Synthesis and characterization of the wide band-gap compound Pr2Te4O11

Article abstract of DOI:10.1016/S0925-8388(03)00190-7

Single crystals of Pr₂Te₄O₁₁ have been grown through the reaction of Pr₆O₁₁ and TeO₂ in a CsCl flux at 1123 K. This compound, which is isostructural with Nd₂Te₄O₁₁ and Ho₂Te₄O₁₁, crystallizes in space group C2/c of the monoclinic system with four formula units in a cell of dimensions at 153 K of a = 12.6880(6) ?, b = 5.2361(3) ?, c = 16.2920(8) ?, β = 106.052(1)°, and V = 1040.17(9) ?³. The three dimensional structure is made from the interconnection between _∞²[Pr₂O₁₀^14-] and _∞²[Te₄O₁₁^6-] networks. Pr₂Te₄O₁₁ is a Curie-Weiss paramagnetic with an effective magnetic moment of 3.59(2) μ_B. Optical diffuse reflectance spectroscopy shows typical 4f-4f optical transitions for Pr(III), in addition to a wide band gap of 3.65 eV.

Full text of DOI:10.1016/S0925-8388(03)00190-7

Journal of Alloys and Compounds 354 (2003) 115–119
L
www.elsevier.com/locate/jallcom
Synthesis and characterization of the wide band-gap compound Pr2Te4O11
*
Ismail Ijjaali, Christine Flaschenriem, James A. Ibers
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113, USA
Received 20 August 2002; accepted 13 January 2003
Abstract
Single crystals of Pr2Te4O11 have been grown through the reaction of Pr6O11 and TeO2 in a CsCl flux at 1123 K. This compound,
which is isostructural with Nd2Te4O11 and Ho2Te4O11, crystallizes in space group C2/c of the monoclinic system with four formula
3
˚
˚
˚
˚
units in a cell of dimensions at 153 K of a512.6880(6) A, b55.2361(3) A, c516.2920(8) A, b5106.052(1)8, and V51040.17(9) A .
The three dimensional structure is made from the interconnection between 2`[Pr2O142] and `2 [Te4O62] networks. Pr2Te4O11 is a
10
11
Curie–Weiss paramagnetic with an effective magnetic moment of 3.59(2) mB. Optical diffuse reflectance spectroscopy shows typical
4f–4f optical transitions for Pr(III), in addition to a wide band gap of 3.65 eV.
2003 Elsevier Science B.V. All rights reserved.
Keywords: Synthesis; Crystal structure; Solid-state compound; Praseodymium; Tellurite
1. Introduction
addition of NH4OH. The resultant precipitate of TeO2 was
filtered and washed with deionized water. Its purity was
established by means of a powder X-ray diffraction
pattern. A CsCl (Strem, 99.9%) flux in approximately five
times molar excess was used to promote single-crystal
growth. The starting materials were mixed together and
placed in a fused-silica tube, which was then evacuated to
|1025 Torr and sealed. The tube was heated to 1123 K at
10 K/h, kept at 1123 K for 6 days, cooled at 2.5 K/h to
873 K, and then the furnace was turned off. The reaction
mixture was washed free of chloride salts with water and
then dried with acetone. Pale green plate-like crystals of
Pr2Te4O11 suitable for X-ray structure analysis were
produced in very high yield. The compound is stable in air
and water. Semiquantitative analyses performed with a
Hitachi 3500N SEM confirmed the presence of Pr and Te
in the approximate ratio of 1:2.75, in reasonable agreement
with that of 1:2 from the X-ray structure determination.
Oxygen was detected but could not be quantified.
Rare-earth oxides of the general formula Ln2Te4O11
(Ln5La–Lu) afford the systematic study of the spectro-
scopic properties of the trivalent lanthanide ions [1–6].
Moreover, oxides containing a metal center with a lone
pair of electrons, including the present case of Te(IV), are
important luminescence activators [7,8]. Structural studies
on the Ln2Te4O11 (Ln5La–Lu) compounds are confined
to X-ray powder diffraction measurements except for
single-crystal studies of Nd2Te4O11 [9] and Ho2Te4O11
[10]. Here we describe the synthesis and structure of
Pr2Te4O11, together with its magnetic and optical prop-
erties.
2. Experimental
2.1. Synthesis
Single crystals of Pr2Te4O11 were prepared from the
reaction of Pr6O11 (Aldrich, 99.999%) and TeO2 in a
molar ratio of 1:4. TeO2 was synthesized by dissolving Te
metal in aqua regia at 353 K, followed by the slow
2.2. Crystallography
A single crystal of Pr2Te4O11 was mounted on the end
of a glass fiber and placed in the cold stream of a Bruker
SMART-1000 CCD diffractometer [11]. The crystal was
kept at 153 K throughout the data collection. X-ray
diffraction data were collected with the use of mono-
*
Corresponding author. Tel.: 11-847-491-5449; fax: 11-847-491-
2976.
E-mail address: ibers@chem.northwestern.edu (J.A. Ibers).
˚
chromatized Mo Ka radiation (l50.71073 A). The dif-
0925-8388/03/$ – see front matter
doi:10.1016/S0925-8388(03)00190-7
2003 Elsevier Science B.V. All rights reserved.

116
I. Ijjaali et al. / Journal of Alloys and Compounds 354 (2003) 115–119
Table 1
2.3. Magnetic susceptibility measurements
Crystal data and structure refinement for Pr2Te4O11
Formula weight
Space group
968.22
C2/c
A 100.7-mg sample of Pr2Te4O11 single crystals was
ground to a fine powder; the purity of the sample was
checked by powder X-ray diffraction methods. The powder
was loaded into a gelatine capsule. Magnetic measure-
ments were carried out with a SQUID magnetometer
(MPMS5, Quantum Design). The magnetic susceptibility
measurements were made at 1000 G over the temperature
range 5–300 K with a zero-field cooling (ZFC) procedure.
Data were corrected for the diamagnetic contributions of
the atomic cores [14].
˚
a (A)
12.6880(6)
5.2361(3)
16.2920(8)
106.052(1)
1040.17(9)
4
153(2)
0.71073
6.183
˚
b (A)
˚
c (A)
b (8)
Volume (A )
3
˚
Z
T (K)
˚
l (Mo Ka) (A)
rc (g/cm3)
Crystal dimensions (mm)
0.1230.1230.08
203.0
m (cm21
)
Transmission factors
0.107–0.259
5725/1268
0.0185
2.4. Diffuse reflectance spectroscopy
Total reflections/unique reflections
R(F)a (Fo2 .2s(F2o))
Rw(F2o)b (all data)
0.0507
An optical diffuse reflectance spectrum was measured at
293 K with a Cary 1E UV–Visible spectrophotometer
equipped with a diffuse reflectance apparatus and a Varian
Halon plate as a reference. Data were collected in the
wavelength range 200–900 nm.
a R(F)5SuuFou2uFcuu/SuFou.
b Rw(F2o)5[S w(Fo2 2Fc2)2 /S wF4o]1 / 2, w215s2(F2o)1(0.023Fo2)2 for
F2o .0; w215s2(F2o) for F2o #0.
fracted intensities generated by a scan of 0.38 in v were
recorded on sets of 606 frames at f angles of 08, 908, 1808,
and 2708. The exposure time was 15 s/frame. Final unit
cell parameters were obtained by a global refinement of the
positions of 4597 reflections with the use of the processing
program SAINT Plus [11]. A face-indexed absorption
correction was applied with the use of the program XPREP
[12] and the program SADABS [11], which relies on
redundancy in the data, was then used to apply some
semi-empirical corrections for frame variations. The struc-
ture was solved with the use of the direct-methods program
SHELXS and refined by full-matrix least-squares tech-
niques with the use of the program SHELXL of the
SHELXTL suite of programs [12]. The program STRUC-
TURE TIDY [13] was used to standardize the positional
parameters. The final refinement included anisotropic
displacement parameters. Additional crystallographic pa-
rameters are given in Table 1. Positional parameters and
equivalent isotropic displacement parameters are given in
Table 2.
3. Results and discussion
Pr2Te4O11 is isostructural with Nd2Te4O11 [9], prepared
in the same manner, and with Ho2Te4O11 [10], prepared in
an excess of TeO2 as the flux. The structure of Pr2Te4O11
viewed down the b-axis is displayed in Fig. 1. The PrO8
polyhedron is a distorted square antiprism. The PrO8
polyhedra share three O–O edges to form a network
parallel to the (a,b) plane that can be formulated
2`[Pr2O142] (Fig. 2). Within this bidimensional network,
10
each PrO8 polyhedron shares three edges with three similar
Table 2
Atomic coordinates and equivalent isotropic displacement parameters for
Pr2Te4O11
a
Atom
x
y
z
Ueq
Pr
0.118934(16)
0.119832(18)
0.370479(18)
0.02662(15)
0.20675(15)
0.25055(15)
0.35236(16)
0.42763(15)
0
0.24417(3)
0.22957(6)
0.27757(7)
0.0948(4)
0.0394(3)
0.0664(4)
0.0547(3)
0.1205(3)
0.3542(5)
0.537623(13)
0.203026(16)
0.128620(14)
0.10135(12)
0.44219(12)
0.12967(12)
0.32733(12)
0.04715(12)
1/4
0.00484(9)
0.00552(9)
0.00439(9)
0.0093(4)
0.0096(4)
0.0102(4)
0.0104(4)
0.0102(4)
0.0114(6)
Te(1)
Te(2)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
a Ueq is defined as one third of the trace of the orthogonalized Uij
tensor.
Fig. 1. View of the structure of Pr2Te4O11 down the b-axis.

I. Ijjaali et al. / Journal of Alloys and Compounds 354 (2003) 115–119
117
˚
distance at 2.451(2) A; the analogous distances from atom
˚
Te(2) are 1.870(2), 1.883(2), 1.885(2), and 2.714(2) A. As
shown in Table 4, these Te–O distances are comparable to
those found in Nd2Te4O11 [9] and Ho2Te4O11 [10] when
the lanthanide contraction is taken into account. By sharing
2
`
62
11
O corners, the TeO4 polyhedra link to generate [Te4O
]
layers parallel to the (a,b) plane (Fig. 3). These layers
2
142
connect through corners to the [Pr2O ] network to form
`
10
the three-dimensional structure.
To interpret the Te coordination, the bond valence
model [17–19] can be applied. When only the three short
Te–O interactions are considered, bond valences of 3.71
e2 for Te(1) and 3.91 e2 for Te(2) result. If the fourth
distance is taken into account, then the bond valences are
3.99 e2 and 4.05 e2, in good agreement with that expected
for Te(IV) and with the values of 3.97 and 3.92 e2
calculated previously for Nd2Te4O11 [9].
The magnetic susceptibility and the reciprocal suscep-
tibility as a function of temperature for Pr2Te4O11 are
shown in Fig. 4. The compound shows Curie–Weiss
paramagnetism over the temperature range 90–300 K, with
x 5 x0 1 C/(T 2up) where x0 55.47.1025 emu mol21
,
C53.23(1) emu K21 mol21, and up 5215.2(2) K. The
effective magnetic moment per Pr(III) cation obtained
from the Curie constant C is 3.59(2) mB, in excellent
agreement with the theoretical value of 3.58 mB [20]. The
deviation from Curie–Weiss behavior at low temperature
reflects the splitting of the ground state under the crystal
field effect [20]. The negative value of the paramagnetic
temperature up is probably indicative of non-cooperative
magnetic interactions [21–23]; to verify this would require
a low-temperature neutron diffraction study.
Fig. 2. View down [001] of the 2`[Pr2O142] network in Pr2Te4O11
.
10
polyhedra. The Pr–O distances range from 2.375(2) to
˚
The absorption spectrum of Pr2Te4O11 (Fig. 5) shows a
complex structure of sharp and well-resolved peaks be-
tween 420 and 520 nm. These bands originate from 4f–4f
2.615(2) A (Table 3). These agree well with literature
˚
values, i.e. 2.418–2.628 A in PrPO4 [15] and 2.341–2.652
˚
A in Pr2W2O9 [16].
3
3
transitions of the H4 ground state to the P0, 3P1, 1I6, and
Atom Te(1) is coordinated to three O atoms at distances
3
˚
of 1.834(2), 1.886(2), and 1.992(1) A, with a longer
3P2 excited states. The most intense transition is P2 →3H4.
These transitions are responsible for the green color
observed in Pr(III) compounds [24,25], whereas the
1D2 →3H4 transition around 600 nm has no influence on
the color. In addition to these narrow spectral lines that
arise from the f–f transitions of Pr(III), an optical absorp-
tion threshold at |340 nm is observed. This corresponds to
3.65 eV, as deduced by means of a straightforward extrapo-
lation method [26]. Comparable band gaps were recently
found in the hydrothermally synthesized compounds
M2Te3O8 (M5Co: 3.60 eV; Ni: 3.84 eV; Cu: 2.64 eV)
[27]. Wide band-gap semiconductors have attracted sub-
stantial interest for applications in solid-state electronics
and optics [28–31].
Table 3
˚
Selected interatomic distances (A) and angles (8) for Pr2Te4O11
Pr–O(4)
Pr–O(5)
Pr–O(2)
Pr–O(2)
Pr–O(1)
Pr–O(3)
Pr–O(5)
Pr–O(1)
1.834(2)
1.886(2)
1.992(1)
2.451(2)
103.36(8)
91.36(9)
95.75(6)
89.17(7)
77.63(7)
173.29(5)
2.375(2)
2.385(2)
2.401(2)
2.425(2)
2.505(2)
2.509(2)
2.574(2)
2.615(2)
Te(2)–O(5)
Te(2)–O(2)
Te(2)–O(3)
Te(2)–O(1)
O(5)–Te(2)–O(2)
O(5)–Te(2)–O(3)
O(2)–Te(2)–O(3)
O(5)–Te(2)–O(1)
O(2)–Te(2)–O(1)
O(3)–Te(2)–O(1)
Te(1)–O(4)
Te(1)–O(1)
Te(1)–O(6)
Te(1)–O(3)
O(4)–Te(1)–O(1)
O(4)–Te(1)–O(6)
O(1)–Te(1)–O(6)
O(4)–Te(1)–O(3)
O(1)–Te(1)–O(3)
O(6)–Te(1)–O(3)
1.870(2)
1.883(2)
1.885(2)
2.714(2)
97.09(8)
102.77(8)
98.52(8)
73.08(7)
74.56(7)
171.13(7)
Acknowledgements
This research was supported by the U.S. National
Science Foundation under Grant DMR00-96676. Use was

118
I. Ijjaali et al. / Journal of Alloys and Compounds 354 (2003) 115–119
Table 4
Comparisons among Ln2Te4O11 compounds (Ln5Pr, Nd, Ho)
Pr2Te4O11
Nd2Te4O11
Ho2Te4O11
Reference
Present work
CsCl flux, 1123 K
2.375–2.615
1.834–1.992
(2.451)
[9]
[10]
Preparation
TeO2 excess, 919 K
2.385–2.603
1.830–1.989
(2.434)
1.863–1.883
(2.694)
CsCl flux, 1073 K
2.259–2.514
1.834–2.010
(2.357)
1.866–1.894
(2.609)
˚
Ln–O (A)
˚
Te(1)–O (A)
˚
Te(2)–O (A)
1.870–1.885
(2.714)
Fig. 5. Diffuse reflectance spectrum for Pr2Te4O11
.
made of the MRL Central Facilities supported by the
National Science Foundation at the Materials Research
Center of Northwestern University under Grant No.
DMR00-76097.
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