Metal borides

      Solid state metal borides are characteristically extremely hard, involatile, high melting and chemically inert materials which are industrially important with uses as refractory materials and in rocket cones and turbine blades, i.e. components that must withstand extreme stress, shock and high temperatures. Preparative routes to metal borides are varied, as are their structures. Some may be made by direct combination of the elements at high temperatures, and others from metal oxides.

   Metal borides may be boron- or metal-rich, and general families include MB₃, MB₄, MB₆ , MB₁₀, MB₁₂, M₂B₅ and M₃B₄ (B-rich), and M₃B, M₄B, M₅B, M₃B₂ and M₇B₃ (Mrich).

The formulae bear no relation to those expected on the basis of the formal oxidation states of boron and metal.  The structural diversity of these materials is so great as to preclude a full discussion here, but we can conveniently consider them in terms of the categories shown in Table 1.1, which are identified in terms of the arrangement of the B atoms within a host metal lattice. The structure type of the MB₆ borides (e.g. CaB₆) can be envisaged by likening it to that of a CsCl lattice with B₆-units (Table 1.1) replacing Cl^- ions. However, the B^_B distances between adjacent B₆-octahedra are similar to those within each unit and so a ‘discrete ion’ model is not actually appropriate.

   The structure type of MB₁₂ (e.g. UB₁₂) can similarly be described in terms of an NaCl lattice in which the Cl^- ions are replaced by B₁₂-icosahedra (Table 1.1).  Although this summary of metal borides is brief, it illustrates the complexity of structures frequently encountered in the chemistry of boron. Research interest in metal borides has been stimulated since 2001 by the discovery that MgB₂ is a superconductor with a critical temperature, Tc, of 39K.

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