The sidebands of the zero phonon band of the phosphorescence transition are investigated in Cr 3+ :Alha 6 J 3 single crystals for several Cr (III)-concentrations. At low Cr (III)-concentrations of 2 mol-% the electronic transitions can be assigned to the zero phonon transition of the phosphorescence and the corresponding phonon sidebands with the aid of polarisation measurements, the temperature behaviour of the bands and the isotope effect. Under the influence of a trigonal field D 3 combined with spin-orbit coupling, both transitions and are observed. The phonon sidebands and the internal vibrational sidebands of the transition are compared with infrared and Raman vibrational bands. At a Chromium (III) concentration of 26 mol-%, some additional bands are measured nearby the transition, which cannot be explained in the single ion crystal field model. There exists a super exchange interaction between the Cr 3+ -ions across the urea ligands. The interaction can be described by a Hamiltonian H ex = -JS 1 · S 2 , which splits the 4 A 2 -ground level into four levels | 4 A 2 · 4 A 2 〉 S = 0,1,2,3 with energy differences |J|; 2|J| and 3 |J|. In the phosphorescence spectrum the four transitions | 2 E · 4 A 2 )〉 S' = 1 → | 4 A 2 · 4 A 2 〉 s = 0,1,2,3 are observed and the value of the exchange integral is calculated.