Low temperature microhardness of Xe-intercalated fullerite C₆₀
The Vickers microhardness of Xe-intercalated polycrystalline fullerite C₆₀ (XexC₆₀, x ≃ 0.35) is measured in a moderately low temperature range of 77 to 300 K. A high increase in the microhardness of the material (by a factor of 2 to 3) as compared to that of pure C₆₀ single crystals is observed....
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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irk-123456789-1214672017-06-15T03:05:23Z Low temperature microhardness of Xe-intercalated fullerite C₆₀ Fomenko, L.S. Lubenets, S.V. Natsik, V.D. Cassidy, D. Gadd, G.E. Moricca, S. Sundqvist, B. Низкотемпературная физика пластичности и прочности The Vickers microhardness of Xe-intercalated polycrystalline fullerite C₆₀ (XexC₆₀, x ≃ 0.35) is measured in a moderately low temperature range of 77 to 300 K. A high increase in the microhardness of the material (by a factor of 2 to 3) as compared to that of pure C₆₀ single crystals is observed. It is shown that the step-like anomaly in the temperature dependences of the microhardness of pure C₆₀ single crystals recorded under the orientational fcc-sc phase transition (Tc ≃ 260 K) is also qualitatively retained for XexC₆₀, but its onset is shifted by 40 K towards lower temperatures and the step becomes less distinct and more smeared. This behavior of ̅NV(T) correlates with x-ray diffraction data, the analysis of which revealed a considerable influence of xenon interstitial atoms on the peculiar features of fullerite thermal expansion due to orientational phase transitions (see the paper by A.I. Prokhvatilov et al. in this issue). 2005 Article Low temperature microhardness of Xe-intercalated fullerite C₆₀ / L.S. Fomenko, S.V. Lubenets1, V.D. Natsik, D. Cassidy, G.E. Gadd, S. Moricca, and B. Sundqvist // Физика низких температур. — 2005. — Т. 31, № 5. — С. 596-601. — Бібліогр.: 25 назв. — англ. 0132-6414 PACS: 81.05.Tp, 62.20.Qp, 81.40.Cd http://dspace.nbuv.gov.ua/handle/123456789/121467 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Низкотемпературная физика пластичности и прочности Низкотемпературная физика пластичности и прочности |
| spellingShingle |
Низкотемпературная физика пластичности и прочности Низкотемпературная физика пластичности и прочности Fomenko, L.S. Lubenets, S.V. Natsik, V.D. Cassidy, D. Gadd, G.E. Moricca, S. Sundqvist, B. Low temperature microhardness of Xe-intercalated fullerite C₆₀ Физика низких температур |
| description |
The Vickers microhardness of Xe-intercalated polycrystalline fullerite C₆₀ (XexC₆₀, x ≃ 0.35)
is measured in a moderately low temperature range of 77 to 300 K. A high increase in the microhardness
of the material (by a factor of 2 to 3) as compared to that of pure C₆₀ single crystals is observed.
It is shown that the step-like anomaly in the temperature dependences of the microhardness
of pure C₆₀ single crystals recorded under the orientational fcc-sc phase transition (Tc ≃ 260 K) is
also qualitatively retained for XexC₆₀, but its onset is shifted by 40 K towards lower temperatures
and the step becomes less distinct and more smeared. This behavior of ̅NV(T) correlates with x-ray
diffraction data, the analysis of which revealed a considerable influence of xenon interstitial atoms
on the peculiar features of fullerite thermal expansion due to orientational phase transitions (see
the paper by A.I. Prokhvatilov et al. in this issue). |
| format |
Article |
| author |
Fomenko, L.S. Lubenets, S.V. Natsik, V.D. Cassidy, D. Gadd, G.E. Moricca, S. Sundqvist, B. |
| author_facet |
Fomenko, L.S. Lubenets, S.V. Natsik, V.D. Cassidy, D. Gadd, G.E. Moricca, S. Sundqvist, B. |
| author_sort |
Fomenko, L.S. |
| title |
Low temperature microhardness of Xe-intercalated fullerite C₆₀ |
| title_short |
Low temperature microhardness of Xe-intercalated fullerite C₆₀ |
| title_full |
Low temperature microhardness of Xe-intercalated fullerite C₆₀ |
| title_fullStr |
Low temperature microhardness of Xe-intercalated fullerite C₆₀ |
| title_full_unstemmed |
Low temperature microhardness of Xe-intercalated fullerite C₆₀ |
| title_sort |
low temperature microhardness of xe-intercalated fullerite c₆₀ |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2005 |
| topic_facet |
Низкотемпературная физика пластичности и прочности |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/121467 |
| citation_txt |
Low temperature microhardness of Xe-intercalated fullerite C₆₀ / L.S. Fomenko, S.V. Lubenets1, V.D. Natsik, D. Cassidy, G.E. Gadd, S. Moricca, and B. Sundqvist // Физика низких температур. — 2005. — Т. 31, № 5. — С. 596-601. — Бібліогр.: 25 назв. — англ. |
| series |
Физика низких температур |
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2025-07-08T19:57:22Z |
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2025-07-08T19:57:22Z |
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| fulltext |
Fizika Nizkikh Temperatur, 2005, v. 31, No. 5, p. 596–601
Low temperature microhardness of Xe-intercalated
fullerite C60
L.S. Fomenko1, S.V. Lubenets1, V.D. Natsik1, D. Cassidy2,
G.E. Gadd2, S. Moricca2, and B. Sundqvist3
1B. Verkin Institute for Low Temperature Physics and Engineering
of the National Academy of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: fomenko@ilt.kharkov.ua
2Australian Nuclear Science and Technology Organization, NSW 2234, Australia
3Department of Experimental Physics, Umea University, SE-90187 Umea, Sweden
Received July 28, 2004
The Vickers microhardness of Xe-intercalated polycrystalline fullerite C60 (Xe Cx 60, x � 0.35)
is measured in a moderately low temperature range of 77 to 300 K. A high increase in the micro-
hardness of the material (by a factor of 2 to 3) as compared to that of pure C60 single crystals is ob-
served. It is shown that the step-like anomaly in the temperature dependences of the microhardness
of pure C60 single crystals recorded under the orientational fcc-sc phase transition (Tc � 260 K) is
also qualitatively retained for Xe Cx 60, but its onset is shifted by 40 K towards lower temperatures
and the step becomes less distinct and more smeared. This behavior of H TV( ) correlates with x-ray
diffraction data, the analysis of which revealed a considerable influence of xenon interstitial atoms
on the peculiar features of fullerite thermal expansion due to orientational phase transitions (see
the paper by A.I. Prokhvatilov et al. in this issue).
PACS: 81.05.Tp, 62.20.Qp, 81.40.Cd
Introduction
The physical and mechanical properties of crystal-
line C60 are considerably affected by intercalation of
the crystal by different impurities, where the environ-
ment itself may be considered as an impurity source
[1]. This is related to the weak van der Waals interac-
tion between the C60 molecules as well as the presence
of comparatively large interstitial sites in the fullerite
lattice. As a result, the crystal easily absorbs impuri-
ties and is easily amenable to intercalation. The most
dramatic effect due to intercalation is the supercon-
ductivity that occurs with alkali metals [2].
Of possible intercalants, gases occupy a highly im-
portant place for C60. The high interest in this type of
impurity is dictated by several facts [1]. Gases are
commonly used to transmit pressure to solid C60. In
this case gas atoms and molecules can diffuse into the
cavities of the fullerite lattice and form interstitial so-
lutions. Considering the arrangement of the centers of
gravity of its molecules, fullerite C60 forms a face-cen-
tered cubic (fcc) lattice through the whole range of
existence of its solid phase. For each C60 molecule in
the lattice there are two tetrahedral sites as well as
one octahedral site, which may be occupied by impu-
rity atoms or molecules. Besides their occupation,
chemical interaction between impurities and the C60
molecules (including polymer bond formation) is pos-
sible.
This paper is concerned with the influence of inter-
stitial gaseous impurities on the mechanical properties
of solid C60. In particular, xenon was selected because
being a rare gas eliminates the intercalant-matrix
chemical interaction. The atomic diameter of Xe
(3.92 Å) is almost two times larger than the effective
diameter of a tetrahedral site (2.2 Å) and close to that
of an octahedral one (4.1 Å), so that one might expect
xenon to only occupy the octahedral sites of the C60
lattice. The large diameter of the Xe atom makes its
entry into the lattice difficult and possible only at ele-
vated pressure and temperature [3]. The effect of xe-
© L.S. Fomenko, S.V. Lubenets, V.D. Natsik, D. Cassidy, G.E. Gadd, S. Moricca, and B. Sundqvist, 2005
non on the mechanical properties of fullerite C60 has
not previously been studied. In fact the only related
study so far, has been in [4], that has shown the re-
markable result of almost a hundredfold increase in
the microhardness of a C60 single crystal grown in
vacuum, and kept in an argon atmosphere.
The mechanical properties of Xe Cx 60 were studied
by using the microindentation technique. As shown in
the previous papers [5–8], the temperature depend-
ence of microhardness of pure C60 single crystals dis-
plays a step-like anomaly, the position of which is cor-
related with the temperature Tc � 260 K of the fcc–sc
phase transition. It is known [9,10] that intercalation
of an impurity into the C60 lattice results, as a rule, in
a decrease of Tc and a broadening of the phase transi-
tion temperature range. The research reported in this
paper concerns the influence of xenon saturation of
the C60 lattice on both the microhardness value and
the anomaly of its temperature dependence in the vi-
cinity of the phase transition point.
Experimental procedure
C60 powder was saturated with xenon at a Xe pres-
sure p � 200 MPa and a temperature T � 575 �C, for
36 hr [3]. To prepare the specimens for micro-indenta-
tion studies, the saturated powder of Xe Cx 60 was
pressed into tablet-shaped samples � 10 mm in dia and
� 5 mm in height, using hydrostatic pressure
(p � 0.5–1.0 GPa) [11,12]. From the x-ray structural
data [13], the degree of filling of the octahedral sites
with xenon amounted to (35 � 5) % (x � 0.35). Prior
to measuring, the tablet surface was polished on a ben-
zol-moistened chamois leather.
The Vickers microhardness at room temperature
was measured with a standard unit PMT-3 and in the
temperature range of 77–300 K with a freely sus-
pended indenter system [14]. The time of load endur-
ance was 10 s. To reveal the relation between micro-
hardness and indentation load, the load, P, was
varied between 0.005 and 0.2 N, at room temperature.
The temperature dependence H TV ( ) was measured at
P � 0.05 N as it was found that there was no influence
of indentation load on microhardness in the vicinity of
this load. The measurement was made in the course of
cooling. The microhardness was calculated by the ex-
pression H P/ aV � 1854 2 2. ( ) , where 2a is the impres-
sion diagonal. For each temperature or load 10 indents
were applied to the surface, and then the values of
microhardness were averaged over those indentations.
The averaged value, H TV ( ), was considered statisti-
cally to be a representative mechanical property of the
material studied.
Experimental results
A typical impression on the polished surface of a
Xe Cx 60 polycrystal is shown in Fig. 1. The impression
exhibited good faceting, indicating a high micro-
plasticity of the material in question. The impressions
remained sharp with decreasing temperature down to
77 K. At the same time cracks were commonly formed
round the observed impressions, even at low load.
These are mainly lateral (secondary) cracks, dipping
at a low angle to the sample surface (clearly seen in
Fig. 1). One may speculate that the cracks propagate
along the grain boundaries which are the weakest sites
of the pressed material due to a high density of de-
fects, particularly pores, in them.
The average values of microhardness, HV (T �
= 300 K), of Xe Cx 60 polycrystal for different loads
are shown in Fig. 2. As is evident, HV (T � 300 K) is
almost independent of indentation load, and only at a
high value of P � 0.15 N can one observe reduced
microhardness, supposedly due to active crack propa-
gation. To avoid the influence of crack formation and
consider H TV ( ) as a characteristic of plasticity, mea-
surements of the temperature dependence of micro-
hardness were made at a low load of P � 0.05 N.
The temperature dependences of microhardness for
the Xe Cx 60 polycrystal and for the pure C60 single
crystal studied in [6] (indentation plane (001)) are il-
lustrated in Fig. 3. A comparison of these curves al-
lows us to recognize the principal differences in the
micromechanical behavior between the two materials.
These differences are:
1. At room temperature the microhardness of the
Xe Cx 60 polycrystal is approximately three times
higher than that of the pure C60 single crystal; and as
the temperature is decreased down to 77 K, this dis-
crepancy reduces to two times.
Low temperature microhardness of Xe-intercalated fullerite C60
Fizika Nizkikh Temperatur, 2005, v. 31, No. 5 597
Fig. 1. The impression on the Xe Cx 60 polycrystal surface
for indenter load P � 0.2 N and room temperature. The
area of brittle fracture in the form of lateral (secondary)
cracks is marked by arrows. The impression diagonal is
about 30 �m.
2. As temperature is decreased from 300 to 225 K,
the microhardness of Xe Cx 60 polycrystal increases
linearly by � 20 %, while for the fcc single crystal of
pure C60 it remains almost uncharged (down to
260 K).
3. The step-like anomaly in the dependence H TV ( )
for pure C60, clearly seen under the transition from fcc
to the sc phase below 260 K, changes significantly.
Such an anomaly is observed for Xe Cx 60 too, but the
step edge is shifted towards lower temperatures (down
to 220 K) and the step itself becomes less steep and
highly smeared (� �T � 60 K for the intercalated
fullerite as compared to �T � 20 K for pure C60).
4. In the case of pure crystalline C60, the low tem-
perature edge of the step in the H TV ( ) curve is adja-
cent to an area of rather slight variations in micro-
hardness. Below 150 K this area turns into a region
of drastic growth in H TV ( ). As for the Xe Cx 60
polycrystal, an analog of the first area can also be ob-
served below 160 K, but we could not detect the sec-
ond range because of lack of microhardness measuring
technique for temperatures below 77 K.
Discussion
Before proceeding to a discussion of the anomalies
observed in the microhardness of C60 on its saturation
with xenon, some important results obtained in com-
parative x-ray diffraction studies of pure C60 and
Xe Cx 60 polycrystals should be mentioned [13]. Intro-
duction of Xe conserves the fcc structure of the greater
part of the material (90 %), but in Xe Cx 60 there also
exists � 10 % of another phase (more probably, poly-
merized C60 molecules). Besides, considerable distor-
tions of the fcc lattice by the Xe atoms cause the fcc-sc
transition temperature, Tc, and the glass transition
point, Tg , to decrease. This correlates with the anoma-
lies in the temperature dependence of microhardness:
The saturation with xenon results in a substantial
hardening and an embrittlement of the fullerite and
similarly, in a shift of the anomalies in the H TV ( )
curve.
Investigations performed by several teams of re-
searchers (see [8] and the references ibid.) have re-
vealed that macro- and microplastic deformation of
fullerite C60 at room and at moderately low tempera-
tures is determined by the glide of dislocations in the
{111}
110 slip system; the system is kept efficient
even after the fcc–sc transition. As the saturation of
fullerite with xenon produces no change in its lattice
structure, the scientific treatment of the observed dif-
ference in microhardness between the pure C60 single
crystals and Xe Cx 60 polycrystals should be limited to
a discussion of some possible differences in those
structural factors and mechanisms which dictate the
dislocation mobility in these materials.
On the qualitative level, one should distinguish
two effects caused by the influence of intercalation.
These are (1) a rather substantial increase in the abso-
lute values (background) of microhardness HV and
(2) a certain transformation of subtle peculiarities in
598 Fizika Nizkikh Temperatur, 2005, v. 31, No. 5
L.S. Fomenko, S.V. Lubenets, V.D. Natsik, D. Cassidy, G.E. Gadd, S. Moricca, and B. Sundqvist
0 0.05 0.10 0.15 0.20
0.3
0.4
0.5
0.6
P, N
H
,G
P
a
v
–
Fig. 2. The dependence of average microhardness HV of
Xe Cx 60 on indenter load P at room temperature. The stan-
dard deviations for HV are indicated as bars.
50 100 150 200 250 300
0.4
0.5
0.6
0.7
0.8
0.9
T, K
0.2
0.3
0.4
0.5
0.6
50 100 150 200 250 300
C 60
T. K
C
60
Xe x
H
,G
P
a
v
H
,G
P
a
v
– –
�T’
�T
Fig. 3. The temperature dependence of HV for the Xe Cx 60
polycrystal (upper curve) and pure C60 single crystal
(lower curve) [6]. The standard deviations for HV are
about 3 %.
the temperature dependence H TV ( ) under the fcc–sc
transition. The origin of these effects may generally be
accounted for by different physical and structural fac-
tors. The first effect may be caused by the different
morphology of the Xe-intercalated sample and by the
influence of Xe atoms on the elastic properties and the
mobility of the dislocations. The other one is probably
connected with the influence of the Xe impurity on
the dynamics of the orientational degrees of freedom
of the C60 molecules, and therefore, on the disloca-
tion-orientational interaction. Below is discussed the
influence of these factors in detail.
First of all, we shall consider a possible influence
of the morphologies of the materials compared. One of
these is a reasonably perfect single crystal, while the
other is a polycrystal with inclusions of another phase.
The difference in morphology between the two materi-
als is due to the specific features of the intercalation
technology [3,12]; unfortunately, at present there is
no other way of preparing macroscopically homoge-
neous single crystals adequately saturated with xenon.
The very high microhardness of the Xe Cx 60
polycrystal as compared to that of the C60 single crys-
tal may be caused by two factors: (i) the hardening ef-
fects of both the intergrain boundaries and secondly
(ii) the minor second phase particles within the crys-
talline grains. By the Hall–Petch relation [15], the
yield stress and microhardness of a polycrystal are
higher than those of a single crystal by an amount in-
versely proportional to the square root of polycrystal
grain size. The second-phase fine particles form local
barriers which impede dislocation slip, reducing ap-
preciably the plastic compliance and increasing the
microhardness of the material (that mechanism of
hardening is well known in the physics of ageing alloy
plasticity [16]). It should also be noted that any po-
rosity induced from pressing the specimens should
have an opposite effect, deteriorating the mechanical
properties [17,18]. The softening due to this factor ap-
pears to be slight.
We now turn to the effect of gaseous impurities on
dislocation mobility in the bulk of a separate crystal-
line grain. A high Xe concentration in the lattice, as
well as its almost identical size with that of the octa-
hedral site which it occupies, suggest that as a first ap-
proximation by concentration, the intercalation does
not disturb the macrostructural homogeneity of the
material inside the grains. Because of this, separate
impurity atoms produce no effective local barriers to
dislocation slip in the sense of the term «local barrier»
used in the physics of plasticity of weakly and moder-
ately concentrated solid solutions [16]. At the same
time, in a highly concentrated solid solution second
order-in-concentration effects, and in particular, the
presence of microclusters with a short-range order of
impurities, are supposed to be of a certain importance.
For example, in some unit cells of the lattice the impu-
rity atoms occupy all octahedral sites while other unit
cells remain unoccupied. Such microclusters may be
effective centers of dislocation drag, thus enhancing
the material’s microhardness.
One more thing should be mentioned: for fullerite
C60 and many atomic cubic crystals, a significant in-
fluence on the plastic compliance may be exerted by
dislocation drag by the Peierls relief which becomes
essential for a low density of local barriers with low
height [19]. It may therefore appear that the effect of
gaseous impurities on the microhardness of fullerite is
partially due to the change of the Peierls relief caused
by the influence of a great number of interstitial atoms
on the lattice parameter and intermolecular interac-
tion. Unfortunately, even the sign of that influence
cannot be predicted without detailed microscopic cal-
culations.
Finally, one more important factor should be
pointed out, namely, a considerable enhancement in
the elastic moduli at the transition from pure crystals
to highly concentrated solid solutions. According to a
well known law in materials science [20], the basic
value of the microhardness of a material as a local
characteristic of plasticity is proportional to the
Young modulus. Independent measurements of the
elastic moduli of intercalated fullerites (for example,
by acoustic techniques or by Brillouin-light-scattering
measurements) would enable one to determine the in-
dividual contributions to enhancement of their hard-
ness but at present such data is not available.
Above we have discussed the principal factors that
dictate the order of magnitude of the yield stress and
microhardness and the general background of their
temperature dependences in a wide range of moder-
ately low temperatures for both pure and atomic im-
purity-intercalated fullerite C60. From the general
conclusions of the theory of thermally activated dislo-
cation motion [21,22], the background should be
rather smooth and involve no distinct features like
kinks which are seen in the dependences H TV ( ) for
pure and intercalated fullerites C60. It would there-
fore appear reasonable that these observed anomalies
are caused by the effect of the orientational degrees of
freedom of the C60 molecule on the dislocation mobil-
ity [6,23]. A direct support for this suggestion may be
the correlations between the anomalies in the H TV ( )
dependences and the anomalies of thermal and acous-
tic properties which may be considered as an indicator
of the orientational phase transitions in fullerite C60:
the fcc-sc phase transition and the orientational glass
transition. The absolute contribution of disloca-
Low temperature microhardness of Xe-intercalated fullerite C60
Fizika Nizkikh Temperatur, 2005, v. 31, No. 5 599
tion-orientation interaction to dislocation drag forces
is not high but it has peculiarities as mentioned above
and therefore can be qualitatively identified in the
background of contributions from other factors. In
conclusion, the effect of interaction on subtle anoma-
lies like kinks in the temperature dependences of the
lattice parameter, a(T), and the microhardness,
H TV ( ), is considered to be caused by the influence of
gaseous impurities on the libration and rotation dy-
namics of C60 molecules.
It is evident that intercalation of the fullerite lat-
tice by a great number of interstitial atomic impurities
has an appreciable effect on the dynamics of the
orientational degrees of freedom of the C60 molecules,
producing a shift along the temperature scale and a
smearing of the orientational fcc-sc and orientational
glass transitions. This is supported by the x-ray dif-
fraction data for Xe Cx 60 [13] testifying that the val-
ues of temperature Tc and the glass transition point Tg
decrease by 30–40 K. The comparison between the
dependences H TV ( ) for pure and Xe-saturated
fullerites shown in Fig. 3 and their correlation with
the x-ray diffraction data for the temperature
dependences of the lattice parameter, a(T) [13], indi-
cate that the dislocation-orientational interaction de-
termines the characteristic anomalies in the tempera-
ture dependence of the microhardness even in the
presence of gaseous impurities. The influence of gas-
eous impurities on the orientation dynamics of the C60
molecules and the parameters of the lattice-orienta-
tion interaction can be estimated quantitatively by an-
alyzing simultaneously x-ray diffraction, thermal, me-
chanical and acoustic data [24]. Although such data is
available for pure C60 fullerite [6,25], no such data is
available for Xe Cx 60, making these estimations diffi-
cult at present.
The authors would like to express their gratitude to
V.G. Manzhelii, M.A. Strzhemechny and A.I. Prokh-
vatilov for their helpful discussion of the results and
to the STCU for partial financing of the research un-
der Project No. 2669.
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Temp. Phys. 29, 440 (2003)].
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