Fluorescence and phosphorescence of Cr3+ in cubic hosts

The low-temperature well-resolved 4T2g → 4A2g fluorescence spectrum of Cs2NaYF6:Cr3+ is presented and interpreted. Strong vibrational progressions in the α1g symmetric stretching and εg stretching modes are based upon the zero phonon line and odd-parity vibronic origins. The Jahn–Teller stabilization energy in the 4T2g state is 255 cm−1. The Cr3+ emission has previously been assigned to Tm3+ upconversion. The 4T2g barycentre is strongly linearly correlated with metal–halogen distance for Cr3+ diluted into elpasolite hosts. More generally, for Cr3+ in other hosts, the metal–ligand distance divided by ligand formal charge exhibits a fairly linear relation with 4T2g–4A2g energy separation, by contrast with the independence upon 2Eg–4A2g energy separation.