Journal of Physics and Chemistry of Solids 64 (2003) 847–853
www.elsevier.com/locate/jpcs
Diffuse phase transition in Sr5RTi3Nb7O30 (R ¼ La, Nd and Sm)
M.R. Ranga Raju, R.N.P. Choudhary*
Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302, India
Received 13 June 2002; accepted 13 September 2002
Abstract
Polycrystalline samples of Sr5RTi3Nb7O30, (R ¼ La, Nd and Sm) compounds were prepared by a high-temperature solidstate reaction technique. The formation and structure of the compounds were checked by preliminary X-ray studies. Detailed
dielectric and electrical properties of the compounds as a function of frequency (1– 100 kHz) and temperature (150– 650 K)
were studied. The temperature dependence of dielectric constant suggests that the compounds undergo ferro – paraelectric phase
transition of diffuse type. Measurements of conductivity as a function of temperature suggest that the compounds have negative
temperature coefficient of conductivity above 373 K.
q 2003 Elsevier Science Ltd. All rights reserved.
Keywords: C. X-ray diffraction; D. Ferroelectricity
1. Introduction
Since the discovery of ferroelectricity and related properties in BaTiO3 in 1945 [1], a large amount of research work on
oxides, in search of new materials for industrial applications,
has been done. High performance dielectric ceramics act as
key materials for resonators and temperature compensated
capacitors. Some ferroelectric oxides are also very important
due to the rapid progress in microwave telecommunications,
satellite broadcasting and other related devices [2 – 5].
Among many ferroelectric oxides, especially some oxides
with tungsten bronze (TB) structural family of a general
formula (A1)2(A2)4(C)4(B1)2(B2)8O30 are in the forefront
both in the area of research as well as in industrial
applications. A wide variety of cations substitution is
possible in the TB-structure because of the presence of
many interstices A, B, and C [6]. The distribution of metal
cations in different interstices can improve physical properties, such as electro-optic, nonlinear, elasto-optic and pyroelectric properties [7]. It largely depends on the crystallite
size, morphology of the specimen and the method of
synthesis. Singh et al. [9] studied the structural properties
of Ba2Na3RNb10O30 (R ¼ rare earth) series by using Naþ
* Corresponding author.
E-mail address: crnpfl@phy.iitkgp.ernet.in (R.N.P. Choudhary).
metal cations in A sites. Panigrahi et al. [8] extended the
systematic studies for the series Ba5RTi3Nb7O30 by varying
R3þ cations and reported the phase transition of diffuse type
over a wide temperature range, and relaxor properties. More
recently, Li et al. [10] modified the dielectric properties of TB
structured compound by cosubstituting Sr2þ cations at the Asites and Ti4þ at the B-sites. Chen and Yang [11] studied the
BaO– Nd2O3 – TiO2 –Ta2O5 system and reported its dielectric properties. Chen et al. [12] also studied the TB structure
compound of BaO – Sm2O3 – TiO2 – Ta2O5 system and
reported the dielectric properties of the compound. Xu and
Chen [13] studied the effect of Ca and Sr substitution on the
dielectric properties in Ba3Sm3Ti5Ta5O30. Extensive literature survey on TB compounds reveals that, even though a lot
of work has been done on the compounds of the family, not
much has been reported on the titled compounds. Hence, we
have extensively studied structural and some ferroelectric
properties of Sr5RTi3Nb7O30 (R ¼ La, Nd and Sm) compounds. In this paper, we report preliminary structural and
detailed dielectric/electrical properties of the compounds.
2. Experimental
Polycrystalline samples of Sr5LaTi3Nb7O30, Sr5NdTi3Nb7O30
and Sr5SmTi3Nb7O30 were prepared by a high-temperature
0022-3697/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 2 - 3 6 9 7 ( 0 2 ) 0 0 4 1 7 - 1
848
M.R. Ranga Raju, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 64 (2003) 847–853
Fig. 1. Comparison of X-ray diffraction profile for Sr5RTi3Nb7O30
(R ¼ La, Nd and Sm) recorded at room temperature.
Table 1
Comparison of lattice parameters, a, b and c (nm), volume (nm3)
and other related dielectric data in the compounds Sr5RTi3Nb7O30
(R ¼ La, Nd and Sm)
Parameters Sr5LaTi3Nb7O30 Sr5NdTi3Nb7O30 Sr5SmTi3Nb7O30
a
b
c
V
Ea (eV)
g
Tc (K)
1RT
1max
tan dRT
tan dTc
1.4462(02)
0.8026(02)
1.1293(02)
1.3108
0.73
1.21
283
514
531
0.011
0.024
1.4463(02)
0.8026(02)
1.1291(02)
1.3107
1.15
1.27
459
320
331
0.015
0.002
1.4452(02)
0.8023(02)
1.1290(02)
1.3091
1.43
1.67
512
387
458
0.031
0.002
solid-state reaction technique using high purity starting raw
materials: SrCO3 (99.9%, M/s B. D. H Chemicals), La2O3,
Nd2O3 and Sm2O3 (99.9%, M/s Indian Rare-earth), TiO2
(99.9%, M/s Sd. fine Chemicals) and Nb2O5 (99.9%, M/s
Loba Chemie. Co. India). The stoichiometric mixture of
ingredients was thoroughly mixed and grounded in wet
medium (methanol) for 1 h, and in dry condition for 1 h in
an agate mortar and pestle. Calcination was done in an
alumina crucible at 1150 8C for 15 h in air atmosphere. The
calcined powders were mixed again and recalcined in the
same condition. The process of calcination and mixing was
repeated until the formation of the compound was
confirmed by XRD. The fine and homogeneous powders
of the compounds, Sr5RTi3Nb7O30 (R ¼ La, Nd and Sm)
were mixed with polyvinyl alcohol (as binder) to reduce
the brittleness and compacted into cylindrical pellets of
10 mm diameter and 1– 2 mm thickness by applying a
pressure of 6 MPa in a hydraulic press. The pellets were
sintered at 1200 8C for 15 h in air using alumina crucible.
The organic binder was burnt out during the process of
sintering. The sintered pellets were then polished by fine
emery paper to make both faces flat and parallel. The
pellets were then electroded with high purity silver paste
for electrical characterizations. For preliminary structural
analysis XRD spectra were taken on calcined powders over
a wide range of Bragg angles (208 # 2u # 808) at room
temperature using a Philips (PW 1710) X-ray diffractometer (Cu Ka radiation, l ¼ 0.15418 nm). The dielectric
constant (1 ) and dielectric loss ðtan dÞ of the compounds
Sr5RTi3Nb7O30 (R ¼ La, Nd and Sm) were obtained by
measuring the capacitance and dissipation factor of the
samples as a function of frequency (1 – 100 kHz) and at
different temperatures (150– 650 K) using HIOKI 3532
Table 2
Comparison of dobs and dcal values with relative intensities ðI=I0 Þ of some reflections Sr5RTi3Nb7O3 (R ¼ La, Nd and Sm)
Sr5LaTi3Nb7O30
dhkl
Sr5NdTi3Nb7O30
I/I0
Obs
Cal
0.3866
0.3406
0.3164
0.2983
0.2889
0.2743
0.2560
0.2113
0.1959
0.1935
0.1836
0.1814
0.1741
0.1723
0.1626
0.1589
0.1491
0.1386
0.1296
0.1239
0.3866
0.3406
0.3164
0.2980
0.2892
0.2745
0.2562
0.2113
0.1959
0.1935
0.1837
0.1813
0.1741
0.1722
0.1625
0.1589
0.1492
0.1386
0.1296
0.1239
19
39
63
70
29
100
21
14
20
24
14
14
20
16
17
27
07
08
08
07
hkl
120
013
411
222
500
023
131
431
514
531
720
721
533
802
144
051
920
445
162
247
dhkl
Sr5SmTi3Nb7O30
I/I0
Obs
Cal
0.3867
0.3408
0.3164
0.2983
0.2889
0.2743
0.2560
0.2113
0.1959
0.1935
0.1836
0.1811
0.1738
0.1720
0.1626
0.1586
0.1491
0.1386
0.1296
0.1239
0.3867
0.3408
0.3164
0.2980
0.2892
0.2743
0.2562
0.2113
0.1959
0.1935
0.1837
0.1811
0.1738
0.1719
0.1625
0.1589
0.1492
0.1385
0.1296
0.1239
18
39
66
62
30
100
21
15
29
13
15
16
14
20
26
08
04
07
07
05
hkl
120
013
411
222
500
023
131
431
514
531
720
703
515
243
144
051
920
445
162
247
dhkl
Obs
Cal
0.3868
0.3407
0.3164
0.2983
0.2889
0.2743
0.2560
0.2113
0.1959
0.1935
0.1836
0.1811
0.1738
0.1718
0.1626
0.1588
0.1491
0.1385
0.1296
0.1239
0.3865
0.3407
0.3163
0.2979
0.2891
0.2745
0.2561
0.2111
0.1958
0.1934
0.1836
0.1810
0.1737
0.1719
0.1627
0.1589
0.1492
0.1386
0.1296
0.1239
I/I0
hkl
19
38
61
60
33
100
22
15
24
13
15
18
16
18
27
08
08
08
08
10
120
013
411
222
500
023
131
431
514
531
720
703
515
243
144
051
920
445
162
247
M.R. Ranga Raju, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 64 (2003) 847–853
LCR Hitester along with a laboratory-made sample-holder
and heating arrangement. Chromel – alumel thermo-couple
and AGRONIC-161 digital millivoltmeter were used to
measure the temperature. The resistivity measurement was
done using a programmable electrometer (Model Keithly617) at a constant applied field (4 V/cm) and a laboratorymade heating arrangement.
849
electronic component. Therefore at higher frequencies 1
becomes almost constant and frequency independent. The
tan d increases with frequency, which is shown in Fig. 4.
The values of 1 and tan d at different temperatures
(175– 650 K) at 10 kHz are shown in Fig. 5, respectively. It
is observed that La, Nd and Sm containing compounds
3. Results and discussion
In Fig. 1 we have compared X-ray diffraction profiles of
Sr5RTi3Nb7O30 (R ¼ La, Nd and Sm) recorded at room
temperature. All the three diffractograms are almost same
with small variations in the relative intensities and/or peak
positions. The sharp and single reflection peaks of the X-ray
diffraction profiles suggest the formation of new compound
in a single-phase. All the observed peaks are indexed and the
unit cell parameters were calculated in different crystal
systems and cell configurations using a standard computer
program package POWD MULT [14]. On the basis of the
best agreement between dobs and dcal, a suitable unit cell of
the compounds was selected in orthorhombic crystal system.
The least-squares refined cell parameters of Sr5RTi3Nb7O30
(R ¼ La, Nd and Sm) are given in Table 1. The observed
(dobs) and calculated (dcal) values of reflections along with
relative intensities are given in Table 2. The crystallite/
particle size of the powder samples was calculated from the
broadening of reflections using the Debye – Scherrer’s
equation [15], Dhkl ¼ 0:89l=ðb1=2 cos uÞ; where b1/2 ¼ half
peak width of the reflection. As the powder samples are
used, the broadening due to strain, instrumental error, beam
divergence and other effects are ignored. The average
crystallite/particle size were found to be 28, 40 and 41 nm
for La, Nd and Sm containing compounds, respectively.
To study the surface morphology of the sintered pellets,
the surfaces were made flat and parallel. On the flat and
clean surfaces gold coating was done by a sputtering
technique to improve the resolution of the image. The
surface morphology was studied by scanning electron
microscopy (SEM. JEOL JSM-5800). The average grain
size was determined by the linear intercept method. The
SEM micrographs show the uniform distribution of
the grains on the entire surface. The grain size calculated
of the compounds was found to be in the range of 3 – 3.5 mm.
The SEM micrographs of the compounds are given in Fig. 2.
Fig. 3 shows the variation in the dielectric constant (1 )
and dielectric loss ðtan dÞ as a function of frequency (1 –
102 kHz) at room temperature. A decrease in 1 has been
observed with increase in frequency. At low frequencies and
temperature, all types of polarizations (viz. interfacial,
dipolar, atomic and electronic polarizations) contribute to
the this value of 1. At higher frequency (.105 Hz) the
contributions from interfacial, dipolar and atomic polarization become zero, and hence we have only left with
Fig. 2. SEM micrographs of Sr5RTi3Nb7O30, (R ¼ La, Nd and Sm).
850
M.R. Ranga Raju, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 64 (2003) 847–853
Fig. 3. Frequency dependence of 1 of Sr5RTi3Nb7O30, (R ¼ La, Nd and Sm).
Fig. 4. Frequency dependence of tan d of Sr5RTi3Nb7O30, (R ¼ La, Nd and Sm).
M.R. Ranga Raju, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 64 (2003) 847–853
Fig. 5. Variation of 1 and tan d of Sr5RTi3Nb7O30 (R ¼ La, Nd and Sm) with temperature.
Fig. 6. Temperature dependence of s in Sr5RTi3Nb7O30, (R ¼ La, Nd and Sm).
851
852
M.R. Ranga Raju, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 64 (2003) 847–853
Fig. 7. Variation of lnðð1=1Þ 2 ð1=1max ÞÞ with ln ðT 2 Tc Þ in Sr5RTi3Nb7O30, (R ¼ La, Nd and Sm).
undergo the ferroelectric – paraelectric phase transition of
diffuse type. The transition temperature (Tc) of La containing compound is below the room temperature (283 K),
where as that of Nd and Sm containing compounds show the
transition temperature well above the room temperature (i.e.
459 and 512 K). The broadening of the dielectric peak was
due to disordering and defects present in the system. The
difference in the values of dielectric peak of Nd and Sm
containing compounds is smaller (D1 ¼ 31 and 102)
compared to that of La containing compound (D1 ¼ 186).
It was found that the Tc increases with increasing atomic
number (Z ) of R3þ cations. The tan d value of all the
compounds shows the same behavior. The tan d changes
considerably at low temperature as well as at high
temperature. But the change in tan d values is relatively
smaller in the temperature range from 350– 550 K.
The temperature variation of conductivity (ln s with
inverse of temperature (1/T )) is shown in Fig. 6. It follows the
usual Arrhenius relation s ¼ s0 expð2Ea =2kTÞ; where k
Boltzmann constant, Ea activation energy, s0 the conductivity at absolute zero temperature. As temperature increases
the value s increases, showing the semiconductor behavior
of the compounds. From the slope of the curves the activation
energy for all compounds was calculated. The value of
activation energy was found to be 0.73, 1.15 and 1.43 eV for
La, Nd and Sm containing compounds, respectively. It was
interesting to observe that the Ea value also increases with
increasing atomic number (Z ) of R3þ cations.
The degree of disorderliness or diffusivity (g ) in
Sr5RTi3Nb7O30 (R ¼ La, Nd and Sm) compounds was
evaluated using an empirical relation lnðð1=1Þ 2
ð1=1max ÞÞ ¼ g lnðT 2 Tc Þ þ constant [16], where 1max is
the maximum 1 value at T ¼ Tc : The value of g for all the
compounds at 10 kHz is obtained from the slope of the
curve lnðð1=1Þ 2 ð1=1max ÞÞ Vs lnðT 2 Tc Þ (Fig. 7). The value
of g for R ¼ La, Nd and Sm was found to be 1.21, 1.27 and
1.67, respectively. The value of g ¼ 1 obeying Curie–
Weiss law and 2 for completely disordered system. It
indicates that the compounds exhibit some characteristics
of diffuse phase transition.
4. Conclusions
Finally, it is concluded that inspite of some similarities in
the crystal parameters of the above compounds, there is a
marked difference in their physical properties at room
temperature. All the compounds have orthorhombic crystal
structure even after changing R3þ cations and show diffuse
phase transition. Also we found that the transition
temperature increases with increasing atomic number (Z ).
We also observed low activation energy and increase in the
conductivity with rise in temperature, which indicates the
semiconductor behavior of the compounds.
M.R. Ranga Raju, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 64 (2003) 847–853
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