6.3: Borides

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Andrew R. Barron
Rice University via CNX

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The non-metallic nature of boron means that it makes a number of binary compounds with elements more electropositive than itself (i.e., metals). These compounds are called, borides, and some are also formed with metalloid elements as well (e.g., arsenic). In this regard, borides may be considered similar to carbides, silicides, and some phosphides.

Borides are prepared in a number of ways, however, direct combination of the elements, (6.3.1), is the simplest. Other routes include, electrolysis of the fused salts, and the reduction of the metal oxide with a mixture of carbon and boron carbide.

\[ \text{M + x B} \rightarrow \text{MB}_\text{x}\]

Metal borides are generally refractory in character and chemically inert, while they often have properties better that of the constituent elements. For example, the thermal conductivity of TiB₂ is about ten times greater than that of titanium, and the melting point is significantly higher (Table \(\PageIndex{1}\)).

Table \(\PageIndex{1}\): The melting points of Group 4 metals and their borides.
Element	Melting point (°C)	Boride	Melting point (°C)
Ti	1725	TiB₂	3225
Zr	1855	ZrB₂	2990
Hf	2233	HfB₂	3100

The structures of metal borides depends on the M:B ratio. Borides with an isolated boron atom have a low B:M ratio: M₄B, M₃B, M₂B, M₅B₂, and M₇B₃. In such compounds the boron atom is normally in a triangular-prismatic or square-antiprismatic hole in a metal lattice. Borides with equal or near equal metal and boron ratio have structures with either pairs of boron atoms (as in V₃B₂), single boron chains (seen in all MB compounds), or double boron chains (observed for many M₃B₄ compounds). Increasing the boron content results in two-dimensional structures. For example, MB₂ usually consists of alternate hexagonal layers of metal and boron (Figure \(\PageIndex{1}\)). Finally, boron rich borides (e.g., MB₄, MB₆, and MB₁₂) all have three-dimensional structures.

Figure \(\PageIndex{1}\): Alternate layers of metal atoms (large circles) and boron (small circles) in MB₂.