Tantalum boride
# Tantalum boride

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

## Properties

The Vickers hardness of TaB and $TaB_2$ films and crystals is ~30 GPa. Those materials are stable to oxidation below 700 °C and to acid corrosion.

$TaB_2$ has the same hexagonal structure as most diborides $(AlB_2,$ $MgB_2,$ etc.). The mentioned borides have the following space groups: TaB (orthorhombic, Thallium(I) iodide-type, Cmcm), $Ta_5B_6$ (Cmmm), $Ta_3B_4$ (Immm), $TaB_2$ (hexagonal, aluminum diboride-type, P6/mmm).

## Preparation

Single crystals of TaB, $Ta_5B_6,$ $Ta_3B_4$ or $TaB_2$ (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 $TaCl_5-BCl_3-H_2-Ar$ in the temperature range 540–800 °C. $TaB_2$ (single-phase) is deposited at a source gas flow ratio $(BCl_3/TaCl_5)$ of six and a temperature above 600 °C. TaB (single-phase) is deposited at $BCl_3/TaCl_5$ = 2–4 and T = 600–700 °C.

Nanocrystals of $TaB_2$ were successfully synthesized by the reduction of $Ta_2O_5$ with $NaBH_4$ using a molar ratio M:B of 1:4 at 700-900 °C for 30 min under argon flow.

    $Ta_2O_5$ + 6.5 $NaBH_4$ → 2 $TaB_2$ + 4 Na(g,l) + 2.5 $NaBO_2+$ 13 $H_2(g)$
