Preparation and crystal structures of Cu2HfS3

Article abstract of DOI:10.1016/S0925-8388(00)01330-X

The phase relations of the compound Cu₂HfS₃ have been studied by XRD and DSC methods. The following three types were found to exist in the temperature range from 500 to 1300°C: (a) monoclinic α-Cu₂HfS₃ phase with a = 12.271(3) A?, b = 11.162(3) A?, c = 6.429(2) A?, β = 100.04(2)° below 650°C, (b) monoclinic β-Cu₂HfS₃ phase with a = 9.611(2) A?, b = 6.444(1) A?, c = 7.127(3) A?, β = 98.05(2)° in the temperature range of 650-780°C, and (c) trigonal γ-Cu₂HfS₃ phase with a = 6.456 A? and c = 12.188 A? above 787°C. Structure determination of β-Cu₂HfS₃ phase was attempted using the Rietveld method. Electrical resistivity measurements were carried out for the trigonal γ-Cu₂HfS₃ phase.

Full text of DOI:10.1016/S0925-8388(00)01330-X

Journal of Alloys and Compounds 317–318 (2001) 217–221
L
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Preparation and crystal structures of Cu2HfS3
*
H. Wada , A. Sato, H. Nozaki
National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Abstract
The phase relations of the compound Cu2HfS3 have been studied by XRD and DSC methods. The following three types were found to
˚
˚
exist in the temperature range from 500 to 13008C: (a) monoclinic a-Cu2HfS3 phase with a512.271(3) A, b511.162(3) A, c56.429(2)
˚
˚
˚
˚
A, b5100.04(2)8 below 6508C, (b) monoclinic b-Cu2HfS3 phase with a59.611(2) A, b56.444(1) A, c57.127(3) A, b598.05(2)8 in the
temperature range of 650–7808C, and (c) trigonal g-Cu2HfS3 phase with a56.456 A and c512.188 A above 7878C. Structure
determination of b-Cu2HfS3 phase was attempted using the Rietveld method. Electrical resistivity measurements were carried out for the
˚
˚
trigonal g-Cu2HfS3 phase.
2001 Elsevier Science B.V. All rights reserved.
Keywords: Copper hafnium sulfide; Phase relation; Crystal structure; Phase transition; Electrical resistivity
1. Introduction
elements and the related binary compounds. The mixtures
were ground in an agate mortar, pressed into pellets
(diameter 7 mm), and sealed in evacuated silica tubes at a
pressure of less than 1023 Torr (1 Torr5133.322 Pa). Heat
treatments were carried out at 550|13008C for 5 h|45 d,
followed by quenching.
In the case of high-temperature experiments above
9008C an alumina crucible was used as the sample
container inside the silica tube to avoid the reaction of
silica and pellet. Samples obtained were identified with
X-ray powder and single crystal diffraction methods. The
structure refinement of the compound was performed using
the total pattern fit program RIETAN [8].
Differential thermal analysis was carried out using
SETARAM DSC-111 instruments at a heating/cooling rate
of 108C min21. Less than 100 mg of powder samples was
put into small quartz ampoules, sealed in vacuum and
used.
Recently we have reported the syntheses and structures
of new ternary hafnium sulfides with the composition
A2HfS3 (A5Ag, Cu) [1,2]. The common feature of crystal
structures of these compounds is the existence of HfS6
octahedra and AS4 tetrahedra. These octahedra are inter-
connected horizontally to make the edge-sharing [HfS3]
octahedral layers. The three-dimensional edge-sharing
HfS6 octahedra exist in Ag4Hf3S8 [3,4]. Double chains of
edge-sharing octahedra have been known for the com-
pounds of Cu2HfTe3 [5], TlCuHfS3 [6] and Tl2Cu2Hf3Se8
[7]. In spite of much knowledge of the structures con-
cerning ternary and/or quaternary hafnium chalcogenides
little is known about their phase relations. We studied the
Cu–Hf–S system and found the new modifications of
Cu2HfS3 prepared at temperatures ranging from 500 to
8008C.
In this work we report the preparation, the phase
transition and the crystal structure determination from
X-ray powder diffraction data by using the Rietveld
method of the low-temperature phase of Cu2HfS3, and its
resistivity measurement.
Electrical resistivity measurements were taken on rec-
tangular shaped, cold-pressed polycrystalline samples
(2.132.435 mm). Electrical contacts were made by silver
paste.
3. Results and discussion
2. Experimental
3.1. Phase relations of Cu2HfS3
Copper hafnium sulfides were prepared from the pure
Several kinds of copper hafnium sulfides with different
compositions were prepared in the subsolidus region.
*Corresponding author. Tel./fax: 181-298-58-5641.
E-mail address: wadah@nirim.go.jp (H. Wada).
0925-8388/01/$ – see front matter
PII: S0925-8388(00)01330-X
2001 Elsevier Science B.V. All rights reserved.

218
H. Wada et al. / Journal of Alloys and Compounds 317–318 (2001) 217–221
Representative powder X-ray diffraction (XRD) patterns of
Cu2HfS3 obtained at 550, 650, 700 and 9008C are shown
in Fig. 1. Patterns (a), (c) and (d) belong to the single
phases with different structures. Pattern (a) is from the
convenience we designate here these modifications as g-,
b- and a-phases, respectively.
The differential thermal analyses of the a- and b-phases
prepared were carried out in order to check the stability
and phase relations of Cu2HfS3. Differential scanning
calorimetry (DSC) curves are shown in Fig. 2. On heating
the first endothermic peak appears at around 5158C. This
probably corresponds to the decomposition of a small
amount of coexisting CuS impurity (1.8CuS→Cu1.8S1
0.8S). The second endothermic peak with an enthalpy of
6.77 J g21 appears at 6588C where the a-phase changes to
b-phase owing to the irreversible phase transition. On
subsequent heating up to the temperature limit of 8308C
for the apparatus the third endothermic process with an
high-temperature phase of Cu2HfS3 with the space group
]
of P31c which has been reported before [2]. Patterns (c)
and (d) are from the low-temperature phases of Cu2HfS3
which have been studied in this work. The structure
difference between (a) and (c) is considered very small
because both diffraction profiles resemble each other
especially concerning strong main reflections. Pattern (b)
represents the mixture of high- and low-temperature
Cu2HfS3 phases. The unit cell parameters of the Cu2HfS3-
related compounds are listed in Table 1. For the sake of
Fig. 1. Typical XRD patterns of Cu2HfS3: (a) prepared at 9008C, (b) prepared at 7008C, (c) prepared at 6508C and (d) prepared at 5508C.

H. Wada et al. / Journal of Alloys and Compounds 317–318 (2001) 217–221
219
Table 1
Summary of crystal structures of Cu2HfS3
Compound
Crystal system
Monoclinic
Space group
–
Z
Lattice parameters
Comments
Ref.
˚
a-Cu2HfS3
8
a512.271(3) A
Stable below 6508C
rcalc56.15 g cm23
This work
˚
b511.162(3) A
˚
c56.429(2) A
b5100.048
˚
˚
b-Cu2HfS3
Monoclinic
4
a59.611(2) A
Stable at 650|7608C
rcalc56.10 g cm23
This work
P21 /n
b56.444(1) A
˚
c57.127(3) A
b598.058
]
˚
g-Cu2HfS3
Trigonal
Trigonal
P31c
4
1
a56.4588 A
Prepared above 8008C
[2]
[9]
˚
c512.1943 A
]
˚
a53.625 A
c55.846 A
HfS2
P3m1
˚
enthalpy of 4.69 J g21 observed at 7878C where the phase
transition of b- to g-phase occurs. The final products after
the DSC run were checked by X-ray powder diffraction
and confirmed as g-phase. In the case of the DSC curve of
the b-phase prepared at 6508C for 14 days, however, no
endothermic peaks appear at temperatures from 5508C to
7608C. There exists a similar phase transition from b- to
g-phase at 7878C as in the sample of 5508C. These results
suggest that the a-phase is stable below 6508C, b-phase is
stable in the range of temperature between 650 and 7608C
and g-phase is formed above 7878C.
3.2. Structure determination of b-Cu2HfS3
The X-ray diffraction intensity data were collected with
a step scan procedure on a Rigaku diffractometer
(Rint2000) using graphite-monochromated CuKa radia-
tion. It was found that the systematic reflection conditions
(hhl; l52n, 0k0; k52n) are consistent with the possible
monoclinic space group, P21 /c (No. 14). As a first step, a
structure model of b-Cu2HfS3 was embodied on the basis
˚
of crystal structure of g-Cu2HfS3 with a56.456 A and
˚
c512.188 A [2], which is related to those of trigonal
Cd(OH)2-type HfS2 (a0, c0) [9] by ag|œ3a0 and cg|
œ3c0. Lattice parameters of b-Cu2HfS3 are related to
those of g-Cu2HfS3 by ab(2/33(ag2bg)21/23cg,
bb(ag1bg, cb(1/33(ag2bg)11/23cg. As shown in
Fig. 3, the starting structure was constructed on the basis
of the assumption that Hf atoms form similar Hf layers as
in g-Cu2HfS3 and the stacking of atoms in the vertical
direction against layer is –S–Hf–S–Cu–S–Hf–S–Cu– in
order. The intensity data in the range of d-spacings from
˚
8.855 to 1.3456 A were used and 21 positional parameters,
three isotropic thermal parameters, one occupation parame-
ter and one scaling factor were refined from the initial
parameters. The agreement of observed and calculated
patterns was satisfactory: Rwp516.3%, Rp2512.5%, RI5
˚
9.8%, and RF56.3% for B(Hf)51.5(0.2) A , B(Cu)54(1)
2
2
˚
˚
A , and B(S)53(1) A . The observed and calculated X-ray
powder diffraction profiles of Cu2HfS3 are shown in Fig.
4, where the experimental data are plotted by dots and
Bragg positions are indicated as vertical lines. Final values
Fig. 2. DSC heating curves of Cu2HfS3: (a) b-phase prepared at 6508C
and (b) a-phase prepared at 5508C.

220
H. Wada et al. / Journal of Alloys and Compounds 317–318 (2001) 217–221
˚
distances for Ag2HfS3 of 2.549 A [1] and Ag4Hf3S8 of
˚
2.541 A [3]. These values are rather close to those of
˚
˚
binary phases, 2.63 A in Hf2S [10] and 2.622 A in HfS3
˚
[11]. The Cu–S distances ranges from 2.15 to 2.84 A.
However, further work on single crystals would be abso-
lutely necessary to clarify the details of the true crystal
structure.
3.3. Electrical resistivity of g-Cu2HfS3
The electrical resistivity of a sintered polycrystalline
sample of g-Cu2HfS3 was measured from liquid N2
temperature to 300 K. The uncertainty of the resistivity
was estimated to be 61 in value owing to the experimental
limitation. As shown in Fig. 5, a plot of r in V cm vs. T in
K for g-Cu2HfS3 reflects a resistivity anomaly behavior.
On cooling from 298 to 78 K, the sample shows a
resistivity hump around 252 K. The resistivity slightly
increases from 21 to 28.4 V cm in the temperature range
298–252 K, and decreases gradually to 11.7 V cm of the
minimum until 204 K, and again increases to 24.8 V cm
with temperature up to 78 K in the semiconducting
behavior. A similar resistivity anomaly has been reported
in the compounds of ACu72xS4 (A5Tl, K, Rb) [12].
Fig. 3. Lattice relation of Cu2HfS3 between the g- and b-phases. The
section (110) plane of the g-plane is presented. Arrows correspond to a
and c axes of the b-phase.
of all structural parameters are given in Table 2. The
observed and calculated d-spacings of reflection peaks are
listed in Table 3, together with the observed intensity.
The HfS6 octahedra are largely distorted when com-
pared with high-temperature phase. The Hf–S distances
4. Summary
˚
˚
are an average 2.78A in Hf(1)–S and average 2.63 A in
Hf(2)–S which is slightly longer than the corresponding
The phase relations of Cu2HfS3 at 500–13008C were
determined by sealed silica tube experiments. There exist
Fig. 4. The observed (dots) and calculated (solid line) XRD patterns of Cu2HfS3. The difference between the observed and calculated profiles is plotted
below on the same scale.

H. Wada et al. / Journal of Alloys and Compounds 317–318 (2001) 217–221
221
Table 2
Structure parameters of b-Cu2HfS3
˚
˚
˚
Space group: P21 /n (No. 14); a59.611(2) A, b56.444(1) A, c57.127(3) A, b598.05(2)8
2
˚
Atom
Position
Ocp
x
y
z
Beq (A )
Cu(1)
Cu(2)
Cu(3)
4e
4e
4e
0.39(4)
0.92
0.69
0.575(11)
0.505(6)
0.207(8)
0.480(28)
0.262(5)
0.639(13)
0.302(13)
0.976(8)
0.523(10)
4(1)
–
–
Hf(1)
Hf(2)
2a
4e
0.5
0.75
0
0
0
1.5(2)
–
0.339(2)
0.006(5)
0.328(4)
S(1)
S(2)
S(3)
4e
4e
4e
1.0
1.0
1.0
0.250(10)
0.404(12)
0.929(10)
0.853(15)
0.845(17)
0.335(20)
0.054(14)
0.729(12)
0.180(12)
3(1)
–
–
Table 3
X-Ray powder diffraction data of b-Cu2HfS3
˚
˚
h
k
l
d0 (A)
dc (A)
I0
1
1
2
2
0
2
3
2
4
2
3
2
3
4
2
1
5
0
4
1
2
6
5
0
0
0
1
2
0
1
0
0
2
1
2
0
2
3
3
1
4
3
4
4
1
3
21
1
0
21
0
22
0
2
0
22
2
2
23
0
1
2
22
0
21
21
22
23
22
6.087
5.323
4.751
3.537
3.218
3.046
2.847
2.662
2.3805
2.2130
2.0888
2.0527
2.0300
1.9126
1.8596
1.7776
1.7179
1.6112
1.5874
1.5571
1.4243
1.3775
1.3722
6.090
5.323
4.758
3.536
3.221
3.045
2.846
2.661
2.3791
2.2131
2.0905
2.0519
2.0302
1.9138
1.8591
1.7776
1.7181
1.6109
1.5874
1.5574
1.4240
1.3771
1.3717
18
7
5
5
26
8
100
4
5
39
4
4
7
5
41
9
17
5
5
14
8
7
9
Fig. 5. Electrical resistivity of g-Cu2HfS3.
[2] H. Wada, A. Sato, J. Alloys Comp. 279 (1998) 215.
[3] O. Amiel, H. Wada, J. Solid State Chem. 115 (1995) 112.
three modifications: (a) monoclinic a-Cu2HfS3 phase
below 6508C (b) monoclinic b-Cu2HfS3 phase in the
temperature range of 650–7808C, and (c) trigonal g-
Cu2HfS3 phase above 7878C. The structure of the b-
Cu2HfS3 phase was determined using the Rietveld method.
The g-Cu2HfS3 phase pellet showed a resistivity hump
around at 252 K and became a semiconductor below 200
K.
[4] H. Wada, O. Amiel, A. Sato, Solid State Ionics 70 (1995) 129.
[5] P.M. Keane, J.A. Ibers, J. Solid State Chem. 93 (1991) 291.
[6] M.F. Mansuetto, P.M. Keane, J.A. Ibers, J. Solid State Chem. 105
(1993) 580.
[7] K.O. Klepp, D. Grurtner, J. Alloys Comp. 239 (1996) 1.
[8] F. Izumi, in: R.A. Young (Ed.), The Rietveld Method, Oxford
University Press, Oxford, 1993, pp. 236–345, Chapter 13.
[9] F.K. McTaggart, A.D. Wadsley, Australian J. Chem. 11 (1958) 445.
[10] H.F. Franzen, J. Graham, Z. Kristallogr. 123 (1996) 133.
[11] S. Furuseth, L. Brattas, A. Kjekshus, Acta Chem. Scand. A29
(1975) 623.
References
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[1] H. Wada, O. Amiel, A. Sato, J. Alloys Comp. 219 (1995) 55.