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

Abstract

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.