Mixed valence tungsten (IV,V) compounds with layered structures (Part III)[2]: Synthesis and crystal structure of Ag1-x[W2O 2Cl6] and investigations on the Ag+ ion mobility

Article abstract of DOI:10.1002/zaac.201000080

The reaction of metallic silver with tungsten tetrachlorideoxide WOCl4 or the reaction of AgCl with WOCl₃, both at 690 K, lead to Ag _1-x[W₂O₂Cl₆] as black lustrous crystal needles. The compound shows a substantial phase field width in the silver content depending on reaction temperature and reaction time and crystals with x = 0, 0.38 and 0.82 were isolated. The crystal structure determinations (monoclinic, C2/m) show the structure to be isotypic to those of Tl[W ₂O₂Cl₆] and K_0.84[W ₂O₂Cl₆] with the presence of 1D polymeric [W₂O₂Cl₆]_n strands and Ag+ ions with a variety in the occupation of the respective crystallographic site. Ag _1-x[W₂O₂Cl₆] represents a mixed valence compound with a variable ratio of W(IV) and W(V) in the W2 dumbbells with a short W-W separation of 2.85 A. In Fourier maps the electron density of the Ag ion appears smeared over a large area. By structure determinations of stoichiometric Ag_1.0[W₂O ₂Cl₆] at temperatures of123, 193, and 293 K a double minimum potential was found with the silver ion dynamically disordered over two flat basins with a thermal activation barrier of 13 meV. No long range ion mobility is present since the Ag⁺ ions are trapped within a coordinating cage consisting of ten chlorine atoms of the surrounding [W ₂O₂Cl₆] strands forming a distorted bicapped cube. By MAS-NMR spectroscopic measurements on the ¹⁰⁹Ag nuclei at different temperatures, the spin lattice relaxation times were determined giving a thermally activated barrier of 15 meV. The electronic conductivity of pressed powder samples of Ag[W₂O₂Cl₆] by a four-probe measurement applying direct current is 1Ω^-1·cm ^-1 at room temperature and 10^-3Ω ^-1·cm^-1 at 25 K. For the conduction mechanism at low temperatures a variable range hopping mechanism is suggested, whereas at higher temperatures a standard semi-conductivity with a bandgap of 60 meV is present.

Full text of DOI:10.1002/zaac.201000080

ARTICLE
DOI: 10.1002/zaac.201000080
Mixed Valence Tungsten (IV,V) Compounds with Layered Structures (Part
III)[2]: Synthesis and Crystal Structure of Ag1–x[W2O2Cl6] and
Investigations on the Ag+ Ion Mobility
Johannes Beck,*[a] Wilfried Hoffbauer,[a] Christian Kusterer,[a] and Marcel Schieweling[a]
Dedicated to Professor Bernd Harbrecht on the Occasion of His 60th Birthday
Keywords: Tungsten; Mixed-valent compounds; Nonstoichiometry; Ion mobility; Solid state NMR spectroscopy;
Small gap semiconductivity
Abstract. The reaction of metallic silver with tungsten tetrachlorideox- 123, 193, and 293 K a double minimum potential was found with the
ide WOCl4 or the reaction of AgCl with WOCl3, both at 690 K, lead silver ion dynamically disordered over two flat basins with a thermal
to Ag1–x[W2O2Cl6] as black lustrous crystal needles. The compound activation barrier of 13 meV. No long range ion mobility is present
shows a substantial phase field width in the silver content depending since the Ag+ ions are trapped within a coordinating cage consisting
on reaction temperature and reaction time and crystals with x = 0, 0.38 of ten chlorine atoms of the surrounding [W2O2Cl6] strands forming a
and 0.82 were isolated. The crystal structure determinations (mono- distorted bicapped cube. By MAS-NMR spectroscopic measurements
clinic, C2/m) show the structure to be isotypic to those of Tl[W2O2Cl6] on the 109Ag nuclei at different temperatures, the spin lattice relaxation
and K0.84[W2O2Cl6] with the presence of 1D polymeric [W2O2Cl6]n times were determined giving a thermally activated barrier of 15 meV.
strands and Ag+ ions with a variety in the occupation of the respective The electronic conductivity of pressed powder samples of
crystallographic site. Ag1–x[W2O2Cl6] represents a mixed valence com- Ag[W2O2Cl6] by a four-probe measurement applying direct current is
pound with a variable ratio of W(IV) and W(V) in the W2 dumbbells 1 Ω–1·cm–1 at room temperature and 10–3 Ω–1·cm–1 at 25 K. For the
with a short W–W separation of 2.85 Å. In Fourier maps the electron conduction mechanism at low temperatures a variable range hopping
density of the Ag ion appears smeared over a large area. By structure mechanism is suggested, whereas at higher temperatures a standard
determinations of stoichiometric Ag1.0[W2O2Cl6] at temperatures of semi-conductivity with a bandgap of 60 meV is present.
Cu1–x[W4O4Cl10], as intermediates between WOX3 and WOX2.
The tungsten(IV) dihalideoxides crystallize most probably iso-
Introduction
In preceding works we reported on the structure of WOCl3
and the structures and properties of the chlorooxotungstates
Mx[W2O2Cl6] with M = Tl+, K+, Pb2+, Hg2+ and the non-stoi-
chiometric phases Ag0.8[W2O2Br6] [1], Cu1–x[W2O2Cl6], and
Cu1–x[W4O4Cl10] [2]. All compounds represent a family of
mixed valence compounds with structures built of negatively
charged infinite W2O2X6 double strands consisting of WO2X4
octahedra condensed by two edge-sharing halogen atoms to
form double octahedra and corner-sharing oxygen atoms to
build 1D strands. The mono or divalent cations are located
between the strands and connect them to layers. The first repre-
sentatives of this structure type were the bromooxotungstates
Na[W2O2Br6] [3] and the non-stoichiometric Ag0.74[W2O2Br6]
[4]. The structures of these compounds can be understood in
principle as intercalates with WOX3 (X = Cl, Br) acting as
host for atoms of the corresponding metals or, in the case of
typic to the structure of MoOCl2, which was investigated by
Thiele et al. [5, 6] despite the structure of WOCl2 is not known
in detail. WOCl2 is a product of thermal decomposition of
WOCl3.
Several of the compounds investigated so far show non-stoi-
chiometric compositions, the cations M are observed to under-
occupy their positions. The compounds were found to be
weakly paramagnetic with temperature independent Pauli char-
acteristics and show a remarkably high electrical conductivity
of about 0.1 Ω–1·cm–1 or higher at room temperature. The tem-
perature dependence of the conductivity corresponds to semi-
conducting behavior with a very small thermal activation bar-
rier for the conduction process of about 0.03 eV at room
temperature. The physical properties of the compounds can be
understood by an interpretation as being one-dimensional met-
als. The real structure of the crystals, however, with a high
number of discontinuances of the one-dimensional building
units, limits the conductivity in close analogy to Krogmann’s
salts [7].
* Prof. Dr. J. Beck
E-Mail: j.beck@uni-bonn.de
[a] Institut für Anorganische Chemie
Universität Bonn
Herein we report on Ag1–x[W2O2Cl6] as the silver containing
representative of the (M+)x[W2O2Cl6] family of compounds
and its remarkable properties.
Gerhard-Domagk-Str. 1
53121 Bonn, Germany
Z. Anorg. Allg. Chem. 2010, 636, 1827–1833
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J. Beck et al.
ARTICLE
materials, chemical transport could not be observed. Crystals were sep-
arated manually and the residual ampoule content was grinded and
sealed once more in an evacuated ampoule for further reaction at
690 K. A second crop of crystals could be separated manually.
Experimental Section
All starting materials and reaction products were handled in an argon
filled glove box. Ampoules were made of borosilicate glass and flame-
dried in vacuo prior to use. WOCl4 was prepared from freshly precipi-
tated WO3, which was obtained from a NaWO4 solution (Merck) by
precipitation with concentrated hydrochloric acid. WO3 was heated un-
der reflux in SOCl2 (Merck) and the orange solid was isolated by
filtration under argon. Before use, WOCl4 was sublimated twice [8,
9]. WOCl3 was prepared by reduction of WOCl4 with aluminum pow-
der in evacuated borosilicate glass tubes at 500 K. AlCl3 formed in the
reaction was sublimated to the cold tip of the ampoule before opening
in the glove box. AgCl was freshly prepared by precipitation from
AgNO3 and hydrochloric acid. It was handled in the dark and dried
with P4O10.
Method B: Reduction of WOCl4 with silver: WOCl4 (1965 mg,
5.752 mmol) and silver powder (463 mg, 4.292 mmol) were filled into
an ampoule of 15 cm length an 1.8 cm diameter, which was sealed,
placed in a horizontal tube furnace and heated to a temperature of
690 K for a period of 7 days. The product was obtained as black nee-
dles with metallic luster. The crystals were separated manually for
crystal structure analysis and solid state NMR investigations.
Crystal Structure Determinations
Crystals from different synthetic runs were mounted in thin-walled
glass capillaries, which were sealed. Data were collected with an En-
raf–Nonius-Kappa-CCD diffractometer using graphite monochroma-
tized Mo-Kα radiation. Low temperature was maintained with a nitro-
gen gas flow cooling system (Oxford Cryosystems). Temperatures
were calibrated with a micro-thermocouple mounted in the position of
a crystal. Data sets were recorded at ambient temperature and addition-
Synthesis
Ag1–x[W2O2Cl6] was prepared by two ways, either by reaction of
WOCl3 with AgCl (Method A) or by the reduction of WOCl4 with
metallic silver (Method B), respectively.
Method A: Reaction of WOCl3 with silver chloride: WOCl3 (136 mg, ally at 193 K and 123 K. Due to the high absorption coefficients and
0.45 mmol) and AgCl (70 mg, 0.49 mmol, dried in vacuo) were filled the anisotropic shape of the crystals a numerical absorption correction
together with a small amount WCl6 (32 mg, 0.08 mmol) into an am- with the aid of the program HABITUS [10] was applied to all data
poule of 13 cm length and 6 mm diameter. This ampoule was placed sets. The optimization of the crystal shape was performed by minimi-
in a horizontal tube furnace and heated starting from 510 K up to zation of the reliability factor of averaging symmetry equivalent reflec-
700 K during a period of 40 days. Along the ampoule, a temperature tions. Structure models were obtained by direct methods and refined
gradient of 20 K was present with the starting materials positioned in against F2 using SHELX97 [11] as implemented in the WINGX pro-
the hotter zone. At a temperature of 620 K, crystallization of black gram package [12]. Table 1 contains the crystallographic data and de-
needles started. The crystals grew on the surface of the sintered starting tails of the structure analyses, positional parameters are given in Ta-
Table 1. Crystallographic data and details of the crystal structure determination of Ag1.0[W2O2Cl6]. The refinement is based on the model for
the silver electron density by 4 (295 K und 193 K) or 3 (123 K) closely neighboured sites. Silver atoms are refined with isotropic displacement
parameters. The standard deviations in brackets refer to the last quoted digit.
Formula
Ag[W2O2Cl6]
Crystal system, space group
Temperature of measurement /K
Lattice parameters /Å, °
monoclinic, C2/m
295(2)
193(3)
123(3)
a = 12.9211(3)
b = 3.7631(1)
c = 9.8663(3)
β = 109.064(1)
453.42(2)
a = 12.8737(3)
b = 3.7629(1)
c = 9.8525(2)
β = 109.161(1)
450.84(2)
a = 12.8499(6)
b = 3.7626(2)
c = 9.8477(5)
β = 109.227(2)
449.57(4)
Cell volume /Å3
Number of formula units
Calculated density /g·cm–3
Crystal size /mm3
Radiation
2
5.276
5.306
5.321
0.019 × 0.112 × 0.018
Mo-Kα (λ = 0.71073 Å)
6.7 ≤ 2θ ≤ 55.0
Data collection range /°
min. / max. h k l
6.7 ≤ 2θ ≤ 55.0
9.1 ≤ 2θ ≤ 54.9
–16 ≤ h ≤ 16, –4 ≤ k ≤ 4, – –16 ≤ h ≤ 16, 4 ≤ k ≤ 4, –12 –16 ≤ h ≤ 16, –4 ≤ k ≤ 4, –
12 ≤ l ≤ 12
5736
≤ l ≤ 12
5303
12 ≤ l ≤ 12
2395
Number of measured reflections
Number of independent reflections
Data quality Rint/Rsigma
Number of refined parameters
Relation reflections / parameters
Absorption coefficient μ /cm–1
Min. / max. transmission
Quality of refinement
598
594
584
0.051 / 0.019
46
0.058 / 0.025
46
0.088 / 0.056
44
13.0
12.9
293.2
13.3
291.5
294.0
0.2599 / 0.7014
0.2561 / 0.7672
0.2857 / 0.7640
R(|F|)
0.015
0.015
0.025
R(|F|) [n(Fo)>4σ(Fo)]
0.013 (566)
0.029
0.014 (577)
0.032
0.024 (562)
0.054
wR(F2)
GooF
1.04
1.16
1.07
max./min. residual electron density /e·Å–3
+0.92 / –0.87
+0.89 / –0.84
+1.30 / –1.49
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Synthesis of Ag1–x[W2O2Cl6] and Investigations on the Ag+ Ion Mobility
ble 2. Additionally, the crystal structures were refined against F2 using
the program JANA2000 [13] and the Gram–Charlier expansion to cal-
culate anharmonic displacement tensors.
Whereas reaction (A) gives the product in higher purity, re-
action (B) gives a higher yield according to the higher reactiv-
ity of WOCl4. The higher purity in reaction (A) is a conse-
quence of the high volatility of WOCl4, which can be
sublimated off from the product, whereas AgCl as the side-
product of reaction (B) is not volatile. Additionally, the thermal
decomposition of WOCl3 to WOCl2 and WOCl4 at the applied
temperatures is a distracting rival to the desired reaction and
thus lessens the yield. It can be suppressed by adding some
WCl6 or some excess WOCl4 into the reaction ampoule. The
two different methods of synthesis show a distinct influence
on the composition of the products. As indicated by EDX anal-
yses and the results of the single crystal analyses, crystals ob-
tained by method A show constantly a silver tungsten ratio
of 1:2, indicating stoichiometric composition according to the
formula Ag[W2O2Cl6]. Crystals obtained by reaction B showed
a variable silver tungsten ratio of 1:2, 0.66:2, 0.62:2, and
0.18:2, respectively, according to the site occupancy factors
obtained by crystal structure determinations on crystals from
different runs. The ratio 1:2, however, was only found in crys-
tals obtained after opening the ampoules after the first reaction
cycle, grinding the contents, re-sealing and heating for a sec-
ond time, as described in the Experimental Section. The occur-
rence of such a large variety in the silver content is remarkable.
The consequence for the charge balance of the crystal is the
presence of a corresponding variety in the oxidation state of
the tungsten atoms, ranging between +5 in WOCl3 = W2O2Cl6
and +4.5 in (Ag+)1.0[W2O2Cl6].
Table 2. Positional parameters and equivalent isotropic displacement
factors U of the atoms in the structure of Ag[W2O2Cl6] at 123 K.
Values in brackets are standard deviations referring to the last quoted
digit. All atoms are located on Wyckoff site 4i with site symmetry m.
SOF is the respective site occupation factor for the split positions of
the silver atom. The sum of the occupation factors amounts
∑SOF2Ag(2)+2Ag(3)+2Ag(4) = 1.02(6).
Atom
SOF
x
y
z
Ueq /Å2
W
0.01132(3)
0.0096(5)
0
0.14890(3) 0.00698(18)
O
½
0
0.1466(5)
0.0503(2)
0.3716(2)
0.0085(13)
0.0101(4)
0.0141(5)
0.0151(4)
Cl(1)
Cl(2)
Cl(3)
Ag(2)
Ag(3)
Ag(4)
0.15815(18)
0.15555(18)
0
–0.10774(19) 0 0.2928(2)
0.023(3)
0.265(17) 0.058(2)
0.114(13) 0.099(2)
0.13(2)
½
½
½
0.4962(11) 0.016(2)
0.5025(8) 0.0131(15)
0.5240(15) 0.016(3)
Further details of the crystal structure investigations have been deposited
and may be obtained from the Fachinformationszentrum Karlsruhe, 76344
Eggenstein-Leopoldshafen, Germany (E-Mail: crysdata@fiz-karlsruhe.de),
on quoting the deposition numbers CSD-421449 (293 K), -421448
(193 K) and -421447 (123 K) for Ag[W2O2Cl6].
Nuclear Magnetic Resonance Analysis
An amount of selected crystals obtained by method B was grinded and
filled into a rotor and sealed by a vespel cap. The rotor was placed
inside a 4 mm MAS probe (Chemagnetics) allowing for a maximum
spinning rate of 18 kHz in the temperature range from 120 to 520 K.
Data were collected with a 400 MHz spectrometer (Varian Infinity+).
A 9 mol·L–1 AgNO3 solution spiked with 0.25 mol·L–1 Fe(NO3)3 for
acceleration of the relaxation process was used as reference.
Crystal Structure of Ag1–x[W2O2Cl6]
The crystal structures have been determined for several se-
lected crystals and, for
a crystal of the composition
Ag1.0[W2O2Cl6], at three different temperatures. All of these
crystallize in the monoclinic space group C2/m with structures
isotypic to those of Tl[W2O2Cl6] or K0.84[W2O2Cl6] [1]. The
Conductivity Measurements
1
basic building blocks are ∞[W2O2Cl6] strands and Ag+ ions
A sample of Ag[W2O2Cl6], prepared by method A, was grinded and
pressed to a pellet of 6 mm diameter. The bulk sample was checked
by X-ray powder diffractometry and EDX analysis for purity. Only
marginal reflections of AgCl were present with a maximal relative
intensity of 1 %. The silver content was found to be stoichiometric
by EDX. Conductivity measurements were performed by four-probe
technique applying direct current within the temperature range from
10 K to 300 K, in both directions with cooling and heating.
coordinated by terminal chlorine atoms of the negatively
charged W2O4/2Cl6 groups (Figure 1). The refined occupation
factors of the silver atoms reveal the large phase field width
of the silver content. The investigated crystals were found to
have the empiric formulae Ag0.18[W2O2Cl6], Ag0.62[W2O2Cl6],
and Ag1.0[W2O2Cl6], respectively. As a consequence, the mean
oxidation states of the tungsten atoms span almost the com-
plete region between +4.5 and +5, which has to be interpreted
as a dynamically intervalent state since all tungsten atoms oc-
cupy the same crystallographic site 4i. Neighbored strands are
bound together by Ag+ ions to layers in the bc plane (Fig-
ure 2).
Results and Discussion
Synthesis
Refinement of anisotropic displacement parameters for Ag+
ions on (0, ½, ½) resulted in an extremely elongated displace-
ment ellipsoid and a high residual electron density. Splitting
the position of the Ag+ ion over two closely neighbored sites
(0.048, ½, 0.501) did not solve the problem: though the resid-
ual electron density decreased clearly, no change in the elonga-
tion of the displacement ellipsoids of the Ag+ ions could be
(B) obtained. An electron density map in the (010) plane shows a
The silver oxochlorotungstate Ag[W2O2Cl6] can be obtained
by two different methods. In the presence of AgCl thermal
decomposition of WOCl3 occurs according to Equation (A):
AgCl + 3WOCl3 → Ag[W2O2Cl6] + WOCl4
(A)
Metallic silver reduces WOCl4 according to Equation (B)
with formation of AgCl:
3Ag + 2WOCl4 → Ag[W2O2Cl6] + 2AgCl
Z. Anorg. Allg. Chem. 2010, 1827–1833
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J. Beck et al.
ARTICLE
Figure 2. Representation of the electron density of the silver ion in
the structure of Ag1.0[W2O2Cl6] (left, based on Fourier coefficients Fo,
axis labellings refer to crystal coordinates, contour lines are graded by
2 e·Å–3), sections of the electron density function through the maxima
parallel to the crystallographic c axis (middle) and representations of
the anharmonic displacements of the silver atoms, drawn to include a
probability density of 70 % (right). On bottom the respective double-
minimum potential function for the Ag+ ion at 140 K is shown.
Figure 1. Section of the chain of edge sharing pairs of [WO2/2Cl2Cl2/2
]
octahedra in the structure of Ag1.0[W2O2Cl6] at 123 K (top), the unit
cell of Ag1.0[W2O2Cl6] in a view along the crystallographic b axis
(middle, the silver ions are represented by their harmonic displacement
components Uij), and the coordination of the dynamically disordered
silver ion by surrounding chlorine atoms (bottom, the silver ions are
represented by their isotropic displacement components U). The dis-
placement spheres and ellipsoids comply with a probability of localiza-
tion of 90 %. Bond lengths /Å and angles /° at 123 K: W–WI
2.8501(4), W–O, W–OII 1.8815(1), W–Cl(1) 2.390(2), W–Cl(1)I
2.403(2), W–Cl(2) 2.404(2), W–Cl(3) 2.360(2), O–W–OII 178.5(4),
Cl(1)–W–Cl(1)I 107.03(6), Cl(2)–W–Cl(3) 84.83(7), W–Cl(1)–WI
72.97(6). Symmetry generated atoms are marked by superscript indices
: I = –x, y, –z; II = x, –1+y, z; III = –x, y, 1–z; IV = x, y, 1–z; V = x,
1+y, z; VI = ½–x, ½+y, 1–z; VII = –x, 1+y, 1–z.
The actual electron density distribution was described better by
this structure model, even though strong correlations between
parameters of the split silver atoms occurred. To gain further
insight into the phenomenon data sets at low temperature were
recorded. At 123 K the site occupation factor of the “central”
silver site in (0, ½, ½) became less than its error indicating a
decreasing electron density in the central point. So this site
broad region of electron density smeared along [100] with two was set to “not occupied” on refinements based on the data
closely neighbored maxima close to the special position 2d (2/ collected at this temperature and the model contained six silver
m) at (0, ½, ½). An alternative model for the refinement was sites (Figure 1). The occupation factors were allowed for free
set up, implying four closely neighbored (symmetry generated refinement and finally reached a sum corresponding to the for-
seven) silver sites using isotropic displacement parameters. mula Ag1.02(6)[W2O2Cl6] (Table 2). In a third model, the elec-
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Synthesis of Ag1–x[W2O2Cl6] and Investigations on the Ag+ Ion Mobility
tron density of the silver ion was refined using an anharmonic amounts 2.8631(5) Å and for x = 0.82 it enlarges to
displacement model with the Gram–Charlier expansion of the 2.8949(4) Å. The decreasing Ag+ content lowers the content
harmonic displacement tensor (program JANA2000 [13]). The of WIV with respect to WV. The W–W distance thus enlarges
silver site was split into two symmetric equivalent positions to reach finally the value found for WOCl3, containing only
close to (0, ½, ½), each of them refined with a displacement WV [1].
tensor of 3rd order. The residual electron density decreased to
an acceptable level but some correlations between parameters
Nuclear Magnetic Resonance Spectroscopic Analysis
persisted. Nonetheless it seems to be a method inherent phe-
nomenon that parameters of the displacement tensor are corre-
lated [14]. Figure 2 displays electron density charts of the sil-
ver ion and a section along the maxima of the electron density
at 295 K, 193 K, and 123 K, and the shape of the anharmonic
displacement parameters. At ambient temperature, the electron
density of the silver ion is blurred over a distance of nearly
4 Å with two flat maxima, representing preferred positions for
the silver ion. On decreasing temperature the two maxima of
electron density persist but these maxima become sharper and
the distance between the maxima increases from 1.24(2) Å at
295 K to 1.473(8) Å at 123 K. The silver ions are actually
located in a double-minimum potential with a saddle point of
low energy. According to Boltzmann’s distribution theorem
To gain further insight into the ionic motion process a nu-
clear resonance analysis was performed. MAS-NMR spectro-
scopic measurements on 109Ag nuclei were performed to obtain
the isotropic chemical shift of the silver ions. The MAS-NMR
spectrum shows one signal at 232.0 ppm referenced to AgNO3
(Figure 3). This result is in accordance with the results of the
X-ray single crystal structure analysis, which showed the
equivalence of the two minima of the double minimum poten-
tial. Further measurements aimed on the longitudinal relaxa-
tion time T1. A sequence of pulses was used to make the mag-
netization fade out. After a pulse delay (pd) a rectangular pulse
was set to excite 109Ag nuclei. The free induction decay FID
was plotted against pd. This procedure was performed at dif-
ferent temperatures of 265, 300, 330, and 355 K. By using
the Bloch equation [17] the spin lattice relaxation time was
calculated. The obtained relaxation times were plotted semi-
logarithmic against the reciprocal values of the corresponding
temperatures (Figure 3). By fitting a linear slope to the data
points, the Arrhenius equation can be applied and the activa-
tion energy of the Ag+ ion motion thus be calculated. The Ar-
E2+–1 In the crystal structure, the Ag+ ions exhibit a distorted
bicapped cuboidal coordination environment. The coordination
polyhedron consists of eight Cl(2) and Cl(3) atoms with cu-
boidal shape, which is capped by two additional Cl(3) atoms
(Figure 1). This is caused by the lattice centering, which shifts
the planar W2Cl6 groups alternately by half of the lattice con-
stant in the b direction and brings two Cl(3) atoms of two
neighboring [W2O2Cl6] strands in positions over two opposite
faces of the cuboidal void in the crystallographic a direction.
Thus the migration paths of the Ag+ ions are blocked and no
long range ion mobility is possible since the structure of the
anionic lattice does not offer non-intermittent migration paths
for ionic conduction and only a local mobility is observed. The
Ag–Cl distances amount 2.582(6) [Ag(4)–Cl(2)VII], 2.653(6)
[Ag(4)–Cl(3)], and 2.976(6) Å [Ag(4)–Cl(3)VI] for the differ-
ent silver sites and are comparable for example with the Ag–
Cl distances in Ag3YCl6 (2.570 to 2.733 Å) [15]. Ag3YCl6 also
contains disordered silver ions with an elongated electron den-
sity inside a pair of face sharing octahedra. A similar situation
is observed for one of the copper ions in the structure of tetra-
hedrite, Cu12Sb4S13 [16]. The copper ion is located in a trigo-
nal bipyramidal coordination environment with three sulfur at-
oms in the equatorial plane and two antimony atoms at the
apexes. A dynamic disorder perpendicular to the triangular
plane was observed by temperature dependent structure deter-
minations.
rhenius plot showed a linear alignment following log T1
=
0.6908 + 175.96 K·T–1. According to
the activation energy is calculated to 15.1 meV.
Electrical Conductivity
Like the others so far examined members of the
Mx[W2O2Cl6] series, Ag[W2O2Cl6] is a good electrical con-
ductor. At room temperature, a specific conductivity of about
1 Ω–1·cm–1 was found on a pressed powder sample. The tem-
perature characteristics is semi-conducting, since the conduc-
tivity decreases with decreasing temperature and reaches about
10–3 Ω–1·cm–1 at 25 K (Figure 4).
From the Arrhenius equation for the activation energy of cur-
rent transport in semiconductors one gets
From the structure analyses of the silver deficient crystals
Accordingly, –ln σ = f(T–1) is expected as a linear function.
the tendency is present that on decreasing silver content an As Figure 4 shows, a linear function is actually present in the
increasing W–W distance follows. With fully occupied silver temperature range between 70 K and 300 K. From the slope
sites in Ag1–x[W2O2Cl6], i.e. x = 0, a W–W distance of the energy of the thermal activation is calculated to
2.8572(2) Å is found, whereas for x = 0.38 this distance 0.0298(1) eV. This corresponds to a bandgap of ΔE =
Z. Anorg. Allg. Chem. 2010, 1827–1833
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Beck et al.
ARTICLE
Figure 4. Electrical conductivity of
a
compressed sample of
Ag1.0[W2O2Cl6] as a function of temperature. The conductivity is
shown on a logarithmic scale (top) and the respective Arrhenius func-
tion –ln σ = f(T–1) (bottom). Two different functions have been fitted
to the data, a linear fit for the high temperature region (a) and a fit
according the VRH model (b).
Figure 3. 109Ag MAS NMR spectrum of Ag1.0[W2O2Cl6], recorded
with a rotation frequency of 10 kHz (top) and the function of the spin
lattice relaxation time at different temperatures versus the reciprocal
temperature including the corresponding linear regression function
(bottom).
from full occupation of the Ag+ site down to an occupation of
merely 18 %. The Ag+ ions are mobile within their cuboidal
coordination sphere in a double minimum potential and are
dynamically disordered with an activation energy in the order
of kT, shown by temperature dependent structure analyses and
the determination of the spin lattice relaxation times based on
109Ag solid state NMR spectra. The crystal structure, however,
does not allow for a long range mobility of the Ag+ ions, since
there are no continuous migration paths for the Ag+ ions. Like
all other members of the A1–x[W2O2X6] family of compounds,
Ag[W2O2Cl6] exhibits a remarkable high electrical conductiv-
ity with semiconducting characteristics and a small bandgap of
approximately 60 meV.
0.0596(2) eV. In the range below 70 K the activation energy
decreases with decreasing temperature and a linear function is
no longer present. This behavior may be interpreted with a
variable range hopping mechanism (VRH), describing the low
temperature behavior of the conductivity in disordered solids.
The temperature dependence of the conductivity follows the
expression [18]:
Initially developed for amorphous solids, the model is nowa-
days used also for crystalline semiconductors [19]. The expo-
nent α is critical towards the dimensionality of the process and
is assumed to be 0.5 for 1D systems [20]. As can be seen from
Figure 4, a satisfying fit to the conductivity data can be ob-
tained with the exponent α = 0.53(2).
Acknowledgement
We are much obliged to J. Daniels for measuring the X-ray diffraction
data sets and to Professor M. Jansen, Max-Planck-Institute for Solid
State Research in Stuttgart, Germany, for giving the opportunity for
the conductivity measurements.
Conclusions
References
Ag1–x[W2O2Cl6] represents
1–x[W2O2X6] family of compounds. A large phase field width
has been achieved for the silver containing compound, ranging
a further member of the
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