Tantalum boride

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Structure of TaB2
Structure of TaB

Tantalum borides are compounds of tantalum and boron most remarkable for their extreme hardness.

Properties

The Vickers hardness of TaB and TaB2 films and crystals is ~30 GPa.[1][2][3] Those materials are stable to oxidation below 700°C and to acid corrosion.[1][3]

TaB2 has the same hexagonal structure as most diborides (AlB2, MgB2, etc.).[4] The mentioned borides have the following space groups: TaB (orthorhombic, Thallium(I) iodide-type, Cmcm), Ta5B6 (Cmmm), Ta3B4 (Immm), TaB2 (hexagonal, aluminum diboride-type, P6/mmm).[3]

Preparation

Single crystals of TaB, Ta5B6, Ta3B4 or TaB2 (about 1 cm diameter, 6 cm length) can be produced by the floating zone method.[2][3]

Tantalum boride films can be deposited from a gas mixture of TaCl5-BCl3-H2-Ar in the temperature range 540–800°C. TaB2 (single-phase) is deposited at a source gas flow ratio (BCl3/TaCl5) of six and a temperature above 600°C. TaB (single-phase) is deposited at BCl3/TaCl5 = 2–4 and T = 600–700°C.[1]

References

  1. ↑ 1.0 1.1 1.2 S. Motojima et al. "Low-temperature deposition of TaB and TaB2 by chemical vapor deposition" J. Nucl. Mater. 105 (1982) 262
  2. ↑ 2.0 2.1 S. Otani et al. "Floating zone growth and high-temperature hardness of NbB2 and TaB2 single crystals" J. Cryst. Growth 194 (1998) 430
  3. ↑ 3.0 3.1 3.2 3.3 S. Okada et al. "Single crystals of TaB, Ta5B6, Ta3B4 and TAB2, as obtained from high-temperature metal solutions, and their properties" J. Cryst. Growth 128 (1993) 1120
  4. ↑ X. Chen et al. "Electronic and Structural Origin of Ultraincompressibility of 5d Transition-Metal Diborides" Phys. Rev. Lett. 100 (2008) 196403
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