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

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 eta-Fe2C and Co2C with the space group Pnnm rather than the closely related zeta-Fe2N with the space group Pbcn. The refined lattice parameters of Co2N in the Pnnm structure are a = 4.6108(1) angstrom, b = 4.3498(1) angstrom, c = 2.85592(7) angstrom, obtained from X-ray diffraction using synchrotron radiation. Furthermore, differential scanning calorimetry (DSC) with subsequent diffraction experiments reveal an endothermal transition to an e-type order at 398 similar to C followed by an exothermal decomposition at 446 degrees 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. (C) 2020 The Authors. Published by Elsevier B.V.

Automated summary

This study revisits the crystal structure and phase stability of Co2N using experiments and first-principles calculations, confirming the stable crystal structure of Co2N as an isotype of eta-Fe2C and Co2C with the space group Pnnm. The research also highlights the importance of considering strong electron correlation in transition metal nitrides for predicting correct experimental structures and magnetic states, with an effective value of U-eff = 2.75 eV identified for Co2N.