Crystal structure determination of CoGeTe from powder diffraction data

Article abstract of DOI:10.1016/j.jallcom.2007.06.048

The crystal structure of cobalt germanium telluride CoGeTe has been determined by direct methods using integrate intensities of conventional X-ray powder diffraction data and subsequently refined with the Rietveld method. The title compound was prepared by heating of stoichiometric amount of Co, Ge and Te in silica glass tube at 670 °C. CoGeTe adopts orthorhombic symmetry, space group Pbca with unit cell parameters a = 6.1892(4) A?, b = 6.2285(4) A?, c = 11.1240(6) A?, V = 428.8(1) A?³ and Z = 8. Its crystal structure is formed by [CoGe₃Te₃] octahedra sharing both edges and corners. CoGeTe represents a ternary ordered variant of α-NiAs₂ type structure. An important feature present in CoGeTe is an occurrence of short Co-Co distance across the shared edge of [CoGe₃Te₃] octahedra. Differential thermal analysis (DTA) has revealed that CoGeTe melts incongruently at about 725 °C; CoGeTe decomposes into GeTe, CoGe and CoTe₂. Temperature dependence of the electrical conductivity and value of Seebeck coefficient at 300 K are also reported.

Full text of DOI:10.1016/j.jallcom.2007.06.048

Journal of Alloys and Compounds 460 (2008) 155–159
Crystal structure determination of CoGeTe from powder diffraction data
F. Laufeka,∗, J. Navratil , J. Plasil , T. Plechacek
b
c
b
´
´ˇ
´ˇ
a Czech Geological Survey, Geologicka´ 6, 15200 Praha 5, Czech Republic
b
ˇ
Joint Laboratory of Solid State Chemistry of IMC AS CR and University of Pardubice, Studentska´
84, 53210 Pardubice, Czech Republic
c Faculty of Science, Charles University, Albertov 6, 12843 Praha 2, Czech Republic
Received 24 April 2007; received in revised form 8 June 2007; accepted 13 June 2007
Available online 17 June 2007
Abstract
The crystal structure of cobalt germanium telluride CoGeTe has been determined by direct methods using integrate intensities of conventional
X-ray powder diffraction data and subsequently refined with the Rietveld method. The title compound was prepared by heating of stoichiometric
amount of Co, Ge and Te in silica glass tube at 670 ◦C.
˚
˚
˚
CoGeTe adopts orthorhombic symmetry, space group Pbca with unit cell parameters a = 6.1892(4) A, b = 6.2285(4) A, c = 11.1240(6) A,
3
˚
V = 428.8(1) A and Z = 8. Its crystal structure is formed by [CoGe3Te3] octahedra sharing both edges and corners. CoGeTe represents a ternary
ordered variant of ␣-NiAs2 type structure. An important feature present in CoGeTe is an occurrence of short Co–Co distance across the shared edge
of [CoGe3Te3] octahedra. Differential thermal analysis (DTA) has revealed that CoGeTe melts incongruently at about 725 ◦C; CoGeTe decomposes
into GeTe, CoGe and CoTe2. Temperature dependence of the electrical conductivity and value of Seebeck coefficient at 300 K are also reported.
© 2007 Elsevier B.V. All rights reserved.
Keywords: CoGeTe; Crystal structure; X-ray powder diffraction; Direct methods; Rietveld refinement
1. Introduction
tion. Data obtained by differential thermal analysis are also
reported. In spite of the relatively intensive research effort on
the Co–Ge–Te system [3–5], it is surprising that this compound
has not been previously reported.
The present paper is a part of systematic investigations on
the crystal structures of the MXCh compounds of cobalt-group
metals (M = Co, Ir and Rh) and main groups IV and VI elements
(X = Si, Ge and Sn; Ch = S, Se and Te). With regard to ternary
cobalt bearing germanide chalcogenides (Ch = S, Se and Te),
the examples encountered so far are CoGe1.5S1.5, CoGe1.5Se1.5
[1,2] and CoGe1.5Te1.5 [3]. The crystal structures of these com-
pounds can be viewed as a modification of skutterudite structure
MX3 (M = Co, Rh or Ir; X = P, As or Sb), where Ge and Ch
atoms exhibit long-range ordering, which results in a lower-
ing of the symmetry from original cubic to rhombohedral [2].
These phases are of interest in materials science because of their
possible thermoelectric applications.
2. Experimental
2.1. Synthesis, chemical composition and differential thermal
analysis
The CoGeTe ternary compound was synthesised from the elements by high
temperature solid-state reactions. Co powder was first heated at 900 ◦C for 2 h in
H2/Ar (15:85) atmosphere to remove possible oxides. Stoichiometric amounts
of Co (99.99%), Ge (99.99%) and Te (99.99%) were sealed in evacuated quartz
tubes and heated at 1150 ◦C for 4 h. Following this, the sample was ground
in argon atmosphere using the agate mortar and pestle, and cold pressed into
cylindrical pellet. The pellet was then sealed into evacuated quartz tube and
heated at 550 ◦C for 3 days. The resultant material was once again ground and
heated at 670 ◦C for 6 days. After annealing, the sample was quenched in cold
water.
In this work, we present a detailed study of the crystal struc-
ture of the new ternary compound CoGeTe, which is a distorted
variant of ␣-NiAs2, using conventional powder X-ray diffrac-
The chemical composition of the CoGeTe was characterised in the polished
section by an energy-dispersive system Oxford Link ISIS 300 connected to the
CamScan IV scanning electron microscope. The accelerating voltage was set to
20 kV and the sample current to 2.5 nA. Data were collected from several spots
on several different crystals and then averaged. As measured, the ratio of the
∗
Corresponding author. Fax: +420 251 818 748.
E-mail address: laufek@cgu.cz (F. Laufek).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.06.048

156
F. Laufek et al. / Journal of Alloys and Compounds 460 (2008) 155–159
Table 1
elements Co:Ge:Te was 1:0.98:1.04 (32.9(2)% Co, 32.5(2)% Ge and 34.49(3)%
Te) which is extremely close to our final crystal structure solution of 1:1:1.
The differential thermal analysis (DTA) was carried out using the instrument
R.M.I.-DTA 003 (Electronic Measuring Instruments, Czech Republic) at a non-
isothermal regime in the temperature range 25–800 ◦C. Small quartz ampoule
with powdered sample (about 40 mg weight) was evacuated down to 10−3 Pa
and heated with rate of 5 K min−1. The calibration was made with the help of In,
Al, Zn, Pb and Sn in order to eliminate the differences between the temperature
of the thermocouples in the furnace and in the vicinity of the sample. Pure Al2O3
was used as a standard.
Data collection and Rietveld analysis
Data collection
Radiation type, source
Generator settings
Data collection temperature
Range in 2θ (◦)
X-ray, CuK␣
40 kV, 30 mA
Room temperature
13–109
Step size (◦)
0.0167273
Crystal data
Space group
Unit cell content
Unit cell parameters (A)
Pbca (no. 61)
CoGeTe, Z = 8
a = 6.1892(4)
b = 6.2285(4)
c = 11.1240(6)
2.2. Structure determination and refinement
˚
Preliminary scanning-electron observations showed that the crystals have
plate-like habit, typically up to 5 ␮m, rarely up to 15 ␮m. Therefore, a single-
crystal study was impracticable, and the crystal structure of CoGeTe was solved
and refined from powder data ab initio.
Rietveld analysis
No. of reflections
268
The X-ray diffraction pattern used for the structure determination was col-
lected in Bragg–Brentano geometry on X’Pert Pro PANalytical diffractometer,
equipped with X’Celerator detector using CuK␣ radiation. The sample was
found to be phase pure; no other compounds were detected. To minimize back-
ground, the sample was placed on a flat low-background silicon wafer. The data
were collected in the range between 13 and 109◦ 2θ. The details of the data
collection and basic crystallographic facts are given in Table 1.
The indexing of the X-ray powder pattern was performed using the
DICLVOL04 program [6]. The first 20 lines, with an absolute error of 0.03◦
2θ on peak positions, were indexed on the basis of the orthorhombic cell listed
in Table 1. The figures of merit M20 [7] and F20 [8] for assessing the quality of
the solution were M20 = 69 and F20 = 79.9 (0.0037 and 68). The analysis of sys-
tematic absences pointed to the space group Pbca. The extraction of the integral
intensities as well as the structure solution by direct methods was accomplished
using the EXPO2004 program [9]. After the subtraction of the background scat-
tering, the integral intensities up to 2θ = 62◦ were extracted with the Pearson VII
function using the modified LeBail method. All three atoms were found from
the first four peaks generated from an E-map with the highest figure of merit.
All of them are in a general Wyckoff position 8c of the space group Pbca; no
special positions are occupied.
No. of structural parameters
No. of profile parameters
RF
RB
Rp
Rwp
χ2
12
7
0.051
0.065
0.065
0.083
2.42
1/y0
Weighting scheme
R agreement factors defined according to McCuster et al. [23].
line width was accounted for by using the Cagliotti function [12]. Intensities
within 15 times the full width at half maximum of a peak were considered to
contribute to the central reflection. The background was determined by the linear
interpolation between consecutive breakpoints in the pattern. The convergence
criterion, ε, forcing the termination of the refinement when the parameter shifts
<ε × σ, was set to 0.1. The refinement involved the following parameters: 12
atomic parameters (including 3 isotropic atomic displacement parameters), 1
scale factor, 1 zero-point, 3 cell parameters, 3 half-widths, 2 asymmetry param-
eters, 2 variables for the angular variation of η and parameter G1 linked to the
preferred orientation along [0 0 1] according to the March–Dollase [13] func-
tion. The refinement converged to satisfactory residual factors RBragg = 0.065
and Rwp = 0.082. The details of the refinement are reported in Table 1; Fig. 1
shows the final Rietveld plot. Atomic coordinates and isotropic displacement
parameters are listed inTable 2.
The structural model found by EXPO2004 [9] was introduced into the Full-
Prof program [10] for a Rietveld analysis. The Rietveld refinement was carried
out in the angular range 13–109◦ 2θ containing 268 reflections. A pseudo-Voight
function was selected to describe individual line profiles, with a variation of the
mixing factor η defining the Lorentzian and the Gaussian character of the peaks’
shape. The corrections taking into account the asymmetry of the peaks caused
by axial divergence were used according to [11]. The angular variation of the
Fig. 1. The Rietveld plot of CoGeTe. The upper trace shows the observed data as dots and the calculated pattern is shown by solid line. The lower trace is a plot of
the difference: observed − calculated. The vertical bars indicate the positions of Bragg peaks.

F. Laufek et al. / Journal of Alloys and Compounds 460 (2008) 155–159
157
Table 2
Atomic coordinates and isotropic displacement parameters for the CoGeTe compound
2
˚
Atom
x
y
z
Biso (A )
Co
Ge
Te
0.0055(6)
0.6633(4)
0.1157(2)
0.6069(6)
0.7698(6)
0.9887(3)
0.1119(3)
0.0710(2)
0.1740(3)
1.41(6)
1.57(5)
1.23(3)
Table 3
2.3. Electrical conductivity
˚
Selected bond distances (A) for CoGeTe and CoAsSe [21]
Electrical conductivity was measured with four-probe method using Lock-In
Amplifier (EG&G model 5209). The Seebeck coefficient was determined using
temperature gradient on the samples not exceeding 10 K at the temperature
300 K.
CoGeTe
CoAsSe
Co–Ch(i)
Co–Ch
2.552(4)
2.568(4)
2.602(3)
2.574
2.385(3)
2.388(5)
2.392(5)
2.388
2.358(7)
2.375(6)
2.37(1)
2.367
2.34(1)
2.348(6)
2.331(6)
2.339
Co–Ch(ii)
Mean Co–Ch
Co–X(iii)
Co–X(iv)
Co–X
3. Results and discussion
3.1. Crystal structure
Mean Co–X
Co–Co(v)
2.825(4)
3.50(1)
CoGeTe is the second cobalt germanide telluride discov-
ered in the Co–Ge–Te system, where 11 phases were known
before. The crystal structure of CoGeTe is depicted in Fig. 2.
Each cobalt atom is octahedrally coordinated by three ger-
manium and three tellurium atoms, with Co–Ge and Co–Te
distances of 2.385(3)–2.392(5) A and 2.552(4)–2.602(3) A (see
Table 3), respectively, comparable to those observed in cobalt
binary tellurides [14] and germanides [15]. X–M–X (X = Te
and Ge) angles in CoGeTe, ranging from 79.8(1)◦ to 107.5(1)◦,
exhibit a significant deviation from the ideal octahedral geome-
try. When compared with the octahedra in binary tellurides such
as Co0.63Te [16], the octahedra in CoGeTe are more distorted. In
CoGeTe structure, one Ge–Ge edge of [CoGe3Te3] octahedron
is shared with an adjacent [CoGe3Te3] octahedron, and all cor-
ners are shared with two other [CoGe3Te3] octahedra. The Co
atoms across the shared Ge–Ge and shared corners are separated
Symmetry codes: (i) 1/2 − x, y − 1/2, z; (ii) −x, y − 1/2, 1/2 − z; (iii) x − 1/2,
1/2 − y, −z; (iv) 1/2 − x, y − 1/2, z; (v) −x, −y, −z.
can be supposed. Apart from this Ge–Te bond, each Ge and Te
anion is also surrounded by three Co atoms showing distorted
tetrahedral coordination.
˚
˚
The crystal structure of CoGeTe can be directly compared
to that of PtSiSb which has been recently described by Wang
et al. [17]. As suggested by Wang et al. [17], these structures
can be derived from that of ␣-NiAs2, which is also known as a
mineral pararammelsbergite [18]. All compounds have the same
space group (Pbca) and display a very similar structural arrange-
ment. Fig. 2b depicts the linking of [CoGe3Te3] octahedra along
the c-axis. Analogically to the PtSiSb, CoGeTe can be viewed
as a ternary ordered variant of ␣-NiAs2, which is transitional
between the marcasite-type and the pyrite-type structures. The
major difference displayed by these three structures is the way
the octahedra are connected together. Whereas, in pyrite struc-
˚
˚
by of 2.823(4) A and by 4.361(4) A, respectively. The structure
of the CoGeTe compound displays the presence of Ge–Te anion
˚
pairs with the distance of 2.713(3) A, so a Ge–Te single bond
Fig. 2. Two representations of the CoGeTe structure: (a) ball and stick representation; (b) polyhedral representation showing the [CoGe3Te3] octahedra.

158
F. Laufek et al. / Journal of Alloys and Compounds 460 (2008) 155–159
Table 4
˚
Selected distortion parameters and Co–Co distance (A) for CoGeTe compared with those for CoAsSe [21], CoSbS [19], CoPSe [21] and ␣-NiAs2 [21]
QE
OAV
Co–Co distance
CoGeTe
CoAsSe
CoSbS
CoPSe
␣-NiAs2
1.022
1.008
1.010
1.009
1.008
75.59
28.68
26.46
28.58
26.54
2.825(4)
3.50(1)
3.350(2)
3.448(3)
3.531(9)
ꢀ
ꢀ
11
i=1
6
i=1
OAV (octahedral angle variance) (◦) =
(θi − 90)2/11 [22]; QE (quadratic elongation):=
(di − d0)2/6 (d0 is the centre-to-vertex distance of a regular
octahedron of the same volume) [22].
ture the octahedra share only corners, in marcasite structure they
share both edges and corners, forming chains of linked octa-
hedra. The CoGeTe structure is composed of layers of atoms
perpendicular to the c-axis with stacking alternating between
pyrite- and marcasite-type of blocks, as shown in Fig. 2b. In the
CoGeTe structure, these alternating layers have every second
marcasite-type layer in the reverse orientation. This arrange-
ment has been described for the structure of PtSiSb and CoSbS
by Wang et al. [17] and by Rowland et al. [19], respectively.
Similarly to the PtSiSb, the centres of Co–Co pairs form a face
centred cubic type arrangement elongated along c-axis.
geometry. Table 4 shows octahedral angle variance, quadratic
elongation and Co–Co distance of Co-bearing NiAs2-type com-
pounds. It is obvious that the [CoGe3Te3] octahedra are more
deviated from their ideal geometry than octahedra in CoAsSe,
CoSbS and CoPSe. This corresponds to the shortening of the
Co–Co distance in CoGeTe, which, as a consequence, deforms
the [CoGe3Te3] octahedron (more specifically, it lowers one of
the angles of X–Co–X (X = Ge, As, Sb and P). Fig. 3 depicts
both the contraction of the Co–Co distance and the deformation
of the X–Co–X angle; moreover, it shows a comparison with
CoAsSe and ␣-NiAs2.
An important feature present in CoGeTe is a short Co–Co
distance across the shared edge of [CoGe3Te3] octahedra.
The Co atoms are displaced from the centre of the octahedra
towards each other, reducing the Co–Co distance to the value of
3.2. Differential thermal analysis
The DTA curve is reproduced in Fig. 4. The sharp endother-
mic peak at about 725 ◦C represents the incongruent melting of
CoGeTe; it decomposes into GeTe (which is present in a liquid
form), CoGe and CoTe2. These phases were identified by XRD
after heating the sample of CoGeTe in an evacuated silica glass
tube at 750 ◦C and consequently rapid quenching to the room
temperature.
˚
2.825(4) A. This distance is somewhat longer than the Co–Co
˚
distance of 2.506 A observed in elementary Co [20], but can
˚
be compared with the Pt–Pt distance of 2.8565(8) A found in
PtSiSb [17], which adopts also ␣-NiAs2 type structure. The
metal–metal distance of other ␣-NiAs2-type compounds con-
taining Co is significantly longer (see Table 4); the difference
between the Co–Co distance in CoGeTe and that in CoAsSe [21]
˚
is more than 0.6 A.
3.3. Electrical conductivity
To assess the displacement of the metal atom from the centres
of octahedra, we applied an approach of the so-called distortion
parameters. Such distortion indices as octahedral angle variance
and quadratic elongation [22] are commonly used to characterise
the departure of an irregular octahedron from the ideal regular
Very weak temperature dependence of the electrical conduc-
tivity of CoGeTe sample (see Fig. 5) and quite high value of
the Seebeck coefficient at 300 K (S300 ∼ −180 ␮V K−1) sug-
gests semiconducting behaviour of the CoGeTe sample. The
Fig. 3. Comparison of the edge-sharing octahedra in (a) CoGeTe; (b) CoAsSe [21]; (c) ␣-NiAs2 [21].

F. Laufek et al. / Journal of Alloys and Compounds 460 (2008) 155–159
159
with the exception of PtSiSb, all hitherto known ␣-NiAs2 type
compounds have 20 valence electrons per formula unit. As
opposed to that, PtSiSb and CoGeTe have 19 valence electrons
per formula unit. This change in number of electrons per for-
mula unit is accompanied by creating one Co–Co bond formed
to each Co atom. It might be revealing to study the develop-
ment of metal–metal interaction depending on the continual
change of the electron count, for instance, in the solid solution
CoxNi1−xGeTe. Thisinvestigationiscurrentlyinprogress. Other
solid solutions derived from a different ␣-NiAs2-type could be
an interesting subject to further analyses.
Acknowledgements
Fig. 4. Differential thermal curve for CoGeTe compound.
This work was supported by the internal project of Czech
Geological Survey (project number 3230) and by the Czech
Science Foundation (project number 203/07/0267).
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To conclude, CoGeTe represents a ternary ordered variant
of ␣-NiAs2 structure, the most striking difference being the
presence of metal–metal bonded pairs. As discussed in [17],