Eva M. Heppke, Stefan Berendts, Martin Lerch
Crystal structure of mechanochemically
synthesized Ag2CdSnS4
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Heppke, E. M., Berendts, S., Lerch, M. (2020). Crystal structure of mechanochemically synthesized
Ag2CdSnS4. In Zeitschrift für Naturforschung B (Vol. 75, Issue 4, pp. 393–402). Walter de Gruyter GmbH.
https://doi.org/10.1515/znb-2020-0022
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Z. Naturforsch. 2020; 75(4)b: 393–402
Eva M. Heppke, Stefan Berendts and Martin Lerch*
Crystal structure of mechanochemically
synthesized Ag2CdSnS4
https://doi.org/10.1515/znb-2020-0022
Received January 27, 2020; accepted February 20, 2020
Abstract: Ag2CdSnS4 was synthesized by a two step mech-
anochemical synthesis route. From a detailed analysis
of the observed reflections in the X-ray powder diffrac-
tion pattern, the crystal structure proposed in the litera-
ture (space group Cmc21 [E. Parthé, K. Yvon, R. H. Deitch,
Acta Crystallogr. 1969, B25, 1164–1174; O. V. Parasyuk, I.
D. Olekseyuk, L. V. Piskach, S. V. Volkov, V. I. Pekhnyo,
J. Alloys Compd. 2005, 399, 173–177]) is questionable. Our
structural investigations presented in this contribution
point to the fact that Ag2CdSnS4 crystallizes in the mono-
clinic wurtzkesterite-type structure (space group Pn). At
around T = 200°C, a phase transition to the orthorhom-
bic wurtzstannite-type structure (space group Pmn21) is
observed.
Keywords: Ag2CdSnS4; HT-XRD; mechanochemical
synthesis; phase transition; Rietveld refinement.
Dedicated to: Professor Rüdiger Kniep on the Occasion of his 75th
birthday.
1 Introduction
A2
IBIICIVX4 (X2− = S, Se, Te) compounds are semiconduc-
tors exhibiting either a cubic or a hexagonal diamond-
related structure [1–3]. The most prominent group of
A2
IBIICIVX4 semiconductors are Cu-bearing compounds
[4–9]. Among them, Cu2ZnSnS4 has gained much atten-
tion as a potential candidate for absorber layers in thin
film solar cells [10–12]. Ag-containing compounds are
more difficult to synthesize and less stable. Neverthe-
less, a few A2
IBIICIVX4 phases with A = Ag are known so
far [4, 13–19]. For most Ag compounds, a stannite- or
wurtzstannite-/wurtzkesterite-type structure has been
presented in the literature. However, there are some
compounds that deviate from these two structures. For
Ag2CdSnS4, the space group Cmc21 is proposed which can
be considered as a hexagonal diamond-/wurtzite-derived
structure with a statistical distribution of Ag, Cd, and Sn
on Wyckoff position 4a with (0, 0.167, 0.370); sulfur is
located at another 4a position with (0, 0.176, 0.997) [4,
20]. The results of our investigations on the aforemen-
tioned compound indicate a different crystal structure.
Due to additional reflections in the X-ray powder dif-
fraction pattern of our synthesized sample, space group
Cmc21 can be excluded. Ag2CdSnS4 was synthesized via
a mechanochemical process which has proven its suit-
ability for the successful preparation of phase-pure and
well crystallized quaternary A2
IBIICIVS4 compounds such
as Cu2ZnSnS4 [21]. The crystal structure of our mecha-
nochemically prepared Ag2CdSnS4 has been determined
using powder X-ray diffraction. Additionally, UV/Vis
and DTA measurements were performed. Surprisingly, a
phase transition at around T = 200°C could be observed
and the crystal structures of both the high- and low-
temperature phases were investigated by in-situ XRD.
2 Results and discussion
The mechanochemical synthesis approach with a subse-
quent annealing step as described in the experimental
section results in the formation of phase-pure and well
crystallized Ag2CdSnS4. Phase composition was deter-
mined by EDX and combustion analysis and is in accord-
ance with the theoretical one (Table 1). EDX mapping also
confirmed a homogenous distribution of Ag, Cd, Sn, and S
in the individual particles (Fig. 1).
Interestingly, in the DTA curve of our Ag2CdSnS4
sample thermal effects were observed. An endothermic
peak occurs in the heating process (Tp = ~205°C) and an
exothermic one is observed during cooling (Tp = ~199°C).
This points to a reversible phase transition at around
200°C (Fig. 2). The reversibility of the phase transition was
also proven by in-situ X-ray diffraction.
*Corresponding author: Martin Lerch, Institut für Chemie,
Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin,
Germany, Fax: +493031479656, E-mail: martin.lerch@tu-berlin.de
Eva M. Heppke and Stefan Berendts: Institut für Chemie, Technische
Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany
394 E.M. Heppke etal.: Crystal structure of mechanochemically synthesized Ag2CdSnS4
2.1 Structural relationship between
hexagonal diamond and the
wurtzstannite-/wurtzkesterite-type
structure
As it can be seen in the Bärnighausen tree [22, 23], eluci-
dating the relationship between the hexagonal diamond
(lonsdaleite) and the wurtzstannite-/wurtzkesterite-type
structures (Fig. 3), space group Cmc21, as a subgroup to
that of the wurtzite-type structure, is proposed either for
binary compounds as well as for compounds with a statis-
tical distribution of their cations/anions on one position.
For Ag2CdSnS4, space group Cmc21 is mentioned in the lit-
erature with Ag, Cd, and Sn distributed statistically on the
position 4a with (0, 0.167, 0.370) and S on another 4a posi-
tion with (0, 0.176, 0.997) [20].
Similar compounds containing silver either crystal-
lize in cubic diamond/sphalerite-related structures such
as the stannite- or kesterite-type structure (such as Ag2Zn-
SnS4 [14], Ag2FeSnS4 [13], Ag2ZnGeS4 [24]) or in crystal struc-
tures derived from the hexagonal diamond/wurtzite type
providing non-equivalent positions for the cations/anions
(the wurtzstannite-type structure (space group Pmn21)
and its derivatives with space group Pn (wurtzkesterite-
type structure) or Pna21). Compounds crystallizing in
the wurtzstannite-type structure are Ag2CdGeS4 [4] and
Ag2HgSnS4 [15], whereas Ag2MnSnS4 [18] and Si-bearing
compounds such as Ag2FeSiS4 [16] and Ag2ZnSiS4 [17]
crystallize in the wurtzkesterite-type structure. Due to
the close structural relationships between these three
structures, it can be quite difficult to distinguish between
them when using conventional methods. For Ag2CdGeS4,
two polymorphs have been reported exhibiting the space
groups Pmn21 [4] and Pna21 [25], respectively.
2.2 The high-temperature phase of
Ag2CdSnS4
Starting with the high-temperature phase, it should be
noted that an orthorhombic unit cell could be found by
the Werner algorithm using the program WinXPOW 1.2
(STOE & Cie GmbH, Darmstadt, Germany) [29] for the
sample at T = 300°C. The lattice parameters were refined
to a = 8.2263 Å, b = 7.0655 Å, and c = 6.7051 Å leaving no
reflection behind. The volume of the unit cell was calcu-
lated to 389.7 Å3. It should be mentioned that the lattice
parameter a given by the Werner algorithm is two times
larger than that of the proposed crystal structure with
space group Cmc21 for Ag2CdSnS4 [4, 20]. Examples of
A2
IBIICIVX4 compounds with a lattice parameter a of around
8.23 Å (and similar values for the lattice parameters b and
c) within the orthorhombic crystal system have also been
described by other authors [15].
For the Rietveld refinement of the high-temperature
phase space group Pmn21 (wurtzstannite-type structure)
Fig. 1: SEM images of Ag2CdSnS4 particles and EDX mapping of homogenously distributed Ag (blue), Cd (red), Sn (green), and S (yellow) in
the Ag2CdSnS4 particles.
Table 1: Phase composition of Ag2CdSnS4 calculated from EDX data
and sulfur content determined by elemental analysis.
Ag2CdSnS4
Ideal (wt.-%) Measured (wt.-%)
Ag 37.5 36.4
Cd 19.5 20.7
Sn 20.6 21.2
S (EDX) 22.3 21.7
S (EA) 22.3 22.0
E.M. Heppke etal.: Crystal structure of mechanochemically synthesized Ag2CdSnS4 395
is used. As depicted in Fig. 4, the experimental pattern
is in good agreement with the theoretical one. Crystal-
lographic details as well as atomic and structural para-
meters are listed in Tables 2 and 3. The Debye Waller
factors of sulfur were set to a value of 1 and not refined,
also the site occupation factors of all atoms. As Pmn21 is a
polar space group and therefore possesses one origin-free
direction (the z direction), the z parameter of the Ag atom
position was fixed. As Ag, Cd, and Sn are not distinguish-
able using conventional X-ray diffraction methods, the
distribution of these cations in the wurtzstannite- (and
also wurtzkesterite-) type structure was set to that of the
related compounds described in the literature.
The wurtzstannite-type structure (space group Pmn21)
of the high-temperature phase of Ag2CdSnS4 can be
described as a hexagonal diamond/wurtzite-derived struc-
ture with a hexagonal closest packing of sulfur anions.
Ag is located at a fourfold position whereas Cd and Sn
occupy two non-equivalent twofold positions. Three crys-
tallographically independent sulfur positions are present,
where S1 is located on a fourfold position and S2 as well
as S3 atoms occupy two twofold positions. Considering
the honeycomb set-up, the wurtzstannite-type structure is
built up of consecutive Ag and Cd/Sn layers (in general: AI
and BII/CIV layers). This is manifested in alternating layers
of Ag-centered and Cd/Sn-centered polyhedra along the b
axis (Fig. 5). These alternating stacking sequence appears
to be in line with that of the stannite type [5, 30], the anal-
ogous structure type derived from the cubic diamond/
sphalerite-type structure. All atoms in the wurtzstannite-
type structure are surrounded tetrahedrally. Each cation
is coordinated by two S1 atoms, one S2 atom, and one S3
atom. The sulfur atoms are coordinated by two Ag and one
Cd atom as well as one Sn atom.
2.3 The low-temperature phase of
Ag2CdSnS4
For the crystal structure determination of the low-
temperature phase, all four direct subgroups of Pmn21
(P21, Pm, Pn, and Pna21) (Fig. 6) were considered and
tested as refinement models. For example, the symme-
try reduction from space group Pmn21 to Pn is expressed
by the splitting of the fourfold Ag atom position 4b into
two non-equivalent twofold positions 2a. The atomic and
lattice parameters for the refinements in the four sub-
groups were generated by the program Transtru found
on the Bilbao Crystallographic Server [31–33]. For the
space groups Pna21 and Pn, we also used reported struc-
tural parameter of related compounds [25, 28] as starting
values in the Rietveld refinements.
In the subgroups P21, Pm, and Pna21 convergence
problems occurred during the refinements (profile,
atomic positions, Debye-Waller factors). However, a suc-
cessful refinement was achieved in space group Pn, the
wurtzkesterite-type structure. The experimental diffrac-
tion pattern is in good agreement with the theoretical
one (Fig. 7). Crystallographic details are summarized in
Table2; atomic and structural parameters are presented
in Table 4. The Debye-Waller factors of the sulfur atoms
were kept fixed at a value of 1, and the site occupation
factors were not refined. Space group Pn also belongs
to the group of polar/origin-free space groups. This par-
ticular space group exhibits two origin-free directions (x
andz). For the refinement in Pn, the x and z parameters of
the Ag1 atom position were kept fixed to define the origin.
The low-temperature phase of Ag2CdSnS4 crystal-
lizes in the space group Pn (wurtzkesterite-type structure)
with a = 6.704, b = 7.038, c = 8.217 Å, and β = 90.16°. The
wurtzkesterite-type structure can be derived from the hex-
agonal diamond structure and is composed of a hexagonal
closest packed arrangement of the sulfur anions (Fig. 8).
The crystal structure contains two independent Ag posi-
tions Ag1 and Ag2, one Cd, and one Sn position where all
cations are located on twofold positions. This setup occurs
also for the anions with in total four independent sulfur
positions on twofold positions. Keeping in mind the hon-
eycomb set-up, alternating Ag/Cd and Ag/Sn layers (in
general AI/BII and AI/CIV layers) are distinguishable which
is analogous to the kesterite-type structure [5] derived from
the cubic diamond structure. Interatomic Ag–S distances
range from 2.46(3) to 2.52(4) Å with an average of 2.50 Å for
Ag1–S and from 2.52(3) to 2.58(2) Å with an average of 2.55Å
for Ag2–S. The Ag–S bond lengths correlate quite well
with those known from Ag2CdGeS4 (Ag–S: 2.52–2.57 Å) [34]
and are slightly smaller than those reported for Ag2FeSiS4
050100 150 200 250 300
-8
-6
-4
-2
0
2
DTA (uV)
temperature (°C)
heating
cooling
exo
Fig. 2: DTA curve of Ag2CdSnS4 under nitrogen atmosphere.
396 E.M. Heppke etal.: Crystal structure of mechanochemically synthesized Ag2CdSnS4
(Ag1–S: 2.52–2.58 Å; Ag2–S: 2.54–2.61 Å) [16]. Cd–S bond
lengths range from 2.51(3) to 2.58(3) Å with an average
of 2.55 Å and are in good agreement with those found in
Cu2CdGeS4 (Cd–S: 2.51–2.60 Å) [4] and Ag2CdGeS4 (Cd–S:
2.49–2.57 Å) [34]. Interatomic Sn–S distances vary from
2.39(3) to 2.51(3) Å and are in average 2.43 Å which is
Fig. 3: Group-subgroup scheme (Bärnighausen formalism) for the group-theoretical relation between hexagonal diamond (lonsdaleite) and
the wurtzstannite-/wurtzkesterite-type structure with data taken from [26] for lonsdaleite, [27] for wurtzite, [20] for space group Cmc21, [4]
for wurtzstannite-type Cu2CdGeS4, and [28] for wurtzkesterite-type Cu2ZnSiS4.
E.M. Heppke etal.: Crystal structure of mechanochemically synthesized Ag2CdSnS4 397
somewhat longer than those in Li2CdSnS4 (Sn–S: 2.38–
2.39Å) [35] and Li2ZnSnS4 (Sn–S: 2.35–2.42 Å) [36].
2.4 Stabilization of the high-temperature
phase at room temperature
The observed phase transition seems to be of first order.
Consequently, it is no surprise that the stabilization of the
high-temperature phase of Ag2CdSnS4 at room temperature
was feasible by annealing the sample at T = 300°C for 3h
in an evacuated and closed silica tube in a vertically posi-
tioned tube furnace (Nabertherm RT 50-250/13; C450 con-
troller) and subsequently quenching it in dry ice. Rietveld
refinements were done in space group Pmn21 as well as in
Pn and confirmed that the quenched sample crystallizes
in the orthorhombic space group Pmn21. As expected, the
difference of the residual values Rwp between the space
groups Pmn21 and Pn is not significant (Rwp = 10.9 for Pmn21
and Rwp = 10.7 for Pn, respectively). Additionally, con-
vergence problems occurred for the refinement in space
group Pn. The powder diffraction pattern of the stabilized
high-temperature phase of Ag2CdSnS4 quenched to room
temperature is shown in Fig. 9. Crystallographic details as
well as atomic and structural parameters are presented in
Tables 2 and 5. The refinement strategy was similar to that
of the sample measured in-situ.
The interatomic Ag–S distances in the quenched high-
temperature phase vary from 2.505(12) to 2.521(15) Å with
an average of 2.52 Å and correlate well with the Ag1–S
distances (2.46(3)–2.52(4) Å) found in the low-temperature
phase of Ag2CdSnS4. However, they are somewhat smaller
than the Ag2–S distances (2.52(3)–2.58(2) Å) reported for
Ag2CdGeS4 (Ag–S: 2.52–2.57 Å) [34] and Ag2FeSiS4 (Ag1–S:
2.52–2.58 Å; Ag2–S: 2.54–2.61 Å) [16]. Cd–S bond lengths
range from 2.531(17) to 2.59(3) Å with an average value of
2.56 Å. This correlates pretty well with reported Cd–S dis-
tances for Cu2CdGeS4 (Cd–S: 2.51–2.60 Å) [4] and Ag2CdGeS4
(Cd–S: 2.49–2.57 Å) [34] as well with the ones in our
Fig. 4: X-ray diffraction pattern of the high-temperature phase
(measured at T = 300°C) of Ag2CdSnS4 with the results of the Rietveld
refinement in space group Pmn21 (red: measured; black: calculated;
blue: measured–calculated).
Table 2: Results of the refinements for the high- and low-temperature phases of Ag2CdSnS4 (standard deviations in parenthesis).
Ag2CdSnS4
High-temperature phase T = 300°C Quenched high-temperature phase at r. t. Low-temperature phase
Crystal system Orthorhombic Orthorhombic Monoclinic
Space group Pmn21Pmn21Pn
Z2 2 2
Unit cell dimensions a = 8.2171(6) Å a = 8.2137(4) Å a = 6.7036(2) Å
b = 7.0641(5) Å b = 7.0403(4) Å b = 7.0375(3) Å
c = 6.7029(5) Å c = 6.7033(2) Å c = 8.2166(3) Å
β = 90.1577(9)°
Cell volume, Å3389.08(5) 387.63(3) 387.63(2)
Calculated density, g cm−34.91 4.93 4.93
Diffractometer RIGAKU SmartLab 3kW system
Radiation CuKα1 radiation
Wavelength, Å λ = 1.54060 Å
Number of refined parameters 34 34 48
Rp0.096 0.081 0.100
Rwp 0.128 0.109 0.138
Rexp 0.138 0.098 0.152
RBragg 0.049 0.049 0.050
S0.93 1.13 0.91
398 E.M. Heppke etal.: Crystal structure of mechanochemically synthesized Ag2CdSnS4
low-temperature phase of Ag2CdSnS4 (2.51(3)–2.58(3) Å).
For the Sn–S distances, the values vary from 2.39(3) to
2.491(17) Å with an average of 2.45 Å. They are in agreement
with those found in the low-temperature phase (2.39(3)–
2.51(3) Å) and also with those reported for Li2CdSnS4 (Sn–S:
2.38–2.39 Å) [35] and Li2ZnSnS4 (Sn–S: 2.35–2.42 Å) [36].
For the high- and the low-temperature phase of
our Ag2CdSnS4 sample, additional refinements were
undertaken in different space groups. These include
the centrosymmetric space group Pnma as well as the
non-centrosymmetric space group Pmc21. In addition,
refinements in the triclinic crystal system have been per-
formed. All refinements in these aforementioned space
groups resulted in severe convergence problems. Conse-
quently, we exclude all these space groups as possible
candidates for Ag2CdSnS4.
Rietveld refinements in Pmn21 and Pn were done
for all temperatures (25–300°C, 25K steps). The mono-
clinic angle β was plotted against the temperature for
the whole temperature range (black dots in Fig. 10). The
plotted results point to the fact that β abruptly becomes
90°, the value of the orthorhombic high-temperature
phase, at about 200°C. This again indicates a first order
Table 3: Refined atomic parameters for the high-temperature phase
(T = 300°C) of Ag2CdSnS4 in space group Pmn21 (standard deviations
in parenthesis).
Atom Wyckoff x y z s.o.f Biso (Å2)
Ag 4b0.2543(7) 0.3242(7) –0.01018a1 3.8(2)
Cd 2a0 0.8440(10) 0.988(3) 0.5 5.3(3)
Sn 2a0 0.1770(8) 0.487(3) 0.5 1.0(2)
S1 4b0.239(2) 0.336(2) 0.378(3) 1 1
S2 2a0 0.180(4) 0.834(3) 0.5 1
S3 2a0 0.855(4) 0.363(4) 0.5 1
aFixed z value.
Fig. 5: Crystal structure of the high-temperature phase of Ag2CdSnS4 (Pmn21) with cation-centered polyhedra viewed along the [010]
direction (left) and the honeycomb set-up with stacking sequence of alternating Ag and Cd/Sn layers viewed along the [001] direction
(right).
Fig. 6: Group-theoretical relation (Bärnighausen formalism)
between space group Pmn21 (wurtzstannite-type structure) and its
direct subgroups P21, Pm, Pn, and Pna21.
Fig. 7: X-ray diffraction pattern of the low-temperature phase
(measured at T = 25°C) of Ag2CdSnS4 with the results of the Rietveld
refinement in space group Pn (red: measured; black: calculated;
blue: measured–calculated).
E.M. Heppke etal.: Crystal structure of mechanochemically synthesized Ag2CdSnS4 399
transition from the wurtzkesterite-type structure to the
wurtzstannite-type structure. The area around 200–225°C
may be considered as two-phase region. The error bars at
temperatures ≤200°C are smaller than those >200°C. For
the refined cell volume, no significant jump in the region
around 200°C can be observed (red squares in Fig. 10).
2.5 UV/Vis spectroscopy
The optical properties of Ag2CdSnS4 were measured
in reflectance mode, and the optical band gaps were
determined using the Tauc plot method [37, 38]. For
the direct optical band gap a value of Eg = 1.93 eV was
calculated whereas a narrower indirect optical band
gap of Eg = 1.82eV was obtained (Fig. 11). The color of
our sample (black with violet accents) points to a direct
optical band gap of Eg = 1.93 eV which is in good agree-
ment with the experimentally determined band gap
mentioned in literature [39].
3 Conclusions
Phase-pure and well crystallized Ag2CdSnS4 was
synthesized by ball milling (350rpm, 5h) and subse-
quent annealing at T = 550°C under H2S atmosphere for
2h in a tube furnace. The observed reflections in the
X-ray powder diffraction pattern are not in agreement
with the crystal structure proposed in literature (space
group Cmc21) where all cations are statistically distrib-
uted on one position. Structural investigations includ-
ing Rietveld refinements using our mechanochemically
prepared Ag2CdSnS4 sample point, in accordance to the
calculations of Chen etal. [42], to the presence of the
wurtzkesterite-type structure (space group Pn) with
an ordered arrangement of the cations. A reversible
Table 4: Refined atomic parameters for the low-temperature phase
of Ag2CdSnS4 (T = 25°C) in space group Pn (standard deviations in
parenthesis).
Atom Wyckoff x y z s.o.f Biso (Å2)
Ag1 2a0.91670a0.675(4) 0.27782a1 1.5(6)
Ag2 2a0.407(3) 0.8454(8) 0.533(3) 1 2.5(3)
Cd1 2a0.913(2) 0.674(4) 0.773(3) 1 1.99(6)
Sn1 2a0.908(2) 0.1759(7) 0.523(2) 1 0.22(13)
S1 2a0.298(7) 0.665(7) 0.783(4) 1 1
S2 2a0.284(6) 0.669(8) 0.278(5) 1 1
S3 2a0.267(4) 0.182(3) 0.526(5) 1 1
S4 2a0.791(4) 0.851(3) 0.521(4) 1 1
aFixed x and z values.
Fig. 8: Crystal structure of the low-temperature phase of Ag2CdSnS4 crystallizing in space group Pn with cation-centered polyhedra (view
along the [010] direction, left) and the honeycomb set-up with a stacking sequence of alternating Ag/Cd and Ag/Sn layers viewed along the
[100] direction.
Fig. 9: X-ray diffraction pattern of the high-temperature phase of
Ag2CdSnS4 stabilized at room temperature with the results of the
Rietveld refinement in space group Pmn21 (red: measured; black:
calculated; blue: measured–calculated).
400 E.M. Heppke etal.: Crystal structure of mechanochemically synthesized Ag2CdSnS4
first-order phase transition at around 200°C from a
low-temperature wurtzkesterite- to a high-temperature
wurtzstannite-type structure has been observed. Addi-
tionally, a direct optical band gap of Eg = 1.93 eV, which
is in agreement to that reported in the literature, was
found. In the present work, the mechanochemical syn-
thetic route was again successful for the preparation of
phase-pure multinary sulfides. This particular way of
synthesis appears to have a strong effect on the crystal
structure of the resulting compounds. This may be an
explanation for our different observations compared to
the literature results presented in [4, 20].
4 Experimental Section
4.1 Synthesis
Ag2CdSnS4 was synthesized by ball milling in a high-
energy planetary Mono Mill (Pulverisette 6, Fritsch, Idar-
Oberstein, Germany), followed by an annealing step
under an atmosphere of H2S. As starting materials Ag2S
(Schuchardt), CdS, SnS, and sulfur (Fluka, 99.99%) were
used which were filled in a 45mL zirconia-made grinding
beaker with 6 zirconia balls with a diameter of 15mm. A
rotational speed of 350rpm and a milling time of 5h were
set. In order to obtain a highly crystalline product, the
grounded powder was annealed at T = 550°C for 2h under
H2S atmosphere. A 0.1 m Cd(CH3CO2)2 solution and H2S (Air
Liquide, 99.5%) were used for precipitation of CdS. SnS
was synthesized using the high-temperature solid-state
reaction of the elements tin (Merck, 99.9%) and sulfur
(Fluka, 99.99%) in an evacuated and sealed SiO2 ampoule.
4.2 Structural and chemical characterization
X-ray powder diffraction investigations as well as in-situ high-
temperature X-ray diffraction experiments (25–300°C, N2
atmosphere) were carried out with a RIGAKU SmartLab 3kW
system equipped with a Kα1 unit (Johansson-type Ge crystal,
CuKα1 radiation, λ = 1.54060 Å). The diffraction data were
obtained in Bragg-Brentano geometry over an angular range
Table 5: Refined atomic parameters for the high-temperature phase
of Ag2CdSnS4 in space group Pmn21 quenched to room temperature
(standard deviations in parenthesis).
Atom Wyckoff x y z s.o.f Biso (Å2)
Ag1 4b0.2532(9) 0.3255(8) –0.00662a1 1.5(2)
Cd1 2a0 0.8445(11) 0.983(3) 0.5 2.4(3)
Sn1 2a0 0.1767(9) 0.491(3) 0.5 0.28(15)
S1 4b0.251(3) 0.337(3) 0.368(5) 1 1
S2 2a0 0.184(4) 0.847(4) 0.5 1
S3 2a0 0.851(4) 0.369(6) 0.5 1
aFixed z value.
050100 150 200 250 300
89,95
90,00
90,05
90,10
90,15
90,20
temperature (°C)
β
(deg)
387,5
388,0
388,5
389,0
389,5
volume (g/cm
3
)
Fig. 10: Monoclinic angle β and cell volume plotted against
temperature (from Rietveld refinements in space group Pn for β and
from refinements in Pn (T ≤ 200°C) and Pmn21 (T > 200°C) for the cell
volume).
-2
0
2
4
6
8
10
12
14
16
1,21,4 1,61,8 2,02,2 2,42,6 1,21,4 1,61,8 2,02,2 2,4
0,0
0,5
1,0
1,5
2,0
hν (eV) hν (eV)
(F(R)·hν)2
(F(R)·hν)1/2
Fig. 11: UV/Vis spectra of Ag2CdSnS4 with Tauc plot determinations of the direct (left) and indirect (right) optical band gap.
E.M. Heppke etal.: Crystal structure of mechanochemically synthesized Ag2CdSnS4 401
of 2θ = 10–120° for room temperature and 2θ = 10–80° for
high-temperature measurements. Rietveld refinements [40]
were performed using the program FullProf [41] by apply-
ing a pseudo-Voigt function. Backgrounds were fitted using
a set of various background points with refinable heights.
It should be mentioned that anisotropic strain broadening
terms were used due to anisotropic reflection broadening.
Further details of the crystal structure investiga-
tion may be obtained from Fachinformationszentrum
Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany
(fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.
de, http://www.fiz-informationsdienste.de/en/DB/icsd/
depot_anforderung.html) on quoting the deposition
numbers CSD-1979198 and CSD-1979199.
For the chemical characterization of Ag2CdSnS4,
energy dispersive X-ray spectroscopy (EDX) using a DSM
982 GEMINI spectrometer (Carl Zeiss AG, Oberkochen,
Germany) equipped with a XFlash 6 | 60 detector (Bruker,
Billerica, USA) were performed. For the determination of
the sulfur content, an instrumental error of 5% is given.
The EDX measurements were carried out at the Zentrum
für Elektronenmikroskopie (ZELMI) of the TU Berlin. For
the confirmation of the sulfur content, additional analy-
ses were carried out using a FlashEA 1112 elemental ana-
lyzer (Thermo Scientific™, Waltham, USA). Hereby, a
device error of 2% is assumed.
Thermoanalytical analysis (DTA) were carried out
using a STA 7300 thermogravimeter (Hitachi, Chiyoda,
Japan). These measurements were performed under nitro-
gen atmosphere up to 300°C with a heating and cooling
rate of 3K min−1; alumina crucibles were used as reference
and sample container.
4.3 UV/Vis spectroscopy
The optical band gaps were determined by UV/Vis meas-
urements using a V670 UV/Vis-NIR spectrometer (Jasco
Deutschland GmbH, Pfungstadt, Germany). The spectra
obtained in diffuse reflectance mode were converted
into absorption spectra by the Kubelka-Munk function;
the optical band gaps were received from the absorption
spectra using the Tauc plot method [37, 38]. The standard
deviation for Eg was estimated close to 0.05 eV. For sample
preparation, Ag2CdSnS4 was mixed with a white standard
(MgO) in a mass ratio of 1:1.
Acknowledgements: Special thanks to the Zentrum für
Elektronemikroskopie (ZELMI) of the TU Berlin giving
access to the EDX measurements. EDX measurements
were carried out by Rodrigo Beltran-Suito; combustion
analysis was performed by Juana Krone (both TU Berlin).
DTA and UV/Vis measurements were carried out by Dr.
Nina Genz (TU Berlin). This project was supported by the
German Science Foundation (DFG, LE 781/19-1).
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