Indian Journal of ChemistryVol. 35A,January 1996,pp.12"l?

Gamma ray induced decomposition of barium and strontium nitratesdispersed in sulphate and carbonate matrices in solid state

N G Joshi & A N Garg"Deartment of Chemistry, Nagpur University, Nagpur 440010

Received 3 May 1994; revised 9 February 1995; accepted 12 August 1995

The G(N02-) values in gamma radiolytic decomposition of barium and strontium nitrates are en-hanced by their respective sulphate.and carbonate additives, their concentration and the absorbed dose.The enh.an~ement is more significant at > 95 mol % of the additive after which the increasing trend isn?t so SIgnificant. Thermolummescence and ESR spectral studies suggest the formation of radical spe-cies such as S04-' S03-' O2-, C03-, CO2-, etc. which may interact with the radical species of nitrates(NO~-, 'N02 et~). causing enhanced decomposition by energy transfer. A comparison of the carbonateand sulphate additives shows the former to be more efficient medium of energy transfer.

Gamma radio lysis of inorganic nitrates is affectedby many factors such as nature and size ofcation'F, water of crystallization">, free space vo-lume in the crystal lattice, lattice energy, absorbeddose", nature and concentration of the additives'".Khare and Johnson? first investigated the energytransfer in several systems composed of KN03 inalkali halides, metal oxides etc. and found thatG(NOi) values are enhanced by 100 times com-pared to pure salt in the concentration range0.005-5.0 wt% of the nitrate. In our laboratory,extensive studies on gamma radio lysis of alkalimetal nitrates with halide, sulphate, carbonate andphosphate additives have shown enhanceddecomposinon'v-t'". It has been suggested thatenergy transfer involving radical species may oc-cur. In the present study, we report on the gammaradio lytic decomposition of barium and stroniumnitrates dispersed in respective sulphate and car-bonate matrices in the concentration range 0.5-S0mol % of the nitrate up to an absorbed dose of250 kGy.

Materials and MethodsAll the nitrate salts and other chemicals were of

AR/GR grade. Sulphates and carbonates of bar-ium and strontium were prepared by precipitationfrom chloride salts with concentrated sulphuric ac-id and aqueous solution of ammonium carbonate.The precipitate was washed thoroughly. and dried.

The binary mixtures of the nitrates with respec-tive sulphate and carbonate additives were pre-pared by grinding together the weighed amountsin an agate mortar. Samples were sieved (50 mesh)

to get uniform particle size and irradiated in un-sealed Corning glass tube in a 60CO GammaChamber-900 at a dose rate of O.SO kGy h -1.

Dose rate was assayed by Fricke dosimeter usingG(Fe3+ )= 15.6 and alanine dosimeter 14• Radiolyticproduct NOi was estimated spectrophotometri-cally using modified Shin's method'! of diazotiza-tion by employing Shimadzu UV- 240 spectropho-tometer. G-values were calculated on the basis ofcorrected dose obtained on the base of its electronfraction 7 in the total system i.e.-(absorbed dose) x (molfraction of niture)

electrons per mol in the nitratex--------~-----------------electrons per mol in the total system

Thermoluminescence glow curves of the gammairradiated solid samples were recorded by heatingat a linear heating rate of 150 K min=L, using In-dotherm temperature controller in conjunctionwith a RCA-931A photomultiplier and a millivoltrecorder. ESR spectra of the gamma irradiatedsamples were recorded on a Brucker ESP-SOOESR X band spectrometer using microwave fre-quency of 9.74 GHz and DPPH as the fieldmarker.

Results and DiscussionGamma radiolytic decomposition of barium and

strontium nitrates and their binary mixtures at var-ying concentrations were studied at various doses.

In Fig. 1 are plotted the variations of G(NOi)with mol % of strontium nitrate in the binary mix-

JOSHI et al.: GAMMA RADIOLYSIS OF BARIUM & STRONTIUM NITRATES 13

t-ON

~C!I

.50 kGyo lOOkGy'6200kGy

10

5 10 15MOl"',Sr(N03~ -

-1~~--~ __~~~~ __~ __~~ __~ __-L__~o 20 40 60 80.. 100

MOl"lSr(Non' -

Fig. i-Variation of G(NOi) with mol % of Sr(NQ3lz inSrS04 matrix at 50, 100 and 200 kGy

r-'1'0

~

o - SO kGy~ - 170kGyc - 230kGy

10

, I •oc

Fig. 2-Dependence of G(NOi) with mol % of Sr(N03lz inSrCOl at absorbed doses of 50, 170 and 230 kGy

Table I-Gamma ray induced decomposition of Ba(NO]h andSr(NO,), with respective sulphate and carbonate matrices at var-

ious compositionsMol % of Ba(N03h + BaS04, Sr(N03h + SrCO"

nitrate 250 kGy 100 kGy

0.51.0

5.020.060.0

100.0

[NOi],ppm1433 ± 22139S±161321 ± 16100S ± 171096 ± 14969± 10

G(NO,)

396 ± 2.2194±2.23.".6 ± 2.3S.9 ± 1.01.9±0.1O

0.S2 ±0.06

[NOi],ppm G(NOi)4690;t41 lS35±4.12782± 13 544±J.0

21S±4 4.7±O.1224±3 2A±O.1227 ± 4 0.64 ± 0.06226±30.47±0.04

li~~----------------------------~~

10

210

2·0mol~

~500mol'/.

~10mol'/.

1 a: 0 0- 0-40111011-

sr::::::::t:::: : ~ 80mol1-~ 100 mol1.

Go.l0 100 200

ABSORBED DOSE (kGyJ-

Fig. 3-:-Variation of G(NOi) with absorbed dose for Sr(NO]),in SrCOJ matrix at different nitrate compositions

tures with SrS04 at various doses. Concentrationsof N02- and G(N02-) values for various composi-tions of Ba(N03lz and BaS04 at an absorbed doseof 250 kGy and of .Sr(N03)2 with SrCOj at 100kGy are listed in Table 1. In Fig. 2 are plottedvariations of G(N02-) with mol % of Sr(N03lz inSrC03 at various doses. Variation of G(N02-) va-lues with the absorbed dose in Sr(N03h + SrC03system at various mol % compositions are plottedin Fig. 3.

Fig. 4-Thermoluminescence (TL) glow curves ~or (A) BaSb~+ 1 mol % Ba(NOJh at 100 kGy; (B) SrS04

+ 1 mol % Sr(NO) at170 kGy and (C) BaeO) + 1 mol % Ba(NO))2 at 270 kGy ) 2

Thermoluminescence (TL) glow plots were re-corded for irradiated samples of pure sulphatesand carbonates and its binary mixtures with 1 mol% of respective nitrates at several doses. TypicalTL glow curves for BaS04 + Ba(N03h at 100kGy, SrS04 + Sr(N03h at 170 kQy and BaC03+ Ba(N03h at 270 kGy are shown ill Fig. 4.

Enhancement in the G(N02-) values in alkaliand alkaline earth metal nitrates by the additivessuch as alkali metal halidesl-"!", silica", metal6xidesl7-20 etc. is well known in literature. Earlierwe have observed that addition of respectivesulphate and carbonate/-? in the alkali metal ni-trates increases its decomposition by several folds.Further, the dicomposition was found to be com-position and dose dependent. Khare and John-son? have proposed an energy transfer mechanisminvolving excitons. On the other hand, Sagert andRobinson?', and Wong and Willard-? explainedenergy transfer process in terms of electron migra-tion. Nitrate ion absorbs energy and it requires aminimum of - 4.0 eV to cause dissociation.Therefore, only the matrix materials having an en-ergy gap> 4 eV are effective in causing the ener-gy transfer. In continuation of our earlier work8,9

we have investigated the effect of sulphate andcarbonate additives in strontium and bariumni-trates at various doses arid compositions.

Decomposition 0/ barium and strontium nitrates insulphate matrix

A perusal of data in Table 1 and Fig. 1 showsthat G(NOi) values are enhanced by sulphate ma-trix in both cases of barium and strontium nitratesat all doses and compositions. The figure alsoshows that G(NOi) values are very high for < 20mol % of Sr(N03h and lia(N03h in. respective

sutphates at all the doses. The inset of this figurefurther indicates that in low concentration rangeup to 10 mol % of the nitrate, G(N02-) values de-crease rapidly and beyond that only slowly. Fur-ther, it has been observed that at low concentr-ations up to 5 mol % of the nitrate, G(N02-) ishigher by two or three orders of magnitude ascompared to the pure nitrate. With decreasingamount of sulphate additives, the enhancement inG(NOi) is much smaller. It appears that the me-chanism of radiolytic decomposition is differentfor lower concentration range of 0.5 to 5 mol %than for higher concentration range of > 20 mol% sulphate. This observation could possibly be at-tributed to the different modes of interaction ofradical species or defect centres formed'" in thesulphate matrix with the nitrate ions. Even thoughin both cases energy is transferred from the sulph-ate matrix to the nitrate but in the low concentra-tion range the defect centres seem to play a domi-nating role, At low concentrations of nitrates(i.e.> 95 mole % sulphate), NO; may be in closecontact with the sulphate matrix leading to the for-mation of lattice defects. In such cases, energytransfer I!lay take place at the interfacial bounda-ries of. the particles of two components thus en-hancing decomposition. However, at higher con-centration range of nitrate (i.e. < 80 mole %sulphate) energy is transferred through excitons. Itis well known that on gamma irradiation of sulph-ate, different radical species such SOi, SOi,S02- ,0; etc. are formed24-26•

Mass spectrometric studies of y-irradiatedSrS04 have shown. the excess of SOi and SO.,-formed by irradiation and interaction of electronstrapped in SrSO 4 lattice with the radical ion SO 4'(ref. '27). Gromov and Karaseva'" studied ESR

rosm et al: GAMMA RADIOLYSIS OF BARIUM & STRONTIUM NITRATES

spectra of Sr35S04 and Ba35S04 and y-irradiatednon-radioactive powders and observed signals forparamagnetic species such as S03- and S04' ESRspectra of y-irradiated sodium and strontium ni-trates show the presence of radical species 'N03and 'N02 (ref. 29). Similarly ESR spectra of y-irradiated barium nitrate show signals for 'NO,N03 and 0- (ref. 30). In order to identify theradical species in y-irradiated systems, ESR spect-ra of BaS04 and BaS04 + 0.5 mol % Ba(N03hwere recorded at an absorbed dose of 200 kGy.The g-values and identification of radical speciesare listed in Table 2. Thermoluminescence mea-surements of y-irradiated BaS04 and SrSO 4 alongwith their respective binary mixtures with 1 mol %nitrates at different absorbed doses of 100, 170and 250 kGy further support the formation ofradical species. Figure 4 shows a shoulder at 385K and a peak at - 450 K, which rimy be assignedto radical species S04 and 0; respectively", Athigher doses the shoulder disappears. However,TL peak intensity at 450 K though decreases withincreasing dose but the intensity is always higherw.r.t. binary mixture with 1 mol % Ba(N03)2 'asshown in Fig. 5. Further, Fig. 5 shows that the in-tensity difference between this peak compared tothat of its binary mixture remains almost constant.These evidences suggest that S04 radical speciesare totally consumed whereas a part of 0; speciesare also used up by interacting with NO; at alldoses during decomposition process. The mechan-ism of decomposition involving interaction of radi-cal species S04-' S03-' and 03- may be represent-ed as follows:

Primary process;

SOl -S04-' SO;, SO;z-, 0;, e-,etc. . .. (1)-

NO; ""-+ NO~~, 'N03, 'N02, NOi, etc. . .. (2)

Secondary process;

SO; + N03- - 'N02 + SO~-

S04- + N03- - NO.3 + SO~-

03- +N02- - NOI- + O2

SOi + NOI- - NOi + SOI-

NO.3 + NOI - - 2N02- + O2

N02+e- -N02-

S04- + e- -SO~- + hv

N03- + hv-N02- +0

· .. (3)

· .. (4)

· .. (5)

· .• (6)

· .. (7)

· .. (8)

· .. (9)

... (10)

15

Table 2-ESR identification of radical species and their gvalues in gamma irradiated BaS04 and BaS04 + 0.5 mol %

Ba(N03h at an absorbed dose of 200 kGy

System Probable gayvalue Referenceradicalspecies

BaS04 S03- 2.003 26O2 l.999· 24

BaS04+ 0.5 mol % SO; 2.003 26Ba(N03)2 N02 2.001 24

,2r---- __-----,

Glow Peakal450 K

~ 9IIIZw!i.......J•..4

·L BaS04+lmol"t. Ba<NOh

100 200 300ABSORBED DOSE <kGyl

Fig. 5- Variation of TL peak intensity at 450 K with absorbeddose in BaS04 and its binary mixture with 1 mol % Ba(N03h

A comparison of the present observations withthose of our earlier studies on alkali metal ni-trates9•12,13 suggests decrease in decomposition ofalkaline earth metal nitrates. Similarly a comparis-on amongst two sulphates suggests BaS04 to bemore effective medium for energy transfer com-pared to SrS04 in the decomposition of respectivenitrates. It seems that besides anionic radical spe-cies and defect centres, the nature and size of ca-tion also seems to play an important role in thedecomposition process.

Decomposition of barium and strontium nitrates incarbonate matrix

It has been observed that decomposition of ni-trates in respective carbonate additives in general,is enhanced for all the compositions and at all thedoses. Figure 2 shows the decreasing trends ofG(N02-) with mol % of SrC03• Inset shows inter-crossing of the plots suggesting decompositionprocess to be dose sensitive and dual role of themechanisms of energy transfer. G(NOi) I valuesdecrease rapidly for the lower mol"% of the ni-trates (- 5 mol %). For higher concentrationrange, however, decreasing trend becomes slow.Again, similar to the case of sulphate system, dif-ferent mechanism seems to operate in the lower

16 INDIAN J CHEM. SEe. A, JANUARY 1996

(< 2 mol %) and higher composition range(~5 mol%).

A cursory look at the data in Table 1 suggestsmanifold increase (by 2-3 orders of magnitude) inG(N02-) values on adding .large amounts of car-bonate. Enhancement in G(NOi) is more signifi-cant in 0.5-5 mol % of the nitrate in both thecases. Also the rate of decrease in G{N02-) be-comes slow at higher absorbed doses (Fig. 3). Thisdecreasing trend may be explained due to buildingup of trapped defect species in carbonate lattice.The number of defect species increases with ab-sorbed dose and reach saturation at a certain dosedepending upon concentration of the nitrate in thebinary mixture. At this stage, radiation annealingmay occur causing decrease in G values", There-fore, energy transfer ability of the additive maycease beyond an absorbed dose when saturationlevel reaches. ESR spectra of y-irradiated SrC03and SrCOj + 0.5 mol % Sr(NOjlz systems at 50kGy in Fig. 6 confirm the presence of CCi(gav= 1.9980)31 and N05- (gav= 2.010)24 radicalspecies. Natarajan et al." have observed the for-mation of radical species CO;, O2- and COi forirradiated BaC03. In order to further confirm theformation of radical species in strontium and bar-ium carbonates and their respective binary mix-tures with 1 mol % nitrate, TL studies were carri-ed out.at different doses (Table 3). Earlier, Natar-ajan et aL31 observed the TL peaks at 383, 430and 570 K in gamma irradiated BaCOj and as-signed to the formation of O2-, CO; and CO2- re-spectively. We have observed three. peaks at 380;430 and 570 K for BaC03 at 270 kGy (Fig. 4C)in confirmation with the observations of Natarajanet al.", However, in the case of binary mixturewith 1 mol % Ba(N03)2 (Fig. 4C) a third peak at605 K was observed which may be due to CO2-.

They have suggested that CO2- could be stable

even after 570 K. Thus, all these evidences sug-gest the formation of radical species CO;, CO

2-

and O2- in both the carbonate matrices. Theseradical species may interact with N03- causing en-hanced decomposition". A probable mechanismmay be represented as follows:

Primary process;

COj -.. C03- , CO2- , O2- , e - etc.

Secondary process;

N03- + CO]- ~ NO*] + COi-

CO2 + NOj- ~ N02- + CO}-

fA)

t

... (11)

... (12)

... (13}

Fig. 6-Room temperature ESR spectra for (A) srC0J; (B)SrCOJ + 0.5 mol % Sr(NOJ)2 after y-irradiation at an ab-

sorbed dose of 50 kGy.

System Dose Peak Intensity/ Probable Reference(kGy) temperature 10 mg x 10-7 radical

(±lOK) species

SrS04 100 425 30 0; 26SrS04 + Sr(NOJlz 100 420 33 0; 26BaC03 270 380 98 0; 31

430 (shoulder) 36 CO; 31570 50 CO; 31

BaCOJ + Ba{NOJlz 270 380 86 0; 31450 (shoulder) 28 CO; 31

605 85 CO; 31

A8!iOR8Eo DOSE.so loGyOPPH t

CO2

(8) ..., 'SOG'

H IJ'PH .UOHG

t

Table 3-Peak temperatures and intensities in TL glow curves of gamma irradiated sulphates and carbonates and their binarymixtures with I mol % nitrates

rosm et aL: GAMMA RADIOLYSIS OF BARIUM & SlRONTIllM NITRATES 17

Table 4-A comparison of G(NOi) values for alkaline earthmetal and alkali metal nitrates dispersed in respective carbo-nate and sulphate matrices

System

BaC03 + 2 mol % Ba(N03hKZC03 + 2 mol % KN03

SrS04 +0.5 mol % Sr(N03hBaSO. + 0.5 mol % Ba(N03hNazSO. + 0.5 mol % NaN03

KZS04 +0.5 mol % KN03

srCo3 + 5 mol % Sr(N03hKzCO, + 5 mol % KN03

Oose(kGy)

100100100100100100230225

G(NOi

122288 (ref. 9)

200360

4000 (ref. 9)1300 (ref. 9)

1065 (ref. 9)

o; + NO; -NOj- +02

NO.3 + NO~- - 2N02- + O2

... (14)

... (15)

Other reactions remaining same as in the case ofsulphate matrix.

A comparison of G(NOi ) values for(Sr(N03h + srC03) and (Ba(N03h + BaC03) withthat of (KN03 + K2C03) binary mixtures (Table 4)suggests alkaline earth metal carbonates to be lessefficient media for energy transfer. However, whencompared amongst themselves, BaC03 seems tobe better medium similar to the case of BaS04•

Further, a comparison of data in Tables 1 and 4and of Figs. 1 and 2 suggests that G(NOi) valuesin carbonate matrix are higher by more than orderof magnitude compared to respective sulphates.Thus, carbonates are better media for energytransfer than sulphates. This is in accordance withour earlier observations for alkali metal nitratesv".The enhancement in decomposition is explaineddue to the interaction of the radical species of sul-phatesl carbonates and the nitrate. ESR and 11evidences support the formation of radical species.

AcknowledgementWe are thankful to Drs M D Sastry and V Na-

tarajan of Radiochemistry Division, BARC, Bom-bay for ESR measurements. One of us (NGJ)thanks the CSIR, New Delhi, for the award of asenior research fellowship.

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