High-pressure behaviour of lawsonite a phase transition at 8.6 GPa
High-pressure behaviour of lawsonite a phase transition at 8.6 GPa
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硬钠石在 8.6 GPa 下的高压行为
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
M. Hanfland
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作者：
I. Daniel;G. Fiquet;P. Gillet;M. Schmidt;M. Hanfland
lawsonite CaAl2Si2O7(OH)2的结构行为。利用角色散x射线粉末衍射和拉曼光谱研究了室温下18 GPa的准静力条件下金刚石砧池中的H2O。随着压力的增加，我们观察到在P = 8.6(3) GPa处发生了相变，其特征是：(1)衍射线分裂，(2)新的拉曼带出现，(3)羟基O-H拉伸模式的频移发生了显著变化。这种转变是位移的，完全可逆的，没有任何可检测到的迟滞。高压相为lawsonite III，在P = 10.6 GPa时，a = 5.6833(3), b = 8.5944(4), c = 12.8773(5) a, Y = 91.42(4)°，V = 628.80(4) A3为单斜晶胞。褐铁矿III的空间群可能为c1121 / m，这是与低压Cmcm对称有关的p21 / m的非常规表示。假设空间群从Cmcm到c1121 / m的变化，根据点群从mmm到2/ m的变化，分析了自发应变张量的结果分量。对称破缺分量e64的四次幂的压力依赖性是线性的，表明了过渡的临界特征。x -射线衍射和拉曼光谱结果表明，在过渡过程中，lawsonite的整体铝硅酸盐骨架被保留。然而，单斜lawsonite III的可压缩性比低压正交晶型低40%。拉曼光谱结果与最近的红外结果一致，表明可压缩性的降低可能与结构中羟基的氢键强度的增加有关，而水分子周围的氢键系统在转变时似乎没有改变。
The structural behaviour of lawsonite CaAl2Si2O7(OH)2.H2O, has been studied under quasi-hydrostatic conditions in a diamond-anvil cell to 18 GPa at room temperature, using angle-dispersive X-ray powder diffraction and Raman spectroscopy. With increasing pressure, we observe a phase transition at P = 8.6(3) GPa, characterized by (1) the splitting of diffraction lines, (2) the emergence of new Raman bands, and (3) significant changes in the frequency shifts of the hydroxyl O-H stretching modes. The transition is displacive and fully reversible, without any detectable hysteresis. The high-pressure phase, referred to here as lawsonite III can be indexed into a monoclinic unit-cell, with a = 5.6833(3), b = 8.5944(4), c = 12.8773(5) A, Y = 91.42(4)° and V = 628.80(4) A3 at P = 10.6 GPa. The space-group of lawsonite III is likely to be C 1121/ m , which is the unconventional representation of P 21/ m related to the low-pressure Cmcm symmetry. Assuming the change in space-group from Cmcm to C 1121/ m , the resulting components of the spontaneous strain tensor are analyzed in terms of the change in point group from mmm to 2/ m . The pressure-dependence of the fourth power of the symmetry-breaking component e 64 is linear, indicating a tricritical character for the transition. Both X-ray diffraction and Raman spectroscopy indicate that the overall aluminosilicate framework of lawsonite is retained through the transition. However, monoclinic lawsonite III is ∼ 40% less compressible than the low-pressure orthorhombic polymorph. The Raman spectroscopic results, in good agreement with recent infrared ones reveal that this decrease in compressibility is likely to be related to the increase in the hydrogen bond strength involving the hydroxyl groups in the structure, whereas the hydrogen bond system around the water molecule does not appear to be modified at the transition.