Pressure-Induced Discovery of a Topological Phase of Bi₄Br₄ with AA''-Stacking

Bismuth halides Bi₄X₄ (X = Br, I) consist of quasi-one-dimensional van der Waals chains whose interchain stacking can be reshaped by pressure, potentially reprogramming topology. Using CALYPSO structure searches with first-principles enthalpy ranking, we uncover a previously unreported C2/m phase of Bi₄Br₄ featuring a novel AA″ stacking that becomes stable above ∼4 GPa and persists to at least 15 GPa. We map the transformation from known β-Bi₄Br₄ (AA stacking): under compression β develops an imaginary phonon mode, appears as a first-order saddle point in the crystal-solution landscape, and relaxes into the C2/m-AA″ structure with an estimated barrier of ∼40 meV/atom, explaining pressure-driven stacking rearrangement. With spin-orbit coupling, a band gap opens at symmetry crossings and a nontrivial inversion yields Z₂ = (001; 0), identifying the AA″ phase as a weak topological insulator with gapless (100)/(010) surface states.