Time-resolved spectroscopy in LiCaAlF6 doped with Cr3+: dynamical Jahn-Teller effect and thermal shifts associated with the 4T2 excited state

This work investigates the centre distribution of the Cr3+ impurity, the dynamical Jahn-Teller effect in the first 4T2 excited state and the thermal shifts of the absorption and emission peaks in LiCaAlF6:Cr3+ by means of time-resolved emission spectroscopy. The electronic and vibrational fine structure observed in both the absorption and emission spectra at low temperature are assigned according to the vibrational modes of the internal (CrF6)3- complex and the lattice modes. Zero-phonon lines associated with 4T2→4A2 and E2→4A2 transitions were detected and assigned on the basis of available high pressure data in LiCaAlF6:Cr3+. We have identified the vibrational coupled modes responsible for the vibrational structure of the low temperature emission spectrum and the reduction of the zero-phonon line (ZPL) splitting caused by the dynamical Jahn-Teller effect in the 4T2 excited state (Huang-Rhys factor, Se = 0.92). In addition, from the temperature variation of the emission intensity I(T), transition energy E(T) and bandwidth H(T), we obtained the vibrational modes that are coupled to the emitting state. We have evaluated the two main contributions to the photoluminescence thermal shift through thermal expansion and high pressure measurements: the implicit contribution induced by changes of thermal population and the explicit contribution induced by thermal expansion effects-40% and 60% of the total shift, respectively.