The dehydration of SrTeO3(H2O) - A topotactic reaction for preparation of the new metastable strontium oxotellurate(IV) phase ε-SrTeO3

Article abstract of DOI:10.1039/c0dt01715a

Microcrystalline single-phase strontium oxotellurate(IV) monohydrate, SrTeO₃(H₂O), was obtained by microwave-assisted hydrothermal synthesis under alkaline conditions at 180 °C for 30 min. A temperature of 220 °C and longer reaction

Full text of DOI:10.1039/c0dt01715a

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PAPER
The dehydration of SrTeO3(H2O) - a topotactic reaction for preparation of
the new metastable strontium oxotellurate(IV) phase e-SrTeO3†
Berthold Sto¨ger,a Matthias Weil,*a Enrique J. Baran,b Ana C. Gonza´lez-Baro´,b Sylvie Malo,c
Jean Michel Rueff,c Sebastien Petit,c Marie Bernadette Lepetit,c Bernard Raveauc and Nicolas Barrier*c
Received 7th December 2010, Accepted 17th March 2011
DOI: 10.1039/c0dt01715a
Microcrystalline single-phase strontium oxotellurate(IV) monohydrate, SrTeO3(H2O), was obtained by
microwave-assisted hydrothermal synthesis under alkaline conditions at 180 ◦C for 30 min. A
temperature of 220 ◦C and longer reaction times led to single crystal growth of this material. The
crystal structure of SrTeO3(H2O) was determined from single crystal X-ray diffraction data: P21/c,
◦
3
˚
˚
Z = 4, a = 7.7669(5), b = 7.1739(4), c = 8.3311(5) A, b = 107.210(1) , V = 443.42(5) A , 1403 structure
factors, 63 parameters, R[F2>2s(F2)] = 0.0208, wR(F2 all) = 0.0516, S = 1.031. SrTeO3(H2O) is isotypic
with the homologous BaTeO3(H2O) and is characterised by a layered assembly parallel to (100) of
edge-sharing [SrO6(H2O)] polyhedra capped on each side of the layer by trigonal-prismatic [TeO3] units.
The cohesion of the structure is accomplished by moderate O–H ◊ ◊ ◊ O hydrogen bonding interactions
between donor water molecules and acceptor O atoms of adjacent layers. In a topochemical reaction,
SrTeO3(H2O) condensates above 150 ◦C to the metastable phase e-SrTeO3 and transforms upon further
heating to d-SrTeO3. The crystal structure of e-SrTeO3, the fifth known polymorph of this composition,
was determined from combined electron microscopy and laboratory X-ray◦powder diffraction studies:
3
˚
˚
P21/c, Z = 4, a = 6.7759(1), b = 7.2188(1), c = 8.6773(2) A, b = 126.4980(7) , V = 341.20(18) A , RFobs
0.0166, RBobs = 0.0318, Rwp = 0.0733, Goof = 1.38. The structure of e-SrTeO3 shows the same basic
=
set-up as SrTeO3(H2O), but the layered arrangement of the hydrous phase transforms into a framework
structure after elimination of water. The structural studies of SrTeO3(H2O) and e-SrTeO3 are
complemented by thermal analysis and vibrational spectroscopic measurements.
cationic, Y is the anionic component and E denotes the electron
1
Introduction
lone pair). Other potential building blocks for crystal engineering
of such materials are oxoanions of the type XO3 (X = AsIII,
n-
The market for materials exhibiting ferroelectric or nonlinear
optical (NLO) properties is steadily growing. The underlying
compounds have extensive technical applications in the fields
of capacitors, lasers, frequency-doubling and other electric and
optical devices. The basic prerequisite for such a class of
compounds is a non-centrosymmetric crystal structure and the
presence of a high polarisability. Ions with non-bonded s-electrons
such as SnII, SbIII, PbII, TlI or BiIII have a strong preference
for a peculiar stereochemistry and are frequently found in an
asymmetric environment in the crystal structures due to formation
of [MYnE] polyhedra missing a centre of symmetry (M is the
SIV, TeIV, SeIV), resulting in strongly distorted XO3E polyhedra.
For various technological applications the desired materials must
have both chemical and thermal stability over a wide temperature
range, ruling out most of the oxoarsenate(III), oxosulfate(IV)
and oxoselenate(IV) compounds which are all strongly prone to
hydrolysis, oxidation or premature thermal decomposition. The
most promising behaviour with respect to the desired stability
ranges of the above mentioned compounds is shown by metal
oxotellurates(IV), in particular by the heavier alkaline earth
elements with formula MTeO3 (M = Ca, Sr, Ba).
In fact, CaTeO3 and SrTeO3 show ferroelectric properties.1–3
Numerous studies have been performed on single crystal growth
and measurements of physical properties of CaTeO3 and SrTeO3,
including ferroelectric, piezoelectric and optical behaviour,4
dielectric5 and optical properties,6 electric conductivity,7 and
phase transition points.8 Nevertheless, the corresponding crystal
structures of the polymorphic MTeO3 phases were determined
only recently for M = Ca9 and M = Sr.10–13 In the majority
of cases, these MTeO3 (M = Ca, Sr) phases were prepared
aInstitute for Chemical Technologies and Analytics, Division of Structural
Chemistry, Vienna University of Technology, Getreidemarkt 9/164-SC, A-
1060, Vienna, Austria
bCentro de Qu´ımica Inorga´nica (CEQUINOR, CONICET/UNLP), Facul-
tad de Ciencias Exactas, Universidad Nacional de La Plata, C. Correo 962,
1900, La Plata, Argentina
cLaboratoire CRISMAT, UMR 6508 CNRS ENSICAEN, 6 bd Mare´chal
Juin, 14050, CAEN Cedex 4, France
† ICSD reference numbers 422355 and 422356.
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by typical solid state reactions with TeO2 and corresponding
alkaline earth carbo◦nates as reagents. Since at the required
temperatures (> 800 C) TeO2 has a noticeable vapour pressure
and can be oxidized under atmospheric conditions to Te(VI),
the reaction control for preparation of single phase products is
relatively complex. A more suitable method for the preparation
of microcrystalline MTeO3 (M = Ca, Sr) phases makes use of
the dehydration reaction of corresponding MTeO3(H2O) phases at
comparatively low temperatures (“chimie douce” reaction route)14
15
and has originally been applied for preparation of CaTeO3
and SrTeO3.16 However, when we made use of this method to
prepare microcrystalline SrTeO3 by thermal decomposition of
SrTeO3(H2O) at temperatures above 350 ◦C, we obtained a powder
X-ray diffraction pattern whose lines could neither be indexed on
basis of the crystal structures of one of the four known SrTeO3
polymorphs10–13 nor of any other known phase in the system Sr–
Te–O, viz. Sr3Te4O11,17 SrTe5O11,18 SrTeO4 or SrTe3O8.20
19
We therefore attempted to determine the crystal structure of
the obtained decomposition product from X-ray powder data and
were eventually able to show that it is in fact the fifth SrTeO3
polymorph which we will denote in the following as e-SrTeO3.
In this communication we also report on preparation, crystal
structure determination, thermal analysis and vibrational spectra
of the hydrous phase SrTeO3(H2O) which was employed as the
precursor material for preparation of e-SrTeO3.
2
Experimental
2.1 Synthesis of SrTeO3(H2O)
2.1.1 Microcrystalline material.
-by precipitation. The reagent Na2TeO3 was prepared according
to Brauer21 by melting stoichiometric amounts of TeO2 and
Na2CO3 at 800 ◦C. In a typical experiment for preparation of
SrTeO3(H2O), a solution of 378 mg (1.3 mmol) Sr(NO3)2 in 20 mL
water was added to a solution of 353 mg (1.4 mmol) Na2TeO3 in
10 mL water which resulted in an instant precipitation of a slightly
off-white solid. The suspension was boiled for ten minutes, and
the precipitate was then filtered off and subsequently washed with
mother liquor and acetone, and was finally dried in a desiccator
overnight. As can be seen from scanning electron microscopy
images (Fig. 1a), the peculiar feature of the microcrystalline
product is the very thin sheet-like form of the crystals which
assemble into rosebud-like agglomerates. This was also noted in
the powder diffraction pattern that shows a pronounced preferred
orientation along [100].
Fig. 1 Typical views of the forms of microcrystalline SrTeO3(H2O) as
observed by scanning electron microscopy: platy crystals forming rosebud–
like agglomerates and obtained from precipitation (top); plate-like crystals
obtained from microwave-assisted hydrothermal synthesis (bottom).
filtered off, washed with water, rinsed with ethanol and dried in
air. The crystallites showed likewise a plate-like form (Fig. 1b) but
appear thicker than those obtained from precipitation.
2.1.2 Single crystal material. Hydrothermal treatment of
a mixture of 221 mg Na2TeO3 and 267 mg SrCl2·6H2O in
3 mL of an aqueous 20%wt NH4OH solution in a Teflon-lined
autoclave at 220 ◦C for one week yielded different phases in single
crystalline form. Besides black needles of elementary tellurium,
17
colourless blocks of Sr3Te4O11 and colourless plates and blocks
-by hydrothermal microwave-assisted synthesis. Microcrystalline
SrTeO3(H2O) has also been obtained by microwave-assisted hy-
drothermal synthesis using a MARS 5 CEM m Waves Digestion
System according to the following procedure. In a Teflon-lined au-
toclave (XP1500+) one equivalent of strontium nitrate (Sr(NO3)2,
1.326 g, 6.27 mmol), one equivalent of tellurium dioxide (TeO2, 1 g,
6.27 mmol) and three equivalents of sodium hydroxide (NaOH,
0.752 g, 18.8 mmol) were mixed in 30 mL of distilled water. The
of SrTeO3(H2O) were obtained. The crystals were separated under
a polarising microscope and their composition determined by
single crystal diffraction. Phase analysis based on XRPD data
of the ground bulk material revealed an approximate phase
composition of 42%wt SrTeO3(H2O), 53%wt Sr3Te4O11 and 5%wt
Te. Some additional weak reflections could not be assigned to any
known phase.
◦
autoclave was then heated from room temperature to 180 C in
2.1.2 Synthesis of e-SrTeO3. Microcrystalline e-SrTeO3 was
prepared in situ by dehydration of SrTeO3(H2O) (obtained from
the hydrothermal microwave-assisted reaction route) at 390 ◦C
under vacuum for 4 h in a Anton Paar HTK1200N high
temperature chamber. At this temperature a sufficient quantity
30 min with a selected power of 400 W, held at this temperature
at the same power during 5 h and was finally cooled to room
temperature within 30 min. The final compound SrTeO3(H2O)
was obtained as a pure microcrystalline white powder and was
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of the dehydrated phase with a reasonable crystallinity necessary
for subsequent structure refinements based on electron diffraction
and Rietveld refinement could be prepared.
were collected in a continuous scan mode with a step scan
of ~0.009◦/2q ranging from 5 to 140◦/2q. Diagrams from the
temperature-dependent diffraction experiments were collected
under primary vacuum (~ 8.10-3 mbar) using a step scan of
~0.014◦/2q ranging from 9 to 80◦/2q, and were recorded every
20 ◦C between 30◦ and 610 ◦C.
2.2 Electron microscopy
Electron diffraction (ED) studies of microcrystalline SrTeO3 were
carried out with a JEOL 200CX electron microscope fitted with
an eucentric goniometer ( 60◦) and equipped with an energy
dispersive spectroscopy (EDS) analyzer. For the transmission
electron microscopy studies, the samples were reground in alcohol
and deposited on a perforated carbon membrane supported by a
copper grid. Simulations of ED patterns were calculated with the
JEMS v3.4705U2009 software.22
The electron diffraction measurements were carried out at room
temperature. The reconstruction of the reciprocal space, done
by tilting around crystallographic axes, evidenced a monoclinic
symmetry with at least a screw axis. The pronounced preferred
orientation of the crystallites did not allow recording all the basic
planes. Nonetheless, the derived monoclinic unit cell characterized
◦
˚
˚
˚
by a ª 6.8 A, b ª 7.1 A, c ª 8.7 A and b ª 127 , without any reflection
conditions, allowed to index all the ED patterns observed (Fig. 2a–
Scanning electron microscopy (SEM) was performed using a
Zeiss supra 55 electron microscope. The Sr : Te ratio was deter-
mined by energy-dispersive X-ray spectroscopy (EDS) analyses on
numerous crystallites and was close to the nominal composition.
c).
2.3 Thermal analysis
Thermogravimetric (TG) and differential scanning calorimetry
(DSC) measurements were carried out with a heating rate of 10 K
min-1 in a flowing nitrogen atmosphere (20 mL min-1). For the TG
measurements, a NETZSCH TG 209 F9 Tarsus system, working
with open aluminium oxide crucibles, was used. For the DSC
measurements, a NETZSCH DSC 200 F3 Maia and a Mettler-
Toledo DSC 25 system, both working with aluminium pans with
pierced lids, were employed.
2.4 X-ray diffraction
2.4.1 Single crystal X-ray diffraction of SrTeO3(H2O). Single
crystals suitable for the X-ray studies were pre-selected under a
polarizing microscope. Intensity measurements were then per-
formed at room temperature on a Bruker SMART-CCD area
diffractometer system, using Mo-Ka radiation and a graphite
monochromator. For the finally selected crystal, a complete
reciprocal sphere up to 62◦/2q was recorded. Intensity data
were processed with the SMART and SAINT software,23 and
absorption effects were corrected using the SADABS algorithm
that is based on a multi-scan approach.24 The crystal structure
of SrTeO3(H2O) was solved with direct methods and was sub-
sequently refined with full-matrix least-squares refinements on F2
with the SHELXTL software suite.25 In the final refinement cycles,
the displacement parameters of all non-H atoms were refined
anisotropically. The protons of the water molecule were clearly
discernible from difference maps. Their positions were refined with
Fig. 2 e-SrTeO3: Experimental ED patterns recorded at room tempera-
ture along a) [100] b) [101] c) [201], and simulated ED patterns with the
structural model determined from XRPD considering double diffraction
along (d) [100], (e) [101], and (f) [201].
The structure of e-SrTeO3 was solved from room temperature
X-ray powder diffraction data. A first pattern matching of the
experimental X-ray powder diffraction diagram on the basis
of the cell as determined from electron diffraction studies was
performed with the program JANA200626 in space group P2. A
careful observation of the indexed pattern showed that all (0k0)
reflections with k = 2n+1 and all (h0l) reflections with l = 2n+1 are
systematically absent. These reflection conditions are consistent
with space group P21/c. It should be noted that two weak
reflections at 25.16◦/2q and 25.81◦/2q could not be indexed and
presumably correspond to small amounts of an yet unidentified
by-product. The extraction of the structure factors using the Le
Bail method27 was carried out with JANA2006. The structure was
solved using Endeavour.28 The best solution was obtained with a
reliability factor of RF = 0.301 and all positions of the Sr, Te and
O atoms were directly found. Subsequent Rietveld refinements of
this model were performed with JANA2006. Preferred orientation
of reflections along the [100] axis was corrected assuming a
March and Dollase function.29 The refined preferred orientation
parameter is 0.690(7) which is consistent with the plate-like
crystallite form (Fig. 1). Anisotropic strain broadening of peaks
was corrected by the tensor method30 and peak asymmetry due
to axial divergence was also corrected according to the Finger,
˚
˚
distance restraints of d(O–H) = 0.95 A and d(H ◊ ◊ ◊ H) = 1.50 A and
a common Uiso(H) parameter. The largest residual electron density
peaks in the final difference map were inconspicuous, indicating
no additional atomic sites.
2.4.2 X-ray powder diffraction and crystal structure determi-
nation of e-SrTeO3. X-ray powder diffraction patterns were
collected on a Bruker D8 diffractometer using Cu-Ka1 radiation
˚
(1.54056 A), selected by an incident germanium monochromator,
and equipped with a Lynx-Eye detector and an Anton Paar
HTK1200N high temperature chamber. At room temperature
(without mounting the high-temperature chamber) the diagrams
5540 | Dalton Trans., 2011, 40, 5538–5548
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˚
Fig. 3 X-ray powder diffraction pattern of e-SrTeO3 (Cu–Ka1 radiation; 1.5406 A), with observed (dots), calculated (solid line), Bragg reflections (ticks)
and difference profiles obtained after Rietveld refinement.
Table 1 SrTeO3(H2O). Details of single crystal data collection and
structure refinement
Table 2 e-SrTeO3. Details of X-ray powder diffraction data and and
structure refinement
Diffractometer
SMART CCD (Siemens)
Diffractometer
Bruker D8 advance vario1
Cu-Ka1 radiation/1.5406
5–140
0.009
20(1)
March & Dollase - along [100]
axial divergence - Finger, Cox &
Jephcoat
Stephens
P21/c, 14
4
6.7759(1)
7.2188(1)
8.6773(2)
126.4980(7)
341.20(18)
263.22
˚
˚
¯
Radiation; l/A
Mo Ka; 0.71073
Radiation; l/A
T/◦C
22(1)
Angular range/◦2q
Step size/◦2q
Crystal dimensions/mm
Crystal color; shape
Space group, no.
Formula units Z
0.120 ¥ 0.072 ¥ 0.024
colourless block
P21/c, 14
4
7.7669(5)
7.1739(4)
8.3311(5)
107.210(1)
443.42(5)
281.24
T/◦C
Pref. orient. correction
Asymmetry correction
˚
a/A
˚
b/A
Anisotropic strain broadering
Space group, no.
˚
c/A
b/◦
Formula units Z
3
˚
˚
V/A
a/A
˚
Formula weight
b/A
m/mm-1
18.485
4.213
2.75–31.00
c/A
˚
X-ray density/g cm-3
Range qmin–qmax
Range
b (◦)
3
˚
V/A
Formula weight
h
-11 → 11
-10 → 10
-12 → 11
4956
1403
1320
X-ray density/g cm-3
Rietveld agreement indices
profile, Rp
5.122
k
l
0.0512
0.0733
12 Legendre polynoms
0.0948
0.0318
1.38
Measured reflections
Independent reflections
Obs. reflections [I > 2s(I)]
Rint
weighted profile, Rwp
background
corrected profile, cRp
derived Bragg R-factor, RBobs
Goof
0.033
Absorption correction
Trans. coef. Tmin; Tmax
Number of parameters
Ext. coef. (SHELXL97)
Diff. elec. dens. max; min
SADABS
0.2151; 0.6653
63
0.0019(4)
1.22 (0.76, Te);
–1.30 (1.14, Te)
0.0208
0.0516
1.031
422355
CSD number
422356
-
-3
˚
˚
[e A ] (dist./A, atom)
R[F2 > 2s(F2)]
wR2(F2 all)
dinates and isotropic displacement parameters of the structures of
SrTeO3(H2O) and e-SrTeO3 are given in Table 3, and anisotropic
displacement parameters for SrTeO3(H2O) are gathered in Table
4. Selected interatomic distances and angles of the two structures
are summarized in Table 5. The structure model of e-SrTeO3 as
determined from XRPD data was used for the calculation of the
ED patterns of the basal planes represented in Fig. 2d–f. The
variation of the intensity of the spots observed on the experimental
ED patterns (Fig. 2a–c) is similar to the one observed on the
simulated patterns (Fig. 2d–f), which confirms the validity of the
structural model. Drawings of structural details were produced
using the software DIAMOND.32
Goof
CSD number
Cox & Jephcoat approach.31 The final refinement converged with
reliability factors of RBobs = 0.0318 and profile factors of Rp =
0.0512; Rwp = 0.073; Goof = 1.38. The resulting X-ray diffraction
diagram is shown in Fig. 3.
Further details of the single crystal and powder data collections
and refinements are summarized in Tables 1 and 2. Atomic coor-
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2
˚
Table 3 SrTeO3(H2O) and e-SrTeO3. Atomic coordinates and equivalent isotropic displacement parameters Ueq = (1/3)RiRjUijai*aj*ai·aj/A
Atom
Wyckoff position
x
y
z
Ueq
SrTeO3(H2O)
Te
Sr
O1
O2
O3
OW
HW1
HW2
e-SrTeO3
Te
4 e
4 e
4 e
4 e
4 e
4 e
4 e
4 e
0.78482(2)
0.39762(4)
0.5674(3)
0.6973(3)
0.7776(3)
0.1432(3)
0.188(6)
0.17353(3)
0.35827(4)
0.3052(3)
-0.0211(3)
0.0758(3)
0.4112(4)
0.440(7)
0.50726(2)
0.66278(3)
0.4498(3)
0.3564(3)
0.7117(2)
0.3855(3)
0.303(5)
0.01131(8)
0.01280(8)
0.0151(4)
0.0166(4)
0.0216(5)
0.0279(6)
0.058(12)
0.058(12)
0.021(5)
0.414(8)
0.342(6)
4 e
4 e
4 e
4 e
4 e
0.37129(14)
0.86465(19)
0.2282(7)
0.3892(8)
0.0853(9)
0.16973(13)
0.37061(15)
-0.0291(9)
0.1085(11)
0.1461(10)
0.10244(12)
0.1324(2)
-0.0583(9)
0.3072(11)
0.4851(9)
0.0108(4)
0.0140(5)
0.010(19)
0.017(2)
0.020(2)
Sr
O1
O2
O3
-2
˚
Table 4 SrTeO3(H2O). Anisotropic displacement parameters A
Atom
U11
U22
U33
U23
U13
U12
Te
Sr
O1
O2
O3
OW
0.01030(11)
0.01497(14)
0.0133(10)
0.0203(10)
0.0220(11)
0.0171(11)
0.01264(11)
0.01133(14)
0.0146(10)
0.0126(10)
0.0294(13)
0.0473(17)
0.01116(11)
0.01183(14)
0.0177(10)
0.0167(9)
0.0135(9)
0.0176(11)
-0.00004(6)
0.00342(7)
0.00356(11)
0.0049(8)
0.0052(8)
0.0052(8)
0.0027(9)
-0.00074(6)
-0.00026(9)
0.0027(7)
-0.0015(8)
-0.0059(10)
-0.0082(11)
0.00000(8)
0.0003(8)
-0.0016(8)
0.0029(9)
0.0029(11)
◦
˚
Table 5 SrTeO3(H2O) and e-SrTeO3. Selected interatomic distances (A) angles ( ) for both structures as well as hydrogen bonding interactions for
SrTeO3(H2O)
SrTeO3(H2O)
Sr
—
—
—
—
—
—
—
O3
O2
O1
OW
O1
O1
O2
2.495(2)
2.520(2)
2.533(2)
2.586(2)
2.633(2)
2.639(2)
2.676(2)
2.583
Te
ø
—
—
—
O3
O2
O1
1.858(2)
1.867(2)
1.870(2)
1.865
3.113(3)
103.37(9)
98.95(9)
94.84(9)
99.05
—
Te
Te
Te
OW
O2
O1
O1
O3
O3
O2
ø
ø
D
OW
OW
H
HW1
HW2
A
O2
O3
D–H
0.88(3)
0.91(3)
H–A
1.83(3)
1.88(4)
D–A
2.703(3)
2.781(3)
∠(DHA)
173(5)
169(4)
e-SrTeO3
Sr
—
—
—
—
—
—
—
O3
O3
O2
O2
O1
O1
O3
2.350(8)
2.481(9)
2.531(6)
2.591(5)
2.694(9)
2.748(5)
2.964(7)
2.623
Te
ø
—
—
—
O1
O2
O3
1.763(10)
1.827(6)
2.052(6)
1.881
O1
O1
O2
ø
Te
Te
Te
O3
O2
O3
102.4(2)
105.0(3)
98.1(3)
101.8
ø
Additional crystallographic information on each structure
can be obtained from the Fachinformationszentrum
Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany,
E-mail: crysdata@fiz-karlsruhe.de, by quoting the reference
2.5 Electronic lone pair localisation
Ab initio calculations based on the Density Functional Theory
(DFT) were performed in order to accurately locate the 5s2
electron lone pairs of the tellurium atoms in both e-SrTeO3 and
SrTeO3(H2O) structures. The electronic structures were calculated
and the CSD-numbers listed at the end of Tables
and 2.
1
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with the CRYSTAL software33 using the B3LYP approximation,
Stuttgart core pseudo-potentials (ECP28MDF) and a valence
double zeta basis set for tellurium atoms,34 Hay-Wadt small-core
ECP with a 311(1d)G basis set for strontium atoms,35 a 8-411(1d)G
basis set for oxygen atoms,35 and 3-1(p)G basis set for hydrogen
atoms.36 The localization of the electron lone pairs was carried out
by an electron localization function (ELF) analysis with a code
provided by J. Contreras-Garcia.37 The 3D plots of the ELF were
produced with the software vmd.38
is that of the water molecule with a separation of 3.113(3) A which,
˚
owing to the remote distance, is considered as non-bonding.41
The SrTeO3(H2O) structure can be described as being con-
structed of edge-sharing [SrO6(H2O)] polyhedra forming extended
layers parallel to the (100) plane. The isolated [TeO3] units are
attached on each side of these layers which in turn are held
together along [100] by O–H ◊ ◊ ◊ O hydrogen bonding interactions
˚
at 2.703(3) and 2.781(3) A between the donor water molecules
and acceptor O atoms of the adjacent layers (Table 5). Like in
most of the known oxotellurate(IV) structures,42 the electron lone
pairs E are stereochemically active and are situated between the
layers (Fig. 5). Under consideration of the space requirement of
the electron lone pair E, the environment around tellurium can be
described as a distorted [TeO3E] y-tetrahedron. The electron lone
pair was located using the electron localization function (ELF)
method37 after performing DFT calculations (see section 2.5). The
lone pair points out close to the [100] direction: Te–E = 0.18458a +
0,0717b + 0.03824c (Fig. 5b). The geometric center of the electron
2.6 Vibrational Spectroscopy
The infrared spectra of microcrystalline SrTeO3(H2O) obtained
from the microwave-assisted hydrothermal method, were recorded
in the spectral range between 4000–400 cm-1 with a Bruker IFS 66
FTIR instrument, with the powdered samples dispersed in KBr.
Raman spectra were recorded with a FRA 106 Raman accessory
of the same instrument, using the 1064 nm line of a Nd:YAG solid
state laser for excitation. Far IR spectra were measured with the
compound dispersed in polyethylene discs, using a Perkin Elmer
System 2000 FTIR instrument.
˚
lone pair is at a distance of 1.46 A from the Te atom. The calculated
electronic charge associated with the lone pair is equal to 2.22
3
˚
electrons within a volume of 17.4 A .
The e-SrTeO3 phase forms in a topotactic dehydration reaction
from SrTeO3(H2O) above 150 ◦C (Fig. 4). After elimination of the
water molecules that are located at x ª 12 , the layer structure of
SrTeO3(H2O) is re-organised by condensation of adjacent layers.
The novel structure of e-SrTeO3 is closely related to that of
SrTeO3(H2O) but shows a considerable distortion of the complete
unit cell and in particular of the monoclinic angle (see Tables 1 and
2). Like SrTeO3(H2O), this dehydrated form can be described as
built up from SrO7 polyhedra and distorted y-[TeO3E] tetrahedra
as main building blocks. The [TeO3] units exhibit a similar but
distorted geometry to that of SrTeO3(H2O), with Te–O distances
3
Results and Discussion
3.1 Crystal Structures
SrTeO3(H2O) crystallizes isotypically with the structure of the
higher homologue BaTeO3(H2O).39‡ It is built up of distorted
[SrO6(H2O)] polyhedra and isolated [TeIVO3] units as the main
building blocks (Fig. 4a). The coordination around the alkaline
earth cations might be described as an intermediate between a
monocapped octahedron and a pentagonal bipyramid with Sr–
˚
O distances ranging from 2.495(2) to 2.676(2) A and an average
¯
˚
ranging from 1.764(10) to 2.052(6) A with a mean d(Te–O) distance
¯
˚
distance of d(Sr–O) = 2.583 A. The latter compares well with
◦
˚
and an (O–Te–O) angle of 1.881 A and 101.8 , respectively. The Sr–
the sum of the ionic radii of 2.59 A. The [TeIVO3] unit displays
40
˚
O distances exhibit a much larger variation in e-SrTeO3, ranging
the well-known trigonal-pyramidal geometry of XO3E groups
˚
from 2.350(8) to 2.964(7) A. This difference originates from the
¯
˚
showing an average distance of d(Te–O) = 1.865 A and a mean O–
three-dimensional nature of the [SrO3]• framework which induces
strong distortion of the strontium-oxygen polyhedra. In fact, the
[SrO3]• framework (Fig. 6a) can be described as built up of
distorted edge-sharing [SrO7] polyhedra that are best described
as monocapped SrO6 octahedra with one considerably longer Sr–
O distance. This 3-D framework delimits small tunnels running
along the b-axis where the Te4+ cations reside (Fig. 6b). The
Te–O angle of 99.05◦. The next-nearest O atom around the Te atom
‡ The structure of BaTeO3(H2O) was originally described in the non-
standard setting P21/a of space group no. 14. For better comparison
with the structure of e-SrTeO3, we chose the standard setting in P21/c
for description of the SrTeO3(H2O) structure.
Fig. 4 The crystal structures of SrTeO3(H2O) (a) and of e-SrTeO3 (b) in projections along their b-axes. The [TeO3] units are displayed as blue polyhedra
and O–H ◊ ◊ ◊ O hydrogen bonding interactions are shown as dashed lines.
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Compared with SrTeO3(H2O), the electronic lone pair in e-
˚
SrTeO3 was localised at a shorter distance of 1.38 A from the
tellurium atom, but likewise points out close to the [100] direction:
Te-E = 0.23481a + 0.07147b + 0.09576c. Moreover, the calculated
electronic charge increases to 2.65 electrons within a slightly
3
˚
smaller volume of 16.6 A . Like in SrTeO3(H2O), the y-[TeO3E]
tetrahedra are isolated and form double rows running along the
c-axis with two different orientations of the “O3” basal plane (Fig.
7a). A view of the assembly of the y-[TeO3E] tetrahedra along
[101] shows the orientation of the electronic lone pairs of Te4+ in
this structure (Fig. 7b).
For both SrTeO3(H2O) and e-SrTeO3 structures, the bond
valence sums43 were calculated with the parameters provided by
Brese and O¢Keeffe.44 As can be seen in Table 6, the individual
bond valence sums for each of the comparable atoms are very
similar (except the water molecule which behaves differently) and
are close to the expected value of 2 valence units (v.u.) for Sr, 4
v.u. for Te and 2 v.u. for the O atoms.
3.2 Thermal Behaviour
Two batches of microcrystalline SrTeO3(H2O), prepared by pre-
cipitation as well as by microwave-assisted hydrothermal synthesis,
were subjected to thermal analysis measurements.
SrTeO3(H2O) starts to decomposes above 150 ◦C under elim-
ination of the water molecule and subsequent crystallisation of
e-SrTeO3, identified by the endothermic DSC effect and in situ
powder X-ray diffraction (Fig. 8, 9). The observed mass losses of
6.95% of the precipitated and of 6.61% of the hydrothermally syn-
thesized material are in reasonable agreement with the theoretical
value of 6.41%. The slightly higher experimental values are most
likely caused by adsorbed water.
Differences in onset temperatures, both in the TG and DSC
experiments, pertain to the different crystallite size of the products
from the two batches. The precipitated product presents a smaller
crystallite thickness as showed in Fig. 1, and the water molecule is
released at slightly lower temperatures than in the corresponding
hydrothermally prepared product. The TG/DSC onsets for the
Fig. 5 a) Distribution of the y-[TeO3E] tetrahedra in SrTeO3(H2O)
viewed in a projection along the c-axis; b) 3-D plot of the ELF with a
isovalue of 0.75 along the c-axis.
main structural set-up of e-SrTeO3 is related to that of the four
previously reported SrTeO3 polymorphs, where irregular [SrOn]
polyhedra (n = 6,7,8) share corners or faces and are further
connected to isolated trigonal-pyramidal TeO3 units via edge-
sharing. The resulting channels have slightly different sizes and
shapes and also provide the required space for the lone-electron
pairs within the structures.10–13
Fig. 6 e-SrTeO3 : a) three-dimensional framework of [SrO3]• built up of corner- and edge-sharing SrO7 polyhedra along the b-axis; b) three-dimensional
framework of [SrO3]• forming small tunnels running along the c-axis where the Te4+ cations are situated.
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Fig. 7 Distribution of the y-[TeO3E] tetrahedra in e-SrTeO3: a) along the c-axis, forming double rows; c) along the [101] direction showing the electron
lone pairs E of the Te4+ atoms alternatively oriented up and down; b) 3-D plot of the ELF with a isovalue of 0.75 along the c-axis.
Table 6 SrTeO3(H2O) and e-SrTeO3. Individual bond valence sums (v.u.)
SrTeO3(H2O)
e-SrTeO3
Te
Sr
O1
O2
4.06
2.02
2.16
2.15
1.96
2.36
1.34
1.21
Te
Sr
O1
O2
O3
4.19
2.01
2.15
2.22
1.83
O3
OW
HW1
HW2
˚
˚
Bond valence parameters: Te–O: R0 = 1.97 A; B = 0.37 A; H–O: R0
=
˚
˚
˚
˚
˚
0.907 A; B = 0.28 A for d(H–O) < 1.05 A Sr–O: R0 = 2.118 A; B = 0.37 A
H–O: R0 = 0.990 A; B = 0.59 A for d(H–O) > 1.70 A. Bond valence =
exp((R0 - R)/B).
˚
˚
˚
precipitated material are 149 ◦C/173 ◦C and the TG/DSC onsets
for the hydrothermally prepared material are 161 ◦C/215 ◦C;
the peak maxima for both measurements are quite similar
with 308 ◦C for the precipitated material and 319 ◦C for the
hydrothermally prepared compound. The temperature-dependent
X-ray measurements reveal transformations into the anhydrous
phase above 170 ◦C for both samples.
Another endothermic effect corresponding to a phase transition
of the e-SrTeO3 polymorph is observed for both samples with
onsets at 580 ◦C for the precipitated material and at 594 ◦C for the
hydrothermally prepared material. This phase transition is clearly
visible in the temperature-dependent X-ray diffraction patterns for
both samples above 570 ◦C. The broadening of the reflection peak
widths and a decrease of the intensities are characteristic for a
loss of crystallinity of the formed phases. These features, together
with the similarity of the individual diffraction patterns of the
four other polymorphs (a - d) of SrTeO3, make it very difficult
to unambiguously assign the obtained phases. Nevertheless, some
differences in the resulting powder patterns of the two samples are
obvious. For both samples metastable e-SrTeO3 is transformed
primarily into d-SrTeO3,13 plus small amounts of a-SrTeO3 (II)10
for the original precipitated material and of g -SrTeO3 (II)12 for
Fig. 8 TG (blue) and DSC (black) curves of hydrothermally prepared
SrTeO3(H2O) (top) and of precipitated SrTeO3(H2O) (bottom).
the hydrothermally prepared material. We ascribe the observed
differences in phase formation likewise to the different crystallite
sizes of the original materials.
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Fig. 9 Powder X-ray diffraction diagrams recorded between 30 and 610 ◦C of the hydrothermally prepared SrTeO3(H2O) sample. The green lines at
170 ◦C and 230 ◦C show the beginning and the end of the dehydration process of SrTeO3(H2O); the green line 570 ◦C shows the beginning of the phase
transition of e-SrTeO3 into d-SrTeO3.
3.3 Vibrational spectroscopy of SrTeO3(H2O)
and all vibrational modes originate from four components, two
active in the infrared and the other two in the Raman spectrum.
The FTIR spectrum in the spectral range between 1600 and 400
cm-1 is shown in Fig. 10 and the Raman spectrum in the 1000–
200 cm-1 range in Fig. 11. The proposed band assignments are
presented in Table 8 and briefly commented, as follows:
In the free state, the isolated trigonal-pyramidal [TeO32-] anion
possesses C3v symmetry and its fundamental vibrations have
been assigned from Raman measurements in solution, as follows:
symmetric stretching (n1, A1) = 758 cm-1, symmetric bending (n2,
A1) = 364 cm-1, antisymmetric stretching (n3, E) = 703 cm-1,
antisymmetric bending (n4, E) = 326 cm-1.45 The four vibrational
modes are active in both the Raman and IR spectra.46
For the analysis of the spectra of microcrystalline SrTeO3(H2O),
we have performed a factor-group analysis of the unit cell,46–48 on
the basis of the described structure, correlating the symmetry of
the “free” anion (C3v), its site symmetry (C1) and its factor group
(C2h). The results of this analysis are presented in Table 7. As can
be seen in the crystalline compound, all degeneracies are removed
The stretching vibrations of the water molecule (not shown
in Fig. 9) generate a very broad feature centred at 2962 cm-1,
with a relatively broad and undefined shoulder at ca. 3230 cm-1.
The position of this band confirms the existence of relatively
short and moderate hydrogen bonds45,49 with O ◊ ◊ ◊ O separations
˚
of 2.703(3) and 2.781(3) A, respectively. On the other hand, the
water deformational mode, found at 1479 cm-1 in the IR spectrum,
presents an unusually low value.50 Finally, in the low energy
spectral range we have tentatively assigned the medium intensity
Table 7 SrTeO3(H2O). Factor group analysis of the TeO3 vibrations (P21/c (C2h) and Z = 4)a
2-
Vibrational mode
“Free” ion/C3v
Site symm./C1
Factor group/C2h
Symm. stretching (n1)
Symm. bending (n2)
Antisymm. stretching (n3)
Antisymm. bending (n4)
A1
A1
E
A
A
2A
2A
Ag + Bg + Au + Bu
Ag + Bg + Au + Bu
Ag + Bg + Au + Bu
Ag + Bg + Au + Bu
E
a Activity under factor-group symmetry: Ag, Bg: Raman active; Au, Bu: IR active.
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Table 8 SrTeO3(H2O). Assignment of the bands in the vibrational spectra
valuable criterion for the evaluation of the strength of coupling
effects in the unit cell51,52 and show that these effects are relatively
important in the present case. Interestingly, the overall Raman
pattern of SrTeO3(H2O) in this region shows a certain resemblance
to that of the recently reported Raman spectrum of the mineral
teineite, CuTeO3·2H2O.53
(band positions in cm-1)
Infrared
Raman
Assignment
ca. 3230 sh,br; 2962 s
1479 m
855 w, 833 m
755 sh, 687 vs
399 w, 357 s
323 vw
291 m
241s, 238 s, 232s
212 vw, 202 w
n(O–H) H2O
d(H2O)
823 w, 759 vs
727 w, 695 s
355 w
310 m
282 vw, 270 vw
238 w
ns(TeO3) (n1)
nas(TeO3) (n3)
ds(TeO3) (n2)
das(TeO3) (n4)
nL(H2O)
In the case of the TeO32- deformational modes the factor group
expectations are not totally fulfilled, probably due to the relatively
low intensity of the involved bands. The expected splitting is only
observed for the n2-mode in the IR spectrum. On the other hand,
both vibrations (n2 and n4) are particularly weak in the Raman
spectrum, as also observed in the case of teineite.53
See text
See text
205 vw
The groups of bands found below 250 cm-1 can surely by
assigned to external (lattice) vibrations.
vs: very strong; s: strong; m: medium; w: weak; vw: very weak; br: broad;
sh: shoulder
Acknowledgements
This work was supported in part by the Consejo Nacional de
Investigaciones Cient´ıficas y Te´cnicas de la Repu´blica Argentina
(CONICET) and the Universidad Nacional de La Plata. E.J.B. and
A.C.G.B. are members of the Research Career from CONICET.
Financial support from the Austrian Fonds zur Fo¨rderung der
wissenschaftlichen Forschung (FWF) under project P19099-N17
is gratefully acknowledged. The authors thank J. Contreras-
Garcıa for providing them the ELF program.
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