Abstract
High-pressure powder x-ray diffraction experiments have revealed that sodium and gold react at room temperature and form new Na-Au intermetallic compounds under high pressure. We have identified four intermetallic phases up to 60 GPa. The first phase (phase I) is the known Na2Au with the tetragonal CuAl2-type structure. It changed to the second phase (phase II) at about 0.8 GPa, which has the composition Na3Au with the trigonal Cu3As-type or hexagonal Cu3P-type structure. Phase II further transformed to phase III at 3.6 GPa. Phase III has the same composition Na3Au with the cubic BiF3-type structure. Finally, phase III changed to phase IV at around 54 GPa. Phase IV gives broad diffraction peaks, indicating large structural disorder.
Highlights
Alloys of gold and alkali metals have attracted the interest of chemists early in the 20th century
The minimum pressure required for the reaction was not determined in the present experiments, since some pressure was necessary to seal the gasket of the diamond-anvil cell (DAC)
An important finding of the present work is the formation of Na-Au compounds at room temperature under high pressure
Summary
Alloys of gold and alkali metals have attracted the interest of chemists early in the 20th century. Sommer [2] confirmed the formation of alkali metal-gold compounds, and mentioned transparent CsAu to be quite interesting. Following these studies, alkali metal-gold systems have widely been investigated. The electronic structures of light (Li, Na) and heavy (K, Rb, Cs) alkali metals largely change under high pressure induced by the s-p, s-d, or p-d electronic transitions [9, 10]. We identified four phases at high pressures, among which three phases are new for the Na-Au system This finding suggests that a rich variety of phase diagrams should exist for alkali metal-gold systems under high pressure. We could not study the hydrostaticity of sodium by using gold itself, the cubic phase III of Na-Au gave alternative information on the hydrostaticity of sodium
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