Tantalum(IV) sulfide

Tantalum(IV) sulfide is an inorganic compound with the formula TaS₂. It is a layered compound with three-coordinate sulfide centres and trigonal prismatic or octahedral metal centres. It is structurally similar to molybdenum disulfide MoS₂, and numerous other transition metal dichalcogenides. Tantalum disulfide has three polymorphs 1T-TaS2, 2H-TaS2, and 3R-TaS2, representing trigonal, hexagonal, and rhombohedral respectively.

The properties of the 1T-TaS₂ polytype have been described. In common with many other transition metal dichalcogenide (TMD) compounds, which are metallic at high temperatures, it exhibits a series of charge-density-wave (CDW) phase transitions from 550 K to 50 K. It is unusual amongst them in showing a low-temperature insulating state below 200 K, which is believed to arise from electron correlations, similar to many oxides. The insulating state is commonly attributed to a Mott state. It is also superconducting under pressure or upon doping, with a familiar dome-like phase diagram as a function of dopant, or substituted isovalent element concentration.

Metastability. 1T-TaS₂ is unique, not only amongst TMDs but also amongst 'quantum materials' in general, in showing a metastable metallic state at low temperatures. Switching from the insulating to the metallic state can be achieved either optically or by the application of electrical pulses. The metallic state is persistent below ~20K, but its lifetime can be tuned by changing the temperature. The metastable state lifetime can also be tuned by strain. The electrically-induced switching between states is of current interest, because it can be used for ultrafast energy-efficient memory devices.

Because of the frustrated triangular arrangement of localized electrons, the material is suspected of supporting some form of quantum spin liquid state. It has been the subject of numerous studies as a host for intercalation of electron donors.