Nonequivalence of chemical and hydrostatic pressures:: A1g and Eg frequencies and Stokes shift of Mn2+-doped perovskites -: art. no. 064441

The frequency of A(1g) and E-g modes of MnF64- in cubic ABF(3) (A: K; B: Mg, Zn and A: Rb, Cs; B: Cd, Ca) perovskites has been derived through density functional calculations on MnF(6)A(8)B(6)(16+) clusters which reproduce the experimental impurity-ligand distance R,. Both frequencies are found to experience a drastic decrement on passing from KMgF3:Mn2+ ((h) over bar omega(A)=556 cm(-1), (h) over bar omega(E)=456 cm(-1)) to CsCdF3:Mn2+ ((h) over bar omega(A) = 317 cm(-1), (h) over bar omega(E) = 239 cm(-1)) despite the fact that R-e increases only by 5%, leading to effective Gruneisen constants (called gamma(A)(c) and gamma(E)(c)) around 3.0 along the series. This figure is 60% higher than the usual Gruneisen constant gamma(A) calculated for a given system like KMaF(3):Mn2+ or KZnF3:Mn2+ when hydrostatic pressure is applied. For supporting this relevant result the value of gamma(A) for CrF63- in a fluoroelpasolite has been calculated as well. The obtained value gamma(A) = 2.1 is close to the experimental one (gamma(A) = 1.9) derived in K2NaGaF6:Cr3+ by combined optical and Raman measurements. As a salient feature, the increase of the Stokes shift when R, increases observed along the ABF(3):Mn2+ series is now well explained through the bigger variations of A(1g) and E-g frequencies induced by the chemical pressure in comparison to those coming from an hydrostatic pressure on a given system. The difference between gamma(A)(c) and gamma(A) is discussed through a simple model that emphasizes the role played by the coupling of the MnF64- complex to the lattice. The influence of chemical and hydrostatic pressures upon the luminescence efficiency is also briefly discussed. Attention is addressed to the method of calculating impurity-associated Gruneisen constants using clusters. Recent results on Cr3+ in several cubic chloroelpabolites that support the present conclusions are briefly discussed as well.