Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction
The lattice parameter a of fullerite C₆₀ intercalated with N₂ molecules is investigated in the temperature interval 6–295 K by x-ray diffraction. It is found that the interstitial molecular N₂ has a considerable effect on both the temperatures, Tc of the orientational phase transition and Tg of...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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| Цитувати: | Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction / N.N. Galtsov, A.I. Prokhvatilov, G.N. Dolgova, D. Cassidy, G.E. Gadd, S. Moricca, B. Sundqvist // Физика низких температур. — 2007. — Т. 33, № 10. — С. 1159–1165. — Бібліогр.: 37 назв. — англ. |
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irk-123456789-1209572017-06-14T03:02:26Z Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction Galtsov, N.N. Prokhvatilov, A.I. Dolgova, G.N. Cassidy, D. Gadd, G.E. Moricca, S. Sundqvist, B. Динамика кристаллической решетки The lattice parameter a of fullerite C₆₀ intercalated with N₂ molecules is investigated in the temperature interval 6–295 K by x-ray diffraction. It is found that the interstitial molecular N₂ has a considerable effect on both the temperatures, Tc of the orientational phase transition and Tg of the orientational glass formation. Hysteresis of a(T) has been detected in the Tc and Tg regions, besides, the abrupt change in the volume over the region defining Tc. Complete intercalation of C₆₀ with N₂ molecules results in a 0.2% increase in the lattice parameter, which persists over the whole temperature range. Evidence is also obtained that the interstitial guest molecular N₂ induces a slight deformation of the cubic symmetry of the host C₆₀ lattice. 2007 Article Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction / N.N. Galtsov, A.I. Prokhvatilov, G.N. Dolgova, D. Cassidy, G.E. Gadd, S. Moricca, B. Sundqvist // Физика низких температур. — 2007. — Т. 33, № 10. — С. 1159–1165. — Бібліогр.: 37 назв. — англ. 0132-6414 PACS: 61.10.Nz, 81.05.Tp, 64.70.–p http://dspace.nbuv.gov.ua/handle/123456789/120957 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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English |
| topic |
Динамика кристаллической решетки Динамика кристаллической решетки |
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Динамика кристаллической решетки Динамика кристаллической решетки Galtsov, N.N. Prokhvatilov, A.I. Dolgova, G.N. Cassidy, D. Gadd, G.E. Moricca, S. Sundqvist, B. Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction Физика низких температур |
| description |
The lattice parameter a of fullerite C₆₀ intercalated with N₂ molecules is investigated in the temperature
interval 6–295 K by x-ray diffraction. It is found that the interstitial molecular N₂ has a considerable effect
on both the temperatures, Tc of the orientational phase transition and Tg of the orientational glass formation.
Hysteresis of a(T) has been detected in the Tc and Tg regions, besides, the abrupt change in the volume over
the region defining Tc. Complete intercalation of C₆₀ with N₂ molecules results in a 0.2% increase in the lattice
parameter, which persists over the whole temperature range. Evidence is also obtained that the interstitial
guest molecular N₂ induces a slight deformation of the cubic symmetry of the host C₆₀ lattice. |
| format |
Article |
| author |
Galtsov, N.N. Prokhvatilov, A.I. Dolgova, G.N. Cassidy, D. Gadd, G.E. Moricca, S. Sundqvist, B. |
| author_facet |
Galtsov, N.N. Prokhvatilov, A.I. Dolgova, G.N. Cassidy, D. Gadd, G.E. Moricca, S. Sundqvist, B. |
| author_sort |
Galtsov, N.N. |
| title |
Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction |
| title_short |
Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction |
| title_full |
Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction |
| title_fullStr |
Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction |
| title_full_unstemmed |
Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction |
| title_sort |
intercalation of fullerite c₆₀ with n₂ molecules. an investigation by x-ray powder diffraction |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2007 |
| topic_facet |
Динамика кристаллической решетки |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120957 |
| citation_txt |
Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction / N.N. Galtsov, A.I. Prokhvatilov, G.N. Dolgova, D. Cassidy, G.E. Gadd, S. Moricca, B. Sundqvist // Физика низких температур. — 2007. — Т. 33, № 10. — С. 1159–1165. — Бібліогр.: 37 назв. — англ. |
| series |
Физика низких температур |
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| fulltext |
Fizika Nizkikh Temperatur, 2007, v. 33, No. 10, p. 1159–1165
Intercalation of fullerite C60 with N2 molecules.
An investigation by x-ray powder diffraction
N.N. Galtsov, A.I. Prokhvatilov, and G.N. Dolgova
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine
47 Lenin Ave., Kharkov 61103, Ukraine
E–mail: galtsov@ilt.kharkov.ua
D. Cassidy, G.E. Gadd, and S. Moricca
Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Manai, NSW 2234, Australia
B. Sundqvist
Department of Experimental Physics, Umea University, S–901 87 Umea, Sweden
Received March 29, 2007, revised April 24, 2007
The lattice parameter a of fullerite C60 intercalated with N2 molecules is investigated in the temperature
interval 6–295 K by x-ray diffraction. It is found that the interstitial molecular N2 has a considerable effect
on both the temperatures, Tc of the orientational phase transition and Tg of the orientational glass formation.
Hysteresis of a(T) has been detected in the Tc and Tg regions, besides, the abrupt change in the volume over
the region defining Tc. Complete intercalation of C60 with N2 molecules results in a 0.2% increase in the lat-
tice parameter, which persists over the whole temperature range. Evidence is also obtained that the intersti-
tial guest molecular N2 induces a slight deformation of the cubic symmetry of the host C60 lattice.
PACS: 61.10.Nz X-ray diffraction;
81.05.Tp Fullerenes and related materials;
64.70.–p Specific phase transitions.
Keywords: fullerite C60 , x-ray diffraction method, phase transitions, processes of orientational ordering.
Introduction
Intercalation of fullerite C60 with dielectric admix-
tures can cause significant changes in its properties.
These changes are predominantly related to the consider-
able accompanying expansion of the crystal volume that
occurs with this interaction, as though a negative pressure
was applied to the solid matrix. An experimental condi-
tion such as this is hardly possible to apply to a pure sub-
stance in any real situation. The most interesting and im-
portant alterations of the properties are observed at the
temperature of the orientational phase transition Tc and in
the region of existence of the orientational glass. Nor-
mally, when inert gas atoms and simple molecules occupy
the octahedral and even sometimes the tetrahedral sites of
the fcc lattice, the noncentral molecular interactions grow
weaker, with the effect that the rotational motion of the
C60 molecules gain more freedom. As a result, the tem-
perature of the orientational phase transition decreases
[1–4]. This is illustrated clearly by the experimental re-
sults in Table 1. At temperatures above a respective Tc
most molecular impurities (N2, O2, CO, CH4, etc.) are
orientationally disordered and rotate quite freely in the in-
terstitial sites of the lattice [7,12–15]. However, below Tc
the nonspherical S6 symmetry of the force field associated
with the octahedral sites, with its specific noncentral in-
teraction with the surrounding C60 molecules, promote
localization of the interstitial guest molecule, as well as
inducing it to align in certain orientations with respect to
the symmetry axes of the C60 lattice [13–15]. This in turn
can influence the lattice dynamics, the phase transition
and even the structure of the C60 host matrix. The latter is
illustrated most clearly by the (CO2)xC60 system, in
which the interstitial CO2 molecule modifies the nature of
the phase transition and below 200 K it is found that these
solid solutions form a monoclinic lattice exhibiting P21/n
© N.N. Galtsov, A.I. Prokhvatilov, G.N. Dolgova, D. Cassidy, G.E. Gadd, S. Moricca, and B. Sundqvist, 2007
symmetry [16]. The «negative» pressure (lattice volume
expansion) is therefore not the only factor responsible for
property modifications of the C60 lattice when interca-
lated with various, molecular species. In developing an
understanding of such a system, it is important to consider
not only the distribution of the guest molecules over all
the possible interstitial sites but also to consider all the
energetically allowed orientations of the molecule within
these crystal sites and what effect these can have on the
noncentral interaction in the solid solution. Small-size
atomic or molecular impurities such as He atoms or H2
molecules produce rather weak effects, and their vibra-
tional and, for the case of H2, also rotational motion in the
octahedral sites of the C60 lattice are relatively free even
at the lowest temperatures. In contrast to this, an example
of where the interaction between C60 and molecular spe-
cies in the solid state is very strong, having a dramatic in-
fluence on both the final solid state structure and its asso-
ciated properties, is the example of C60 interacting with
cubane C8H8 [17–19]. In this case a hetero-molecular
crystal results with «immovable» molecules of cubane
and with «free» rotation of the C60 molecules! On cooling
the sample just below 140 K, the rotation of the C60 mole-
cules freezes out, with a lowering of the crystal symmetry
to orthorhombic.
The intercalation with van der Waals species has other
effects in addition to the suppression of the orientational
phase transition temperature Tc. For example, no glass
phase transition was detected in C60 intercalated with ei-
ther CO or NO molecules [8–10]. A similar result (a much
lower Tc and a nearly completely suppressed glass forma-
tion) was observed in dilatometric and neutron diffraction
measurements [11] of C60 intercalated with N2 and O2
molecules. It was found [8–12] that at the lowest (liquid
helium) temperatures studied, nearly all of the C60 mole-
cules (90%) in the solid solution formed the lower energy
pentagon–pentagon arrangements between neighboring
C60 molecules. Over this temperature region, the pres-
ence of atomic and molecular species can also signifi-
cantly alter the temperature behavior of the thermal ex-
pansion as compared to that of the pure C60 lattice, as well
as introducing hysteresis effects [20,21].
Although N2 and O2 have closely similar vdW molecu-
lar diameters and lengths (3.0 and 4.1 � for N2; 2.8 and
4.0 � for O2 , respectively) it is found that N2 molecules
diffuse much more slowly into the C60 lattice as well as
being much harder to remove from it, as compared to O2
molecules. It is proposed that the O2 molecules interact
less strongly with C60 molecules and subsequently be-
have more freely within the C60 lattice. It is interesting
note that their atomic spacing are reversed in the order of
their size length: N–N 1.095 �, O–O — 1.208 � [22–24].
In this study we have investigated the effect of N2
upon the phase transitions, the processes of orientational
ordering and glass formation in C60 by the x-ray powder
diffraction method and over the temperature range of
6–295 K. Data were taken during both the heating and
cooling of the samples. The XRD patterns obtained were
analyzed to detect possible effects of a nonspherical guest
molecule on the structure of the C60 host matrix.
Experimental technique
Polycrystalline C60 powder was saturated with N2 us-
ing hot isostatic pressing (HIP) and under a gas pressure
of 200 MPa at T = 723 K. It was then compacted into pel-
let form using hydrostatic compression at P = 1 GPa. The
pellet samples prepared (8 mm high, 10 mm in diameter)
were used to investigate the thermal expansion, the hard-
ness and structural characterization as in this study. The
chosen technique of saturation made it possible to
achieve, as validated by thermo-gravimetric analysis
(TGA) and measured weight loss, a 100% occupancy of
the octahedral sites in the C60 crystal.
The x-ray investigation was performed using a
DRON–3M diffractometer (K� emission with � =
= 1.54178 � from copper anode). The low temperature
measurement was made using an x-ray helium cryostat for
2–293 K. The temperature measurement and stabilization
were accurate within � 0.1 K. The error in the lattice pa-
rameter was � 0.02 %.
1160 Fizika Nizkikh Temperatur, 2007, v. 33, No. 10
N.N. Galtsov, A.I. Prokhvatilov, G.N. Dolgova, D. Cassidy, G.E. Gadd, S. Moricca, and B. Sundqvist
Table 1. Lattice parameter a, orientational phase transition tem-
perature Tc and glass formation temperature Tg for pure C60 and
C60 intercalated with atomic and molecular impurities.
Substance Òc, K à, � Òg , K Source
Pure Ñ60 260 14.161 90 [25,26]
He1C60 246 14.223 85 [4,5]
Ne0.49C60 258 14.168 90 [5,27]
Ar1C60 251 14.173 80 [2–4]
Kr0.8Ñ60 238 14.207 – [2]
Xe0.34C60 230 14.240 70 [6]
Xe0.46C60 220 14.236 – [2]
Xe0.66C60 210 14.36 – [2]
(N2)0.6C60 239 14.183 80 [7]
(O2)0.7C60 240 14.174 – [5,11]
(CH4)0.92C60 241 14.187 – [1,13]
(CD4)0.88C60 235 14.184 – [1]
CO0.67C60 245 14.179 – [5,8–9,14–15]
(ÑÎ2)0.67Ñ60 < 200 14. 224 – [16]
NO0,1C60 245 – – [5,9–10]
Results and discussion
A typical x-ray diffractogram of the compacted
polycrystalline C60–N2 samples is shown in Fig. 1. Apart
from the distinct intensity reflections from the fcc C60 lat-
tice, the diffraction pattern contains several additional
weak ones that are not typical for pure fcc C60. Among
them are the rather weak (200) and (400) reflections,
which account firstly for the occupancy of the octahedral
sites in C60 with N2 molecules and, secondly, for a notice-
able deformation of the C60 crystal. According to [28],
pertaining to the x-ray diffraction of pure nondeformed
C60, the reflections from (h00) planes have nearly zero in-
tensity at room temperature. The other weak reflections
observed are marked with arrows in Fig. 1, and these may
correspond to diffraction by an additional phase formed
(like in [6]) by partial polymerization of the C60 as a re-
sult of the high-pressure compaction. This assumption is
supported by the fact that the x-ray diffraction patterns
taken of the powder resulting from gently crushing a com-
pacted samples showed only the reflections from fcc
C60–N2 alloys, with these weak reflections now absent.
The intensity distributions of the medium to strong
x-ray reflections from both solids of the C60–N2 solution
and pure C60 are very similar quantitatively. However, the
half–widths of the reflections from C60–N2 (0.4 � 0.01
�
)
are between 2.0 and 2.5 times larger than those for an-
nealed pure C60. These higher half-widths are typical for
both substitutional and interstitial solid solutions and are
due to nonuniform static displacements of the trapped
atomic or molecular species in the host lattice. An addi-
tional contribution could also be made from residual plas-
tic deformation of the samples as a result of the high-pres-
sure compaction.
The magnitude of the lattice parameter of the C60–N2
solution at room temperature was estimated from the pro-
cessing of all the x-ray diffraction patterns taken at this
temperature. The derived lattice parameters, with one cal-
culated from the �hkl of each reflection, were plotted
against the extrapolation function 1/2(cos
2
�/ sin � +
+ cos
2
�/�) [29] (Fig. 2). The set of points was linearly
fitted, and then extrapolated to intercept the vertical lat-
tice parameter axis. The intercept was taken as the lattice
parameter of the (N2)xC60 solution. This procedure mini-
mizes random and systematic errors and yields a highly
accurate estimate of the true parameter. Using this pro-
cedure, we processed results of several experiments and
obtained the average lattice parameter a = 14.1906 �
� 0.003 � for the (N2)xC60 solution. This exceeds the
value for pure C60 by 0.029 �0.003 � [25,26].
Figure 2 reveals considerable scatter of the calculated
ahkl — values about the linear fit. Scatter like this is im-
possible for a nondeformed fcc lattice. Indeed, as follows
from our experiments on pure C60, the lattice parameters
calculated from individual (hkl) reflections fall quite ac-
curately (within the experimental error) on the extrapo-
lated straight line (Fig. 2). Besides, the linear dependence
a = f(�) is also typical for binary solutions of C60 with
spherically symmetrical species such as rare gas atoms.
We have analyzed in the same way as above, diffraction
parameters of C60 solutions with He, Ne, Ar and Kr and
have found that the scatter of points is no worse than that
of pure C60.
The above results suggest that certain inhomogeneous
systematic changes in the interplaner spacing of the fcc
lattice of C60 occurs when intercalated with linear N2
molecules. It is most likely that the intercalation with the
linear N2 molecule causes a weak tetragonal deformation
of the cubic C60 lattice. The largest deviations from the
extrapolated linear dependence are observed for the
Intercalation of fullerite C60 with N2 molecules. An investigation by x-ray powder diffraction
Fizika Nizkikh Temperatur, 2007, v. 33, No. 10 1161
9 18 27
2500
5000
7500
10000
12500
(5
1
1
)
(4
2
2
)
(4
2
0
)
(3
3
1
)
(4
0
0
)
(2
2
2
)
(2
2
0
)
(2
0
0
)
(3
1
1
)
(1
1
1
)
In
te
n
si
ty
2 , deg�
C – N , T = 293 K60 2
Fig. 1. Typical x-ray diffraction pattern of the compacted poly-
crystalline (N2)xC60 samples. The arrows show an unknown
second phase.
0 1 2 3 4 5 6 7 8 9
14.12
14.16
14.20
14.24
14.28
14.32
14.36
(5
1
1
)
(4
2
2
) (4
2
0
)
(3
3
1
)
(2
2
2
)
(3
1
1
)
(2
2
0
)
(1
1
1
)
Pure C60
C – N60 2
a,
�
1/2 (cos / sin )
2
� � � � �cos /
2
Fig. 2. Lattice parameters calculated from individual experi-
mental reflections as a function of diffraction angle: two dif-
ferent measurements on C60–N2 alloy (�, �); pure C60 (�).
(hk0), (hk1), and other reflections which bifurcate in the
case of tetragonal deformation. It is also known that lin-
ear molecules (e.g., N2) in simple substitutional solid so-
lutions cause local tetragonal distortions of the host lat-
tice [30–32]. The weak tetragonality assumed for the
(N2)xC60 lattice offers another possibility for explaining
the comparatively large broadening of individual reflec-
tions that can exhibit a weak doublet splitting. Attributing
the cubic symmetry violation to a noncentral component
in the N2–C60 molecular interaction, we might expect en-
hancement of the effect in the low–temperature region,
where the N2 molecules will become orientationally or-
dered. Surprisingly, with this ordering of the N2 mole-
cules, no further distortion is observed at the coldest tem-
perature studied from the observed distortion at room
temperature.
The temperature dependence of the lattice parameter
measured on cooling and heating the C60–N2 solutions
are shown in Fig. 3. An appreciable hysteresis of a(T) is
observed in the region of the orientational phase transi-
tion Tc. The temperatures of the orientational phase tran-
sition on cooling and heating the C60–N2 solution differ
nearly by 10 K, and Tc values are 245 and 255 K, respec-
tively (the phase transitions are marked with solid arrows
in Fig. 3). This also applies to heating and cooling the
sample in the temperature region where an orientational
glass exists, below T = 60 K. It is important to realize that
these differences observed between the lattice parameters
measured on cooling subsequent heating exceed the pos-
sible error in the measurement. These effects, when con-
verted into thermal expansion coefficients, agree with the
directly measured dilatometric data for this solution [33].
As well as these observed phenomena, it is also found that
both Tc and Tg are displaced considerably towards lower
temperatures for the C60–N2 solutions in comparison to
the corresponding values for pure C60 (Fig. 3). The lattice
parameters we have measured on cooling agree quite well
(within the total error of both experiments) with the data
from Renker et al. for (N2)0.6C60 [7], also measured over
a similar temperature range in this work. On cooling over
this temperature interval the lattice parameter of our
(N2)0.85C60 solution changes by�a = 0.1294 �, whereas
the ( N2)0.6C60 solution of Renker et al. [7] shows a some-
what smaller change of�a = 0.1269 �, though compara-
ble within experimental error. Both of the values, how-
ever, slightly exceed the lattice parameter change
typically observed for cooling of pure C60 over a similar
temperature change, namely �a = 0.1190 �, for [26].
This means that the thermal expansion of nitrogen-doped
C60 within the temperature range studied is 8.5% larger
compared to pristine fullerite. However, on heating the
thermal expansion of the sample appeared to be much
smaller than that on cooling. The change in the lattice pa-
rameter is comparable with or even slightly lower than in
the case of pure C60 ( Fig. 3). As a result, at room temper-
ature the lattice parameter does not reach its initial value
observed at the beginning of the cycle. It is likely that the
hysteresis loop is closed at higher temperatures. At the
same time we could observe the effect of the lattice pa-
rameter relaxation. During the exposure of the sample to
room temperature (15–20 hours) the lattice parameter re-
turns to its initial value. However, no relaxation of the pa-
rameter a was observed during the same period at temper-
atures below that of the phase transition. We are planning
a detailed investigation of the relaxation processes in the
C60–N2 solution.
It is interesting to compare the lattice parameters for
the solutions of C60 with N2 to those of C60 with other
species. This comparison is very illuminating when we
compare the dependences of the room temperature lattice
parameter with occupancy of the octahedral sites by these
species. The room-temperature lattice parameters for
both our C60–N2 and the C60–60 % N2 solution of Renker
et al. [7] are shown in Fig. 4, together with the room tem-
perature values arising from a range of other C60 solu-
tions with differing trapped species as well as varying
stoichiometry. It is seen that the linear lattice parameter
versus concentration relationship holds for C60 with Ne,
as was thoroughly investigated by Morosin et al. [27].
The value of the lattice parameter obtained in this study, if
assumed to fall on another straight line formed between
the data point formed from the lattice parameter and occu-
pancy of the sample of [7], predicts the occupancy of our
sample to be 85 % (Fig. 4). This value was confirmed by
analyses of integral diffraction intensities of studying al-
loy. The decrease in the concentration N2 of our samples
from 100 to 85 % might come from rather long (several
weeks) exposure of the sample to vacuum during its prep-
1162 Fizika Nizkikh Temperatur, 2007, v. 33, No. 10
N.N. Galtsov, A.I. Prokhvatilov, G.N. Dolgova, D. Cassidy, G.E. Gadd, S. Moricca, and B. Sundqvist
0 60 120 180 240 300
14.04
14.08
14.12
14.16
14.20
14.24
Tg
Tc
T, K
– cooling (this work)
heating (this work)–
heating (this work)–
heating pure C– 60 [26]
Renker et al. [7,11]–
a,
�
Fig. 3. Temperature dependences of lattice parameters of
C60–N2 solution on cooling (O) and heating (�, �) (results this
work). For comparison, the data by Renker et al. [7,11] (�) and
for pure C60 [26] ( ) are shown.
aration and measurement of the thermal expansion coeffi-
cients [33]. The obtained linear dependence is slightly
higher but still close to the a(x) values for C60–Ar [2–4]
solutions. The similarity of the lattice parameters of the
C60–N2 and C60–Ar solutions may indicate nearly equal
effects of these species with very similar gas–kinetic di-
ameters [23] upon the lattice of C60. Also of interest is the
lattice parameter for C60–He [4], which if plotted with a
100 % occupancy appears to be rather high compared to
the other gases.
The increase in the parameter a and volume of the C60
lattice by intercalation with van der Waals species sup-
presses the C60–C60 intermolecular interaction, which in
turn enhances the rotational motion of the C60 molecules
and lowers the temperature of orientational ordering.
Using the presently available data on the structure of
binary C60 solutions with atomic and molecular species,
we have been able to plot in Fig. 5 the dependence of the
orientational phase transition temperature (Tc) for C60 on
the size of the lattice parameter (a), measured at the onset
of the phase transition, when cooling the sample down
from room temperature. Figure 5 illustrates the general
tendency of Tc to decrease with growing lattice parameter
for C60. The tendency holds within a single system (e.g.,
Xe–C60) and for the whole collection of binary solutions.
Only one value (for C60–He [4]) appears outside the
smooth dependence Tc (a) (see the Hex point in the inset
in Fig. 5). The reason can be as follows. In the course of a
prolonged exposure of the samples to the He atmosphere,
the atoms have enough time to occupy not only the octa-
hedral sites but also the tetrahedral ones [4,34–36]. It is
most likely that the larger-than-expected lattice parame-
ters of these solutions is a direct result of this ability of He
to be able to occupy both types of interstitial sites and
therefore exert a greater than normal internal pressure on
the C60 lattice. When a monatomic species (such as Ne or
Ar) occupies (completely) only octahedral sites, the
changes in a and Tc are much smaller. Indeed, assuming
that the initial part of the curve shown in reference [4] that
describes the time variation of the parameter a during in-
tercalation of C60 with He [4], corresponds to occupation
of the octahedral sites only, we obtain the smallest lattice
parameter change measure for intercalating species with
C60 and if assumed to be occupying 100 % of the octahe-
dral sites. The lattice parameter changes by only �a =
= 0.012 �, whilst Tc decreases only by 2 K. The point
He1.0 in Fig. 5 corresponds to these results [4,5] and falls
quite accurately on the averaged smooth dependence.
When He atoms occupy 100 % of only the octahedral
sites, their effect on both the lattice parameter and the Tc
of the C60 matrix is very close to that arising from Ne at-
oms occupying the C60 lattice but with a 49 % occupancy.
Conclusions
C60–N2 solid solutions have been investigated over in
a wide temperature range (6–293 K) using the x-ray dif-
fraction method. It is found that an interstitial molecular
species has a considerable effect upon the structural prop-
erties, the orientational phase transition (Tc) and the
orientational glass temperature (Tg) in C60. In contrast to
atoms the N2 molecules intercalated into the C60 lattice
cause some deformation of the cubic cell as well as ap-
Intercalation of fullerite C60 with N2 molecules. An investigation by x-ray powder diffraction
Fizika Nizkikh Temperatur, 2007, v. 33, No. 10 1163
0 10 20 30 40 50 60 70 80 90 100
14.12
14.14
14.16
14.18
14.20
14.22
14.24
NeAr
N2
CO2
CO O2
N2
He
Ne
293 K
Pure C60
Ar
Octahedral sites occupancy, %
a,
�
Fig. 4. The FCC lattice parameter of C60 at 293 K as a function
of the occupancy of octahedral sites by inert gas atoms and
sample molecules: He (�) [4], Ne (�) [27], Ne (�) [35],Ar
(�) [3], (�) Ar [4], CO2 (�) [16], CO (�) [14,15], O2 ( )
[5,11], N2 (
) [7] and this study, pure C60 (�) [26].
14.2 14.3 14.4 14.5 14.6 14.7
140
160
180
200
220
240
260
14.20
220
240
260
Xe0.66
(C H )8 8 1
T
K
c
,
T
K
c
, Hex
Xe0.44
Xe0.35
Kr0.84
(N )2 0.6
(N )2 0.85
(CD )4 0.88
(CH )4 0.92
(O )2 0.7
Ar1
Pure C60
Ne0.49
He1.0
a, �
a, �
CO0.67
Fig. 5. Effects of increasing the cubic unit cell parameter of
C60 on Tc in binary substitutional solid solutions for different
atomic and molecular species: Ne0.49C60 (�) [27]; Ar1.0C60
( ) [3]; CO0.67C60 (�) [15]; (O2)0.7C60 (�) [5,11];
(CH4)0.92C60 (�) [1]; (CD4)0.88C60 (�) [1]; He1.0C60 and
HexC60 (�,
) [4,34]; (N2)0.6C60 (
) [7,11]; (N2)0.85C60 (�)
[this study]; Kr0.84C60 (�) [2]; Xe0.35,0.44,0.66C60 (�) [2];
(Ñ8Ð8)1.0C60 (�) [17 –19]; pure C60 (�) [26,37].
proximately a 0.2% increase in the lattice parameter at
room temperature. This excess over the value for pure C60
persists when cooling the sample down to a temperature
of T = 6 K. On heating the sample back to ambient temper-
atures, the lattice parameter of the solid solution ap-
proaches the a value of pure C60 in the region of Tc. It is
observed that the onset of the phase transition at Tc, as ob-
served on the C60–N2 sample from room temperature, is
lower than that corresponding to pure C60. Both Tc and
the associated change in the lattice parameter exhibit hys-
teresis. The distinct behavior of solid C60–N2 solutions
and solutions with atomic species is largely determined
by the noncentral component in the C60–C60 intermo-
lecular interaction, as well from possible ordering of the
molecular species in the octahedral sites at lower tempe-
ratures.
The authors are indebted to M.A. Strzhemechny for
helpful discussions at every stage of the investigation and
helpful advice in the course of the paper preparation and
to T.Yu. Shcherban’ for help in carrying out the experi-
ments.
The study was partially supported by the International
STCU Foundation, Grant ¹ 2669.
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