Specific features of thermal expansion and polyamorphism in CH4–C60 solutions at low temperatures

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PostprintThis is the accepted version of a paper published in Low temperature physics (Woodbury, N.Y., Print). This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination. Citation for the original published paper (version of record):Bagatskii, M., Sumarokov, V., Dolbin, A., Sundqvist, B. (2012) Low-temperature heat capacity of fullerite C-60 doped with deuteromethane.Low temperature physics (Woodbury, N.Y., Print) The heat capacity C of fullerite doped with deuteromethane (CD 4 ) 0.4 (C 60 ) has been investigated in the temperature interval 1.2 -120 K. The contribution 4 CD ΔC of the CD 4 molecules to the heat capacity C has been separated. It is shown that at T ≈ 120 K the rotational motion of CD 4 molecules in the octahedral cavities of the C 60 lattice is weakly hindered. As the temperature decreases to 80 K, the rotational motion of the CD 4 molecules changes from weakly hindered rotation to libration. In the range T = 1. Keywords:Heat capacity, fullerite С 60 , rotational dynamics of CD 4 IntroductionThe discovery of new forms of carbon (fullerite, carbon nanotubes, graphene) has attracted much interest from researchers working in different fields of physics, chemistry and materials science. The interest is first of all provoked by the wide-range diversity of the physical and chemical properties of these objects. A comprehensive investigation of the physical properties of these materials is of great importance for both fundamental science and practical applications.The thermal properties of pure fullerite have been investigated in numerous studies. Unfortunately, there have been few studies of the dynamics of impurities in C 60 crystals.In the low temperature phase there is one octahedral and two tetrahedral cavities for each C 60 molecule in the lattice. The average sizes of these are ≈ 4.12 Ǻ and ≈ 2.2 Ǻ, respectively [1,2]. Under certain conditions the octahedral cavities can be occupied by impurity atoms or molecules whose sizes are smaller than or similar to the size of the cavity. By now various experimental methods have been used to investigate fullerite C 60 doped with inert gas atoms [3][4][5][6][7][8] and simple molecules [9][10][11][12][13][14][15] of different symmetries and sizes. Doping affects significantly the physical properties of fullerite at low temperatures [16][17][18].The quantitative and qualitative features of the behavior of A x C 60 (A = 4 He, Ne, Ar, Kr, Xe) and M x C 60 (M = H 2 , N 2 , O 2 , CO, CH 4 , CD 4 ) systems depend in particular on the concentration x (x is the fraction of the octahedral cavities occupied by admixtures) and on the atomic / molecular parameters of the admixtures. Note that in a 4 He -C 60 solution the 4 He atoms can also occupy the tetrahedral cavities. The masses, the moments of inertia, the central and noncentral forces of the pair interaction of H 2 , N 2 , O 2 , CO, CH 4 and CD 4 molecules are much smaller than those of C 60 molecules. As a result, admixture of these molecules to fullerite ...

Section: Introductionmentioning

confidence: 99%

Low-temperature heat capacity of fullerite C60 doped with deuteromethane

Bagatskii

Sumarokov

Долбин

et al. 2012

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“…10. Temperature dependence of the characteristic times for the positive contribution to thermal expansivity [22][23][24] in C 60 doped with CO and the methanes: (CO) 0.9 -C 60 (circles); (CH 4 ) 0.5 -C 60 (squares); (CD 4 ) 0.5 -C 60 (crosses). Fig.…”

Section: Diffusion Of Light-mass Particles In C 60mentioning

confidence: 99%

Novel carbon materials: New tunneling systems (Review Article)

Strzhemechny

Долбин

2013

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This review covers recent achievements in the studies of quantum properties of the novel carbon materials (fullerite C 60 and bundles of single-walled nanotubes (SWNT)) saturated with such light-mass species as helium isotopes, the homonuclear molecular hydrogens, and neon. It is shown that even some heavy dopants demonstrate kinetic phenomena, in which coherent effects play an essential role. Two theoretical concepts are surveyed which have been suggested for the explanation of the anomalous phenomena in saturation kinetics and linear thermal expansion of doped C 60 . Most unusual effects have been also observed in the low-temperature radial expansion of bundles of single-walled carbon nanotubes saturated with the helium isotopes. First, it was shown that low-temperature radial expansion of pure SWNT is negative, i.e., a nanotube shrinks with warming. Second, saturation of SWNT bundles with the helium isotopes entails a huge increase of the negative expansion effect, when the dopant is He. So far, no detailed physical picture has been put forward. It is worth mentioning that the dynamics of a single helium atom on an isolated nanotube corresponds to that of a tight-bound quasiparticle with a band width of about 10 K.

“…It should be noted that the role of chemically neutral interstitial impurities in the modification of rotational states of C 60 is not completely understood. For example, arguments exist that such impurities facilitate polyamorphic first-order transformations between different states of the orientational glass [18,19]. The above considerations and conclusions should be checked using an independent experimental method.…”

Section: Introductionmentioning

confidence: 99%

Orientational glassification in fullerite C60 saturated with H2: Photoluminescence studies

Zinoviev

Zoryansky

Silaeva

et al. 2012

Using one-photon excitation we studied photoluminescence of C 60 saturated with molecular hydrogen over a temperature range 10 to 230 K. Saturation of samples was done at a pressure of 30 atm and at temperatures low enough (T < 250 °C) to exclude chemical sorption. The samples were saturated during periods of varied duration τ to reach different occupancy levels. To check reliability of our luminescence results and interpretation, our spectra for pure C 60 were compared with data known in the art, demonstrating good compatibility. The luminescence spectra were attributed according to the approach of Akimoto and Kan , no by separating total spectra in two components of different origin. The A-type spectra, which are associated with exciton transport to deep traps, above 70 K become prevail over the B-type emission. Until saturation times did not exceed a certain value (for one, 50 h for a saturation temperature of 200 °C) the integrated intensity I as a function of the temperature T of luminescence measurements, I(T), remained at a constant level up to the orientational vitrification point of about 100 K and then went rather steeply down with increasing T. However, at longer τ the intensity I(Τ) persisted in constancy to higher T (the higher, the longer τ) and then dropped with increasing T. This finding made us to reexamine more closely the lattice parameter vs saturation time dependence for saturation temperatures 200 and 230 °C. As a result, additional evidence allowed us to infer that after completion of the single-molecule filling of O-voids (specifically, after roughly 50 h for T sat = 200 °C) a slower process of double filling sets in. Double filling entails an anisotropic deformation of the octahedral cage, which modifies rotational dynamics stronger than single filling. Further, we argue that singlet exciton transport to traps (which is responsible for the A-type emission) can be crucially hampered by rotational jumps of one of the molecules over which a travelling exciton is spread. Such jumps break coherence and the exciton stops thereby increasing the probability of emissionless deactivation. If so, then the temperature, at which the rotational jumps occur sufficiently frequently, may be by inference considered the unfreezing point for the orientational glass state (essentially coinciding with the inverse critical point T g where the rotational system freezes into the orientational glass). This treatment of T g differs from that existing in the art according to which the glass state is destroyed owing to the increased density of phonon states. Keeping to our reasoning, we conclude that the orientational glass state does not disappear but, instead, is conserved almost unchanged under one-molecule feeling and persists to appreciably higher temperatures in the case of double filling, which affects exciton dynamics stronger.