Stable Calcium Nitrides at Ambient and High Pressures

Area of Science:

• Materials Science
• Solid-State Chemistry
• Computational Chemistry

Background:

• Alkaline-earth metal nitrides, particularly calcium nitrides, are crucial for understanding nitrogen bonding and energy storage applications.
• Three crystalline forms of calcium nitride (CaN2, Ca2N, Ca3N2) were previously known at ambient conditions.

Purpose of the Study:

• To theoretically explore the phase diagram of calcium-nitrogen (CaNx) compounds.
• To identify novel, stable stoichiometries of calcium nitrides beyond those currently known.
• To investigate the potential of these new compounds as high-energy-density materials.

Main Methods:

• Utilized a global structure searching method to explore the CaNx phase diagram.
• Performed theoretical calculations to determine thermodynamic stability at ambient and high pressures.
• Analyzed the resulting structures for polynitrogen forms and potential energy release.

Main Results:

• Discovered a new thermodynamically stable CaN phase at ambient conditions, synthesizable from CaN2 and Ca2N.
• Identified four additional high-pressure stable stoichiometries: Ca2N3, CaN3, CaN4, and CaN5.
• Predicted the presence of diverse polynitrogen structures (N2, N4, N5, N6, N∞) within the new CaNx compounds.

Conclusions:

• The study expands the known phase diagram of calcium nitrides, revealing new stable forms.
• The predicted CaNx compounds, featuring polynitrogen moieties, are promising candidates for high-energy-density materials due to their exothermic dissociation potential.