Pressure-induced polymorphism in SrB6 and deformation mechanisms of covalent networks

We study the high-pressure structures of ${\mathrm{SrB}}_{6}$ up to 200 GPa using first-principles structure prediction calculations and high-pressure x-ray diffraction experiments. The computations show that the ambient-pressure cubic phase transforms to an orthorhombic structure $(Cmmm)$ at 48 GPa, and then to a tetragonal structure $(I4/mmm)$ at 60 GPa. The high-pressure experiments are consistent with the theoretically predicted tetragonal structure, which was quenched successfully to ambient conditions. Pressure induces simple boron octahedra to form complex networks in which the electrons are delocalized, leading to metallic ground states with large density of states at the Fermi level. Calculated stress-strain relations for the $I4/mmm$ structure of ${\mathrm{SrB}}_{6}$ demonstrate its intrinsic hard nature with an estimated Vickers hardness of 15 GPa, and reveal a novel deformation mechanism with transient multicenter bonding that results in the combination of high strength and high ductility. Our findings offer valuable insights for understanding the rich and complex crystal structures of ${\mathrm{SrB}}_{6}$, which have broad implications for further explorations of hexaboride materials.