1. CERAMICS
INTERNATIONAL
Available online at www.sciencedirect.com
Ceramics International 41 (2015) 3693–3700
Microwave dielectric properties of double perovskite ceramics
Ba2Zn1ÀxCaxWO6 ðx ¼ 0À0:4Þ
Anurag Gandhi, Sunita Keshrin
Department of Physics, Birla Institute of Technology, Mesra, Ranchi 835215, India
Received 7 September 2014; received in revised form 6 November 2014; accepted 7 November 2014
Available online 20 November 2014
Abstract
The structural, vibrational and microwave dielectric properties of double perovskite ceramics Ba2Zn1ÀxCaxWO6 ðx ¼ 0À0:4Þ were investigated.
The samples were sintered at different temperatures in the range 1300–1400 1C for 4 h. The grown samples were characterized by means of X–ray
diffraction, Raman spectroscopy, scanning electron microscopy and energy dispersive X–ray spectroscopy analysis. Microwave dielectric properties
of the samples were measured using the TE01δ resonance mode of the cylindrical pellets. The relative permittivity ðεrÞ, calculated using Clausius–
Mossotti equation is found to be comparable with the experimental results. Our analysis shows that the tolerance factor ðtÞ as well as the temperature
coefficient of resonant frequency ðτf Þ of these perovskites decreases with the increase in Ca content. The value of τf is zero for the samples with x ¼
0.3 and 0.4.
& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Double perovskite; Microwave dielectric properties; Relative permittivity; Quality factor
1. Introduction
Dielectric oxide ceramics have revolutionized the microwave
wireless communication industry by reducing the size and cost of
filter, oscillator and antenna components useful for different
applications, ranging from cellular phones to global positioning
systems. The constant need for miniaturization of such compo-
nents provides a continuing driving force for the discovery and
development of increasingly sophisticated materials to perform
the same or improved function with decreased size and weight.
The search of suitable materials working in higher resonant
frequency is also an important aspect of wireless communication
industry. For such applications, the microwave dielectric ceramics
should have high dielectric constant for size miniaturization, high
quality factor Q Â fð Þ for high frequency selectivity and nearly
zero temperature coefficient of resonant frequency ðτf Þ for
thermal stable circuits. Many dielectric ceramics of single [1–4]
and double perovskite [5–8] structures have been developed and
modified for specific applications. However, the development of
good materials is still in high demand.
The double perovskite structure A2BB0
O6 (where A is an
alkaline cation, and B and B0
are two heterovalent transition-
metal elements that have attracted much interest as they exhibit
rich structural and physical properties [5–9]. Mixed alkaline earth
tungstate double perovskites with the general formula A2BWO6
(A¼Ba, Sr, Ca; B¼Mg, Co, Zn, Ni) are of interest for their
complete 1:1 ordered state due to their larger differences in
charge and ionic radii on B–site [10]. In most of these compounds
the octahedrally coordinated cations are ordered so that succes-
sive crystal planes, in the direction of cubic close packing, are
solely occupied either by an alkaline–earth or a tungsten cation
[11]. Recent structural study on A2MWO6 (A¼Ba, Sr, Ca;
M¼Ni, Co) compositions [12] demonstrated that progressively
increasing the effective size of the A–type cation by chemical
substitution of the alkaline earth cation resulted in the sequence of
structural phase transitions: monoclinic–tetragonal–cubic [13,14].
Microwave dielectric properties of A2BWO6 were widely
investigated by several researchers [15,16]. All these researches
were focused on the effect of variation of A–site and B–site ions
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0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
n
Corresponding author. Tel.: þ91 651 2275444x4500;
fax: þ91 651 2275401.
E-mail addresses: s_keshri@bitmesra.ac.in,
sskeshri@rediffmail.com (S. Keshri).

2. as well as sintering temperature on the microwave dielectric
properties. These parameters have played a major role in altering
the dielectric properties [10,11]. For example, Bian et al. [16] have
studied the structural and microwave dielectric properties of A1 À
3x=2LaxðMg1=2W1=2ÞO3 (A¼Ba, Sr, Ca; 0rxr0:05) cera-
mics for different sintering temperatures. Good combination of
microwave dielectric properties was obtained for Ba1À3x=2Lax
ðMg1=2W1=2ÞO3 sample with x¼0.04 sintered at 1450 1C for 2 h:
εr $ 20, QÂ f ¼ 87,680 GHz and τf ¼ À1:2 ppm=1C. On the
other hand, an excellent combination of microwave dielectric pro-
perties was obtained for Ba2Mg0:9Ca0:1WO6 ceramic with εr ¼
20:8, QÂ f ¼ 120,729 GHz, and τf ¼ 0 ppm=1C [10]. However,
most of these samples were grown by sintering them in the
temperature range 1450–1550 1C. Obviously, high sintering tem-
perature of these ceramics limits their practical application and the
reduction of the sintering temperature is desirable.
The literature review [9,17–19] on tungstate double perovs-
kites Ba2ZnWO6 reveals that there is a considerable difference
in the reported values of dielectric parameters of this ceramic.
For example, Jancar et al. [20] have reported that the dielectric
losses of Ba2ZnWO6 based ceramics strongly depend on the
processing parameters; the sample sintered at 1350 1C for 2 h
shows dielectric properties: εr ¼ 26, Q Â f ¼ 65,000 GHz, and
τf ¼ À34 ppm=1C. For the same composition Khalyavin
et al. [18] measured larger εr (¼28), a lower Q Â f value
(¼19,800 GHz) and a large temperature coefficient of permit-
tivity (¼90 ppm/K). As per best of our knowledge, there are
only few reports regarding such perovskites with doping in B-
site. In this article, we report a systematic study on the
structural, vibrational, morphological and microwave dielectric
properties of Ba2Zn1 ÀxCaxWO6 ðx ¼ 0À0:4Þ ceramics. We
chose larger sized Ca2þ
(0.99 Å) as dopant partially substituting
for Zn2þ
(0.84 Å) so as to reduce the tolerance factor ðtÞ [10].
In general, this factor is used as a measure of the stability of the
perovskite phases, which influences the structure and also the
dielectric properties [15,21].
2. Experimental
The Ba2Zn1 ÀxCaxWO6 (x = 0, 0.1, 0.2, 0.3 and 0.4) ceramics
were prepared by the method of solid state reaction, using the
starting chemicals BaCO3 (99.7%, Merck), ZnO (99.7%, Merck),
CaCO3 (99.5%, Merck) and WO3 (99.5%, Alfa Aesar). The
stoichiometrically calculated reagents were thoroughly mixed
with isopropanol and grinded using agate mortar and pestle for
4 h and the dried powder was calcined at 1100 1C for 2 h. The
obtained mixture was then dried, mixed with 7–10 wt.% poly-
vinyl alcohol and uniaxially pressed into pellets. The pellets were
sintered at 1300–1400 1C for 4 h at heating rates of 5 1C/min.
The pellets were sintered at 1425 1C also, but unfortunately those
were found melted. In order to verify the crystalline behavior of
the sample, powder X–ray diffraction using Rigaku diffractometer
with CuKα radiation from 101 to 601 with a step size of 0.021
was carried out. Raman measurement in backscattering geometry
was carried out at room temperature by a Reinshaw Invia Raman
microscope attached with four standard Leica objective lenses.
An Argon ion laser source of excitation wavelength 514 nm was
used for this measurement. The spectra of 100–1000 cmÀ1
were
collected using 2400 lines per mm grating, with a 20 s data point
acquisition time. The morphological and elemental analyses of
the samples were obtained by JEOL–6330F scanning electron
microscope (SEM) equipped with Oxford INCA energy disper-
sive X–ray (EDX) spectrometer. Microwave dielectric properties
of the composites were measured using the TE01δ resonance
mode of the cylindrical pellets inserted in a shielding cavity by
Agilent PNA N5230A network analyzer in the transmission setup
with a moderate coupling [22,23]. Numerical calculations of the
dielectric parameters from the measured resonance frequencies
and Q-factors were based on the electrodynamics analysis [22,23].
The temperature coefficient of resonant frequency ðτf Þ of all the
samples was measured by introducing the cavity in a temperature–
controlled chamber. The following equation was used for this
calculation:
τf ¼
f 2 Àf 1
f 1 T2 ÀT1ð Þ
ð1Þ
where f 1 and f 2 represent the resonant frequencies at two different
temperatures T1($25 1C) and T2($75 1C), respectively.
3. Results and discussion
3.1. XRD Studies
The room temperature XRD patterns of Ba2Zn1ÀxCaxWO6
ðx ¼ 0À0:4Þ ceramics, sintered at 1400 1C, are shown in Fig. 1(a).
The indexing of the XRD patterns and calculation of lattice
parameters were done using ‘Checkcell’ software; no additional
phases were detected. It is observed that the samples do not show
any phase transition and have monoclinic structure with space
group P2=m. The variation of calculated lattice parameters as a
function of Zn concentration is shown in Fig. 1(b). It is noted that
the peak positions shifts to lower 2θ value with the increase of x,
which means that the cell volume increases with increasing x due to
the substitution of larger Ca2 þ
ðR ¼ 0:99 nmÞ in smaller
Zn2 þ
ðR ¼ 0:88 nmÞ site. It is noteworthy to mention that Prakash
et al. [19] have reported a monoclinic phase for the similar
compound BaðZn1=2W1=2ÞO3. Zhou et al. [15] have demonstrated
a phase transition from cubic–rhombohedral–monoclinic for the
compositions Ba2Ca1ÀxSrxWO6 ðx ¼ 0À1Þ. The XRD patterns
of Ba2Zn0:9Ca0:1WO6 sample, sintered at different temperatures,
are shown in Fig. 2(a) and the variation of corresponding lattice
parameters with sintering temperature are demonstrated in Fig. 2(b).
From this plot it is evident that the lattice parameters of this sample
do not show any significant change with respect to sintering
temperature 1300–1400 1C.
3.2. Raman studies
The Raman spectra of Ba2Zn1 ÀxCaxWO6 ceramics are shown
in Fig. 3(a). The observed five bands correspond to F2g Bað Þ,
F2g Oð Þ, Eg Oð Þ, A1g Oð Þ and ν1 Ag
À Á
modes, which are in agree-
ment with previous literature [24]. The band at $115 cmÀ1
and
421 cmÀ1
could be ascribed to triple degenerate F2g Bað Þ and
F2g Oð Þ modes, respectively [10]. According to Blasse's analysis,
A. Gandhi, S. Keshri / Ceramics International 41 (2015) 3693–37003694

3. the band at 816 cmÀ1
could be ascribed to the A1g Oð Þ mode of a
WO6 octahedron surrounded by six Zn2þ
ions whereas the band
at 845 cmÀ1
was assigned to the Eg Oð Þ mode of a WO6 octahe-
dron surrounded by five Zn2þ
ions and one Ca2þ
ion [25–27].
For x ¼ 0 sample, the band centered at 816 cmÀ1
corresponds to
A1g Oð Þ mode. However for x ¼ 0:1 sample, the intensity of
this band decreases and a new band Eg Oð Þ emerges at about
845 cmÀ1
. Similar result was observed by Jia et al. [10] for the
composition Ba2Mg0:95Ca0:05WO6. For other three compositions
the Eg Oð Þ mode is observed at $845 cmÀ1
. The small peak
observed at $926 cmÀ1
could be associated with the stretching
vibrational mode ν1 Ag
À Á
[28]. It is worthy to note that with the
increase in Ca content, the F2g Bað Þ and F2g Oð Þ modes shift
slightly to lower frequency (Fig. 3(b)) which may be because of
the decrease in the BaÀO bond strength.
3.3. Morphological and elemental studies
The SEM micrographs of the samples, sintered at 1300 1C for
4 h, are presented in Fig. 4, together with the EDX patterns. It
can be observed from these figures that the grains of all samples
have well–defined boundaries. From EDX results, it is clear that
the samples contain all the compositional elements. The Pt peaks
in the EDX spectra comes from the coating of platinum over
the surface of sample to avoid charging. The parent sample
Ba2ZnWO6, has various sizes of grains and the grain size lies in
the range of $4–12 μm. For x ¼ 0:1; 0:2 and 0:4 samples,
grains are smaller in size and lie in the range of $2–4 μm.
However for x ¼ 0:3 sample, two types of grains (bigger and
smaller) were observed. The SEM images of x ¼ 0:1 sample,
sintered at different temperatures (1300–1400 1C), are shown in
Fig. 5. From this figure it is evident that the grains became larger
and more compact with the increase in sintering temperature.
3.4. Microwave dielectric behaviors
The relative permittivity ðεrÞ and quality factor ðQ Â f Þ of
Ba2Zn1ÀxCaxWO6 (x ¼ 0À0.4) ceramics are shown in Fig. 6(a)
Fig. 1. (a) XRD pattern of all compositions, sintered at 1400 1C. (b) Variation
of cell parameters as a function of x, sintered at 1400 1C.
Fig. 2. (a) XRD patterns of x ¼ 0.1 composition, sintered at 1300–1400 1C.
(b) Variation of cell parameters as a function of sintering temperature for
x ¼ 0:1 composition.
A. Gandhi, S. Keshri / Ceramics International 41 (2015) 3693–3700 3695

4. and (b) respectively as a function of sintering temperature. For all
temperatures, the value of εr decreases with the increase in x
except for x ¼ 0.1 sample. Its highest value is 19.32 correspond-
ing to x ¼ 0.1 sample sintered at 1400 1C. The relationship
between dielectric constant and microscopic polarizability for cubic
perovskites can be expressed through the Clausius–Mossotti
formula [29]
εr À1
εr þ2
¼
4π
3
αm
Vm
ð2Þ
where εr is the relative dielectric constant, αm is the macroscopic
polarizability and Vm is the molar volume. The values of αm for
Ca2þ
and Zn2þ
are 3.16 and 2.04 Å3
respectively [30,31]. The
theoretical dielectric constant εtheo calculated from Eq. (2) is also
shown in Table 1. A similar method has also been used by Pang
et al. [32] for a novel Ca3WO6 microwave dielectric ceramic with
a complex perovskite structure. The deviation between theoretical
and experimental values of εr is found to be less which could be
because of the distortion from the cubic structure; the larger the
deviation from cubic symmetry, the larger the difference between
theoretical and experimental values. From Fig. 6(b) it is evident
that a small decrease in Q Â f values is observed with the increase
in x which may be because of inhomogeneous grain growth. For
all compositions the values of εr and Q Â f increase with the
increase of sintering temperature. This may be because of the fact
that porosity of the sample decreases with the increase in sintering
temperature as evident from SEM images (Fig. 5). The change in
temperature coefficient of resonant frequency ðτf Þ with x is shown
in Fig. 7(a) for all samples sintered at 1400 1C. As expected, the
doping of Ca tuned τf value from negative to zero value. The
values of resonant frequency ðf 0Þ, quality factor ðQ Â f Þ,
temperature coefficient of resonant frequency ðτf Þ and dielectric
loss ðtan δÞ of all samples are shown in Table 1. It is interesting to
note that for the samples with x ¼ 0:3 and 0.4, the value of τf is
zero. Even for other samples its value is very small, whereas in
other report [20] its value is $ À34 ppm/K. However, the values
of Q Â f are not so high; its value is maximum ð20; 162 GHzÞ for
the parent sample Ba2ZnWO6.
Since τf depends on the tolerance factor ðtÞ of the perovs-
kites, we have calculated tolerance factor using the following
formula [15]
t ¼ R Bað ÞþRðOÞ½ Š=
ffiffiffi
2
p 1Àxð ÞR Znð ÞþxR Cað ÞþR Wð Þ
2
þR Oð Þ
!
ð3Þ
where RðBaÞ; RðZnÞ; RðCaÞ; RðWÞ, and RðOÞ correspond to the
ionic radii of Ba2þ
, Zn2þ
, Ca2þ
, W6þ
and O2À
respectively.
Using this formula it is understood that with the increase in Ca
content, t decreases (Table 1). In the above equation the values
of ionic radii were taken from Shannon's paper [33]. We have
also calculated the temperature coefficient of dielectric constant
ðτεÞ using the following equation [34]:
τε ¼ À2 τf þαL
À Á
ð4Þ
where αL is the coefficient of linear thermal expansion. The αL
values are usually almost independent of composition for most
ceramics; hence, a common value of 8 ppm/1C was used as also
considered by Reaney et al. [34]. The dependence of τε on
tolerance factor ðtÞ is demonstrated in Fig. 7(b), from which it is
evident that the value of τε for all samples is considerably less
as compared to the reported value ð90 ppm=KÞ of Khalyavin
et al. [18]. The figure also shows that with the increase in t, it
varies slightly and becomes more negative.
4. Conclusions
In this article we report the structural, vibrational and micro-
wave dielectric properties of Ba2Zn1ÀxCaxWO6 ðx ¼ 0À0:4Þ
dielectric ceramics. From XRD studies, it is observed that the
samples do not show any phase transition and have monoclinic
structure with space group P2=m. The Raman spectra show five
bands corresponding to F2g Bað Þ, F2g Oð Þ, Eg Oð Þ, A1g Oð Þ and
ν1 Ag
À Á
modes. The theoretical dielectric constant ðεtheoÞ calcu-
lated from the Clausius–Mossotti equation shows small deviation
from experimental value of εr which could be because of the
Fig. 3. (a) Raman Spectra of Ba2Zn1ÀxCaxWO6 ceramics, sintered at 1400 1C.
(b) Raman shift as a function of x.
A. Gandhi, S. Keshri / Ceramics International 41 (2015) 3693–37003696

5. distortion from the cubic structure. By analyzing the tolerance
factor ðtÞ of these perovskites it is understood that with the increase
in Ca content, t decreases. The sample with x ¼ 0.4 of this series,
sintered at 1400 1C, is found to possess microwave dielectric
parameters: εr $ 15:19, Q Â f $ 13; 351 (at 8.43 GHz) and
τf $ 0 ppm=K. The parent sample Ba2ZnWO6, sintered at same
Fig. 4. SEM micrographs of Ba2Zn1ÀxCaxWO6 ceramics, sintered at 1300 1C for the compositions: (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.2, (d) x ¼ 0.3 and (e) x ¼
0.4; (f)–(j) their corresponding EDX results.
A. Gandhi, S. Keshri / Ceramics International 41 (2015) 3693–3700 3697

6. Fig. 5. (a)–(e) SEM micrographs of x ¼ 0.1 composition, sintered at 1300–1400 1C; (f)–(j) their corresponding EDX results.
A. Gandhi, S. Keshri / Ceramics International 41 (2015) 3693–37003698

7. temperature, also shows good combinations of these parame
ters: εr $ 16:22, Q Â f $ 20; 162 GHz (at 8.56 GHz) and
τf $ À 0:03 ppm=K. The compositions with zero τf values can
be found suitable for designing antenna. However, more work in
this direction is still required to improve the quality factor of these
compositions.
Acknowledgments
The author, S. Keshri gratefully acknowledges Department of
Science and Technology (DST), India for funding the project
(SR/S2/CMPÀ74/2012). DST is also acknowledged for sanc-
tioning FIST fund to the Department of Physics, Birla Institute
of Technology (BIT), for procuring Raman Spectrometer. The
authors are thankful to the members of Central Instrumentation
Facility Lab, BIT for SEM measurements. XRD work of this
study was performed at UGC–DAE Consortium for Scientific
Research, Indore; Dr. Mukul Gupta is acknowledged for
supporting this work. The authors are thankful to Prof. Vibha
Rani Gupta and Prof. Nisha Gupta, Department of ECE, BIT for
extending the support of dielectric measurements.
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