Unusually strong heteroatomic bonding in the complex polyanion of intermetallic Ba₆Pt₂₂Al₅₃

The new intermetallic compound Ba₆Pt₂₂Al₅₃ was obtained from reaction of the elements. The title compound crystallizes in the hexagonal crystal system with lattice parameters of a = 1427.5(3) and c = 1614.3(4) pm and space group P6₃/mcm (Z = 2) and adopts a new structure type (Pearson code hP162, Wyckoff sequence l²k³j²i²hg²ecb). In the structure, the Pt and Al atoms form a complex polyanionic [Pt₂₂Al₅₃]^δ- network, while the Ba cations reside in cavities. The structure can be described as a packing of layers, formed by the Pt@Al_n polyhedra parallel to (001). The layers are stacked in an ABCB fashion. Within the [Pt₂₂Al₅₃] framework, one of the shortest Pt-Al distances (239 pm) reported is found, suggesting extremely strong covalent bonding interactions. Due to slightly enhanced displacement parameters of the Al9 site, temperature dependent single-crystal X-ray diffraction (285 and 90 K) and low-temperature heat capacity experiments were conducted. However, no structural phase transition was observed down to 2 K. With the help of DFT calculations, the bonding situation was elucidated in detail, verifying the strong heteroatomic bonding and the destabilization of the structure upon in silico distortions. Mulliken charge calculations confirm the picture of the polyanionic framework and cationic Ba atoms within the cavities.