The low-temperature well-resolved T-4(2g) --> (4)A(2g) fluorescence spectrum of Cs2NaYF6:Cr3+ is presented and interpreted. Strong vibrational progressions in the alpha(1g) symmetric stretching and e, stretching modes are based upon the zero phonon line and odd-parity vibronic origins. The Jahn-Teller stabilization energy in the T-4(2g) state is 255 cm(-1). The Cr3+ emission has previously been assigned to Tm3+ upconversion. The T-4(2g) 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 T-4(2g) - (4)A(2g) energy separation, by contrast with the independence upon E-2(g)-(4)A(2g) energy separation. (C) 2004 Elsevier B.V. All rights reserved.