Locating hydrogen in the Mg₅Bi₃H_x Zintl phase

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Zintl phases with strong spin-orbit coupling are of interest for their electronic properties, particularly in superconductivity and topological phenomena.
• Bismuth intermetallic compounds are gaining attention, but magnesium-based Zintl phases are less common than those with heavier alkaline-earth metals.

Purpose of the Study:

• To develop a convenient synthesis route for magnesium-bismuth Zintl phases.
• To characterize the structure and bonding of a newly synthesized magnesium bismuth hydride, Mg₅Bi₃H_x.
• To investigate the electronic properties and the role of interstitial hydrogen in stabilizing the intermetallic phase.

Main Methods:

• High-temperature, high-pressure synthesis.
• X-ray diffraction for crystal structure determination (space group Pnma).
• Thermal decomposition coupled with mass spectroscopy for trace hydrogen detection.
• Direct space analysis of chemical bonding.
• Quantum chemical calculations for band structure analysis.

Main Results:

• Successful synthesis of Mg₅Bi₃H_x under moderate conditions.
• The compound adopts a Ca₅Sb₃F-type structure.
• Trace amounts of hydrogen were detected at partially occupied interstitial sites, forming polar Mg-H bonds.
• Calculated band structures show significant electronic reorganization upon hydrogen insertion.
• Strongly polar Mg-Bi and Mg-H bonds are consistent with the Zintl concept.

Conclusions:

• High-temperature, high-pressure synthesis is an effective method for preparing Mg₅Bi₃H_x.
• Interstitial hydrogen plays a crucial role in stabilizing this intermetallic Zintl phase.
• The combination of analytical and computational methods efficiently identifies and characterizes trace interstitial atoms in intermetallic compounds.