Sodium–gold binaries: novel structures for ionic compounds from an ab initio structural search

Abstract

Intermetallic compounds made of alkali metals and gold have intriguing electronic and structural properties that have not been extensively explored. We perform a systematic study of the phase diagram of one binary system belonging to this family, namely Na _x Au _1− _x , using the ab initio minima hopping structural prediction method. We discover that the most stable composition is NaAu _2 , in agreement with available experimental data. We also confirm the crystal structures of NaAu _2 and Na _2 Au, that were fully characterized in experiments, and identify a candidate ground-state structure for the experimental stoichiometry NaAu. Moreover, we obtain three other stoichiometries, namely Na _3 Au _2 , Na _3 Au and Na _5 Au, that could be thermodynamically stable. We do not find any evidence for the existence of the experimentally proposed composition NaAu _5 . Finally, we perform phonon calculations to check the dynamical stability of all reported phases and we simulate x-ray diffraction spectra for comparison with future experimental data.