Tantalum(IV) sulfide is an inorganic compound with the formula Ta S₂. 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 dichalcogenide materials. The 1T-TaS₂ polytype exhibits some unusual properties. 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 transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states o ...

s 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 at low temperatures (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.

Preparation

Sample of tantalum disulfide with spatula tip for scale.jpg, Sample polycrystalline TaS₂ Photo of 1T-TaS2 crystal.jpg, 1T-TaS₂ crystals grown by transport reaction CDW in 1T-TaS2.jpg, upright=1.7, (a): Schematic of the David star pattern in 1T-TaS₂ where green atoms are S and purple are Ta. (b) and (c) are STM images (6.5 K) before and after application of 2.8 V pulses through the STM tip. Insets show ~10 times magnified images. Atomic resolution image of Tantalum disulfide.png, Atomic resolution image of 1T-TaS₂ (298 K). Acquired using HAADF STEM. Scale bar 2nm. TaS₂ is prepared by reaction of powdered

tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...

and sulfur at ~900 °C. It is purified and crystallized by

chemical vapor transport In chemistry, a chemical transport reaction describes a process for purification and crystallization of non- volatile solids. The process is also responsible for certain aspects of mineral growth from the effluent of volcanoes. The technique ...

using iodine as the transporting agent: :TaS₂ + 2 I₂ TaI₄ + 2 S It can be easily cleaved and has a characteristic golden sheen. Upon extended exposure to air, the formation of an oxide layer causes darkening of the surface. Thin films can be prepared by chemical vapour deposition and molecular beam epitaxy.

Properties

Three major crystalline phases are known for TaS₂:

trigonal In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crystal ...

1T with one S-Ta-S sheet per

unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...

, hexagonal 2H with two S-Ta-S sheets, and

rhombohedral In geometry, a rhombohedron (also called a rhombic hexahedron or, inaccurately, a rhomboid) is a three-dimensional figure with six faces which are rhombi. It is a special case of a parallelepiped where all edges are the same length. It can be us ...

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 T_C = 0.5 K, which increases to 2.2 K in flakes with a thickness of a few atomic layers. The bulk T_C 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 T_C quickly rises up to 5 K at ~4 GPa and then saturates. At ambient pressure and low temperatures 1T-TaS₂ is a

Mott insulator Mott insulators are a class of materials that are expected to conduct electricity according to conventional band theories, but turn out to be insulators (particularly at low temperatures). These insulators fail to be correctly described by band ...

. Upon heating it changes to a Triclinic

charge density wave A charge density wave (CDW) is an ordered quantum fluid of electrons in a linear chain compound or layered crystal. The electrons within a CDW form a standing wave pattern and sometimes collectively carry an electric current. The electrons in such ...

(TCDW) state at T_TCDW ~ 220 K, to a nearly commensurate

(NCCDW) state at T_NCCDW ~ 280 K, to an incommensurate CDW (ICCDW) state at T_ICCDW ~ 350 K, and to a metallic state at T_M ~ 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 A scanning tunneling microscope (STM) is a type of microscope used for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer, then at IBM Zürich, the Nobel Prize in Physics in 1986 ...

(STM) to the CDW state causes it to drop electrical resistance and creates a "mosaic" or domain 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.

Memory devices and other potential applications

Switching of the material to and from the "mosaic", or domain state, by optical or electrical pulses is used for "Charge configuration memory" (CCM) devices. The distinguishing feature of such devices is that they exhibit very efficient and fast non-thermal resistance switching at low temperatures. Room temperature operation of a charge-density-wave oscillator and thermally-driven GHz modulation of the CDW state have been demonstrated.

References

{{Sulfides Sulfides Tantalum compounds Transition metal dichalcogenides Monolayers