Photochemical Reactions in Potassium Chloride Crystals Grown from Reagent Grade KCl

Bars of single crystal NaCl have been deformed by four‐point bending and the sign of the charge on dislocations has been determined. This sign was found to be negative in nominally pure (i.e. analytical reagent grade) material at room temperature in agreement with theory. Crystals have also been prepared by doping with NaOH in the melt. Dislocations in NaCl crystals having an OH concentration of less than 3 × 10−4 mole fraction were found to be negatively charged and in crystals with an OH concentration greater than 3 × 10−4 mole fraction positively charged. The effect of annealing in an atmosphere of chlorine has been studied and a reversal of sign has been observed in crystals heavily doped with NaOH. Doped crystals were quenched and sign reversals also observed. Possible chemical reactions which serve to interpret these phenomena have been discussed. Bars of crystal already bent in the apparatus were turned over and bent in the opposite direction. The Bauschinger effect was observed in ‘pure’ and doped crystals and also a reversal in the polarity of the potential measured on opposite faces. This has been interpreted in terms of the expected movements of charged dislocations.

Charged Dislocations in Sodium Chloride Crystals Containing Sodium Hydroxide

Caffyn

Freitas

Goodfellow

1965

Photochemical Properties of SH⁻ Ions in KCl and KBr Crystals

Facey

Jacobs

1967

Self Cite

The photochemistry of KCl and KBr containing SH-impurity ions has been investigated. In addition to the formation of S-and U,-centres reported previously, U-centres arc formed in high yield and F-centres in low yield. The thermal contribution to the photochemical process is slight ( M 0.013 eV). The kinetics of the decay of Us-centres has been shown to be bimolecular and to correspond mainly to the reaction S-+ Hi + SH-, where Hi denotes on interstitial hydrogen atom. A model involving the utilization of the thermal energy released during a radiationless transition from the excited state is succossfiilly used t o give a qualitative explanation of the data.Es werden photochemische Prozesse in KCl-und KBr-Kristallen, die SH--Fremdionen enthalten, untersucht. Zusatzlich zur Bildung von S--und U,-Zcntren, woriiber schon fruher bcrichtet wurdc, werden mit hoher Ausbeute U-Zentren und mit niedriger Ausbeute F-Zentren gebildet. Der thermische Beitrag zum photochemischen ProzeB ist gcring (0,013 cV). Es zeigt sich, daB die Kinetik des Zerfalls der U,-Zentren bimolckular ist und hauptsachlich der Reaktion S-+ Hi c SH-entspricht, wobei Hi ein Wasserstoffatom auf Zwischengitterplatz bezeichnet. Ein Modell, das die wiihrend de8 strahlungslosen Ubergangs vom angcrcgten Zustand freiwerdende thermische Energie mit beriicksichtigt, wird mit Erfolg eur qualitativen Deutung der Ergebnisse benutzt.

OH⁻ Dissociation and U₂ Decomposition in Hydroxide‐Doped KCl and KBr Crystals

Kurz

1969

We report quantitative results concerning the dissociation of OH-, supporting the conception given first by Kerkhoff. U, centres created by this reaction are thernially stable below about 100 "K. It is shown that U, centres can be destroyed by optical bleaching. A t 4.5 "K, U and H centres are created simultaneously; this is described as h v ( n z ) HP,, (U,) -+ H; (U) + halogen; ,(H) .LVc suggest for the underlying mechanism that the first process after U, excitation is a noncubic relaxation of the lattice (static Jahn-Teller effect) followed by a (110) replacement sequence as described by Williams to form a H-centre-vacancy pair, the vacancy finally being occupied by the Hion. -In KCI a reverse reaction can be induced either by thermal activation or by bleaching with H light. -Oscillator strengths of the U,, H, and OHbands are calculated from the results using the Smakula equation.Die quantitativen Ergebnisse dieser Arbeit uber die Dissoziation von OH--Ionen stutzcn eine erstmalig von Kerkhoff angegebene Reaktionsgleichung. Die dabei erzeugten U,-Zcntren sind unterhalb etwa 100 OK thermisch stabil. Es wird gezeigt, daB U,-Zentren durch optisches Bleichen zerstort werden konnen. Bei 4,5 " K entstehen dabei paarweise U-und H-Zentren. d. h.Als Erzeugungsmechanismus schlagen wir vor : Nach der U,-Anregung erfolgt zunachst eine nichtkubische Relaxation des Gitters (statischer Jahn-Teller-Effekt). Anschliefiend fuhrt ein von Williams beschriebener ProzeB iiber eine (1 10)-Verschiebungssequenz zur Erzeugung eines Paares H-Zentrum und Anionliicke. Das H--Ion stabilisiert sich schliel3lich in der Liicke als U-Zentrum. -I n KCl kann eine Riickreaktion entweder durch Zufuhrung thermischer Energie oder durch optisches Bleichen mit H-Licht ausgelost werden. ~ Unter Zugrundelegung der Smakula-Gleichung werden aus den Ergebnissen die Oszillatorenstarken der U2-, H-iind OH --Bande errechnet.