EXCITATION AND EMISSION THERMAL SHIFTS IN ABF(3)MN2- COUPLING WITH IMPURITY VIBRATIONAL-MODES( PEROVSKITES )

The thermal shifts undergone by the first moment of the (6)A(1g)(S) -- > T-4(1g)(G) excitation band and the associated emission band of Mn2+- doped ABF(3) perovskites are investigated in the 9-300 K temperature r ange. It is found that these shifts are similar for the whole series a nd have average values of +150 and +450 cm(-1) for excitation and emis sion, respectively. Both the sign and the magnitude of these different thermal shifts are explained in terms of (i) the phonon assistance me chanism required to gain intensity of the parity-forbidden transitions , (ii) the quadratic electron-phonon coupling and (iii) thermal expans ion effects. To achieve this analysis a previous discussion upon the n ature of the vibrational modes seen in the optical spectra is carried out. It is stressed that the impurity vibrational mode displaying HBAR omega(g) = 570 cm(-1) in the emission spectrum of KMgF3:Mn2+ exhibits a value of 540 cm(-1) in the corresponding excitation spectrum. This situation, which is also found for other modes seen in the optical spe ctra of KMgF3:Mn2+, indicates that the mode (though associated with th e LO(3) branch of KMgF3) is not a pure mode of the lattice but display s a kind of resonant character. As a salient feature the calculated th ermal shifts are based on the experimental shifts experienced by the f requencies of the optical and acoustic modes on going from the ground (6)A(1g) to the excited T-4(1g) state of MnF64-. At variance with find ings for the R lines in Cr3+ and V2+, it is clearly demonstrated that the explicit and implicit contributions to the thermal shift of the ze ro-phonon line in MnF64- are similar and both induce red shifts upon h eating. Moreover the present analysis reveals that the explicit contri bution to the thermal shift undergone by the zero-phonon line of KMgF3 :Mn2+ is mainly dominated by the odd-parity low-energy modes. The calc ulated thermal shifts reproduce reasonably well the experimental data.