High-pressure behavior of åkermanite and gehlenite and phase stability of the normal structure in melilites
High-pressure behavior of åkermanite and gehlenite and phase stability of the normal structure in melilites
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奥克锰长石和钙黄长石的高压行为以及黄长石中正常结构的相稳定性
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
W. Crichton
中科院分区：
文献类型：
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作者：
M. Merlini;M. Gemmi;M. Hanfland;W. Crichton
摘要采用同步辐射粉末法和单晶衍射法研究了Åkermanite （Ca2MgSi2O7）和辉长石（Ca2Al2SiO7）在高达30gpa的高压下的结构。在2gpa左右，非相应调制结构（IC）转变为正常结构(N)。N结构的体积模量，与Birch Murnhagan EoS对粉末数据的拟合，为93.5(5)GPa。压缩性是各向异性的，在垂直于结构的四面体层的方向上，沿着c轴压缩性更大。在15gpa以上发生相变，表现为弹性行为的不连续，粉末衍射峰的强度和半峰全宽（FWHM）的变化很小。衍射模式是相对于Nmelilite结构的四边形电池高达30gpa索引。在减压过程中观察到弹性行为的滞后。相比之下，单晶数据显示在15gpa以上出现了新的单斜相。单位胞参数为a = 8.82(1) Å， b = 7.34(1) Å， c = 9.13(1) Å， β = 115.1(2)°。该晶胞与文献报道的Ca2ZnGe1.25Si0.75O7相似。利用空间群P21/n中相应的模型进行细化，考虑到晶体是双晶且嵌合性较大，对单晶数据进行了合理的RBragg = 15%的拟合。辉长石的体积模量为106.1(4)GPa，高于<s:1>角麻石。可压缩性是各向异性的，其行为与<s:1> kermanite相似，但Al在四面体位置的存在降低了平行于（001）平面的可压缩性。辉长石的结构在25 GPa以下是稳定的，发生相变。
Abstract Åkermanite (Ca2MgSi2O7) and gehlenite (Ca2Al2SiO7) have been studied at high pressure by synchrotron radiation powder and single-crystal diffraction up to 30 GPa. At about 2 GPa, the incommensurately modulated structure (IC) transforms to a normal structure (N). The bulk modulus for the N structure, fitted with a Birch Murnhagan EoS on powder data, is 93.5(5) GPa. The compressibility is anisotropic, and it is greater along the c axis, in the direction perpendicular to the tetrahedral layers of the structure. Above 15 GPa, a phase transition is observed, marked by a discontinuity in the elastic behavior and a small change in intensity and in the full-width at half maximum (FWHM) of the powder diffraction peaks. The diffraction patterns are indexed with respect to tetragonal cell of the Nmelilite structure up to 30 GPa. A hysteresis in the elastic behavior is observed during decompression. In contrast, single-crystal data show a new monoclinic phase appearing above 15 GPa. The unit-cell parameters are a = 8.82(1) Å, b = 7.34(1) Å, c = 9.13(1) Å, β = 115.1(2)°. This unit cell is similar to that of Ca2ZnGe1.25Si0.75O7 reported in the literature. A refinement using the corresponding model in space group P21/n fits the single-crystal data with a reasonable RBragg = 15%, considering that the crystal is twinned and the mosaicity is large. Gehlenite has a higher bulk modulus, 106.1(4) GPa, than does åkermanite. The compressibility is anisotropic, and the behavior is similar to that of åkermanite, but the presence of Al in tetrahedral sites decreases the compressibility parallel to the (001) plane. The structure of gehlenite is stable up to 25 GPa, when a phase transition occurs.