Order-disorder and ionic conductivity in calcium nitride-hydride

Area of Science:

• Materials Science
• Catalysis
• Solid-state Chemistry

Background:

• Nitrogen-hydrogen compounds, like calcium nitride hydride (Ca2NH), are emerging co-catalysts for ammonia synthesis under mild conditions.
• Ca2NH functions as a hydrogen (H2) sink, incorporating lattice hydrogen into ammonia (NH3) product.
• Understanding ionic transport and diffusion in these co-catalysts is key to optimizing ammonia synthesis.

Purpose of the Study:

• To investigate hydride ion conduction properties in distinct calcium nitride-hydride (Ca2NH) phases.
• To correlate material structure and synthesis method with ionic conductivity.
• To elucidate the mechanism of ion transport in conductive Ca2NH phases.

Main Methods:

• Synthesis of two distinct Ca2NH phases using different routes.
• Measurement of ionic conductivity at elevated temperatures (600°C).
• In situ combined analysis techniques to probe ion transport mechanisms.

Main Results:

• Two Ca2NH phases exhibited significantly different ionic properties.
• The beta (β) phase demonstrated fast hydride ion conduction (0.08 S/cm at 600°C), surpassing CaH2 by tenfold.
• The alpha (α) phase showed 100-fold lower conductivity compared to the β-phase.
• The β-phase conducts ions via a vacancy-mediated mechanism, dependent on secondary site ion concentration and main site vacancy concentration.

Conclusions:

• Hydride ion conduction is confirmed in calcium nitride-hydride materials.
• The synthesis route critically influences the ionic transport properties of Ca2NH.
• The β-phase exhibits promising ionic conductivity for potential applications in ammonia synthesis catalysis.