Jahn-teller and A1g vibrations for Mn 2+ doped fluoroperovskites:: Stokes shift

The totally symmetric and Jahn-Teller vibrations due to a Mn2+ impurity in cubic fluoroperovskites are explored by means of Density Functional calculations on 21 atom clusters. In addition to reproduce the experimental Mn2+-F- distances at zero pressure, R, the present calculations indicate that on passing from KMgF3:Mn2+ (R-o=2.06Angstrom) to CsCdF3:Mn2+ (R-o=2.15Angstrom) both omega(E) and omega(A) are reduced by about 45%. As salient feature this huge reduction is found to be practically twice that calculated for a system like KMgF3:Mn2+ when the equilibrium distance is forced to change 9 pm due to an applied hydrostatic pressure. Therefore this result stresses that though chemical and hydrostatic pressures produce the same effects on the electronic properties of a localized center in an insulating material, this conclusion is not necessarily true when local vibrations are considered. The relevance of this result with regard to the Stokes shift is discussed.