Crystal Structure of High Temperature Phase and Ionic Conductivity Mechanism of CuHgSX (X = Cl, Br)

Article abstract of DOI:10.1246/bcsj.76.2111

We found a new ionic conduction phase in CuHgSCl above 373 K and in CuHgSBr above 346 K. The crystal structures of these novel phases have been determined by Rietveld refinement of powder X-ray diffraction patterns. The electric conductivity at 500 K measured by AC impedance method was 1.4 × 10^-5 S cm^-1 for CuHgSCl and 4.0 × 10^-6 S cm^-1 for CuHgSBr. The activation enthalpy was determined to be 53 kJ mol^-1 for CuHgSCl and 67 kJ mol^-1 for CuHgSBr. The ionic transport number measurements indicated that Cu⁺ ions constitute the majority charge carriers in these samples. The electronic contribution to the conduction process is small in comparison with the Cu⁺ ionic contribution. The charge density analysis by the maximum entropy method (MEM) combined with Rietveld analysis clearly showed that the Cu⁺ ionic conduction path was along the crystallographic (100) direction.

Full text of DOI:10.1246/bcsj.76.2111

Ó 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 2111–2115 (2003) 2111
Crystal Structure of High Temperature Phase and Ionic Conductivity
Mechanism of CuHgSX (X = Cl, Br)
ꢀ
Masakazu Moro’oka, Hiroshi Ohki, Koji Yamada, and Tsutomu Okuda
Department of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526
Received April 14, 2003; E-mail: morooka@hiroshima-u.ac.jp
We found a new ionic conduction phase in CuHgSCl above 373 K and in CuHgSBr above 346 K. The crystal struc-
tures of these novel phases have been determined by Rietveld refinement of powder X-ray diffraction patterns. The elec-
tric conductivity at 500 K measured by AC impedance method was 1:4 ꢁ 10ꢂ5 S cmꢂ1 for CuHgSCl and 4:0 ꢁ 10ꢂ6
S cmꢂ1 for CuHgSBr. The activation enthalpy was determined to be 53 kJ molꢂ1 for CuHgSCl and 67 kJ molꢂ1 for
CuHgSBr. The ionic transport number measurements indicated that Cuþ ions constitute the majority charge carriers
in these samples. The electronic contribution to the conduction process is small in comparison with the Cuþ ionic
contribution. The charge density analysis by the maximum entropy method (MEM) combined with Rietveld analysis
clearly showed that the Cuþ ionic conduction path was along the crystallographic (100) direction.
It is well known that copper and silver halogenides are cat-
ionic conductors. Many double salts of these halogenides with
other compounds show higher conductivity at lower
temperature. For example, Cu2HgI4 and Ag2HgI4, which are
synthesized by the reaction of CuI or AgI with HgI2, undergo
phase transitions at 342 K and 323 K, respectively, and the elec-
tric conductivity ꢀ jumps one to two orders of magnitude.
In the family of MHgSX (M = Cu, Ag; X = Cl, Br, I), which
were prepared by combination of copper and silver halogenides
with HgS, only a few compounds have been characterized.
Gullio et al. determined the crystal structures of CuHgSCl
and CuHgSBr.1 They showed the electrical conductivity and
activation energy only at 298 K for CuHgSBr. Blachnik et
al. synthesized AgHgSBr and AgHgSI, and made a phase dia-
gram of the AgI–HgS system.2 They synthesized AgHgSCl and
examined the equilibrium diagrams of the CuCl–HgS and
AgCl–HgS systems as well.3 On the other hand, Beck et al.
reported the structures of CuHgSeBr, AgHgSBr and AgHgSI
synthesized by hydrothermal synthesis.4 Recently, they report-
ed new structures of CuHgSCl and CuHgSBr, which are differ-
ent from those given by Gullio et al.5
In this paper, we report the conductivity and the mechanism
for the high temperature phases of CuHgSCl and CuHgSBr.
These compounds were easily synthesized by solid-state
reaction. Although the structure of the high temperature phase
of CuHgSBr was determined by Beck et al., the structure and
the phase transition temperature for CuHgSCl are unknown.
Therefore, we have determined the structure of CuHgSCl by
Rietveld refinement of powder X-ray diffraction pattern. More-
over, the Cuþ ion behavior at high temperatures was studied by
maximum entropy method (MEM)/Rietveld method. We will
discuss the ion conduction mechanism for CuHgSX (X = Cl,
Br).
Experimental
CuHgSX (X = Cl, Br) were prepared by heating stoichiometric
mixtures of CuX (Kojundo Chemical Lab. Co.) and red-HgS
(Kojundo Chemical Lab. Co.) in an evacuated Pyrex tube at ca.
573 K for 2 weeks. CuX was synthesized by a solid-state reaction
between CuX2 (Kojundo Chemical Lab. Co.) and powdered Cu
metal (Soekawa Chemicals Co.).
Differential thermal analysis (DTA) measurements were carried
out by use of a homemade apparatus. The powder X-ray diffrac-
tion patterns were measured on a Rigaku Rint 2000 system using
Cu-Kꢁ radiation with scan rate = 0.5 degree/min and scan step
of 0.02 degree.
The crystal structure of the high temperature phase was initially
guessed by the Monte Carlo Method,6 and refined by Rietveld
method using the RIETAN2000 program.7
The electron densities of these crystals were analyzed by Max-
imum Entropy Method (MEM) combined with the Rietveld pattern
fitting.8 During the electron density analysis, the unit cell of the
ꢀ
sample was divided into edges of ca. 0.1 A each.
The electric conductivity was determined by AC impedance
method. The complex impedance measurements were performed
by the two-terminal method using a computer-interfaced HIOKI
3532 LCR meter (42 Hz–5 MHz). The pellet sizes were 13 mm
diameter and 1.5–1.8 mm thickness. Carbon paint was applied
on both sides of the pellet and electrodes to insure a good contact
with each other. The temperature of the sample was controlled
within ꢃ1 K using a Chino KP1000 temperature controller equip-
ped with Cu–Constantan thermocouples.
The ionic transport measurements were carried out using an ion-
blocking method and a non-blocking method. (i) In the non-block-
ing method, the two pellets of the sample were sandwiched be-
tween copper electrodes. A constant DC current of 0.01 mA
was applied across the sample for 5 days at 500 K using an AD-
VANTEST programmable DC voltage/current generator. (ii) In
the ion-blocking method, the two pellets of the sample were sand-
Published on the web November 15, 2003; DOI 10.1246/bcsj.76.2111

2112 Bull. Chem. Soc. Jpn., 76, No. 11 (2003)
Ionic Conductivity of CuHgSX (X = Cl, Br)
Table 1. Experimental Conditions and Crystallographic Da-
ta for CuHgSCl at 400 K
Compound
Space group
Crystal system
Color
CuHgSCl
Pmam (No. 51)
Orthorhombic
Orange
a ¼ 9:8870ð3Þ
b ¼ 8:8554ð3Þ
c ¼ 4:1036ð1Þ
359.29(2)
6.13
ꢀ
Lattice constants/A
ꢀ 3
Volume/A
dcal/g cmꢂ3
Z
4
5–100
0.02
3.54
Range 2ꢂ/ꢄ
Scan step/ꢄ
Rp/%
Rwp/%
Rexp/%
RBragg/%
S
4.62
3.12
4.10
1.48
Fig. 1. The DTA curves of CuHgSX (X = Cl, Br).
Table 2. Positional Parameters of CuHgSCl at 400 K
ꢀ 2
Atom
Position
x
y
z
Uiso/A
Cu
4(i)
2(b)
2(d)
4(j)
2(e)
2(e)
0.0825(3)
1/2
1/4
0.0072(4)
1/4
1/4
0.3032(3)
1/2
0.7172(2)
0.7238(5)
0.0495(7)
0.4743(7)
0
0.079(1)
0.0433(6)
0.0323(6)
0.026(2)
0.044(2)
0.032(2)
Hg1
Hg2
S
Cl1
Cl2
1/2
1/2
1/2
0
0
ꢄ
ꢀ
Table 3. Interatomic Distance (A) and Bond Angles ( ) in
CuHgSCl at 400 K
wiched between copper electrodes with carbon paint on both sides
of the pellets. A constant DC voltage of 10 V was applied across
the sample for 3 days at room temperature using an ADVANTEST
programmable DC voltage/current generator.
Cu–X1
Cu–X2
Cu–S
Hg1–X2
Hg1–S
Hg2–X1
Hg2–S
2.787(2)
2.252(2)
2.246(3)
2.974(3)
2.404(2)
3.242(3)
2.442(2)
Results
DTA. Figure 1 shows the DTA curves for CuHgSX (X =
Cl, Br). For CuHgSCl, an endothermic peak appeared at 373
K on heating and an exothermic one at the same temperature
on cooling. As for CuHgSBr, endothermic and exothermic
peaks were seen at 346 K on both heating and cooling
processes. The thermal decomposition takes place above 727
K for CuHgSCl and above 627 K for CuHgSBr.
Rietveld Analysis. Figure 2 shows the result of the Rietveld
refinement using powder X-ray diffraction patterns of
CuHgSCl at 400 K. The space group and the lattice parameters
are summarized in Table 1 along with the refinement
conditions. Table 2 lists the atomic positions and the isotropic
thermal parameters determined through the refinement, while
Table 3 shows the selected interatomic bond distances and
angles. Figure 3 depicts the crystal structures in the low and
high temperature phases of CuHgSCl.
S–Cu–S
131.9(4)
X1–Cu–X2
S–Hg1–S
S–Hg2–S
96.2(1)
177.0(4)
180
7, respectively.
Conductivity. Figure 8 represents the electric conductivity
plotted against the inverse temperature for CuHgSX. With
increasing temperature, the conductivity increased mono-
tonously. For CuHgSCl, no obvious change was observed at
the phase transition temperature. From the slopes of these
curves, the activation enthalpies for conduction were deter-
mined to be 53 kJ molꢂ1 for CuHgSCl and 67 kJ molꢂ1 for
CuHgSBr.
MEM Analysis. Figure 4 shows the electron density maps
along (001) plane for CuHgSCl at room temperature. The elec-
tron distribution on each atom is shown. The contour lines
were drawn in log scale intervals. Figure 5 indicates the charge
density map at 500 K.
Ionic Transport.
The measurements by non-blocking
method indicate that Cuþ ions constitute the majority charge
carriers in these samples. A copper metallic luster was separat-
ed out in between sample and electrode. Moreover, the ionic
The electron density maps for CuHgSBr are deduced and the
results at room temperature and 500 K are shown in Figs. 6 and

M. Moro’oka et al.
Bull. Chem. Soc. Jpn., 76, No. 11 (2003) 2113
Fig. 2. Observed (cross) and calculated (solid line) X-ray diffraction pattern of CuHgSCl at 400 K. Tick marks indicate the positions
of allowed Bragg reflections. The difference line, observed minus calculated, is located at the bottom of the figure.
Fig. 4. The charge density maps determined by MEM along
(001) plane for CuHgSCl at room temperature (a) at Cu–Cl
plane and (b) at Hg–S plane. The contour lines are drawn
Fig. 3. Crystal structure of (a) low-temperature phase
reported by Beck et al. and (b) high-temperature phase at
400 K for CuHgSCl.
ꢀ ꢂ3
(a) from 5.0 to 240.0 e A and (b) from 5.0 to 1200.0
ꢀ ꢂ3
e A at log scale intervals, respectively.

2114 Bull. Chem. Soc. Jpn., 76, No. 11 (2003)
Ionic Conductivity of CuHgSX (X = Cl, Br)
Fig. 5. The charge density maps determined by MEM along
(001) plane for CuHgSCl at 500 K (a) at Cu–Cl plane and
(b) at Hg–S plane. Both planes display four unit cells. The
ꢀ ꢂ3
contour lines are drawn (a) from 5.0 to 240.0 e A and (b)
ꢀ ꢂ3
from 5.0 to 1200.0 e A at log scale intervals, respectively.
Fig. 6. The charge density maps determined by MEM along
(001) plane for CuHgSBr at room temperature (a) at Cu–Br
plane and (b) at Hg–S plane. The contour lines are drawn
transport number values using ion-blocking method revealed
that the electronic contribution to the conduction process is
about 10%, in comparison with about 90% of the Cuþ ionic
contribution. Ionic transport number (tion) of the sample was
examined using the relation tion ¼ Iion=It, where Iion is the cur-
rent due to the mobile ions and It is the total current due to all
the mobile species.
ꢀ ꢂ3
(a) from 5.0 to 240.0 e A and (b) from 5.0 to 1200.0
ꢀ ꢂ3
e A at log scale intervals, respectively.
1=2 plane. Cu is surrounded by S and Cl, and is set in the center
of a CuS2Cl2 tetrahedra. As shown in Fig. 3, the atomic posi-
tions were not changed so much. This finding also satisfies a
displacive-type phase transition.
Ionic Conductivity and the Mechanism. As shown in
Fig. 8, the conductivity for CuHgSCl was higher than that of
CuHgSBr. The activation enthalpy is obtained from the follow-
ing equation.9
Discussion
DTA. We observed a reversible phase transition with no
thermal hysteresis at 373 K for CuHgSCl and at 346 K for
CuHgSBr. The crystal structure reported by Beck et al.4 corre-
sponds to the low temperature phase. These findings indicate
that the phase transitions observed in both compounds are of
second order. Thus the space group of the low temperature
phase should be a subgroup of the high temperature phase. This
will be described below.
Crystal Structure of High Temperature Phase for
CuHgSCl. The space group notation Pmam was used here,
which is different from Pmma (No. 51), because the length of
b-axis in the high temperature phase is half of that in the low
temperature phase. The lengths of a- and c-axes are
unchanged. Moreover, Pmam belongs to a subgroup of Pbma
that is consistent with the above estimation. Hg and S are lo-
cated in the z ¼ 0 plane and form zig-zag chains along a-
axis. On the other hand, Cu and Cl are located in the z ¼
ꢀðTÞ ¼ ꢀ0 expðꢂꢁH=RTÞ;
ð1Þ
where ꢀ is the electric conductivity. By fitting Eq. 1 to the ob-
served data, the activation enthalpy ꢁH was evaluated to be 53
kJ molꢂ1 for CuHgSCl and 67 kJ molꢂ1 for CuHgSBr. These
ꢁH values would correspond to the Cu motion and the forma-
tion for defects, since all Cu sites in CuHgSX are completely
occupied.9
We tried to identify the motion of the Cuþ ion using
63Cu NMR, but failed to detect the signal, due probably to large
quadruple interaction of Cu. Therefore, the evidence has been
obtained from the comparison of the charge density maps of

M. Moro’oka et al.
Bull. Chem. Soc. Jpn., 76, No. 11 (2003) 2115
Fig. 8. Temperature dependence of electric conductivity in
CuHgSX (X = Cl, Br).
346 K, respectively. The unit cell in the high temperature phase
is halved compared with that in the low temperature one. The
ionic conductions of CuHgSX (X = Cl, Br) were carried out by
Cuþ ion jumping to the neighboring Cu sites along (100)
direction. The Cuþ ion motion and the formation of defects
need 53 kJ molꢂ1 for CuHgSCl and 67 kJ molꢂ1 in CuHgSBr.
The conductivity of CuHgSCl appears higher than CuHgSBr,
since bigger Br blocks the Cuþ ion conduction path.
Fig. 7. The charge density maps determined by MEM along
(001) plane for CuHgSBr at 500 K (a) at Cu–Br plane and
(b) at Hg–S plane. Both planes display four unit cells. The
ꢀ ꢂ3
contour lines are drawn (a) from 5.0 to 240.0 e A and (b)
ꢀ ꢂ3
from 5.0 to 1200.0 e A at log scale intervals, respectively.
CuHgSCl at room temperature and at 500 K. As shown in
Fig. 4, the density distribution on each atom is clearly
localized. This indicates that all atoms are static at room
temperature. However, in Fig. 5, the clear overlap of electron
densities between the neighboring Cu atoms at 500 K show the
presence of ionic conduction, although the electron densities on
Cl, Hg and S atoms are still localized. Therefore, Cu atoms
transfer along the (100) direction, and it can be concluded that
CuHgSCl is an ionic conductor.
For CuHgSBr, the similar consideration can be applied, as
shown in Figs. 6 and 7. Hence, CuHgSBr is also an ionic con-
ductor, which has the same conduction mechanism as CuHgSCl
has. We showed in the previous section that the ionic conduc-
tivity for CuHgSBr is lower than that for CuHgSCl. The pres-
ence of bigger Br ion leads to a shrinkage of the bottle-neck of
the conduction path.
References
1 M. Guillo, B. Mercey, and A. Deschanvres, Mater. Res.
Bull., 14, 947 (1979).
2 R. Blachnik and H. A. Dreisbach, Monatsh. Chem., 117,
305 (1986).
3
(1990).
4
R. Blachnik and K. Lytze, Thermochim. Acta, 190, 79
J. Beck, H. Keller, M. Rompel, and L. Wimbert, Z. Anorg.
Allg. Chem., 627, 2289 (2001).
5 J. Beck and M. Rompel, Z. Anorg. Allg. Chem., 629, 421
(2003).
6
7
H. Miura and T. Kikuchi, J. Chem. Software, 5, 163 (1999).
F. Izumi, ‘‘The Rietveld Method,’’ ed by R. A. Young, Uni-
versity Press, Oxford (1993), pp. 236–253.
Conclusion
8
9
F. Izumi, Rigaku J., 17, 34 (2000).
S. Chandra, ‘‘Superionc Solid, Principles and Applica-
CuHgSX (X = Cl and Br) show the ionic conduction phase
accompanied by the second-order phase transition at 373 and
tions,’’ North-Holland Pub. Co., Amsterdam (1981), p. 225.