High-pressure single-crystal X-ray diffraction and infrared spectroscopic studies of the C2/m-P21/m phase transition in cummingtonite
High-pressure single-crystal X-ray diffraction and infrared spectroscopic studies of the C2/m-P21/m phase transition in cummingtonite
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角闪石中 C2/m-P21/m 相变的高压单晶 X 射线衍射和红外光谱研究
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发表时间：
1998
期刊：
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通讯作者：
R. Hemley
中科院分区：
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作者：
Hexiong Yang;R. Hazen;C. Prewitt;L. Finger;R. Lu;R. Hemley
摘要：利用单晶 X 射线衍射和高达 7.90 GPa 的各种压力和高达 8.63 GPa 的红外光谱研究了 (Fe + Mn)/(Fe + Mn + Mg) ≈ 0.50 的角闪石中与 C2/m-P21/m 相变相关的结构变化。随着压力的增加，晶体在~1.21 GPa 时从 C2/m 对称性转变为 P21/m 对称性，这是由违反 C2/m 空间群的反射的出现所决定的。红外光谱为相变提供了额外的证据：OH 拉伸带的明显分裂是由一个不等价 OH 位置增加到两个的结果。 C2/m-P21/m 转变具有弱位移一阶或三临界特征，在轴比 a/b 和 a/c 图中随压力变化的斜率明显变化。晶胞压缩具有相当大的各向异性，C2/m 和 P21/m 相中的 a 尺寸是最可压缩的。 C2/m-P21/m 转变的主要结构变化包括：(1) 一条晶体学上不同的硅酸盐链变成不连续的两条，通过 M4-O5 键以及 M4-O6 分裂成两个非等价键，以及 (2) M4-阳离子配位从六倍增加到七倍。更重要的是，我们观察到 A 链的旋转方向发生了变化，同时晶体结构保持 P21/m 对称性：与 B 链一样，它在 1.32 GPa 下进行 O 旋转，但在 2.97 GPa 和更高的压力下进行 S 旋转。随着压力从 1.32 GPa 增加到 7.90 GPa，与 M4 阳离子配位的最近的桥接 O 原子发生切换：M4-O5B 距离从 2.944 收缩到 2.551 Å，而 M4-O6B 距离从 2.754 A° 增加到 2.903 A°。低压和高压多晶型物的压缩机制似乎略有不同。在C2/m相中，A和M4位点的行为控制结构的压缩，而M1、M2和M3八面体对压力的响应也在决定P21/m结构的压缩中发挥作用。相变被认为主要是由 M4 和 T 位点之间的差异压缩驱动的，对称性破缺为 M4 位点提供了必要的更紧密的协调。根据我们的数据，Zhang 和 Hafner (1992) 观察到的 1.0 和 3.4 GPa 之间的镁铝榴石中 57Fe 超精细参数的明显变化可能是由 C2/m-P21/m 结构转变引起的。
Abstract The structural changes associated with the C2/m-P21/m phase transition in cummingtonite with (Fe + Mn)/(Fe + Mn + Mg) ≈ 0.50 have been studied with single-crystal Xray diffraction at various pressures up to 7.90 GPa and infrared spectroscopy up to 8.63 GPa. With increasing pressure, the crystal transforms from C2/m to P21/m symmetry at ∼1.21 GPa, as determined by the appearance of reflections violating the C2/m space group. Infrared spectra provide additional evidence for the phase transition: A distinct splitting of OH stretching bands results from an increase from one to two nonequivalent OH positions. The C2/m-P21/m transition is of weakly displacive first-order or tricritical character with apparent slope changes in the plots of the axial ratios a/b and a/c as a function of pressure. The unit-cell compression is considerably anisotropic with the a dimension in both C2/m and P21/m phases being the most compressible. Major structural changes for the C2/m-P21/m transition include: (1) One crystallographically distinct silicate chain becomes two discontinuously, coupled by the splitting of the M4-O5 bond, as well as M4- O6, into two nonequivalent bonds, and (2) the M4-cation coordination increases from sixfold to sevenfold. More importantly, we observed a change in the sense of rotation for the A chain while the crystal structure maintains P21/m symmetry: It is O rotated, as the B chain, at 1.32 GPa, but S-rotated at 2.97 GPa and higher pressures. As pressure increases from 1.32 to 7.90 GPa, there is a switching of the nearest bridging O atoms coordinated with the M4 cation: The M4-O5B distance contracts from 2.944 to 2.551 Å , whereas the M4-O6B distance increases from 2.754 to 2.903 A° . Compression mechanisms for the lowand high-pressure polymorphs appear to be slightly different. In the C2/m phase, the behavior of the A and M4 sites controls the compression of the structure, whereas the response of the M1, M2, and M3 octahedra to pressure also plays a role in determining the compression of the P21/m structure. The phase transition is regarded as primarily driven by the differential compression between the M4 and T sites, and the symmetry breaking provides a necessary tighter coordination for the M4 site. Based on our data, the obvious changes in the hyperfine parameters of 57Fe in grunerite between 1.0 and 3.4 GPa, observed by Zhang and Hafner (1992), are likely to result from the C2/m-P21/m structural transformation.