A new pressure-densified orthogonal hard superconducting phase of RhB₄

A new hard superconducting phase of RhB₄ with the space group Cmca is predicted, and the phase transition and mechanical and superconducting properties of RhB₄ under 300 GPa are studied using first principles. We predict a new high-pressure phase of RhB₄ by substituting the most stable Cmca structure of OsB₄, known for its excellent mechanical properties. The calculated enthalpy shows that above 112.6 GPa, Cmca is superior to Pmmn as was previously predicted by particle swarm optimization. The stability of the predicted phase is checked using formation enthalpy, elastic constant and phonon dispersion. Additionally, the convex hull of the Rh-B system confirms that the phase is expected to be synthesized experimentally. The Cmca phase is an incompressible hard material with a hardness of 23.75 GPa at 300 GPa attributed to strong intralayer covalent B-B bonds. Furthermore, the phase is a relatively pressure-insensitive superconductor, with a T_c of 8.6 K at 112.6 GPa and a pressure-dependent coefficient of -0.03 K GPa^-1. The finding reveals a superconducting hard material that is well-suited for extreme high-pressure environments.