Beryllium hexaboride (BeB6) is a compound with unique properties that can be used for a variety of applications. It is known for its ultra-hardness and superconductivity, properties that contribute to the development of advanced materials with tunable multifunctional properties.

BeB6 has three different crystal structures, α-BeB6, β-BeB6, and γ-BeB6, which are stable at both atmospheric and high pressures. The α-BeB6 structure features rhomboid B4 units, which are interconnected to extended B∞ ribbons and intercalated by beryllium (Figure a,b). 

In BeB6, the Be atom in fact participates in the (partially covalent) Be?B bonding rather than severing as an electron donor, which dictates that the structure of BeB6 differs from other Zintl?Klemm compounds. The rhomboid B4 in αBeB6 has two distinct edge lengths, 1.776 and 1.893 ?, while neighboring units are connected by a shorter bond of 1.674 ?. This range of bond lengths is comparable to that of α-B (1.669?2.003 ?) and γ-B (1.661?1.903 ?).

The β-BeB6 features cage-like boron slabs intercalated by beryllium (Figure c,d). The B?B bond lengths in the slab range from 1.740 to 1.977 ?, and within the interslabs, the B?B bonds are varied as well (1.800, 1.805, and 1.892 ?), while the Be?B bond lengths change from 1.952 to2.203 ?, slightly greater than those in the α-BeB6.

The γ-BeB6 (Figure e,f) features buckled boron sheets similar to the 2D boron allotrope. The formation of these sheets is a result of competition between two- and three-center bonding, which induces a B3 isosceles triangle (with bond lengths of 1.789 and 1.843 ?) connected by distorted B4 rhomboids (with bond lengths of 1.745?2.023 ?). Remarkably, the shortest Be?B bond length in γ-BeB6 is 1.790 ?, much shorter than those in αBeB6 and β-BeB6. This bond length is between typical single (1.87 ?) and double (1.68 ?) Be?B covalent length, implying the relatively strong Be?B bonding in γ-BeB6.

References:

[1] LAILEI WU. Coexistence of Superconductivity and Superhardness in Beryllium Hexaboride Driven by Inherent Multicenter Bonding.[J]. The Journal of Physical Chemistry Letters, 2016. DOI:10.1021/ACS.JPCLETT.6B02444.