Ab Initio Thermodynamics and the Relationship between Octahedral Distortion, Lattice Structure, and Proton Substitution Defects in Malachite/Rosasite Group Endmember Pokrovskite Mg2CO3(OH)2

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• 作者：Anne M. Chaka
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：December 29, 2016
• 年：2016
• 卷：120
• 期：51
• 页码：10181-10195
• 全文大小：731K
• ISSN：1520-5215

文摘

Divalent metal hydroxycarbonates with M₂CO₃(OH)₂ stoichiometry are widely used in industry and are abundant in nature as the malachite/rosasite group of minerals. Essential to their performance as catalytic precursors and in nanoelectronics, these materials and minerals exhibit a high degree of cation ordering in mixed metal systems due to differences in distortion of the octahedral metal sites. Density-functional theory (DFT) calculations on pokrovskite Mg₂CO₃(OH)₂ in the rosasite structure and Mg analogues of monoclinic and orthorhombic forms of malachite determine that the octahedral sites are innately distorted, and that d⁹ Cu(II) Jahn–Teller distortion accommodates this distortion rather than causes it, leading to the significant preference of Cu for the type I octahedral sites. This distortion also leads to a high propensity for formation of cation vacancies charge balanced by proton substitution. Ab initio thermodynamics is used to determine that there are conditions under which proton substitution defects are slightly more stable than the stoichiometric structure, consistent with the widespread observation of such defects in pokrovskite in nature. Pokrovskite itself is most likely to form under CO₂-rich/low water conditions, particularly those utilizing supercritical CO₂ for carbon sequestration and is sufficiently thermodynamically stable to trap CO₂ under geological conditions. Low temperature and high water concentration promotes the formation of proton substitution defects, which has implications for synthesis of any material where octahedral strain may be relieved by proton substitution defects.