Hydrogen Incorporation in Crystalline Jadeite: Insight from First Principles Calculations

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• 作者：ZHANG Xiaoling ; FAN Xiaoyu ; MENG Dawei ; WU Xiuling and LIU Weiping
• 刊名：Acta Geologica Sinica - English Edition
• 出版年：2016
• 出版时间：June 2016
• 年：2016
• 卷：90
• 期：3
• 页码：939-945
• 全文大小：5879K
• ISSN：1755-6724

文摘

Hydrogen incorporation is critical for explaining defect energies, structure parameters and other physical characteristics of minerals and understanding mantle dynamics. This work analyzed the hydrogen complex defects in jadeite by the plane-wave pseudo-potential method based on density functional theory, and optimized the atomic positions and lattice constants in all configurations (different defective systems). Incorporation mechanisms considered for hydrogen (H) in jadeite include: (1) hydrogen incorporating with the 02 site oxygen and coexisting with M2 vacancy; (2) one H atom combined with an Al atom replacing Si in tetrahedron; (3) 4H atoms directly replacing Si in tetrahedron and (4) 3H atoms replacing Al on the Ml site. The four incorporation mechanisms mentioned above form the corresponding V_Na-H_i, Al_Si-H_i, V_Si-4H_i and V_Al-3H_i point defects. The molecular dynamics simulation to the ideal, V_Na-H_i, Al_si-H_i, V_Si-4Hi and V_Al-3H_i point defects under the P-T conditions of 900 K, 2 GPa, the V_Na-H_i and Al_Si-H_i point defects under different pressures at T = 900 K, and Al_Si-H_i point defects under different temperatures at P = 3 GPa was performed to examine the preferential mode of hydrogen incorporation in jadeite by means of first-principles calculations. The calculations show that the averaged O–H bond-length in the hydrogen point defects system decreased in the order of Al_Si-H_i, V_Na-H_i, V_sr-4H_i and V_Al 3H_i. V_Na-H_i complex defects result in a contraction of the jadeite volume and the presence of Al_Si-H_i, V_Si-4H_i and V_Al-3H_i defects could increase the supercell volume, which is the most obvious in the V_Al-3H_i defects. The energy of formation of AI_Si-H_i and V_Al-3H_i complex defects was much lower than that of other defect systems. The V_Al-3H_i defects system has the lowest energy and the shortest O–H bond-length, suggesting that this system is the most favorable. The analytical results of vacancy formation energy, O-H bond-length, and the stability of the hydrogen defects in jadeite have suggested that the preferred hydration incorporation mode in jadeite is V_Al-3H_i complex defect.