Rietveld structure refinement of MgGeO3 post-perovskite phase to 1 Mbar
Rietveld structure refinement of MgGeO3 post-perovskite phase to 1 Mbar
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DOI：
10.2138/am.2008.2691
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发表时间：
2006-12
影响因子：
3.1
通讯作者：
A. Kubo;B. Kiefer;S. Shim;G. Shen;V. Prakapenka;T. Duffy
中科院分区：
文献类型：
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作者：
A. Kubo;B. Kiefer;S. Shim;G. Shen;V. Prakapenka;T. Duffy
摘要采用CaIrO 3型结构模型（空间群Cmcm），基于Rietveld方法和基于密度泛函理论的第一性原理计算，在78-109 GPa下确定了MgGeO 3后钙钛矿（pPv）相的晶格参数和原子位置。不同样品获得的结构参数的再现性，与理论计算的一致性，以及与基于结构相似材料的预期键长的良好一致性，都为CaIrO 3型结构模型的有效性提供了证据，该模型用于MgGeO 3中超过1 Mbar的pPv相，并且通过Rietveld改进接近1 Mbar获得的结构参数的可靠性。MgGeO 3 pPv相在轴向压缩性方面表现出很强的各向异性，其中b轴是最可压缩的。GeO_6八面体的多面体体积模量是MgO_8十一面体的1.9倍。对相邻O-O距离的分析表明，沿沿着a方向排列的O-O距离最长，沿沿着c方向排列的O-O距离最短，这可能与沿沿着c方向的压缩性比沿a方向低有关。MgGeO 3 pPv与MgSiO 3、NaMgF 3和CaIrO 3 pPv的结构特征比较表明，MgSiO 3 pPv与NaMgF 3和MgGeO 3 pPv在八面体畸变程度等参数上比CaIrO 3 pPv更相似，这意味着NaMgF 3和MgGeO 3 pPv都比CaIrO 3 pPv更类似于MgSiO 3 pPv。
Abstract Using the CaIrO3-type structure model (space group Cmcm), lattice parameters and atomic positions of the MgGeO3 post-perovskite (pPv) phase were determined based on Rietveld refinements at 78-109 GPa and first-principles calculations based on density functional theory. The reproducibility of structural parameters obtained for different samples, consistency with theoretical calculations, and good agreement with expected bond lengths based on structurally similar materials all provide evidence for both validity of CaIrO3-type structure model for the pPv phase in MgGeO3 exceeding 1 Mbar and reliability of structural parameters obtained by Rietveld refinements approaching 1 Mbar. The MgGeO3 pPv phase exhibits strong anisotropy in axial compressibility, with the b-axis being most compressible. The polyhedral bulk modulus for the GeO6 octahedron is 1.9× larger than that for the MgO8 hendecahedron. Examination of neighboring O-O distances shows that the O-O distance aligned along the a direction is one of the longest and that aligned along c is one of the shortest, and these may be related to the lower compressibility along c compared with a. Comparison of structural features of MgGeO3 pPv with those for MgSiO3, NaMgF3, and CaIrO3 pPv show that MgSiO3 pPv has more similarity with NaMgF3 and MgGeO3 pPv than with CaIrO3 pPv in such parameters as degree of octahedral distortion, implying that both NaMgF3 and MgGeO3 pPv are better analogs to MgSiO3 pPv than CaIrO3 pPv.