First-principles study of the pressure-induced rutile-CaCl2 phase transition in MgF2

Ab initio calculations based on the density functional theory are performed to investigate the pressure-induced rutile-CaCl2 phase transition in MgF2. The phase transition is characterized as a second-order nature by evidence of the continuous changes of the cell volume and lattice constants at the transition, consistent with the experimental observation. Under compression, the B-1g Raman active mode in the rutile phase is predicted to soften, signifying the dynamical instability. The softening of shear modulus C-s with increasing pressure is also identified through the elastic constants calculation. The transition pressures derived from the free energy, soft mode, and elastic constants calculations are in satisfactory agreement with the experimental value. The current calculations have demonstrated that the rutile-CaCl2 phase transition is driven by the coupling between the Raman active B-1g mode and shear modulus C-s. (C) 2007 Elsevier Ltd. All rights reserved.