J. Phys. Chem. 1981, 85,3514-3523
3514
Chemical Effects of Nuclear Transformations in Mixed Crystals. 7. Chemical Effects of the 35Cl(n,y)38CINuclear Reaction in K2ReCI,-K2ReBr8 Mixed Crystals' Horst Muller," ELH.
M. Dlefallah,* and S.
Martin
Chemisches Laboratoriumder Universitat, 0-7800Frelburg i.Br., Federal Republic of Qermany (Received: October 29, 1980; In Final Form: May 22, 1981)
The solid-state reactions occurring during the moderation of recoiling %C1,produced by the (n,?) reaction, have been studied in KZReCl6-K2ReBr6 mixed crystals. The main reaction products are Re36C1C162-, ReS6C1Br6", and %I-, but the more intimately mixed species ReWC1C1,Br,2- (n = 1,2,3,4)are found in significant amounts. The production of the different recoil-labeled species can be explained by elementary impact models: 6% of the recoils do not leave their original lattice site (primary retention); between 6% and 23%, dependent upon the mixed-crystal composition,appear as interstitials; 31-48% give rise to direct displacement reactions of one halide ligand; and 40% produce larger disruption by substitution of at least two halide ligands. The results have been compared with Rossler's 3sClrecoil experiments.
Introduction Radioactive recoil atoms, produced by nuclear reactions in suitable inorganic solids, can be used for examining solid-state damage by chemical methods. A family of compounds very well suited for this purpose is the hexahalogenometallate complexes, K2MeX6(Me = Re, Os, Ir, Pt;X = F, C1, Br, I). In some cases homogeneous mixed crystals covering the whole composition range may be prepared, e.g., KzReC16-K2ReBr6or K2ReBr6-K2SnCls.3*4 Such substances have been used to investigate questions as the recoil range, the size of the damage zone, the kind of disorder produced, the amount of recoil atoms trapped as interstitials, the annealing reactions, and primary retention.6fj It is true that the investigations cannot be performed without dissolution, but most of the damage centers of the solid are transformed in a simple manner; e.g., halide interstitials appear as free halide ions, and all ReC1,BrG_,2- species persist upon dissolution. This work was undertaken because of the great differences that have been obtained for 82Brand 38Clrecoils in KZReCl6-K2ReBr6mixed cry~tals.~-l~ As circumstances did not allow us to use short-lived nuclides, we have chosen the 36Cl(n,y)reaction producing the 300 OOO-yr %C1for our investigation. Experimental Section Materials and Irradiation. The preparation of the KzReC16-K2ReBr6mixed crystals has been described el~ewhere.~ Neutron activation of 100-mg samples sealed in quartz ampules has been carried out in the Karlsruhe (1) (a) Part 6: H. Miiller and D. Cramer, Radiochirn. Acta, 14, 78 (1970); (b) presented in part at the 9th International Hot Atom Chemistry Symposium at Virginia Polytechnic Institute and State University, Blacksburg, VA, Sept 18-23,1977. (2) On leave from the Department of Chemistry, Assiut University, Assiut, Egypt. Present address: Department of Chemistry, Zaqazig University at Banha, Banha, Egypt. (3) H. Miiller and S. Martin, 2. Anorg. Allg. Chern., 445, 47 (1978). (4) H. Muller, 2. Anorg. Allg. Chern., 336, 24 (1965). (5) H. Muller, J . Inorg. Nucl. Chern., 27, 1745 (1965). (6) H. Miiller, Radiochirn. Acta, 9, 167 (1968). (7) H. Muller and S. Martin, Inorg.Nucl. Chem. Lett., 5,761 (1969). (8) (a) R. Bell, K. Rossler, G. Stocklin, and S.R. Upadhyay, Report JOL-625-RC (1969); (b) R. Bell, K. Rossler, G. Stocklin and S. R. Upadhyay, J. Inorg. Nucl. Chern.,. 34, 461 (1972). (9) J. Otterbach, Report JUL-832-RC (1972). (10) K. Msler, J. Otterbach, and G. Stiicklin, J. Phyys. Chem., 76,2499 (1972). 0022-3654/81/2085-3514$01.25/0
research reactor FR 2. The details are summarized in Table V. Usually 10 samples have been irradiated simultaneously. They have been stored for at least 3 months at room temperature before further treatment. After this cooling period the main activities were 32P,36S,and 194Cs (from the Cs content of the potassium) besides the small 36Clactivity. Product Separation and Radiometric Analysis. The separation of the %C1recoil products was accomplished at room temperature by column chromatography. A glass column of 20-mm inner diameter and 500-mm length was filled with DEAE Cellulose SH 0.85 (Serva, Heidelberg). For the elution we used 1.8 M HC104 (3-bar pressure, 480 mL h-' flow rate). At the beginning of each separation, a 5-mL solution containing 50 mg of the activated material together with 50 mg of a mixture of all KzReBr,CIG_, complex salts in 1.8 M HCIOI was injected. The KzReBr,CIG_,mixture was prepared by heating mixtures of KzReC16and KzReBr6in glass ampules for 24 h at 400 O C . When the reaction products of experiments with different starting ratios were mixed in a proper way, the final mixture contained the seven species in comparable amounts. The injected solution contained in addition 1 mg of KBr and 1 mg of KRe04. After leaving the column, the solution was passed continuously through a photometer adjusted to 217 nm. Without the addition of the inactive carrier substances, only ReC1,'- and ReBr2- could have been located. Figure 1 shows a typical elution diagram. The eluate was collected in 5-mL portions, and those containing the same recoil-labeled species have been assembled. From these fractions the %C1has been separated in small volume by using the following reactions: Re36C1C1,Br5-n2-
(1)CT + Br(2) OH-+ H202
Reo4- + 36Cl-+ Cl-
(3) H+
+ Br-
-+ &Nos
Ag36C1.1+ AgCll + Agl iS6Cl-+ C1- + Br-J
AgBrl After addition of 3 mL of 1 M KC1 + 3 mL of 1 M KBr + 5 drops of H202(30%)+ 10 drops of phenolphthalein, 16 M NaOH was added with stirring until the solution at first became red and then changed again to colorless. The solution was boiled for 5 min, 5 drops of H202(30%) was added, and the boiling was continued for 5 min further. After cooling to 40 "C and addition of 2 M HzS04until 0 1981 American Chemical Society
Nuclear Transformations in Mixed Crystals
The Journal of Physical Chemistty, Vol. 85, No. 23, 1981 3515
TABLE I: Numerical Evaluation of the Experimental Results Corresponding to the Equation Y = p o t p l y
36~1Re36C1Cl,zRe 36 ClCI,Br2Re36C1C13Br,2Re36C1C1,Br32Re 36C1C1Br,2 Re36C1Br,Z4
n-
1
Re36C1C1nBr,.n2~
Po = yo + 22.9 ( 1 7 ) t 5 . 7 (12) +4.8(8) +0.7 ( 9 ) +2.6 (6) +3.8 (6) t60.0(16) + 11.4 ( 1 9 )
P1 - 52 ( 1 0 ) t 4 8 (7) +17 (5) +29 (5) + l l( 4 ) +23 (4) -82(10) +87 (12)
AP,>
Ap,,
% -
%
7 21 17 116 22 17 3
18 15 29 18 31 16 12
27 18 24 17 25 13 51
4.0 2.8 1.9 2.0 1.3 1.5 3.9
17
13
12
4.5
APO, %
Pz +35(10) + 4 4 (8) -20 ( 5 ) -31(5) -15 (4) - 29 ( 4 ) t 2 1 (10) -103(12)
+ p2yZa
mean error of single measurement
Y,,,
+ 6.4
+ 97.0
t 1.5
- 1.6 - 1.0 - 1.4 - 1.2
-4.3
a Y is the calculated yield in percent; y is the mole fraction of K,ReCI,; ( ) indicates mean error of p , , p l , a n d p , in units of the last digit; A p , , A p , , and a p , are mean errors of the respective coefficients in percent;p, = Y ois the calculated yield for is the calculated yield for pure K,ReC16 (y = 1). pure KzReBr6(y = 0); Y,,,
for ReC162-and ReBr6'-. After neutron activation of the paper strips, activity has only been found at the positions of ReCl2- and ReBr62-; the activity of each of the five mixed species was smaller than our detection limit of 0.5% of the total activity.
t
200
400
600 volume ( m l l
0.5 1.0 1.5 time (h) Flgure 1. Chromatographic separation of mixed ligand complex ions ReCI,Br,-,2-, Reo,, Br-, and CI-.
the solution became acidic, 0.1 M AgN03 was added dropwise until all C1- and Br- was precipitated, and the silver halide was centrifuged down, washed 3 times with water, and centrifuged again. Water was then decanted, 500 mg of Zn dust and 10 drops of 3 M HzS04were added, and, after 30 min, 5 mL of 3 M HzSO4 was added. When all Zn had been dissolved, the precipitated Ag was removed by filtration, the solution was diluted to 20 mL, and aliquots were used for counting. All activities besides 36Clare removed by this procedure, and the yield of 36Cl-is quantitative. Counting of the 0.7-MeV 36Cl0-rays has been accomplished by a GM liquid counter, with 24 h for each sample. Activity rates were between 20 and 600 cpm (background rate, 11-13 cpm). Therefore, the statistical error of a counting rate of a sample is in most cases much smaller than 1'70 of the total activity. The reproducibility of the measured activities is usually within a 3% range or less, as can be seen from Figure 2a-d for parallel runs. In some cases the fraction leaving the column between the Cl-/Br- fraction and ReC12- has been investigated for its 36Clactivity, which was found to be negligible. This shows that during the runs no noticeable hydrolysis of Re36C1C1,Br5-2- species resulting in free 36Cl-occurs. It shows further that the concentration of labeled ligand deficient species in the crystal which should be transformed into aquation products must be very small. We have further shown that no gross ligand exchange occurs between ReC16'- and ReBr62- resulting in mixed ReC1,Br6-,z- species during the activations. For this purpose the activated samples have been dissolved without addition of inactive carrier and have been separated by electrophoresis. Colored spots have only been observed
Results and Discussion The radiochemical yields, Y , of the individual 36Cl-labeled products have been determined as functions of the mixed-crystal composition. Experiments where the separation was unsatisfactory for some or for all of the species have been omitted. Yields are expressed in percent of the total induced 36Clactivity and are shown in Figure 2a-e. As, contrary to Rossler'slo results for 38Cl,no significant differences have been found for unannealed samples and those annealed 5 h at 180 "C, all experiments are presented together. In order to find an unambiguous way for constructing the curves for the different yields, functions of second order with least-squares fit have been calculated. Table I shows the numerical results for the equation
y = Po + Ply + PzY2 where Y is the percentage yield of the respective labeled species and y is the mole fraction of KzReCI, in the mixed crystals. Included are the absolute and the percentage errors of the three coefficients po, pl, and p z , the mean error of the single measurement, and the extrapolated yields for mixed crystals with zero content (y = 0) of No K'ReC1, (Yo = po) and for pure (y = 1)K2ReC1,(Yloo), corrections have been applied to give rise to a zero-yield Y,, for the bromine-containing complex ions. These yields are mainly of negative sign but still in the region of the respective mean errors. The mean errors are mainly caused by the insufficient separation of the different %Nabeled species; their values show the extent of the overlapping. Rossler'slo results for 38Clrecoil products are included in Figure 2a-e. A qualitative difference is that Rossler observed thermal annealing at least for KzReC&-richmixed crystals while we have no indication of thermal annealing at all. A reasonable assumption is that in our experiments radiation annealing has taken place caused by the large accompanying radiation during the (n,y) activation; therefore, our results should be compared only with Rossler's annealing results. While the experimental results for the Re*ClCl?- yield are rather similar, the other yields are characteristically different: (1) Rossler's *C1- yield increases to 95% for vanishing KZReC&content while we found not more than 23%; the highest single value obtained was 30%. For mixed crystals with high K2ReC16 concentrations, the results are similar. (2) Fore Re*ClBr:Rossler's and our results are consistent for large K'ReC1, content. With decreasing KzReC16 content our yield in-
Mijller et al. A
\, \, 75yo
\, \
I
\ 50 -
\ 10
O S
6
6
'
\-"; P
25
J
50
75
i
100
Mol -% K 2ReCl6
E
25
50 MOL-%
75 K2ReCL6
40
100
Figure 2. Yield of 3sCI-labeledspecies resultlng from the 35Cl(n,y)3sCI nuclear process in K,ReCI,-K,ReBr,
mixed crystals: (X) without annealing;
(0) with annealing. Included are Rossler'sio curves for his 37Cl(n,y)%I experiments: (---) wbhout annealing; (.--.) with anneallng. No annealing has been observed for Re3'CIBrs2-.
creases, while Rossler found a decrease to 0% for vanishing K2ReC&concentration. (3) Rossler did not find more than 3% of the mixed species Re*C1C1,Br5-2- (n = 1,2, 3,4), while we found up to 30% for mixed crystals with similar amounts of the components and considerable yields for all concentrations. It is worthwhile comparing the results of the 36Cland 38Clrecoil reactions with the results of the 8zBrrecoil reactions in the same mixed-crystal system K2ReC16-K2ReBr6 as obtained by MUer and by R o s ~ l e r . ~The J ~ results are shown for comparison in Figure 3a-c. The summed yield of mixed species ReszBrBr,Clg-2- (n = 1 , 2 , 3,4) is
smaller than 5% and is not shown. Minor discrepancies may be caused by the different extent of annealing and should not be overestimated, but it is quite obvious that the 82Brrecoil results of Muller and Rossler and our 36Cl experiments show the same behavior while Rossler's %C1 experiments differ widely. Our %C1experiments can be explained in the same way as the 82Br experiments which have been discussed by using the solid-state physicist's language. Y(Re%lC&-) so is the primary retention which represehts that part of tke recoil 36Clatoms which did not leave their original lattice place or which recombined after a temporary bond
Nuclear Transformatlons In Mlxed Crystals
The Journal of Physical Chemistty, Vol. 85,No. 23, 1981 3517
25t- - - - - - - - - - - - - - - - 25
50 75 Mol -% KZReBr6
--.
100
14% of its energy and has a real chance not to be captured by the just-produced ligand vacancy but to escape and survive as an interstitial. Re36C1C162-and Re36C1Br62-are produced mainly by a displacement substitution of one C1 in ReC162-or one Br in ReBr2- by the 36Clrecoil atom. Two basically very similar mechanisms are discussed (a) direct displacement reaction (billiard-ball sub~titution)~ 36Cl- + Rex6’- Re3T1X?- + XX = C1, Br
-
and (b) delayed displacement reaction (after formation of a ligand vacancy still connected with the recoil atom in the cage of the surrounding lattice)&1° 36c1-+ (ReX50- + 36c1-)+ XRe3T1X?- XX = C1, Br
-
25
%
75
50 75 M o l - % K2ReBr6
100
15
100
I
h \
50 -
\
25 -
25
50 M o l - % K2 ReBrg
Flgure 3. Yield of 82Br-labeled species resulting from the 8’Br(n,y)82Br
nuclear rocess in K,ReCI,-K,ReBr, mixed crystals: (-) Muller, ’ Rossler‘g(- - -) without annealing and (.--.) with annealing. Experimental points have been omitted.
breaking with the remaining otherwise undisturbed R e C l p entity. The primary retention amounts to 6%. Although this value has been determined directly only for mixed crystals with vanishing K,ReC16 concentration, it is usually thought to be valid for all mixed crystals. Y(36Cl-)represents those recoil atoms which after the recoil event are located a t interstitial lattice sites or are forming other defects. During the dissolution of the irradiated crystals, all of these atoms appear as 36Cl-. The free-halide yield is 6% for pure K2ReC16and 23% for mixed crystals with vanishing K2ReCI, concentration. The increase of the free-halide yield with decreasing KzReC16 content can easily be understood: the elastic impact of a C1 upon another C1 results in a mean energy loss of 50% compared with 43% when Br is hit. Still more evident are the figures for the central impacts. During a C1 C1 impact 100%of the energy is transferred and the energetic primary C1 atom is left without any kinetic energy; during a C1- Br impact the energetic primary C1 atom retains
-
+
-
A more convincing formulation of the delayed displacement reaction must take into account that the ReX,oentity can react not only with the recoil atom but likewise with the lost ligand X-, all three usually being located in the same cage. If in a first approximation we disregard isotope effects and confine ourselves to pure K,ReC&, the probabilities of the two reaction paths should be equal. Taking into account the primary retention of 6%, we would expect 47% 36Cl-and 53% Re36C1C162-;the experiment showed 6 % and 94 % , respectively. This inconsistency can only be avoided by assuming that the unreacted %C1ions after some diffusion have repeatedly a new chance to react with other ReC12- or ReC150- entities. Before attempting to calculate the extent of the displacement reactions from our data, we have to discuss the mechanism of the formation of the mixed species Re36C1C1,Br5-,2- ( n = 1,2, 3, 4). Rosslerlo in his 38Clexperiments has found only a few percent for the sum of these four species, and similar results have been obtained for Res2BrBr,C15-,2- ( n = 1, 2, 3, 4) in the s2Br experim e n t ~ . ~InJ ~ our 36Clexperiments summed yields up to 30% have been obtained. For the formation of the more intimately mixed species, it must be assumed that processes producing more disorder than simple displacement reactions do occur. In the past the so-called “hot spot process” has been discussed as producing disorder resembling a melt with a resulting distribution of C1 and Br around the central Re atom controlled mainly by statisti~s.~J’The distribution of the four species considered here does not follow such a rule. More qualitatively we assume that in some processes the primary %C1produces more than one ligand vacancy-two or three at most and in rare cases even more-and that one of these vacancies is filled by the %C1recoil atom while the others are filled partly by ligands from surrounding complex ions and partly by the ejected ligands. We are on the way to develop a quantitative model for this concept discriminating between the different extents of disorder, The maximum yield of the mixed species amounts to 30% for mixed crystals with comparable amounts of both components. We must take into account, however, that. mechanisms of larger disorder will also produce some ReMC1Cl;- and Re36Cl13r52which cannot be distinguished from the direct displacement reaction. From our experimental figure of 30%, we estimate that the mechanism of larger disorder is operative for 40% of all %C1recoil events. Even for mixed crystals with compositions close to zero content of K2ReC16,11%of mixed species are formed. This can be understood only by assuming that for this part the %C1recoil atom is still bonded to its original Re central (11) G. Harbottle and N. Sutin, J. Phys. Chem., 62, 1344 (1968).
3518
The Journal of Physical Chemistry, Vol. 85,
No. 23,
Muller et al.
1981
TABLE 11: Estimated Maximum and Mean Recoil Energy of (n,7 )-Produced Nuclides nuclear reaction
ER(max), eV
ER(mean), eV
1096 528
650 294
35Cl(n,7)36C11231 37Cl(n,7)38C114-16
nuclear reaction
* IBr( n,7 )s2Br' Re( n ,-y )' Re'
ER(max), eV
ER(mean), eV
374 112
100 50
TABLE 111: Compilation of C1 and Br Recoil Experiments in K,ReBr, and K,ReCl, and Mixed Crystalsa free-halide yield (extrapolated) nuclear reaction 35Cl(n,r)36C1 (this work) 37Cl(n,y)38Cl( Rosslerlo.l') computer simulation (300-eV C1) ( R o b i n ~ o n 'and ~ Rossler17) 35C1(n,-y )36C11 37C1(n,7 ) 3 8 1 Br( n,7 )82Br ( Muller7) 81Br(n,y)82Brb(Rossler10v17) computer simulation (100-eV Br) (Robinsonls and RosslerI7) Br( n,r )82Br89'
direct displacement (extrapolated)
primary retention
pure substanceC
diluted substanced
pure substanceC
diluted substanced
mechanism of larger disorder
6 5 3, [ 41
[GI 39, ( 7 ) , [ I O 1 6
23 95, ( 9 5 ) 14
48 56 49
31 0
40
