High-pressure phases of BaCN2 explored using a genetic algorithm

Nitrogen-containing polymers have attracted considerable research attention for their application to high energy density materials, where the energy is internally stored in their triple bonds. In this regard, it is intriguing to examine whether such polymerized phases appear in the high-pressure phase of metal carbodiimide MCN2, which have been synthesized recently. However, very few studies have investigated crystal structures up to 10/15 GPa in experiments/simulations. In this work, we adopted a structure search based on genetic algorithm coupled with ab initio electronic structure calculations to investigate possible crystal structures that may appear in the high-pressure phase of BaCN2. Furthermore, we reproduced the previously reported crystal structures in the lower pressure range similar to 6 GPa. With confirmed reliability of its predictive capability, we used the genetic search and predicted a polymerized phase with Ima2 appearing at higher pressure above 42 GPa. The polymerized phase has a linear network structure of CN3 planar triangular units. It has been found that the anion site units CN2, which are close to each other under high pressure, form direct covalent bonds and stabilize the phase.