Local symmetry change in BaF2:Mn2+ at approximately 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 BaF(2):Mn(2+) at approximately 50 K is explored by means of density functional theory calculations. It is found that while the a(2u) vibrational frequencies calculated for MnF(8) (6-) in CaF(2) (168 cm(-1)) and SrF(2) (132 cm(-1)) are real, in the case of BaF(2):Mn(2+), 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 Mn(2+)-F(-) distance in BaF(2):Mn(2+) 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.