Symmetry-Enforced Dirac Fermions and Semimetal–Semiconductor Transition in Pentagonal XTe₂ Monolayers

The recent synthesis of monolayer pentagonal PdTe₂ [L. Liu et al., Nat. Mater. 23, 1339–1346 (2024)] has sparked interest in exploring two-dimensional transition-metal ditellurides whose electronic properties are shaped by symmetry. Using density-functional calculations and symmetry analysis, we find that nonsymmorphic symmetries enforce symmetry-protected degeneracies along the Brillouin-zone boundaries, leading to band degeneracy without spin–orbit coupling and fourfold Dirac nodes pinned at X and Y when spin–orbit and PT symmetries are included. Following the structural evolution from the hexagonal to the pentagonal phase, we find a symmetry-driven transition from semimetallic to semiconducting behavior, associated with changes in coordination and lattice geometry that modify d–p orbital hybridization. These findings identify XTe₂ monolayers (X = Ni, Pt, Pd) as a family of two-dimensional systems in which symmetry dictates the emergence of Dirac fermions and structural distortions tune their electronic character.