Pressure-Induced Discovery of a Topological Phase of Bi(4)Br(4) with AA"-Stacking.

Bismuth halides BiX 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 2/ phase of BiBr 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-BiBr (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 2/-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 = (001; 0), identifying the AA″ phase as a weak topological insulator with gapless (100)/(010) surface states.