The heat capacity Cm of polycrystalline fullerite C₆₀ doped with nitrogen has been measured in

the temperature interval 2–13 K. The contributions to the heat capacity from translational lattice

vibrations (Debye contribution), orientational vibrations of the C₆₀ molecules (Einstein contribution),

and from the motion of the N₂ molecules in the octahedral cavities of the C₆₀ lattice have

been estimated. However, we could not find (beyond the experimental error limits) any indications

of the first-order phase transformation that had been detected earlier in the dilatometric investigation

of the orientational N₂–C₆₀ glass. A possible explanation of this fact is proposed

Cassidy, D.

Dolbin, A.V.

Gadd, G.E.

Gurevich, A.M.

Kondrashev, D.S.

Moricca, S.

Sundqvist, B.

Terekhov, A.V.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

Fizika Nizkikh Temperatur, 2006, v. 32, No. 10, p. 1275–1277Short NotesLow-temperature heat capacity of fullerite C60 doped withnitrogenA.M. Gurevich1, A.V. Terekhov1, D.S. Kondrashev1, A.V. Dolbin1,D. Cassidy2, G.E. Gadd2, S. Moricca2, and B. Sundqvist31 B. Verkin Institute for Low Temperature Physics and Engineeringof the National Academy of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, UkraineE-mail: dolbin@ilt.kharkov.ua2 Australian Nuclear Science & Technology Organisation, Menai, NSW 2234, Australia3 Department of Physics, Umea University, SE-901 87 Umea, SwedenReceived January 23, 2006, revised March 2, 2006The heat capacity Cm of polycrystalline fullerite C60 doped with nitrogen has been measured inthe temperature interval 2–13 K. The contributions to the heat capacity from translational latticevibrations (Debye contribution), orientational vibrations of the C60 molecules (Einstein contribu-tion), and from the motion of the N2 molecules in the octahedral cavities of the C60 lattice havebeen estimated. However, we could not find (beyond the experimental error limits) any indica-tions of the first-order phase transformation that had been detected earlier in the dilatometric in-vestigation of the orientational N2–C60 glass. A possible explanation of this fact is proposed.PACS: 74.70.WzKeywords: heat capacity, doped fullerite, nitrogen, impurity effect.The doping of fullerites can affect their propertiessignificantly and thus extend their applications. Theimpurity effect upon the properties of C60 has beenstudied most extensively. In particular, the effects ofsome gases (He, H2, D2, Ne, Ar, Kr, Xe, N2) upon thethermal expansion and the structure of C60 at lowtemperatures have been studied in sufficient detail[1–3]. The most interesting results include in particu-lar the detection of the first-order phase transitionstimulated by the gas impurities in the orientationalC60 glass [2,3]. Unfortunately, until now the heat ca-pacity of the gas mixtures in fullerites has escaped theattention of researchers. However, it is interesting toknow how the dissolved gases influence the heat ca-pacity of C60 or how the first-order phase transition inthe glass manifests itself in the behavior of the heatcapacity. This knowledge is very important, too, be-cause it is practically impossible to avoid contamina-tion of C60 with air gases. We think that it is reason-able to start investigation of the low-temperature heatcapacity of the gases dissolved in fullerites with aN2–C60 solution because nitrogen is the main constitu-ent of air. This is the basic objective of this study.At room temperature C60 has a fcc lattice with oneoctahedral and two tetrahedral interstitial cavities perC60 molecule. The octahedral cavities are sufficientlylarge in size (4.14  [4]) to house molecules of manygas impurities, including, for example, N2 molecules,with a gas-kinetic diameter  = 3.7  [5].The sample was prepared from high-purity (99,99%)C60 powder with average grain size  100 mm (SES,USA). First, the powder was intercalated with N2 andthen compacted. The intercalation was performed atthe Australian Nuclear Science and Technology Or-ganization (ANSTO, Australia) under conditions ofP  200 MPa, T = 575 C and t = 36 h. The intercala-tion technique is described in [6]. The thermalgravimetric analysis results (ANSTO) showed that theoctahedral cavities of C60 were filled with N2 to prac-tically 100%. The N2-intercalated C60 powder was© A.M. Gurevich, A.V. Terekhov, D.S. Kondrashev, A.V. Dolbin, D. Cassidy, G.E. Gadd, S. Moricca, and B. Sundqvist, 2006compacted at Umea University, Sweden, by the tech-nique described in [1]. The investigation was startedtwelve months after preparation of the sample. As aresult, according to x-ray analysis, the N2 concentra-tion decreased to 20% [7].The heat capacity of the C60 sample intercalatedwith N2 was measured by the method of absolute calo-rimetry in the interval 2–13 K. The calorimetric cellwas a rectangular copper-foil plate with a germaniumresistance thermometer and a film heater (R = 100 )fixed on it. The sample, with a mass of 0.3215 g, wasmounted on the free side of the plate, covered with athin layer of vacuum grease. The mass of the calori-metric cell without a sample was 1.081 g. The heat ca-pacity of the cell was measured separately. The mea-surement error in the heat capacity of the sample was3–7% (depending on the temperature interval).The sample was cooled down to liquid helium tem-perature by cold conduction through the wires (with-out using helium gas as heat exchanging gas) in va-cuum 10–2 Torr at room temperature and 10–6 Torr at4.2 K. The heat capacity was measured in several se-ries (Fig. 1).The cooling to helium temperature took 24 hours.Then the heat capacity of the calorimetric cell withthe sample was measured in a stepwise heating proce-dure.We described the temperature dependence of theheat capacity using the following simplified scheme.The heat capacity measured in the interval of experi-mental temperatures was considered taking into ac-count the contributions from all types of motion: theDebye contribution, due to translational fullerite lat-tice vibrations and the Einstein contributions, due toorientational vibrations of the C60 molecules and theinfluence of the N2 impurity molecules. The experi-mental results are described fairly well using a Debyeterm with the characteristic temperature D = 45 Kand two Einstein terms with E1 = 37 K and E2 == 53 K (Fig. 2). The most reliable value for pure C60is D = 54 K in [1,8]. It is quite reasonable that ourD is lower because (i) the intercalation with N2 in-creases the molar volume of the C60 crystal and (ii)the added impurity increases the effective molarweight of the system.Our model ignores two other contributions to theheat capacity. One of them is the linear contributionto the heat capacity of glasses. It can be responsiblefor the discrepancy between the experimental and cal-culated data at the lowest temperatures of the experi-ment. The intramolecular vibrations of C60 moleculesalso contribute to the heat capacity of C60. Their con-tribution may account for the excess of the experimen-tal values over the calculated ones in the high-temper-ature region of our investigation.It is known from the literature [1–3,9] that thetemperature dependence of the thermal expansion ofC60 intercalated with some gases at the temperaturesof liquid helium and nitrogen has a hysteresis. Adia-batic calorimetry precludes measurement of the heatcapacity at decreasing temperature. However, a hys-teresis can appear in heat capacity measurement evenon heating provided that the thermal prehistory of thesample and the kinetic parameters of the experimentare modified. To detect a hysteresis, the measurementseries were made at different starting temperaturesand with different rates of heating to the starting tem-perature. The temperature interval («step») of mea-surement was also varied. The sample was cycled at acertain pre-assigned temperature. In this case the heatcapacity of the sample was measured at temperatureT1. The sample was then cooled down by 1–2 degreesto T2 and the heat capacity was measured at T2.1276 Fizika Nizkikh Temperatur, 2006, v. 32, No. 10A.M. Gurevich et al.2 4 6 8 10 1205101520T,KC,J/mol·KFig. 1. Molar heat capacity of polycrystalline C60 dopedwith N2. The measurement series are indicated by differentsymbols.0 40 80 120 1600,51,01,55 15 250,20,40,6T , K2 2T , K2 2C/T,J/mol·Km2C/T, J/mol·Km2Fig. 2. Calculated and measured temperature dependencesof molar heat capacity of polycrystal C60 doped with N2:measured N2–C60 heat capacity data (); calculatedN2–C60 heat capacity (curve).Finally, the sample was heated and the heat capacitywas measured at T1 again.The measured heat capacities are within the limitsdetermined by the experimental error. The data ob-tained suggest that the heat capacity of the N2–C60system is insensitive to the processes provoking thehysteresis effect. This may be due to the fact that, ac-cording to [1,2], the hysteresis is caused by a first-or-der phase transition between the orientational glassesbased on gas-doped fullerite. However, the differencebetween the molar volumes of the phases in the tem-perature interval 2–24 K is only 0.02% [1]. In thiscase the transition-related change in the heat capacitycan be smaller than the experimental error. It shouldbe remembered that the phase transition in glassesdoes not proceed at a constant temperature; it occursover a rather wide temperature interval. At the sametime the phase transformation is clearly indicated by achange in the linear expansion coefficient [1,2]. Thereason may be that that the phase transition consid-ered above is a tunnel transition during which thechange in the volume expansion coefficient can be sev-eral orders of magnitude larger than the relativechange in the heat capacity [10]. It correlates wellwith the very high Gruniesen coefficients in tunnel en-ergy spectra [11,12].To conclude, we emphasize that this study is thefirst attempt to estimate the effect of gas impuritiesupon the heat capacity of C60 and to find out the pos-sibilities of further research in this direction. Since themolecules of gas impurities have small sizes andmasses, we can hardly expect a significant impact ofthe dissolved gas upon the heat capacity of C60 at tem-peratures far from the phase transition interval. Ne-vertheless, the increase in the heat capacity of C60 atlow temperatures observed after introduction of 20%N2 exceeds considerably (by approximately 15%) theeffect that might be caused by the impurity-inducedchange in the molar volume and the effective mass ofC60.Our attempt to reveal the effect of the phase transi-tion on the heat capacity in the orientational glassN2–C60 has gone unrewarded.It is evident that further investigations should bemade on solutions with higher concentrations of gasimpurities in which the molecules have larger sizesand masses. With these conditions met, both the im-purity contribution and the effect produced by thephase transition are expected to increase appreciably.It is also necessary to improve the technique of calori-metric measurement, in particular, to ensure the possi-bility of long-term maintenance of the solution sam-ples at a constant temperature. We are planningfurther investigations to pursue this goal.We wish to thank V.G. Manzhelii and M.I. Ba-gatskii for helpful participation in a discussion of theresults.1. A.N. Aleksandrovskii, A.S Bakai, A.V. Dolbin, V.B.Esel’son, G.E. Gadd, V.G. Gavrilko, V.G. Manzhelii,S. Moricca, B. Sundqvist, and B.G. Udovidchenko,Fiz. Nizk. Temp. 29, 432 (2003) [Low Temp. Phys.29, 324 (2003)].2. A.N. Aleksandrovskii, A.S. Bakai, D. Cassidy, A.V.Dolbin, V.B. Esel’son, G.E. Gadd, V.G. Gavrilko,V.G. Manzhelii, S. Moricca, and B. Sundqvist, Fiz.Nizk. Temp. 31, 565 (2005) [Low Temp. Phys. 31,429 (2005)].3. V.G. Manzhelii, A.V. Dolbin, V.B. Esel’son, V.G.Gavrilko, G.E. Gadd, S. Moricca, D. Cassidy, and B.Sundqvist, Fiz. Nizk. Temp. 32, 913 (2006) [LowTemp. Phys. 32,  (2006)].4. M.S. Dresselhaus, G. Dresselhaus, and P.C. Eklund,Science of Fullerenes and Carbon Nanotubes, Aca-demic Press, San Diego, California (1996).5. V.G. Manzhelii, M.A. Strzhemechny, Yu.A. Freiman,A.I. Erenburg, and V.A. Slusarev, Physics of Cryo-crystals, AIP Press, American Institute of Physics,Woodbury, New York (1997).6. G.E. Gadd, S. Moricca, S.J. Kennedy, M.M. El-combe, P.J. Evans, M. Blackford, D. Cassidy, C.J.Howard, P. Prasad, J.V. Hanna, A. Burchwood, andD. Levi, J. Phys. Chem. Solids 56, 1823 (1997).7. A.I. Prokhvatilov, private communication.8. N.A. Aksenova, A.P. Isakina, A.I. Prokhvatilov, andM.A. Strzhemechny, Fiz. Nizk. Temp. 25, 964 (1999)[Low Temp. Phys. 25, 724 (1999)].9. A.I. Prokhvatilov, N.N. Galtsov, I.V. Legchenkova,M.A. Strzhemechny, D. Cassidy, G.E. Gadd, S. Mo-ricca, B. Sundqvist, and N.A. Aksenova, Fiz. Nizk.Temp. 31, 585 (2005) [Low Temp. Phys. 31, 445(2005)].10. Yu.A. Freiman, Fiz. Nizk. Temp. 9, 657 (1983) [Sov.J. Low Temp. Phys. 9, 335 (1983)].11. C.R. Case, K.O. McLean, C.A. Swenson, and G.K.White, Thermal Expansion-1971, AIP ConferenceProc., New York (1972), p. 312.12. C.R. Case and C.A. Swenson, Phys. Rev. B9, 4506(1974).Low temperature heat capacity of fullerite C60 doped with nitrogenFizika Nizkikh Temperatur, 2006, v. 32, No. 10 1277

Low-temperature heat capacity of fullerite C₆₀ doped with nitrogen

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Low-temperature heat capacity of fullerite C₆₀ doped with nitrogen

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