Structural Phase Transitions and Physical Properties of TaB₄ under High Pressure

Crystal structure searches based on the particle swarm optimization algorithm combined with first-principles calculations were performed to systematically investigate the crystal structures, stability, electronic properties, mechanical properties, and superconductivity of the boron-rich transition-metal boride TaB₄ under high pressure, with particular emphasis on the intrinsic relationship between crystal structure and superconductivity. The results show that the C2/m-Ⅰ phase, which is stable at ambient pressure, undergoes a structural phase transition at approximately 2.5 GPa, giving rise to a new monoclinic C2/m-Ⅱ phase. Phonon calculations reveal that this new phase remains dynamically stable after decompression to ambient pressure, suggesting the possibility of its metastable existence under ambient conditions. Electronic structure calculations indicate that C2/m-Ⅱ-TaB₄ exhibits metallic behavior. Electron-phonon coupling calculations further demonstrate that this phase exhibits phonon-mediated superconductivity, with a superconducting transition temperature of up to 13.5 K at ambient pressure. The electron-phonon coupling is mainly associated with low-frequency phonon modes involving Ta atoms. Mechanical property calculations show that C2/m-Ⅱ-TaB₄ has a Vickers hardness of approximately 14.5 GPa, indicating a moderate hardness. These results expand our understanding of the structural diversity of boron-rich compounds in the Ta-B system and provide a theoretical basis for elucidating the intrinsic relationships among crystal structure, lattice vibrations, and superconductivity in transition-metal borides.