与两个PTM中的静水力变化有关的这些结构转变方面似乎不如XRD灵敏。这些结果揭示了两个PTM中压力引起的富集过渡，并通过直接的结构证据阐明了它们对静水压力的影响。在这项工作中获得的两个PTM的高压XRD和拉曼数据也可能有助于区分样品信号和未来高压实验中来自PTM的信号。