“…An intense nanosecond X-ray luminescence of free excitons in CsPbX 3 (X = Cl, Br, I) at 77 K was reported by Voloshinovskii et al already in 1993. 21 The light yield at room temperature, however, was too low (o500 ph MeV À1 ) 22 for their application in conventional scintillation detectors. In 2004, interest increased again when sub-nanosecond scintillation decay at room temperature was found in the layered hybrid metal-halide compound (C 6 H 13 NH 3 ) 2 PbI 4 , 23 however, with a light yield of only B6000 ph MeV À1 .…”