The linear thermal expansion of compacted Ar-doped fullerite C₆₀(ArxC₆₀) is investigated at 2-12 K using dilatometric method. The thermal expansion of ArxC₆₀ was also studied after partial desaturation of argon from fullerite. It is revealed that argon doping resulted in the considerable change of the temperature dependence of the thermal expansion of fullerite. An explanation of the observed effects is proposed

Aleksandrovskii, A.N.

Esel`son, V.B.

Gavrilko, V.G.

Maletskiy, V.P.

Manzhelii, V.G.

Sundqvist, B.

Udovidchenko, B.G.

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Fizika Nizkikh Temperatur, 2001, v. 27, No. 3, p. 333–335Aleksan dro vskii A. N.,  Ga vr ilko  V . G., Ese l’ son V. B., Manzhelii V . G., Su ndqvist B., Udovidche nko B. G., and  Maletskiy V. P.Ar go n effe ct on th erm al expan sio n of fuller it e C6 0 at helium  t emp era tur esAleksa ndr ovski A.  N. , Gavrilko V. G., E sel’son V.  B.,  Manzhe lii V. G., Sundq vist B., Udovidch enko B. G., an d Malet skiy V.  P.A rg on eff ect on  t her mal expa nsion of fu ler ite C60 at he lium  tem per atur esLetters to the EditorArgon effect on thermal expansion of fullerite C60at helium temperaturesA. N. Aleksandrovskii1, V. G. Gavrilko1, V. B. Esel’son1, V. G. Manzhelii1,B. Sundqvist2, B. G. Udovidchenko1, and V. P. Maletskiy11B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine,Lenin Ave. 47, 61164, Kharkov, Ukraine 2 Umea University, Department of Experimental Physics, 90187, Umea, SwedenReceived December 26, 2000The linear thermal expansion of compacted Ar-doped fullerite C60 (ArxC60) is investigated at 2–12 Kusing dilatometric method. The thermal expansion of ArxC60 was also studied after partial desaturationof argon from fullerite. It is revealed that argon doping resulted in the considerable change of thetemperature dependence of the thermal expansion of fullerite. An explanation of the observed effects isproposed.   PACS: 74.70.WzA. N . Aleksandrovskii et al.We have already reported the detection andinvestigation of a negative linear thermal expansioncoefficient α of fullerite C60 at helium tempera-tures [1,2]. The effect was tentatively attributed totunneling transitions between energetically equiva-lent orientations of C60 molecules. To test thisassumption, we have studied the thermal expansionof Ar-doped C60 at liquid helium temperatures. Theresults of these studies are presented in this commu-nication. In a fullerite crystal each C60 molecule isassociated with two tetrahedral and one octahedralinterstitial cavities [3] whose average linear dimen-sions are about 2.2 A°  and 4.2 A° , respectively [4].According to x-ray [5] and neutron diffraction [6, 8]data, the Ar atoms with the gas-kinetic diameter3.405 A°  [7] occupy only the octahedral cavities. Itshould also be noted that at 15 K the lattice pa-rameter of a saturated ArxC60 solution is 0.006 A°smaller than that of fullerite [6]. We assumed thatthe Ar atoms occupying the octahedral intersticeswould increase the potential barrier impeding rota-tion of the C60 molecules and thus diminish theprobability of rotational tunnel transitions and con-sequently the tunneling splitting of the groundstate of the molecules [2]. If this assumption iscorrect, the total negative thermal expansion∫ α dT should decrease and the region of negativeexpansion will shift towards lower temperaturesafter doping.An Ar-doped C60 sample was studied at 2–12 Kusing a high-sensitivity capacitance dilatometer [9]and with the same procedure as was applied to pureC60 earlier [1,2]. The sample was prepared by com-pacting high-purity (not worse than 99.98% C60)powder under about 1 kbar. The grain sizes were0.1–0.3 mm. The resulting C60 sample was a cylin-der 9 mm high and 10 mm in diameter. The thermalexpansion coefficient along the cylinder axis wasfirst measured at 2–12 K before doping. The evacu-ated sealed measuring cell with the sample has beenwarmed to room temperature and filled with argonunder atmospheric pressure. The doping lasted for19 days.When the doping process was completed, theAr-filled measuring cell with the sample was slowlycooled to helium temperatures. In this case both thephase transitions of C60 (at 260 and 90 K) occurredin an Ar atmosphere. Figure 1 shows the measuredcoefficients before (curve 1) and after (curve 2)Ar-doping. It is seen that the doping not only leadsto the expected decrease in the negative thermalexpansion and its shift towards lower temperatures© A. N. Aleksandrovskii, V. G. Gavrilko, V. B. Esel’son, V. G. Manzhelii, B. Sundqvist, B. G. Udovidchenko, and V. P. Maletskiy, 2001but that it also reduces strongly the (positive)thermal expansion coefficient above 5.5 K.It seems natural to assume that the Ar-inducedincrease in the barrier impeding rotational motionof the C60 molecules should also enhance the angu-lar dependence of the non-central forces acting uponthe C60 molecules. As a result, the frequencies ofthe orientational oscillations of the moleculesshould increase and hence the normal (positive)thermal expansion coefficient dependent on theseoscillations should decrease. This is what we ob-served experimentally at T > 5.5 K. The experi-mental results can thus be explained qualitativelyproceeding from the assumption that the atomic Arimpurity introduced to the octahedral interstices ofC60 suppresses the splitting of the ground state ofthe C60 molecules and modifies the orientationaloscillation spectrum of the molecules.It appears that dissolved Ar atoms influence verystrongly the thermal expansion even though theyare able to move quite freely inside the octahedrallattice interstices. We should also bear in mind thatin our experiment the Ar atoms occupy only a partof the octahedral interstices. We did not estimatethe quantity of the dissolved Ar. According toMorosin et al. [10], neon occupies only 21% of theoctahedral interstices under identical conditions(room temperatures, atmospheric pressure). Takinginto account that in a simple cubic lattice each ofC60 molecules is surrounded by six octahedral inter-stices, the 21% occupancy implies that with ran-domly distributed impurity atoms about 75% of theC60 molecules have Ar atoms nearby. However,because the Ar atoms are larger than Ne atoms, thisnumber must be considered an upper limit of occu-pancy only.Another important consideration here is that webelieve that only a small fraction of the C60 mole-cules (the so-called «defects») for which the rota-tion-impending barrier Uϕ is quite low contributesto the negative thermal expansion of fullerite [2].Correspondingly the doping-induced change in thenegative thermal expansion is determined only bythe Ar atoms neighboring these «defects». At thesame time, the positive thermal expansion is af-fected by all the dissolved Ar atoms.To obtain more information, we studied how thethermal expansion coefficient changed when thedoping atoms were removed from the sample. Forthis purpose, the measuring cell with the samplewas warmed to room temperature and evacuated to1⋅10−3 mm Hg. The gas evacuation at room tem-perature lasted for 3 days. The thermal expansionwas then measured at low temperatures. The resultsare shown in Fig. 1 (curve 3). It is seen that thethermal expansion coefficient changes only slightlyabove 5 K but below 3.5 K the negative thermalexpansion again has the minimum typical for un-doped C60 . The measuring cell with the sample waswarmed again to room temperature and gas evacu-ation was continued for 42 days. The thermal ex-pansion coefficient was then measured with theresults shown in Fig. 1 (curve 4). Note, in particu-lar, that after a total of 45 days evacuation of argonthe «high-temperature» part of the thermal expan-sion coefficient was restored completely. The nega-tive thermal expansion in the range 2.5–5 K, how-ever, still differed from that of the initial puresample.This can be accounted for assuming the follow-ing. The octahedral voids adjacent to defects, i.e.,C60 molecules with low Uϕ barriers, form deeperpotential wells for the impurity atoms than theregular octahedral interstices do. It is thereforemore difficult to remove the impurities from thesenear-defect regions, and the residual impurities con-centrated around defects are precisely those respon-sible for the negative thermal expansion of fullerite.There is also another fact supporting this assump-tion. The thermal expansion coefficients α of all ourC60 samples, both used in Refs. 1, 2 and in thisstudy, agree quite well above 5 K but they differconsiderably in the temperature region where α isnegative. These samples were prepared under diffe-rent conditions and vary in quality and in theamount of residual impurities.The proposed qualitative explanation of the ef-fect observed cannot replace a consistent theoreticalFig. 1. Temperature dependences of the thermal expansion ofcompacted fulletite C60 : pure fullerite before doping (1); Ar-doped fullerite (2); fullerite after evacuation of Ar for 3 days(3) and for 45 days (4).A. N. Aleksandrovskii et al.334 Fizika Nizkikh Temperatur, 2001, v. 27, No. 3interpretation. Several interesting ideas have beenpublished to date, which are concerned with atentative mechanism of the negative thermal expan-sion of molecular crystals [11]. In the case offullerite, we decide in favor of our explanation sinceit accounts for the unusually high Gruneisen coeffi-cients, which were observed experimentally.We wish to thank Yu. A. Freiman, V. M. Lok-tev, V. D. Natsik, A. I. Prokhvatilov, and M. A.Strzhemechny for participation in the discussion ofthe results.The authors are indebted to the Science andTechnology Center of Ukraine and the Royal Aca-demy of Sweden for support. 1. A. N. Aleksandrovskii, V. B. Esel’son, V. G. Manzhelii,A. Soldatov, B. Sundqvist, and B. G. Udovidchenko, Fiz.Nizk. Temp. 23, 1256 (1997) [Low Temp. Phys. 23, 943,(1997)]. 2. A. N. Aleksandrovskii, V. B. Esel’son, V. G. Manzhelii,A. Soldatov, B. Sundqvist, and B. G. Udovidchenko, Fiz.Nizk. Temp. 26, 100 (2000) [Low Temp. Phys. 26, 75(2000)]. 3. P. A. Heiney, J. Phys. Chem. Solids 53, 1333 (1992). 4. C. H. Pennington and V. A. Stenger, Rev. Mod. Phys. 68,855 (1996). 5. G. E. Gadd, M. James, S. Moricca, P. J. Evans and R. L.Davis, Fullerene Sci. and Technol. 4, 853 (1996). 6. G. E. Gadd, S. J. Kennedy, S. Moricca, C. J. Howard,M. M. Elcombe, P. J. Evans, and M. James, Phys. Rev.B55, 14794 (1997). 7. V. G. Manzhelii, A. I. Prokhvatilov, I. Ya. Minchina, andL. D. Yantsevich, in: Handbook of Binary Solutions ofCryocrystals, Begell House Inc., New York, Wallingford(UK) (1996). 8. G. E. Gadd, P. J. Evans, D. J. Hurwood, J. Wood, andM. James, Chem. Phys. Lett. 261, 221 (1996). 9. A. M. Tolkachev, A. N. Aleksandrovskii, and V. I. Kuch-nev, Cryogenics, 9, 547 (1975).10. B. Morosin, J. D. Jorgenson, S. Short, G. H. Kwei, andJ. E. Shirber, Phys. Rev. B53, 1675 (1996).11. V. M. Loktev, Fiz. Nizk. Temp. 25, 1099 (1999) [LowTemp. Phys. 25, 823 (1999)].Argon effect on thermal expansion of fullerite C60 at helium temperaturesFizika Nizkikh Temperatur, 2001, v. 27, No. 3 335

Argon effect on thermal expansion of fullerite C₆₀

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Argon effect on thermal expansion of fullerite C₆₀

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