We have investigated the influence of electron beam generated defects on the structure of periodic lattice distortions (PLDs) which accompany charge density wave modulations in $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ and $1T\text{−}\mathrm{TaS}{\mathrm{e}}_2$. Lattice defects were generated through irradiation with high-energy electrons in a transmission electron microscope (TEM). Using atomically resolved high-resolution TEM imaging, we investigate the PLD structure and the changes in this structure with prolonged exposure to the electron beam. We observe the formation of dislocationlike topological defects in the PLD structure. Prolonged exposure to the electron beam also leads to an increase in density of these defects. This is also accompanied by an increase in structural disorder of the PLD. Density functional theory calculations were also performed in order to understand sulfur (S) and selenium (Se) vacancy defect formation in $1T\text{−}\mathrm{TaS}{\mathrm{e}}_2$ and $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ and their effects on the PLD structure. The formation energy of Se/S vacancies was calculated to be lowest for the highly displaced S/Se atoms in the vicinity of PLD maxima. Vacancies formed at the less displaced sites near the PLD minima were found to have lower formation energy. The calculations also showed that an increase in the S/Se vacancies leads to the formation of dislocations and an increase in disorder in the PLD structures. This supports the experimental observations.

• Received 2 January 2018
• Revised 10 October 2018

DOI:https://doi.org/10.1103/PhysRevB.99.024101