Dynamics and crystal chemistry of tellurites - 1. Raman spectra of thallium tellurites: Tl2TeO3, Tl2Te2O5 and Tl2Te3O7

Article abstract of DOI:10.1016/S0022-3697(01)00192-5

Raman spectra of the three entitled crystals are analysed within the framework of a lattice-dynamical model treatment using preliminary obtained X-ray diffraction data. The short range atomic arrangement and spectrochemical peculiarities of these structures are jointly discussed, which is considered as an initial step for studying the nature of the glass phases in the xTl₂O+(1-x)TeO₂ system. The charged TeO₃^2- groups and the neutral TeO₂ quasi-molecules are proposed as the basic units forming the complex tellurite anions. However, no relevant characteristic frequencies can be indicated in the spectra since the interatomic separation in those units are highly variables and their vibrational states are mixed and delocalised.

Full text of DOI:10.1016/S0022-3697(01)00192-5

Journal of Physics and Chemistry of Solids 63 12002) 545±554
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Dynamics and crystal chemistry of tellurites
1. Raman spectra of thallium tellurites:
Tl2TeO3, Tl2Te2O5 and Tl2Te3O7
*
A.P. Mirgorodsky, T. Merle-Mejean , P. Thomas, J.-C. Champarnaud-Mesjard, B. Frit
Â
    Â
Laboratoire de Science des Procedes Ceramiques et de Traitements de Surface, UMR 6638 CNRS, Universite de Limoges, Faculte des Sciences,
123, avenue Albert Thomas, 87060 Limoges Cedex, France.
Received 10 April 2001; accepted 10 July 2001
Abstract
Raman spectra of the three entitled crystals are analysed within the framework of a lattice-dynamical model treatment using
preliminary obtained X-ray diffraction data. The short range atomic arrangement and spectrochemical peculiarities of these
structures are jointly discussed, which is considered as an initial step for studying the nature of the glass phases in the
xTl2O 1 11 2 x)TeO2 system. The charged TeO322 groups and the neutral TeO2 quasi-molecules are proposed as the basic
units forming the complex tellurite anions. However, no relevant characteristic frequencies can be indicated in the spectra since
the interatomic separation in those units are highly variables and their vibrational states are mixed and delocalised. q 2002
Elsevier Science Ltd. All rights reserved.
Keywords: D. Crystal structure
1. Introduction
electron distribution, i.e. to the peculiarities in the short-
range atomic arrangement.
Tellurites 1Te IV) and the related compounds are rather
promising materials for the optical device technology. They
manifest good visible and infrared light transmittance, and
have exceptionally high non-linear refractive index value n2.
Recent studies of TeO2-based glasses have revealed that the
latter value was the highest one found for oxide glasses and
could be up to 100 times as large as that of SiO2 [1,2]. This
becomes the most pronounced when the modi®ers contain
elements having lone electron pairs 1as example, for light
wavelength 1.5 mm, the n2 value of a commercial SiO2
1Herasil) equals 0.12 £ 10219 m2/W, and that for the
73.9mol%TeO2±23.5mol%TlO0.5±2.6mol% BiO1.5 glass is
about 9.1 £ 10219 m2/W). The origin of those properties is of
great interest for material science. Although its microscopic
nature is still far from the ®nal clarity, however, it is likely
that it would directly relate to the peculiarities of the valence
This is one of the points due to which the tellurite
structures can be considered among the most challenging
objects of the modern physics and chemistry of solids.
Actually, generally 1if not always), they possess peculia-
rities mainly originating from the individual electron
con®guration of Te atoms involving a lone electron pair
1E-pair). Because of the stereochemical activity of E-pairs,
the Te±O chemical bonds are situated on one side of the Te
atom, whereas the other side is a 'dead' zone of E-electrons
which form `empty' cavities of different shapes and
volumes. Due to such a sharp asymmetry of the atomic
arrangement of the ®rst coordination spheres, the TeOn
polyhedra have strong dipole moments which provide
considerable dipole±dipole contribution to the inter-
anion potentials. This factor differentiates the energetics
of the tellurium 1IV)±oxygen systems from most of the
other solid oxides.
The above mentioned features predispose the tellurite
structures to be microscopically labile with respect to the
cation±anion interactions, mechanically ¯exible with
respect to the macroscopic strains, and very sensitive to
* Corresponding author. Tel.: 133-55-45-72-00; fax: 133-55-45-
72-70.
Â
E-mail address: merle@unilim.fr 1T. Merle-Mejean).
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0022-3697101)00192-5

546
A.P. Mirgorodsky et al. / Journal of Physics and Chemistry of Solids 63 62002) 545±554
external electric ®elds, which displays itself in the extraor-
dinary polarisation characteristics.
1,100 mW) of the excitation line in order to avoid the
destruction of the samples. The spectral resolution was
about 4.5 cm21 at the exciting line.
Thallium tellurites are compounds in which all the cations
have E-pairs. Their crystal structures have complicated
geometry and low symmetry. To our knowledge, the data
on the vibrational spectra of thallium tellurite crystals are
limited to that of Tl2TeO3 presented in Refs. [3,4] in the
frame of a Raman study of several TeO2-based glassy
compounds. The vibrational properties of such crystals
have never been discussed within the framework of a treat-
ment including both aspects of the lattice dynamics and
crystal chemistry. Little is known about the atomic arrange-
ments in the relevant glasses, and their relations with the
structural and chemical peculiarities of the crystals are not
clear. In this situation, we have conducted a comparative
study of the vibrational and structural properties of crystals
and glasses for the xTl2O 1 11 2 x)TeO2 system.
In this paper, we present the Raman spectra of all the
crystalline phases found in that system. We analyse them
by using the results of the lattice-dynamical modelling
jointly with the previously published X-ray diffraction
1XRD) data on Tl2TeO3 [5], Tl2Te2O5 [6] and Tl2Te3O7
[7]. Methodologically, this can be regarded as a continua-
tion of our preceding structural and spectrochemical studies
of tellurium oxide polymorphs [8,9]. The properties of the
related glasses and the mechanisms of their crystallisation
will be discussed elsewhere [10].
3. Crystal structure
In thoroughly studying the xTl2O±11 2 x)TeO2 system by
XRD and Raman spectroscopy techniques, we have found
four crystalline phases lying in the interval 0.5 $ x . 0. The
crystal structures were de®nitively determined in our labora-
tory by X-ray single crystal diffraction methods for the three
of them: Tl2TeO3 [5], a-Tl2Te2O5 [6] and Tl2Te3O7 [7]. For
the fourth phase, b-Tl2Te2O5, only the space group and the
unit cell dimensions were determined [11]. The three former
structures have been exhaustively characterised and all the
experimental details relating to the XRD measurements
were given in the original papers [5±7]. Therefore, only a
brief description of those structures 1necessary for the
further discussion) are given below. Those structure are
shown in Figs. 1±3, and some of their characteristics are
given in Table 1.
Tl2TeO3 crystallises with the orthorhombic symmetry
1space group: Pban-D24h, unit cell parameters:
Ê
Ê
Ê
a ˆ 16.6011) A, b ˆ 11.07816) A, c ˆ 5.23818) A, Z ˆ 8).
As mentioned above, due to the strong stereochemical
activity of the lone pairs E, Te and Tl atoms have very
asymmetric coordination polyhedra: Tl12)O4 disphenoids,
Tl11)O3 and TeO3 pyramids. By sharing either edges or
corners, those polyhedra constitute Tl2TeO3 sheets along
1010) between which the lone pairs E are located 1Fig. 1a).
Within those sheets, the coordination polyhedra of Te atoms
can be considered as independent pyramidal ortho-anions
[TeO3]22 1Fig. 1b). The three nearly identical Te±O bonds
1see Table 1) form O±Te±O angles of values 96.2, 98.0 and
98.28. The `dead' zone around the lone pair E of the Te atom
involves three non-bonding Te´´´O contacts 13.06, 3.10 and
2. Experimental
All the samples 1Tl2TeO3, a-Tl2Te2O5, b-Tl2Te2O5, and
Tl2Te3O7) were prepared from various mixtures of high
purity Tl2CO3 and TeO2. The former was a commercial
product 1Aldrich, 99.9%). TeO2 was prepared by decompos-
ing commercial H6TeO6 1Aldrich, 99.9%) at 5508C. The
synthesis process implied ®rst heating 128C/min) at 3508C
for 18 h and then cooling down 128C/min) at 2508C for 24 h
the various mixtures in a gold crucible under pure ¯owing
nitrogen. Well crystallised powder samples thus were
obtained 1with grain size in the range about 2±10 m) and
directly used for the Raman spectra recording. The latter
was performed at ambient conditions in the 80±1200cm21
range using a Dilor spectrometer 1XY model) equipped with
a CCD detector and an Ar1 laser 1514.5 nm exciting line) in
a backscattering geometry. With such a detector, a good
signal/noise ratio needed one or at most two scans 1during
40±60 s). Several samples of each compound have been
studied, and the spectra were repeatedly recorded from the
different impact points of any sample. This was visually
controlled through a microscope 1 £ 100), which had
enabled us to analyse a surface with diameter of about
2 mm. The diameter of the laser spot focused on the sample
was about 1 mm. Beautiful reproducibility of the results thus
obtained was a proof of identity and homogeneity of the
samples. Measurements were made at a low enough power
Ê
3.37 A).
a-Tl2Te2O5 has monoclinic symmetry 1space group:
5
Ê
P21/n-C2h, unit cell parameters: a ˆ 7.11911) A,
Ê
Ê
b ˆ 12.13812) A, c ˆ 8.43912) A, b ˆ 114.2813)8, Z ˆ
4). All the atoms are in general positions 4c of the
P21/n space group. As in the Tl2TeO3 crystal structure,
the distribution of oxygen atoms around each cation is
highly anisotropic. For example, the shortest 11.8±
Ê
2.4 A) Te±O distances 1Table 1) correspond to distorted
disphenoids TeO4 with the lone pair E forming the ®fth
equatorial corner of a TeO4E trigonal bipyramid 1Table
1 and Fig.2). By sharing O11) and O12) corners and
O13)±O13) edges, those TeO4 disphenoids constitute
the Te2O5 two-dimensional network visualised in
Fig. 2a. However, the Te11)±O13) and Te12)±O11)
Ê
axial bonds with lengths of 2.16 and 2.33 A, respec-
tively, are clearly longer than the three other bonds of
each TeO4 disphenoid 1see Table 1); so taking into
account the preliminary considerations in Refs. [8,9]

A.P. Mirgorodsky et al. / Journal of Physics and Chemistry of Solids 63 62002) 545±554
547
Fig. 1. 1a) Projection onto 1001) of the Tl2TeO3 crystal structure 1arrows visualise the direction towards which are located the lone pairs E of Te
and Tl atoms). 1b) View of the independent pyramidal anion [TeO3]22
.
they can be regarded as electrostatic contacts separating
the structural fragments shown in Fig 2b.
[Te2O5]22 diortho-anions. In a-Tl2Te2O5, those complex
anions are interrelated by the inversion symmetry operation,
thus forming via the long Te11)´´´O13) contacts, so-called
Te2O2 double bridges, similar to those observed in b-TeO2
[12].
These fragments can be characterised as isolated diortho-
anion [Te2O5]22. Whereas their four terminal bonds 11.870,
Ê
1.873, 1.908 and 1.925 A) manifest a considerable asymme-
try, the intra-anion Te11)±O12)±Te12) bridge is quite
Tl2Te3O7 crystallises with the triclinic symmetry
1space group: P1-Ci1, unit cell parameters a ˆ
Ê
`balanced' 12.03 and 2.04 A). We wish to underline that
Ê
Ê
Ê
these bridges are formed by two axial bonds 1Te±axOax±
Te) which, to our knowledge, is a particular feature of the
6.83911) A, b ˆ 7.43211) A, c ˆ 9.92012) A, a ˆ 92.00
13)8, b ˆ 108.9513)8, g ˆ 112.8513)8, Z ˆ 2). The
Fig. 2. The a-Tl2Te2O5 crystal structure. 1a) spatial view visualising the Te2O5 sheets parallel to 11 0 21); 1b) the [Te11)Te12)O5]22 diortho-
anion.

548
A.P. Mirgorodsky et al. / Journal of Physics and Chemistry of Solids 63 62002) 545±554
Fig. 3. The Tl2Te3O7 crystal structure. 1a) A perspective view; 1b) the [Te3O7] fragment.
inversion centre is a single nontrivial symmetry opera-
tion. All the atoms are in general 2i positions. Of the
three crystallographically different tellurium atoms
1Te11), Te12), and Te13)), Te12) is bonded to three
oxygen atoms only 1thus forming TeO3 pyramid),
whereas the Te11), and Te13) are summits of distorted
disphenoids TeO4. By sharing either edges 1Te11)O4
polyhedra between them) or corners 1Te13)O4 polyhedra
with Te12)O3 pyramids on one side and Te11)O4 poly-
hedra on the other side), those polyhedra constitute
highly corrugated strips extending in®nitely along the
a axis 1Fig. 3a). However, the bond length criteria
used above to specify the isolated [Te2O5]22 anions in
the a-Tl2Te2O5 crystal structure allow us to propose a
three-fold coordination for the Te11) and Te12) atoms,
since their fourth neighbours, O16) and O17), are distant
Consequently, an independent complex anion [Te3O7]22
containing three TeO3 polyhedra 1Fig. 3b) can be
isolated.
Its rather asymmetric con®guration can be attributed to
the anisotropy of the short-range anion±anion and overall
cation±anion forces. Actually, the Te11)±O11) and Te13)±
O17) potentials provide the strongest interactions between
the complex anions 1via the
Oꢀ1†
Teꢀ1† S 2Teꢀ1†
Oꢀ6†
double bridges and the Te13)´´´O17) contacts respectively),
which is not the case for the Te12) atoms. Moreover, of the
two oxygen atoms O13) and O12), forming the two intra-
anion Te±O±Te bridges, the former is coordinated to a Tl
atom, whereas the latter is not, so that the two Te±O13)
Ê
from them by about 2.22 and 2.15 A, respectively.
Ê
bridging bonds are more disparate 11.912 and 2.108 A),
Table 1
Main tellurium-oxygen and thallium-oxygen distances in crystalline a-, b- and g-TeO2 and Tl-Te mixed oxides
Ê
Shortest Te±O distances 1A)
Ê
Shortest Tl±O distances 1A)
Reference
[17]
[12]
[8,9]
[7]
a-TeO2
b-TeO2
g-TeO2
Tl2Te3O7
2.12 1 £ 2); 1.88 1 £ 2)
1.87/1.89/2.07/2.15
1.858/1.945/2.022/2.202
Te11): 1.833/1.912/2.032/2.223
Te12): 1.837/1.898/1.912
Te13): 1.834/1.943/2.108/2.154
Te11): 1.908/1.925/2.034/2.162
Te12): 1.870/1.873/2.041/2.327
1.868/1.871/1.877
Tl11): 2.471/2.588/2.700/2.828
Tl12): 2.488/2.652/2.736/2.942
[6]
[5]
a-Tl2Te2O5
Tl11): 2.580/2.584/2.646/2.840
Tl12): 2.681/2.775/2.799/2.813
Tl11): 2.548/2.746/2.810
Tl2TeO3
Tl12): 2.508/2.636/2.663/2.870

A.P. Mirgorodsky et al. / Journal of Physics and Chemistry of Solids 63 62002) 545±554
549
and their average length is longer than that of Te±O12) ones
Ê
12.032 and 1.943 A). As for the ®ve non-bridging oxygen
of atoms per primitive cell. The values thus obtained were
the frequencies v of the longwave vibrations and their
shapes 1eigenvectors), the elastic constants Cik and related
values, and the derivatives dv/dK and dCik/dK.
atoms, three of them are bonded to only one tellurium
atom 1the O11), O14) and O15) atoms). Consequently the
corresponding three terminal Te±O bonds 1Te11)±O11),
Te12)±O15), and Te13)±O14)) are the shortest and have
First, we can note in brief that according to the model
estimations, the thallium tellurite crystals seem to be much
more mechanically compliant than the condensed TeO2
phases: their theoretical bulk moduli lie in the interval
10±12 GPa, whereas they are about 40 GPa for a- and b-
TeO2 [8]. This fact can be attributed to the weakness of the
Tl±O potentials essentially contributing to the compression
mechanism of the structures in question. However, the limits
of this paper do not allow us to analyse all the calculation
results, and we shall focus our attention on the consideration
of the Raman-active vibrations.
Ê
lengths of about 1.83 A which practically coincide with
those observed in the isolated gaseous TeO2 molecules [13].
The above lattices, Tl2TeO3, Tl2Te2O5 and Tl2Te3O7,
characterise three points of the structural evolution of the
xTl2O 1 11 2 x)TeO2 system, for x varying from 0.5 to 0.
The ®nal point of this evolution is TeO2, whose crystalline
phases are traditionally described as three-dimensional
networks built up from corner- or edge-sharing TeO4
disphenoids [8,9].
Keeping this traditional point of view, it can be said that
the decreasing x would drive the basic TeOn polyhedra to
change from a slightly distorted trigonal pyramid TeO3
The measured Raman spectrum of the Tl2TeO3 crystal in
Fig. 4A is in good correspondence with the previous data of
Refs. [3,4]. Since all the 48 atoms of the unit cell are in
general positions, the 144 Brillouin zone centre normal
coordinates are uniformly distributed between the eight irre-
ductible representations of the D2h group. Consequently, the
longwave vibration spectrum of Tl2TeO3 1144 2 3 ˆ 141
modes) has the following symmetry properties:
1distorted TeO3E tetrahedron) to
a TeO4 disphenoid
1TeO4E trigonal bipyramid), via an intermediary TeO311
unit in which one of the two axial bonds 1on average longer
than the two equatorial ones) lengthens up to about
Ê
2.22±2.33 A 1see Table 1). At the same time, the degree
of condensation of those TeOn polyhedra would increase
from isolated [TeO3]22 ortho-anions to a three-dimensional
network 1TeO2)1, via isolated [Te2O5]22 and [Te3O7]22
complex anions.
18 Ag 1 18 B1g 1 18 B2g 1 18 B3g 1 18 Au 1 17 B1u
117 B2u 1 17 B3u:
It is important to note that the island-like Tl2TeO3 crystal
structure consists of independent and very heavy structural
fragments: eight complex anions [TeO3]22 and 16 cations
Tl1 per unit cell. Their relative motions are governed by
forces corresponding to interatomic distances longer than
4. Raman spectra and lattice dynamics
The Raman spectra for all the above mentioned thallium
tellurite lattices are presented in Fig. 4. The high frequency
bands between 600 and 800 cm21 can be unambiguously
attributed to the Te±O stretching vibrations, mainly corre-
sponding to the motions of oxygen atoms, whereas the
lowest parts of the spectra 1,200 cm21) can be associated
with the motions of the heavy atoms Tl and Te. The origin of
most of the bands in the interval 200±600 cm21 is not so
evident. To make the situation more transparent and infor-
mative, lattice dynamical calculations are necessary.
Ê
2.5 A. Therefore, those forces are weak and the vibrations
of those fragments as rigid units would be considerably
softer than the 48 internal vibrations of the TeO3 pyramids.
Correspondingly, of the 72 Raman-active calculated
modes, 48 are situated below 200cm21. The 12 vibrations
lying in the interval 280±350 cm21 are chie¯y related to the
O±Te±O bending deformations of the TeO3 pyramids. The
most intense of them 1near 290 cm21) corresponds to the Ag
species. According to the calculations, the cation and rigid
complex anion motions are essentially mixed, and it can be
Such calculations have been conducted within the modi-
®ed Valence Force Field Model in which the two-body short
range Te±O, Tl±O and O±O interactions were taken into
account jointly with the three-body interactions inside the
O±Te±O and Te±O±Te bridges described via the diagonal
bending force constants of these bridges and via the non-
diagonal Te±O/Te±O force constants through atoms Te and
O. The two-body constants were estimated from the empiri-
cally found dependences of the relevant force constants K
on the interatomic distances 1Fig. 5). The bending force
constant values and those of the non-diagonal interactions
were transferred from the lattice-dynamical model, earlier
elaborated by us for the crystalline TeO2 polymorphs [8,9].
The calculations were performed by using the code
CRYME [14] expanded to the case of an arbitrary number
Ê
said that the strongest Tl±O forces 12.5±2.7 A) mainly
contribute to the modes between 120 and 200 cm21, whereas
the lowest part of the spectrum is dominated by weak inter-
pyramidal O±O interactions.
The 12 stretching vibrations of the Te±O bonds which
occupy the interval 600±750 cm21, are worth closer exam-
ination. Actually, although the TeO3 pyramid has the C1
trivial symmetry, the Te±O bond lengths, as well as the
values of the O±Te±O angles, do not differ drastically.
Therefore, its three stretching and three bending vibrations
can be described as the A1 1 E ones in the symmetry terms
of the C3v pyramid-like molecule. According to the
calculation, the group of highest frequency vibrations
1700±725 cm21) 1see Fig. 4A) corresponds to the stretching

550
A.P. Mirgorodsky et al. / Journal of Physics and Chemistry of Solids 63 62002) 545±554
Fig. 4. Raman spectra of the thallium tellurites. 1A) Tl2TeO3; 1B) b-Tl2Te2O5,; 1C) a-Tl2Te2O5,; 1D) Tl2Te3O7 and of the a- 1E), b- 1F) and g-
form 1G) of TeO2. The inset exhibits the details of 400±600 cm21 range.

A.P. Mirgorodsky et al. / Journal of Physics and Chemistry of Solids 63 62002) 545±554
551
Ê
Fig. 5. Force constants K 1mdyn/A) versus bond lengths l 1A) for Te±O, O±O and Tl±O potentials 1from top to bottom, respectively).
Ê
pulsations 1A1) of the TeO3 pyramids. Their totally
symmetric combination 1Ag species) produces the strongest
Raman band at 724 cm21. The other three combinations,
B1g, B2g, and B3g are practically not split and are situated
near 705 cm21. The range 600±700 cm21 contains the eight
lines related to the E stretching motions of the TeO3
pyramids.
The 105 longwave vibrations of the a-Tl2Te2O5 lattice
are distributed between the symmetry species as follows: 27
Ag 1 27 Bg 1 26 Au 1 25 Bu. The Raman spectrum of
a-Tl2Te2O5 contains fewer vibrations than that of Tl2TeO3
1see above), but in Fig. 4C it displays more lines since its
structure has a less regular short range organisation, which
favours the splitting of the vibrational modes. According to

552
A.P. Mirgorodsky et al. / Journal of Physics and Chemistry of Solids 63 62002) 545±554
the calculations, the vibrations forming the highest
frequency part of the spectrum 1600±720 cm21) are combi-
nations of the stretching motions of the terminal Te±O
1600±800 cm21). The calculations show that the most
intense of them can be associated with the synchronous
Ê
stretching motions of the Te±O bonds shorter than 1.95 A.
Ê
bonds 11.870, 1.873, 1.908 and 1.925 A). One of them,
Some of those bonds contribute to the Te±axOeq±Te bridges,
and their vibrations can be formally regarded as the asym-
metric ones of such bridges. However, if these bridges had
been built from pairs of covalent bonds, their asymmetric
vibrations could not be strong in the Raman spectra 1this
point is discussed in detail in Ref. [8]). Therefore, the strong
Raman intensities observed for the highest-frequency part of
the spectra in Fig. 4A±D indicate that the relevant bands
relate to the vibrations of the essentially terminal bonds.
Thus, from the spectrochemical point of view, all the
Te±Oeq bonds should be considered as terminal ones. This
means that the Te±axOeq±Te linkages in the Tl2Te2O5 and
Tl2Te3O7 crystals contain only one covalent bond 1Te±Oeq),
and the Te±Oax contacts cannot be attributed to this
category.
corresponding to the synchronous motions 1but with differ-
ent amplitudes) of those bonds, produces the strongest line
near 664 cm21
.
The asymmetric stretching vibrations of the intra-
anion Te±axOax±Te bridges have very weak intensity,
and lie near 500 cm21 1exceptionally low, in our
opinion), whereas the symmetric ones can be associated
with the intense band near 273 cm21. The symmetric
vibrations of the Te11)2O2 double bridges are situated
near 410 cm21, slightly lower than for their homologue
in b-TeO2 [8]. The bands in the interval 200±400 cm21
mainly originate from the deformations of the O±Te±O
angles, and the region below 200 cm21 can be attributed
to the motions of the heavy structural fragments 1as in
the spectrum of Tl2TeO3).
Firstly, such an issue allows us to recognise the Te2O5 and
Te3O7 groups in those two crystals as the isolated structural
fragments previously determined in Sec. II by using the
bond-length criteria. So the spectrochemical characteristics
of the high-frequency atomic vibrations of the thallium tell-
urites support the point of view according to which the
Te±Oax `bonds' 1longer than the sum of the covalent radii
of O and Te) should be considered as electrostatic contacts.
The existence of such contacts seems to be natural 1and
sometime even inevitable) for the tellurites and condensed
TeO2 crystal structures. Actually, any asymmetric 1polar)
TeOn polyhedron would necessarily have positive 1Te) and
negative 1O) extremities. The oppositely charged extremi-
ties of the neighbouring polyhedra would attract each other,
thus forming inter-polyhedron Te´´´O contacts longer than
The Raman spectrum of the metastable b-form of
Tl2Te2O5 is shown in Fig. 4B. Since the detailed crystal
structure is still unknown, no serious analysis of this spec-
trum can be done. However, it is likely that the position of
the strongest line 1751 cm21) indicates the presence in this
structure of rather short terminal bonds.
The 69 longwave vibrations of Tl2Te3O7 lattice belong to
the two symmetry species, 36 Ag1 33 Au. Most of the
Raman-active modes 136 Ag) seems to be seen in Fig. 4D.
According to the calculations, the high-frequency part of the
spectrum 1600±800 cm21) can be interpreted in terms of the
stretching vibrations of the [Te3O7]22 fragment, containing
nine Te±O bonds. Five of them with lengths in the range
Ê
1.83±1.91 A 1see Table 1 and Fig. 3) are terminal ones. The
four remaining bonds form the intra-anion Te±O±Te
Ê
2.0 A, which minimise the electrostatic 1dipole±dipole)
Ê
bridges via oxygen atoms O13) 11.912±2.108 A) and O12)
energy of the lattice. It is interesting to note that the so-
called Te2O2 double bridges 1i.e, the parallelepipeds in
which two opposite edges are short Te±O bonds, and the
other two edges are the Te´´´O contacts), as a rule, have
inversion symmetry, thus indicating an anti-parallel orienta-
tion of the dipole moments of the neighbouring units, i.e. the
largely electrostatic character of their interactions.
Ê
11.943±2.032 A). The calculations show that the highest
frequency band 1740±770 cm21) in Fig. 4D mainly origi-
nates from the vibrations of seven bonds: the above
mentioned ®ve terminal Te±O bonds and the two shortest
Ê
bridging ones 1Te12)±O13) ˆ 1.912 A and Te13)±
Ê
O12) ˆ 1.94 A). The strongest peak in that band can be
associated with the synchronous stretchings of all those
bonds. The region 400±600 cm21 can be mainly associated
with the Te±O±Te bridge vibrations. The lower-frequency
part of the spectrum, like in the above considered cases, can
be attributed to the deformations of the complex anions and
to their motions jointly with Tl1 cations.
Secondly, the following question arises: is our above
conclusion about the absence of the Te±O±Te covalent
bonding in Tl2Te2O5 and Tl2Te3O7 consistent with other
vibrational properties of these structures?
Therefore, we focus on the middle-frequency range
1400±600 cm21). Actually, the Raman spectra of ionic-
covalent crystalline oxides having the X±O±X bridges
manifest in this range a very strong band 1if this is
permitted by rules of symmetry) associated with the
symmetric stretching vibrations of such bridges. 1A
classic example of this is the spectra of SiO2 poly-
morphs 1see Ref. [8] for references).) A high Raman
intensity is inherent for these sorts of vibrations 1see
Section 4 in Ref. [8]).
5. Discussion
According to the lattice-dynamical modelling of the
Raman spectra of thallium tellurite lattices, the bands
above 200 cm21 in Fig. 4A±D can be reliably associated
with the internal deformations of the complex anions.
We will ®rst concentrate on the highest-frequency modes
We wish to recall now that the structural evolution of our

A.P. Mirgorodsky et al. / Journal of Physics and Chemistry of Solids 63 62002) 545±554
553
system from Tl2TeO3 to TeO2 corresponds to an increasing
condensation of the TeOn polyhedra from the isolated
[TeO3]22 ortho-anions to the [TeO2]1 networks of the
TeO2 polymorphs. Due to this, the number of the
Te±O±Te fragments inside the complex anions would
also increase. Consequently, on average, the intensity of
the middle frequency Raman lines would increase. Such a
general tendency can be seen in Fig. 4 1compare the spec-
trum of Tl2TeO3 consisting of isolated ortho-anions to that
of g-TeO2 containing polymerised Te±O±Te chains).
At the same time, no strong bands are present in the
interval 400±600 cm21 in the spectra of the Tl2Te2O5 and
Tl2Te3O7 crystals 1Fig. 4B±D) in which the symmetric
vibrations of the covalent Te±O±Te bridges had to lie
[8,9] 1similar to a strong band near 430 cm21 in Fig. 4G).
Thus, one more time, the spectrochemical evidence indi-
cates the absence of the Te±O±Te covalent bridges in
those crystals. So, it can be concluded that the condensation
of the TeOn polyhedra into the complex thallium tellurite
anions is not associated with their polymerisation 1i.e. with
the formation of the Te±O±Te covalent bridges).
Keeping in mind the above mentioned results, we will
concentrate now on the questions: 1a) how do the TeOn
polyhedra change during the structural evolution of our
system from Tl2TeO3 to TeO2? 1b) What are the elementary
units forming the Te2O5 and Te3O7 isolated anions, and how
are those units interconnected? 1c) How is the valence elec-
tron distribution arranged within the complex tellurite
anions?
According to the point of view implying the molecular
nature of the a- and b-form of TeO2, the question 1a) could
be readily answered: `they evolve from the charged
[TeO3]22 group to the neutral TeO2 quasi-molecule'.
Under the former term, we shall understand the TeO3 pyra-
Ê
mid with three short Te±O bonds of about 1.83±1.93 A; and
the latter term will correspond to the TeO4 disphenoid, simi-
lar to those present in the a- and b-TeO2 crystal structures,
in which two O atoms form the Te±Oeq bonds with lengths
Ê
around 1.86±1.90 A, and the two Oax atoms are separated
Ê
from Te by about 2.10 A and more.
Answering question 1b), it can be said that the structural
units forming the complex tellurite anions are: 1i) the
[TeO3]22 pyramids; 1ii) the TeO2 quasi-molecules; and
1iii) the [Te2O5]22 groups which can be regarded either as
diortho-anions 1i.e. two adjusted TeO3 pyramids sharing one
corner) or as two TeO2 pseudo-molecules keeping between
them an O22 anion 1TeO2 1 O22 1 TeO2). In the complex
tellurite anions, these three types of units can coexist being
mutually linked by Te´´´O potentials similar to those which
keep together the `pseudo-molecules' in a- and b-TeO2
crystals.
We have already analysed such a phenomenon in the
frame of a lattice-dynamical treatment of the TeO2 lattices
[8,9]. Traditionally the a- and b-TeO2 polymorphs are
considered as three-dimensional networks of TeO4 disphe-
noids sharing only corners in a-TeO2 and both corners and
edges in b-TeO2 [12,15]. Each TeO4 disphenoid contains
Ê
two equatorial bonds 12 £ 1.88 A in a-TeO2, 1.87 and
Ê
Ê
1.89 A in b-TeO2) and two axial bonds 12 £ 2.12 A in a-
Ê
TeO2, 2.07 and 2.15 A in b-TeO2) which form the single or
double dissymetric Te±eqOax±Te bridges. Such a de®nition
of the coordination polyhedra around Te atoms is usually
argued by the electrostatic estimations of the bond valence
values [16].
In connection with question 1c), we venture the hypoth-
esis that there are the following charge±structure correla-
tions for such units: 1i) the higher the TeO3 group is charged,
the more it resembles the isolated [TeO3]22 ortho-anion and
vice versa 1this seems to be in line with the quantum chem-
istry calculations [18]); and 1ii) the closer to zero the charge
of the TeOn polyhedron, the more its atomic arrangement
corresponds to the TeO2 quasi-molecule.
By looking at the [Te3O7]22 complex anion in Fig. 3b, we
can note that only the Te12) atom unequivocally forms a TeO3
group. Thus, according to the above hypothesis, this group
would concentrate the most part of the anion charge. There-
fore, we could conventionally decompose the [Te3O7]22 anion
as the sum of the three units: [TeO3]22 1 TeO2 1 TeO2, in
relating the ®rst unit with atom Te12), and the two TeO2
pseudo-molecules with the Te11) and Te13) atoms. Such an
interpretation is in agreement with the model analysis of the
vibrational spectrum of Tl2Te3O7: as was noticed in Section 3,
the strongest line in Fig. 4D 1near 744 cm21) can be associated
with the synchronous stretching motions of the seven Te±O
bonds forming the above three units. In the same way, it is
possible to assume that, as an alternative to the Te2O5 diortho-
group,the[Te2O5]22 anioncouldexistasthe[TeO3]22 1 TeO2
combination; it may be that this situation is realised in
b-Tl2Te2O5, the band at 751 cm21 in Fig. 4B corresponding
then to the pulsation of the TeO3 pyramid. In such a case, the
Another point of view comes from the lattice-dynamical
treatment of these two crystal structures [8,9]. Actually,
from the spectrochemical position, the difference between
the equatorial and the axial Te±O bonds is so important that
the existence of isolated TeO2eq `pseudo-molecules' 1inter-
connected via electrostatic Te´´´Oax contacts) in a-TeO2 and
b-TeO2 is more pronounced than the Te±eqOax±Te three-
dimensional polymerisation. Such an issue is not something
new. It just argues for the previously proposed opinion that
paratellurite 1a-TeO2) could be regarded as a molecular
crystal 1see Ref. [17] and references therein) rather than a
three-dimensional network.
At the same time, for the new metastable form g-TeO2
[8,9] in which half of the Te±Oax bonds shorten 1in compar-
ison with the values in the a- and b-polymorphs) to about
Ê
Ê
2.0 A and half lengthen up to 2.2 A, a well pronounced
effect of a one-dimensional Te±eqOax±Te polymerisation
can be seen in the Raman spectrum 1Fig. 4G). This allowed
us to consider the g-TeO2 crystal structure as a chain
structure 1like crystalline SeO2) in which each Te atom is
three-coordinated at the apex of a distorted TeO3 trigonal
pyramid.

554
A.P. Mirgorodsky et al. / Journal of Physics and Chemistry of Solids 63 62002) 545±554
Ê
Te±O bond lengths can be estimated to be about 1.85 A and
shorter. It is interesting to note that differently charged
complex anions as [Te5O11]22 and [Te3O8]42 in lead tellurites
[19,20] or [Te4O11]62 in chromium [21] and gallium [22]
tellurites can be regarded as various combinations of the
elementary structural units [TeO3]22 and TeO2.
quality of invited researcher of the CNRS of France. He
thanks Professor J.F. Baumard, director of Laboratory
SPCTS, for the invitation and for interest in this work.
References
Evidently, the proposed scheme is rather simpli®ed, but it
seems to be capable of drawing the main trends of the charge
distribution inside the complex tellurite anions, and of the
`structure±charge' correlation for the tellurium±oxygen
coordination spheres.
[1] S.H. Kim, T. Yoko, S. Sakka, J. Am. Ceram. Soc. 76 11993)
1061.
[2] B. Jeansannetas, S. Blanchandin, P. Thomas, P. Marchet,
Â
J.C. Champarnaud-Mesjard, T. Merle-Mejean, B. Frit,
V. Nazabal, E. Fargin, G. Le Flem, M.O. Martin, B. Bousquet,
L. Canioni, S. Le Boiteux, P. Segonds, L. Sarger, J. Sol. State
Chem. 146 11999) 329.
6. Concluding remarks
[3] J. Dexpert-Ghys, B. Piriou, S. Rossignol, J.M. Reau,
B. Tanguy, J.J. Videau, J. Portier, J. Non-Cryst. Solids 170
11994) 167.
By using lattice-dynamical and spectrochemical
arguments, we propose an interpretation of the structural
and chemical peculiarities of thallium tellurites, which
differs from the traditional description of the tellurium-
oxygen systems. Within the framework of this approach,
the tellurite 1IV) anions 1with exceptions for the mixed
TeIV-TeVI oxides in which the coordination of tellurium
atoms can be rather particular [23]) are regarded as consist-
ing of two types of basic structural units: the charged
[TeO3]2- groups; and the neutral TeO2 quasi-molecules.
Theoretically, it suggests that in the general case, the
`quasi-molecular' tellurium dioxide in reacting with a modi-
®er can provide the tellurite structures in which some TeO2
molecules are transformed into the charged TeO3 units, and
some molecules remain in the initial 1neutral) state.
Although this approach is schematic and simpli®ed, it
[4] T. Sekiya, N. Mochida, A. Ohtsuka, M. Tonokawa, J. Non-
Cryst. Solids 144 11992) 128.
[5] B. Frit, D. Mercurio, P. Thomas, J.-C. Champarnaud-Mesjard,
Z. Kristallogr. 214 11999) 439.
[6] P. Thomas, B. Jeansannetas, P. Marchet, J.C. Champarnaud-
Mesjard, B. Frit, Mat. Res. Bull. 33 11998) 1709.
[7] B. Jeansannetas, P. Thomas, J.-C. Champarnaud-Mesjard, B.
Frit, Mat. Res. Bull. 32 11997) 51.
Â
[8] A.P. Mirgorodsky, T. Merle-Mejean, J.-C. Champarnaud-
Mesjard, P. Thomas, B. Frit, J. Phys. Chem. Solids 61
12000) 501.
[9] J.-C. Champarnaud-Mesjard, S. Blanchandin, P. Thomas,
Â
A.P. Mirgorodsky, T. Merle-Mejean, B. Frit, J.Phys.Chem.
Solids 61 12000) 1499.
Â
[10] T. Merle-Mejean, A.P. Mirgorodsky, J.-C. Champarnaud-
allows us to clarify the main tendency in
a non-
Mesjard, P. Thomas, B. Frit, 1to be published).
[11] B. Jeansannetas, Thesis, University of Limoges, 1998.
[12] V.H. Beyer, Z. Kristallogr. 124 11967) 228.
[13] D.W. Muenov, J.W. Hastie, R. Hauge, Trans. Faraday Soc. 65
11969) 3210.
homogeneous distribution of the valence electron charge
within the complex tellurite anions.
Thus, it can be said, that, unlike such traditional glass-
former complex oxides as silicates, germanates, phosphates,
etc., in which the XO4 units vary their charges, but always
1independently of cations) keep the tetrahedron geometry
1and thus keep the characteristic peculiarities in their vibra-
tional spectra), in the tellurite anions, both charges and
atomic arrangements of the structural polyhedra are essen-
tially variable. Consequently, it is likely that the high
frequency stretching vibration spectra of the tellurites 1the
most informative from the point of view of the crystal chem-
istry) have not any conservative features typical for a given
structural type of the tellurite anions, and can be considered
as personalised characteristics of their short-range atomic
arrangements very sensitive to the inter-ion interactions.
[14] M.B. Smirnov, A.P. Mirgorodsky, P.E. Quintard, J. Mol.
Structure 348 11995) 159.
[15] O. Lindquist, Acta Chem. Scan. 22 11968) 977.
[16] I.D. Brown, J. Solid State Chem. 11 11974) 214.
[17] P.A. Thomas, J. Phys. C: Solid State Phys. 21 11988) 4611.
[18] S. Suehara, S. Hishita, S. Inoue, A. Nukui, Phys. Rev. B 58
11998) 14124.
[19] A. Oufkir, M. Dutreilh, P. Thomas, J.-C. Champarnaud-
Mesjard, P. Marchet, B. Frit, Mat. Res. Bull. 36 12001) 693.
[20] J.C. Devan, A.J. Edwards, G.R. Jones, I.M. Young, J.C.S.
Dalton 11978) 1528.
[21] G. Meunier, J. Frit, J. Galy, Acta Cryst. B32 11976) 175.
[22] M. Dutreilh, P. Thomas, J.C. Champarnaud-Mesjard, B. Frit,
Solid State Sciences 3 12001) 423.
[23] I. Alves Weil, Thesis, University of Bordeaux I 1France),
2000.
Acknowledgements
One of the authors 1A.P.M.) participated in this work in