Superconducting boron allotrope featuring pentagonal bipyramid at ambient pressure

Elemental boron has evoked substantial interest owing to its chemical complexity in nature. It can form multicenter bonds due to its electron deficiency, which induces the formation of various stable and metastable allotropes. The search for allotropes is attractive for finding functional materials with fascinating properties. Using first-principles calculations with evolutionary structure search, we have explored boron-rich K-B binary compounds under pressure. A series of dynamically stable structures (Pmm2 KB₅, Pmma KB₇, Immm KB₉, and Pmmm KB₁₀) containing boron framework with open channels are predicted, which can possibly be synthesized under high pressure and high temperature conditions. After the removal of K atoms, we obtain four novel boron allotropes, o-B₁₄, o-B₁₅, o-B₃₆, and o-B₁₀, which exhibit dynamical, thermal, and mechanical stability at ambient pressure. Among them, o-B₁₄ contains an unusual B₇ pentagonal bipyramid and appears in a bonding combination of seven-center-two-electron (7c-2e) B-B π bonds, which is the first time to be identified in three-dimensional boron allotropes. Interestingly, our calculation reveals that o-B₁₄ can act as a superconductor with a T_c value of 29.1 K under ambient conditions.