Structure prediction and targeted synthesis: a new Na(n)N2 diazenide crystalline structure

Area of Science:

• Materials Science
• Computational Chemistry
• Solid State Chemistry

Background:

• Advances in computational methods enable the prediction of stable crystal structures.
• Previous predictions of sodium nitride (Na3N) showed positive formation enthalpy and heteropolar bonds.
• Experimental synthesis attempts of predicted structures can yield unexpected results.

Purpose of the Study:

• To predict stable crystal structures and stoichiometries using first-principles calculations.
• To identify the first alkali diazenide compound, sodium diazenide (Na2N2).
• To investigate the bonding characteristics and stability of predicted alkali nitride structures.

Main Methods:

• Global Space-Group Optimization (GSGO) approach for locating ground-state structures.
• First-principles energy functional calculations.
• Objective starting points with random lattice vectors and atomic positions.

Main Results:

• Prediction of the first alkali diazenide, Na2N2, with a negative formation enthalpy.
• Na2N2 exhibits homopolar N-N bonds, contrasting with heteropolar bonds in previously studied Na3N.
• The predicted Na2N2 completes the series of known diazenides (BaN2, SrN2) with charge transfer into the N2 antibonding orbital.

Conclusions:

• Na2N2 represents a new class of alkali nitrides with distinct homopolar bonding.
• The study underscores the complexities and potential pitfalls in correlating computational predictions with experimental synthesis.
• Further synthesis attempts of the predicted stable Na2N2 are warranted.