The absorption spectra of the ferrate (VI) ion (FeO2-4) 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‚Üí 1E and 3A2 ¬†‚Üí 1A1. The lowest excited state is the¬†1E state, serving as initial state for¬†1E ‚Üí¬†3A2 sharp-line luminescence at around 6200 cm-1. Another luminescence is observed centered at 9000 cm-1, which is assigned to the¬†3T2¬†‚Üí¬†3A2¬†transition. It is rather broad and three orders of magnitude weaker than the¬†1E luminescence at 30K 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 ‚Üí¬†3A2¬†luminescence is understood within a complex scheme of radiative and non-radiative processes.
The two title compounds were synthesized and investigated with the inelastic-neutron-scattering (INS) technique. They contain mixed YbMBr93- (M=Cr3+, Ho3+) dimers as discrete units, and the magnetic excitations of mixed Yb3+-Cr3+ and Yb3+-Ho3+ dimers could thus be observed. The Yb3+-Cr3+ dimer has three INS transitions, for which anisotropic exchange, as well as zero-field splitting of Cr3+, has to be included in the exchange Hamiltonian. For the Yb3+-Ho3+ dimer the effect of the exchange interaction manifests itself as a broadening and a splitting of the crystal-electric-field levels of the isolated Ho3+ ion. Taking into account the full (2J¬†+ 1) ground-state multiplet of Ho3+, as well as anisotropic exchange, gives a satisfactory description of this dimer.