“…These general considerations are confirmed experimentally and theoretically for Y 3 Fe 5 O 12 , where the weak absorption between 1.2 and 2.2 eV and abrupt absorption rise above 2.4 eV have been assigned to 6 S → 4 G crystal-field transitions, respectively, in Fe 3+ O 6 octahedra and Fe 3+ O 4 tetrahedra. 16,17,19 In the time scale of phonon vibrations an electronic transition is essentially instantaneous, which means that immediately after the transition the atomic positions are the same as in the initial state. The final state, however, requires different equilibrium position, which is achieved by the FranckCondon process of phonon emission as illustrated in Fig.…”