Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity
The linear expansion coefficients of solid bulk samples of N₂O with Kr 5% impurity are measured by an absolute dilatometric method in comparison with pure N₂O in the temperature range 80–150 K. An additional unusual orientational effect is discussed. An analysis of the data from measurements of the...
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| Zitieren: | Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity / N.N. Zholonko, V.V. Tsibulin, I. Sarwar // Физика низких температур. — 2003. — Т. 29, № 9-10. — С. 1018-1020. — Бібліогр.: 16 назв. — англ. |
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irk-123456789-1289242018-01-15T03:04:29Z Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity Zholonko, N.N. Tsibulin, V.V. Sarwar, I. Low-Temperature Thermodynamics and Structure The linear expansion coefficients of solid bulk samples of N₂O with Kr 5% impurity are measured by an absolute dilatometric method in comparison with pure N₂O in the temperature range 80–150 K. An additional unusual orientational effect is discussed. An analysis of the data from measurements of the linear expansion coefficients of pure solid SF₆, CHCl₃,C₆H₆, CCl₄ in comparison with solid Xe in the temperature range 80–170 K is carried out in order to determine the role of molecules` orientational disordering in the thermal expansion of the given condensed systems. The results are discussed in connection with the problem of determining the contribution of orientational subsystems with different types of molecular symmetry to the total thermal expansion and its behavior in various temperature intervals of solid phase existence. 2003 Article Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity / N.N. Zholonko, V.V. Tsibulin, I. Sarwar // Физика низких температур. — 2003. — Т. 29, № 9-10. — С. 1018-1020. — Бібліогр.: 16 назв. — англ. 0132-6414 PACS: 65.70.+y http://dspace.nbuv.gov.ua/handle/123456789/128924 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Low-Temperature Thermodynamics and Structure Low-Temperature Thermodynamics and Structure |
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Low-Temperature Thermodynamics and Structure Low-Temperature Thermodynamics and Structure Zholonko, N.N. Tsibulin, V.V. Sarwar, I. Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity Физика низких температур |
| description |
The linear expansion coefficients of solid bulk samples of N₂O with Kr 5% impurity are measured by an absolute dilatometric method in comparison with pure N₂O in the temperature range 80–150 K. An additional unusual orientational effect is discussed. An analysis of the data from measurements of the linear expansion coefficients of pure solid SF₆, CHCl₃,C₆H₆, CCl₄ in comparison with solid Xe in the temperature range 80–170 K is carried out in order to determine the role of molecules` orientational disordering in the thermal expansion of the given condensed systems. The results are discussed in connection with the problem of determining the contribution of orientational subsystems with different types of molecular symmetry to the total thermal expansion and its behavior in various temperature intervals of solid phase existence. |
| format |
Article |
| author |
Zholonko, N.N. Tsibulin, V.V. Sarwar, I. |
| author_facet |
Zholonko, N.N. Tsibulin, V.V. Sarwar, I. |
| author_sort |
Zholonko, N.N. |
| title |
Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity |
| title_short |
Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity |
| title_full |
Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity |
| title_fullStr |
Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity |
| title_full_unstemmed |
Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity |
| title_sort |
role of the orientational subsystem in the expansion of pure cf₆, chcl₃, c₆h₆, ccl₄ and n₂o with kr impurity |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2003 |
| topic_facet |
Low-Temperature Thermodynamics and Structure |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/128924 |
| citation_txt |
Role of the orientational subsystem in the expansion of pure CF₆, CHCl₃, C₆H₆, CCl₄ and N₂O with Kr impurity / N.N. Zholonko, V.V. Tsibulin, I. Sarwar // Физика низких температур. — 2003. — Т. 29, № 9-10. — С. 1018-1020. — Бібліогр.: 16 назв. — англ. |
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Физика низких температур |
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2025-07-09T10:14:26Z |
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| fulltext |
Fizika Nizkikh Temperatur, 2003, v. 29, Nos. 9/10, p. 1018–1020
Role of the orientational subsystem in the expansion of
pure CF6, CHCl3, C6H6, CCl4 and N2O with Kr impurity
N.N. Zholonko and V.V. Tsibulin
Cherkasy State University of Technology, 460 Shevchenko Blvd., Cherkasy 18006, Ukraine
E-mail: zholonko@yahoo.com
I. Sarwar
National Technical University of Ukraine «KPI», 37 Peremogi Ave., Kiev, Ukraine
The linear expansion coefficients of solid bulk samples of N O2 with Kr 5% impurity are mea-
sured by an absolute dilatometric method in comparison with pure N O2 in the temperature range
80–150 K. An additional unusual orientational effect is discussed. An analysis of the data from
measurements of the linear expansion coefficients of pure solid SF6, CHCl3, C H6 6, CCl4 in compari-
son with solid Xe in the temperature range 80–170 K is carried out in order to determine the role
of molecules’ orientational disordering in the thermal expansion of the given condensed systems.
The results are discussed in connection with the problem of determining the contribution of
orientational subsystems with different types of molecular symmetry to the total thermal expan-
sion and its behavior in various temperature intervals of solid phase existence.
PACS: 65.70.+y
Introduction
Unlike the hardened inert gases, the interaction be-
tween molecules in molecular crystals has a more com-
plicated character due to the existence of angular evo-
lutions of the orientational subsystem and also
fluctuational intramolecular oscillations. In the analy-
sis of crystal lattice dynamics the last factors can
rather often be neglected [1] because of the signifi-
cantly larger energy of bonding in a molecule as com-
pared to the energy of sublimation E. It allows one to
simplify the analysis and to concentrate attention on
the peculiarities of the interaction between the collec-
tive translational excitations (phonons) and the orien-
tational molecular movements.
Depending on the molecular symmetry and tem-
perature, the orientation excitations in molecular
crystals can have the collective character of waves
(librons) or even almost free rotation of separate mo-
lecules. In some molecular crystals (N O2 , CO2,
CHCl3) the anisotropic interaction is so strong that
the crystal melts before complete orientational disor-
dering occurs. When diluted by simple atomic parti-
cles of nearly the same size, such systems often can be-
have in a rather unusual way. On the other hand,
there still exist rather complicated problems in under-
standing of the temperature dependences of the ther-
mal properties for many pure molecular crystals. This
paper is devoted to further study of high-temperature
behavior of the linear expansion of molecular crystals
with different symmetry of the particles.
Main part
The studies of the thermal expansion coefficients of
solid SF6, CHCl3, CCl4 and C H6 6 were carried out in
an interval of temperatures 85–170 K on an optical la-
ser Michelson interferometric dilatometer. We mea-
sured the linear thermal expansion factors for four
samples of solid SF6, 6 mm in diameter and 10 cm
long, each grown from the gas, and also two samples
of CCl4, three samples of CHCl3, and two samples of
C H6 6, obtained from the liquid. The total error
of determination of the linear thermal expansion is
10–15 %.
The high-temperature � phase of solid SF6 [2–9]
has a cubic bcc lattice of space symmetry Im3m. The
existence region of the� phase is extraordinarily large:
the crystallization of SF6 occurs at 222.4 K, and the
phase transition lowering the symmetry of the trans-
lational and orientational subsystems of the crystal
© N.N. Zholonko, V.V. Tsibulin, and I. Sarwar, 2003
does not occur until 94.3 K. The interaction between
the nearest neighbors in the bcc phase tends to order
the molecules so that their S–F bonds lie along
the {100} direction, and in the interaction with the
next-nearest neighbors a repulsion between the fluo-
rine atoms predominates. The data from recent struc-
tural studies indicate a strict orientational order in
SF6 above the phase transition temperature. This
makes SF6 different from such plastic crystals as
CH , CCl4 4, adamantane, etc., in which the destruc-
tion of the long-range order occurs immediately after
the phase transition. The intensive growth of processes
of orientational disordering begins in SF6 only at tem-
peratures above 150 K and is of a dynamic nature. The
SF6 molecule has octahedral symmetry which means a
spherical rotator with three perpendicular main axes.
The CCl4 molecule is also highly symmetric (tetrahe-
dral) and is a spherical rotator with nonperpendicular
main axes. As opposed to this, the CHCl3 and C H6 6
molecules are less symmetrical objects. The first have
an even nonzero dipole moment. They both are sym-
metrical rotators only (not spherical). Therefore it
will cause larger barriers interfering rotation.
The contribution of rotary movement to the ther-
mal expansion can be appraised by a comparison of the
corresponding properties of molecular crystals and
hardened inert gases. The properties of the latter are
connected with translational movement of molecules
only, without rotations. To allocate the contribution
of the rotary subsystem to the thermal expansion of
the molecular crystals studied, we used the already
known temperature dependence of the volume thermal
expansion coefficient of solid Xe [10]. The latter has a
mass close to that of the molecular crystals studied.
The experimental dependences for all the investigated
substances in comparison with Xe are plotted in re-
duced coordinates in Fig. 1, where � is the coefficient
of linear expansion, Å is the sublimation energy and Tt
is the temperature of three-phase equilibrium.
In Fig. 1 we can see the known phase transition for
solid SF6 (near 94,3 K), near which a jumplike drop is
observed on the temperature growth of the thermal ex-
pansion coefficient. Such anomalies of the thermody-
namic properties are caused by inhancement of the
correlation of rotary movement [10]. Our dilatometric
SF6 results are in good agreement with x-ray diffrac-
tion data [11]. This allows us to check our experimen-
tal equipment. Active transitions from orientational
molecular oscillations to rotations of molecules in
solid SF6 give a weak additional contribution to ther-
mal expansion in the form a somewhat larger slope of
the smoothed line in comparison with Xe. In the
low-temperature phase (lower than 94,3 Ê), where
the free rotation is absent altogether, the above-men-
tioned effect appears significant. Thus, the weak de-
pendence of thermal expansion coefficient for solid
SF6 at temperatures higher than the phase transition
temperature in the given coordinates indicates the
transition to rotational molecular movement. In other
words, SF6 molecule rotational correlations are weak-
ened above the phase transition.
The low-temperature phase of solid CCl4 is mono-
clinic (Ñ2/ñ–C h2
6 ) [13] and contains 32 molecules per
unit cell. The molecular centers of interaction have
tetrahedral symmetry and a slightly deformed cubic
bcc lattice. The close values of the slopes of the re-
duced curves for solid CCl4, Xe and SF6 may also in-
dicate the transition of the movement of some of the
molecules to hindered rotation in the monoclinic
phase in the temperature range 80–170 Ê. Similar be-
havior of a rotary subsystem of CCl4 crystals is caused
by the high tetrahedral symmetry of their molecules.
The high-temperature phase of solid chloroform
CHCl3 has orthorhombic structure of space symmetry
Pnma D h� 2
16 with four molecules per unit cell, tri-
ple-point temperature Tt = 209.5 K [14]. The non-
central forces in CHCl3 crystals are great. The ori-
entational order is maintained up to the melting
temperature and the rotary movement constitutes li-
brations. The reorientation frequencies do not exceed
104 per second [15,16].
As can be seen from Fig. 1, for the asymmetrical
chloroform molecules, with a nonzero dipole moment
in the low-temperature region, the additional effect is
absent, as is indicated by the insignificant increase of
the libration amplitude with the temperature. How-
ever, with further increase in temperature we have
a stronger dependence of the thermal expansion of
CHCl3 crystals in reduced coordinates. This can be
Role of the orientational subsystem
Fizika Nizkikh Temperatur, 2003, v. 29, Nos. 9/10 1019
60
40
20
0
0.3 0.5 0.7 0.9
T/T
Xe
CHC
CC
3
4
SF6
SF6 C H6 6
t
E
, J
/(
m
o
l·K
)
�
Fig. 1. Temperature dependence of the linear expansion
coefficients � for solid SF6, CHCl3, C H6 6, CCl4 in redu-
ced coordinates, where E is the sublimation energy and Tt
is the triple point.
seen from a comparison with solid CCl4, which has a
central interaction of about the same value but a lower
rotary barrier. This last circumstance indicates appre-
ciable growth of the libration amplitudes.
At pressures lower than 1.2 ÌPà solid benzene
C6H6 exists in one crystalline form corresponding to
space group Pbca D h� 2
15 with four molecules per unit
cell. A weak orientation effect is present near the tran-
sition of C H6 6 to rotation about the sixfold symmetry
axis (90–120 Ê). At Ò > 120 K, however, the depend-
ence of the thermal expansion in C H6 6 in reduced co-
ordinates is stronger even than in CHCl3. This may
also attest to an increase of the libration amplitudes of
the benzene molecules about two other twofold to the
symmetry axes lying in the plane of the benzene ring.
As CHCl3 and N O2 molecules, they are far from
spherical symmetry and have a dipole moment. Stu-
dies of the thermal expansion coefficients of solid bulk
samples of N O2 with 5% Kr were carried out in an in-
terval of temperatures 85–150 K on the same equip-
ment. We measured the linear thermal expansion fac-
tors for two samples. The experimental dependence in
comparison with pure N O2 is shown in Fig. 2. As we
can see, dilution with atomic Kr causes a weakening of
the dependence. The difference of the thermal expan-
sion coefficients is seen to grow with increasing tem-
perature. This anomaly of the thermodynamic proper-
ties may be caused by easing of the free rotation due to
the replacement of one-fifth of the linear by spherical
atoms. It should be emphasized that there are unpub-
lished data indicating that the solubility of Kr in N O2
is not less than 5 percent.
Conclusions
The thermal expansion orientational effect in re-
duced coordinates is weakly expressed for the more
symmetric molecules SF6 and CCl4 in the high-tem-
perature region investigated, while for the less sym-
metrical molecules CHCl3 and C H6 6 the effect is sig-
nificant. This could be explained by the fact that for
growth of the libration amplitudes the octahedral
molecules SF6 and tetrahedral CCl4 require smaller
additional volumes, and the transition to their hin-
dered rotation is not accompanied by an appreciable
volume increase. At the same time CHCl3 and C H6 6
molecules do not pass to the state of hindered rotation
at the temperatures of experiment (except for rotation
around the sixfold axis for solid benzene), and for
increase of the libration amplitudes and jumps to adja-
cent equilibrium orientations with higher energy le-
vels they require significant additional volumes. Sub-
stitution of the spherically symmetric impurity Kr for
the N O2 molecules leads to a weaker growth of the
linear expansion with the temperature then for pure
solid N O2 . This may indicate that the high-tempera-
ture volume effects in solid N O2 are influenced more
strongly by the rotational states than by the trans-
lational states.
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1020 Fizika Nizkikh Temperatur, 2003, v. 29, Nos. 9/10
N.N. Zholonko, V.V. Tsibulin, and I. Sarwar
2.5
2.0
1.5
1.0
70 80 90 100 110 120 130 140 150
T , K
, 1
0
K
–
4
–
1
�
Fig. 2. Temperature dependence of the linear expansion co-
efficients � for solid pure N O2 (top curve) and N O2 with
5% Kr (bottom curve).
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