Local structure changes in 1T-TaS_2-2xSe_2x systems

The transition metal dichalcogenides (TMD) materials are prototypical examples of charge density wave (CDW) instability where the CDW state is in close proximity to superconductivity. In this project, we investigate the CDW phases in 1T-TaX₂ (X=S, Se) class of TMDs. It is reported that 1T-TaX₂ forms a √13×√13 super cell, with a large distortion, about 10% of the lattice constant, in the commensurate CDW (CCDW) state. 1T-TaS₂ undergoes a series of phase transitions upon cooling and achieves a CCDW phase below 183K. Simultaneously, electron correlation effects set in and localize the unpaired electron at the center of star, leading to a Mott insulating state. 1T-TaSe₂ on the other hand, undergoes CCDW transition at a higher temperature of 473K and remains metallic upon cooling to low temperatures. No metal-to-insulator transition is observed in 1T-TaSe₂ which implies that the spin on the unpaired electron is not localized within the star-of-David and implies a different distortion is at play, even though the average symmetries and periodic modulations are the same for TaS₂ and TaSe₂. We investigate the local atomic correlations in 1T- TaS₂ and TaSe₂ which is very important to study the lattice, spin and electronic correlations in this system.