Structures, phase transformations, and dielectric properties of (1-x)Bi2Zn2/3Nb4/3O7-xBi1.5NiNb1.5O7 pyrochlore ceramics prepared by aqueous sol-gel method

Article abstract of DOI:10.1016/j.jallcom.2014.10.021

As a candidate of thermostable low temperature co-fired ceramics (LTCC) material, (1-x)Bi₂Zn_2/3Nb_4/3O₇-xBi_1.5NiNb_1.5O₇ (0.0 ≤ x ≤ 1.0) ceramics with improved dielectric properties have been prepared via aqueous sol-gel method. The relations of phase equilibrium, crystal structure and dielectric properties of the composites were investigated systematically. Phase transformation, from orthorhombic zirconolite-like to cubic pyrochlore structure, occured with the increasing Bi_1.5NiNb_1.5O₇ content. The phase stability of the orthorhombic and cubic pyrochlore phase in the (1-x)β-BZN-xBNN system was dependent on the Bi³⁺ content as well as the distribution and variety of divalent cations, such as Ni²⁺/Zn²⁺ ratio. The phase boundaries were located around x = 0.1 and x = 0.6 for orthorhombic and cubic phases, respectively. Near-zero temperature coefficient of dielectric constant (τ_ε) was obtained and the dielectric constant (ε_r) was in the range of 80-165 in this system, which were strongly correlated with phase composition. The (1-x)β-BZN-xBNN ceramic with x = 0.2 satisfied the EIA (Electronic Industries Association) specification NP0 (τ_ε≤± 30 ppm/°C between -55 and 125 °C) exhibited excellent dielectric properties of ε_r = 105.6, small dielectric tangent (tan δ) ～ 10^-4, τ_ε = -11.1 ppm/°C with the low-firing temperature of 900°C within the two-phase region, which can be a promising candidate for LTCC and multilayer components applications in high frequency and microwave range.

Full text of DOI:10.1016/j.jallcom.2014.10.021

Journal of Alloys and Compounds 622 (2015) 200–205
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Structures, phase transformations, and dielectric properties
of (1ꢀx)Bi Zn Nb O –xBi NiNb O pyrochlore ceramics
2
2/3
4/3
7
1.5
1.5 7
prepared by aqueous sol–gel method
⇑
Y.X. Jin, L.X. Li , H.L. Dong, S.H. Yu, D. Xu
School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2014
Received in revised form 28 September 2014
Accepted 3 October 2014
Available online 17 October 2014
As
a
candidate of thermostable low temperature co-fired ceramics (LTCC) material,
–xBi1.5NiNb1.5 (0.0 6 x 6 1.0) ceramics with improved dielectric properties have
(1ꢀx)Bi
2
Zn2/3Nb4/3
O
7
O
7
been prepared via aqueous sol–gel method. The relations of phase equilibrium, crystal structure and
dielectric properties of the composites were investigated systematically. Phase transformation,
from orthorhombic zirconolite-like to cubic pyrochlore structure, occured with the increasing
7
Bi1.5NiNb1.5O content. The phase stability of the orthorhombic and cubic pyrochlore phase in the
Keywords:
Dielectrics
Ceramics
Sol–gel process
Phase transformation
Microstructure
Dielectric properties
3+
(
1ꢀx)b-BZN–xBNN system was dependent on the Bi content as well as the distribution and variety of
2+ 2+
divalent cations, such as Ni /Zn ratio. The phase boundaries were located around x = 0.1 and x = 0.6
for orthorhombic and cubic phases, respectively. Near-zero temperature coefficient of dielectric constant
ð
s
e
Þ was obtained and the dielectric constant ð
strongly correlated with phase composition. The (1ꢀx)b-BZN–xBNN ceramic with x = 0.2 satisfied the EIA
(Electronic Industries Association) specification NP0 (
ꢁ ꢂ 30 ppm/°C between ꢀ55 and 125 °C) exhib-
e
r
Þ was in the range of 80–165 in this system, which were
s
e
ꢀ4
ited excellent dielectric properties of
e
r
= 105.6, small dielectric tangent (tand) ꢃ 10
e
, s = ꢀ11.1 ppm/°C
with the low-firing temperature of 900 °C within the two-phase region, which can be a promising candi-
date for LTCC and multilayer components applications in high frequency and microwave range.
Ó 2014 Elsevier B.V. All rights reserved.
1
. Introduction
Low-temperature co-fired ceramic (LTCC) materials are the base
prefers a site coordination number of eight and a smaller tetrava-
lent or pentavalent cation B that typically occurs in octahedral sites
[8–10]. It has considerable compositional and structural flexibility,
including the possibility to incorporate a wide range of dopant ions
materials for the development of multilayer components and
devices operating at high frequency and microwave range. They
provide a solution to the integration of passive components, such
as capacitor, resistor, inductor, resonator and filter, etc. into a
multilayer ceramic module [1–3]. Bismuth-based pyrochlores,
2
+
2+
2+
4+
4+
[11–14]. Thus ion substitution, such as Cu , Mg , Sr , Ce , Zr
and Ti
4
+
[15,16], is normally used to improve the dielectric
properties, especially to enhance dielectric constant and adjust
temperature coefficient of dielectric constant.
Bi
because of their superior dielectric properties, i.e. high dielectric
constants ( ) in the range of 80–210, small dielectric tangent
2
O
3
–MO–Nb
2
O
5
(M = Zn, Ni, Mg, etc.), are promising ones
Members of Bi
family, Bi1.5Zn1.0Nb1.5
Bi1.5Ni1.0Nb1.5 (BNN), Bi1.5Mg1.0Nb1.5
the most particular attention [17–19]. The crystal structure of
the -BZN, BNN and BMN pyrochlores are cubic structure with
space group Fd3m, while it shows to adopt an orthorhombic dis-
torted pyrochlore structure in the b-BZN ceramic [17,19]. Note that
these two phase pyrochlores have opposite sign of the temperature
2
O
3
–MO–Nb
2
O
5
(M = Zn, Ni, Mg, etc.) pyrochlores
-BZN), Bi (b-BZN),
(BMN), have received
O
7
(a
2 7
(Zn2/3Nb4/3)O
O
7
e
r
4
O
7
ꢀ
(
tand) ꢃ 10 , controllable temperature coefficients of dielectric
constant together with the low sintering temperature
<1000 °C) and compatibility with Ag electrodes to replace the
(s
e
)
a
(
poor conductivity and high cost Ag–Pd electrode which can greatly
reduce the production cost [4–7].
The pyrochlore structure, of general formula A
2
2
B O
6
O’, is found
e
coefficient of dielectric constant (s ): negative and positive, respec-
to contain a large trivalent or bivalent cation A that typically
tively, therefore, temperature stable ceramics can be fabricated by
mixing them in different stoichiometric proportions sintered at
suitable temperatures. In our previous work, the BMN and b-BZN
pyrochlores were mixed together to investigate structural stability
⇑
Corresponding author. Tel./fax: +86 22 27402838.
E-mail address: lilingxia91@163.com (L.X. Li).
http://dx.doi.org/10.1016/j.jallcom.2014.10.021
925-8388/Ó 2014 Elsevier B.V. All rights reserved.
0

Y.X. Jin et al. / Journal of Alloys and Compounds 622 (2015) 200–205
201
water for 12 h before pressing into disks with 10 mm in diameter and 1 mm in
thickness. Then these disks were sintered at temperatures ranging from 875 °C to
of the pyrochlore system [20]. In this paper, we report on our
attempts to obtain high dielectric constant LTCC materials with
950 °C for 4 h in air.
e
NP0 (s ꢁ ꢂ 30 ppm/°C between ꢀ55 and 125 °C) characteristics
The crystalline structures of the sintered samples were identified via Powder
through the preparation of (1ꢀx)b-BZN–xBNN (0.0 6 x 6 1.0)
ceramics.
X-ray diffraction (XRD), using a diffractometer (D8-Focus; Bruker AXS GmbH,
Karlsruhe, German) with Cu Ka radiation. The microstructure observation on
ceramic surfaces of the sintered samples was performed and analyzed with field
emission scanning electron microscopy (FE-SEM, S-4800; Hitachi, Ltd., Tokyo,
Japan). For the electrical property measurements, the disks were coated with silver
paste on both sides, followed by a heat treatment at 800 °C for 15 min to ensure
good electrical contact. The dielectric properties versus frequency at room temper-
ature were measured using a wide frequency LCR meter (TH2828; Tonghui, Changz-
hou, Jiangsu, China) in conjunction with Agilent 4285 A precision LCR meter (Palo
Alto, USA.) from 100 Hz up to 30 MHz. A capacitance meter (HP4278A; Hewlett–
Packard, Santa Clara, California, USA.) in conjunction with a temperature chamber
(GZ-ESPEC) was used in dielectric measurements over a temperature range of
ꢀ55 to 150 °C at four different frequencies (1 kHz, 10 kHz, 100 kHz, and 1 MHz).
BZN-based dielectric ceramics prepared by the solid-state reac-
tion method have been systematically studied [21–23]. It is known
that the solid state reaction method can lead to local chemical het-
erogeneity and large sized particles, which may result in multi-
phase powders. However, chemical methods have offered an
alternative to produce smaller sized, chemically homogeneous par-
ticles [24]. Therefore, the sol–gel process in which reacting species
homogenized at the atomic level was employed to prepare
chemically homogeneous and phase-pure specimens with a nar-
row particles size distribution and low sintering temperatures.
The relations of the crystal structures, stability, and dielectric prop-
erties of the (1ꢀx)b-BZN–xBNN ceramics were investigated
systematically.
The temperature coefficient of dielectric constant (
by using the following equation:
s
e
, ppm/°C) can be calculated
ð
e
ðT
1
ꢀ
e
ꢀ T
2
Þ
e
s ¼
ð1Þ
e
1
2
1
Þ
where
e
1
and
2 1 2
e are the dielectric constant at temperatures T and T respectively.
The average crystallite sizes were calculated using the Scherrer formula:
2
. Experimental procedures
Kk
B cosðhÞ
d ¼
ð2Þ
The (1ꢀx)Bi
the sol–gel process. Analytically pure (AR) Bi(NO
99.00%), Ni(NO O (99.99%) and Nb (99.99%) were used as the raw mate-
ꢄ5H
rials for preparation of the Bi (Zn2/3Nb4/3)O (b-BZN) and Bi1.5NiNb1.5 (BNN) pre-
2
Zn2/3Nb4/3
O
7
–xBi1.5NiNb1.5
O
7
nanopowders were synthesized by
)
3 3
ꢄ5H O (99.00%), Zn(NO ꢄ6H
2
3
)
2
2
O
where d, k, B, and h are crystallite size, Cu Ka wavelength, full width at half-maxi-
(
3
)
2
2
2 5
O
mum intensity (FWHM) in radians, and Bragg’s diffraction angle, respectively.
2
7
O
7
cursor solution, respectively. Citric acid (99.00%) and ethylene glycol (99.00%) were
used as a chelating agent and reaction medium. The experimental procedure for
preparing the nanopowders is shown in Fig. 1. Firstly, stoichiometric Nb O was
2 5
3
. Results and discussion
dissolved in the appropriate amount of HF at 80 °C for 20 h, and then ammonia
water was added into the above solutions until the pH value was 9.0 to form chem-
Fig. 2 shows the XRD profiles of the (1ꢀx)Bi
2
Zn2/3Nb4/3
O
7
–xBi1.5
ical precipitate of niobic hydroxide [Nb(OH)
5
]. The precipitates were filtered off and
NiNb1.5
O
7
(0.0 6 x 6 1.0) samples. The Bi
2
Zn2/3Nb4/3O (b-BZN,
7
+
+
washed with distilled water to remove the H and F ions. The Nb(OH)
were dissolved completely in citric acid water solution under stirring at 80 °C for
h. Subsequently, stoichiometric amounts of Zn(NO O, Ni(NO ꢄ5H O and
ꢄ6H
Bi(NO O were added to the niobium citrate solution to form the (Bi, Zn,
5
precipitates
x = 0) composition is indexed with orthorhombic symmetry of space
group C2c according to the JCPDS card (Joint Committee on Powder
Diffraction Standard) No. 54-0972. A single-phase cubic pyrochlore
structure (JCPDS #54-0971) belonging to the space group Fd3m is
obtained for the Bi1.5NiNb1.5O7 (x = 1.0) composition. The
4
)
3 2
2
)
3 2
2
3
)
3
ꢄ5H
2
Nb) and (Bi, Ni, Nb) complex precursors, respectively. The solutions was kept under
stirring at 60 °C for 1 h, and the pH was adjusted to 8–9 with ethylenediamine,
2 7
resulting in clear pale yellow and green solutions. The Bi Zn2/3Nb4/3O solution
and Bi1.5NiNb1.5 solution were then homogeneously mixed at nominal stoichiom-
O
7
etry to obtain the (1ꢀx)b-BZN–xBNN solution. Ethylene glycol was added to those
solutions to promote polymerization of the mixed citrate. By heating the solutions
in a water bath at 80–90 °C for 10 h, viscous gels were obtained. Finally, the gels
were dried at 200 °C and then calcined at 650 °C in air for 2 h to obtain the (1ꢀx)
2 7 7
Bi Zn2/3Nb4/3O –xBi1.5NiNb1.5O nanopowders. The calcined powders were granu-
lated by adding 0.75 wt% PVA solution as an organic binder and milled in deionized
Fig. 1. Flow chart for preparing the (1ꢀx)b-BZN–xBNN nanopowders by the sol–gel
process.
Fig. 2. XRD profiles of the (1ꢀx)b-BZN–xBNN samples for x = 0.0–1.0.

2
02
Y.X. Jin et al. / Journal of Alloys and Compounds 622 (2015) 200–205
intermediate compositions (0 < x < 1) exhibit three distinct regions
as the symmetry transformed from orthorhombic to cubic symme-
try with the increasing x. A two-phase region is found in the
0
.15 6 x 6 0.5 range, and corresponding orthorhombic and cubic
solid solution ranges are observed for x 6 0.1 and x P 0.6 respec-
tively. From this, we can determine the solid solution limits (i.e.,
phase boundaries between solid solution and two-phase regions)
to be located around x = 0.1 and x = 0.6 in this (1ꢀx)b-BZN–xBNN
system. The XRD characterization reveals the phase evolution with
the increase of the x value. The cell parameters and unit-cell vol-
umes (Vunit) of the (1ꢀx)b-BZN–xBNN (0.0 6 x 6 1.0) samples can
be obtained from the Rietveld refinement based on the XRD pat-
terns, and are listed in Table. 1. Within these solubility limits, the
unit-cell volume of both the orthorhombic (1ꢀx)b-BZN–xBNN
phases (0.0 6 x 6 0.1) and the cubic (1ꢀx)b-BZN–xBNN phases
(
0.6 6 x 6 1.0) decreased with the increasing x value. These results
3+
are caused by the decreasing Bi content with large ionic radii
VIII
2+
VI
(r = 1.17 Å ) and the increasing Ni (r = 0.69 Å ) content with
2+
VI
smaller ionic radii than Zn (r = 0.74 Å ) [25], that decreased the
unit cell volumes.
Fig. 3. The dependence of the phase evolution of the (1ꢀx)b-BZN–xBNN
2+
2+
compositions on the stoichiometric of Bi content and Ni /Zn ratio.
To investigate the phase evolution mechanisms for the
(
1ꢀx)b-BZN–xBNN system, it is essential to know the primarily dif-
ference between the detailed structural characterization of the two
Table 2
phases. The pyrochlore structure, of general formula A
2
B
2
O
7
(B
2 6
O
Phase stability field of the orthorhombic and cubic in the (1ꢀx)b-BZN–xBNN
0
A
2
O ), is built of two interpenetrating networks: [BO
6
] octahedra-
(0.0 6 x 6 1.0) compositions.
sharing vertices form a three-dimensional network based on a dia-
Phases
x
Bi content
Ni2+/Zn2+ ratio
0
mond net and a second cuprite-like A
2
O tetrahedral net. For the
Orthorhombic (O)
C&O
Pseudo cubic
Cubic (C)
0–0.1
1.95–2
0–0.1667
ꢃ0.2647–0.6429
ꢃ1–1.5
distorted pyrochlores Bi
long-range coupling of the 6s lone-pair electrons of Bi cations
on the A-site position, which can be affected by the character of
2
Zn2/3Nb4/3
7
O (b-phase) resulted from the
ꢃ0.15–0.3
ꢃ0.4–0.5
ꢃ0.6–1
1.85–1.925
1.75–1.8
1.5–1.7
2
>2.25
2 7
B site ions, has a structure similar to that of CaZrTi O [26–29]. A
detailed structural description revealed that the b-phase was of
the orthorhombic zirconolite type in which alternating layers of
Bi atoms in parallel to [110] direction were located between hex-
zirconolite-like to cubic pyrochlore structure, occurs with the
decreasing Bi content and increasing Ni /Zn ratio. The phase
structure is orthorhombic zirconolite-like in the specimens with
3+
2+
2+
6
agonal tungsten bronze (HTB) – like (001) sheets of ZnNbO octa-
2
+
hedra containing Zn in the centers of its six membered rings [29].
The slightly off-center Zn2 could be distributed between two half
occupied (5 + 1) – fold coordinated sites near these rings and/or
3+
2+
2+
x 6 0.1 with a high Bi content and a small Ni /Zn ratio. As
+
3+
2+
2+
Bi content decreasing and Ni /Zn ratio increasing, mixture
phases coexist in the specimen with x = 0.15–0.3. As Bi content
further increasing and Ni /Zn ratio increasing, the main phase
changes into cubic pyrochlore, however, the reflections of ortho-
rhombic peaks do not disappear completely in the specimen with
x = 0.4–0.5. Then, we assumed the structure of the specimen with
x = 0.4–0.5 as pseudo cubic pyrochlore. As Bi content decreases
to 1.7 and Ni /Zn ratio increases to 2.25, the cubic phase is
obtained when x P 0.6.
3+
5+
found together with the smaller Nb in a 6-fold coordination envi-
ronment. Meanwhile, the larger Bi3 with a coordination number of
seven or eight fully occupied A-sites [29]. However, for the cubic
pyrochlores, the interstices formed by the six-membered rings of
octahedra in HTB layers are occupied by the (5 + 3) – fold coordi-
nated A (Bi/Zn)-cations [29].
2+
2+
+
3+
2+
2+
3+
In this (1ꢀx)b-BZN–xBNN system, Bi occupies A-site and the
2
+
2+
divalent cations of Zn or Ni can occupy both the A- and
B-sites. Based on the structural difference between the two
phases, Bi content as well as the distribution and variety of diva-
Fig. 4 shows the SEM micrograph of the 0.8 b-BZN-0.2 BNN
nanopowders prepared by the sol–gel method. The crystallites
possess spherical morphology with a narrow size distribution of
2
+
2+
lent cations, such as Ni /Zn ratio exhibit a strong effect on the
phase evolution in this pyrochlore system. The dependence of
the phase evolution of the (1ꢀx)b-BZN–xBNN compositions on
the stoichiometric of Bi content and Ni2 /Zn ratio are shown
+
2+
in Fig. 3 and Table. 2. Phase transformation, from orthorhombic
Table 1
Crystallographic data obtained from Rietveld refinement for the (1ꢀx)b-BZN–xBNN
(0.0 6 x 6 1.0) compositions.
Compounds
Lattice parameters (Å)
V
unit (Å3)
a
b
c
Bi
2
Zn2/3Nb4/3
O
7
8.1641
8.1653
10.5153
10.5123
10.5059
10.4981
10.7561
10.6845
10.5153
10.5123
10.5059
10.4981
6.4884
6.4667
10.5153
10.5123
10.5059
10.4981
527.17
523.9
1162.68
1161.68
1159.57
1156.98
0.9b-BZN–0.1BNN
0.4b-BZN–0.6BNN
0.3b-BZN–0.7BNN
0.2b-BZN–0.8BNN
Fig. 4. SEM micrograph of the 0.8b-BZN–0.2 nanopowders prepared by the sol–gel
process.
Bi3/2NiNb3/2O
7

Y.X. Jin et al. / Journal of Alloys and Compounds 622 (2015) 200–205
203
3
0–55 nm in average diameter at the nanoscale. Typical surface
morphology of the (1ꢀx)b-BZN–xBNN ceramics with x = 0.0, 0.2,
.0 are illustrated in Fig. 5. The SEM micrograph indicates that
value for orthorhombic phase and the negative
s
e
value for cubic
phase offer the potential for low s value in the two-phase region.
e
1
The frequency dependence of dielectric properties of the b-BZN
(x = 0.0) and BNN (x = 1.0) compositions at room temperature have
been presented in Fig. 6 (100 Hz–30 MHz). Dispersive behaviors
can be observed in both the dielectric constant and dielectric loss
dense microstructures with well-defined and closely packed grains
can be obtained in all the compositions. For x = 0.0 (orthorhombic
phase), it reveals a well-sintered polycrystalline sample with irreg-
ular (spherulite-like) grains due to its low crystallization tempera-
ture caused by the large bismuth content and their grain sizes are
2 7 7
tangent of the Bi Zn2/3Nb4/3O and Bi1.5NiNb1.5O compositions.
In addition, the decrease in dielectric loss tangent is rapid at lower
frequencies and becomes slow at higher frequencies, which takes
place when the jumping frequency of electric charge carriers can’t
follow the alternation of applied AC electric field beyond a certain
critical frequency [31]. And for the BZN pyrochlore ceramics, the
performances of the dielectric properties in higher frequency range
still need to be further investigated.
found in the range of 0.16–0.55 lm (Fig. 5(a)) [30]. As the x value
increasing, two kinds of grains, small grains and big grains corre-
sponding to orthorhombic and cubic phase respectively are
observed (Fig. 5(b)). For the BNN composition with cubic symme-
try (x = 1.0), dense microstructure with well-defined bigger polyg-
onal grains can be seen in Fig. 5(c) and their grain sizes are found in
the range of 0.67–1.2
BNN ceramics estimated by Scherrer method are 35 nm and
1 nm respectively, which indicate that the grains consist of hun-
l
m. The crystallite sizes of the b-BZN and
Fig. 7 shows the temperature dependence of the dielectric prop-
erties as a function of frequency for x = 0.0, 1.0 compositions. For
pure Bi Zn2/3Nb4/3O composition, the dielectric constant has a lin-
2 7
5
dreds of small crystallites.
ear temperature dependence with no relaxation, it increases
Table. 3 summarizes the dielectric properties of Bi
2
Zn2/3Nb4/3
O
7
slightly within the measurement temperature range (ꢀ55 °C to
(
x = 0 for orthorhombic phase), Bi1.5NiNb1.5
phase) end members and a low composition (x = 0.2). The cubic
phase exhibits much higher dielectric constant of = 122 than the
orthorhombic phase of = 83.3. All of them obtain relatively small
dielectric loss tangent (tand < 1 ꢅ 10 ) at 1 MHz. The positive
O
7
(x = 1.0 for cubic
+150 °C) as shown in Fig. 7(a). However, for the Bi1.5NiNb1.5O
7
s
e
composition (x = 1.0), the dielectric constant diminishes slowly, it
has a maximum around ꢀ50 °C at 1 MHz, which may shift to lower
temperature with decreasing frequency. Furthermore, as the fre-
quency increasing, the dielectric loss tangent increases and the
onset of dielectric loss tangent shifts toward higher temperature
below the room temperature as shown in Fig. 7(b). This drop
should be ascribed to the dielectric relaxation observed in cubic
e
r
e
r
ꢀ3
s
e
pyrochlores at lower temperatures and the T
at which the maximum in dielectric constant occurs) of
has been reported to be located blow 150 K [19].
This dielectric relaxation phenomenon has also been observed in
other bismuth pyrochlores, such as Bi ScNbO and Bi InNbO
and has been attributed to dipolar glasslike mechanism
32–34]. The upturns in dielectric loss tangent at higher tempera-
m
(the temperature
Bi1.5NiNb1.5O
7
2
7
2
7
,
a
[
ture are typically associated with losses by conduction. Such
variation is strongly dependent on the frequencies with losses at
high frequencies lower than those occurring at low frequencies.
With the temperature increasing, electrical conductivity increases
due to the increase in thermally activated drift mobility of electric
charge carriers according to the hopping conduction mechanism [31].
According to the infrared and Raman spectra data for the cubic
BZN pyrochlore, each A atom occupies one of 6 closely spaced pos-
0
sible positions and each O atom is disordered among 12 positions
[
7
34]. The relaxation behavior observed in the Bi1.5NiNb1.5O com-
positon might stem from the hopping of dynamically disordered
0
A (Bi/Ni) and O atoms among the closely spaced possible positions
[
34,35]. The origin of dynamic dielectric polarization relaxation in
the cubic BZN pyrochlore has also been investigated by Tan et al.
through the Raman spectroscopy and photoluminescence analysis
[
36]. The investigations demonstrate that A site vacancies (Zn defi-
ciency) and associated oxygen vacancies always exist in BZN
pyrochlores [28,35]. The existence of these oxygen vacancy sites
can lead to the creation of
a Frenkel defect notated by
0
0
O
o
$ O þ V o€ . The Frenkel defects associated with the existence
i
0
of vacancies on the A and O sites may result in the dielectric relax-
ation in the cubic BZN pyrochlore. However, the closely related
2 7
orthorhombic Bi Zn2/3Nb4/3O
which has completely ordered Bi3+
2
+
with no Zn substitution on the A sites as already mentioned,
exhibits no dielectric relaxation below phonon frequencies. The
absence of dielectric relaxation for the orthorhombic zirconolite
structure, in contrast to the cubic pyrochlore dielectric properties,
demonstrates that the primary structural differences between
these phases are responsible.
Fig. 8 presents the variations of the dielectric constants (
and temperature coefficients of dielectric constant ( ) of the
(1ꢀx)b-BZN–xBNN ceramics as a function of BNN content at 1 MHz.
r
e )
s
e
Fig. 5. SEM micrographs of the (1ꢀx)b-BZN–xBNN ceramics: (a) x = 0.0, (b) x = 0.2,
and (c) x = 1.0.

2
04
Y.X. Jin et al. / Journal of Alloys and Compounds 622 (2015) 200–205
Table 3
Dielectric properties of the (1ꢀx)b-BZN–xBNN (0.0 6 x 6 1.0) compositions at 1 MHz.
ꢀ
Composition
Bi (Zn2/3Nb4/3)O
.8b-BZN–0.2BNN
Bi3/2NiNb3/2 (x = 1.0)
Crystal structure
e
r
tand (10 4
)
e
s (ppm/°C)
Sintering condition
2
7
(x = 0.0)
Orthorhombic
Orthorhombic + cubic
Cubic
83.3
1.9
9.3
3.1
170.5
900 °C, 4 h
900 °C, 4 h
950 °C, 4 h
0
105.6
122.0
ꢀ11.1
O
7
ꢀ361.5
Fig. 6. Frequency dependence of dielectric properties of the (1ꢀx)b-BZN–xBNN
ceramics for x = 0.0 (b-BZN) and x = 1.0 (BNN) compositions (100 Hz–30 MHz).
Within the orthorhombic (1ꢀx)b-BZN–xBNN solid solution
region (0.0 6 x 6 0.1), the
e
s values remain unchanged at about
70 ppm/°C. As the BNN content increases, the s shifts signifi-
e
1
cantly to negative region from an initial value of 61.9 ppm/°C for
x = 0.15 to ꢀ554.8 ppm/°C for x = 0.5 accompanied by the dilution
e
of orthorhombic phase. The changes of slope in s versus composi-
tion (x) locate between the phase boundaries in the figure. The
dependence of on composition within the two-phase region
s
e
can be attributed to a logarithmic mixing rule in which the relative
amounts of cubic and orthorhombic solid solution phases control
the dielectric properties. Moreover, a near-zero
x = 0.2) which has a
of ꢀ11.1 ppm/°C, a dielectric constant of
05.6 and a small dielectric loss tangent of tand ꢃ 10 , is found
within the two-phase region. For 0.6 6 x 6 1.0, this trend is
reversed adopts less-negative values. It shifts from
e
s composition
(
1
s
e
Fig. 7. Temperature dependence of dielectric properties of the (1ꢀx)b-BZN–xBNN
ceramics for (a) x = 0.0 (b-BZN), (b) x = 1.0 (BNN) compositions (dielectric constant
e and dielectric loss tangent tand at 1, 10, and 100 kHz and 1 MHz).
r
ꢀ
4
–
s
e
ꢀ
584.9 ppm/°C to ꢀ361.5 ppm/°C with the increasing BNN content
within the cubic phase region. Valant et al. suggested that a corre-
lation exists between the ratio of the A- and B-site ionic radii
(
s
(
R
A
/R
values decrease as R
1ꢀx)b-BZN–xBNN solid solution range (0.6 6 x 6 1.0), as the x
B
) and
s
e
in a chemically substituted cubic BZN system: the
e
A
B
/R increase [37]. Within the cubic
3+
value increasing, A-site Bi with large ionic radii decreases which
decreases R /R that leads to the increasing of the
The dielectric constants show almost no change and are very
similar to the parent compound Bi = 83.3) for
the orthorhombic (1ꢀx)b-BZN–xBNN solid solution (0.0 6 x 6
.1). Within the cubic (1ꢀx)b-BZN–xBNN solid solution range
A
B
e
s .
2 7
Zn2/3Nb4/3O (e
r
0
(
0.6 6 x 6 1.0), the dielectric constants decrease with the increas-
ing x value from 165.6 for x = 0.6–122 for x = 1.0 as the composi-
tion leaves the phase boundary. Generally, the dielectric constant
e
r
is dependent on the ionic polarizabilities of the composition,
the decrease in dielectric constant within the cubic phase region
3
+
may be due to the decreasing Bi with high ionic polarizability
(
Fig. 8. Dielectric constant (
r
e ) and temperature coefficient of dielectric constant
3
6.12 Å ) [38]. The dependence of the dielectric constant on
(s
e
), as a function of x in the (1ꢀx)b-BZN–xBNN (0.0 6 x 6 1.0) system at 1 MHz.

Y.X. Jin et al. / Journal of Alloys and Compounds 622 (2015) 200–205
205
composition within the two-phase region also exhibits good agree-
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The sol–gel process was employed to synthesize
(
1ꢀx)b-BZN–xBNN (0.0 6 x 6 1.0) ceramics. The phase composi-
tions, microstructures and dielectric properties were found to be
strongly correlated with the x value. X-ray diffraction results indi-
cated that phase transformation, from orthorhombic zirconolite-
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[
[
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4
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