X-ray production of α-centers in KCl at liquid helium temperature

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The X-ray production and thermal annealing a-centres (isolated negative ion vacancies) are studied in KC1 as a function of purity and the temperature of irradiation. At any temperature the a-centre production results from competition between the formation process and thermally activated destruction of negative ion vacancies. Since the latter process is strongly temperature and structure dependent, the a-centre production is structure sensitive or insensitive depending on the temperature of irradiation. From the point of view of thermal stability the a-centres produced by X-rays can be divided into two groups.The first group includes a-centres which decay below 77 "K and whose thermal stability does not depend on the purity of the sample. The second group includes a-centres which decay in the 100 to 250 "K temperature range and whose thermal stability is strongly structure sensitive. Finally, some results on the influence of electron traps on a-centre formation are reported.Nous avons etude la formation par rayons X et la destruction thermique des centres a (lacunes de ion negatif isol6es) drtns KC1 en fonction de la purete et de la tempbrature d'irradiation de l'bchantillon. A toute temp6rature la production des centers a est le &sultat de deux processus, un de formation et l'autre de destruction (ce dernier active thermiquement) de lacunes de ion nbgatif. Puisque ce dernier processus depend de la temperature et de la pure% de l'bhentillon, la production des centres a depend ou non de la pure% de 1'6chantillon selon la temperature B laquelle 1'6chantillon eat irradie. Du point de vue de la stabilite thermique, les centres a produits par lea rayons X peuvent 6tre divide entre deux groupes. Le premier groupe contient lea centres a qui se dbtruisent au dessous de 77 O K et dont la stabilite thermique ne depend pas de la purete de 1'8chantillon. Le second groupe contient lea centres a qui se detruisent dam l'interval de temperature compris entre 100 et 250 O K et dont la stabilite thermique depend fortement de la purete de l'bchantillon. Enfin, nous reportons quelques &sultates sur la production des centres a en cristaux contenants des piAges d'blectron.

Section: Growth and Decay Of The U-bandmentioning

confidence: 95%

X‐Ray production and thermal annealing of frenkel pairs in KCl

Giuliani

Reguzzoni

1968

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The Influence of Electron‐Trapping Impurity on Defect Creation and Bleaching in KCl Irradiated at 78 °K

Sibley

Sonder

1967

The excitonic mechanism for creation of halogen Frenkel-type defects in alkali halides predicts an early positive curvature in tho defect growth cnrve. The model also predicts that annihilation of VK-centers by the optical release of electrons from traps would produce Frenkel pairs in the halide sublattice. The first prediction has been checked in KCI: TI+ and KCI:Pb++ irradiated at 78 OK with 1.7 MeV Van de Graaff electrons, and thesecondupon ir bleaching of an irradiated Tl+-doped crystal a t 78 "K. If one takes the F-center concentration as a measure of the Frenkel-defect density, both tests were positive. A carefnl monitoring of the cc-band region before and after bleaching, however, indicates that the F-center concentration is not a valid measure of the vacancy density. I n a crystal containing 48 ppm Tl", the a-center concentration after irradiation a t 80 "K was 2.5 times that for F-centers. Moreover, ir bleaching of T1" and VK, while enhancing the F-band = 300/,, destroyed nearly all of the a-centers. The last observation reveals that more empty halide vacancies are destroyed than F-centers are produced by the bleaching light.Der Exzitonmechanismus fur die Bildung yon Halogendefekten vom Frenkeltyp in Alkalihalogeniden sagt eine friihe positive Krummung der Defektwachstumskurve vorher. Auch sagt das Modell aus, daR die Annihilation von VK-Zent,ren durch optische Befreiung von Elektronen aus Haftstellen Frenkelpaare im Halogenuntergitter erzeugen sollte. Die erste Aussage wurde in KCI:Tl 'und KCI:Pb++, die bei 78 OK mit 1,7 MeV-Van de Graaff Elektronen bestrahlt wurden, uberpruft, die zweite dnrch Infrarotbleichen eines bestrahlten. Tl+-dotierten Kristalls bei 78 OK. Wenn man die F-Zentrenkonzentration als Ma13 fur die Frenkeldefektdichte ansieht, waren beide Tests positiv. Eine sorgfaltige Kontrolle der sc-Bande vor und nach dem Bleichen zeigte jedoch, da13 die F-Zentrenkonzentration kein zuverliissiges Ma13 fur die Leerstellendichte ist. I n einem Kristall. der 48 ppm TIo enthielt. war die a-Zentrenkonzentration nach der Bestrahlung bei 80 OK 2.5mal so hoch wie die der F-Zentren. Daruberhinaus ergab das Infrarotbleichen von T1" und Vl; eine Erhohung der F-Bande um m 30%. wahrend sie nahezu alle a-Zent.ren beseitigte. Die letzte Beobachtung zeigt, da13 durch das bleichende Licht mehr leere Halogenleerstellen vernichtet als F-Zentren produziert werden.

Evidence for Frenkel Defect Recombination During Thermal Annealing of Low‐Temperature X‐Irradiated KCl

Balzer

Peisl

Waidelich

1968

Now it seems to be well established that F'renkel defects are created during irradiation of KCl by low-energetic X-rays (1 to 4). The defects created at low temperatures are thermally unstable at higher temperatures (5 to 8). The purpose of this paper is to report about the annealing mechanism evaluated from combined measurements of lattice parameter and length change after isochronal annealing of irradiated KCl.llPurell Harshaw KCl single crystals were used for this investigation. Relative lattice parameter and length change were measured with the same setup briefly described in a preceding paper (3). During irradiation and all X-ray measurements the crystal was kept at constant temperature of (8 5 C ! , 5) OK. Isochronal annealing was performed by warming the crystal for 10 min to successive higher temperatures Tn. Temperature steps were chosen that AT/T = 0.1; AT = Tn -Tn-l ,change as a function of annealing temperature for KCl aftar 18.8 h X-irradiation.The remaining changes are given in percent of the total change after irradiation given in the caption.