First-principles study of phase transitions in antiferromagnetic [formula omitted] ( [formula omitted], Co and Ni)

Abstract

Ab initio calculations based on the density functional theory are performed in order to reveal the physical origin of the pressure-induced rutile → CaCl 2 transitions in antiferromagnetic XF 2 (X=Fe, Co and Ni). These phase transitions have been characterized as second order, as evidenced by the continuous changes of the cell volume and lattice constants in the transitions. We found a universal feature for these compounds: that the B 1 g Raman active mode and the shear modulus C s in the rutile phase soften with increasing pressure, signifying pressure-induced dynamical and mechanical instability. Our results imply that the physically driven force of the rutile → CaCl 2 phase transition in antiferromagnetic XF 2 might be in close relation with the coupling between the unstable Raman B 1 g modes and the shear modulus C s .