Robustness of Superatoms and Their Potential as Building Blocks of Materials: Al₁₃^- vs B(CN)₄^-

Superatomic clusters, due to their unique size- and composition-specific properties, could form the building blocks of a new class of cluster-assembled materials, provided they can retain their structure and properties when assembled. One such cluster that has been studied extensively is Al₁₃^-, which has a total of 40 valence electrons and is stable according to the jellium rule (electron shell closings in a spherical jellium background model). However, the stability of KAl₁₃ salt, both as a molecular species and as a bulk material, has resulted in conflicting results. While KAl₁₃ cluster is stable with Al₁₃ behaving like a halogen, the crystalline form of KAl₁₃ is unstable. To determine whether changing the cation from a metal to a nonmetallic one would result in stabilizing the Al₁₃^- icosahedral geometry, we studied the stability of [(CH₃)₄N⁺][Al₁₃^-] crystal. Note that (CH₃)₄N, with an ionization potential of 3.27 eV, can be classified as a superalkali. While (CH₃)₄N was found to maintain its geometry, the Al₁₃ cluster anions began to coalesce, thus destroying their initial icosahedral geometry. However, the results are different when the anion is chosen to be a nonmetal superhalogen, e.g., the bison anion B(CN)₄^-, which is stabilized by the octet rule, i.e., the neighboring nonmetal atoms share electron pairs to form covalent bonds. [(CH₃)₄N⁺][B(CN)₄^-] is found to be a charge-transfer super-salt, where both (CH₃)₄N⁺ and B(CN)₄^- maintain their individual structures, even at room temperature (300 K). This finding suggests a competition between the octet rule stabilizing the nonmetallic molecular ion B(CN)₄^- versus the “jellium rule” stabilizing the metallic molecular ion Al₁₃^-, which could be the key for the design and synthesis of building blocks for cluster-assembled materials.