Local symmetry change in BaF2:Mn2+ at ∼50 K:: Microscopic insight

The microscopic origin of the abrupt cubic-tetrahedral symmetry change associated with the local a(2u) vibrational mode observed by electron paramagnetic resonance in BaF2:Mn2+ at similar to 50 K is explored by means of density functional theory calculations. It is found that while the a(2u) vibrational frequencies calculated for MnF86- in CaF2 (168 cm(-1)) and SrF2 (132 cm(-1)) are real, in the case of BaF2:Mn2+, the adiabatic potential curve along this mode exhibits a double well with a small barrier of 50 cm(-1). Although the ground and first excited vibrational states are localized around the energy minima, the rest of the excited states resemble those of a harmonic oscillator centered at Q(a(2u)) = 0. Moreover, only the inclusion of the anharmonic coupling between a(2u) and t(1u) modes allows one to understand the T-d-O-h transition temperature. It is shown that both the unusually high Mn2+-F- distance in BaF2:Mn2+ and the pseudo-Jahn-Teller interaction of the t(2g)(xy; xz; yz) antibonding orbital with filled t(1u) orbitals favor the a(2u) instability. The calculated a(2u) force constant for different electronic states supports this conclusion. (C) 2008 American Institute of Physics.