Structural variety beyond appearance: high-pressure phases of CrB4 in comparison with FeB4

Abstract

Employing particle swarm optimization (PSO) combined with first-principles calculations, we systemically studied high-pressure behaviors of hard CrB₄. Our predictions reveal a distinct structural evolution under pressure for CrB₄ despite having the same initial structure as FeB₄. CrB₄ is found to adopt a new P2/m structure above 196 GPa, another Pm structure at a pressure range of 261–294 GPa and then a Pmma structure beyond 294 GPa. Instead of puckering boron sheets in the initial structure, the high-pressure phases have planar boron sheets with different motifs upon compression. Comparatively, FeB₄ prefers an I4₁/acd structure over 48 GPa with tetrahedron B₄ units and a P2₁3 structure above 231 GPa having equilateral triangle B₃ units. Significantly, CrB₄ exhibits persistent metallic behavior in contrast with the semiconducting features of FeB₄ upon compression. The varied pressure response of hard tetraborides studied here is of importance for understanding boron-rich compounds and designing new materials with superlative properties.