The absorption spectra of the ferrate (VI) ion (FeO^2-₄) 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→ ¹E and ³A_{2 Â} â†’ 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 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 → 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 YbMBr₉^3- (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.