First-principles study of phase transitions in antiferromagnetic XF2 (X=Fe, 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 B1gB1g Raman active mode and the shear modulus CsCs 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 B1gB1g modes and the shear modulus CsCs.

► We reveal the physical origin of the rutile →→ CaCl2 phase transitions in XF2 (Fe, Co, Ni). ► The shear modulus CsCs and B1gB1g Raman mode of the rutile phase are softened at high pressures. ► The rutile →→ CaCl2 transitions could be driven by the coupling between B1gB1g modes and CsCs.