Ralph P. Schenker, Thomas C. Brunold and Hans U. Güdel
Synthesis and Optical Spectroscopy of MnO₄^2--Doped Crystals of Cs₂CrO₄, SrCrO₄, CsBr and CsI
Inorg. Chem. 37, 918-927 (1998)                 

Abstract: The first low-temperature absorption and luminescence spectra of MnO₄^2--doped crystals of Cs₂CrO₄, SrCrO₄, CsBr, and CsI are reported. Single crystals of Cs₂CrO₄:MnO₄^2-, which crystallize in the orthorhombic b-K₂SO₄ structure, were grown from basic aqueous solutions. An alkali-chloride flux composition was used to obtain single crystals of SrCrO₄:MnO₄^2- with the monoclinic monazite structure. Crystals of CsBr:MnO₄^2- and CsI:MnO₄^2- were prepared by slowly cooling the melt. The site symmetry in SrCrO₄ is C₁, but the deviations from D₂ symmetry are small, and the spectra are analyzed in the approximate D₂ symmetry. The ²E ²T₂ ligand-field transitions are strongly c polarized, and in absorption only the transition to the lowest-energy component of the split ²T₂ state is observed with its origin at 10303 cm^–1. Luminescence is exclusively observed to the lower-energy component of the ²E state. By a combination of absorption and luminescence data, it is shown that, in the cubic CsBr and CsI lattices, MnO₄^2- is partly incorporated in the form of Cs₂MnO₄ crystallites. The observed nonexponential luminescence decay for these systems indicates the existence of several manganese(VI) species with different relaxation times. Within isomorphous host lattices, a correlation is established between the temperature-quenching behavior and the size of the M⁶⁺ host ion: the efficiency of multiphonon relaxation processes increases with increasing ionic radius of M⁶⁺.