The Luminescence of K 4 Bi 2 Br 10 · 4 H 2 O and Rb 3 BiBr 6 Single Crystals

The luminescence properties of K 4 Bi 2 Br 10 · 4 H 2 O and Rb 3 BiBr 6 are reported. Both compounds show blue luminescence at low temperatures. For Rb 3 BiBr 6 the luminescence is ascribed to the 3 P 0 → 1 S 0 transition on the isolated Bi 3+ centre, while in K 4 Bi 2 Br 10 · 4 H 2 O the luminescence originates from the [Bi 2 Br 10 ] 4− dimers. The energy level diagram of the dimer centre is approached by using simple MO‐theory. The compound K 4 Bi 2 Br 10 · 4 H 2 O shows also an emission band in the red, which is ascribed to a charge‐transfer transition. The quenching of the luminescence is caused by energy migration among the dimers in K 4 Bi 2 Br 10 · 4 H 2 O, while in Rb 3 BiBr 6 the luminescence is quenched via the charge‐transfer state.