Mixed-valence thallium(I,III) bromides the crystal structure of α-Tl2Br3

Article abstract of DOI:10.1002/1521-3749(200212)628:12<2675::AID-ZAAC2675>3.0.CO;2-4

Thallium sesquibromide Tl₂Br₃ is dimorphic. Scarlet coloured crystals of α-Tl₂Br₃ were obtained by reactions of aqueous solutions of TlBr₃ and Tl₂SO₄ in agarose gel. In case of rapid crystallisation of hydrous TlBr₃/TlBr solutions and from TlBr/TlBr₂ melts β-Tl₂Br₃ is formed as scarlet coloured, extremely thin lamellae. The crystal structures of both forms are very similar and can be described as mixed-valence thallium(I)-hexabromothallates(III) Tl₃ [TlBr₆]. In the monoclinic unit cell of α-Tl₃[TlBr₆] (a = 26.763(7) A; b = 15.311(6) A; c = 27.375(6) A; β = 108.63(2)°, Z = 32, space gr. C2/c) the 32 Tl^III-cations are found in strongly distorted octahedral TlBr₆ groups. The 96 Tl^I cations are surrounded either by four or six TlBr₆ groups with contacts to 8 or 9 Br neighbors. Crystals of β-Tl₃[TlBr₆] by contrast show almost hexagonal metrics (a = 13.124(4) A, b = 13.130(4) A, c = 25.550(7) A, γ = 119.91(9)°, Z = 12, P2₁/m). Refinements of the parameters revealed structural disorder of TlBr₆ units, possibly resulting from multiple twinning. Both structures are composed of Tl₂[TlBr₆]^- and Tl₄ [TlBr₆]⁺ multilayers, which alternate parallel (001). The structural relationships of the complicated structures of α- and β-Tl₃[TlBr₆] to the three polymorphous forms of Tl₂Cl₃ as well as to the structures of monoclinic hexachlorothallates M₃TlCl₆ (M = K,Rb) and the cubic elpasolites are discussed.

Full text of DOI:10.1002/1521-3749(200212)628:12<2675::AID-ZAAC2675>3.0.CO;2-4

Mixed-Valence Thallium(I,III) Bromides
The Crystal Structure of α-Tl2Br3
Rupprecht Ackermann, Christian Hirschle, Heinz W. Rotter, and Gerhard Thiele*
Freiburg i. Br., Institut für Anorganische und Analytische Chemie und Materialforschungsinstitut FMF der Universität
Received May 27th, 2002.
Professor Rudolf Hoppe zum 80. Geburtstag gewidmet
˚
˚
˚
Abstract. Thallium sesquibromide Tl2Br3 is dimorphic. Scarlet col-
oured crystals of α-Tl2Br3 were obtained by reactions of aqueous
solutions of TlBr3 and Tl2SO4 in agarose gel. In case of rapid crys-
tallisation of hydrous TlBr3/TlBr solutions and from TlBr/TlBr2
melts ß-Tl2Br3 is formed as scarlet coloured, extremely thin lamel-
lae. The crystal structures of both forms are very similar and can
be described as mixed-valence thallium(I)-hexabromothallates(III)
Tl3[TlBr6]. In the monoclinic unit cell of α-Tl3[TlBr6] (a ϭ 26.763(7)
A; b ϭ 15.311(6) A; c ϭ 27.375(6) A; ß ϭ 108.63(2)°, Z ϭ 32,
space gr. C2/c) the 32 TlIII-cations are found in strongly distorted
octahedral TlBr6 groups. The 96 TlI cations are surrounded either
by four or six TlBr6 groups with contacts to 8 or 9 Br neighbors.
Crystals of ß-Tl3[TlBr6] by contrast show almost hexagonal metrics
(a ϭ 13.124(4) A, b ϭ 13.130(4) A, c ϭ 25.550(7) A, γ ϭ 119.91(9)°,
Z ϭ 12, P21/m). Refinements of the parameters revealed structural
disorder of TlBr6 units, possibly resulting from multiple twinning.
Both structures are composed of Tl2[TlBr6]Ϫ and Tl4[TlBr6]ϩ multi-
layers, which alternate parallel (001). The structural relationships
of the complicated structures of α- and ß-Tl3[TlBr6] to the three
polymorphous forms of Tl2Cl3 as well as to the structures of mono-
clinic hexachlorothallates M3TlCl6 (M ϭ K,Rb) and the cubic elpa-
solites are discussed.
˚
˚
˚
Keywords: Thallium; Thalliumbromides; Bromothallates; Mixed
valence compounds, Crystal structure
Gemischtvalente Thallium(I,III)-bromide. Die Kristallstruktur von α-Tl2Br3
InhaltsübersichtThalliumsesquibromid Tl2Br3 ist dimorph. Dunkel-
rote Kristallblättchen von α-Tl2Br3 wurden durch Umsetzungen
von wässrigen Tl2SO4 /TlBr3-Lösungen in Agarose-Gel erhalten. β-
Tl2Br3 entsteht in Form extrem dünner, roter Lamellen bei raschen
Kristallisationen aus wässrigen TlBr/TlBr3-Lösungen oder TlBr/
TlBr2-Schmelzen. Beide Formen besitzen einen ähnlichen Aufbau;
sie sind als gemischtvalente Thallium(I)-hexabromothallate(III)
Tl3[TlBr6] zu beschreiben. In der monoklinen Elementarzelle von
α-Tl2Br3 (a ϭ 26,763(7) A; b ϭ 15,311(6) A; c ϭ 27,375(6) A; β ϭ
108,63(2)°, Z ϭ 32, Raumgruppe C2/c) sind 32 TlIII-Kationen in
stark verzerrten, oktaedrischen TlBr6-Baugruppen enthalten. Die
96 TlI-Kationen sind von vier oder sechs TlBr6-Baugruppen umge-
ben und bilden Kontakte zu 8 oder 9 Br-Nachbarn aus. Bei Kristal-
len von β-Tl2Br3 wurde eine monokline Zelle mit nahezu exakt he-
˚
˚
xagonaler Metrik gefunden (a ϭ 13.124(4) A, b ϭ 13.130 (4) A ,
˚
c ϭ 25.550(7) A, γ ϭ 119.91(9)°, Z ϭ 12, Raumgruppe P21/m). Die
Verfeinerungen der Parameter ergaben strukturelle Fehlordnungen
für TlBr6-Baugruppen, was auf das Vorliegen von Viellingen zu-
rückgeführt wird. Beide Strukturen sind aus Schichtpaketen
Tl2[TlBr6]Ϫ und Tl4[TlBr6]ϩ aufgebaut, die alternierend parallel
(001) verlaufen. Die strukturellen Verwandtschaftsbeziehungen der
komplizierten Strukturen von α- und β-Tl3[TlBr6] zu den drei poly-
morphen Formen von Tl2Cl3 sowie zum Aufbau der monoklinen
Hexachlorothallate A3TlCl6 (A ϭ K, Rb) und zum kubischen Elpa-
solith-Typ werden diskutiert.
˚
˚
˚
Introduction
elemental halogens as thallium(I)Ϫhexahalogenothalla-
tes(III), Tl3[TlX6]. According to this procedure, aqueous
TlX3 solutions are heated with a huge surplus of solid TlX
whereby the insoluble precipitates are formed. Benrath as-
sumed that the formulae Tl2X3 were correct [2, 3], however
the samples contained always varying amounts of TlX and
owing to this, it was not possible to determine the chemical
composition exactly.
Thallium prefers the oxidation states ϩI and ϩIII , but also
halides with compositions between TlX3 and TlX (X ϭ hal-
ogen) have been known for a long time [1Ϫ8]. Benrath
[2, 3] described the yellow and red products he obtained by
his trials to oxidise the insoluble halides TlCl and TlBr with
The extremely hygroscopic and thermally very unstable
trihalides are available only by high pressure halogenation
[9]. They lose halogen by heating to temperatures below
100°C and degrade to the dihalides TlX2 which were ident-
ified as thallium(I)Ϫtetrahalogenothallates(III) Tl[TlX4]
[5Ϫ7]. The water soluble dihalides were obtained by crystal-
lisation from aqueous solutions that contained approxi-
* Prof. Dr. G. Thiele
Institut für Anorganische Chemie und Materialforschungszentrum
FMF der Albert-Ludwigs-Universität Freiburg i.Br.
Albertstr. 21,
D-79104 Freiburg
Fax-Nr. 0049-761-2036012
e-mail: gerhard.thiele@ac.uni-freiburg.de
Z. Anorg. Allg. Chem. 2002, 628, 2675Ϫ2682  2002 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 0044Ϫ2313/02/628/2675Ϫ2682 $ 20.00ϩ.50/0
2675

R. Ackermann, C. Hirschle, H. W. Rotter, G. Thiele
mately equal amounts of TlCl and TlX3 [5, 6]. In water or
in the presence of complexing ligands it is possible to oxi-
dise the monohalides completely to halogenocomplexes of
thallium(III). From the solutions, the hydrates TlX3·4H2O
[10], or in the presence of suitable counter cations halogeno-
thallates(III) can be crystallised. Thus numerous halogeno-
thallates(III) that contain univalent cations of the alkali me-
tals were synthesised and characterised, besides the tetra-
halogeno- and hexahalogenothallates(III), in particular
those of the type A2[TlX5] [11, 12] and A3[Tl2X9] [13, 14].
The corresponding mixed-valence thallium halides Tl3X5 ϭ
TlI2[TlX5] and Tl5X9 ϭ TlI3[Tl2X9] are unknown yet. By
reactions of aqueous TlX3 solutions with a surplus of TlX
(X ϭ Cl, Br) hexagonally shaped extremely thin lamellae
are formed, that are unsuitable for XϪray structure
analysis.
sulted in red samples containing TlBr as the main ingredi-
ent. The complete total bromination of TlBr to TlBr3 was
succeessful only by working with a surplus of bromine in
sealed glass ampoules at approximately 200°C. The re-
sulting samples are extremely hygroscopic and thermally
unstable. On removing the excess of bromine, TlBr3 decays
to TlBr2, which on the other side is easier to produce by
crystallisation from aqueous solutions [5]. In further at-
tempts TlBr2 was used as a TlBr3 source for reactions in
melts.
In search of further compounds, 100 mg of mixtures of
TlBr/TlBr2 with different compositions (e.g. Tl3[Tl2Br9] and
Tl2[TlBr5]) were heated within sealed quartz glass ampoules
at 200°C and subsequently cooled to ambient temperature.
The melting points of the end-member phases were deter-
mined to 162°C (TlBr2) and 460°C (TlBr). DTA investi-
gations gave noindications for the formation of new com-
pounds. The resulting phase diagram showed an eutectic
point at 10( 1) mol Ϫ % TlBr with TM ϭ 120( 1)°C. How-
ever, by X-ray powder investigations of the samples of
around 50 mol Ϫ % TlBr additional reflections were found,
which could be assigned to Tl2Br3 as described by Benrath
(β-Tl2Br3). The thermal behaviour shows that β-Tl2Br3 is
formed below the melting temperature of the eutectic mix-
ture in the subsolidus regions by slow cooling.
First attempts in solving the structures of Tl2Cl3 by Hägg
and Jereslev [4] resulted in the determination of a trigonal
˚
˚
cell with a ϭ 14.3 A, c ϭ 25.1 A only. We found that on
rapid cooling of aqueous TlCl/TlCl3 solutions Ϫ especially
under the conditions of hydrothermal synthesis Ϫ pale yel-
low needles crystallise. The X-ray crystal structure analysis
showed an orthorhombic structure and confirmed the com-
position Tl3[TlCl6] [8]. As the hexagonally shaped platelets
show different powder patterns they have to be regarded
as a different modification. Several attempts to solve the
structures of the thin platelets of β-Tl2Cl3 and also of the
homologous bromine compound failed [9]. Even though the
pseudotrigonal symmetry character of the crystal structures
was identified. Big problems occurred with regard to the
refinement of the site parameters due to superstructures,
pseudosymmetry and twinning, so that attempts for a struc-
ture solution were interrupted. We started further investi-
gations [15] of the systems TlClϪTlCl3 and TlBrϪTlBr3
after the unusual compositions and bonding properties of
the corresponding indium halides became published
[16Ϫ27]. Structures like those of In2Br3 and In5Br7 with
ethane-analogous, covalent InIIϪInII-bonds containing
[In2Br6]2ϪϪgroups have been detected [18Ϫ27]. Therefore
the question arose, whether thallium(II) compounds with
TlIIϪTlII would exist or not. Within the scope of the recent
investigations, besides the clarification of crystal structures,
was to identify further compounds in these systems. In the
following we report on the characterisation of mixed-val-
ence thallium bromides. In particular on the production of
single crystals and the structural analysis of Tl2Br3.
Trials of crystallisation from aqueous solutions led to
microcrystalline red precipitates when thallium(I) salts
(Tl2SO4) were added to TlBr3 solutions. The X-ray powder
diagrams revealed the existence of mixtures that consisted
of β-Tl2Br3 and a significant share of TlBr. By heating
aqueous TlBr3 solutions together with solid TlBr within
sealed quartz glass ampoules at temperatures of approxi-
mately 150°C and subsequent quick cooling (30 K/h) crys-
tals were obtained as very thin, red and hexagonally shaped
lamellae. Attempts for a X-ray structure solution (c.f. fol-
lowing section) led to a trigonal unit cell with parameters
˚
˚
of a (b) ϭ 13.13 A and c ϭ 25.5 A. On trying to grow single
crystals by crystallization from agarose-gel well shaped
pseudohexagonal platelets of α-Tl2Br3 with monoclinic
symmetry were obtained.
Experimental
Synthesis
Aqueous solutions of TlBr3 were prepared by oxidizing a suspen-
sion of TlBr (Fa. ACROS, purity 99 %) in water with bromine
until all solid had dissolved. A stream of nitrogen was then passed
through the solution in order to remove the excess bromine. For
gel crystallization an U-tube (diameter 1.5 cm, distance of the col-
umns 4 cm) was filled with 20 mL of an agarose gel (3.5% agarose,
Fa. Aldrich, Agarose rein) as a matrix. To one of the column about
15 ml of an aqueous solution of Tl2SO4 ( conc. 0.07 mol/l, Fa.
EGA-Chemie, 99.5 %) was added, to the other column about 15 ml
of a solution of TlBr3 (conc. 0.4 mol/l). After 14 days dark red,
very thin platelets were found mainly in the space between the gel
and the side of the tube. The crystallisation was stopped at the
earliest when conglomerates of crystals developed. Individuals were
removed mechanically from the conglomerates.
Preparation of Thallium Bromides
Attempting to grow single crystals of Tl2Br3 and in order to
detect new thallium bromides melt reactions of TlBr/TlBr2
mixtures and crystallisations from aqueous solutions were
carried out. For phase analysis the products were character-
ised by thermal analysis, X-ray powder diffractometry and
Raman spectroscopy. By the method of Benrath [2, 3], no
consistent products were available. The treatment of TlBr
with bromine at ambient pressure in aqueous solutions re-
2676
Z. Anorg. Allg. Chem. 2002, 628, 2675Ϫ2682

Mixed-Valence Thallium(I,III) Bromides
Table 1 Crystallographic data and their determination.
Table 2 α-Tl2Br3, Positional parameters and mean isotropic dis-
placement parameters (A x 10 )
2
3
˚
Compound
Systematic name: Thalliumsesquibromide Tl2Br3; structural formula: α-tri-
Atom
Wyckoff Sitesym-
x
y
z
U(eq)
thallium(I)-hexabromothallate(III) α-Tl3[TlBr6]; molar mass: 1296.94 g molϪ1
.
Position
metry
Crystallographic Data
Tl31
Br11
Br12
Br13
Tl32
Br21
Br22
Br23
Tl33
Br31
Br32
Br33
Br34
Br35
Br36
Tl34
Br41
Br42
Br43
Tl35
Br51
Br52
Br53
Tl36
Br61
Br62
Br63
Tl37
Br71
Br72
Br73
Tl11
Tl12
Tl13
Tl14
Tl15
Tl16
Tl17
Tl18
Tl19
Tl20
Tl21
Tl22
4 e
8 f
8 f
8 f
4 e
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
4 c
8 f
8 f
8 f
4 d
8 f
8 f
8 f
4 a
8 f
8 f
8 f
4 b
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
8 f
2
1
1
1
2
1
1
1
1
1
1
1
1
1
1
0
0.2238(1)
0.2247(2)
0.8532(2)
0.5971(2)
0.2341(1)
0.1081(2)
0.2645(2)
0.3590(2)
0.5009(1)
0.9625(2)
0.8800(2)
0.1275(2)
0.0326(2)
0.1220(2)
0.8781(2)
0.25
0.3502(2)
0.1558(2)
0.1422(2)
0.75
0.7387(2)
0.5703(2)
0.7616(2)
0.5
0.25
21(1)
37(1)
34(1)
38(1)
33(1)
54(1)
61(1)
58(1)
23(1)
33(1)
38(1)
39(1)
35(1)
38(1)
36(1)
22(1)
34(1)
31(1)
33(1)
22(1)
33(1)
31(1)
34(1)
24(1)
49(1)
37(1)
34(1)
27(1)
51(1)
48(1)
92(2)
46(1)
44(1)
47(1)
47(1)
50(1)
40(1)
82(1)
66(1)
48(1)
58(1)
56(1)
79(1)
Crystal system: monoclinic; Space group: C 2/c (No. 15); Lattice constants
0.0809(1)
0.4522(1)
0.4514(2)
0.5
0.2090(2)
0.1825(2)
0.1814(2)
0.25
˚
˚
˚
a ϭ 26.763(7) A; b ϭ 15.311(6) A; c ϭ27.375(6) A; β ϭ 108.63(2)° ; Cell
3
˚
volume: 10630(6) A ; Formula units: 32; F(0 0 0): 17088; Calc. density:
6.483 g cmϪ3, µ ϭ 66.35 mmϪ1
.
0.4352(2)
0.5518(2)
0.4206(2)
0.7599(1)
0.3149(1)
0.1870(1)
0.2053(1)
0.2020(1)
0.3312(1)
0.3180(2)
0.25
0.2384(1)
0.1622(1)
0.3085(1)
0.25
0.3425(1)
0.2390(1)
0.2062(1)
0.5
0.1946(2)
0.1867(2)
0.1812(2)
0.2486(1)
0.1791(2)
0.2004(2)
0.1844(2)
0.3156(2)
0.2976(2)
0.3127(2)
0
0.0776(2)
0.9981(2)
0.0769(2)
0
0.0776(2)
0.0010(2)
0.0783(2)
0
Data collection
Temperature 293( 2) K; Instrument: Nonius CAD4, graphit monochroma-
tor; Radiation: Mo Kα; Range for data collection: 3° ␽ < 21°, Ϫ25 Յ h Յ
26, Ϫ15 Յ k Յ 8, Ϫ27 Յ l Յ 18; Scan type: ω-scan; Scan width: 0.51 ϩ 0.33
tanθ; Reflections: 9695 measured, 5668 unique, 3796 with F > 2σ(F).
Structure solution and refinement
Program System: SHELXS97-2 / SHELXL97-2 [28] Crystal shape: hexago-
nal platelet; Absorption correction: numerical, face method; 8 faces, min. /
max. transmission 0.01261 / 0.51080; Number of parameters: 369; Weighting
scheme: W ϭ 1/[σ2(Fo2) ϩ (0.0774 P)2] with P ϭ (Fo2 ϩ 2Fc2)/3; Min. / max.
¯
1
1
1
1
residual electron density: Ϫ3.867 / 3.054 e AϪ3; R-values R1 / wR2(F2):
˚
¯
1
0.0542 / 0.1203 for 3796 reflexions with F > 2σ(F), 0.0977 / 0.1401 for 5668
unique reflexions.
1
1
1
¯
1
1
1
1
0.0588(2)
0.9770(1)
0.9095(1)
0.5
0.0213(2)
0.8311(2)
0.0672(2)
0
0.0998(2)
0.0144(2)
0.0142(2)
0
X-ray structure determination and refinement
¯
1
After the quality of the crystals was checked by film methods, a
fragment of a hexagonal platelet with defined edges was mounted
on an CAD4 diffractometer. For α-Tl2Br3 the cell search led di-
rectly to a C-centred monoclinic cell. The precise lattice parameters
were determined with the data of 25 high angle reflections (θ >20°).
The C-centering and the ratio a/c near √3agrees with the metrics
of a hexagonal cell, but the Laue symmetry group was determined
to be 2/m. The structure was solved by direct methods for space
group C2/c and refined anisotropically. The maxima and minima
of the residual electron density were located very close to the heavy
atoms and cannot be brought in connection with additional atoms.
The anisotropic displacement parameters show no indications of
disorder or under-occupation of sites. Further details of the crystal
structure investigation are available from the Fachinformationszen-
trum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany),
on quoting the depository number CSD-391171, the name of the
authors, and citation of the paper.
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0.0888(2)
0.4305(2)
0.5207(2)
0.1205(1)
0.8248(1)
0.5707(1)
0.9756(1)
0.9255(1)
0.3469(1)
0.2982(1)
0.0937(1)
0.2028(1)
0.0816(1)
0.1245(1)
0.3157(1)
0.3978(2)
0.8620(2)
0.0257(4)
0.4614(1)
0.4640(1)
0.7777(1)
0.2156(1)
0.4617(1)
0.4983(1)
0.7499(1)
0.7933(1)
0.9896(1)
0.9950(1)
0.2363(1)
0.2503(1)
0.0145(2)
0.0004(2)
0.0991(2)
0.4159(1)
0.9098(1)
0.9083(1)
0.0920(1)
0.2804(1)
0.0923(1)
0.1974(1)
0.0890(1)
0.0850(1)
0.2249(1)
0.0915(1)
0.1904(1)
Structure Description for α-Tl2Br3
The composition of the red crystals corresponds to a thal-
lium sesquibromide but the detected coordination po-
lyhedra constitute a description as trithallium(I)-hexa-
bromothallate(III) in accordance with Benrath [2,3] and the
formula TlI3[TlIIIBr6]. The monoclinic crystal structure is
unusually complicated both for a binary and for a ternary
metal halide. Therefore, the clarification of crystallochemi-
cal relationships is of particular importance for an under-
standing of the structural principle.
The monoclinic unit cell (see Fig. 1) contains 320 atoms
distributed to 43 crystallographically independent sites (see
table 2). 32 of the 128 Tl atoms are distributed over 7 sites
(Tl 31 Ϫ Tl 37) each of them surrounded octahedrally by
Br atoms while the coordination polyhedra around the re-
maining 96 Tl atoms (sites Tl11ϪTl22) are significantly
more expanded and also fairly irregular.
Fig. 1 Crystal structure of Tl3[TlBr6]. Monoclinic unit cell with
an alternating sequence of cationic and anionic layers.
Although the majority of the TlϪBr distances within the
12 crystallograhically different TlBr6 groups lies close to the
Z. Anorg. Allg. Chem. 2002, 628, 2675Ϫ2682
2677

R. Ackermann, C. Hirschle, H. W. Rotter, G. Thiele
˚
˚
tances in the range between 2.62 and 2.98 A were detected
[12, 14].
Table 3 α-Tl2Br3, Interatomic distances/A and angles/° within the
TlIIIBr6-polyhedra.
In the case of the polyhedra around the 96 TlI cations
(sites Tl 11 Ϫ Tl 22) either 8 or 9 bromine neighbours exist
in the distance range between 3.19 and 4.3 A (see table 4).
Considering the number of the flanking TlBr6 groups it is
possible to differentiate clearly between two kinds of TlI
cations with Br neighbours stemming either from four or
six octahedra that add to the coordination by an apex, an
edge or a face (see Fig. 2). The TlBr6 octahedra form either
an octahedral or tetrahedral arrangement. Herein the struc-
tural relationship to the cubic elpasolite type arises, which
will be discussed in the following section.
Central atom
Ligand
Distances
Angles BrϪTlϪBr, range
87.07(16) Ϫ 91.87(11)
˚
Tl 31
Average: 2.73
Br 11
Br 12
Br 13
2.737(3)
2.732(4)
2.725(4)
Tl 32
Average: 2.77
Br 21
Br 22
Br 23
2.711(4)
2.583(4)
3.028(5)
83.22(13)Ϫ101.63(17)
88.50(11)Ϫ92.32(11)
Tl 33
Average: 2.74
Br 31
Br 32
Br 33
Br 34
Br 35
Br 36
2.814(4)
2.708(4)
2.710(4)
2.797(4)
2.692(4)
2.699(4)
Tl 34
Average: 2.73
Br 41
Br 42
Br 43
2.717(3)
2.744(3)
2.740(4)
85.37(11)Ϫ94.63(11)
84.93(11)Ϫ95.07(11)
85.14(10)Ϫ94.86(10)
83.49(11)Ϫ96.51(11)
Tl 35
Average: 2.74
Br 51
Br 52
Br 53
2.690(4)
2.768(3)
2.758(4)
Tl 36
Average: 2.73
Br 61
Br 62
Br 63
2.701(4)
2.716(3)
2.771(3)
Tl 37
Average: 2.73
Br 71
Br 72
Br 73
2.766(4)
2.818(3)
2.618(5)
Fig. 2 Sourrounding of the TlI-cations Tl11 and Tl12 as examples
for the coordination polyhedra arround the 12 crystallographically
different TlI-cations. Left: Polyhedron arround Tl 11: 9 Br-atoms
stem from 6 TlBr6-polyhedra that add to the coordination by a face
(Tl 37), by an edge (Tl 35) and 4 by an apex (Tl 31ϪTl 34).
Right: Polyhedron arround Tl 12: 9 Br-atoms stem from 4 TlBr6-
polyhedra that add to the coordination by faces (Tl 33, Tl 37), by
an edge (Tl35) or by an apex (Tl 34).
Table 4 α-Tl2Br3, Coordination polyhedra around the TlI-cations.
All Br-neighbours in the distance range up to 4.3 A were taken
into account.
˚
Central Number of
Distance
Number of
neighbouring
TlBr6-
Number of common
apices edges faces
atom
neighbouring range
Br-Atoms
Polyhedra
Tl 11
Tl 12
Tl 13
Tl 14
Tl 15
Tl 16
Tl 17
Tl 18
Tl 19
Tl 20
Tl 21
Tl 22
9
9
8
8
9
8
9
8
9
8
8
9
2.231Ϫ3.798
3.325Ϫ4.222
3.257Ϫ3.843
3.227Ϫ3.680
3.299Ϫ4.214
3.233Ϫ3.703
3.346Ϫ4.016
3.154Ϫ4.071
3.297Ϫ4.031
3.306Ϫ3.695
3.191Ϫ3.773
3.342Ϫ4.055
6
4
4
4
4
6
4
6
4
4
6
4
4
1
1
2
1
4
1
5
Ϫ
1
4
Ϫ
1
1
2
Ϫ
1
2
1
Ϫ
3
2
2
3
1
2
1
2
2
Ϫ
2
1
1
1
Ϫ
1
Fig. 3 Crystal structure of Tl3[TlBr6]. Set-up of the layer packages
with the composition Tl4[TlBr6]ϩ (left, z ϭ 0 and 1/2 resp.) and
Tl2[TlBr6]Ϫ (right, z ϭ 1/4 and 3/4 resp.) running parallel to (001).
The outlines of the C-centered monoclinic unit cell parallel to the
the ab-plane are plotted by solid lines, that of a hexagonal pseudo
cell by dashed lines.
˚
average value of 2.73 A, some of the polyhedra are more
distorted (see table 3). Especially the polyhedron around
the atom Tl32 is unusually (2ϩ2ϩ2) distorted; it shows two
˚
˚
very short (2.58 A) and two very long (3.03 A) bonds. Dis-
tances in the range of 2.47 to 2.56 A are known from tetra-
˚
hedral TlBr4Ϫ anions which are structural units in the crys-
tal structures of alkali tetrabromothallates (A ϭ K to Cs)
[29] and thallium(I)-tetrabromothallate(III) [5]. Trustworthy
reliable comparative values of hexabromothallates that con-
tain isolated TlBr6 anions are missing. In the case of the
crystal structures of Cs2TlBr5 and Cs3Tl2Br9, which contain
TlBr6-groups, even strong distorted polyhedra with dis-
Although the coordination polyhedra of the Tl atoms are
connected to a threeϪdimensional framework, the charac-
teristics of a layer type structure prevails (see Fig. 1). An
alternating succession of anionic layer packages with the
composition Tl2[TlBr6]Ϫ and cationic packages Tl4[TlBr6]ϩ
2678
Z. Anorg. Allg. Chem. 2002, 628, 2675Ϫ2682

Mixed-Valence Thallium(I,III) Bromides
already comparable to the circumstances known from the
structural chemistry of the d10-cation mercury(II) [30].
Structural Relationships
Differences between the two structures of α-Tl3[TlCl6] and
α-Tl3[TlBr6] exist in the succession of the layer packages
TlI4[TlIIIX6]ϩ and TlI2[TlIIIX6]Ϫ. The translation perpen-
dicular to the layers include two cationic Ϫ and two anionic
layers in the case of the chloride [8]; but six in the case of
the bromide (see Fig. 5).
Fig. 4 Environment of two differently oriented TlBr6 polyhedra
The array of the layer packages is similar to closest pack-
ings. Thus it is also possible to describe the structure of α-
Tl3[TlBr6] idealized in terms of a cubic sequence AB and
the structures of α-Tl3[TlCl6] and β-Tl3[TlBr6] in terms of
a hexagonal sequence ABC where the terms A, B and C
symbolyze for a cationic-anionic double layer with the com-
position Tl3[TlX6]. The cubic succession that includes 3
double layers (i.e. 6 packages) is already known from the
K3[MoCl6]-type [31] that is adopted by the ambient-tem-
perature modifications of the hexachlorothallates A3[TlCl6]
with A ϭ K, Rb [9, 32, 33]. The crystal chemical ionic radii
of Rbϩ and Tlϩ are almost the same; therefore the detected
within the cationic layers TlI4[TlIIIX6]ϩ
is found parallel to the crystallographic plane (001) (see
Fig. 1).
Fig. 3 illustrates the set-up up of the two different layer
packages that almost fulfill the conditions of threefold sym-
metry in both cases. The axis ratio a/b ϭ 1.745 of the
monoclinic C-centred unit cell deviates hardly at all from
the ideal value √3 of a hexagonal cell (dashed lines). In the
case of the TlI2TlIIIBr6 packages one TlI cation is closest
packed together with three Br atoms; between two layers
with the composition TlIBr3, the cations TlIII occupy the
octahedral voids that are enclosed by Br atoms. This pack-
age is representing a detail of the perowskite-type structure,
all the participating TlBr6 groups are orientated similarly.
In contrast to this, the cationic layers with the composition
[TlI4TlIIIBr6]ϩ contain four differently orientated, crystallo-
graphically independent TlBr6 complex anions. They are lo-
cated within the centres of hexagonal prisms of TlI that are
connected with each other by their lateral faces. Two of the
TlBr6-groups (Tl34, Tl35) point with edges to the base
planes. The octahedra are rotated approximately 120°
against each other, with the apices pointing along two a-
directions of the hexagonal pseudocell. The two other oc-
tahedra (Tl36, Tl37) don’t point with their apices into the
direction of the third a-axis but approximately parallel to c.
In spite of the distortion of the arrangement, the metrics of
the hexagonal pseudocell is maintained within these layers.
I
I
˚
Restricting the Tl ϪTl distances to 6 A and including the
additional TlI-atoms of the anionic layers above and be-
neath the cationic package, polyhedra in the form of bi-
capped hexagonal prisms around the TlBr6 anions result
which consist of 14 TlI atoms. Due to the incorporation of
the bulky TlBr6 groups, the TlI14-polyhedra are differently
distorted as demonstrated by means of Fig. 4. It emerges
from the distances that TlIϪTlI interactions don’t play a
˚
role because the contacts are longer than 4 A and therefore
out of range of bonding interactions. The realised space-
saving integration of the TlBr6 octahedra into the TlI14-po-
lyhedra results inevitably in rather short TlIϪBr distances
and as a consequence of this in more intense distortions
of the TlBr6 anions. Albeit, as mentioned above, in case of
Cs2TlBr5 and Cs3Tl2Br9 [12,14] intensely deformed TlBr6
groups were detected with distortions even drastically, and
Fig. 5 Detail of the crystal structure of Tl3[TlBr6] with view along
direction b at y ϭ 0. The direction of the perpendicular of the
layers corresponds to [103]. The translation amounts to 77.8 A and
˚
includes a succsession of 12 packages, i.e. 6 packages TlI4[TlIIIX6]ϩ
and 6 packages TlI2[TlIIIX6]Ϫ. The outlines of a detail that corre-
sponds to the unit cell of the K3[MoCl6]-type is plotted by das-
hed lines.
Z. Anorg. Allg. Chem. 2002, 628, 2675Ϫ2682
2679

R. Ackermann, C. Hirschle, H. W. Rotter, G. Thiele
differences concerning the geometric arrangement seem to
be due to polarization influences. Both structures may be
regarded as a distorted form of a monoclinic structure de-
rived from the cubic elpasolite-array [34] with the ratio of
the lattice parameters a:b:c of √3 : 1: √3 and the β-angle
corresponding to the tetrahedral angle. The group theoreti-
cal relations are composed in the following scheme:
Deformation Perowskite Ǟ A3BX6 structure: F m3m Ǟ
C 2/m, Z ϭ 4,
hexachlorothallates(III) of the alkali metals (K, Cs) adopt
the monoclinic K3[MoCl6]-type [32, 33]. On heating phase
transformations to cubic high temperature variants of the
elpasolit-type are observed [32], but unusually high thermal
displacement parameters indicate structural disorder of the
TlCl6 octahedra. The disorder may be associated with a
variation of the coordination of the cations [34]. In case of
the ordered elpasolite structure A2BMX6, the cations Aϩ
and Bϩ feature very different coordinations. The Aϩ cations
adopt sites with cuboctahedral, 12-fold , coordination
whereas the the Bϩ cations are coordinated octahedrally
like the M3ϩϪcations. The monoclinic structures of
K3[MoCl6] and Tl3[TlBr6] demonstrate how an equalization
of the coordination numbers is achieved by rotations of the
MX6 octahedra. Fig. 7 displays a detail of the monoclinic
α-Tl3[TlBr6]-structure that corresponds to an elpasolite-like
cubic face centered cell. By group theoretical relations the
calculations result in a cell with almost cubic dimensions of
am ϭ 1/2[Ϫaϩbϩ2c], bm ϭ 1/2[aϩb], cm ϭ 1/2[ϪaϩbϪ2c].
Deformation A3BX6 Ǟ K3MoCl6: C 2/m Ǟ k(2) Ǟ P 21/c,
Z ϭ 4.
Deformation A3BX6 Ǟ α-Tl3TlBr6: C2/m Ǟ k(2) Ǟ C2/c,
cЈ ϭ 2cm, C2/c Ǟ e(4) Ǟ C2/c, aЈ ϭ 2am, bЈ ϭ 2bm, Z ϭ 32.
In comparison to the monoclinic K3[MoCl6]-type the three
base translations a, b and c are doubled in the case of the
structure of α-Tl3[TlBr6]. As shown in Fig. 5 the doubled
translation c results because the corresponding TlBr6 po-
lyhedra are additionally slightly tilted against each other
(e.g. Tl36 and Tl37, marked by arrows). For the structure
of LTϪRb3[TlCl6] similar layer packages as in α-Tl3[TlBr6]
may be defined. As shown in Fig. 6 the package
Rb4[TlCl6]ϩ contains only two symmetry equivalent TlCl6
octahedra that differ in their orientations. In contrast to
this the corresponding package of Tl4[TlBr6]ϩ includes 8
differently distorted TlBr6 polyhedra. Accordingly, the
package with the composition Rb2[TlCl6]Ϫ in the structure
of Rb3[TlCl6] contains only two symmetry equivalent TlCl6
octahedra, whereas in the case of Tl3[TlBr6] eight TlBr6 oc-
tahedra are distributed to three symmetry independent gen-
eral sites.
˚
˚
˚
aЈ ϭ 10.998 A, bЈ ϭ 10.998 A, cЈ ϭ 10.994 A, αЈ ϭ 89.019°,
βЈ ϭ 90.981° and γЈ ϭ 91.772° by a cell transformation in
the terms of aЈ ϭ a/4ϩb/2ϩc/4; bЈ ϭ ؊a/4ϩb/2Ϫc/4; cЈ ϭ
a/4Ϫc/4.
Fig. 7 Illustration of a detail of the crystal structure of Tl3[TlBr6]
(left) corresponding to an unit cell of the cubic elpasolite type
structure (right).
Because Tl3[TlBr6] does not exist in a cubic HT-modifi-
cation the data of the HT-form of Rb3[TlBr6] with a ϭ
˚
10.89 A are used in Fig. 7 for comparison. The centres of
Fig. 6 Comparison of the arrangements for layers with the com-
position A4[TlX6]ϩ in the crystal structures of LTϪRb3[TlCl6] (left)
and Tl3[TlBr6] (right). View approximately parallel to the
(001)Ϫplane at z ϭ 0. All the TlX6 octahedra are located at inver-
sion centres.
the TlX6 groups nearly coincide for both structures, but the
rotations of the octahedra and the considerable displace-
ment of the TlI cations, that result on average in comparable
coordination numbers (see table 4), are in evidence. The
Figures of three different TlBr6 groups (see Fig. 8) exemp-
lify the rotations and the displacements of the atom sites in
comparison to the ideal elpasolite array. The orientations
of the octahedra that are assigned to the anionic Tl2[TlBr6]
layers (see Fig. 3) are retained almost unchanged, whereas
the TlBr6 octahedra within the cationic Tl4[TlBr6]-layers are
rotated much more (see Fig. 8). The crystal structure of α-
Tl3[TlBr6] exemplifies how the motif of a highly symmetric
basic structure with well balanced electrostatic charge dis-
tribution is maintained, even when ball-shaped anions are
substituted by complex groups. But this leads to a compli-
cated structure with seven crystallographically independent
TlBr6 octahedra and twelve different TlI cations.
The structural relationship to the cubic elpasolite array
¯
(A2BMF6, Spg.: Fm3m) [34] arises by the centres of the
TlBr6 groups forming an almost perfect cubic face centered
array. Furthermore, 2/3 of the TlI cations are surrounded
tetrahedrally by four TlBr6 groups wheras the remaining
third is surrounded octahedrally by six TlBr6 groups (see
Fig. 2). The structural relationship between the two struc-
ture types can be reconstructed by crystallographic group-
subgroup relations against the real background of phase
transitions that pass very quickly and nearly without hyster-
esis. At ambient temperature, Rb3[TlCl6] and other
2680
Z. Anorg. Allg. Chem. 2002, 628, 2675Ϫ2682

Mixed-Valence Thallium(I,III) Bromides
reliable clues for symmetry reduction either way. Ordered
models would require either a reduction of the trigonal
symmetry and the implementation of twin models or an
enlargement of the unit cell with retained trigonal sym-
metry. Accurate examinations of the data sets and also in-
spections of the reciprocal space by film methods displayed
no indications for superstructure reflections or splitting of
high angle reflection spots. The refinements based on a trip-
let model in the monoclinic space group P21/m converged
satisfactorily in the range of R1 ϭ 6%. But the parameters
were afflicted with high errors. These calculations led to
indications for a small surplus of bromine and for the com-
position Tl11Br17. Apparently the disorders within the cat-
ionic layers Tl4[TlX6]ϩ are conditioned by structural
reasons because similar problems occured in the case of all
structure analyses of the different forms the of mixed-val-
ence halides Tl2Cl3 and Tl2Br3. The single crystal data of
five different phases are compiled in Table 5. Up to now it
was possible to isolate three different forms of the chloride
and two forms of the bromide. Single crystals that were lar-
gely free from defects were found only in the case of the
two α-variants. Attempts for structure solutions of the β-
and γ-forms resulted in the problems of disorder within the
cationic layers Tl4[TlX6]ϩ as described for β-Tl2Br3, whereas
it was possible to refine the parameters of the atoms within
the layers Tl2[TlX6]Ϫ without any problems. These anionic
layers contain arrays with almost perfect trigonal symmetry
which determine the dimensions of the hexagonal pseudo-
cells. For the cationic layers Tl4[TlX6]ϩ no array with tri-
gonal symmetry is possible because the orientations of the
TlX6 polyhedra within the hexagonal prisms cannot fulfill
the conditions of trigonal symmetry due to shortage of
space (see Fig. 2). By loss of the three-fold rotation axis
it is possible to insert the octahedra within the layers as
demonstrated for the structures of the α-forms. The ob-
served disorder results from at least three different options
of orientation that exist for each of the layers.
Fig 8 Different orientation of the TlBr6 octahedra in relation
to the ideal positions of the octahedra within the elpasolite type
structure (dashed bonds). Tl 32: octahedra from anionic layers
TlI2[TlIIIBr6]Ϫ, no rotation. Tl 35 and Tl 36: octahedra from catio-
nic layers TlI4[TlIIIBr6]ϩ rotated about 45° along different axes.
Characterization of the β-Phase of Tl2Br3
The lattice parameters that were determined by single crys-
tal investigations suggest a structure very similar to that of
α-Tl3[TlCl6]. Due to the large error margins in the case of
all applied methods of determination, small differences in
the chemical composition may not be excluded because
larger samples contained always impurities of insoluble
TlBr and in the case of single crystal structure analyses un-
der occupations due to split positions cannot be ruled out.
Until now it was not possible to achieve a satisfactory solu-
tion of the crystal structure, although several attempts with
different crystals were undertaken, because the data sets
were falsified due to the formation of twins in conjunction
with trigonal pseudo symmetry. The measurements always
led to a monoclinic unit cell with hexagonal metrics that
normally should contain 12 formula units of Tl3[TlX6] (see
table 5). The structural models that were developed, dis-
played a close relationship to the set-up of α-Tl2Cl3, but
an AB-stacking sequence of double layers of Tl2[TlX6]Ϫ
/
Tl4[TlX6]ϩ along the direction c of the unit cell were de-
tected likewise. But the refinements of the parameters in
different space groups (e.g. C2/c, P2/m, P21/m) led to split
positions for some of the TlX6 polyhedra within the cationic
layers Tl4[TlX6]ϩ (see. fig. 3) in conjunction with unrealistic
displacement parameters and too short interatomic dis-
tances. Attempts to resolve the structural disorder and to
develope a regular structure model with lower symmetry
did not lead to satisfactory results because there were no
Support by the Deutsche Forschungsgemeinschaft, Bonn, and the
Fonds der Chemischen Indutrie, Frankfurt a. M. is gratefully
acknowledged.
References
[1] A. G. Lee, The Chemistry of Thallium, Elsevier Publishing
Company, Amsterdam-London-New York 1971, p 20-22.
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Table 5 Crystal data of thallium sequichlorides and bromides.
Compound
α-Tl2Br3
26.763
15.311
27.375
108.63
10630
β-Tl2Br3
13.124
13.130
25.550
α-Tl2Cl3
12.68
14.70
β-Tl2Cl3
25.581
14.763
26.393
108.84
γ-Tl2Cl3
12.658(9)
12.686(9)
25.04(2)
119.96(1)
3478
(ϳ√3)
12
*P21/m
290
˚
a/A
˚
b/A
˚
c/A
25.09
β/γ/°
119,91(1) 90
3
˚
V/A
3838
(ϳ√3)
12
*P21/m
320
4677
9433
a/b (√ ϭ 1.732) 1.748
1.725 (2a/b) 1.7328
16
P n a m
291.7
Z / Tl3[TlX6]
Space gr.
32
C2/c
323.2
32
C2/c
294.8
3
˚
VEZ/Z /A
Layers
Tl3[TlX6]
6
2
2
6
2
* Setting unique axis c
Z. Anorg. Allg. Chem. 2002, 628, 2675Ϫ2682
2681

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2682
Z. Anorg. Allg. Chem. 2002, 628, 2675Ϫ2682