Crystalline and glassy phases in the ternary system Tl/Bi/Cl: Synthesis and crystal structures of the thallium(I) chloridobismutates(III) Tl3BiCl 6 and TlBi2Cl7

Article abstract of DOI:10.1002/zaac.200900567

Slow cooling of melts composed of TlCl and BiCl₃ allows for the isolation of the compounds Tl₃BiCl₆ (1) and TlBi ₂Cl₇ (2). Compound 1 is formed by sublimation at 480 °C from the black melt of 3 TlCl + 1 BiCl₃ as colourless crystals. The crystal structure determination (tetragonal, P4₂/m) consists of nearly regular octahedral [BiCl₆]^3- anions and two independent Tl⁺ cations, which have coordination number 8 in form of a slightly distorted cube and 10 in form of an Edshamar polyhedron, respectively. The structure is not isotypic with the recently reported naturally occurring form of Tl₃BiCl₆, the mineral steropesite. Compound 2 is obtained from a dark red melt of composition TlCl + 2 BiCl ₃. On rapid cooling, this melt solidifies to a metasta-ble dark red glass which at ambient temperature crystallises to a light amber crystalline powder within some weeks. The structure of 2 was determined by powder diffraction (triclinic, P1). A distinct lone pair effect is present causing an irregular coordination on the two independent bismuth atoms. Taking Bi-Cl bonds up to 3.5 A into account, both bismuth atoms gain coordination number seven. ²⁰³T1 and ²⁰⁵Tl solid state NMR and XANES spectra on the Bi and Tl-L_III edges of both glassy and crystalline TlBi₂Cl₇ show that a close structural similarity exists between both forms. In contrast, the Raman spectra show distinct differences in the bands of the Bi-Cl vibrations region.

Full text of DOI:10.1002/zaac.200900567

ARTICLE
DOI: 10.1002/zaac.200900567
Crystalline and Glassy Phases in the Ternary System Tl/Bi/Cl: Synthesis and
Crystal Structures of the Thallium(I) Chloridobismutates(III) Tl3BiCl6 and
TlBi2Cl7
Johannes Beck*[a] and Sebastian Benz[a]
Dedicated to Professor Rüdiger Kniep on the Occasion of His 65th Birthday
Keywords: Thallium; Chloridobismutates; Lone pair effect; NMR spectroscopy; X-ray absorption spectroscopy; Glass formation
Abstract. Slow cooling of melts composed of TlCl and BiCl3 allows ble dark red glass which at ambient temperature crystallises to a light
for the isolation of the compounds Tl3BiCl6 (1) and TlBi2Cl7 (2). Com- amber crystalline powder within some weeks. The structure of 2 was
¯
pound 1 is formed by sublimation at 480 °C from the black melt of 3 determined by powder diffraction (triclinic, P1). A distinct lone pair
TlCl + 1 BiCl3 as colourless crystals. The crystal structure determina- effect is present causing an irregular coordination on the two independ-
tion (tetragonal, P42/m) consists of nearly regular octahedral [BiCl6]3– ent bismuth atoms. Taking Bi–Cl bonds up to 3.5 Å into account, both
anions and two independent Tl+ cations, which have coordination num- bismuth atoms gain coordination number seven. 203Tl and 205Tl solid
ber 8 in form of a slightly distorted cube and 10 in form of an Edsha- state NMR and XANES spectra on the Bi and Tl-LIII edges of both
mar polyhedron, respectively. The structure is not isotypic with the glassy and crystalline TlBi2Cl7 show that a close structural similarity
recently reported naturally occurring form of Tl3BiCl6, the mineral exists between both forms. In contrast, the Raman spectra show dis-
steropesite. Compound 2 is obtained from a dark red melt of composi- tinct differences in the bands of the Bi–Cl vibrations region.
tion TlCl + 2 BiCl3. On rapid cooling, this melt solidifies to a metasta-
a potentially steric active lone-pair. Looking for substances
Introduction
with nonlinear optical properties, Hagemann and Weber pre-
The crystal chemistry of the halides of trivalent bismuth is
pared crystals of the compounds Tl7Bi3I16 and Tl3BiI6. The
strongly influenced by the stereochemical activity of the lone
crystal structures, however, could not be determined due to
electron pair on Bi3+. Its structural influence is generally strong
among the fluorides, but decreases with the heavier halogens.
twinning problems [6]. Later, Ruck and co-workers were able
to characterise structurally the phases Tl7Bi3I16, Tl3BiI6 [7],
In the structure of BiF3, the Bi3+ ion has a distorted square-
and Tl3Bi2I9 [8] by X-ray diffraction. All three iodidobismu-
antiprismatic coordination [1]. In BiCl3, a strongly distorted
tates have bismuth in sixfold coordination. The BiI6 coordina-
3+3 octahedral coordination with three short and three long
tion octahedra are distorted, which is assigned to the effect of
Bi–Cl bonds is present [2], whereas for BiI3 an almost undis-
the lone pair.
torted octahedral coordination environment was found [3].
Only one compound in the ternary system Tl/Bi/Cl was de-
During the last two decades, a number of alkali halogenidobis-
scribed so far. The mineral steropesite of the approximated for-
mutates have been characterised, which show a large structural
mula Tl3BiCl6 was recently found in volcanic high-temperature
variability, mainly caused by the absence or presence of the
fumaroles [9]. The lack of knowledge on this ternary system
lone-pair effect of the trivalent bismuth atoms. Examples are
led us to explore the formation and structures of phases among
CsBi2F7 [4] containing a Bi/F framework, in which the bis-
the thallium chloridobismutates.
muth atoms have a strongly distorted six to eight fold coordi-
nation by fluoride anions. On the other hand, the structure of
Cs3BiCl6 [5] contains discrete BiCl6 units in which the Bi–Cl
coordination is nearly ideally octahedral.
This structural variability can be further expanded by the
substitution of alkali ions by such ions which themselves have
Experimental Section
Starting Materials
BiCl3 was obtained by chlorination of bismuth at 400 °C followed by
several cycles of sublimation in vacuo. TlCl was precipitated from an
aqueous solution of TlNO3 with hydrochloric acid, washed with water
and dried in dynamic vacuum for several days. Glass ampoules of
10 cm length and 1.5 cm diameter were used as reaction vessels. All
manipulations with the hygroscopic BiCl3 including charging and
opening of the reaction ampoules were performed in an argon filled
glove box.
* Prof. Dr. J. Beck
E-Mail: j.beck@uni-bonn.de
[a] Institut für Anorganische Chemie
Universität Bonn
Gerhard-Domagk-Str. 1
53121 Bonn, Germany
928
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 636, 928–935

The Thallium(I) Chloridobismutates(III) Tl3BiCl6 and TlBi2Cl7
technique. Samples were packed in a rotor of 4 mm diameter and a
rotation frequency of 12 kHz was applied. The spectrum was cali-
brated against a solution of TlNO3 with a concentration of 0.1 mol·L–1.
The resonance frequencies were 229.36 MHz for 203Tl and
231.62 MHz for 205Tl. Differential scanning calorimetry mesurements
were carried out with a DSC 204 Phoenix instrument (Netzsch Inc.)
in closed aluminium crucibles with samples of about 15 mg.
Synthesis of Tl3BiCl6
Tl3BiCl6 was synthesised by heating a melt of TlCl and BiCl3 in 3:1
molar ratio to 480 °C for ten hours in evacuated glass ampoules and
slow cooling (5 °C·h–1) the black-brown melt to ambient temperature.
In a typical experiment, an ampoule of 12 cm length and 1.5 cm diam-
eter was loaded with the TlCl/BiCl3 mixture (700 mg). Besides a black
amorphous residue, colourless crystals of high refractivity deposited at
the inner walls of the ampoule in the cooler region above the melt.
Synthesis of TlBi2Cl7
Crystal Structure Determination of Tl3BiCl6
Polycrystalline samples of TlBi2Cl7 were prepared by heating a mix-
ture of TlCl and BiCl3 in the molar ratio 1:2 to 250 °C in evacuated
glass ampoules and slowly cooling (10 °C·h–1) the dark red melt to
The crystals of Tl3BiCl6, deposited from the gas phase as described
above, were notoriously multiple twinned with diffraction patterns that
ambient temperature. Trying to obtain single crystals by sublimation could not be indexed and it took several attempts until a suitable single
of the product only lead to sublimation of BiCl3 leaving behind the
less volatile TlCl.
crystal specimen was found. Diffraction data were recorded at ambient
temperature using a Bruker Nonius Kappa-CCD diffractometer with
Mo-Kα radiation. The tetragonal symmetry and the systematic absences
led to the space group P42/m. An empirical absorption correction was
applied to the data set [10]. The structure model was obtained by direct
methods and refined based on F2 with anisotropic displacement param-
eters for all atoms [11]. The crystallographic data and details of data
collection are summarised in Table 1, atomic coordinates in Table 2.
The powder diffractograms of bulk samples can, however, be com-
pletely indexed and simulated on the basis of the tetragonal unit cell
and the positional parameters obtained by the single crystal structure
analysis. No reflections of additional phases are present indicating that
Analytical Methods
Raman spectra were recorded using a BRUKER FT Raman spectrome-
ter RFS100 in backscattering geometry, a Nd:YAG laser (λ = 1036 nm)
and a resolution of 4 cm–1. The laser power was adjusted between 30
and 300 mW. Measurements were done on powdered samples enclosed
in sealed glass capillaries of 1 mm diameter. X-ray absorption spectra
were measured at the synchrotron light source ANKA of the For-
schungszentrum Karlsruhe, Germany. Solid state NMR spectra were
recorded with a VARIAN Infinity Plus spectrometer applying the MAS under the applied reaction conditions only one single phase is formed.
Table 1. Crystallographic data and details of the structure determination for Tl3BiCl6 and TlBi2Cl7.
Compound
Tl3BiCl6
TlBi2Cl7
Bi2Cl7Tl
Formula
BiCl6Tl3
tetragonal, P42/m
2
¯
Crystal system, space group
triclinic, P1
2
Number of formula units, Z
a /Å
8.9690(2)
8.8832(1)
b /Å
10.7632(1)
c /Å
7.4876(2)
6.9329(1)
α /°
107.523(1)
β /°
107.334(1)
γ /°
75.684(1)
V /Å3
602.32(2)
5.706
594.30(1)
Density (calcd.) /g·cm–3
Absorption coefficient μ /mm–1
Diffractometer type
Radiation, wavelength /Å
Temperature /K
2θmax /°
4.86
55.84 (Mo-Kα)
Bruker-Nonius κ-CCD
Mo-Kα, 0.7107
293
96.89 (Cu-Kα)
Bruker D8 Advance
Cu-Kα1, 1.540598
293
54.94°
4° < 2θ < 81°
0 ≤ h ≤ 7, –8 ≤ k ≤ 9, –5 ≤ l ≤ 5
cylinder correction
0.060/0.107
1484
hkl range
Absorption correction
h 11, k 8, l 9
empirical [9]
0.005/0065
1356
T
min/Tmax
Reflections collected
Independent reflections, Rmerg
No. of data and parameters
Goodness-of-fit on F2
wR(F2)
730, 0.027
730 and 31
1.066
1480 and 41
1.55
0.043
0.070
R(|F|) for [n] Fo > 4σ(Fo)
R(|F|) for all data
0.028 [682]
0.031
RProfiles
0.031
wRProfiles
0.043
wR(F2)
0.079
Largest difference peak and hole /e·Å–3
+1.30 and –1.57
+1.88 and –1.61
Z. Anorg. Allg. Chem. 2010, 928–935
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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929

J. Beck, S. Benz
ARTICLE
Table 2. Positional coordinates and equivalent isotropic displacement
parameters Ueq /Å2 for the atoms in the structure of Tl3BiCl6. Standard
deviations refer to the last significant digit.
Atom
x
y
z
Ueq
Tl(1) 0.07685(7)
Tl(2) 0.5
0.26533(6)
0.5
0
0.0540(3)
0.0373(3)
0.0221(2)
0.0382(5)
0.059(1)
0
Bi
0
0.5
0.5
0.2463(3)
0.5
Cl(1) 0.2037(2)
Cl(2) 0.0644(4)
0.5612(3)
0.2068(4)
Crystal Structure Determination of TlBi2Cl7
To protect the slightly hygroscopic powder of TlBi2Cl7 from moisture,
samples were filled in glass capillaries with a diameter of 0.2 mm after
thoroughly grinding with an equal amount of glass powder. Data were
collected at ambient temperature using a Bruker-D8 Advance powder
diffractometer with Cu-Kα radiation. 10482 data points were recorded
in the 2θ range 4 to 81° with increments of 0.00735°. Indexing of the
diffractogram and determination of the lattice parameters were per-
formed using the program WinXPow [12]. The diffraction pattern
could be indexed by a triclinic cell. A first kind Chebyshev background
function with 30 terms was applied as implemented in the GSAS pro-
gram system [13]. A Pseudovoigt function with 18 terms was used to
fit the reflection profiles for all 1480 identified reflections. Structure
models were obtained with the aid of the program Endeavour [14]. Due
to the closely similar atom scattering factors of bismuth and thallium, it
turned out to be advantageous to assume both kinds of atoms as occu-
pied by Pb in the structure solution process and in the early stages of
refinement. Out of the suggested structure models those were chosen
fulfilling the restraints dmin(Pb–Pb) = 3,5 Å, dmin(Pb–Cl) = 2.5 Å,
Figure 1. Powder diffractogram of TlBi2Cl7. Shown are the measured
intensities, the calculated intensities, the background function, and the
difference curve Iobs–Icalcd. The inset shows an enlarged view of the
high angle region of the diffractogram.
Tl3BiCl6 and CSD-421318 for TlBi2Cl7 [15]. The graphical representa-
tions were drawn with the use of the programs DIAMOND [16] and
POV-RAY [17].
Results and Discussion
min(Cl–Cl) = 3.0 Å. Eliminating all models with high coordination Synthesis of Thallium(I) Chloridobismutates
d
numbers of cations by cations or anions by anions, one centrosymmet-
ric model was left, which was refined by the Rietveld method using
the program GSAS [12]. Finally, an assignment of the different thal-
lium and bismuth positions could be made unequivocally based on the
In a series of experiments, the formation of discrete phases
in the ternary system Tl/Bi/Cl was investigated by reactions of
TlCl and BiCl3 in different molar ratios from 3:1 to 1:3. With
respective cation–Cl bond lengths. Isotropic displacement parameters intermediate compositions no formation of compounds was ob-
for all atoms were chosen and refined as free parameters. Table 1 con-
tains the crystallographic data and details of the refinement, Table 3
the positional parameters of the atoms. Figure 1 shows the observed
the calculated powder diffractogram.
served. Powder diffractograms of solidified melts of composi-
tions between 2:1 and 1:1 showed beside reflections of new
phases reflections of the starting materials TlCl and BiCl3. An-
other common feature is the widespread formation of dark col-
oured glassy solids on solidification of the melts. The forma-
tion of ternary compounds was only observed on the border
areas of the investigated compositions, in the thallium rich and
the bismuth rich region. Two different thallium(I) chloridobis-
mutates, Tl3BiCl6 and TlBi2Cl7, composed of TlCl and BiCl3
in the stoichiometric amounts 3:1 and 1:2, could be synthesised
and isolated as discrete phases. With both compositions as a
typical feature dark coloured melts are formed. In the case of
the 3:1 mixture, the deposition of crystals of Tl3BiCl6 from the
gas phase above the melt in a small temperature gradient dur-
ing the slow cooling process from 480 °C to ambient tempera-
ture could be achieved. The deposited colourless crystals of
Tl3BiCl6 are strongly light refracting. The analogous experi-
ments with melts of the composition 1 TlCl:2 BiCl3 gave melts
of dark-red colour. In contrast to the TlCl rich melts, no depo-
sition of crystalline material was observed from the gas phase
above the melts of this composition. Slow cooling of such a
melt from 250 °C to ambient temperature gives an amber col-
Table 3. Positional coordinates and isotropic displacement parameters
U
iso /Å2 for the atoms in the structure of TlBi2Cl7. Standard deviations
refer to the last significant digit.
Atom
Tl
x
y
z
Uiso
0.6739(2)
0.1409(2)
0.5188(2)
0.1583(2)
0.3547(9)
0.0194(8)
0.318(1)
0.1904(9)
–0.0332(8)
0.6431(8)
0.3869(9)
0.9225(3)
0.5569(3)
0.6966(2)
0.795(1)
0.262(1)
0.705(2)
1.094(1)
0.647(1)
0.648(1)
0.311(1)
0.0437(7)
0.0156(5)
0.0209(6)
0.030(3)
0.023(3)
0.053(4)
0.032(3)
0.024(3)
0.027(3)
0.028(3)
Bi(1) –0.2437(2)
Bi(2) 0.1380(2)
Cl(1) –0.158(1)
Cl(2) 0.000(1)
Cl(3) 0.352(1)
Cl(4) 0.256(1)
Cl(5) 0.370(1)
Cl(6) 0.053(1)
Cl(7) –0.445(1)
Further details on the crystal structure investigations may be obtained
from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leo-
poldshafen, Germany (Fax: +49-7247-808-666; E-Mail: crysdata@fiz-
karlsruhe.de), on quoting the depository numbers CSD-421317 for oured microcrystalline solid. By rapid quenching the melt from
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The Thallium(I) Chloridobismutates(III) Tl3BiCl6 and TlBi2Cl7
250 °C to –38 °C, performed by immersing the hot reaction
ampoule into chilled liquid mercury, an amorphous, glassy,
transparent, dark-red product was isolated (Figure 2a). Grind-
ing of the red glass gives a light red to rose powder. The non-
crystallinity was verified by X-ray powder diffraction. Leaving
this glassy material at room temperature under protection from
moist air causes the slow conversion into the light amber col-
oured crystalline phase (Figure 2b).
Figure 2. Photographs of pieces of the rapidly cooled melt of TlBi2Cl7,
forming a dark red, transparent glass (a), and this glass after two weeks
at room temperature with emerging amber coloured crystalline segre-
gations (b).
Crystal Structure of Tl3BiCl6
The tetragonal unit cell of Tl3BiCl6 contains two formula
units (Figure 3). Two bismuth atoms occupying the site sym-
metry 2/m are almost regular octahedrally coordinated by six
chlorine atoms. The two independent Bi–Cl distances are equal
within standard deviations and amount 2.692(2) and
2.693(3) Å (Table 4). The angular distortions within the BiCl6
octahedra are small. A significant deviation from rectangular-
ity is observed for the plane containing the bismuth atoms and
the four Cl(1) atoms, which is inclined by 3° to the Cl(2)–Bi–
Cl(2II) axis. An almost regular shape of BiCl6 octahedra in the
structures of chloridobismutates(III) is not unusual and found
e.g. in the structure of (NH4)3[BiCl6] (Bi–Cl 2.682 to 2.753 Å,
angle deviations from 90° less than 6°) [18].
The tetragonal unit cell of Tl3BiCl6 contains two independent
thallium atoms. The coordination sphere around the Tl(1) at-
oms, located on the Wykoff site 4j in the mirror plane may
be described best by a distorted Edshamar polyhedron with
coordination number of 10 and Tl–Cl distances in the region
between 3.117(3) to 3.802(3) Å, formally built by half a cube
Figure 3. Details of the crystal structure of Tl3BiCl6 (from top to bot-
and half an icosahedron. Tl(2) atoms with site symmetry 2/m tom): The coordination polyhedra of the two independent Tl+ ions
Tl(1) and Tl(2), the octahedral [BiCl6]3– anion, and a view of the unit
are surrounded by eight chlorine atoms in shape of a cube. The
cell along the crystallographic c axis. Thermal ellipsoids are drawn to
actual reduction of the ideal cubic symmetry to 2/m allows for
include a probability density of 50 %. Symmetry operations: I: 1 –y,
distortions which find their expressions in two different Tl–Cl
distances of 3.281(3) and 3.312(3) Å. The cuboidal Tl(2)Cl8
polyhedra are condensed to linear strands along the crystallo-
graphic c axis with the thallium ions being half the length of
x, –0.5+z; II: –x, 1 –y, z; III: –x, 1 –y, –z; IV: x, y, –z; V: 1 –y, x, 0.5–
z; VI: x, y, –1+z; VII: –y, x, –0.5+z; VIII: y, –x, –0.5+z; IX: 1 –x, 1 –
y, z; X: y, 1 –x, –0.5+z; XI: 1 –x, 1 –y, –z; XII: y, 1 –x, 0.5–z; XIII: –
x, 1 –y, 1 –z; XIV: x, y, 1 –z.
Z. Anorg. Allg. Chem. 2010, 928–935
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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931

J. Beck, S. Benz
Table 4. Selected bond lengths /Å in the structures of Tl3BiCl6 and TlBi2Cl7. For symmetry operations see legends of Figure 1 and Figure 2.
ARTICLE
Tl3BiCl6
TlBi2Cl7
Bi–Cl(1), Cl(1II), Cl(1XIII), Cl(1XIV
)
2.692(2)
2.693(3)
3.426(3)
3.486(3)
3.802(3)
3.281(3)
3.312(3)
Bi(1)–Cl(1)
Bi(1)–Cl(3II)
Bi(1)–Cl(4I)
Bi(1)–Cl(6)
Bi(1)–Cl(6II)
Bi(1)–Cl(7)
Bi(1)–Cl(7III)
Bi(2)–Cl(1)
Bi(2)–Cl(2)
Bi(2)–Cl(2IV)
Bi(2)–Cl(3)
Bi(2)–Cl(4)
Bi(2)–Cl(5)
Bi(2)–Cl(6II)
Tl–Cl
2.603(9)
Bi–Cl(2), Cl(2II)
2.705(9)
Tl(1)–Cl(1), Cl(1IV)
3.331(9)
Tl(1)–Cl(1II), Cl(1III)
Tl(1)–Cl(1I), Cl(1V)
3.059(9)
2.533(9)
Tl(2)–Cl(1), Cl(1IV), Cl(1IX), Cl(1XI)
2.470(9)
Tl(2)–Cl(1I), Cl(1V), Cl(1X), Cl(1XII
)
3.010(9)
3.048(10)
2.967(8)
2.636(8)
2.845(10)
2.584(9)
2.551(8)
3.467(9)
3.374(9) to 4.087(9)
the c axis (3.744 Å) apart. The Tl(1)Cl10 polyhedra share two test for a transition into a high temperature form, crystals of
of their square faces and four edges with the Tl(2)Cl8 polyhe- the low temperature form were grinded to a powder, filled into
dra resulting in a three-dimensional network (Figure 3).
a glass capillary, which was sealed, and investigated by X-ray
Since Tl3BiCl6 is of the basic formula type A3BX6, for which powder diffractometry. Above 230 °C the diffraction pattern
numerous representatives are known, we attempted to contrib- changed completely and the reflections of the room tempera-
ute its structure to a known structure type. The actual release ture phase disappeared. Lowering the temperature again to
of the ICSD database [19] contains 479 known crystal struc- 180 °C caused only an incomplete reconversion since the dif-
tures of this formula type. Using the subroutine CMPZ of the fractogram of the room temperature phase appeared again but
program KPLOT [20] a similarity search was performed. The the strongest reflections of the high temperature diffractogram
tolerances were set to high values: for all lattice constants a remained. The powder diffractogram at high temperature
tolerance of 15 %, for deviation of angles 5 %, the inclusion shows a significant higher number of reflections. This behav-
spheres for atom positions were enlarged to 1.0 Å. Surpris- iour may indicate a transition to a high temperature phase with
ingly and despite the wide tolerances, no coincidence with any a structure of lower symmetry. On looking for a structure
of the deposited structures of the A3BX6 formula type com- model for the assumed high temperature phase, we found that
pounds was found. The arrangement of the Tl+ ions in rows the calculated powder diffractogram of monoclinic Cs3BiCl6,
along one crystallographic axis with the remarkably short after substitution of caesium against thallium, shows a striking
Tl+···Tl+ distances is unusual and causes the uniqueness of similarity to the diffractogram of HT-Tl3BiCl6. The complete
Tl3BiCl6 within the structures of this formula type.
indexing of the diffractogram was not successful and a refine-
The recently described mineral steropesite was discovered as ment of the structure model was not possible on basis of the
yellow crystals of the formula Tl3BiCl6 [9]. The structure was observed diffraction data. We cannot, however, rule out the
solved on a crystal containing 10 % bromine on the chlorine possibility of a partial decomposition of the sample.
positions. It crystallises in the monoclinic system with a very
large unit cell having a volume of almost 5000 Å3. Its crystal
Crystal Structure of TlBi2Cl7
structure consists of discrete and slightly distorted octahedral
[BiCl6]3– anions and Tl+ cations with coordination numbers
The triclinic unit cell of TlBi2Cl7 contains two formula units
between six and eight. The reported structure can be consid-
with all atoms occupying general positions. The coordination
ered as a distorted variant of the structure of Cs3BiCl6, but
of the two independent bismuth atoms and the thallium atom
besides the presence of the analogous building units there is
by surrounding Cl atoms is much more irregular in comparison
no direct relation to the tetragonal form of Tl3BiCl6 reported
to the structure of Tl3BiCl6. In Figure 4 the distribution of
here.
these distances is depicted in the form of histograms. For all
three independent cations present in the crystal structure, a dis-
tinct gap is observed which allows for the definition of coordi-
High Temperature Phase of Tl3BiCl6
nation polyhedra. The two bismuth atoms both gain coordina-
The thermal behaviour of Tl3BiCl6 was investigated in the tion number seven with all Bi–Cl distances up to 3.34 Å for
temperature region between ambient temperature and 400 °C Bi(1) and 3.47 Å for Bi(2) taken into account. In each of the
by differential scanning calorimetry at a heating rate of two BiCl7 polyhedra one Cl atom achieves a long distance
10 °C·min–1. Two reproducible, endothermic effects could be which is typical for a pronounced lone pair effect of the three-
detected,
a
solid state phase transition at 189.1 °C valent bismuth atom. The BiCl7 polyhedra are connected by
(0.93 kJ·mol–1), and the melting at 378.4 °C (3.4 kJ·mol–1). To common corners and edges to a three dimensional net, in
932
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Z. Anorg. Allg. Chem. 2010, 928–935

The Thallium(I) Chloridobismutates(III) Tl3BiCl6 and TlBi2Cl7
which the Tl+ ions are embedded. Thallium gains the coordina- Vibrational Spectra
tion number eleven if all Tl–Cl distances up to 4.09 Å are
considered (Figure 4).
Of both compounds, Tl3BiCl6 and TlBi2Cl7, Raman spectra
were recorded. As expected, the spectrum of Tl3BiCl6 shows
only a low number of bands due to the high local symmetry
of the vibrational active BiCl6 group. Three broad bands at 261
(vs), 218 (s), and 117 (vs) cm–1 were observed. These bands
are identical with those of Cs3BiCl6 within the resolution limit
and can be attributed to the fundamental Raman active vibra-
tions A1g, Eg, T2g of the BiCl6 octahedron. In the region be-
tween 300 and 100 cm–1 the Raman spectrum of TlBi2Cl7
shows 10 bands (104(s), 116(m), 121(m), 132(s), 144(w),
172(s), 186(s), 247(s), 269(s), 290(s) cm–1). Due to the irregu-
lar, low symmetric coordination of the cation in this structure,
a definite attribution of these bands cannot be made.
Structural Investigations of Glassy TlBi2Cl7
The thermal behaviour of glassy TlBi2Cl7 was measured in
the temperature region between –20 and 230 °C, the respective
differential scanning calorimetry (DSC) function, obtained at
a heating rate of 10 °C·min–1 is given in Figure 5. The endo-
thermic effect at 41.3 °C (point a in Figure 5, 2.9 kJ·mol–1) is
attributed to the glass transition. The two following exothermic
effects at 59.3 (b and c, –6.0 kJ·mol–1) are attributed to crystal-
lisation processes. The small endothermic effect at 180 °C (d)
was not observable in all measurements and is probably an
artefact. At 189.5 °C melting occurs (e, 30.3 kJ·mol–1). On
cooling the samples after melting, a sharp exothermic signal at
109 °C appears (f, –22.1 kJ·mol–1) originating from crystallisa-
tion of the melt. If the measurement is carried out with a lower
heating rate of 1 °C·min–1, the glass transition shifts to
35.2 °C. According to the empirical relation Tg = A + B logβ
with Tg = observed glass transition temperature, A = true glass
transition temperature, B = empirical factor depending on the
preparation of the glass, β = heating rate, the transition temper-
ature at 32.5 °C is identical with the true transition temperature
[21]. The relation of the absolute glass transition temperature
to the melting temperature allows within limits for information
on the structural properties of amorphous networks. For highly
symmetric polymers made up from small monomeric units, Tg/
Tm is generally 0.5 or lower, for the broad majority of poly-
mers, however, this relation amounts 2/3 [22]. For the amor-
phous TlBi2Cl7 phase Tg/Tm is 308 K/463 K = 0.66. The ob-
served thermal effects do thus not allow for detailed
conclusions on the structural properties of the glass.
205Tl and 203Tl MAS solid state NMR spectra were recorded
in order to find further insight into the structure of the red
glassy form of TlBi2Cl7. The MAS spectra of both, the glassy
and the crystalline TlBi2Cl7 do not show differences. Only one
broad signal centred at 47.7 ppm relative to the standard was
observed (Figure 6). This is in line with the crystallographic
cell, which contains only one symmetry-independent thallium
atom. The similarity of the spectra suggests that the chemical
environment of the thallium atoms in both phases does not
show large differences.
Figure 4. Details of the crystal structure of TlBi2Cl7 (from top to bot-
tom): The Bi–Cl and Tl–Cl distances in the form of histograms, the
coordination polyhedron of the Tl+ ion, the coordination of the two
independent bismuth atoms Bi(1) and Bi(2), and a view of the unit cell
along the crystallographic c axis. In the two detailed representations,
atoms are drawn as spheres with radii corresponding to 50 % of the
probability density of the thermal vibration. Symmetry operations: I:
–x, 1 –y, 2 –z; II: –x, 1 –y, 1 –z; III: – 1 –x, 1 –y, 1 –z; IV: –x, –y, 1
–z; V: 1+x, y, 1+z; VI: 1 –x, –y, 1 –z; VII: 1 –x, –y, 2 –z; VIII: 1+x, y,
z; IX: 1 –x, 1 –y, 2 –z.
The vibrational spectrum of the amorphous TlBi2Cl7 phase
shows, however, distinct differences to the spectrum of the
Z. Anorg. Allg. Chem. 2010, 928–935
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
933

J. Beck, S. Benz
ARTICLE
TlBi2Cl7 in the glassy and the crystalline state both have
closely related coordination environments around the thallium
and bismuth atoms. On the other hand, the Raman spectra with
their differing patterns of vibrational bands indicate a substan-
tially altered local structure.
Figure 5. Differential scanning calorimetry function of TlBi2Cl7. The
solid line represents the heating curve from –10 to 210 °C, the dotted
line the cooling process back to ambient temperature. The observable
thermal effects are indicated by letters a–f, see text. The temperature
rate for heating and for cooling was 10 °C·min–1.
Figure 7. XANES spectrum on the Bi-LIII absorption edge of glassy
and crystalline TlBi2Cl7, and of crystalline Tl3BiCl6.
Conclusions
Two discrete phases were prepared and characterised in the
ternary system Tl/Bi/Cl, both showing structural pecularities.
The TlCl rich compound Tl3BiCl6 is built of only slightly dis-
torted [BiCl6]3– octahedra, whereas the BiCl3 rich phase
TlBi2Cl7 is made of a complex network of connected, irregular
BiCl7 polyhedra. In both structures, the Tl+ ions gain high co-
ordination numbers between eight and eleven. A remarkable
and yet not elucidated detail is the amorphous state of
TlBi2Cl7, which forms a glass on rapid cooling of the melt. The
metastable, transparent glass is of intensive dark-red colour,
whereas the crystalline phase is almost colourless. No explana-
tion for the colour of the glass could be found and the attempts
to find structural relations between the glassy and the crystal-
line state by XANES, NMR and Raman spectra lead to contra-
dictory results.
Figure 6. 205Tl MAS-NMR spectrum of amorphous TlBi2Cl7 at
12 kHz spinning speed. Signals marked with an asterisk are rotational
bands. The respective 203Tl spectrum is of almost identical shape but
shows lower signal intensity.
crystalline form. Eight bands (126(s), 142(m), 157(m), 178(s),
246(m), 268(w), 292(sh, s), 306(vs) cm–1) are present, of
which only three (142, 246, 268 cm–1) are observed at the
same frequencies as for the crystalline state. The spectrum of
amorphous TlBi2Cl7 shows instead a close similarity to the
spectrum of BiCl3 (six bands at 280(vs), 262(m), 252(sh, w),
179(m), 141(m), 119(m)) in the frequency and intensity distri-
bution. For BiCl3 the bands at 280 (A1) and 252 cm–1 (E) can
be attributed to the C3v symmetric molecules, which are
Acknowledgement
present in the solid state. The vibrational spectra lead to the We gratefully acknowledge the measurements of the X-ray absorption
spectra by N. Palina, B. Brendebach and H. Modrow, the collection of
the single crystal data by J. Daniels, the recording of the NMR spectra
by W. Hoffbauer and the dedicated help of R. Hundt with the data base
searches.
suggestion that amorphous TlBi2Cl7 contains BiCl3 building
units as present in the structure of crystalline BiCl3.
Near edge X-ray absorption spectra (XANES) of Tl3BiCl6,
and amorphous and crystalline TlBi2Cl7 were recorded on both
the Tl-LIII and the Bi-LIII edges. Figure 7 shows the spectra on
the Bi-LIII edges as a superposition for all three samples. A
striking similarity is observed between the spectra of glassy
and crystalline TlBi2Cl7, whereas the spectrum of Tl3BiCl6 is
different throughout the entire energy interval. This observa-
tion is also valid for the spectra on the Tl-LIII edge. This result
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Z. Anorg. Allg. Chem. 2010, 928–935
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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