CuAl2 revisited: Composition, crystal structure, chemical bonding, compressibility and Raman spectroscopy

The structure of CuAl2 is usually described as a framework of base condensed tetragonal antiprisms [CuAl8/4]. The appropriate symmetry governed periodic nodal surface (PNS) divides the space of the structure into two labyrinths. All atoms are located in one labyrinth, whereas the second labyrinth seems to be ‘empty’. The bonding of the CuAl2 structure was analyzed by the electron localization function (ELF), crystal orbital Hamiltonian population (COHP) analysis and Raman spectroscopy. From the ELF representation it is seen, that the ‘empty’ labyrinth is in fact the place of important covalent interactions. ELF, COHP in combination with high-pressure X-ray diffraction and Raman spectroscopy show that the CuAl2 structure is described best as a network built of interpenetrating graphite-like nets of three-bonded aluminum atoms with the copper atoms inside the tetragonal-antiprismatic cavities.