Vacancies in the solids of low molecular weight ... - Springer Link

Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 210, No. 2 (1996) 599-.605

Jointly published by Elsev&r Science S. A., Lausanne and Akaddmiai Kiadd, BtMapest

VACANCIES IN THE SOLIDS OF LOW MOLECULAR WEIGHT ORGANIC COMPOUNDS OBSERVED BY POSITRON ANNIHILATION Y. ITO,* HAMDY F. M. MOHAMED, *+ M. SHIOTANI**

*Research Centerfor Nuclear Science and Technology, The University of Tokyo, Tokai, lbaraki 319-11 (Japan) **Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739 (Japan) (Received August 26, 1996)

PAL were measu red for severallow molecular organic compounds, normal and cyclic-hydrocarbons and their pertluorinated ones, from room temperature down to about 40 K, mid information about the vacancies in them has been extracted from l:3 and 13. Normally the size of vacancies were larger in the solids comprising larger molecules, showing that o-Ps can represent the vacancy size. In a special case of perfluorocyclohexane whose solid had to be prepared by sublimation the vacancy size was larger than expected. In all the solid molecules studied two o-Ps states could be obseived and their relative importance could easily change showing a hysteresis-like behavior. At further lower temperatures of around 40 K the o-Ps state with longer lifetime and larger intensity became overwhelming for all molecules. Also r 3 and 13 were not very sensitive to most of the phase transitions, including the melting points.

This study was motivated by an ESR study of radical-cations of organic molecules. 1 Simple organic molecules like cyclohexane (c-C6), methylcyclohexane (Me-cC6) and rnethylsilocyclohexane (Me-cSiCs) were dissolved as guest molecules in a matrix (or host molecules) of perfluorocyclohexane (PFCH, cC6Flz) or perfluoromethylcyclohexane (PFMCH, CFa-cC6Flt) and the solidified mixture was irradiated with gamma-rays. The positive charges produced by the irradiation are transferred to the guest molecules and excess electrons are trapped by the matrix. Hence radical-cations of the guest molecule are stabilized, and it can be observed by optical and ESR spectroscopy. 2 For exmple ESR spectra of the radical-cation of Me-cSiC5 varied with the temperature of the measurement. In an ESR spectral lineshape simulation in which calculation of exchange-broadened lineshape is contained, the frequency of the exchange oscillation of the C-Si-C bondings was involved. The frequency of the oscillation was found to be larger in PFCH than in PFMCH matrix, and this in turn suggests that the space provided for the Me-cSiC 5 cation-radicals is larger in the former matrix) +On leave from E1-Minia University, Egypt.

0236-5731/9o/US $15.0 Copyright 9 1996 AkadFmiai KiadF~,Budapest All rights reserved

Y. r i o et al.: VACANCIES IN THE SOLIDS OF LOW MOLECULAR WEIGHT

This is opposite to what is expected from a simple consideration of molecular size. Because PFCH is smaller in size than PFMCH the vacancy size, if any, should be larger for the latter. The experiments have been carried out with a hope find an answer to this paradox. Several new findings have been obtained during the course of the experiments that (1) o-Ps is not much sensitive to the thermochemical and the crystallographic phase changes, (2) Ps can take two different states, the o-Ps parameters of one of them being as large as those in liquid, and that (3) o-Ps appears to see vacancies in micro-cracks formed at low temperature solids.

Experimental In most cases the purest grade of the chemicals were purchased and used without further purification. For n-hexane the purest grade chemicals were further distilled, or mixture (hexanes) was used to see the effect of impurity. Excepting PFCH, raw liquid samples were put in a glass tube containing a positron source (::Na enveloped in thin Kapton film) and were evacuated by freeze-pump-thaw and then the glass tube was heatsealed. Since PFCH is solid at room temperature, it was Wansferred by sublimation into glass tube containing the positron source. The sealed glass tube containing the sample and the positron source was set in a low temperature cryostat designed for the PAL measurements. The PAL measurements were performed first in a cooling direction from RT down to about 40 K, and then in a heating direction from 40 K to RT at an interval of 10 degrees. At each temperature it took 7 hours to take one speclrum, and about 30 minutes to change and get next stable temperature. The temperature was controlled + 0.2 degree during the measurements.

Results The results for PFCH and PFMCH are shown in Fig. 1, where the size of o-Ps holes estimated by the TAO and ELDRUP's equation 4 is shown on the right ordinate. (1) The solid PFCH made by sublimation showed large r3 and 13 (point A in Fig. 1). However, the next experimental point was settled to a shorter r3 and larger 13. Apparently, the solid PFCH as formed by sublimation had a loose structure. The initial small 13 value should be an experimental artifact since the solid could not be packed well between the source and the glass tube, and many positrons must have annihilated in the wall of the glass tube. (2) The PAL parameters did not show any observable change at the transitions known from the literature. For PFMCH even the melting point was not reflected to the o-Ps parameters. 600

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(3) At temperatures between 140-120 K, the o-Ps parameters show a dramatic decrease, and at a further lower temperature of about 40 K they rose again. Hysteresis appears for the cooling and the heating runs, but the behavior is not unique. Similar data for the non-fluorinated molecules are shown in Fig. 2. For Me-cC6 o-Ps parameters do not change at the melting point. However, for cyclohexane and occasionally for n-hexane the melting point is reflected to PAL. From the experiments using n-hexane samples with different purity, it appears that the results are not due to 601

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Y. ITO et al.: VACANCIESIN THE SOLIDSOF LOW MOLECULARWEIGHT impurity. The o-Ps parameters rarely changed at the melting point. In one experiment, the liquid of pure n-hexane was rapidly solidified in liquid nitrogen (point B in Fig. 2) and, by repeated warming and cooling, non-transparent solid was prepared and PAL was measured. In this case v3 and 13 showed the lower values of Fig. 2 (point B'). The data for benzene (Fig. 2) are similar to the literature data.s The lifetime spectra of the solid benzene could be decomposed into only 2 components, but we plot it on the same graph as the data of v3 and 13 since it may be a kind of positronium. Our data for solid benzene agree with that by GOWOREK et al.6 in that the o-Ps intensity does not decrease eve~fiat very tow temperatures, but differs in detailed behaviors: our data do not show strong temperature dependence while in Reference 6 v3 was seen to decrease below 100 K and 13 showed oscillatory change. Most compounds excepting benzene and cyclohexane showed two different o-Ps states in the solid. In one state both % and 13 change smoothly from the liquid values as if corresponding to super-cooled state. Another is the state with short lifetime (1 - 1.4 ns) and small intensity. Transition from one of these two states to another takes place in a hysteresis-like behavior, but the temperature at which it occurs is not unique.

Discussion In the solids just below the melting point v3 is 2.7 and 3.2 ns for PFCH and PFMCH, which corresponds to the vacancy of 0.68 and 0.76 nm, respectively. Since the size (the long axis) of PFCH and PFMCH is 0.71 and 0.80 nm, respectively, they will not be accomodated in the pre-existing vacancies of PFCH and PFMCH. The vacancy size measured by o-Ps is larger for PFMCH than for PFCH. This is reasonable since the molecular size of PFMCH is larger, but is opposite to the expectation from the ESR study. This paradox is settled if we notice the large r 3 = 3.8 ns (corresponding diameter is 0.82 nm) in PFCH as prepared by sublimation (point A in Fig. 1). This is large enough to accomodate the radical cations of Me-cSiC5 and allow for the C--Si--C exchange oscillation. Since the sample for the ESR measurement was prepared by rapidly cooling the mixture of the guest molecules and the sublimated host PFCH, the space around the guest molecule may be large. The paradox may thus be attributed to the very fact that the solid PFCH was the one as prepared by sublimation. Second important point is the intensivity of o-Ps parameters toward phase changes. For PFCH the transition at 178 K observed by broadline NMR 7 is considered to be the rotational transition, and that at 168 K observed by calorimetry8 is presumed to involve internal molecular motion. For cyclohexane the phase transition at 186 K9 is not observed, too. For PFMCH the change of the o-Ps parameters occurs at temperatures close to but not exactly the same as the two transition temperatures at 174 K and 226 K.lO,II 603'


The intensitivily of o-Ps parameters toward melting point is the most notable of the present results. Such has rarely been reported. The appearance of the long-lived state in the solid should involve vacancies, and the hysteresis-like behavior suggests that such vacancies are controlled by a condition that is unstably realized. The vacancy in question may be similar to those that would be seen in super-cooled liquid. But it is not necessary to assume that the whole of the substance is in a super-cooled state. Even if the solid is mostly crystallized Ps may be trapped in disordered regions, if any. In the solids of low molecular weight compounds, Ps trapped in vacancies may be able to expand the vacancy size in a similar manner as it creates "Ps bubble" in liquids. In well developed good crystals, however, Ps will not find such site to create large vacancies. Existence of short lived o-Ps component of about 1 ns appears to be common for molecular solids and liquids. They are found in many molecular solids and assigned to dense packing in crystals 12 or to atomic size defects. 13 They have also been found in liquid molecular compounds and been assigned to secondary reaction products of Ps in the positron spur. 14 The short lived state ranging 1.0 - 1.4 ns in the present results may be correlated with them. We note that v3 values of these short lived components are although crudely~ reflecting the molecular size (hence the vacancy size), i.e., they are larger for the solids of larger molecules. The appearance of the long-lived and intense o-Ps state at very low temperatures below 40 K is also important. This component looks as if a super-cooled state is connected from liquid to the very low temperature. We have been performing PAL measurements for macromolecules in the same temperature range, too, but have not observed a similar phenomenon. In an old litemturO 5 an increase in the o-Ps lifetime and intensity in polytetrafluoroethylene was observed below 1.2 K, which, although the authors suggested posible phase transition~ has been left unclarified. We suggest that at the very low temperature micro-cracks start to be formed in the crystals and provide sites for Ps trapping and formation of "bubble-like" holes.

Conclusion The large vacancies in perfluorocyclohexane expected from the results of ESR measurements have turned out to be related with the particularly soft spongy structure as prepared by sublimation. This illustrates how the vacancy spectroscopy using Ps can be used for practical purposes. In normal solids composed of well relaxed good crystals the vacancy size measured byo-Ps lifetime roughly reflects the molecular size in accord with the observation of ELDRL,rP et al.16 In the solids of most of the molecules studied Ps can take two different states, one being "bubble-like" Ps state with the Ps hole size and the intensity resembling those in super-cooled liquid, and the other being Ps in 604


much smaller site and lower intensity. At further lower temperatures around 40 K Ps tends to take "bubble-like" state probably due to crack-easy structure of the crystals. These new aspects of o-Ps behavior may be particular to the solids of low molecular weight compounds, in which molecules can be displaced by Ps. We have recently started PAL measurements with much larger statistics than the present results and have been able to decompose into four lifetime components, confirming existence of short lived components in liquids and solids of the molecules studied. Detailed analysis of the four component analysis will be published in due course.

References 1. M. SHIOTANI, M. LINGREN, T. ISHIKAWA, J. Am. Chem. Soc., 112 (1990) 967. 2. M. SHIOTANI, Mag. Res. Rev., 12 (1987) 333; M. LINDGREN, M. SHIOTANI, Radical Ionic Systems-Properties in Condensed Phases, Kluwer, 1991, Chapter 15. 3. M. SHIOTANI, K. KOMAGUCHI, J. OHSHITA, M. ISHIKAWA, Chem. Phys. Lett., 188 (1992), 93; K. KOMAGUCHI, M. SHIOTANI, to be published in J. Phys. Chem. 4. H. NAKANISHI, Y. C. JEAN, in: Positron and Positronium Chemistry, D. M. SCHRADER and Y. C. JEAN (Eds), Elsevier, 1988, Chapter 5. 5. S. Y. CHUANG, S. J. TAO, AppL Phys., 11 (1976) 247. 6. T. GOWOREK, C. RYBKA, J. WAWRYSZCZUK, R. WASIEWICZ, Chem. Phys. Lett, 106 (1984) 482. 7. A. FRATIELLO, D. C. DOUGLASS, J. Chem. Phys., 41 (1964) 974. 8. H. J. CHRISTOFERS, E. C. LINGAFELTER, G. H. CODY, J. Am. Chem. Soc., 69 (1947) 2502. 9. J. G. ASTON, G. J. SZASZ, H. L. FINK, J. Am. Chem. Soc., 65 (1943) 1135. 10. A. DWORKIN, C. R. 269C (1969) 73. 11. BUU BAN, C. CHACHATY, M. RENAUD, R. FOURME, Can. J. Chem., 49 (1971) 2953. 12. D. LIGHTBODY, J. N. SHERWOOD, M. ELDRUP, Chem. Phys., 93 (1985) 475. 13. P. C. JAIN, M. ELDRUP, N. J. PEDERSEN, Chem. Phys., 106 (1986) 303. 14. T. HIRADE, O. E. MOGENSEN, Chem. Phys., 170 (1993) 249, and the references therein. 15. T. M. KELLY, K. F. CANTER, L. O. ROELLIG, Phys. Lea., 18 (1965) 115. 16. M. ELDRUP, D. LIGHTBODY, J. N. SHERWOOD, Chem. Phys., 63 (1981) 51.

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