Al₁₅Ge₄Ni₃: A new intergrowth structure with Cu₃Au- and CaF₂-type building blocks

The new ternary compound Al₁₅Ge₄Ni₃ (τ₂ in the system Al–Ge–Ni) was synthesized in single crystalline form by a special annealing procedure from samples located in the three phase fields [L+Al+τ₂] and [L+Ge+τ₂]. The crystal structure of Al₁₅Ge₄Ni₃ was determined by single-crystal X-ray diffraction. The compound crystallizes in a new structure type in space group I4-bar3m, Pearson Symbol cI88, cubic lattice parameter a=11.405(1) Å. Phase diagram investigations indicate stoichiometric composition without considerable homogeneity range; τ₂ melts peritectically at T=444 °C. The crystal structure of Al₁₅Ge₄Ni₃ shows a unique combination of simple Cu₃Au- and CaF₂-type building blocks: a three dimensional network of CaF₂-type units, formed by Ni and Al atoms, is interspaced by clusters (Al₆Ge₈) resembling unit cells of the Cu₃Au-type. Both structural motifs are connected by Al–Ge bonds. The ground state energy of the compound was obtained by DFT calculations and the densities of states were analyzed in detail. In addition, electron density maps were calculated in four different sections through the unit cell using the full potential linearized augmented plane-wave (FLAPW) method. The bonding situation in Al₁₅Ge₄Ni₃ was discussed combining results from electronic calculations with the analysis of the coordination of atoms. - Graphical abstract: The new compound Al₁₅Ge₄Ni₃ shows a unique combination of simple Cu₃Au- and CaF₂-type building blocks. - Highlights: • The crystal structure of Al₁₅Ge₄Ni₃ (space group I4-bar3m) was determined. • It shows a unique combination of CaF₂- and Cu₃Au-type building blocks. • Electronic (DFT) calculations were performed to gain insight to chemical bonding