The assignment of the low-temperature 5D 4 electronic emission spectra of Cs 2NaTb( 14NO 2) 6 and Cs 2NaTb( 15NO 2) 6 enabled the locations of 7F J (J = 0-6) and 5D 4 crystal field levels to be identified for Tb 3+ at the T h site symmetry in these hexanitrito complexes. The complex vibronic spectra comprised metal-ligand, lattice mode, and two center NO 2 - transitions, and the 14,15N vibrational energy shifts of the latter were most helpful in spectral assignments. Variable-temperature studies enabled symmetry representations to be assigned to some crystal field levels because thermally populated upper 5D 4 levels also produced emission. The crystal field analysis of the energy level data set by five adjustable parameters yielded a mean deviation of 16.6 cm -1. All of the three independent crystal field parameters were negative, with B 62 dominant. The comparison of energy parameters with those from previous studies of Cs 2NaLn(NO 2) 6 (Ln = Pr, Eu) systems showed (i) a reasonably linear increase with lanthanide atomic number Z (Z 2) for the Slater (spin orbit coupling) parameter; and (ii) irregular trends for the crystal field parameters for the three systems, because these are biased by the inclusion of a small number of crystal field levels from several multiplet terms in the data sets. Copyright © 2012 American Chemical Society.