Crystal structure and dynamical properties of a new tellurite: AgTlTeO 3

Article abstract of DOI:10.1016/j.materresbull.2010.09.003

The structure of a newly synthesized AgTlTeO₃ crystalline compound has been solved from single crystal X-ray diffraction data and refined to a final reliability factor R1 = 0.037. It was found having an orthorhombic Iba2 space group symmetry with unit cell parameters a = 14.708(7) , b = 10.745(6) , c = 5.166(3) , Z = 8. Its lattice is divided into separated AgTlTeO₃ sheets parallel to [0 1 0] which are formed by TeO ₃ pyramids, TlO₄ disphenoids and AgO₆ octahedra sharing either corners or edges. At the same time, from the crystal chemistry point of view, it can be classified as a typical island-type compound made up from molecular-like [TeO₃]^2- ortho-anions weakly connected with Ag⁺ and Tl⁺ cations. The vibrational spectra and their interpretations complemented by the calculated elastic properties confirm this classification. The model-estimated piezoelectric constants allow characterizing AgTlTeO₃ as a strong pyroelectric structure.

Full text of DOI:10.1016/j.materresbull.2010.09.003

Materials Research Bulletin 45 (2010) 1883–1888
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Crystal structure and dynamical properties of a new tellurite: AgTlTeO3
b
a
b
b
b
b,
D. Linda a,b, M. Dutreilh-Colas , M. Loukil , A. Mirgorodsky , O. Masson , J.R. Duclere , P. Thomas
*
,
`
A. Kabadou a
a Laboratoire des Sciences des Mate´riaux et de l’Environnement, Universite´ de Sfax, Route de Soukra km 4, 3038 Sfax, Tunisie
b SPCTS UMR 6638 CNRS, Limoges University, Science and Technology Faculty, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France
A R T I C L E I N F O
A B S T R A C T
Article history:
The structure of a newly synthesized AgTlTeO3 crystalline compound has been solved from single crystal
X-ray diffraction data and refined to a final reliability factor R1 = 0.037. It was found having an
Received 8 June 2010
Received in revised form 19 August 2010
Accepted 2 September 2010
Available online 15 September 2010
˚
˚
orthorhombic Iba2 space group symmetry with unit cell parameters a = 14.708(7) A, b = 10.745(6) A,
˚
c = 5.166(3) A, Z = 8. Its lattice is divided into separated AgTlTeO3 sheets parallel to [0 1 0] which are
formed by TeO3 pyramids, TlO4 disphenoids and AgO6 octahedra sharing either corners or edges. At the
same time, from the crystal chemistry point of view, it can be classified as a typical island-type
compound made up from molecular-like [TeO3]2ꢀ ortho-anions weakly connected with Ag+ and Tl+
cations. The vibrational spectra and their interpretations complemented by the calculated elastic
properties confirm this classification. The model-estimated piezoelectric constants allow characterizing
AgTlTeO3 as a strong pyroelectric structure.
Keywords:
A. Oxides
C. X-ray diffraction
C. Raman spectroscopy
D. Crystal structure
D. Lattice dynamics
ß 2010 Elsevier Ltd. All rights reserved.
1. Introduction
anions necessarily involve, at the very least, one TeO3 pyramidal
ortho-group which is indicative of a tellurite compound [10]. The
Over the last years, the extraordinary optical properties of the
TeO2-based materials (which, from the chemical point of view, can
be usually classified as tellurites) have stimulated numerous
researches aimed at finding an optimal ‘‘modifiers-TeO2’’ compo-
sition in which such properties could be combined with quite
satisfactory mechanical and thermal resistance (see [1] and
references therein). Thus, it was found that the Tl2O modifier
addition allows maintaining the high optical non-linearity of the
TeO2-based materials, and, moreover, improving several other
characteristics which, unfortunately, do not include good stability
of the materials [2–4].
simplest of them, having anions with n = 1 (i.e., the [TeO3]2ꢀ ortho-
anions), are the subjects of fundamental quantum-chemistry
investigations [11], and are generally considered as the models
capable of clarifying the constitutions of the ortho-tellurite glasses
by performing relevant lattice-dynamical calculations [10,12,13].
All these compounds are characterized by highly asymmetric
coordination of oxygen atoms around Te4+ and Tl+ cations (MO3
trigonal pyramids and MO4 disphenoids (M = Te or Tl) are
classically observed [2,5–8]). This asymmetry is the direct
‘‘signature’’ of the presence of a stereo-active lone pair ns2 and
is induced by the fact that this lone pair is localized on one side of
the coordination polyhedra pushing the oxygen atoms to the
opposite side. On the contrary, when the lone pair is inert, the ns2
electronic pair remains localized with its center on the tellurium
atom and symmetric coordination environments around the cation
are obtained. These cases have been observed in recent structural
studies on oxide compounds containing lone pair elements (see for
example, Te4+ [14,15], Tl+ [16] Sn2+ [14], Pb2+ [17], I5+ [18]).
In principle, one of the possibilities to ameliorate the above
mentioned shortcomings of TeO2-Tl2O materials is the addition of a
second modifier. Consequently, a series of ternary TeO2-Tl2O-YkOl
systems are now under consideration in the SPCTS laboratory. In
particular, the case Y = Ag was studied, and the data related to the
relevant compounds are exposed in [19]. During that study, a new
crystalline structure AgTlTeO3 was found. In contrast to the
Tl2TeO3 [5,6] and Ag2TeO3 [20] crystalline lattices, AgTlTeO3 has no
During a relevant comprehensive study of the (1 ꢀ x)TeO2-
xTl2O system, three stable (Tl2TeO3 (x = 1/2),
and Tl2Te3O7 (x = 3/4)) and one metastable (
a-Tl2Te2O5 (x = 2/3)
b
-Tl2Te2O5) crystal-
line compounds were identified [2,5–8]. (Recently, the existence of
a high-temperature polymorph of Tl2TeO3 was evidenced in [9]).
2ꢀ
The analysis of the atomic arrangements of their [TenO2n+1
]
complex tellurite-anions, jointly with the corresponding data on
the other crystalline tellurites, allowed concluding that such
´
´
´
* Corresponding author at: Science des Procedes Ceramiques et de Traitements de
´
Surface, UMR 6638 CNRS, Faculte des Sciences et Techniques, 123 Avenue Albert
Thomas, 87060 Limoges Cedex, France. Tel.: +33 5 55 45 74 96,
fax: +33 5 55 45 72 70.
E-mail address: philippe.thomas@unilim.fr (P. Thomas).
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.09.003

1884
D. Linda et al. / Materials Research Bulletin 45 (2010) 1883–1888
inversion center, which potentially makes the scope of its physical
properties much wider, and relatively small volume of its primitive
cell makes it possible to suppose that AgTlTeO3 can be an
intriguing object for the ab initio model investigations similar to
those in Refs [11,21–23].
In the present paper, the experimental data on the structural
and vibrational (Raman and infrared) properties of the AgTlTeO3
lattice are communicated for the first time jointly with the lattice-
dynamical empirical treatment including estimations of its elastic
and piezoelectric constants.
liquid nitrogen-cooled CCD detector. Measurements were made at
low laser power (ꢂ2 mW on the surface of the sample), in order to
avoid any damage of the crystal. The scattered light was collected
through a microscope objective (x50LWD) and confocal filtering
(0.1 mm). The spectral resolution was about 2.5 cmꢀ1 at the
exciting line.
3. Results and discussion
3.1. Structure determination
AgTlTeO3 crystallizes with orthorhombic symmetry (Iba2
space group). The density measured by helium picnometry
2. Experimental methods
(
r
exp = 7.91 ꢃ 0.05 g.cmꢀ3) implies Z = 8 AgTlTeO3 per unit cell
AgTlTeO3 single crystals were synthesized by solid state
reaction of equimolar amounts of Tl2TeO3 and Ag2TeO3 crystalline
compounds. For this, Tl2TeO3 was first synthesized in the
laboratory by heating an intimate mixture of Tl2CO3 (Aldrich,
99.9%) and TeO2 (prepared in the laboratory by decomposition at
550 8C of commercial H6TeO6 (Aldrich, 99.9%)) in a platinum
crucible at 330 8C for 18 h, under pure flowing nitrogen and then
cooling it (5 8C/min) down to room temperature. Ag2TeO3 was also
prepared in the laboratory by heating a mixture of Ag2O (Aldrich,
99%) and TeO2 in a platinum crucible at 500 8C for 12 h under pure
flowing nitrogen and then cooling it (2 8C/min) down to room
temperature. The Tl2TeO3–Ag2TeO3 mixture was first melted in a
sealed gold tube at 600 8C for 12 h and then cooled down (0.02 8C/
min) to room temperature.
(rcalc = 7.94 g.cmꢀ3). The structure was solved by direct method
using the SHELXS 86 program [25]. The atomic coordinates and
anisotropic thermal parameters for all atoms were refined using the
SHELXL-97 program [26].
The structure of AgTlTeO3 was refined by assuming the above
mentioned non-centrosymmetric space group, undertaking and
evaluating the absolute-structure. The values of the Flack
parameter x [27] and its standard uncertainty u were equal to
0.48 and 0.02, respectively. From the full text of Flack and
Bernardinelli [28], it was deduced that the value of u = 0.02
indicates a strong inversion-distinguishing power, and that the
domains around the value of x should be well defined and clearly
distinguishable from one another.
Such a parameter value indicates that the crystal is twinned by
inversion. Consequently, it should be considered as constituted by
a mixture of domains with inverted structures, and its absolute
structure determination is not possible. A final refinement taking
into account these domains by way of the TWIN/BASF instructions
as recommended by SHELXL-97 program converged to final
reliability factors R1 = 0.0374 and wR2 = 0.0986.
Grey, transparent AgTlTeO3 single crystals suitable for X-ray
diffraction study thus were obtained. The sample selected was
prismatic (0.12 ꢁ 0.05 ꢁ 0.02 mm3). Intensity data were collected
with Kappa CCD (Nonius) diffractometer using monochromatized
˚
Mo K radiation (l = 0.71073 A) under conditions reported in
a
Table 1. A multi-scan method (SADABS [24]) was used for
absorption correction.
The refined atomic coordinates and thermal parameters are
presented in Tables 2 and 3. Table 4 reports the main interatomic
distances and angles calculated by using the Brown’s method
[29,30]. DIAMOND program was used for graphical representa-
tions [31].
Infrared absorption spectrum of the crystalline powder (dis-
persed in KBr or polyethylene) was recorded on a ThermoFisher
6700 spectrophotometer in the range 50 to 1000 cmꢀ1
.
Raman spectra of the crystalline compound was recorded in the
10–1100 cmꢀ1 range and in back-scattering mode at 514.532 nm
using a T64000 Jobin–Yvon spectrophotometer operating in triple
substractive configuration (1800 grooves/mm) equipped with a
3.2. Description of the structure
A projection of the unit-cell content structure onto the xOy
plane and its perspective view are shown in Figs. 1 and 2,
respectively. The atomic arrangements of various coordination
polyhedra of cations are represented in Fig. 3. As classically
observed in all thallium tellurium oxide compounds, the distribu-
tion of the chemical bonds around the Te and Tl atoms is highly
anisotropic, being influenced by strong stereochemical activities of
their electronic lone pairs 5 s2 and 6 s2, respectively.
Table 1
Crystal data and structure refinement conditions for AgTeTlO3.
Temperature
293(2) K
487.85 g
Formula weight
Wavelength
˚
0.71073 A
Orthorhombic
Iba2
Crystal system
Space group
˚
Unit cell dimensions
a = 14.708(7) A
˚
b = 10.745(6) A
˚
c = 5.166(3) A
˚
˚
The three shortest Te–O bonds (1.842(10) A, 1.859(15) A and
3
˚
Volume
816.5(8) A
˚
1.90(2) A, bond valence sum of 4.02) coming from each Te atom
Z
8
compose a distorted TeO3 triangular pyramid, so that the E lone
Data collection instrument
Density calculated
Absorption coefficient
F (000)
NONIUS Kappa-CCD
7.937 Mg/m3
51.065 mmꢀ1
Table 2
1632
2
˚
Atomic coordinates and equivalent isotropic displacement parameters (A ) for
u
range for data collection
5.188 to 21.928
ꢀ15 < h < 15, ꢀ11 < k < 11, ꢀ5 < l < 5
490
AgTeTlO3.
Index ranges
Reflections collected
Independent reflections
Refinement method
Atom
site
X
y
Z
Ueq
404 [R (int) = 0.0581]
Full-matrix least-squares on F2
1.055
Tl
8c
8c
8c
8c
8c
8c
0.40918(6)
0.25709(11)
0.39837(12)
0.2727(10)
0.4021(8)
0.12024(5)
0.12125(10)
0.37217(7)
0.3904(9)
0.5062(3)
0.0037(15)
0.0057(12)
0.010(7)
0.0182(6)
0.0252(7)
0.0098(6)
0.033(4)
0.056(5)
0.017(3)
(a)
Goodness-of-fit on F2
Ag
(b)
Final R [I > 2
s
(I)]
R1 = 0.0374, wR2 = 0.0986
R1 = 0.0441, wR2 = 0.1106
0.00034(6)
Te
R (all data)
O(1)
O(2)
O(3)
Extinction coefficient
Flack parameter
P
0.2008(9)
0.010(8)
0.48(2)
0.4006(10)
0.3619(10)
0.639(4)
(a) G.O.F. = { w(F0 ꢀ FC2)/(Nobs ꢀ Nparm)}1/2
.
Ueq is defined as one-third of the trace of the orthogonalized Uij tensor (esd are
given in parentheses)
2
P
P
P
(b) R1 (F) = jjF0j ꢀ jFCjj/jF0j; wR2(F2) = { [w(F0 ꢀ FC2)2]/ [w(F02)]}1/2
.
2

D. Linda et al. / Materials Research Bulletin 45 (2010) 1883–1888
1885
Table 3
Anisotropic displacement parameters (in A ) for AgTlTeO3.
2
˚
Atom
U11
U22
U33
U23
U13
U12
0.0006(3)
Tl
0.0129(7)
0.0109(13)
0.0034(9)
0.017(7)
0.021(7)
0.026(9)
0.0140(7)
0.0335(12)
0.0092(9)
0.036(8)
0.008(6)
0.0277(7)
0.0311(11)
0.0167(10)
0.045(8)
ꢀ0.0006(9)
ꢀ0.0008(15)
0.0022(13)
ꢀ0.036(13)
ꢀ0.039 (19)
ꢀ0.005(6)
ꢀ0.001(3)
0.006(3)
Ag
ꢀ0.0098(5)
ꢀ0.0009(3)
ꢀ0.009(5)
ꢀ0.009(5)
ꢀ0.008(6)
Te
ꢀ0.004(4)
ꢀ0.003(18)
ꢀ0.032(18)
ꢀ0.001(6)
O(1)
O(2)
O(3)
0.140(14)
0.008(7)
0.018(7)
The anisotropic displacement factor exponent takes the form Uij = exp [ꢀ2p2(h2a*2U11 + ꢄꢄꢄ + 2hka*b*U22 + ꢄꢄꢄ)] (Esd’s are given in parentheses)
pair of tellurium atom can be considered as a fourth vertex of a
TeO3E tetrahedron (Fig. 3). Those bonds, forming O–Te–O angles of
values 87.3(14), 91.9(13) and 97.8(5)8 occupy one semi-sphere
around the Te atom, whereas the other one is occupied by the lone
pair E of the Te atom, thus representing some sort of a ‘‘dead zone’’
for chemical bonding. Consequently, three non-bonding Te. . .O
or edges and to two TlO4 disphenoids via O(2) corners; (iii) each
AgO6 octahedron is linked to four TeO3 pyramids and four TlO4
disphenoids via O(1) corners and to six AgO6 octahedra by sharing
O1–O1 edges.
3.3. Dynamical properties
˚
˚
˚
contacts (2.939 A, 3.038 A and 3.274 A) occurring in that zone form
empty cavities (Figs. 1 and 3).
3.3.1. Lattice dynamical model
The coordination polyhedron around the atom of Tl can be
described as a TlO4 disphenoid in which the lone pair E of Tl is
pointing in the direction of the fifth apex of a TlO4E trigonal
bipyramid (Fig. 3). In this disphenoid, the four Tl–O bonds have
The lattice dynamical model treatment was carried out to
interpret the vibrational spectra and to numerically estimate the
piezoelectric and elastic properties of AgTlTeO3. The LADY (LAttice
DYnamics) code, which is an extended version of the CRYME
(CRYstal MEchanics) program described in detail in [32], was used.
˚
lengths ranging from 2.678(15) to 2.74(4) A (bond valence sum of
0.93). Two of them (bonds with O(1) and O(3)) are relatively short,
and two others (with O(2)) are longer. The localisation of the lone
pairs E of thallium atoms in the opposite way of these four Tl–O
bonds results in the formation of empty zones in the structure
(Figs. 1 and 3). In contrast to Te and Tl atoms, Ag atoms (having no
lone pairs) are homogeneously surrounded by six oxygen atoms
forming a distorted octahedron with Ag–O distances ranging from
T[F(.giT)1_IF$DG] he model calculations were performed by using the Valence Force
˚
˚
2.298(11) A to 2.902(10) A (bond valence sum of 0.94). From the
packing-factor point of view; the AgTlTeO3 structure can be
described (analogously to Tl2TeO3 and Ag2TeO3) as a distorted
NaCl-type structure (Fig. 1). From the crystal chemistry side, it is a
typical island-type one made up from well separated fragments,
namely, the molecular-like [TeO3]2ꢀ ortho-anions formed by
mainly covalent Te–O bonds, and Tl+ and Ag+ cations electrostati-
cally linked with the anions.
By sharing either edges or corners, TeO3 pyramids, TlO4
disphenoids and AgO6 octahedra constitute AgTlTeO3 sheets
parallel to the [0 1 0] direction (see Figs. 1 and 4). It is worth
noting that the appearance of such sheets is due to the occurrence
in the structure of the localized lone pairs of Te and Tl atoms which
play the role of ‘‘structural scissors’’. In considering Figs. 1 and 4,
one can see that: (i) the cationic ordering exists thus inducing the
formation of the successive single layers of the TeO3-TlO4/AgO6/
TeO3-TlO4 polyhedra parallel to the [0 1 0] direction; (ii) each TlO4
disphenoid is connected to three TeO3 pyramids by sharing corners
Fig. 1. Projection along c-axis of the AgTlTeO3 unit cell content (arrows visualize the
stereochemically active lone pairs E of Te and Tl; the ‘‘z-positions’’ of Te, Tl and Ag
atoms are indicated).
[F(ig._2)TD$FIG]
Table 4
˚
Selected bond distances (A) and angles (8) in AgTlTeO3 (esd are given in
parentheses).
Tl-O1(i)
Tl-O2(ivi)
Tl-O3
2.678(15)
2.71(4)
O1-Tl-O3(i)
O2-Tl-O2(ivi)
O3-Tl-O2(ivi)
O1-Tl-O2(i)
O1-Ag-O1(i)
O3-Ag-O1(ii)
O2-Ag-O3(ii)
O1-Ag-O1(iii)
O2-Ag-O1(i)
O3-Ag-O1(i)
O2-Te-O1
89.6(4)
88.9(7)
86.0(4)
88.3(8)
160.0(7)
89.9(4)
148.7(8)
174.4(4)
97.2(6)
86.3(10)
97.8(5)
87.3(14)
91.9(13)
2.689(12)
2.74(4)
Tl-O2
Ag-O2
2.298(11)
2.429(16)
2.519(10)
2.59(4)
Ag-O3(ii)
Ag-O1(iii)
Ag-O1(i)
Ag-O1(ii)
Ag-O1
2.65(4)
2.902(10)
1.842(10)
1.859(15)
1.90(2)
Te-O2
Te-O1
O2-Te-O3
Te-O3
O1-Te-O3
Symmetry transformation to generate equivalent atoms: i: ꢀx + 1/2, ꢀy + 1/2, z ꢀ 1/
2; ii: ꢀx +1/2, ꢀy + 1/2, z+ 1/2; iii: ꢀx + 1/2, y ꢀ 1/2, z; ivi: x, y, z ꢀ 1
Fig. 2. Perspective view of the AgTlTeO3 unit cell content.

[
D. Linda et al. / Materials Research B
5
ulletin 45 (2010) 1883–1888
30
25
20
15
10
5
0
200
400
600
800
Wavenumber (cm-1)
Fig. 5. Experimental and calculated Raman spectra of AgTlTeO3.
three-body interactions inside the O–Te–O bridges. The diagonal
approximation was used to describe all the interactions, with
exception for Te–O ones for which non diagonal Te–O/Te–O force
constants were introduced. The Te–O bond constant, the O–O
interactions, the bending force constant values and those of the
non-diagonal interactions were transferred from the lattice-
dynamical models, earlier elaborated for various crystalline
tellurite structures [10,36,37] while the Tl–O and Ag–O force
constant values were estimated as 0.07 and 0.1 mdyn/A, respec-
tively, by fitting the calculated Raman and IR-spectra to the
experimental ones. The vibrational modes were characterized by
their frequencies, Raman– and IR–intensities, and atomic dis-
placement patterns (eigenvectors given in the space of the
Cartesian atomic displacements).
Fig. 3. The various cation coordination polyhedra. Notations for atoms are those
used in Figs. 1 and 2. Arrows vizualize the direction towards which the lone pairs E
are directed.
Field (VFF) approach. The IR– and Raman–intensities of the modes
were estimated by using the Rigid Ion Model (RIM) and Polarizable
Bond Model (PBM) approximations (see e.g. [33] and references
therein), respectively. The elastic and piezo-electric constants
were calculated by using the VFF and RIM parameters and
theoretical formulas obtained in Refs [34,35].
3.3.2. Lattice vibrations
The group-theory analysis shows that the 72 brillouin zone-
center normal modes of the AgTlTeO3 lattice consist of the 69
vibrations distributed between irreducible representations of the
C2v factor group as follows:
During the calculations, two-body short range Te–O, Tl–O, Ag–O
and O–O interactions were taken into account jointly with the
[(Fig._4)TD$FIG]
G
(vibr) = 17A1 + 18A2 + 17B1 + 17B2, to which three rigid lattice
translations along z, x and y (A1 + B1 + B2) must be added. All the
vibrations are Raman-active, whereas the A2 ones are forbidden in
the IR-spectrum. The measured Raman– and IR– spectra of
AgTlTeO3 are presented in Figs. 5 and 6, respectively, jointly with
the model results.
[F(ig._6)TD$FIG]
2,00
1,75
1,50
1,25
1,00
0,75
0,50
0,25
100
200
300
400 500 600
700
800
900 1000
Wavenumber (cm-1)
Fig. 6. Experimental and calculated infrared spectra of AgTlTeO3.
Fig. 4. Perspective view of one AgTlTeO3 sheet parallel to [0 1 0] direction.

D. Linda et al. / Materials Research Bulletin 45 (2010) 1883–1888
1887
Table 5
Calculated piezoelectric dij (10ꢀ11 C/N) and elastic Ckl (GPa) constants for AgTlTeO3.
ij=
15
24
-19.04
33
31
-24.11
31
32
33
dij
8.71
-0.31
0.05
kl=
11
37.3
22
21
32
44
6.1
55
66
Ckl
16.9
41.6
6.3
12.1
3.7
11.1
2.8
Taking into account the island-type nature of the AgTlTeO3
lattice (see Section 3.2), their 69 optically–active vibrations can be
specified in the terms of the internal (stretching and bending)
vibrations of the TeO3 anions, and their external (rotational and
translational) motions to which the translational motions of Tl and
Ag should be added. Consequently the vibrational spectra of
AgTlTeO3 can be described as consisting of 12 Te–O stretchings
(3A1 + 3A2 + 3B1 + 3B2), 12 O-Te-O bendings (3 A1 + 3A2 + 3B1 +
3B2), 12 rotational modes R (3 A1 + 3A2 + 3B1 + 3B2) of the TeO3
units, and 33 translational modes T (8A1 + 9A2 + 8B1 + 8B2) formed
by the relative displacements of those units jointly with the Tl+ and
Ag+ cations. Taking into account that the latter 45 vibrations (12
R + 33 T) would be chiefly governed by weak oxygen-cations forces
(and, moreover, include the motions of heavy bodied like TeO3, Tl
and Ag), their positions should lie in the lowest-frequency domain
of the spectra. This prediction is readily confirmed by the shapes of
the experimental Raman and IR spectra which show that most of
the vibrational bands are located below 200 cmꢀ1, and the data
deduced from the analysis of the calculated eigenvectors are in line
with their above given descriptions.
From this analysis, the highest-frequency bands (600–
800 cmꢀ1) can be unambiguously attributed to the Te–O bond
stretching vibrations, mainly corresponding to the motions of
relatively light oxygen atoms. As to the bands lying in the interval
200–600 cmꢀ1, their origins are not so unequivocal, being related
to the O–Te–O bending forces mixed with those coming from the
strongest O–O, Tl–O and Ag–O interactions. In comparing the three
just indicated domains in Fig. 5 (Raman) and Fig. 6 (IR) one can see
that their positions in the Raman scattering and IR-absorption do
not differ markedly, thus indicating the weakness of the factor-
group splitting.
molecule [38]. However, according to our model calculations, the
high splitting and intensities of the band lying near 719 and
635 cmꢀ1 in the Raman spectrum of AgTlTeO3 originate from the
strong asymmetry of TeO3 pyramids (according to Table 5, the Te–
O bond lengths are very different). Consequently, the language of
the trivial symmetry seems to be much more convenient for this
TeO3 pyramids structure than that of C3v group.
Generally, the Raman and IR– spectra of the AgTlTeO3 lattice are
in full correspondence with its crystal chemistry characteristics
given in Section 3.2. Those spectra, in fact, reflect the properties
intrinsic in ‘‘genuine’’ ortho-tellurite structures (like e.g., Tl2TeO3
and Ag2TeO3) to which the AgTlTeO3 structure should be
apparently attributed.
3.3.3. Elasticity and piezoelectricity
Belonging to the C2v crystallographic class, the AgTlTeO3 lattice
would have a permanent (but temperature-dependent) macro-
scopic polarization defined by a vector P oriented along the Z axis.
The crystal polarisation variations induced by homogeneous
stresses (in the Voigt notations [39]) specify its tensor of
piezoelectric modules dij. The non-zero elastic and piezoelectric
matrix elements for AgTlTeO3 are presented in Table 5.
It can be mentioned that the piezoelectric constant values thus
obtained are close to the relevant characteristics found for the best
oxide piezoelectrics, which can be also regarded as a marker of a
high pyroelectricity of AgTlTeO3. We venture the opinion that, if so,
strong second-harmonic generation effects can then be expected
for it.
The model-estimated values of the Ckl elastic constants given in
Table 5 seem to be quite reasonable, with exception for the shear
constant values which can be suspected as to be underestimated
because of the exclusively two-body character of the forces
considered in coordination polyhedra around Ag and Tl atoms.
According to the calculations, the external vibrations of TeO3, as
well as the Tl and Ag cation motions are mainly governed by weak
˚
forces corresponding to interatomic separation longer than 2.3 A
originating from the anion–anion and anion–cation interactions.
The cation motions are essentially mixed with [TeO3]2ꢀ complex
anion translations and rotations. Since, on the one hand, the
4. Conclusion
The structural and vibrational properties of a new AgTlTeO3
crystal were determined, allowing us to classify it as a typical
island–type compound whose anionic part is formed from the
practically isolated [TeO3]2ꢀ ortho- anions, whereas the cationic
part is made from the Ag+ and Tl+ ions. The model-estimated elastic
constants show a strong anisotropy of the shear elasticity thus in
agreement with the layer-like crystal chemistry constitution of
this structure, whereas the values of calculated piezoelectric
tensor elements specify it as a strong pyro-electric in which,
theoretically, second-order non-linear optical effects can be
generated.
˚
shortest Ag–O interactions (2.3 A), are stronger than the Tl–O ones
˚
(2.6 A), and on the other hand, the Ag atoms are markedly lighter
than the atoms of Tl and the TeO3 units are, the modes lying
between 170–220 cmꢀ1 were found to be mainly related to the
motions of Ag, whereas the lowest part of the spectrum is
dominated by Tl–O bond, and by weak interactions between TeO3
pyramids. The existence of exceptionally low–frequency bands
(down to 20 cmꢀ1) predicted by our model calculations both for
Raman and IR spectra seems to be quite adequate to physical
reality as it can be seen in Fig. 5, and can be correlated to a typical
feature of the ortho-tellurite vibrational spectra due to a great
mass of the TeO3. (The recent ab initio calculations of the
vibrational spectra of the Li2TeO3 lattice [11] revealed several
modes lying near 30 cmꢀ1).
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