First-principles study of phase transitions in antiferromagnetic (, Co and Ni)

Ab initio calculations based on the density functional theory are performed in order to reveal the physical origin of the pressure-induced rutile →  CaCl2 transitions in antiferromagnetic XF2 (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 →  CaCl2 phase transition in antiferromagnetic XF2 might be in close relation with the coupling between the unstable Raman B 1 g modes and the shear modulus C s .