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

Abstract

The microscopic origin of the abrupt cubic-tetrahedral symmetry change associated with the local a2u vibrational mode observed by electron paramagnetic resonance in BaF2:Mn2+ at ∼50K is explored by means of density functional theory calculations. It is found that while the a2u vibrational frequencies calculated for MnF86− in CaF2 (168cm−1) and SrF2 (132cm−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 50cm−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(a2u)=0. Moreover, only the inclusion of the anharmonic coupling between a2u and t1u modes allows one to understand the Td-Oh transition temperature. It is shown that both the unusually high Mn2+–F− distance in BaF2:Mn2+ and the pseudo-Jahn–Teller interaction of the t2g(xy;xz;yz) antibonding orbital with filled t1u orbitals favor the a2u instability. The calculated a2u force constant for different electronic states supports this conclusion.