Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 76

The aluminium diboride family of crystal structures shot to prominence in 2000 when isostructural MgB₂ was discovered to be a superconductor with T _c = 39 K. It has a relatively simple crystal structure, shown in Fig. 1.51, in which Mg atoms form close packed layers stacked in an AAA sequence, which may be referred to as primitive hexagonal packing, hp. The cp Mg layers are separated by B layers arranged as in graphite; hence Mg is 12‐coordinate with hexagonal rings of B atoms above and below. Each B has three B nearest neighbours in a trigonal planar arrangement and six Mg next nearest neighbours arranged in a trigonal prism.

Numerous borides and silicides have the AlB₂ structure, including MB₂: M = Ti, Zr, Nb, Ta, V, Cr, Mo, Mg, U, and MSi₂: M = U, Pu, Th. The crystal structure of some metal hydroxides such as Cd(OH)₂ is also closely related.

The AlB₂ structure is closely related to the NiAs structure. In both structures, the metal atoms form a primitive hexagonal array but only half the trigonal prismatic sites are occupied by As in NiAs whereas all trigonal prismatic sites are occupied by B in AlB₂. Consequently, the coordination of Ni is octahedral in NiAs instead of 12‐coordinate Al in AlB₂.

Figure 1.51 The crystal structure of MgB2 as (a) an oblique projection showing hexagonal rings of B atoms with 12‐coordinate Mg situated between pairs of rings and (b) [001] projection of the crystal structure.