Pressure-induced evolution of stoichiometries and electronic structures of host-guest Na-B compounds

Superionic and electride behaviors in materials, which induce a variety of exotic physical properties of ions and electrons, are of great importance both in fundamental research and for practical applications. However, their coexistence in hot alkali-metal borides has not been observed. In this work, we apply first-principles structure search calculations to identify eight Na-B compounds with host-guest structures, which exhibit a wide range of building blocks and interesting properties linked to the Na/B composition. Among the known borides, Na-rich Na9B stands out as the composition with the highest alkali-metal content, featuring vertex- and face-sharing BNa16 polyhedra. Notably, it exhibits electride characteristics and transforms into a superionic electride at 200 GPa and 2000 K, displaying unusual Na atomic diffusion behavior attributed to the modulation of the interstitial anion electrons. It demonstrates semiconductor behavior in the solid state, and metallic properties associated with Na 3p/3s states in the superionic and liquid regions. On the other hand, B-rich NaB7, consisting of a unique covalent B framework, is predicted to exhibit low-frequency phonon-mediated superconductivity with a Tc of 16.8 K at 55 GPa. Our work advances the understanding of the structures and properties of alkali-metal borides.