C60-C70-C6H5CH3 crystal solvates: Structure and thermal stability

Article abstract of DOI:10.1134/S0036024408130086

Differential scanning calorimetry (DSC) and X-ray diffraction were used to study the thermal stability and structure of C₆₀-C ₇₀-C₆H₅CH₃ crystal solvates synthesized at room temperature. The decomposi

Full text of DOI:10.1134/S0036024408130086

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 13, pp. 2207–2212. © Pleiades Publishing, Ltd., 2008.
PHYSICAL CHEMISTRY
OF NANOTECHNOLOGIES
C60–C70–C6H5CH3 Crystal Solvates: Structure
and Thermal Stability
M. A. Eremina, V. I. Lad’yanov, and R. M. Nikonova
Physicotechnical Institute, Ural Division, Russian Academy of Sciences, ul. Kirova 132, Izhevsk, 426000 Russia
E-mail: mrere@mail.ru, las@pti.udm.ru
Received February 22, 2008
Abstract—Differential scanning calorimetry (DSC) and X-ray diffraction were used to study the thermal sta-
bility and structure of C60–C70–C6H5CH3 crystal solvates synthesized at room temperature. The decomposition
of the crystal solvates generates C60–C70 solid solutions with hexagonal close-packed (hcp) structure.
DOI: 10.1134/S0036024408130086
Fullerenes in the condensed state extracted from a saturated solution of C60 in toluene and solvate II,
fullerene black retain some amount of the solvent. Sol-
vent molecules can reside both on the facets of fuller-
ites [1] and, as clathrates/crystal solvates, in interstices
of van der Waals lattices built of fullerenes [2]. These
solvates are supposed to be responsible for the non-
monotonic behavior of C60 solubility in some organic
solvents [3], this nonmonotonicity being important for
the separation of fullerene mixtures.
which decomposes into C60 and toluene at ~48°C [10].
The orthorhombic C60–toluene (1 : 1) solvate was pre-
pared experimentally [11] with the following parame-
ters: a = 10.37 Å, b = 31.67 Å, c = 10.13 Å.
For the C70–toluene system, the C70 · C6H5CH3 sol-
vate is known with orthorhombic structure (space group
Amm2) and the unit cell parameters a = 10.86(2) Å,
b = 10.55(6) Å, c = 33.09(8) Å [12, 13]. Data on its
thermal stability are few. The endotherm at 130–180°C
observed on the differential scanning calorimetry
(DSC) curve for a toluene-containing sample is associ-
ated with the decomposition of the crystal solvate [14].
It is known [15] that monoclinic C60 · C6H5CH3 and
orthorhombic C70 · C6H5CH3 solvates are quasi-isomor-
phic to the orthorhombic n-pentane solvate of C60 (1 : 1)
[16]. From the quasi-isomorphism of C60 · C6H5CH3
and C70 · C6H5CH3, we can suggest the existence of
mixed solvates (solid solutions) in the C60–C70–
C6H5CH3 system at room temperature; their suggested
composition is (C60)x(C70)1 – x · C6H5CH3.
The extraction of fullerenes by a boiling solvent and
the slow evaporation of fullerene solutions at room
temperature can generate crystal solvates, whose
molecular composition and structure have been studied
insufficiently. The systems of several types of
fullerenes, in particular, in C60–C70–solvent systems,
can generate complex mixed solvates or solid solutions
of solvates. Solvates of the C60–C70–o-C6H5(CH3)2 ter-
nary system form a solid solution, which is isomorphic
to the C60 · 2(o-C6H4(CH3)2) solvate [4, 5]. The struc-
tural alteration of mixtures after the solvent is removed
during heat treatment is not discussed in the above-
cited works. However, the formation of fullerene
C60/C70 solid solutions is quite possible as a result of the
thermally activated demixing of mixed crystal solvates.
The similarity of the molecular sizes of fullerenes C60
and C70 implies the possibility of the existence of sub-
stitutional solid solutions of C60 in C70 and of C70 in C60
with face-centered cubic (fcc) and/or hexagonal close-
packed (hcp) lattices [6, 7]. Complete miscibility was
found in vacuum-sublimed samples [7], and incomplete
miscibility was found in a sample prepared from solu-
tions [7, 8].
EXPERIMENTAL
Here, we studied the structure and thermal stability
of crystal solvates of the C60–C70–C6H5CH3 system
containing 0, 1.9, 4.8, 10, 15, 20, 30, 50, and 100 ( 0.3)
wt % C70 in the C60/C70 starting mixture using DSC and
X-ray diffraction. To prepare test samples, weighed
samples of references C60 (99.95%) and C70 (98%)
were mixed and completely dissolved in toluene (high-
purity grade). Then, the solution was allowed to con-
centrate until dry at room temperature (for ~24 h).
X-ray diffraction was measured on DRON-6 using
CuKα radiation in the angle range 5°–40°. DSC curves
were recorded on PerkinElmer Diamond DSC in alumi-
The C60–toluene (C60–C6H5CH3) system forms two
types of monoclinic crystal solvates C60 · 2C6H5CH3 (I,
C2/m, a = 17.03(2) Å, b = 10.344(4) Å, c = 11.16(1) Å,
β = 107.10(7)°) and C60 · C6H5CH3 (II, a = 10.25(3) Å,
b = 31.3(1) Å, c = 10.17(4) Å, β = 94.0(3)°) [9]. Solvate num crucibles under heating from –50 to 220°C at
I is metastable and melts incongruently at 12°C to yield 20 K/min in flowing argon.
2207

2208
EREMINA et al.
Q, W/g
–0.6
–0.4
–0.2
0
8
1
8
2
9
7
0.2
0.4
0.6
(d)
Endo
6
179.2°C
155.9°C
5
4
3
130
170 T, °C
T, °C
40
60
69.3°C
80
100
67.3°C
120
140
160
(c)
(a)
62.2°C
(b)
76.5°C 82.6°C
88.2°C
100
75.8°C
75.1°C
40 60 80 T, °C
60
80
T, °C
60
80
T, °C
Fig. 1. DSC curves for initially prepared C –C –C H CH samples containing (1) 0, (2) 1.9, (3) 4.8, (4) 10, (5) 15, (6) 20, (7) 30,
60 70
6
5
3
(8) 50, and (9) 100 wt % C in C /C mixtures. Q is heat flow. The insets in the lower part of Fig. 1 are fragments of DSC curves
70
60 70
for initially prepared C –C –C H CH samples containing (a) 1.9, (b) 4.8, (c) 10, and (d) 100 wt % C in C /C mixtures.
60 70
6
5
3
70
60 70
Endotherms are counted from bottom up.
Figure 1 illustrates the results of DSC measure- (Cf. a = 14.17 Å [17]) and the monoclinic crystal sol-
ments for C60–C70–C6H5CH3 samples with various vate with a = 10.38 0.02 Å, b = 32.77 0.02 Å, c =
C60/C70 proportions. Let us start with the C60–C6H5CH3 10.25 0.01 Å, β = 90.6° 0.2°. With account for the
and C70–C6H5CH3 binary systems. The heat flow versus intensities of the solvate and fullerite lines, this X-ray
temperature curve for the C60–toluene system (Fig. 1, diffraction pattern corresponds to fcc C60 concentrations
curve 1) has a small, broad endotherm at ~89°C, appar- within 1–10%. The appearance of fcc fullerite in the
ently associated with the incongruent melting of C60
·
phase composition of this sample is likely due to the high
C6H5CH3 [10]. This sample being prepared at room solvent evaporation rate during sample preparation.
temperature, it is free of C60 · 2C6H5CH3, whose
Heating to 60°C increases the diffraction intensities
decomposition temperature is ~12°C [10]. The endot-
from the fcc fullerite and decreases those from the sol-
herm temperatures and relevant heats for all test sam-
vate (Fig. 2). The solvate lines completely disappear
ples are listed in the table.
upon heating to 109°C, signifying its decomposition in
Figure 2 displays X-ray diffraction patterns for an agreement with data in [10] and DSC results (Fig. 1).
initially prepared C60–toluene sample and for the X-ray diffraction pattern 3 in Fig. 2 shows two phases
same samples after heating to certain temperatures in the sample. Along with the rather narrow and strong
previously determined from the DSC curve. To avoid lines of fcc fullerite C60 (a = 14.187 0.010 Å), broad
oxidation, all samples were thermally treated in peaks characteristic of hcp fullerite C60 [18] are
dynamic vacuum (~10–4 mmHg) at 20 K/min. Data observed in this X-ray diffraction pattern. The consid-
analysis shows that the initially prepared samples con- erable line width can arise from high imperfection of
sist of fcc fullerite C60 with a = 14.187 0.10 Å this phase and/or very small crystallite size. The lines of
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 13 2008

C60–C70–C6H5CH3 CRYSTAL SOLVATES: STRUCTURE AND THERMAL STABILITY
2209
Onset temperatures (Ton) and end temperatures (Tend) and enthal-
pies (∆H) of solvate decomposition for C60–C70–C6H5CH3
samples with various C70 weight percentages in C60/C70 mix-
tures
the hcp phase have noticeably lower intensities than the
fcc phase, and its fraction in the sample can reach 50–
80% [19]. Presumably, hcp fullerite C60 is generated by
the decomposition of the crystal solvate. Hexagonal
close-packed C60 was also observed in samples pre-
pared by salting out and cryoextraction [18]. Samples
retain this structural state after 2-h annealing at 200–
400°C.
For the C70–toluene system, the DSC curve (Fig. 1,
curve 9) shows two endotherms at 156 and 179°C (see
Fig. 1d). The formation of orthorhombic C70 · C6H5CH3
is discussed in [12, 13], but kinetic data on its decom-
position are absent. The DSC curve implies either the
possibility of existence of one more solvate in this sys-
tem under the specified sample-preparation conditions
or the two-stage decomposition of C70 · C6H5CH3.
C70, wt % Ton 0.2, °C Tend 0.2, °C ∆H 1.0, J/g
0
72.7
61.8
89.0
69.3
2.5
12.8
32.7
34.5
27.3
24.7
15.6
4.7
1.9
4.8
60.5
67.9
10
68.6
79.4
15
20
30
50
72.1
83.4
74.0
85.1
77.9
89.5
82.2
95.5
169.6
144.3
169.1
179.8
155.9
179.2
13.0
7.3
Figure 3 displays X-ray diffraction patterns for an
initially prepared C70–C6H5CH3 sample and after heat-
ing. The initially prepared sample contains lines from
the crystal solvate exclusively and does not contain
reflections from fcc or hcp fullerite C70 phases. Com-
parison with data in [13] shows reflections from orthor-
hombic C70 · C6H5CH3 (A) (a = 10.88 Å, b = 10.55 Å, c
= 32.92 Å) and monoclinic solvate (B) (a = 10.63 Å, b
= 10.58 Å, c = 33.47 Å, β = 94.9°). Presumably, the two
DSC peaks (Fig. 1, curve 9) are due to the existence of
two crystal solvates of various symmetries. The inten-
sities of the lines of solvate B decrease noticeably upon
heating to 109°C. After heating to 155°C, reflections
characteristic of fullerite C70 appear, along with the set
of reflections from the orthorhombic solvate. Moderate
intensities of lines and their small number make it
impossible to decide in this case whether the fullerite
structure is fcc or hcp. A further rise in temperature to
208°C makes the crystal solvate disappear, and the
X-ray diffraction pattern corresponds to hcp fullerite
C70 (Fig. 3) with the unit cell parameters a = 10.59 Å,
c = 17.34 Å. The aforesaid agrees with data in [19, 20].
Subsequent anneals at 200–400°C leave the structural
state of the sample unchanged, as for the C60–toluene
system.
100
16.0
130–200°C becomes noticeable; in the 50 wt % C70
sample, this second endotherm prevails over the first
one. Importantly, the temperature of this second peak
almost coincides with the crystal solvate degradation
temperature in the C70–C6H5CH3 system [14].
Addition of C70 to C60–C6H5CH3 samples does not
change the set of lines characteristic of C60 · C6H5CH3,
although the interplanar spacings and, as a result, the
unit cell volume increase (Fig. 5). An additional set of
lines positionally close to the C70 · C6H5CH3 lines but
with smaller unit cell parameters becomes noticeable
only in the mixture containing 50 wt % C70. After heat-
ing to 100°C, the X-ray diffraction pattern retains the
lines from a solvate with the composition close to
orthorhombic C70 · C6H5CH3 but with a smaller unit cell
volume. The intensities of the main lines from the C60
and C70 solvates are approximately equal, but the X-ray
diffraction pattern of the 50 wt % C60 + 50 wt % C70
sample does not coincide with the patterns from the C60
and C70 solvates. In addition, the lines of fcc fullerite
C60 decrease in intensity with rising C70 concentration
and disappear in the 50 wt % C70 sample. The fcc unit
cell parameter is 14.187–14.208 Å, close to the param-
eter of the C60–C6H5CH3 system.
Let us consider the thermal stability and structural
state of C60–C70–C6H5CH3 ternary mixtures. Figure 1
illustrates DSC curves for samples with various C70
proportions in C60/C70 mixtures. The table lists the tem-
peratures of endothermic transformations and their
Presumably, fcc fullerites C60 formed from solution
enthalpies. Figure 4 illustrates the relevant X-ray dif- at room temperature are almost free of fullerenes C70.
fraction patterns. The DSC curves change noticeably The increased unit cell parameter of C60 relative to the
when C70 substitutes for 1.9 wt % C60. An endotherm is reference (aref = 14.166 Å) is likely due to the occupa-
observed within 35–100°C. The peak associated with tion of octahedral pores in the fullerite lattices by gases
the degradation of the crystal solvate is nonsymmetrical and due to high structure imperfection [21, 22]. Its
(Fig. 1a). The enthalpy of transformation increases with imperfection is proven by the absence of the peak from
increasing C70 concentration to 4.8 wt %, while the an orientational phase transition in the low-temperature
peak asymmetry is conserved (Fig. 1b). A further rise in part of DSC curves: this transition is very responsive to
C70 concentration increases the transformation temper- defect density [23]. After annealing in dynamic vacuum
ature, i.e., increases the thermal stability of mixtures at 200–400°C, the fullerite unit cell parameters can
and decreases the enthalpy (table). When C70 concen- remain increased despite the removal of molecular
trations exceed 30 wt %, a second endotherm within gases [24].
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 13 2008

2210
I, pulses/s
600
400
200
EREMINA et al.
I, pulses/s
400
6
6
4
4
2
2
200
(e)
*
(c)
5
9
11
1
7
8
10
11
3
*
9
7
5
10
1
8
3
*
*
0
0
(d)
*
200
600
*
0
400
200
(b)
(a)
(c)
200
*
*
*
*
0
0
400
400
200
0
(b)
200
0
*
orthorh. C60 · C6H5CH3 [11]
400
200
0
monocl. C60 · C6H5CH3 [9]
monocl. C60 · 2C6H5CH3 [9]
fcc C60
(a)
orthorh. C70 · C6H5CH3 [13]
20 30 2θ, deg
10
20
30
2θ, deg
10
Fig. 2. X-ray diffraction patterns of (a) initially prepared
–C H CH samples and (b, c) the same samples after
C
60
6
5
3
Fig. 3. X-ray diffraction patterns of (a) initially prepared
–C H CH samples and (b–e) the same samples after
heating to (b) 60 and (c) 109°C. In the lower part of the fig-
C
ure, X-ray diffraction diagrams for crystal solvate and fcc
70
6
5
3
C
phases; in the upper part, lines of fcc and hcp C
heating to (b) 60, (c) 109, (d) 155, and (e) 208°C. In the
lower part of the figure, the X-ray diffraction diagram for
orthorhombic crystal solvate C · C H CH ; in the upper
60
60
phases are indicated: (1) hcp [100], (2) fcc [111]/hcp [002],
(3) hcp [101], (4) fcc [220]/hcp [110], (5) hcp [103], (6) fcc
[311]/hcp [112], (7) fcc [222]/hcp [004], (8) fcc [331]/hcp
[121], (9) fcc [420]/hcp [114], (10) fcc [422], and (11) hcp
[511]/hcp [333]. Lines from the cell material are marked
with asterisks.
70
6
5
3
part, lines of hcp C are indicated: (1) [100], (2) [002],
70
(3) [101], (4) [110], (5) [103], (6) [112], (7) [004], (8) [121],
(9) [114], (10) [205], and (11) [006]. Lines from the cell
material are marked with asterisks.
The isothermal treatment of all C60–C70–C6H5CH3 cell parameters a = 10.15 Å, c = 16.70 Å. (The refer-
samples at 200°C for 2 h leads to the complete decom-
position of crystal solvates (Fig. 6). Low-C70 samples
are structurally similar to the samples obtained by
annealing the C60 solvate. With rising C70 concentra-
tion, the intensity is reduced and the half-width of fcc-
C60 lines increases, and the X-ray diffraction patterns
resemble more the pattern of the hcp fullerite phase.
However, separate lines of fullerite C70 are absent in all
samples.
ence values for hcp C60 are a = 10.02 Å, c = 16.42 Å
[25]: for C70, a = 10.60 Å, c = 17.30 Å [26]).
In the 50 wt % C70 sample, structural alteration is
even greater. The set of lines shows the fcc and hcp
major phases. The cubic phase has the average unit cell
parameter a ~ 14.24 Å (ranging from 14.197 to 14.368 Å
1
for individual lines). The broad low-intensity line near
19° can indicate the presence of an hcp phase based on
fullerite C60. The second phase has an hcp lattice with
X-ray diffraction patterns for the 30 and 50 wt % C70
samples are of interest. For the 30 wt % C70 sample, the
structure can be characterized by two phases: fcc fuller-
ite C60, which was found in all samples with lower C70
concentrations; and an hcp phase with the average unit
1
This determination of the unit cell parameter based on the last
well recorded line, (113) in the case at hand, is incompletely ade-
quate because of a considerable error and the possible existence
of packing defects.
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C60–C70–C6H5CH3 CRYSTAL SOLVATES: STRUCTURE AND THERMAL STABILITY
2211
I
8
7
6
5
4
3
2
1
5
10
15
20
25
30
35
2θ, deg
Fig. 4. X-ray diffraction patterns of initially prepared C –C –C H CH samples containing (1) 0, (2) 4.8, (3) 10, (4) 15, (5) 20,
60 70
6
5
3
(6) 30, (7) 50, and (8) 100 wt % C in the C /C mixture.
70
60 70
V, Å3
3750
3650
3550
3450
3350
0
20
40 C70, wt %
Fig. 5. Unit cell volume V of the crystal solvate (C , C ) · C H CH vs. C concentration in the C /C mixture.
60 70
6
5
3
70
60 70
average unit cell parameters a ~ 10.45 Å, c ~ 17.16 Å containing ~10 wt % C70, whereas the second hcp phase
(c/a = 1.64). Assuming that the unit cell parameters are
linear functions of concentration, the first phase is a
substitutional solid solution based on fcc fullerite C60
is a C70-based substitutional solid solution containing
~20–30 wt % C60. Annealing at 400°C under the same
conditions gives similar results.
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EREMINA et al.
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