Hidden Thermodynamic Ground State of Calcium Diazenide

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• 作者：Hui Wang ; Yansun Yao ; Yanling Si ; Zhijian Wu ; G. Vaitheeswaran
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：January 9, 2014
• 年：2014
• 卷：118
• 期：1
• 页码：650-656
• 全文大小：379K
• ISSN：1932-7455

文摘

A new member to the family of alkaline earth diazenides, calcium diazenide (CaN₂), was recently synthesized by a controlled decomposition of calcium azide at high-pressure/high-temperature conditions (Schneider et al. Inorg. Chem. 2012, 51, 2366鈥?373). Isotypic to tetragonal calcium carbide, the synthesized CaN₂ phase adopts a tI6 structure in which Ca²⁺ cations and [N₂]^2- anions are arranged in a distorted rocksalt structure. Using an unbiased structure searching method, we investigate the energy landscape of CaN₂ at the first principles level and suggest that the tI6 structure is a metastable form of CaN₂. A new orthorhombic oP12 structure, formed by a distorted bcc lattice of Ca²⁺ cations interpenetrated by [N₂]^2- anions, is predicted to be the thermodynamic ground state of CaN₂. The energy of the oP12 structure is considerably lower than that of the synthesized phase, by 80 meV/CaN₂, while it is also lower than the energies of the candidate structures previously considered. The oP12 structure remains as the thermodynamic ground state of CaN₂ at high pressures up to 2.9 GPa, at where it undergoes a first-order phase transition to a higher density oP12鈥?structure. Above 2.9 GPa, the synthesized tI6 phase becomes the thermodynamic ground state of CaN₂. The formation of the oP12 structure is predicted to be exothermic which suggests that this structure may be synthesized in future experiments with proper experimental conditions.