Atomic-Scale Pentagraphene Ribbons Stabilized with Alkali Metals under Moderate Pressures

Pentagraphene frameworks with sp² carbon atoms have significance in fundamental research studies and material science. Here, we find pentagon ribbons stacked in an AB sequence at the atomic scale within alkali metal atoms. A Pnma phase is favored on the Gibbs free energy landscape at moderate pressures and finite temperatures. Strong electron localization, covalent interactions, weak van der Waals interactions, and electronic repulsive interactions coexist in this ionic structure. Electronic bands with narrow direct gaps are flattened with double degeneracy to produce a small effective mass and van Hove singularity for the density of states, which enhances visible-light absorption and produces a thermoelectric power factor on crystals. Alkali metal atoms strongly scatter acoustic-optical phonons to reduce the lattice thermal conductivity to an ultralow level. These characteristics introduce potential thermoelectric effects into Pnma crystals. In addition, the alkali metal ions exhibit high delocalizations with superionic properties at high temperatures.