First-principles study of crystal structures of under pressure

First-principles calculations employing density functional theory are performed to study ionic crystal structures of NO+NO3−. The pressure dependences of enthalpy, structural parameters, and electronic band gap are investigated for the two experimentally reported phases of NO+NO3−. It is found that these two phases have comparable densities for P<25 GPa and are thus competing ones that may be obtained through different pathways. Moreover, one of the two phases is unlikely the previously proposed orthorhombic P21cn structure. The trend of pressure dependence of the band gap is typical of that for ionic crystals. This study provides insight into different experimental findings.