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

Abstract

In order to help understand the structural stability of KCoO 2 -type ternary nitrides AMN 2 , referring to perovskite structure, a tolerance factor t is proposed to describe the size effect on the phase/symmetry options of the experimentally accessible AMN 2 nitrides. This leads to a range of t values above 0.946 for structurally stable KCoO 2 -type AMN 2 nitrides with t values around 0.970 for the orthorhombic and tetragonal phase boundary. In contrast, most of AMN 2 nitrides exhibit α-NaFeO 2 -type structure with t ∼ 0.898−0.946 and cations ordered or disordered rocksalt structure while t below 0.898. Employing the proposed criterion, the structure formation for other ternary AMN 2 compositions with lanthanum and alkaline earth cations for the A sites were predicted, which was testified through the synthesis attempts and complemented by formation energy evaluations. The efforts to synthesize the ternary Lanthanide and alkaline earth-based AMN 2 nitrides were unsuccessful, which could associate the structural instability with the large formation energies of lanthanide nitrides LaMN 2 and the greater tolerance factor of 1.048 for BaTiN 2 . The experimentally already synthesized AMN 2 nitrides could be categorized into three types with different tolerance factors, and scarce AMN 2 nitrides with lower formation energies would be accessible using different synthetic routes beyond the traditional solid-state synthesis method.