Electronic and bonding origin of enhanced phase stability in layered 1T-MoSSe

The layered material 1T-MoSSe demonstrates enhanced phase stability compared to pristine 1T-MoS₂ and 1T-MoSe₂. To understand the origin of this improved stability, first-principles calculations were employed, focusing on both electronic structure and bonding characteristics. An isomeric design concept was introduced by substituting selenium (Se) atoms into the MoS₆ octahedral repeating unit, enabling the system to adopt different atomic configurations. In particular, the emergence of fac and cis isomers plays a crucial role in modifying the local coordination environment. These isomeric arrangements facilitate Mo-Mo cluster formation, which drives a structural transformation from the ideal octahedral geometry to a skew-trapezoidal bipyramidal (STB) configuration. This rearrangement further induces the formation of the distorted 1T' phase, characterized by a zig-zag chain-like arrangement of Mo atoms. The resulting 1T'-MoSSe structure exhibits pronounced lattice distortion, reflected in a skewed Mo-Mo-Mo angle of 159.25° and significantly shortened bond lengths (Mo-Mo: 2.7518 Å and Mo-S: 2.3538 Å). These values differ notably from those in pristine MoS₂ (Mo-S: 2.4118 Å), indicating stronger interatomic interactions and enhanced orbital overlap. This highly distorted lattice and strengthened bonding contribute to the improved stability of the 1T'-MoSSe single layer. Overall, the S/Se position is the key driving force behind the formation of a highly distorted lattice, leading to enhanced bonding interactions and greater phase stability in 1T-MoSSe.