Crystal structure showing two stacked S-Ta-S sheets
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||245.078 g/mol|
|Melting point||>3000 °C |
|Tantalum telluride |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Tantalum(IV) sulfide is the inorganic compound with the formula TaS2. It is a layered compound with three-coordinate sulfide centres and trigonal prismatic metal centres. It is structurally similar to the more famous material molybdenum disulfide, MoS2. TaS2 is a semiconductor with d1 electron configuration. Although an obscure material otherwise, TaS2 has been the subject of numerous studies because it is a versatile host for intercalation of electron donors, and because it exhibits unusual phase transitions at low temperatures.
TaS2 is prepared by reaction of powdered tantalum and sulfur at ~900 °C. It is purified and crystallized by chemical vapor transport using iodine as the transporting agent:
Three major crystalline phases are known for TaS2: trigonal 1T with one S-Ta-S sheet per unit cell, hexagonal 2H with two S-Ta-S sheets, and rhombohedral 3R with three S-Ta-S sheets per cell; 4H and 6R phases are also observed, but less frequently. These polymorphs mostly differ by the relative arrangement of the S-Ta-S sheet rather than the sheet structure.
2H-TaS2 is a superconductor with the bulk transition temperature TC = 0.5 K, which increases to 2.2 K in flakes with a thickness of a few atomic layers. The bulk TC value increases up to ~8 K at 10 GPa and then saturates with increasing pressure. In contrast, 1T-TaS2 starts superconducting only at ~2 GPa; as a function of pressure its TC quickly rises up to 5 K at ~4 GPa and then saturates.
At ambient pressure and low temperatures 1T-TaS2 is a Mott insulator. Upon heating it changes to a Triclinic charge density wave (TCDW) state at TTCDW ~ 220 K, to a nearly commensurate charge density wave (NCCDW) state at TNCCDW ~ 280 K, to an incommensurate CDW (ICCDW) state at TICCDW ~ 350 K, and to a metallic state at TM ~ 600 K.
In the CDW state the TaS2 lattice deforms to create a periodic Star of David pattern. Application of (e.g. 50fs) optical laser pulses or voltage pulses (~2–3 V) through electrodes or in a scanning tunneling microscope (STM) to the CDW state causes it to drop electrical resistance and creates a "mosaic state" consisting of nanometer-sized domains, where both the domains and their walls exhibit metallic conductivity. This mosaic structure is metastable and gradually disappears upon heating.
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