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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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Electrons shallowly trapped a t intrinsic sites in single-crystal and polycrystalline samples of AgBr, AgCl, and mixed iodohalides are observed by ESR. The results are consistent with a model in which the electron is localised by a silver ion in an effective mass state. The observed ESR linewidths are explained in terms of exchange narrowing. The effect of iodide on the free or nearly-free electron g-value is determined. Addition of iodide to AgCl or AgBr resulted in alloy line broadening. The amount of shallow trapping depended in part on dislocation density. In polycrystalline samples of AgBr, cyclotron resonance of the free electron polaron is observed simultaneously with the ESR signals from localized electrons. From the linewidth of the cyclotron resonance signal, the mobility of the free electron polaron in some polycrystalline samples is determined to be the same as that measured previously in very pure single crystals of AgBr.Elektronen in gittereigenen flachliegenden Haftstellen in Einkristallen und polykristallinen Proben von AgBr, AgCl und Jodid enthaltenden Mischhalogeniden werden mittels ESR beobachtet. Die Ergebnisse stimmen mit einem Modell uberein, bei dem das Silberion das Elektron in einem effektiven Massenzustand lokalisiert. Die mit ESR beobachteten Linienbreiten lassen sich als Austauschverschmalerung erklaren. Der EinfluB der Jodide auf den g-Wert des freien oder beinahe freien Elektrons wird bestimmt. Der Zusatz von Jodid zu AgCl oder AgBr ruft eine ungeordnete Lininenverbreiterung hervor. Die Anzahl der flachliegenden Haftstellen hiingt teilweise von der Versetzungsdichte ab. I n polykristallinen Proben von AgBr wird eine Zyklotronresonanz des freien Elektronenpolarons gleichzeitig mit den ESR-Signalen von lokalisierten Elektronen beobachtet. Auf Grund der Linienbreite des Zyklotronresonanzsignals wird gefunden, da13 die Beweglichkeit des freien Elektronenpolarons in einigen polykristallinen Proben mit der fruher an sehr reinen Einkristallen gemessen iibereinstimmt.

Oxygen contamination introduces hole-trapping centres into barium flumohalides. In the preceding paper, an 0centre WM reported as the product of the reaction between out-of-plane Br; VI; centres and oxide impurities. This paper describes an ENDOR study of this centre which identifies it as 0;. M oxygen ion substituted at a fluoride site. No evidence has been found for the presmce of a neighbouring defect (anion v-cy or interstitid cation) remaining from its charge-compensating role for the proposed p r e m o r O;-.