Experimental observation of mesoscopic fluctuations to identify origin of thermodynamic anomalies of ambient liquid water
Experimental observation of mesoscopic fluctuations to identify origin of thermodynamic anomalies of ambient liquid water
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介观波动的实验观察以确定周围液态水热力学异常的起源
DOI：
10.1103/physrevresearch.5.013120
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发表时间：
2023
影响因子：
4.2
通讯作者：
Baron Alfred Q. R.
中科院分区：
文献类型：
--
作者：
Kajihara Yukio;Inui Masanori;Matsuda Kazuhiro;Ishikawa Daisuke;Tsutsui Satoshi;Baron Alfred Q. R.
我们报告了对环境液态水热力学异常背后的介观波动的实验观察。两种频率差异很大的声速测量方法（即太赫兹频段的非弹性 X 射线散射 (IXS) 和兆赫频段的超声波 (US)）的组合使我们能够研究特征频率介于上述两个频率之间的弛豫现象。我们进行了 IXS 测量，以获得液态水从环境条件到液-气相变 (LGT) 超临界区域的 IXS 声速，并将结果与文献中报道的 US 声速进行比较。我们发现，使用这两种方法获得的与弛豫强度相对应的声速比表现出简单但显着的变化。在高温和低温区域观察到两次明显的上升，这意味着存在两种弛豫现象。在高温区域，在 LGT 临界脊线附近观察到一个峰值，这与密度波动幅度以及等容比热容和等压比热容的变化有关。这一结果表明高温弛豫源于LGT临界涨落，证明该方法对于观察这种介观涨落是有效的。同时，在低温区，从550 K向低温区升高并达到较高值，达到环境条件下的“快声”状态。这一结果表明存在另一种弛豫机制，导致声速异常，包括液态水在环境条件下的“快声”现象。低温区的变化与等容热容的变化有关，这表明这种弛豫导致了众所周知的液态水的热容异常。这种低温弛豫对应于液-液相变（LLT）的临界波动，据推测该临界波动存在于过冷区域。本研究同时观测了LGT和LLT的临界涨落，并通过分析两种相变的异同，全面讨论了热力学与临界涨落之间的关系。
We report an experimental observation of mesoscopic fluctuations underlying the thermodynamic anomalies of ambient liquid water. The combination of two sound-velocity-measurement methods with largely different frequencies, namely inelastic x-ray scattering (IXS) in the terahertz band and ultrasonic (US) in the megahertz band, allows us to investigate a relaxation phenomenon that has a characteristic frequency between the two aforementioned frequencies. We performed IXS measurements to obtain the IXS sound velocity of liquid water from the ambient conditions to the supercritical region of liquid–gas phase transition (LGT) and compared the results with the US sound velocity reported in the literature. We found that the ratio of the sound velocities,, which corresponds to the relaxation strength, obtained using these two methods exhibits a simple but significant change. Two distinct rises were observed in the high- and low-temperature regions, implying that two relaxation phenomena exist. In the high-temperature region, a peak was observed near the LGT critical ridge line, which was linked to changes in the magnitude of density fluctuation and isochoric and isobaric-specific heat capacities. This result indicates that the high-temperature relaxation originates from the LGT critical fluctuation, proving that this method is effective for observing such mesoscopic fluctuations. Meanwhile, in the low-temperature region,increased from 550 K toward the low-temperature region and reached a high value, attaining the “fast sound” state under ambient conditions. This result indicates that another mechanism of relaxation exists, which causes the sound velocity anomaly, including the “fast sound” phenomenon of liquid water under ambient conditions. The change inin the low-temperature region is linked to the change in the isochoric heat capacity, which identified that this relaxation causes the well-known heat capacity anomaly of liquid water. This low-temperature relaxation corresponds to the critical fluctuation of the liquid–liquid phase transition (LLT) that is speculated to exist in the supercooled region. In this study, both LGT and LLT critical fluctuations were observed, and the relationship between thermodynamics and the critical fluctuations was comprehensively discussed by analyzing the similarities and differences between the two phase transitions.
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