Janus layers and electronic structure of 1T-(TiSeS)₂

TiS₂-TiSe₂ is one of the most studied titanium based solid solution systems. However, so far, all research on it has only focused on its disordered phase. Here, we systematically investigate its ordered phases. Using a structure search method based on the particle swarm optimization (CALYPSO) algorithm, we identify TiSeS-156 and discover a new structure (1T-(TiSeS)₂). Based on first principles theory, their phonon spectra, formation energy, mechanical, electronic, thermal, and optical properties, as well as chemical bond analysis and synthetic pathways, have been investigated. The primitive cell of TiSeS-156 has three atoms and has a space group of P3m1 (no. 156). 1T-(TiSeS)₂ has six atoms and has P3̄m1 symmetry (no. 164). TiSeS-156 and 1T-(TiSeS)₂ are constructed by stacking the S-Ti-Se Janus layer materials. TiSeS-156 and 1T-(TiSeS)₂ are narrow-gap semiconductors. The localized nature of the Ti(3d) states of TiSeS-156 and 1T-(TiSeS)₂ leads to their semiconductor properties. 1T-(TiSeS)₂ and TiSeS-156 have very similar mechanical, electronic, thermal, and optical properties of 1T-TiS₂ and 1T-TiSe₂, and are members of the 2D hexagonal lattice transition metal dichalcogenide layered material family. However, compared with 1T-TiS₂ and 1T-TiSe₂, TiSeS-156 and 1T-(TiSeS)₂ have a wider range of potential applications, such as photovoltaic devices and photocatalysis, due to their S-Ti-Se Janus layer structure. They also provide a pathway for the preparation of Janus TiSeS monolayer and multi-layer materials. Moreover, our findings provide crucial insights for understanding the rich and complex crystal structures of the TiS₂-TiSe₂ system, which have broad implications for further exploration of this class of promising materials.