ABSORPTION AND LUMINESCENCE SPECTROSCOPY OF FERRATE(VI) DOPED INTO CRYSTALS OF K2MO4 (M=S, SE, CR, MO)

The absorption spectra of the ferrate (VI) ion (FeO42-) in K2MO4 (M = S, Se, Cr, Mo) host lattices consist of a series of relatively weak bands at low energy, which can be assigned to transitions within the partially filled 3d shell and some intense bands at higher energy, which are assigned to ligand-to-metal charge transfer transitions (LMCT). In the near-infrared (NIR) region sharp lines are observed belonging to the spin-forbidden spin-flip transitions 3A2 --> 1E and 3A2 --> 1A1. The lowest excited state is the 1E state, serving as initial state for 1E --> 3A2 sharp-line luminescence at around 6200 cm-1. Another luminescence is observed centered at 9000 cm-1, which is assigned to the 3T2 --> 3A2 transition. It is rather broad and three orders of magnitude weaker than the 1E luminescence at 30 K as a result of efficient non-radiative relaxation processes to the 1E state. The temperature dependence of the total intensity and the lifetime of the 1E --> 3A2 luminescence is understood within a complex scheme of radiative and non-radiative processes.