Indian Journal of ChemistryVol. 20A, November 1981, pp. 1057-1059

Free Space & Annealing of Chemical Radiation Damage InIonic Nitrates

E. CHANDUNNI, M. S. KRISHNAN & S. M. K. NAIR*Department of Chemistry, University of Calicut, Kerala 673 635

Received 25 February 1981; accepted 31 March 1981

Thermal annealing of chemical radiation damage in various ionic nitrates has been studied. The order ofinitial damage corresponds with the order of increasing free space in the crystals. Annealing in some cases increaseswith increase in free space. Anomalies are-observed in the case of alkali and alkaline earth metal nitrates. In thesesystems annealing increases with decrease in the cationic radii. The broad generalisation that free space in. a crystalenhances thermal annealing of radiolytic damage is strictly not true and each compound must be considered as aunique system for the annealing studies.

FREE space (volume) concept was postulatedto account for the relative ease of decom-position of ionic nitrates and it was consi-dered that materials having a high free volume inthe crystals were fairly susceptible to radiolysis>",The free space in a crystal is an inverse measure ofthe closeness of packing and determines the heightof the energy barrier which the oxygen fragment ofa dissociating excited nitrate ion must surmountto achieve minimum critical separation from thenitrite fragment to escape recombination, and,therefore, the probability of dissociation. Thehigher yields in loose-packed systems follow alsofrom the lower probability of dissipation of energyinto lattice vibrations. The large free space in acrystal should, therefore, not only enhance chemicalradiation damage but also produce appreciablethermal recovery of the damage by facilitating themigration of the damaged oxygen once it is releasedfrom the traps+, It was, therefore, of interest toinvestigate the relation between free space andannealing of chemical radiation damage. Ionicnitrates have been chosen for this purpose becausethe mechanism of annealing in these compounds isfairly well understood's".

Materials and MethodsAnalytical reagent grade (BDH) sodium, potas-

sium, rubidium, caesium, barium, strontium, leadand zirconium nitrates and suprapur (E. Merck)lithium nitrate crystals were used. All the samplesexcept lithium and zirconium nitrates were dried overphosphorus pentoxide in vacuo. Zirconium nitratewas dehydrated by heating to constant weight at110°C and finally over phosphorus pentoxide.Lithium nitrate was dried over phosphorus pento-xide in vacuo for several days and then in a highvacuum system at room temperature until its vapourpressure fell below 10-3 mm of mercury. Finallyit was sieved in a dry box and again dried over phos-phorus pentoxide. The anhydrous crystals (85-100mesh) of all the nitrates sealed in vacuo in pyrex

ampoules, were irradiated with 60CO y-rays to adose of 52 Mrad at a dose rate of 0.2 Mrad hrl.The irradiated samples were preserved over phos-phorus pentoxide before examination or thermaltreatment. Because of the highly hygroscopic natureof lithium nitrate it was always handled in a dry box.

Isothermal annealing runs were carried out inair in a thermo stated electric hot air oven (accuracy± 1.0°C). The damage nitrite was estimated spectro-photometrically=" within ± I % with a Pye UnicamSP 1800 instrument.

Results and DiscussionThe colour induced by irradiation and the bleach-

ing upon thermal annealing of the irradiated crystalswere similar to those reported earlier", The values ofinitial damage (given in parentheses in ppm in termsof NO-;) produced by 52 Mrad 60CO y-rays indifferent nitrates are as follows: LiNOg(28); NaN03(319); KN03 (3310); RbN03 (1021); CsNOg (2338);Ba (NOg)2 (4425); Sr(NOg)2 (830); Pb (N03)2 (1246);and Zr(NOg)4 (1330). The order of magnitude ofinitial damage in terms ofNO-;; in the above nitratesis : LiNOg < NaN03 < Sr(N03)2 < RbN03 <Pb(N03)2 < Zr(N03)4 < CsN03 < KN03 <Ba(N03)2'

The plots of initial damage in the above nitratesas a function offree space in their crystals are shownin Fig. 1. The order in terms of values of initialdamage in the alkali metal nitrates is: LiN03 <NaN03 < RbN03 < CsN03 < KN03, same as theincreasing order of free space in the crystals of thesenitrates as well as the order of G(N02) values foundby Cunningham and Heal" for the alkali metalnitrates.

The order of initial damage in the alkaline earthmetal nitrates [Sr(N03)2 < Ba(N03)2] also followsthe same order as the increasing free space in theircrystals, in agreement with that obtained by Doiganand Davisw,

The plots in Fig. I further show that the valuesof initial damage in strontium, rubidium, lead and

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INDIAN J. CHEM., VOL. 20A, NOVEMBER 1981

zirconium nitrates are also in the increasing orderof free space in the crystals, thus, substantiating theearlier observation 2,10 that the nitrite yield in theradio lysis of ionic nitrates increases with increaseof free space in the crystals. The correlation bet-ween free space and the overall decomposition ofnitrate is, therefore, valid in the case of the alkali,alkaline earth and the other nitrates studied.

The thermal annealing of damage in these nitrateshas been investigated at various temperatures, overa period of 100 hr and at different temperatures,depending on the extent of annealing in each nitrate.The magnitude of annealing is expressed as thefraction (1') annealed

CHANDUNNI et at. : RADIATION DAMAGE IN IONIC NITRATES

TABLE 3 - ENERGY OF ACTIVATION OF THE ANNEALING PROCESSBY DIFFERENT TREATMENTS

f:::"Ea (kcal mol>') by

Fletcher- Waite FirstBrown treatment order

treatment

Secondorder

LiN03RbN03CsN03Ba(NOs).Sr(N03).Pb(N03).Zr(N03).

14.327.025.436.018.322.436.4

13.526.725.434.518.523.835.9

8.911.08.8

12.28.99.58.6

13.520.018.523.915.023.231.7

not correspond with the decreasing order of cationicradii seen in the first three sets.

The annealing data of all the nitrates exceptsodium and potassium nitrates have been analysedon the basis of models developed by Fletcher andBrown13'14 and by Waite15,16 for interstitial vacancyrecombination and also on the basis of conventionalchemical kinetics". In all the cases it is observedthat annealing is a combination of first order andsecond order processes. The values of the energyof activation obtained by different treatments aregiven in Table 3. These values do not bear anycorrelation with the free space in crystals or withthe cationic radii.

In conclusion the correlation existing betweenfree space and radio lytic decomposition in ionicnitrates does not always hold good in the case ofannealing in these systems. The effects of factorslike the polarizability of the cation, a particularcrystal structure (i. e. the radius ratio of the ions),electron trapping centres, stability of the primaryproducts and radiation-induced reaction betweenproducts, the effect of radiation damage on the lattice(changes in lattice parameters) and the immediateelectronic and spatial environment of the decom-posing species, etc. in the radio lysis of inorganicsolids have been reported"; It is probable that these

factors may influence the annealing reactions also.Hence the broad generalization that free space in acrystal enhances thermal annealing of radiolyticdamage is not warranted and each compound mustbe considered as a unique system for the annealingstudies.

Acknowledgement

Grateful thanks of the authors are due to Prof.S. R. Mohanty, Utkal University, Bhubaneswar,and to Mjs. Western India Plywoods, Baliapatam,for the irradiations.

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