Staged evolution of vacancy-type defects in layered TlIn(S 1- x –Se x ) ₂ chalcogenides revealed by positron annihilation lifetime spectroscopy

In this work, layered semiconductor chalcogenide compounds TlIn(S 1-x Se x )2 (x = 0–0.4) were synthesized by melting high-purity constituents in vacuum-sealed quartz ampoules followed by crystallization using a modified Bridgman technique. The influence of isovalent S→Se substitution on the formation of vacancy-type defects was investigated by positron annihilation lifetime spectroscopy (PALS). PALS measurements reveal the presence of multicomponent annihilation processes across the entire concentration range and show that the positron trapping behavior at defects changes with increasing Se content. The obtained positron lifetime parameters demonstrate a strong correlation between the defects and the local electronic environment as well as open-volume characteristics. These results enable a deeper understanding of the mechanisms by which isovalent anion substitution shapes the defect landscape in layered semiconductor systems and confirm the high sensitivity of the PALS method for atomic-scale characterization of such materials.