Homologous silver bismuth chalcogenide halides (N, x)P. I. syntheses and crystal structures of the (0, 1)P compound AgBi 2S2Cl3 and of three members of the (1, x)P solid solution series Ag2xBi4-2xS 6-4xBr4x

Article abstract of DOI:10.1002/zaac.200400131

AgBi₂S₂Cl₃ and three members of the Ag_2xBi_4-2xS_6-4xBr_4x solid solution (x = 0.50 (I), x = 0.64 (II), and x = 0.73 (III)) were synthesized from Bi ₂S₃, AgX and BiSX (X = Cl, Br) at 720 K. X-ray diffraction on single crystals revealed that the compounds crystallize in the monoclinic space groups C2/m (No. 12) with a = 1257.3(3), b = 400.0(1), c = 804.1(2) pm, β = 111.0(1)° for AgBi₂S₂Cl₃, a = 1331.3(5), b = 408.0(2), c = 972.0(3) pm, β = 90.8(1)° for (I), a = 1328.9(4), b = 408.5(1), c = 973.4(4) pm, β = 90.9(1)° for (II), and a = 1332.4(3), b = 409.3(1), c = 974.3(2) pm, β = 90.4(1)° for (III). AgBi₂S₂Cl₃ adopts a variant of the InBi ₂S₄Cl structure type, the isostructural compounds (I), (II) and (III) are related to the orthorhombic FeUS₃ type. All four compounds belong to homologous series with general formula [BiSA] ₂·[Ag_xBi_1-xS_2-2xX _2x-1]_N+1 (X = Cl, Br), which resemble minerals of the pavonite series. They are characterized by the parameters N and x and are denoted ^{(N, x)}P. In the crystal structures, two kinds of layered modules, A and B, alternate. Modules of type B are composed of chains of edge-sharing [MBr₄Z₂] octahedra (M = Ag/Bi; X = Cl, Br; Z = S/X) and vary in thickness N. Modules of type A uniformly consist of rows of paired monocapped trigonal prisms around Bi atoms with [MS₄Z ₂] octahedra around mixed-occupied metal positions (M = Ag/Bi) between them. Ag-Bi₂S₂Cl₃ comprises modules of type A only (N = 0) and is the silver-rich end-member (x = 1) in the series ^{(0, x)}P. (I) to (III) belong to the ^{(1, x)}P series with B modules of thickness N = 1. Calculations of the electronic band structures indicate that the compounds should act as indirect semiconductors with band gaps between 1.5 and 0.6 eV.

Full text of DOI:10.1002/zaac.200400131

Homologous Silver Bismuth Chalcogenide Halides (N, x)P. I .
(
0, 1)
Syntheses and Crystal Structures of the
P Compound AgBi S Cl and of
2
S
2
3
Br
(
1, x)
Three Members of the
P Solid Solution Series Ag Bi
2
x
4؊2x 6؊4x 4x
Pierre Ferdinand Poudeu Poudeu, Tilo Söhnel and Michael Ruck*
Dresden, Institut für Anorganische Chemie der Technischen Universität
Received February 24th, 2004.
Abstract.
Ag2xBi4Ϫ2x
x ϭ 0.73 (III)) were synthesized from Bi
AgBi
6Ϫ4xBr4x solid solution (x ϭ 0.50 (I), x ϭ 0.64 (II), and
, AgX and BiSX (X ϭ
2
S
2
Cl
3
and
three
members
of
the
meters N and x and are denoted (N, x)P. In the crystal structures,
two kinds of layered modules, A and B, alternate. Modules of type
B are composed of chains of edge-sharing [MBr Z ] octahedra
4 2
S
2
S
3
Cl, Br) at 720 K. X-ray diffraction on single crystals revealed that
the compounds crystallize in the monoclinic space groups C2/m
(M ϭ Ag/Bi; X ϭ Cl, Br; Z ϭ S/X) and vary in thickness N. Mod-
ules of type A uniformly consist of rows of paired monocapped
(
No. 12) with a ϭ 1257.3(3), b ϭ 400.0(1), c ϭ 804.1(2) pm, β ϭ
11.0(1)° for AgBi S Cl , a ϭ 1331.3(5), b ϭ 408.0(2), c ϭ
2 2 3
72.0(3) pm, β ϭ 90.8(1)° for (I), a ϭ 1328.9(4), b ϭ 408.5(1), c ϭ
73.4(4) pm, β ϭ 90.9(1)° for (II), and a ϭ 1332.4(3), b ϭ 409.3(1),
trigonal prisms around Bi atoms with [MS
mixed-occupied metal positions (M ϭ Ag/Bi) between them. Ag-
Bi Cl comprises modules of type A only (N ϭ 0) and is the
silver-rich end-member (x ϭ 1) in the series
4 2
Z ] octahedra around
1
9
9
2
S
2
3
(
0, x)
P. (I) to (III) belong
P series with B modules of thickness N ϭ 1. Calculations
(
1, x)
c ϭ 974.3(2) pm, β ϭ 90.4(1)° for (III). AgBi
of the InBi Cl structure type, the isostructural compounds (I),
II) and (III) are related to the orthorhombic FeUS type. All four
compounds belong to homologous series with general formula
BiSX] ·[Ag Nϩ1 (X ϭ Cl, Br), which resemble
minerals of the pavonite series. They are characterized by the para-
2
S
2
Cl
3
adopts a variant
to the
2
S
4
of the electronic band structures indicate that the compounds
should act as indirect semiconductors with band gaps between 1.5
and 0.6 eV.
(
3
[
2
x 2Ϫ2x 2xϪ1
Bi1ϪxS X ]
Keywords: Bismuth; Silver; Solid solutions; Chalcogenides; Halides
Homologe Silber-Bismut-Chalkogenid-Halogenide (N, x)P. I .
(
0, 1)
Synthesen und Kristallstrukturen der
P-Verbindung AgBi S Cl und von drei
2
2
3
(
1, x)
Vertretern der
P-Mischkristallreihe Ag Bi
S
Br
2
x
4؊2x 6؊4x 4x
Inhaltsübersicht. AgBi
stallreihe Ag2xBi4Ϫ2x 6Ϫ4xBr4x(x ϭ 0,50 (I), x ϭ 0,64 (II) und x ϭ
,73 (III)) wurden bei etwa 720 K aus Bi , AgX und BiSX (X ϭ
Cl, Br) synthetisiert. Aus Röntgenbeugungsuntersuchungen an Ein-
kristallen geht hervor, dass die Verbindungen in den monoklinen
Raumgruppen C2/m (Nr. 12) kristallisieren mit a ϭ 1257,3(3), b ϭ
2
S
2
Cl
3
und drei Verbindungen der Mischkri-
Pavonit-Reihe gleichen. Sie werden durch die Parameter N und x
(
N, x)
S
charakterisiert und mit
wechseln sich zwei Typen schichtartiger Bauverbände A und B ab.
Ketten kantenverknüpfter Oktaeder [MX ] (M ϭ Ag/Bi; X ϭ Cl,
P bezeichnet. In den Kristallstrukturen
0
2 3
S
4 2
Z
Br; Z ϭ S/X) bilden die B-Module, welche von unterschiedlicher
Mächtigkeit N sein können. Module des Typs A bestehen einheit-
lich aus Doppelsträngen einfach überdachter trigonaler Prismen
4
1
1
2 2 3
00,0(1), c ϭ 804,1(2) pm, β ϭ 111,0(1)° für AgBi S Cl , a ϭ
331,3(5), b ϭ 408,0(2), c ϭ 972,0(3) pm, β ϭ 90,8(1)° für (I), a ϭ
328,9(4), b ϭ 408,5(1), c ϭ 973,4(4) pm, β ϭ 90,9(1)° für (II) und
um Bi-Atome sowie die Stränge verknüpfenden Oktaedern [MS
um mischbesetzte Metallatompositionen (M ϭ Ag/Bi). AgBi
4
Z
Cl
2
]
2
S
2
3
a ϭ 1332,4(3), b ϭ 409,3(1), c ϭ 974,3(2) pm, β ϭ 90,4(1)° für
III). AgBi Cl kristallisiert in einer Variante des InBi Cl-Typs,
die isotypen Verbindungen (I), (II) und (III) haben dem ortho-
rhombischen FeUS -Typ verwandte Strukturen. Alle vier Verbin-
dungen gehören zu homologen Reihen der allgemeinen Formel
BiSX] ·[Ag Nϩ1 (X ϭ Cl, Br), die Mineralen der
setzt sich ausschließlich aus A-Modulen zusammen (N ϭ 0) und
(
0, x)
(
2
S
2
3
2
S
4
stellt den silberreichsten Vertreter (x ϭ 1) der Reihe
P dar. (I)
(
1, x)
bis (III) gehören zur
P-Reihe, in der die B-Module die Dicke
3
N ϭ 1 haben. Rechnungen zur elektronischen Bandstruktur legen
nahe, dass die Verbindungen indirekte Halbleiter mit Bandlücken
zwischen 1,5 und 0,6 eV sein sollten.
[
2
x 2Ϫ2x 2xϪ1
Bi1ϪxS X ]
Introduction
number of new compounds, for example ABiQ (A ϭ alkali
2
metal; Q ϭ chalcogen) [1Ϫ5], ABi S (A ϭ K, Cs, Rb)
3
5
During the past three decades, investigations of ternary bis-
muth chalcogenides have resulted in the discovery of a great
[
6Ϫ8], CsBi3.67Se [9], CsBi Te [10], A Bi Se (x ϭ 1, 2;
6 4 6 x 4 7
A ϭ Rb, Cs) [11], Ͱ and β-KBi6.33S10 [12, 13], K Bi S [12,
2
8 13
1
3], Ͱ and β-K Bi Se [5, 14], A Bi Se (A ϭ Rb, Cs) [9],
2 8 13 2 8 13
*
Prof. Dr. Michael Ruck
K2.5Bi8.5Se14 [14], Cs Bi Se [15], BaBiSe [16], BaBiTe3
3 7 12 3
Institut für Anorganische Chemie
Technische Universität Dresden
D-01062 Dresden
Fax: (ϩ49) 351-463-37287
E-mail: Michael.Ruck@chemie.tu-dresden.de
[
17], Ͱ and β-BaBi S [18], BaBi Se [9], and Sr Bi Se [19].
2 4 2 4 4 6 13
Within the last five years, quaternary bismuth chalcogen-
ides have also been reported in the systems A/Bi/M/Q (A ϭ
K, Cs, Rb; M ϭ Cu, Ag, Sn, Pb; Q ϭ S, Se). These include
1276
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DOI: 10.1002/zaac.200400131
Z. Anorg. Allg. Chem. 2004, 630, 1276Ϫ1285

Homologous Silver Bismuth Chalcogenide Halides (N, x)P. I
Q ϭ S, Se and X ϭ Cl, Br. The strategy consists of partial
substitution of bivalent sulphur atoms by univalent halogen
atoms in the structures of various known and hypothetical
pavonite homologues in the ternary system Ag/Bi/S. This
operation introduces a variation of charge in the crystal lat-
tice and might open a way to structural ordering of both
cations and anions.
At the beginning of the investigation, only a limited num-
ber of quaternary phases consisting of a transition metal
and several types of p-block elements were known. In the
systems T Ϫ Pn Ϫ Q Ϫ X (T ϭ transition metal; Pn ϭ P,
As, Sb, Bi; Q ϭ S, Se, Te; X ϭ F, Cl, Br, I), the most preva-
lent examples are copper-containing compounds. These in-
clude Cu PS Br (an ion-conducting argyrodite) [27],
6
5
(
(
CuBr) P Se [28], (CuI)P Se4 [29], (CuI) P Se [30],
3 4 4 4 3 4 4
CuI) P Se [31], (CuI) Cu SbS [32], Bi Cu S X (X ϭ Cl,
2
8
3
2
3
3
2
3 4
Br) [33, 34], Cu3.1Bi9.4Se
I
[35], Cu4.5Bi Se
I
[35]
15.3 0.7
10 16.5 1.5
CuHgSeCl [36], CuHgSeBr [37], CuHgSX (X ϭ Cl, Br, I)
[
[
[
38]. Examples with other d-block elements are CdSb S I
6
8 4
39], Hg AsQ X (Q ϭ S, Se; X ϭ Cl, Br, I) [40], MnSbSe I
3
4
2
41], AgHgSX (X ϭ Br, I) [37] and Ag HgSI [42]. The iso-
Fig. 1 Representation of the structure principle of pavonite homo-
logues showing the two types of building units A and B. The thick-
ness of the NaCl-like module, B, increases with the number N of
edge-sharing octahedra. Black atoms: Bi; medium grey atoms: M ϭ
2
2
structural bismuth indides InBi S X (X ϭ Cl, Br) [43] are
also known.
In the quaternary systems M/Bi/Q/X (M ϭ Cu, Ag, Q ϭ
S, Se; X ϭ Cl, Br) we have recently reported the syntheses
and crystal structures of Ag Bi S Cl [44], AgBi Q Cl (Q ϭ
2
4
(Ag, Cu, Pb, Bi, Fe,...); light grey atoms: S.
2
2
3
2
2
3
S, Se) [45] and MBiSCl (M ϭ Cu, Ag) [46]. All of them
2
exhibit the desired ordering of atoms. Our current investi-
gation is expected to elucidate the order or disorder of Ag
and Bi atoms, the type of cation sites influenced by the Ag
and Bi substitution, the type of anion sites affected by the
Q and X substitution, as well as the distortions of the ideal
framework (pavonite homologues) connected with these
phenomena. In addition, the variation in the bonding pat-
tern of Bi as a consequence of the substitution processes
will be examined. Here we report the syntheses, and the
crystal and electronic structures of AgBi S Cl and three
K Bi Cu S [20], CsBi CuS [20], RbBi2.66CuSe5 [20],
3
5
2
10
2
4
CsBiAg2S3 [20], A Bi Cu S (A ϭ Rb, Cs) [21] and
3
5
2 10
KBi CuS [22]. In addition to these synthetic phases, many
2
4
naturally occurring bismuth chalcogenides have been disco-
vered and structurally characterized. Most are in the M/Bi/
S (M ϭ Cu, Ag, Pb, Fe,...) systems. A large number of these
so-called sulphosalts are members of homologous series.
One of them is the pavonite homologous series, which
includes structures that are composed of two alternating
kinds of modules parallel to (001) (Fig. 1). In the slab de-
noted A, [MS ] octahedra alternate with paired [BiS ] mon-
2
2
3
members of the Ag Bi4Ϫ2xS6Ϫ4xBr solid solution series.
2x
4x
6
7
These compounds belong to a new bivariant class of qua-
ocapped trigonal prisms. The NaCl-like slabs, denoted B,
are composed of parallel, N-membered chains of edge-shar-
ing octahedra. Different members of the series are denoted
(
N, x)
ternary solids denoted
BiQX] ·[Ag Bi1ϪxQ2Ϫ2xX
P with general formula
[
]
(Q ϭ S, Se; X ϭ Cl, Br).
2
x
2xϪ1 Nϩ1
(N, x)
N
This first contribution on the chemistry of
P com-
by the symbol P [23]. The general chemical formula is
pounds will be continued soon with reports on (2, x)P,
(3, x)
P,
oct
oct
M
Bi Q [23], where M
represents the octa-
Nϩ1
2
Nϩ5
(
4, x) (5, x)
(7, x)
P,
P and
P compound and a detailed discussion
hedrally coordinated metal positions in the structure. In the
of general principles.
ternary system Ag/Bi/S the series includes “AgBi S ” (N ϭ
6
9
4
) [24], Ag Bi S (N ϭ 5) [20] and Ag Bi S (N ϭ 7) [25].
2 6 10 3 7 12
In their structures, Ag/Bi disorder is generally observed in
the octahedrally coordinated metal sites. In addition to
Experimental Section
Syntheses
these phases, the existence of AgBiS [26] has been reported
2
and Ag/Bi mixed occupancy was also found in the metal
positions (statistical NaCl structure type). All compounds
belong to the pseudo-binary section Ag SϪBi S .
In order to study the nature of the cation disorder ob-
served in the structures of pavonite homologues in the sys-
tem Ag/Bi/S and to explore the possibility of a solid solu-
tion leading to more ordered phases, we have started the
investigation of the quaternary systems Ag/Bi/Q/X where
2 3
The following chemicals were used as starting materials: Bi S ,
AgCl, AgBr, BiSBr, and BiBr . Bi S (Alfa Aesar, 99.9 %), AgCl
3 2 3
(Merck, > 99.4 %) and AgBr (Merck, 99.7 %) were used as ob-
tained without any further treatment for purification.
2
2
3
BiBr
and 1.75 ml of Br
It was necessary to cool the bottom of the ampoule in liquid nitro-
gen during the introduction of Br . The mixture was then allowed
3
: BiBr
3
was synthesized from a mixture of 4.70 g of Bi powder
2
(3.2 g/ml) handled under nitrogen atmosphere.
2
Z. Anorg. Allg. Chem. 2004, 630, 1276Ϫ1285
zaac.wiley-vch.de
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1277

P. F. Poudeu Poudeu, T. Söhnel, M. Ruck
to pre-react by slowly heating the external wall of the ampoule until
the contents melted. The reaction was completed by sublimation
of the BiBr from 620 K to room temperature using a tubular oven.
3
The air-sensitive yellow product obtained was ground into powder
and used for the synthesis of BiSBr.
Single-crystal X-ray crystallography
In order to investigate their symmetry and diffraction qual-
ity, the 0kl, 1kl, hk0, hk1 photographs of single crystals of
all four compounds were recorded on a Buerger precession
camera. The symmetry of the diffraction patterns corre-
sponds to Laue class 2/m. In all cases the reflection con-
dition observed on the photographs was h ϩ k ϭ 2n for
hkl, suggesting C2/m, Cm or C2 as possible space groups.
For information about the collection of intensity data refer
to Table 1.
2 3 3
BiSBr: An equimolar mixture of Bi S and BiBr was thoroughly
ground under a dry argon atmosphere in a glove box and trans-
ferred into a silica tube, which was flame-sealed under a pressure
Ϫ3
of 10 Torr. The tube was heated to 430 K in 5 h, kept at 430 K
for 60 h and further heated to 500 K in 5 h. After 2 days at 500 K,
the ampoule was allowed to cool to room temperature in 10 h. The
dark-red product was ground into powder and used for further re-
actions.
The structure of AgBi S Cl was solved in the centrosym-
2
2
3
metric space group C2/m using direct methods and refined
by the full-matrix least-squares technique [48]. Two crystal-
lographically independent metal positions, M(1) and M(2),
and three anion positions, Z(1), Z(2) and Z(3), were lo-
cated. The M(1) and M(2) sites were easily assigned to Bi
and Ag. The sulphide and chloride anions present almost
the same X-ray scattering factors and therefore could not
be directly differentiated by Fourier synthesis. The assign-
ment of S and Cl positions was done calculating of the
bond valence sums [49]. The structure was refined to a re-
2 2 3
AgBi S Cl : A mixture of AgCl and BiSCl in the molar ratio 1:2
was thoroughly ground and loaded into a silica tube, which was
Ϫ3
flame-sealed under a pressure of 10 Torr. The tube was placed
in a tubular oven, heated at 720 K for 5 days and then quenched
to room temperature. Well-formed black plate-like crystals of
3
AgBi
2
S
2
Cl
3
with dimensions up to 0.5 ϫ 0.25 ϫ 0.05 mm were
found in the polycrystalline product mixture. Crystals of
AgBi Cl were found to be stable in air for at least one year.
Microcrystalline powder of AgBi Cl with approximately 95 %
purity can be obtained by annealing a stoichiometric mixture of
starting materials at 630 K for 5 days. AgBi Cl melts incongru-
ently near 650 K (LABSYS DSC 12, Netzsch).
2
S
2
3
2
S
2
3
sidual value R of about 10 % using this model. This value
S
2
3
1
2
dropped to about 5 % after absorption correction and in-
clusion of anisotropic displacement parameters for all
atoms in the structure. At this stage the difference electron
density around Bi was very high (ϩ7.3 and Ϫ5.0 e/
Ag2xBi4؊2x
S6؊4xBr4x, (I) (x ؍ 0.50): Single crystals of (I) were ob-
tained from a reaction involving Bi , AgBr and BiSBr in the mo-
2 3
S
lar ratio 1 : 1 : 1. A well-ground mixture of the starting materials
was loaded into a silica tube, which was flame-sealed under a press-
6
3
10 pm ), and the ellipsoid for the Bi atom was highly elon-
gated. Therefore, the position was split into two close posi-
tions, Bi(a) and Bi(b), which were constrained to have the
same displacement parameters and a sum of occupancies of
one. Refinement of this model yielded a more acceptable
shape for the Bi atom.
Ϫ3
ure of 10 Torr. The tube was heated to 670 K in 2 days, kept at
that temperature for 2 days and then further heated at 740 K for
15 days. The ampoule was then cooled to room temperature at a
rate of 5 K/h.
Ag2xBi4؊2x
S
6؊4xBr4x, (II) (x ؍ 0.64): Single crystals of (II) were
first obtained from a reaction involving an equimolar mixture of
AgBr and Bi at 720 K. The compound was later synthesized
with better yield starting from Bi , AgBr and BiSBr in the molar
The fractional atomic coordinates and isotropic displace-
ment parameters of all atoms with estimated standard devi-
ations are listed in Table 2. The connectivity between the
cations and anions in the structure and their coordination
numbers are given in Table 3. Interatomic bond distances
are given Table 4.
2 3
S
2 3
S
ratio 13 : 13 : 7. The procedure for the synthesis was similar to the
one described for (I).
Ag2xBi4؊2x
S6؊4xBr4x, (III) (x ؍ 0.73): The synthesis procedure was
similar to that for (I). A mixture of Bi , AgBr and BiSBr in the
2 3
S
The structure of (II) was solved by direct methods in the
space group C2/m and refined with a full-matrix least-
squares program [48]. Three cation positions, Bi(1), M(2)
and M(3), as well as three anion positions, S(1), Br(1) and
Z(2), were found in the asymmetric unit. A disorder model
was applied for the M and Z positions and the sum of the
occupation factors for each site was constrained to be
100 %. The atoms sharing the same site were forced to have
the same coordinates and displacement parameters. The site
occupation factors of the M and Z positions were re-
strained to maintain the electroneutrality of the compound.
The following compositions were obtained: M(2) ϭ
molar ratio 3 : 3 : 1 was used. For (I), (II) and (III), the products
consisted of agglomerates of black plate-like crystals. All of the
Ag2xBi4Ϫ2xS6Ϫ4xBr4x compounds were found to be stable in air.
EDX analysis
The chemical composition of a single crystal of (II) was determined
by EDX analysis. The crystal used for the investigation was first
checked with the Buerger precession camera for identification. The
rough lattice parameters obtained correspond to those of the crys-
tal used for the structure determination. The crystal was then cut
and the analysis was carried out on different points of the new
face. The experiment was performed on a LEO 982 digital scanning
electron microscope. Data were acquired using an accelerating volt-
age of 20 kV and a 1 min accumulation time. The semi-quantitative
analysis of the spectrum was performed with the program
Analyse Voyager [47]. The presence of all four elements in the crys-
[83.0(1) % Ag ϩ 17.0(1) % Bi], M(3) ϭ [45.0(1) % Ag ϩ
55.0(1) % Bi] and Z(2) ϭ [73.0(4) % S ϩ 27.0(4) % Br]. The
formula of the compound based on this refinement was
Ag1.27(1)Bi2.73(1)S3.46(2)Br2.54(2). The final refinement after
absorption correction and including the secondary extinc-
tion correction as well as anisotropic displacement param-
eters for all atoms yielded the R indices given in Table 1.
tal corresponding to the average composition Ag1.3Bi2.7
S3.5Br2.5
was confirmed; the crystal structure determination gives
Ag1.27(1)Bi2.73(1)S3.46(2)Br2.54(2).
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Z. Anorg. Allg. Chem. 2004, 630, 1276Ϫ1285

Homologous Silver Bismuth Chalcogenide Halides (N, x)P. I
Table 1 Selected crystallographic data and details of the structure determinations
Formula sum
AgBi
2
S
2
Cl
3
AgBi
3
S
4
Br
2
(I)
Ag1.27Bi2.73
S
3.46Br2.54 (II)
Ag1.46Bi2.54
6.38
S3.09Br2.91 (III)
Crystal system; Space group; Z
Density (ρcal)/(g/cm )
Crystal size [mm ]; Colour
Lattice parameters/pm; ° a ϭ
monoclinic; C2/m (No. 12); 2
3
6.12
6.43
6.42
3
0.35 ϫ 0.21 ϫ 0.03; black
1257.3(3)
400.0(1)
804.1(2)
110.98(3)
0.08 ϫ 0.05 ϫ 0.03; black
1331.3(4)
408.0(1)
972.0(2)
90.80(3)
0.16 ϫ 0.07 ϫ 0.02 ; black 0.08 ϫ 0.03 ϫ 0.02; black
1328.9(4)
408.50(8)
973.4(4)
90.90(4)
1332.4(3)
409.3(1)
974.3(2)
90.40(3)
b ϭ
c ϭ
β ϭ
Diffractometer
CCD (Mercury);
graphite monochromator
IPDS-I (Stoe); graphite monochromator
Radiation/pm
λ(MoK
200; 0.6; 295(1)
63°; Ϫ17 Յ h Յ 14,
α
) ϭ 71.073
λ(MoK
200; 1.5; 297(1)
56°; Ϫ17 Յ h Յ 16,
α
) ϭ 71.073
λ(AgK ) ϭ 56.087 λ(MoK ) ϭ 71.073
α
α
φ range; ∆ φ/°; T/K
200; 0.8; 293(1)
270; 1.0; 297(1)
57.6°; Ϫ17 Յ h Յ 18,
2θmax; index range
51.6°; Ϫ20 Յ h Յ 20,
Ϫ5 Յ k Յ 3, Ϫ11 Յ l Յ 11 Ϫ4 Յ k Յ 5, Ϫ12 Յ l Յ 12 Ϫ5 Յ k Յ 6, Ϫ14 Յ l Յ 15 Ϫ5 Յ k Յ 5, Ϫ13 Յ l Յ 13
Measured reflections
Unique reflections
2334
598
0.066 / 0.038
1418
600
0.070 / 0.049
3612
1118
0.029 / 0.027
2465
743
0.066 / 0.061
Rint / R
σ
Corrections
background, Lorentz factor, polarization factor
numerical, crystal description optimised using equivalent reflections
Absorption correction
Ϫ1
µ/cm
560
31
600
315
561
2
Structure refinement
least-squares method; full matrix; based on F ;
anisotropic displacement parameters [48]
37
No. of parameters
Extinction correction x ϭ
Difference electron density (e/10 pm ) ϩ2.5 to Ϫ2.9
36
35
6(1) · 10
Ϫ4
Ϫ4
Ϫ4
Ϫ4
22(7) · 10
24(4) · 10
66(2) · 10
6
3
ϩ1.8 to Ϫ1.6
0.046
0.109
ϩ3.3 to Ϫ2.4
0.027
0.042
ϩ1.9 to Ϫ1.7
0.040
0.073
R
1
(F
o
> 4σ(F
o
))
0.035
0.093
wR (all)
2
Table 2 Wyckoff positions (W.p.), site occupation factors (k),
2 2 3
Table 4 Selected interatomic distances (pm) in AgBi S Cl . Stand-
atomic coordinates, and equivalent isotropic displacement param-
eters (Ueq / pm ) for all atoms in the asymmetric unit of
ard deviations corresponding to the last digit are indicated in
brackets. Operators for generating equivalent atoms: (i) x, y, zϪ1;
(ii) 1/2Ϫx, Ϫ1/2Ϫy, 1Ϫz; (iii) 1/2Ϫx, 1/2Ϫy, 1Ϫz; (iv) 1/2ϩx, 1/
2ϩy, z; (v) 1/2ϩx, yϪ1/2, z; (vi) 1Ϫx, Ϫy, 1Ϫz; (vii) 1/2ϩx, 1/2ϩy,
zϪ1; (viii) 1/2ϩx, yϪ1/2, zϪ1
2
AgBi
2 2 3
S Cl . Estimated standard deviations corresponding to the
last digit are indicated in brackets. All atoms are at y ϭ 0
Atom
W.p.
k
x
z
U
eq
Bi(a)ϪS(ii, iii
Bi(a)ϪS
)
267.1(2)
275.8(2)
297.0(6)
310.7(4)
322.2(2)
Bi(b)ϪCl(2i)
254.9(6)
274.4(2)
293.4(3)
322.7(4)
332.2(5)
ii iii
Bi(a)
Bi(b)
Ag
Cl(1)
Cl(2)
S
4i
4i
2b
2c
4i
4i
0.96(1)
0.04(1)
1
1
1
1
0.3036(1)
0.2920(1)
0.5
0.3180(2)
0.2550(4)
0
1/2
0.9484(3)
0.6758(3)
191(3)
191(3)
294(4)
219(7)
179(5)
130(5)
Bi(b)ϪS( , )
i
ii iii
Bi(a)ϪCl(2 )
Bi(b)ϪCl(2 , 2 )
iv
v
Bi(a)ϪCl(1 , 1 )
Bi(b)ϪS
Bi(b)ϪCl(1 , 1 )
ii iii
iv
v
0
Bi(a)ϪCl(2 , 2 )
0.1347(2)
0.3358(2)
AgϪS(i , vi)
268.4(6)
274.7(7)
AgϪCl(2 , 2 , 2vii, 2viii
)
ii iii
substantial mixed occupancy corresponding to the formula
Table 3 Connectivity of atoms in the structure of AgBi
their coordination numbers
2
S
2
Cl
3
and
c.n.
Ag1.46(1)Bi2.54(1)S3.09(2)Br2.91(2)
.
The fractional atomic coordinates and isotropic displace-
ment parameters of all atoms with estimated standard devi-
ations are listed in Table 5. In order to simplify comparison,
the parameters for the three crystals are grouped according
to the Wyckoff positions and with respect to increasing sil-
ver content. The connectivity between the cations and
anions in the structure and their coordination numbers are
given in Table 6. Interatomic bond distances in these three
phases are compared in Table 7.
Atom
Cl(1)
Cl(2)
S
Bi(a/b)
Ag
c.n.
2/4
0/0
4
3/3
4/2
5
3/3
2/1
4
8
6
The structures of (I) and (III) were refined using the
atomic positions of (II) as a starting point. Similar pro-
cedures were used for the refinements. In the case of (I),
significant mixed occupancy was found only in metal posi-
tions M(2) ϭ [64.0(1) % Ag ϩ 36.0(1) % Bi], and M(3) ϭ
Further details of the crystal-structure investigations may be ob-
tained from the Fachinformationszentrum Karlsruhe, D-76344
Eggenstein-Leopoldshafen, Germany, by quoting the depository
2 2 3 3 4 2
numbers CSD-413938 (AgBi S Cl ), CSD-413939 (AgBi S Br ),
[
36.0(1) % Ag ϩ 64.0(1) % Bi], resulting in a composition
CSD-413940 (Ag1.27Bi2.73 3.46Br2.54), and CSD-413941 (Ag1.46-
S
very close to AgBi S Br .
3
4
2
Bi2.54S3.09Br2.91). Graphic representations of the crystal structures
were made with the program Diamond [50], using the ellipsoid rep-
resentation at the 95 % probability level.
In (III), only the M(3) ϭ [45.0(4) % Ag ϩ 55.0(4) % Bi]
and Z(2) ϭ [55.0(4) % S ϩ 45.0(4) % Br] positions show
Z. Anorg. Allg. Chem. 2004, 630, 1276Ϫ1285
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 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1279

P. F. Poudeu Poudeu, T. Söhnel, M. Ruck
Table 5 Wyckoff positions (W.p.), occupancies (q), atomic coordi-
2
nates, and equivalent isotropic displacement parameters (Ueq / pm )
for all atoms in the asymmetric unit of AgBi
3 4 2
S Br (I),
Ag1.27Bi2.73S3.46Br2.54 (II) and Ag1.46Bi2.54S3.09Br2.91 (III). Standard
deviations corresponding to the last digit are indicated in brackets.
All atoms are at y ϭ 0. Occupancy of cation and anion positions:
M ϭ qBi ϩ (1Ϫq)Ag; Z ϭ qS ϩ (1Ϫq)Br with 0 Յ q Յ 1
Atom
Bi(1)
W.p.
q
x
z
U
eq
4i
1
1
1
0.7741(1)
0.7741(1)
0.7681(1)
1/2
1/2
1/2
1/2
1/2
1/2
0.1415(2)
0.1419(1)
0.1413(1)
0.1479(3)
0.1481(1)
0.1479(3)
0.4167(1)
0.4167(1)
0.4179(2)
0.1870(1)
0.1866(1)
0.1819(1)
0
0
0
1/2
1/2
1/2
0.4225(3)
0.4226(1)
0.4230(2)
0.0558(5)
0.0557(2)
0.0571(4)
0.2425(2)
0.2411(2)
0.2391(3)
378(1)
334(1)
386(3)
546(7)
415(3)
680(8)
438(5)
362(2)
475(4)
324(2)
286(2)
325(4)
210(1)
164(3)
207(8)
175(4)
296(4)
340(6)
M(2)
M(3)
Br(1)
S(1)
2b
2d
4i
0.360 (1)
0
0
.172 (1)
0.640(1)
0
0
0
0
0
1
1
1
1
0
0
.554(1)
.545(4)
2 2 3
Fig. 2 A representation of the structure of AgBi S Cl showing the
close relationship with the pavonite structure family (right hand)
and the lillianite family (left hand).
4i
and S. RKMAX and GMAX were equal to 8 and 14 auϪ1, respec-
Z(2)
4i
tively. As density functional, the gradient-corrected functional ac-
cording to Perdew, Bourke and Ernzerhof [52] was used. The Bril-
louin zone for the SCF-cycles was constructed using the tetra-
hedron method of Bloechl et al. [53] with 32 independent k-points,
which corresponds to a number of 200 k-points in the entire Bril-
louin zone. Calculation of the density of states (DOS) was per-
formed using 288 independent k-points (2000 k-points in the Bril-
louin zone).
.711(6)
.545(4)
Table 6 Connectivity of cations and anions in the structure of
Ag2xBi4Ϫ2xS6Ϫ4xBr4x and their coordination numbers
Atom
Br(1)
S(1)
Z(2)
c.n.
Ag2xBi4؊2xS6؊4xBr4x (x ؍ 1/2 and x ؍ 1): The electronic band
Bi(1)
M(2)
M(3)
c.n.
2/2
0/0
4/2
4
3/3
4/2
0/0
5
2/2
2/1
2/1
4
7
6
6
structures and the density of states (DOS) were calculated for the
two hypothetical end members of the series of isostructural com-
pounds, AgBi S Br (x ϭ 1/2) and AgBiSBr (x ϭ 1), in order to
3 4 2 2
investigate the influence of the substitution coefficient x on the elec-
tronic properties. For the calculations, the same lattice parameters
were considered for both compositions. The ordered structure
models used for the calculations had Ag at M(2), Bi at M(3) and
Table 7 Selected interatomic distances (pm) for AgBi
Ag1.27Bi2.73S3.46Br2.54 (II) and Ag1.46Bi2.54S3.09Br2.91 (III). Standard
deviations corresponding to the last digit are indicated in brackets.
Operators for generating equivalent atoms: (i) 1Ϫx, Ϫy, Ϫz; (ii) 1/
3
S
4
Br
2
(I),
S at Z(2) positions for AgBi
Br at Z(2) positions for AgBiSBr
3
S
4
Br
2
, and Ag at M(2) and M(3) with
. For the calculations within the
2
scalar relativistic all-electron local density approximation (LDA)
the program LMTO-ASA [54] was used.
2
1
ϩx, 1/2ϩy, z; (iii) 1/2ϩx, yϪ1/2, z; (iv) 1/2Ϫx, Ϫ1/2Ϫy, Ϫz; (v)
/2Ϫx, 1/2Ϫy, Ϫz; (vi) 1Ϫx, Ϫy, 1Ϫz; (vii) 1/2Ϫx, Ϫ1/2Ϫy, 1Ϫz;
(viii) 1/2Ϫx, 1/2Ϫy, 1Ϫz; (ix) xϪ1/2, yϪ1/2, z; (x) xϪ1/2, 1/2ϩy, z
Results and Discussion
(I)
(II)
(III)
The crystal structure of AgBi S Cl
3
2
2
Bi(1)ϪS(1i)
259.2(3)
283.4(6)
292.4(6)
355.4(7)
259.1(4)
283.1(2)
292.1(3)
355.8(5)
259.1(3)
291.1(2)
286.4(3)
355.1(4)
ii iii
AgBi
S Cl crystallizes in the monoclinic space group
2 2 3
Bi(1) ϪZ(2 , 2 )
ii iii
Bi(1)ϪS(1 , 1 )
C2/m and adopts a variant of the InBi S Cl [43] structure
type (Fig. 2).
The Ag atom is situated in an flattened octahedral en-
vironment (Table 3) distorted towards [2ϩ4] coordination
2
4
ii iii
Bi(1)ϪBr(1 , 1 )
M(2)ϪZ(2, 2i)
261.8(3)
288.2(6)
261.0(4)
288.2(2)
258.0(3)
289.2(2)
ii iii iv
v
M(2)ϪS(1 , 1 , 1 , 1 )
vi
M(3)ϪZ(2, 2 )
272.4(4)
288.4(6)
273.7(5)
288.8(2)
276.0(3)
288.4(2)
(Table 4) with two S atoms in trans positions (268 pm) and
M(3)ϪBr(1 , 1 , 1vii, 1viii
)
ii iii
four Cl atoms in the equatorial plane (275 pm). A similar
type of coordination is generally found for Ag atoms in
I
the structures of silver-containing sulphides. Some typical
Band structure calculations
examples include AgBi3S5 [20], Ag Bi S
[25] and
3
7 12
Ag Bi S Cl [44].
AgBi
2
S
2
Cl
3
: Relativistic DFT (density functional theory) calcu-
2
2
3
2
The Bi atom is surrounded by three S atoms and five Cl
atoms (Fig. 3). The coordination polyhedron can be de-
scribed as a bicapped trigonal prism, where four Cl and
lations were carried out with the program WIEN97 [51] using the
LAPW (linearized augmented plane wave) method. As Muffin tin
radii 2.4 au was used for Ag and Bi, and 2.2 au was selected for Cl
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Homologous Silver Bismuth Chalcogenide Halides (N, x)P. I
2 2 3
Fig. 3 Environments of cations and anions in AgBi S Cl . For bond
distances see Table 5.
Fig. 5 Environments of cations and anions in Ag2xBi4Ϫ2xS6Ϫ4xBr4x.
For bond distances see Table 7.
central Bi atom is shifted from the centro ¨ı d [55] of the coor-
dination polyhedron towards the SϪS edge of the trigonal
prism. The marginal splitting of the Bi position is attributed
to minor statistical disorder of S and Cl over the anion
positions in the coordination sphere.
The S atom is located in a distorted tetrahedral coordi-
nation environment of three Bi atoms and one Ag atom.
Cl(1) displays distorted square coordination by four Bi
atoms, while Cl(2) exhibits square pyramidal coordination
by two Ag and three Bi atoms (Fig. 3).
In the crystal structure of AgBi S Cl , the bicapped trig-
2
2
3
onal prisms around Bi join common edges along [001] and
share their triangular bases along [010] to form double-
chains (Fig. 2), which alternate along [100] with the dis-
torted octahedra around Ag atoms. This structural arrange-
ment is similar to the A module found in the structures
Fig. 4 Projection of the structure of Ag2xBi4Ϫ2x
S6Ϫ4xBr4x (x ϭ 0.64)
of different members of the homologous series [BiQX] ·
2
(II) along [010] highlighting the double-chains of monocapped trig-
[
Ag Bi1ϪxQ2Ϫ2xX
]
(Q ϭ S, Se; X ϭ Cl, Br).
onal prisms around Bi(1). The two alternating types of building
units A and B that form the structure are indicated.
x
2xϪ1 Nϩ1
AgBi
S
Cl
can thus be considered as the first member,
2
2
3
(0, 1)
P, of this series, consisting only of module A. Parallels
with the structure of the fully (Ag,Bi)-substituted lillianite
homologous series (
N1, N2)
L [56] with (N1, N2) ϭ (1, 0) can
two S atoms form the trigonal prism with the two [S Cl ]
2
2
rectangular faces capped by additional S and Cl atoms. The
also be seen (Fig. 2).
Z. Anorg. Allg. Chem. 2004, 630, 1276Ϫ1285
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P. F. Poudeu Poudeu, T. Söhnel, M. Ruck
2
Fig. 6 Group-subgroup relationship between the structures of AgBiSCl and Ag2xBi4Ϫ2xS6Ϫ4xBr4x.
Comparison of the charge-balanced formulations
than the one around M(2). This observation is in agreement
with the analysis of the distribution of Ag (characteristic
[2ϩ4] coordination) and Bi atoms among the octahedral
metal positions in the structures: the M(2) position is highly
preferred by Ag atoms (64 % in (I), 83 % in (II) and 100 %
in (III)) while in the M(3) position Bi atoms slightly domi-
nate.
The S(1) and Br(1) positions show no mixed occupancy.
The S(1) atom is located in a distorted square pyramid,
while the Br(1) atom shows a distorted square coordination
ϩ
3ϩ
2Ϫ
Ϫ
Ϫ
3ϩ
3ϩ
(
(
Ag )(Bi )2(S ) (Cl(2) ) (Cl(1) ) and (In )(Bi ) -
2 2 2
Ϫ
2
Ϫ
2Ϫ
S(1) )2(S(2) ) (Cl ) for AgBi S Cl and InBi S Cl [43]
2
2
2
3
2 4
indicates that AgBi S Cl corresponds to a halogen-rich
2
2
3
analogue of the hypothetical family of isostructural com-
pounds with general formula MBi Z (M ϭ metal; Z ϭ
2
5
anion, primarily chalcogen or halogen) in which the re-
placement of chalcogen atoms by halogen atoms is compen-
sated by the introduction of a metal atom of lower charge.
(
Fig. 5). The nearest additional metal atom, M(2), that
The crystal structures of Ag Bi4Ϫ2xS6Ϫ4xBr4x
2x
could complete a square pyramidal coordination of Br(1) is
situated at a distance of 494 pm. The anion position Z(2)
shows mixed S/Br occupancy and is located in a distorted
tetrahedral environment of cations.
AgBi S Br (I), Ag1.27Bi2.73S3.46Br2.54 (II) and Ag1.46-
3
4
2
Bi2.54S3.09Br2.91 (III) are three members of the series of com-
pounds with general composition Ag Bi4Ϫ2xS6Ϫ4xBr4x (x ϭ
2
x
The structure of Ag Bi4Ϫ2xS Br , as indicated in
0.50, x ϭ 0.64 and x ϭ 0.73). The structural motif associ-
2x
6Ϫ4x 4x
Figure 4, is closely related to that of the pavonite family
described in the introduction and to the structures of
various
ated with this series is shown in Figure 4.
The asymmetric unit of Ag Bi4Ϫ2xS6Ϫ4xBr4x contains
2
x
members
of
the
series
[BiQX]2
·
three crystallographically different metal positions and
three non-metal positions (Table 6). Examination of the Ag/
Bi mixed occupancy in the metal positions reveals that the
sevenfold-coordinated Bi(1) is not substituted by Ag in any
of the compounds. The coordination polyhedron around
Bi(1) is a distorted monocapped trigonal prism (Fig. 5) of
three S atoms, two Br atoms and two anions with varying
character (Z ϭ S/Br). In all compounds, the coordination
polyhedra of the M(2) and M(3) positions exhibit a tetrag-
onal distortion of the octahedron towards a [2ϩ4] coordi-
nation with two short bonds trans to one another (Table 7).
In any case, the octahedron around M(3) is less distorted
[Ag Bi1ϪxQ2Ϫ2xX
]
(Q ϭ S, Se; X ϭ Cl, Br). It can
P member of the quaternary series.
x
2xϪ1 Nϩ1
1, x)
(
be considered as the
In the structures of Ag Bi4Ϫ2xS
cated at the interface of modules A and B in the pavonite
structures are completely replaced by Br atoms.
Br , the S atoms lo-
6Ϫ4x 4x
2x
Relation to the structure of AgBiSCl2
(1,
x)PS,Br compounds
The monoclinic structure of the
Ag Bi4Ϫ2xS6Ϫ4xBr can be derived from the orthorhombic
2x
4x
structure of the (1, 1)PS,Cl compound AgBiSCl [46] by dis-
2
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Homologous Silver Bismuth Chalcogenide Halides (N, x)P. I
carding the mirror planes at z ϭ 1/4 and 3/4. The group-
subgroup relationship between the two structures is de-
picted in Figure 6. The Cl position (8f) of AgBiSCl is di-
2
vided into two independent positions (4i) that are occupied
by different anions, S(1) and Br(1), in the monoclinic struc-
ture. The higher affinity of Bi to S results in a shift of the
Bi atom of 84 pm away from the corresponding position in
AgBiSCl . Thereby, the coordination number of Bi is re-
2
duced from c.n. ϭ 8 (bicapped trigonal prism) in AgBiSCl2
to c.n.
ϭ
7
(monocapped trigonal prism) in
Ag Bi4Ϫ2xS6Ϫ4xBr . The corrugated layer of bicapped
2
x
4x
trigonal prisms found in the orthorhombic structure is
consequently broken into parallel double-chains of mono-
capped trigonal prisms. A similar structure type was re-
cently reported for MnSbSe I [38].
2
Relations between composition and structure
One of the most important structural features of the
Ag Bi4Ϫ2xS6Ϫ4xBr4x compounds is the “strategic” position
2
x
of Z(2) in the structure. This anion position, which has the
particularity of being connected to all metal atoms in the
structure, shows mixed S/Br occupancy and seems to be the
key to assuring the electroneutrality of the compounds. Due
to the multiplicity of the point positions, the occupancies
follow the simple relation q(M(2)) ϩ q(M(3)) ϭ q(Z(2)).
Herein the occupancies of the M and Z positions are de-
fined as 1 ϭ q Bi ϩ (1Ϫq) Ag and 1 ϭ q S ϩ (1Ϫq) Br
with 0 Յ q Յ 1. The amount of S found in the Z(2) position
decreases with increasing Ag content according to the equa-
tion q(Z(2)) ϭ 2(1Ϫx).
Fig. 7 Band structure and density of states (DOS) of AgBi
with “fat-band” representation of the Ag-5s state.
2 2 3
S Cl
M(2) and M(3) by Ag atoms and of Z(2) by Br atoms is ex-
pected.
Another structural characteristic of this class of com-
pounds is the selectivity of the substitution process for cer-
tain cation and anion positions. Substitution is only ob-
served within the network of octahedra that share common
vertices, i.e. it extends exclusively along the
M(2)ϪZ(2)ϪM(3)ϪZ(2) zigzag pattern of trans short
bonds. In the case of minimal silver content (x ϭ 0.5), the
composition of the zigzag pattern corresponds to the se-
quence (Ag/Bi)ϪSϪ(Ag/Bi)ϪS. With maximum silver con-
tent (x ϭ 1), its composition should be AgϪBrϪAgϪBr,
although due to chemical reasons an atomic rearrangement
is expected that will result in the sequence AgϪSϪAgϪS, as
found in the ordered structure of the homoeotypic sulphide
In line with the above crystallographic analysis, the solid
solution is restricted to particular atomic positions in the
structure. The structural adaptation Ϫ i.e. isomorphic sub-
stitution between Bi and Ag due to their similar effective
3
ϩ
ionic radii for c.n. ϭ 6 (103 pm for Bi and 115 pm for
ϩ
Ag ) [57] Ϫ observed in the Ag Bi4Ϫ2xS6Ϫ4xBr4x series is
2
x
facilitated by the flexibility introduced by the occupancy of
the anion position Z(2) with either S or Br atoms. The cou-
pled exchange process of cations and anions may yield a
continuous spectrum of isostructural charge balanced com-
pounds.
According to the structures of the present members, the
series Ag Bi4Ϫ2xS6Ϫ4xBr4x is better represented by the
chloride AgBiSCl [46]. According to the structures of the
2
x
2
structural formula [BiSBr] ·[Ag Bi2(1Ϫx)S4(1Ϫx)Br2(2xϪ1)
]
three phases of the series that we have investigated, it seems
that the structure type is moving towards orthorhombic
symmetry as the Ag content increases (Table 1), which is
consistent with the equivalence of the positions M(2) and
M(3).
2
2x
with 0.5 Յ x Յ 1. The [Ag Bi2(1Ϫx)S4(1Ϫx)Br2(2xϪ1)] part of
2
x
the formula describes the coupled charge-balanced ex-
change process that takes place within the network of
[
MS Z ] and [MBr Z ] octahedra. Following this formula,
4 2 4 2
the composition range of the series is confined between the
One of the most significant structural features of the
two isostructural end members AgBi S Br (x ϭ 0.5) and
Ag Bi4Ϫ2xS6Ϫ4xBr4x series is the strong dependence of the
3
4
2
2x
AgBiSBr (x ϭ 1). As determined from the refinement of
(
order at the metal sites M(2) and M(3), while the Z(2) posi-
tion is fully occupied by S atoms. In the as yet hypothetical
BiϪZ(2) and BiϪS bonds on the substitution coefficient x.
The BiϪZ(2) distance rapidly increases with increasing x,
whereas the BiϪS distance decreases (Table 7). This behav-
2
I), the silver-poorest member (1, 0.5)P still shows Ag/Bi dis-
2Ϫ
iour can be explained by the substitution of S by larger
Ϫ
silver-richest end member AgBiSBr , full occupancy of
and lower charged Br anions in the Z(2) position.
2
Z. Anorg. Allg. Chem. 2004, 630, 1276Ϫ1285
zaac.wiley-vch.de
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1283

P. F. Poudeu Poudeu, T. Söhnel, M. Ruck
Electronic band structures of Ag Bi4Ϫ2xS6Ϫ4xBr4x
2x
with x ϭ 0.5 and x ϭ 1
As can be seen in Figure 8, the electronic band gap (Eg)
is considerably diminished from the silver-poor compound
AgBi S Br (x ϭ 0.5), for which E ϭ 0.9 eV, to the silver-
3
4
2
g
rich phase AgBiSBr (x ϭ 1), for which E ϭ 0.6 eV. Both
2
g
values are consistent with the black colour of the crystals
of Ag Bi4Ϫ2xS6Ϫ4xBr . As above, both compounds present
2x
4x
small dispersion of the valence bands, suggesting large
localization of the electrons. However, the valence band in
AgBiSBr (x ϭ 1) is slightly broader, indicating that the
2
compound, which contains more ions of low charge, may
display more covalent character.
The electronic structure of AgBi S Br exhibits indirect
3
4
2
transitions from Z to M and, with slightly larger energy,
from Z to AϪM, as well as a direct gap at Z. In the case
of AgBiSBr , two indirect transitions with similar energies,
2
from Z to A and from M to A, are possible. The calcu-
lations clearly show that the simultaneous substitution of
Bi by Ag in the metal positions and of S by Br in the anion
positions strongly affects the electronic character of mem-
bers of the solid solution series. The top of the valence band
is influenced by substitutions in the Z(2) and M(2) posi-
tions while the bottom of the conduction band is influenced
by substitution in the M(3) position. Conductivity measure-
ments have yet to be carried out.
Acknowledgements. We would like to thank the Fonds der Chemi-
schen Industrie for financial support, Dr. H. Borrmann (MPI-CPfS,
Dresden) for the X-ray data collection of Ag1.27Bi2.73
S3.46Br2.54 and
Dr. M. Kohout (MPI-CPfS, Dresden) for the calculation of the elec-
tronic band structures of Ag2xBi4Ϫ2xS6Ϫ4xBr4x (x ϭ 0.5 and x ϭ 1).
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