Neodymium hexaboride
| Names | |
|---|---|
| Other names
Neodymium boride | |
| Identifiers | |
| ECHA InfoCard | 100.031.380 |
| EC Number |
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PubChem CID |
|
CompTox Dashboard (EPA) |
|
| Properties | |
| NdB6 | |
| Molar mass | 209.10 g/mol |
| Structure | |
| Cubic, CaB6-type | |
| Pm3m (No. 221) | |
a = 4.1269 Å | |
Formula units (Z) |
1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Neodymium hexaboride is a binary inorganic compound of neodymium and boron with the formula NdB6. It is a refractory rare-earth boride that crystallizes in the cubic calcium hexaboride structure.[1] NdB6 is metallic and undergoes antiferromagnetic ordering at approximately 8 K.[2][3] Its relatively low work function has also led to investigation of the material and its nanostructures for electron-emission applications.[4][5]
Preparation
[edit]Neodymium hexaboride can be prepared by heating mixtures of neodymium(III) oxide and elemental boron at high temperature under vacuum.[1] It can also be synthesized by reduction of neodymium(III) oxide with boron carbide. In one method, single-phase NdB6 powder was produced under a vacuum of approximately 10−2 Pa by heating the starting mixture above 1773 K for 4 hours.[6]
The overall carbothermal reaction can be represented as:
- Nd2O3 + 3B4C → 2NdB6 + 3CO
The resulting particles in this process were typically 2–4 μm in diameter, with larger aggregates of approximately 4–8 μm.[6]
NdB6 has also been prepared under high-pressure and high-temperature conditions by reducing neodymium oxide with elemental boron. This method produces phase-pure NdB6, while residual neodymium oxide and metallic neodymium can be removed by treatment with hydrochloric acid.[7]
Nanocrystalline NdB6 can be prepared at room temperature by mechanochemical reduction of B2O3 with magnesium in the presence of neodymium metal:
- Nd + 3B2O3 + 9Mg → NdB6 + 9MgO
After 15 hours of high-energy milling and removal of the MgO by-product, crystallites with an average size of approximately 18 nm were obtained.[8]
Large single crystals can be grown by the optical floating-zone technique. High-quality [100]-oriented crystals approximately 9 mm in diameter have been produced using two successive floating-zone passes.[4]
Properties
[edit]Crystal structure
[edit]Neodymium hexaboride crystallizes in the cubic crystal system, space group Pm3m (No. 221), with the CaB6-type structure.[1][6] A precise X-ray diffraction study gave a lattice parameter of approximately 4.127 Å.[1]
The structure contains B6 octahedra connected to one another to form a three-dimensional boron framework. The neodymium atoms form a simple cubic sublattice, occupying the large cavities between the B6 octahedra.[6] Each neodymium atom is surrounded by 24 nearby boron atoms, while each B6 octahedron is surrounded by eight neodymium atoms.
Rietveld refinement of nanocrystalline material gave a lattice parameter of 4.1254(2) Å, in close agreement with measurements on bulk material.[8]
Magnetic properties
[edit]NdB6 is paramagnetic at higher temperatures and becomes antiferromagnetic below approximately 8 K. Magnetic-susceptibility measurements found a Néel temperature of 8.45 K.[2]
Neutron diffraction measurements confirmed the antiferromagnetic ordering and gave a Néel temperature of approximately 8.6 K. The magnetic structure involves a doubling of the chemical unit cell along one crystallographic direction. At 4.2 K, the ordered magnetic moment was measured as 1.74 μB per neodymium ion.[3] The ordered state is generally described as an A-type collinear antiferromagnetic structure.[9]
Heat-capacity measurements of single-crystalline NdB6 show a sharp anomaly associated with the magnetic transition. One study found the transition at 7.45 K in zero field; applying a magnetic field of 3 T shifted the anomaly slightly to approximately 7.6 K. A broad Schottky anomaly is also observed around 50 K as a result of the crystalline-electric-field splitting of the Nd3+ states.[9]
Electronic and transport properties
[edit]Neodymium hexaboride is metallic.[4] Its conduction states involve hybridization between the boron 2p states and neodymium electronic states, while the localized neodymium 4f electrons are responsible for much of its magnetic behaviour.
Hall-effect measurements on NdB6 single crystals between 2 and 300 K found a negative Hall coefficient, consistent with electron-like charge carriers. Between approximately 10 and 20 K, the Hall coefficient is nearly temperature-independent at about −3.7×10−4 cm3 C−1. A step-like anomaly occurs near 25 K, substantially above the Néel temperature.[10]
Mechanical properties
[edit]Like other rare-earth hexaborides, NdB6 is a hard refractory ceramic. Measurements on dense hot-pressed material gave a microhardness of 23.8 GPa and a nanoindentation hardness of 28.8 GPa. The elastic modulus was approximately 484 GPa, the fracture toughness was 3.04 MPa m1/2, and the three-point flexural strength was 156 MPa.[11]
Hot pressing NdB6 powder at 1800 °C for 2 hours under vacuum produced material with approximately 93% of its theoretical density. Addition of 5 wt.% B4C as a sintering aid increased the density to 99.9% of the theoretical value.[11]
Electron emission
[edit]Rare-earth hexaborides are of interest as thermionic and field-emission materials because of their relatively low work functions. Measurements by ultraviolet photoelectron spectroscopy on the NdB6 (100) surface gave a work function of 2.806 eV. Thermionic-emission measurements gave an average effective work function of 2.813 ± 0.109 eV, in good agreement with theoretical calculations.[4]
One-dimensional NdB6 nanostructures have consequently been investigated as field emitters. Single-crystalline nanowires prepared from neodymium metal and boron trichloride had a field-emission turn-on field of 5.55 V μm−1 at a current density of 10 μA cm−2 and a field-enhancement factor of 1037.[5]
Vertically aligned NdB6 nanorods prepared by reaction of Nd(OH)3 with NaBH4 at 1000 °C have shown turn-on fields below 4.5 V μm−1 and field-enhancement factors as high as 2263.[12]
References
[edit]- 1 2 3 4 Eick, Harry A.; Gilles, Paul W. (1959). "Precise Lattice Parameters of Selected Rare Earth Tetra- and Hexa-borides". Journal of the American Chemical Society. 81 (19): 5030–5032. doi:10.1021/ja01528a005.
- 1 2 Hacker, H. Jr.; Lin, M. S. (1968). "Magnetic susceptibility of neodymium hexaboride". Solid State Communications. 6 (6): 379–381. doi:10.1016/0038-1098(68)90161-0.
- 1 2 McCarthy, C. M.; Tompson, C. W. (1980). "Magnetic structure of NdB6". Journal of Physics and Chemistry of Solids. 41 (12): 1319–1321. doi:10.1016/0022-3697(80)90135-3.
- 1 2 3 4 Wang, Yan; Xu, Jianfei; Lei, Yawei; Luo, Shifeng; Zhang, Jingwen; Han, Cuiliu; Zhang, Jiuxing (2023). "Determination on the work function of NdB6 (1 0 0) crystal surface by both theory and experiment". Applied Surface Science. 641 158544. doi:10.1016/j.apsusc.2023.158544.
- 1 2 Fan, Qinghua; Zhang, Qinyuan; Zhao, Yanming; Ding, Qiwei (2013). "Field emission from one-dimensional single-crystalline NdB6 nanowires". Journal of Rare Earths. 31 (2): 145–148. doi:10.1016/S1002-0721(12)60248-8.
- 1 2 3 4 Liu, Yanju; Lu, W. J.; Qin, J. N.; Zhang, D. (2007). "A new route for the synthesis of NdB6 powder from Nd2O3–B4C system". Journal of Alloys and Compounds. 431 (1–2): 337–341. doi:10.1016/j.jallcom.2006.05.084.
- ↑ Zhao, Xudong; Liu, Xiaoyang; Lin, Feng; Liu, Weina; Su, Wenhui (1997). "A new route for the synthesis of boron-rich rare-earth boride NdB6 under high pressure and high temperature". Journal of Alloys and Compounds. 249: 247–250. doi:10.1016/S0925-8388(96)02856-3.
- 1 2 "Nano-sized neodymium hexaboride: Room temperature mechanochemical synthesis". Physica B: Condensed Matter. 570: 217–223. 2019. doi:10.1016/j.physb.2019.06.047.
- 1 2 Reiffers, M.; Šebek, J.; Šantavá, E.; Shitsevalova, N.; Gabáni, S.; Pristáš, G.; Flachbart, K. (2007). "Heat capacity of NdB6". Journal of Magnetism and Magnetic Materials. 310 (2): e595–e597. doi:10.1016/j.jmmm.2006.10.587.
- ↑ Anisimov, M. A.; Bogach, A. V.; Glushkov, V. V.; Demishev, S. V.; Samarin, N. A.; Shitsevalova, N. Y.; Sluchanko, N. E. (2009). "Hall Effect in LaB6 and NdB6". Solid State Phenomena. 152–153: 525–528. doi:10.4028/www.scientific.net/SSP.152-153.525.
- 1 2 Sonber, Jitendra Kumar; Murthy, Tammana Sree Rama Chandra; Paul, Bhaskar; Majumdar, Sanjib; Sairam, K.; Sahoo, Deepak; Singh, D. K.; Bathula, Vishwanadh; Kain, Vivekanand (2024). "Studies on synthesis, densification, and characterization of neodymium hexaboride". International Journal of Applied Ceramic Technology. 21 (3): 1425–1437. doi:10.1111/ijac.14659.
- ↑ "Excellent field emission from ultrafine vertically aligned nanorods of NdB6 on silicon substrate". Applied Surface Science. 526 146652. 2020. doi:10.1016/j.apsusc.2020.146652.