An infrared and Raman spectroscopic study of gypsum at high pressures
An infrared and Raman spectroscopic study of gypsum at high pressures
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DOI：
10.1007/s002690100187
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发表时间：
2001-10-01
影响因子：
1.4
通讯作者：
Williams, Q
中科院分区：
文献类型：
--
作者：
Knittle, E;Phillips, W;Williams, Q
我们提供了 300 K 下 21 GPa 石膏的拉曼光谱和红外光谱。我们的测量包括 400 至 1150 cm(-1) 之间的 (SO4)(-4) 基团的内部模式、3200 至 3600 cm(-1) 之间的羟基伸缩振动以及分子 H2O 基团的振动和弯曲振动。硫酸基的所有振动都具有正压力位移，而羟基伸缩振动和弯曲振动具有正压力位移和负压力位移的混合：因此压力对水分子氢键的影响显得很复杂。在接近 5 GPa 时，水分子的两个红外活性弯曲振动合并，并且光谱的羟基伸缩区域的形态发生显着变化。这种行为与石膏在 5-6 GPa 附近的压力诱导相变一致，观察到在减压至零压力时该相变是可逆的。光谱观察结果与石膏中水分子位置无序增加的开始一致：硫酸盐振动很大程度上不受这种转变的影响。硫酸基团的拉曼活性对称拉伸在 4 GPa 附近经历明显的分裂，这被解释为由带有对称弯曲振动泛音的费米共振产生。我们采样的 20 种振动模式的平均模式格鲁内森参数小于 0.05，而体热格鲁内森参数为 1.20。因此，石膏内的水和硫酸盐单元的振动对体积压实高度不敏感。因此，尽管水单元的键合在 5 GPa 附近发生变化，但亚稳压缩石膏在 300 K 时仍能保持分子状单元的牢固结合，达到 20 GPa 以上。
We present Raman and infrared spectra of gypsum to 21 GPa at 300 K. Our measurements encompass the internal modes of the (SO4)(-4) group that lie between 400 and 1150 cm(-1), hydroxyl-stretching vibrations between 3200 and 3600 cm(-1), and a libration and bending vibrations of the molecular H2O group. All vibrations of the sulfate group have positive pressure shifts, while the hydroxyl-stretching and -bending vibrations have a mixture of positive and negative pressure shifts: the effect of pressure on the hydrogen bonding of the water molecule thus appears to be complex. Near 5 GPa, the two infrared-active bending vibrations of the water molecule coalesce, and the morphology of the hydroxyl-stretching region of the spectrum shifts dramatically. This behavior is consistent with a pressure-induced phase transition in gypsum in the vicinity of 5-6 GPa, which is observed to be reversible on decompression to zero pressure. The spectral observations are consistent with the onset of increased disorder in the position of the water molecule in gypsum: the sulfate vibrations are largely unaffected by this transition. The Raman-active symmetric stretch of the sulfate group undergoes an apparent splitting near 4 GPa, which is interpreted to be produced by Fermi resonance with an overtone of the symmetric bending vibration. The average mode Gruneisen parameter of the 20 vibrational modes we sample is less than 0.05, in contrast to the bulk thermal Gruneisen parameter of 1.20. Accordingly, the vibrations of both water and sulfate units within gypsum are highly insensitive to volumetric compaction. Therefore, in spite of the changes in the bonding of the water unit near 5 GPa, metastably compressed gypsum maintains strongly bound molecular-like units to over 20 GPa at 300 K.