Tantalum borides are compounds of tantalum and boron most remarkable for their extreme hardness.
The Vickers hardness of TaB and TaB<sub>2</sub> films and crystals is ~30 GPa. Those materials are stable to oxidation below 700 ðC and to acid corrosion.
TaB<sub>2</sub> has the same hexagonal structure as most diborides (AlB<sub>2</sub>, MgB<sub>2</sub>, etc.). The mentioned borides have the following space groups: TaB (orthorhombic, Thallium(I) iodide-type, Cmcm), Ta<sub>5</sub>B<sub>6</sub> (Cmmm), Ta<sub>3</sub>B<sub>4</sub> (Immm), TaB<sub>2</sub> (hexagonal, aluminum diboride-type, P6/mmm).
Single crystals of TaB, Ta<sub>5</sub>B<sub>6</sub>, Ta<sub>3</sub>B<sub>4</sub> or TaB<sub>2</sub> (about 1 cm diameter, 6 cm length) can be produced by the floating zone method.
Tantalum boride films can be deposited from a gas mixture of TaCl<sub>5</sub>-BCl<sub>3</sub>-H<sub>2</sub>-Ar in the temperature range 540âÂÂ800 ðC. TaB<sub>2</sub> (single-phase) is deposited at a source gas flow ratio (BCl<sub>3</sub>/TaCl<sub>5</sub>) of six and a temperature above 600 ðC. TaB (single-phase) is deposited at BCl<sub>3</sub>/TaCl<sub>5</sub> = 2âÂÂ4 and T = 600âÂÂ700 ðC.
Nanocrystals of TaB<sub>2</sub> were successfully synthesized by the reduction of Ta<sub>2</sub>O<sub>5</sub> with NaBH<sub>4</sub> using a molar ratio M:B of 1:4 at 700-900 ðC for 30 min under argon flow.