Tolerance Factor and Phase Stability of the KCoO₂-Type AMN₂ Nitrides

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Understanding the structural stability of ternary nitrides (AMN2) is crucial for materials design.
• Existing structural classifications for AMN2 compounds include KCoO2-type, alpha-NaFeO2-type, and rocksalt structures.
• The influence of cation size on the phase and symmetry of AMN2 nitrides needs further investigation.

Purpose of the Study:

• To propose and validate a tolerance factor (t) for predicting the structural stability of KCoO2-type ternary nitrides (AMN2).
• To correlate tolerance factor values with different structural types (KCoO2, alpha-NaFeO2, rocksalt) observed in AMN2 nitrides.
• To predict and explore the synthesis of novel AMN2 compositions using the proposed criterion.

Main Methods:

• Development of a tolerance factor (t) based on structural parameters.
• Analysis of existing experimental data for AMN2 nitrides to establish structure-tolerance factor relationships.
• Computational prediction of structure formation for new AMN2 compositions.
• Experimental synthesis attempts for lanthanide and alkaline earth-based AMN2 nitrides.
• Calculation of formation energies to assess thermodynamic stability.

Main Results:

• A tolerance factor (t) criterion was established: t > 0.946 for stable KCoO2-type AMN2, with a phase boundary around t = 0.970.
• AMN2 nitrides with t ~ 0.898-0.946 predominantly exhibit the alpha-NaFeO2-type structure.
• Nitrides with t < 0.898 tend to form cation-ordered or disordered rocksalt structures.
• Synthesis attempts for lanthanide and alkaline earth-based AMN2 nitrides were unsuccessful, indicating potential structural instability.
• High formation energies for LaMN2 and a large tolerance factor for BaTiN2 (1.048) were identified as reasons for instability.

Conclusions:

• The proposed tolerance factor effectively predicts structural stability and phase preferences in AMN2 nitrides.
• Structural instability in lanthanide and alkaline earth-based AMN2 nitrides is linked to high formation energies and large tolerance factors.
• Further research into alternative synthetic routes is needed to access AMN2 nitrides with low formation energies.