Luminescence properties of A2ReCl6 crystals

Abstract

At temperatures below 60 K A₂ReCl₆(A = K, Rb, Cs) crystals emit intense, well resolved luminescence in the 13 900–12 800 cm^–1 region under blue–green excitation. The emission is strongly temperature dependent and also depends somewhat on the excitation wavelength and crystal quality. It is here assigned as the vibronic structure associated with the Γ₇(²T_2g)→Γ₈(⁴A_2g) transition of centrosymmetric and essentially octahedral ReCl₆^2– ions at up to nine distinct defect sites. The compounds (NR₄)₂ReCl₆(R = CH₃, C₂H₅) also luminesce in the same spectral region but more weakly than the alkali-metal salts. [N(n-C₄H₉)₄]₂ ReCl₆ does not luminesce under these conditions. The nature of the defect sites and their excitation processes are discussed.