Prediction of crystal structures and superconductivity of actinium borides at high pressures

Abstract To investigate potential compounds that may emerge in actinium borides at high-pressure conditions. In this work, we carried out systematic crystal structure search procedure on the Ac-B compounds from 0 to 300 GPa conditions based on evolutionary algorithms and density functional theory (DFT). We discovered a stable phase Pm _3 m -AcB 6 at 0 GPa with common structure of hexaboride and three phases R _3 m -AcB, Cmmm -Ac 2 B 11 , and R _3 m -AcB 8 at higher pressures in the Ac-B system. Our results show that AcB 8 and Ac 2 B 11 are superconducting, especially Ac 2 B 11 , which exhibits a Vickers hardness of approximately 31 GPa and a superconducting transition temperature T c of 14.4 K at 0 GPa. The mechanical and dynamical stability of these Ac-B phases have been verified through the application of Born criteria and phonon calculations. Furthermore, their density of states (DOS) indicates that they all exhibit metallic properties. The mechanical properties of AcB 6 are comparable to those of LaB 6 with the same structure, whereas AcB demonstrates the poorest mechanical performance and a low bonding tendency, manifested by its distinct tendency towards brittleness, as evident from its Pugh’s ratio. The structure of Ac 2 B 11 resembles a middle phase evolving from the hexaboride AcB 6 towards a class of pentaboride structures, characterized by the formation of a B 22 cage structure resulting from further boron enrichment based on the B 20 cage, or derived from the partial dimerization of the B 24 cage found in hexaborides. We believe that this configuration may hold significant potential as a parent matrix for the design of interesting ultra-hard materials with relatively high superconducting transition temperatures.