JOSEPH CUNNINGHAM
30
Radiation Chemistry of Ionic Solids. IV.
Modifying Nitrate
Radiolysis in Crystals by Compression’
by Joseph Cuuningham2 Physics Division, IIT Research Institute, Chicago, Illinois, and Chemistry Division, Argonne N a t M Laboratory, Argonw, Illinois (Received February 89, 1966)
Samples of crystalline alkali metal nitrates maintained under hydrostatic compression by several hundred atmospheres of rare gases were exposed to Cow y irradiation simultaneously with unpressurized samples. Growth of radiolytic product was followed by chemical solution analysis by two methods after irradiation. Compression-induced changes depend upon whether the kinetics being observed correspond to: region 0, growth of primary product a$ low dose, which W M increased by 15% in NaN03 irradiated a t 77°K. ; region I, diffusion-controlled growth of secondary products at intermediate doses, which was decreased by 4,9, 23, and 19% in KNO,, NaN03, RbN03, and CsNOa irradiated at 300°K.; or region 11, growth of secondary product altered by crystal modifications at high dose, which was unaffected by compression. Product growth in region 0 at 77°K. or region I at 400°K. was sensitive to low partial pressures of oxygen. Detailed mechanismsof nitrate radiolysis are presented to account for the pressure and oxygen effects.
Introduction Radiolysis of b r o m a t e ~ , ~ Jand ~ perchloratesllJ2 of the alkali metals have been described in terms of new covalently bonded groups found after irradiation, e.g., NO NOS- from NO,-, or ClOzC103C10ClOz from C104-. Efficiency of utilization of radiation energy to produce new groupings from these and other polyatomic anions varies markedly according to the cation, but interpretations of such varying efficiencies of radiolysis have not considered the role of the cations directly. Rather, the probability of radiation-induced unimolecular dissociation of the anions by excitation or electron attachment have been loosely correlated with the closeness of packing of the different crystal lattice^.^^^^*^^ Such correlations have been expressed in terms of “free space” or “free volume” (unit cell volume minus ion core volumes) which is an inverse measure of the crystal packing and has been represented as acting to regulate the probability of dissociation of radiation-activated
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anion^.^,^
It is desirable, if possible, to relate the efficiency of utilization of the radiation energy to a more specific property than this empirical “free space” parameter or, The Journal of Physical Chemistry
conversely, to identify a more basic property of the lattices from which the observed correlations with free space may arise. The extent of cation-anion interactions in the various lattices is one such property deserving study. Alkali metal nitrates were chosen for this study because (a) their radiolyses have been studied in detail, (b) methods of vibrational spectroscopy have been applied with particular succe~sto this series of salts for (1) Work was supported by IITRI Research Fund but executed, in part, with the experimental facilities of Argonne National Laboratory. (2) National Physical Laboratory, Teddington, Middlesex, England. (3) J. Cunningham, J. Phys. C h m . , 65, 628 (1961). (4)J. Cunningham, %%id., 66, 779 (1962). (6) J. Cunningham, ibid., 67, 1772 (1963). (6) H.G.Heal and J. Cunningham, Tram. Faraday Soc., 54, 1366 (1968). (7) H.Zelda and R. Livingston, J. C h m . Phya., 35, 663 (1961). (8)E. R. Johnson and J. Forten, Discussions Faraday SOC.,31, 238 (1961). (9) G. R. Boyd, E. W. Graham, and Q. V. Larson, J . Phys. Chm., 66,300(1962). (IO) G. E. Boyd and Q. V. Larson, ibid., 68, 2627 (1964). (11) H.G. Heal, Can. J. Chm., 37, 979 (1969). (12) L. A. Prince and E. R. Johnson, J. Phys. C h . ,69,369 (1966).
MonIFnNG
NITRATE RAnroLYsIs IN
31
CRYSTALS BY COMPRESSION
investigations of cation-anion interactions,” and (c) published values support, as follows, a correlation h e tween cation-anion interactions and efficiency of radiolysis in the salts: James has shown that the fw quency, vl, or the force constant, F,, for the symmetrical stretching vibrational mode of the nitrate ion can be correlated” with the cationic polarizing power for the alkali metal nitrates. He has interpreted increases in v1 and in Fl with increasing cationic polarizing power in terms of increased polarization of the anion. According to this interpretation v1 provides a measure of cation-anion interaction. If the efficiency of radiolysis of the nitrate ion were also related to the degree of cation-anion interaction, some correlation between GNO~.and v, would be expected, and a plot of reported values is, in fact, linear.I4 Variations in cation-anion interactions also occur when ion separations vary, e.g., upon melting nitrates or subjecting them to compression. Changes in efficiency of nitrate radiolysis by meltinga and by compression have been r e p ~ r t e d . ~ ~The J ~ present paper describes further experiments to determine the effect of compression upon nitrate radiolysis. Experimental Section Compression Technique. Rare gases or nitrogen were used for maintaining the solids under compression during irradiation with Co” y rays. Precautions were taken to ensure that the history and treatments of samples irradiated under kilobar compressions by rare gases dfiered in no other detail from the history and treatments of reference samples similarly irradiated. Equipment for applying and maintaining compression employed a screwdriven, oil-free, 0-ring-sealed pressure intensifier with which inlet pressures of 25W3500 p.8.i. obtained from gas cylinders were increased to 9OOO15,000 p.8.i. The compressed gas was introduced through 0.006-in. i.d. stainless steel capillary tubing into pressure-tight containers with the designs and dimensions shown in Figure la orb. Batches of eight samples were pressurized simultaneously and equilibrated for times greater than 30 min. before the capillary tubmg was crimped shut at three positions and cut above the third crimp. Helium was the preferred compression gas because of ease of testing the sealed capsules for I d s , and, if leakage was detected, the ferrule seals either were opened briefly to reduce the pressure to 1 atm. and then resealed OT were removed. In either case these pressure “rejects” were valuable references for study of radiation effectsin the absence of pressure since their case histories were identical with those of pressurized samples. Sample holders retaining pressure, pressure “rejects,” and an additional eight samples
R
P
Figure 1. Details of vessels for maintaining kilobar compression during irradiation. (a) Small preasure vessels with capacity of 10 mg.: preasure bombs (B) for maintaining !dobar compression over 10 mg. of solid contained in platinum vials (P); tightening screw ( S ) compresses ferrule (F) to make fust pressure-tight seal; compression gas introduced through 0 . W i n . i.d. capiUary tubing (T)is retained by crimping and cutting T. (b) Larger pressure vessels with capacity 5w mg.: pressure bombs (B) for maintaining kilobar compression over 300 mg. of solid contained in thinwalled metal vials (V); screwing the large hexagonal nut (N) makes the first pressure-tight seal at surf= ( S ) using a gold washer (not shown); the metal plug (P) mmes as an adapter to permit the me of tubing for gas int,roductionand senl-off.
in open containers identical in all other respects with those used for compression, were tightly clamped into 16 reproducible positions for y irradiation at 300 or 400°K. in a vessel described previously.’ After irradiation, the pressure remaining in the unopened vessels was measured by expanding the gas at kilobar pressures ahove the samples into an evacuated pressuremeasuring system of cu. 5000 times greater volume. Measured pressure rises, equal to those from sealed samples tested directly after sealing, were taken as sufficient evidence that no additional large (>20%) loss in gas pressure had occurred during irradiition, through leakage of gas from the vessel or occlusion of the gas. Only data from samples in holders thus indicated to hold more than 80% of the original compressed gas are presented here. (13) D. W. James. “Vibrational spectra of Molten Salts,“ O R N G 3413, July 1963. (14) Values for n and & o r at the lowast reported temperat-
should be used for such plota in order to minimize aide eU&
at
highher temperatures and m e to be found in: R. A. Sohroader. C. E. Weir, and E. R. Lippinoott, J . Rea. Natl. Bur. Sld.. A66,407 (1982), or ref. 3, respectively. (15) J. Cunningham and L. R. Steele, Phvs. Reo. Leuers, 9, 47 (1982). (16) T-A.
Chen and E. R. Johnson, J . Phua. C h . ,66,2068 (1962).
Volume 70. N u d m 1 Jonuorv 1966
JOSEPHCUNNINGHAM
32
The composition of the compression gases over the solids was varied as desired by: evacuating the system and sample holders; introducing oxygen or nitrogen or air into the evacuated system to the desired gauge pressure (typically 30 to 100 p.s.i.g.) and maintaining the gas in the sample section while the system was re-evacuated ; introducing helium, argon, or extra dry nitrogen into the system to 3500-2500 p.s.i.g. and, hence, into the sample section; increasing the system gauge pressure to 10,00(314,000 p.s.i. with the compressor. Composition of the compressing gas is specified in the text by the partial pressure in atmospheres of the gas present; e.g., Po2 = 5 PAr = 700 indicates 5 atm. of oxygen in 700 atm. of argon. Commercially available nitrates were purified by recrystallization or by fractional precipitation from aqueous solution with acetone followed by recrystallization. Samples were ground in an agate mortar and dried in vacuo at 340-360°K. Radiolysis of nitrates may be divided into three regions, as is done in Table I on the basis of published
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Table I: Dose Ranges of Initial (0), Intermediate (I), and High-Dose (11),Radiolysis of Nitrates Region I 1021 e.v./g.
Region I1 1021 e.v./g.
0.4+18 0 . 5 - , ( 5 f 2) 4 5 4 4 3, 6, 8
>25 >2 >5
Region 0 1021
Salt
e.v./g.
KNOi
0+0.3 040.4
RbNOa CsNOa Ref.
5
NaNOn
>4 8
rt, 545 400 437 430 30
a Ttis the temperature a t which transformation to the first new crystal phase is complete upon heating.
results. Initial radiolysis, here denoted as region 0, is characterized by a total product of ca. mole/ mole of irradiated salt and by distribution of this product between free-radical and nonradical species. The product may be expressed either, as M(NOz-), the number of nitrite ions which would produce that number of nitrous acid molecules detected by Shinn’s analysis upon dissolution of the irradiated salt in acidified aerated water, or, as M’(NOz-), the nitrite ions equivalent to A [Ce4+J detected upon dissolution of the solid in an aqueous solution 2 X lo-’ M in Ce4+arid 0.4 M in HzS04. Free-radical products including NO, N02z-, and NO?-, are detected more efficiently in the M’ (NOz-) product, but the lower sensitivity of this ceric analysis made necessary the dissolution of 250 mg. to detect the region 0 product by this method. The Journal of P h y s k ? Chemistru
The vessels shown in Figure l b were used for region 0 irradiations and were totally immersed in liquid N2 for 77°K. irradiations. Small samples, 5-20 mg. in platinum holders, were adequate for detecting the M(N02-) product at all doses and were enclosed, together with the 250-mg. samples, in Figure l b holders for region 0 doses or separately in Figure l a holders for region I or I1 doses. Results and Interpretation 1. Region 0, Primary Product Growth at Low Dose. Product detected in samples of NaN03 and KN03 irradiated at 77°K. under compression by helium (1000 atm. of helium compressed and sealed into the vessels at 300°K. would exert a compression of only ea. 250 atm. at 77°K.) are compared in Figures 2 and 3 with product detected in unpressurized samples. For KNOa, M’(N02-) or &‘(NO2-) data for pressurized and unpressurized samples fall upon the same lines Data for NaN03 within the estimated error *S%. indicate that 250 atm. compression had no detectable effect on the rate of M(N02-) formation in region 0 but did increase the rate of formation of N’(NO2-) by (17 I 5%). Some species detected by ceric analysis, which, however, Shinn’s method does not detect or detects with much lower sensitivity than the ceric analysis, must therefore be formed with increased efficiency a t 77°K. in NaN03 samples compressed by 250 atm. of helium during irradiation. The possibility was investigated of obtaining information on this 77”K.-pressure effect through values of the IM’(N02-) :M(NOz-) ratio, obtained by analyzing one part of the irradiated sample bp the ceric method and another by Shinn’s method. It mias observed, howeser, that prolonged exposure to air (during irradiation or for >30 min. at 300°K. after irradiation) invalidated this procedure because reaction with oxygen occurred and reduced the M’(NOZ-) :M(XO2 -) ratio toward unity. Data for M’(NOz-) in Figures 2 and 3 were therefore obtained by handling irradiated samples in an argon atmosphere. 2. Region I , Secondary Product Growth at Intermediate Doses. A comparison of the ratio of the rate of formation of ICI(NOZ-) in nitrate samples evacuated continuously during irradiation with that in samples Pair = 5 subject to compression by PEe = 700 was made in a preliminary p ~ b l i c a t i o n . ~Those ~ results and other data obtained in the same conditions are summarized in Table 11. Reductions in the rate of radiolysis were thus observed in all samples maintained under compression Pair = 5 . Results with the salts by P E ~= 700 enriched to 85y0in l*O did not indicate any significant
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MODIFYINQ NITRATERADIOLYSIS IN CRYSTALS BY COMPRESSION
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Table II: Ratios of the Rate of Product Formation in Pressurized Samples to That in Evacuated Samples at 300°K. Salt
NaNOs
Ratio"
KNOB
RbNOs
Case A: pure powdered samples, natural isotopic abundances, Pxe N 700 atm. 0.910 f0.013 0.955 f 0.009 0.777 f 0.024
Ratiob
Case B: salts 85% enriched in :*O, multiple recrystallization, Pxe 0.87 f0.03b 0.94 f0.029"
Ratiob
Case C: salt doped with 1% Ag+, Pxe 0.83 f0.OSb
-
700
+ P,i,
-
+ P,i,
700
N
+ P,i, N
-
CsNOs
5
0.804f0.032
5
5
" SufEcient data were obtained for these so that mean deviations were calculated by least mean squares fit to line y
=
bx.
* Insuf-
ficient data (
