Stabilization of 9/10-fold structure in bismuth selenide at high pressures

We report joint theoretical and experimental research on the high-pressure structures of bismuth selenide (Bi₂Se₃) up to 50 GPa. Our first-principles structure prediction via calypso methodology meets our high-pressure X-ray diffraction experiments performed in diamond anvil cell. We established that the ambient-pressure rhombohedral phase transforms to a monoclinic C2/m structure at 9.8 GPa, and then to a monoclinic C2/c structure at 12.4 GPa. Above 22.1 GPa, we were able to identify that Bi₂Se ₃ develops into a novel 9/10-fold structure, which was not taken by its other family members Bi₂Te₃ and Sb₂Te ₃. The large differences in atomic core and electronegativity of Bi and Se are suggested to be the physical origin of the stabilization of this 9/10-fold structure. Our research work allows us to reveal a rich chemistry of Bi in the formation of 6, 7, 8, and 9/10-fold covalent bond with Se at elevated pressures.