Chemistry:Niobium ditelluride
| Names | |
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| Other names
Niobium telluride
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| Identifiers | |
3D model (JSmol)
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| EC Number |
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PubChem CID
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| Properties | |
| NbTe2 | |
| Molar mass | 348.11 g/mol |
| Appearance | Black crystals |
| Density | 7.6 g/cm3 |
| Structure | |
| Monoclinic, distorted 1T type | |
| C2/m, No. 12 | |
a = 1.939 nm, b = 0.3642 nm, c = 0.9375 nm α = 90°, β = 134.58°, γ = 90°
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
| Infobox references | |
Niobium ditelluride is an inorganic compound of niobium and tellurium with the chemical formula NbTe2. It is a black, layered crystalline solid and a member of the transition metal dichalcogenide family. Bulk NbTe2 is a semimetal with a distorted monoclinic structure associated with a charge-density wave, and becomes superconducting at very low temperature.[1][2]
Preparation
Niobium ditelluride can be prepared by direct reaction of stoichiometric quantities of elemental niobium and tellurium at high temperature:
- Nb + 2 Te → NbTe2
Single crystals can be grown by chemical vapor transport. Brown obtained crystals of NbTe2 by vapor-transport methods for single-crystal X-ray diffraction.[3]
Structure
Bulk NbTe2 crystallizes in the monoclinic crystal system, in space group C2/m (No. 12).[3] The lattice parameters reported from single-crystal diffraction are a = 19.39 Å, b = 3.642 Å, c = 9.375 Å and β = 134.58°.[3]
The structure is layered and can be regarded as a strongly distorted derivative of the octahedrally coordinated 1T structure common to transition-metal dichalcogenides. Each niobium atom is surrounded by tellurium atoms in a distorted octahedral environment.[3]
The distortion produces metal–metal bonding in which the niobium atoms form groups of three parallel rows within the layers.[3] Modern diffraction and microscopy studies confirm the monoclinic layered structure and substantial distortion of the tellurium sublattice.[4]
Earlier work reported a rhombohedral or trigonal cell for material in the NbSe2–NbTe2 system, but later single-crystal structure determinations established the monoclinic structure for stoichiometric bulk NbTe2.
Electronic and physical properties
NbTe2 is a layered semimetal. Electronic-structure calculations and photoelectron spectroscopy show a pseudogap in the electronic density of states near the Fermi level.[4]
Its monoclinic distortion is closely connected with a charge-density wave. Angle-resolved photoemission measurements show that much of the Fermi surface is pseudogapped, while electronic-structure and phonon calculations indicate that Fermi-surface nesting and electron–phonon coupling contribute to the reduction in symmetry and formation of the charge-density-wave state.[1]
Electrical and magnetic measurements on high-quality single crystals show a superconducting transition at approximately 0.5 K.[2] The superconducting state has an unusually small lower critical field, and the measured diamagnetic response is strongly anisotropic because of the layered crystal structure.[2]
Raman spectroscopy and density-functional calculations identify numerous vibrational modes associated with the distorted monoclinic lattice.[4] Ultraviolet photoelectron spectroscopy gives a work function of approximately 5.32 eV, comparable with that of gold. The comparatively high work function and stability of freshly cleaved surfaces have led to interest in NbTe2 as a possible electrical-contact material in optoelectronics.[4]
Monolayers
Atomically thin NbTe2 has also attracted interest as a two-dimensional material. Several charge-density-wave superstructures have been observed in monolayer NbTe2.
First-principles work published in 2024 indicates that both 1T- and 1H-derived monolayer phases may occur. Calculations associate observed 4 × 1 and 4 × 4 charge-density waves with 1H-derived structures, while other observed phases are consistent with distorted 1T stacking.[5]
The calculations predict competition among charge-density-wave, spin-density-wave and ferromagnetic states and suggest that structural distortions can drive transitions between topological electronic phases.[5]
References
- ↑ 1.0 1.1 Battaglia, Corsin; Cercellier, Hervé; Clerc, Florian (2005). "Fermi-surface-induced lattice distortion in NbTe2". Physical Review B 72. doi:10.1103/PhysRevB.72.195114.
- ↑ 2.0 2.1 2.2 Nagata, Shoichi; Abe, Tsuyoshi; Ebisu, Shuji; Ishihara, Yutaka; Tsutsumi, Kitomi (1993). "Superconductivity in the metallic layered compound NbTe2". Journal of Physics and Chemistry of Solids 54 (8): 895–899. doi:10.1016/0022-3697(93)90215-D.
- ↑ 3.0 3.1 3.2 3.3 3.4 Brown, B. E. (1966). "The crystal structures of NbTe2 and TaTe2". Acta Crystallographica 20 (2): 264–267. doi:10.1107/S0365110X66000501.
- ↑ 4.0 4.1 4.2 4.3 Silva, J. C. González (2018). "Crystalline structure, electronic and lattice-dynamics properties of NbTe2". Scientific Reports 8. doi:10.1038/s41598-018-35308-4. PMID 30451940.
- ↑ 5.0 5.1 Jin, Heng; Wei, Tan; Huang, Bing (2024). "Incognizant 1T/1H Charge-Density-Wave Phases in Monolayer NbTe2". Nano Letters 24 (35): 10892–10898. doi:10.1021/acs.nanolett.4c02621.
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