Radiation-produced electron and hole centres in oxygen-containing BaFBr. II. An ENDOR study of O_F^-

Abstract

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;-.

Electron traps in Ca2+- or Sr2+-doped BaFBr:Eu2+ x-ray storage phosphors

“…After the exciton decay the electron is trapped at that vacancy and the V K center formed migrates away at room temperature and may transfer its hole to another defect ͑note, V K centers are not stable at room temperature 9,15-17 ͒. For example, it reacts with O F 2Ϫ to form O F Ϫ , 15,16 but there may also be another, unknown, hole center ͑see the discussion in Ref. 3-5͒. It seems that for the preferential generation of F͑Br Ϫ ͒ and F A ͑Br Ϫ ͒ centers the origin of the Br Ϫ vacancies is not important.…”

Physics and Current Understanding of X-Ray Storage Phosphors

“…The isotope substitution of 17 O for 16 O proved that the defect centre of Fig. 14, always produced upon X-irradiation of BaFBr, is due to an oxygen contamination [39].…”

“…Above 120 K the V K ðBr À 2 Þ centres become mobile, react with the O 2À F centres and form O À F hole trap centres. Above 200 K the F(Br --) centres can diffuse away and become isolated [39,43]. The microscopic structure of the O À F defect ( Fig.…”

“…Very careful avoidance of oxygen could only reduce the oxygen contamination [37,38]. Oxygen can be incorporated in many ways: It was found by EPR and ENDOR investigations as O 2--in the fluorine (O 2À F ) [39] as well as in the bromine sublattice (O 2À Br ) [40,41]. There was also a molecular oxygen centre ((O F -O Br ) 3--) observed where one oxygen is placed on a fluorine site and the other one on a neighbouring bromine site [41].…”

“…When Xirradiating at low temperature, V K centres also show this correlation with the other 17 O-doped BaFBr after X-irradiation at RT, recorded for B k c at T = 5 K applying a microwave frequency of 9.42 GHz. The bars indicate the 17 O hyperfine splittings [39] centres [43,49]. This leads to the speculation that an exciton decay occurs at the Eu 2+ activator forming electron and hole trap centres and that there is a spatial correlation between the O 2À F -anion vacancy pairs and the Eu 2+ activators generated during the production of the material (Fig.…”