Structural properties of ferromagnetic CaN in a CaCl-type and rock salt structure: A first-principles study

Abstract

The mechanical stability of the synthetic compound, CaN, under high pressure is investigated by first-principles calculations based on density functional theory. We investigate the structural stability of CsCl-type and rock salt (RS) structures, calculating elastic constants and phonon dispersion. The structural stability of the RS structure depends on the choice of computational methods and exchange and correlation functional. The enthalpy of the RS structure crosses that of the CsCl-type one at 15 GPa and, in the higher pressure region than 15 GPa, the enthalpy of the CsCl-type structure is lower than that of the RS structure. The CsCl-type structure is mechanically stable, resulting from both pseudo potential method and full-potential linearized augmented plane wave method for both local density and generalized gradient approximation to calculate the elastic constants and phonon band dispersion within a harmonic approximation. The CsCl-type structure is mechanically stable up to about 65 GPa. In the higher pressure region than 65 GPa, the elastic constant, C 11–C 12, becomes negative. From the results of elastic properties and the enthalpy, CaN in the CsCl-type structure may appear as a metastable state in the range from 15 to 65 GPa.