An electron spin resonance study of iron in silver chloride

Trivalent iron is formed in weakly doped crystals of AgCl:Fe 2+ when these are irradiated at a temperature which allows silver ions to leave sites near to Fe 2+ and diffuse away through the lattice. Fe 2+ normally acts as a hole trap but below 168 °K it behaves as a recombination centre as no better electron-trapping centre exists. The stable configuration for Fe 3+ is the tetrahedral (FeCl 4 ) - complex identified by Hayes, Pilbrow and Slifkin in 1964, but a new spectrum which corresponds to a weak trigonal distortion of this complex by a neighbouring Ag + ion is found during the formation of the more stable complex. Formation of the tetrahedral (FeBr 4 ) - complex by optical irradiation in AgBr:Fe 2+ is also reported.