Crystal structure and phase stability of Co2N: A combined first-principles and experimental study

Highlights: • Co₂N is an isotype of η-Fe₂C (space group Pnnm) rather than ζ-Fe₂N (space group Pbcn). • Co₂N experiences an endothermal phase transition to Co₃N_1+x isotypic to ε-Fe₃N_1+x. • Strong electron correlation is essential for the prediction of phase stability of Co₂N. • Antiferromagnetism must be carefully considered for the description of Pnnm Co₂N. -- Abstract: The crystal structure and phase stability of Co₂N are revisited based on experiments and first-principles calculations. Powder X-ray diffraction (PXRD) measurements and Rietveld refinements clearly confirm that the stable crystal structure of Co₂N is an isotype of η-Fe₂C and Co₂C with the space group Pnnm rather than the closely related ζ-Fe₂N with the space group Pbcn. The refined lattice parameters of Co₂N in the Pnnm structure are a = 4.6108(1) Å, b = 4.3498(1) Å, c = 2.85592(7) Å, obtained from X-ray diffraction using synchrotron radiation. Furthermore, differential scanning calorimetry (DSC) with subsequent diffraction experiments reveal an endothermal transition to an ε-type order at 398 °C followed by an exothermal decomposition at 446 °C. First-principles density-functional-theory (DFT) calculations including the Hubbard U correction (DFT+U) demonstrate that it is essential for transition metal nitrides to consider strong electron correlation to predict the correct experimental structure and magnetic state. In particular, an effective value of U_eff = 2.75 eV can be utilized to obtain an antiferromagnetic Pnnm phase of Co₂N in agreement with experiments.