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.
ASJC Scopus subject areas
- Materials Science(all)