Phase transitions and incommensurate phases in layered thallium sulfide semiconductor - An NMR study

Over the past decade, the physics of low-dimensional materials beyond graphene has been of significant interest to both science and applications. To this end, we report on the nuclear magnetic resonance (NMR) study of the layered two-dimensional compound of thallium monosulfide TlS. The structure of this semiconductor compound consists of the metal-chalcogen layers formed by Tl³⁺S₄²⁻ tetrahedra and chains of univalent Tl¹⁺ ions between the layers. ²⁰⁵Tl NMR measurements reveal phase transitions at 262, 305, 319, 341, and 350 K. In the regions 319–341 K and 262–305 K, the temperature dependence of spin-lattice relaxation time shows plateaus characteristic of incommensurate phases. The results obtained, as well as the microscopic origin of phase transitions are discussed along with literature data on dielectric and X-ray measurements and the electronic structure of the TlS.