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An 0centre formed upon hole trapping by 0'in X-irradiated BaFBr is long-lived at room temperature and has been detected by EPR below 30 K. It can also be produced in low wncentrations by expos-of BaFBr to subbandgap (200400 nm) radiation. The formation of F(Br-) centres at chargecompensating bromide ion vacancies is a corollary of the photoionivltion of Oz-. High eoneentrations of oxide have been successfully introduced into this material by doping with anhydrous BaO. including Ba"O for unambiguous identification of the 0defect.Oxide addition and the formation of 0-produces a series of optical bands that have been assigned using a wmbination of optical and magneboptical techniques. The inBwnce of the material preparation conditions on the concentration of 0'is dircussed and possible mechanisms for the formation of 0are proposed

Ru3+ and Ru2+ ions are substitutionally incorporated into melt-grown crystals of AgBr and AgCl. The d5 trivalent ion predominates in samples prepared in oxidizing environments, such as Br2 and Cl2, and it has been the subject of a detailed magnetic resonance study. It is distributed between four structurally inequivalent lattice sites in both salts. These sites are distinguished by the arrangement of charge-compensating silver-ion vacancies in the two closest cation subshells. Reasonable dopant-vacancy configurations have been deduced from the EPR data.

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