| Summary: | It has been realized that the stoichiometries of compounds may change under high pressure, which is crucial in the discovery of novel materials. This work uses systematic structure exploration and first-principles calculations to consider the stability of different stoichiometries of Na–O compounds with respect to pressure and, thus, construct a high-pressure stability field and convex hull diagram. Four previously unknown stoichiometries (NaO<sub>5</sub>, NaO<sub>4</sub>, Na<sub>4</sub>O, and Na<sub>3</sub>O) are predicted to be thermodynamically stable. Four new phases (<i>P</i>2<i>/m</i> and <i>Cmc</i>2<sub>1</sub> NaO<sub>2</sub> and <i>Immm</i> and <i>C</i>2<i>/m</i> NaO<sub>3</sub>) of known stoichiometries are also found. The O-rich stoichiometries show the remarkable features of all the O atoms existing as quasimolecular O<sub>2</sub> units and being metallic. Calculations of the O–O bond lengths and Bader charges are used to explore the electronic properties and chemical bonding of the O-rich compounds. The Na-rich compounds stabilized at extreme pressures (P > 200 GPa) are electrides with strong interstitial electron localization. The <i>C</i>2<i>/c</i> phase of Na<sub>3</sub>O is found to be a zero-dimensional electride with an insulating character. The <i>Cmca</i> phase of Na<sub>4</sub>O is a one-dimensional metallic electride. These findings of new compounds with unusual chemistry might stimulate future experimental and theoretical investigations.
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