Brillouin Scattering Study on Molecular Liquid under High Pressure and High Temperature Conditions
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摘要: 利用金刚石对顶砧技术，采用180°背向散射和60°前向对称散射两种几何配置, 对水、氨、二水合氨和甲烷等含氢小分子液体进行了高温高压布里渊散射研究，计算了在室温(296 K)和高温(410 K)下的声速，比较了不同小分子液体中的声速及绝热体弹模量随压力的变化关系。在等温条件下，各体系中声速随着压力的增加逐渐增加；在相同温度下，甲烷液体的声速随着压力增加的速率明显高于水、氨及二水合氨液体；在相同的温度和压力条件下，水、氨及二水合氨液体的体弹模量明显高于甲烷液体的体弹模量，表明氢键的存在对于小分子液体弹性具有较大影响。二水合氨的体弹模量斜率在1.5 GPa左右发生改变，表明液体结构可能发生了改变，并分析了氢键对该体系弹性性质的影响。研究有助于理解其他含氢小分子液体中压力和温度诱导的分子结构变化。Abstract: High-pressure and high-temperature Brillouin scattering studies have been performed on molecular liquids, including water (H2O), ammonia (NH3), ammonia dihydrate (NH3·2H2O) and methane (CH4) by using a diamond anvil cell (DAC).The acoustic velocity and adiabatic bulk modulus have been determined from measured Brillouin frequency shifts in both 180° and 60° scattering geometries under pressure up to freezing point, along room temperature (296 K) and high temperature (410 K) isotherms, and the pressure dependence of acoustic velocity and adiabatic bulk modulus were compared in different molecular systems.The acoustic velocities increase smoothly with the increasing pressure, methane possesses the highest velocity increasement and the lowest adiabatic bulk modulus under similar pressure and temperature, indicating the hydrogen bond impact on elasticity in these liquids.A change in the slope of pressure dependence of the bulk modulus in liquid ammonia dihydrate was observed around 1.5 GPa, suggesting a possible structure change in liquid and analysing the effect of hydrogen on elasticity.This study improves our understanding of the pressure- and temperature-induced molecular structure changes in other molecular liquid systems.
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图 1 室温下60°前向对称散射几何配置测得的液态二水合氨布里渊散射谱
Figure 1. Brillouin scattering spectra of ammonia dihydrate in 60° scattering geometry under room temperature
图 2 在410 K时，液态甲烷的布里渊散射谱:(a) 60°前向对称散射几何配置，(b) 180°背向散射几何配置
Figure 2. Brillouin scattering spectra of methane in 60° (a) and 180° (b) scattering geometry under 410 K
R is the peak of Rayleigh scattering, D refers to diamond peak; νL indicates the longitudinal signal of liquid methane; νB indicates the weak backscattering signal; νS indicates the longitudinal signal of solid methane. Solidification of liquid methane was observed at 2.7 GPa under microscope.
图 3 室温(296 K)与高温(410 K)下液态水、氨、二水合氨和甲烷的声速随着压力的变化关系
Figure 3. Pressure dependence of the acoustic velocity of water (H2O), ammonia (NH3), ammonia dihydrate (NH3·2H2O)and methane (CH4) under room temperature (296 K) and high temperature (410 K)
图 4 室温(296 K)与高温(410 K)下液态水、氨、二水合氨和甲烷绝热体弹模量随着压力的变化关系
Figure 4. Pressure dependence of the acoustic bulk modulus of H2O, NH3, NH3·2H2O, CH4under room temperature (296 K) and high temperature (410 K)
表 1 不同温度下液态水、氨、二水合氨和甲烷的常压体弹模量(B0)、体模量随着压力变化的一阶导数(dB/dp)
Table 1. The acoustic bulk modulus of H2O, NH3, NH3·2H2O and CH4 at ambient pressure and pressure dependence under different temperatures
Mater 296 K 410 K B0/(GPa) dB/dp B0/(GPa) dB/dp H2O 2.26(04) 6.92(09) 2.73(11) 6.13(07) NH3 2.09(05) 6.67(11) 1.67(10) 5.94(10) NH3·2H2O[24] 1.50(52) 6.83(29) 5.14(40) 4.41(36) CH4 0.24(12) 5.09(12) 0.00(05) 4.80(03) 
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