As discussed above, there is a counterpart for each lowest-energy
structure of AlBen in the minimum structures of
their corresponding Ben +1 cluster. Besides, as
can be seen in Table 1, the number of Al–Be bonds in
AlBen increases with the increasing cluster size
and reaches a maximum value of six. It implies that the impurity Al atom
is able to bond with six Be atoms at most. These rules may help to
identify the low-lying structures of larger Al-doped
Ben clusters.
The comparison between AlBen and other Al-doped
clusters shows how the relative size of impurity atom affects the
structural evolution of the whole system. For example, the impurity Al
atom always occupies an external position of the host clusters in
AlBen and
AlBn [40] until n =
12, while it gets trapped in the host cage from n = 6 onwards in
AlLin [41] and
AlNan ,[42,43] and fromn = 9 onwards in the
AlTin ,[44]AlScn ,[45]AlPbn ,[46] and
AlYn [47] clusters. These
structural distinctions can be related to the size difference of the
dopant atom versus the host atom. To be specific, the impurity Al atom
(1.25 Å)[48] has larger atomic radius than those
of B (0.85 Å) and Be (1.05 Å) atoms, which hinders it from entering the
Bn or Ben cages. In
contrast, a framework constituted by larger host atoms such as Li (1.45
Å), Na (1.80 Å), Ti (1.40 Å), Sc (1.60 Å), Pb (1.80) and Y (1.80 Å) can
accommodate the dopant Al atom.