Microwave dielectric properties of the (1?x)La(Nb0.9V0.1)O4-xCaMoO4 (0.05?≤?x?≤?0.50) scheelite solid solution ceramics

Article abstract of DOI:10.1016/j.jallcom.2019.03.057

The CaMoO₄ was added into La(Nb_0.9V_0.1)O₄ ceramics by solid state reaction method to adjust the temperature coefficient of frequency (TCF) values. The series of (1?x)La(Nb_0.9V_0.1)O₄-xCaMoO₄ (0.05 ≤ x ≤ 0.5) ceramics were sintered at 1060–1180 °C and found to possess permittivity (ε_r) = 10.38–17.73, Q × f = 31,820–76,570 GHz (Q = 1/dielectric loss, f = resonant frequency lying between 8.5 and 9 GHz), and TCF = ?26.3 ～ +160.7 ppm/°C. The structures of ceramic samples changed from monoclinic fergusonite to tetragonal scheelite continuously at x = 0.20 along with the shift of TCF values from positive to negative. The best microwave dielectric properties with ε_r = 15.71, Q × f = 76,310 GHz (at 8.825 GHz), and TCF = ? 26.3 ppm/°C were obtained in 0.8La(Nb_0.9V_0.1)O₄-0.2CaMoO₄ ceramics sintered at 1160 °C and these ceramics might be good candidates for microwave devices.

Full text of DOI:10.1016/j.jallcom.2019.03.057

Accepted Manuscript
Microwave dielectric properties of the (1−x)La(Nb
V
)O -xCaMoO
0
.9 0.1 4
4
(0.05￿ ꢀ￿ x￿ ꢀ￿ 0.50) scheelite solid solution ceramics
Zhili Ma, Jinxiong Zhao, Di Zhou
PII:
S0925-8388(19)30864-3
DOI:
https://doi.org/10.1016/j.jallcom.2019.03.057
JALCOM 49821
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 8 January 2019
Revised Date: 2 March 2019
Accepted Date: 4 March 2019
Please cite this article as: Z. Ma, J. Zhao, D. Zhou, Microwave dielectric properties of the
1−x)La(Nb )O -xCaMoO (0.05￿ ꢀ￿ x￿ ꢀ￿ 0.50) scheelite solid solution ce rJ ao mu ri cn sa ,l of Alloys
(
V
0
.9 0.1
4
4
and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.03.057.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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ACCEPTED MANUSCRIPT
Microwave dielectric properties of the
(
1−x)La(Nb V )O -xCaMoO (0.05 ≤ x ≤ 0.50)
0
.9 0.1
4
4
scheelite solid solution ceramics
a
b
∗ c
Zhili Ma, Jinxiong Zhao, and Di Zhou �
a
State Grid Gansu Electric Power Company, Lanzhou 730030, China
b
State Grid Gansu Electric Power Research Institute, Lanzhou 730070, China
c
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of
Education & International Center for Dielectric Research, School of Electronic and
Information Engineering, Xi’an Jiaotong University, Xi'an 710049, China
Abstract
The CaMoO was added into La(Nb V )O ceramics by solid state reaction method
4
0.9 0.1
4
to adjust the temperature coefficient of frequency (TCF) values. The series of
1−x)La(Nb V )O -xCaMoO (0.05 ≤ x ≤ 0.5) ceramics were sintered at 1060 ~
(
0
.9 0.1
4
4
o
1
180 C and found to possess permittivity (ε ) = 10.38 ~ 17.73, Q×f = 31,820 ~ 76,570
r
GHz (Q = 1 / dielectric loss, f = resonant frequency lying between 8.5 ~ 9 GHz), and
o
TCF = −26.3 ~ +160.7 ppm/ C. The structures of ceramic samples changed from
monoclinic fergusonite to tetragonal scheelite continuously at x = 0.20 along with the
shift of TCF values from positive to negative. The best microwave dielectric
properties with ε = 15.71, Q×f = 76,310 GHz (at 8.825 GHz), and TCF = − 26.3
r
o
o
ppm/ C were obtained in 0.8La(Nb V )O -0.2CaMoO ceramics sintered at 1160 C
0
.9 0.1
4
4
and these ceramics might be good candidates for microwave devices.
Keywords: Microwave dielectric; LaNbO ; low temperature co-fired ceramic (LTCC)
4
∗
Corresponding author E-mail address: zhoudi1220@gmail.com (Di Zhou).

ACCEPTED MANUSCRIPT
1
. Introduction
Today, microwave dielectric ceramics are being studied by more and more people
because of their universal application as dielectric resonators, filters, RF substrates
and waveguides [1-3]. Generally, ceramics are prepared by conventional solid state
reaction methods. The key point of this approach is mainly the presence of ceramic
processing, which makes the ceramic compact enough. Microwave dielectric ceramics
have been studied for decades to pursue good microwave dielectric properties, namely
high Q×f (Q=1/dielectric and f=resonant frequency), high relative dielectric constant
(εr) and near-zero resonant frequency temperature coefficient (TCF) [4-6]. Compared
with the traditional perovskite (ABO ) materials, recently the universal ABO systems,
3
4
including wolframite (MgWO ), scheelite (CaWO ), fergusonite (LaNbO ), rutile
4
4
4
(
TiO ) and zircon (NdVO ) materials, have attracted more and more attention due to
2 4
the structure adaptability [7-9].
It has been reported that the LaNbO material transitions from fergusonite structure
4
o
to scheeelite structure at about 600 C [10] and at the same time there is remarkable
o
thermal expansion transition. Besides, there is a wide temperature gap 450-750 C,
which can be revealed from Neutron and X-ray diffraction due to the existence of
multicrystals [11,12]. Stubican et al. [13] noticed that the related phase transition
temperature increased with decrease of rare earth ionic radius, and the niobates have
lower phase transition temperatures than the corresponding tantalates. Based on the
literatures’ results, some researchers have provided solutions that use Ta instead of Nb
or reduce the ionic radius of rare earth ions [14,15]. In 1980s, Bastide et al. [16]
concluded that phase transition of ABX ceramics according to the radius ratio of
4
cation and anion. Subsequently, this work was supplemented by Manjon et al., [17]
which mainly aimed at current research focusing on materials including orthorhombic,

ACCEPTED MANUSCRIPT
wolframite, fergusonite scheelite and zircon structures. It is well known that the phase
evolution from high-temperature scheelite to low-temperature multicrystal is
attributed to decrease of B-site cation.[16-18] Brik et al. [18] noticed that the phase
transition temperature of LaNbO ceramics could be adjusted to near room
4
temperature in the manner of replacing by vanadium with smaller radius than niobium.
Therefore, Brandão et al. [19] studied the influence of electrons and protons by
scheelite retention.
Brower and Choi et al. reported microwave dielectric properties of the CaMoO4
o
ceramics sintered at 1100 C with ε = 10.8 ~ 11.7, Q×f = 55,000 ~ 89,700 GHz and τ
r
f
o
=
− 50 ~ − 60 ppm/ C [20-24]. Vidya et al. [23] found that the CaMoO
4
o
nano-ceramics could be densified at a quite low sintering temperature ~ 775 C.
However, the large negative TCF of CaMoO ceramic still limited the practical
4
applications. Recently, a temperature stable LTCC (low temperature co-fired ceramics
technology) materials have been achieved by using TiO to adjust performance of the
2
2
+
CaMoO ceramics [23]. Meanwhile, Ca in CaMoO ceramics could be substituted
4
4
2+
for by (Li Bi ) ions and this resulted in near-zero TCF values [24]. Meanwhile,
0
.5 0.5
the low sintering temperatures of the (Li Bi )MoO -CaMoO solid solution
0
.5 0.5
4
4
ceramics make them good candidates for LTCC technology [24,25]. We recently
reported that the La(Nb V )O ceramics have good microwave dielectric properties
0
.9 0.1
4
including a large positive TCF value [26]. In the present study, we primarily
researched the relation of microwave dielectric properties and structure of the
(
1−x)La(Nb V )O -xCaMoO ceramics.
0.9 0.1 4 4
2
. Experimental Procedure
Reagent-grade raw materials of Nb O (> 99.5%, Guo-Yao Co. Ltd.), La O (>
2
5
2
3
9
9.95%, Guo-Yao Co. Ltd.), V O (> 99%, Fuchen Chemical Reagent, Tianjin, China),
2 5

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CaCO (> 99%, Guo-Yao Co. Ltd.) and MoO (>99%, Guo-Yao Co. Ltd.) were
3
3
measured
according
to
the
stoichiometric
compositions
of
the
(
1−x)La(Nb V )O -xCaMoO (0.05 ≤ x ≤ 0.50). La O powders were treated at
0.9 0.1 4 4 2 3
o
1
000 C for 5 h before weighting to remove carbonates. Powder mixtures were milled
for 5 h using a planetary mill (QM-1F; Nanjing Machine Factory, Nanjing, China) at
2
00 rpm with the milling media (zirconia balls). After drying the mixtures were
o
calcined at 800 C for 5 h and then re-milled at 250 rpm for 5 h to obtain fine powders.
The fine powders were pressed into pellets (11 mm in diameter and 4 ~ 5 mm in
height) with 5 wt. % polyvinyl alcohol (PVA). All the samples were sintered in the air
o
o
at 1000 C ~ 1200 C for 2 h.
Phase compositions and crystal structures of the (1−x)La(Nb V )O -xCaMoO
4
0
.9 0.1
4
(0.05 ≤ x ≤ 0.50) ceramics were investigated using X-ray diffraction (XRD) with Cu
Kα radiation (Rigaku D/MAX-2400 X-ray diffractometry, Tokyo, Japan) and Rietveld
refinement method was used to analyze the details of crystal structures.
Microstructures were studied using a scanning electron microscopy (SEM; Quanta
2
50 F, FEI). Microwave dielectric properties were measured using the shielded
resonant cavity method using the TE01δ mode to calculate the permittivity and Q
values, a network analyzer (8720ES, Agilent, Palo Alto, CA), and a temperature
chamber (Delta 9023, Delta Design, Poway, CA). The TCF / τ values were obtained
f
from the following formula:
fT − f
T0
6
TCF =
×10
(
ppm/°C
)
(1)
f ×(T −T )
T
0
0
o
o
where f and f are the resonant frequencies at 85 C and 25 C, respectively.
T
T0
3
. Results and Discussions
Figure 1 shows XRD patterns of the (1−x)La(Nb V )O -xCaMoO (0 ≤ x ≤ 1)
0
.9 0.1
4
4

ACCEPTED MANUSCRIPT
ceramics sintered at their optimal temperatures for 2 h, which can help us understand
the mechanism of two kinds of phase transition. The lattice parameters of pure
La(Nb V )O ceramic were calculated to be a = 5.508(9) Å, b = 11.603(8) Å, c =
0
.9 0.1
4
o
3
5
.236(0) Å, β = 94.920 , and cell volume = 334.274(4) Å . As the substitution content
increased, the phase transition from fergusonite to tetragonal sheelite structure can be
clearly observed from XRD patterns. In the range 0 ≤ x ≤ 0.1, the
(
1−x)La(Nb V )O -xCaMoO ceramics belong to a fergusonite solid solution with
0.9 0.1 4 4
a space group I2/a. It was clearly seen from Figure 1, XRD reflection peaks (−121)
and (121) of (1−x)La(Nb V )O -xCaMoO (0 ≤ x ≤ 0.1) ceramics moved to each
0
.9 0.1
4
4
other continuously with increase in CaMoO contents. Finally at x = 0.2, (−121) and
4
(121) merged into one peak (112), which indicates that crystal structure of the
(
1−x)La(Nb V )O -xCaMoO ceramics changed from fergusonite to scheelite.
0
.9 0.1
4
4
Figure
2
shows
the
variations
in
lattice
parameters
of
the
(
1−x)La(Nb V )O -xCaMoO ceramics. As CaMoO content increased, lattice
0
.9 0.1
4
4
4
o
parameters a, b and β (β > 90 ) in the solid solution region (x ≤ 0.1) decreased
slowly. Lattice parameters c increased slightly. With further increase of x value, the a
and c values became equal to each other. Meanwhile, the b increased to 11.430(3) Å
at x = 0.2 in tetragonal structure. The variation of lattice parameters clearly suggested
the phase transition from fergusonite to scheelite structure in the
(
1−x)La(Nb V )O -xCaMoO ceramics. These changes are in accordance with
0.9 0.1 4 4
XRD results analysis. In order to research the details of crystal structure, Rietveld
refinements were performed to study the crystal structure of the
0
.9La(Nb V )O -0.1CaMoO and 0.8La(Nb V )O -0.2CaMoO ceramics. The
0.9 0.1 4 4 0.9 0.1 4 4
Rietveld refinement results are presented in Figure 3. The space group, occupancies
and atomic positions are given in the Table 1 (R = 9.94 %, R = 12.9 %, and R
=
exp
p
wp

ACCEPTED MANUSCRIPT
7
0
.06
%)
and
all
the
refinement
results
are
acceptable.
The
.9La(Nb V )O -0.1CaMoO ceramic belongs to single fergusonite phase with a
0
.9 0.1
4
4
space group I2/a. The 0.8La(Nb V )O -0.2CaMoO ceramic belongs to standard
0
.9 0.1
4
4
sheelite structure with a space group I4 /a accordingly.
1
o
SEM results of the (1−x)La(Nb V )O -xCaMoO ceramics sintered at 1160 C are
0
.9 0.1
4
4
shown in Figure 4. The dense micro-structure of the (1−x)La(Nb V )O -xCaMoO
4
0
.9 0.1
4
ceramics can be observed with almost no pores. With the increase of CaMoO4
contents, grains of the ceramics become more homogeneous and very uniform grain
distribution can be observed in the 0.8La(Nb V )O -0.2CaMoO ceramic. Figure 5
0
.9 0.1
4
4
shows the variation of densities of the (1−x)La(Nb V )O -xCaMoO (0.05 ≤ x ≤ 0.5)
0
.9 0.1
4
4
ceramics
as
a
function
of
sintering
temperature.
All
the
(
1−x)La(Nb V )O -xCaMoO ceramics can be well densified in wide sintering
0
.9 0.1
4
4
temperatures. Due to the atomic weight differences, the increase of CaMoO content
4
apparently resulted in decrease of the bulk densities.
Variation of permittivity and Q×f values of the (1−x)La(Nb V )O -xCaMoO
4
0
.9 0.1
4
(0.05≤ x ≤ 0.50) ceramics as a function of sintering temperature are shown in Figure 6.
All the permittivity values keep stable in a quite wide sintering temperature range as
shown in Figure 6(a) and this is in accordance with the change trend of bulk density.
The largest permittivity about 17.63 was obtained in the 0.95
La(Nb V )O -05CaMoO
ceramic. Variation of Q×f values of the
1−x)La(Nb V )O -xCaMoO (0.05 ≤ x ≤ 0.50) ceramics s a function of sintering
0
.9 0.1
4
4
(
0
.9 0.1
4
4
temperature are shown in Figure 6(b) (resonant frequency lying between 8.5 ~ 9 GHz).
Q×f values increase with sintering temperature first, and then decrease due to
secondary grain growth at high temperatures. The 0.8 La(Nb V )O -0.2CaMoO
0
.9 0.1
4
4
o
ceramics sintered at 1160 C posses the highest Q×f values ~ 76,310 GHz at 8.825

ACCEPTED MANUSCRIPT
GHz.
Figure 7 shows microwave dielectric permittivity and Q × f values of the
1−x)La(Nb V )O -xCaMoO (0 ≤ x ≤ 1) ceramics as a function of CaMoO
4
(
0
.9 0.1
4
4
content. Generally, permittivity is determined by dielectric polarization. In range of
microwave frequency, dielectric polarization is the sum of both ionic and electronic
components [27]. Shannon et al. [27] suggested that molecular polarizability could be
estimated as the sum of individual ionic polarizabilities. Hence, polarizabilities (α ) of
x
the (1−x)La(Nb V )O -xCaMoO ceramics could be obtained as follows:
0
.9 0.1
4
4
α x
=
(
1− x
)
α La +
3
(
+ 0.9 1− x
)
α Nb +
5
+ 0.1(1− x)αV +
5
+ xα Ca +
2
+ xα Mo +
6
+ 4α O +
2
(2)
α La3+
,
α Nb5+
,
αV 5+
,
α Ca2+
,
α Mo6+
α O2+
3+
5+
5+
and
where
are polarizabilities of La , Nb , V ,
2
+
6+
2+
Ca , Mo and O ions [27]. Using the Clausius-Mosotti relation, the dielectric
permittivity can be calculated as follows:
3
V +8πα x
x
εx
=
(3)
3V − 4πα x
x
3
in which V is the cell volume (= 336.695 / 4= 84.17 Å ). Figure 7(a) shows that εcal
x
and εr of the (1−x)La(Nb V )O -xCaMoO ceramics gradually decrease as the
0.9
0.1
4
4
2
+
6+
increase of CaMoO content due to the smaller ionic polarizability of Ca and Mo
4
3
+
5+
than that of La and Nb . Q×f values of the CaMoO and La(Nb V )O ceramics
4
0.9 0.1
4
are about 90,000 GHz and 60,000 GHz, respectively, as reported in literatures [21,26].
As shown in Figure 7(b), Q×f values increase with the content of CaMoO as
4
expected. Quite high Q×f values were obtained in the solid solution ceramics due to
the good sinterability that resulted in homogeneous microstructures. The
0
7
.8La(Nb V )O -0.2CaMoO ceramic possesses the highest Q×f value about
0.9 0.1 4 4
6,310 GHz at 8.825 GHz. The TCF values as a function of x value are presented in
Figure 7(c). It can be seen that the TCF values are large positive within monoclinic

ACCEPTED MANUSCRIPT
fergusonite solid solution region (x < 0.2) and this is in accordance with results in the
La(Nb,V)O system.[26] Introduction of CaMoO4 into the La(Nb V )O lowered
4
0.9 0.1
4
the ferroelastic phase transition temperature to near room temperature, resulting in
composition-sensitive and unstable TCF values. When x ≥ 0.2, the TCF values keep
negative and increase with the CaMoO content in scheelite region, which is quite
4
o
normal in solid solution ceramics. The smallest TCF value about − 26.3 ppm/ C was
obtained in the 0.8La(Nb V )O -0.2CaMoO ceramics. The intrinsic microwave
0
.9 0.1
4
4
dielectric properties were studied by far-infrared spectra as shown in Fig. S1 and
Table S1. All the fitted and measured values were found to be in good agreements.
4
. Conclusions
o
The 0.65La(Nb V )O -0.35CaMoO ceramic sintered at 1160 C was found to
0
.9 0.1
4
4
o
possesse a ε = 15.00, a Q×f = 69,720 GHz at 9 GHz, and a TCF/τ = − 30.6 ppm/ C.
r
f
The best microwave dielectric properties were obtained
in the
o
0
.8La(Nb V )O -0.2CaMoO ceramics sintered at 1160 C with a ε = 15.71, a Q×f
0
.9 0.1
4
4
r
o
=
76,310 GHz at 8.825 GHz, and τ = − 26.3 ppm/ C. The XRD analysis reveals that
f
the ceramics changed from monoclinic fergusonite to tetragonal sheelite phase at x =
.2. Far-infrared reflectivity fitting of the 0.65La(Nb V )O -0.35CaMoO ceramic
0
0
.9 0.1
4
4
shows that the main polarization comes from ionic polarization rather than electronic
ones. These materials are very promising candidates for high quality microwave
devices.

ACCEPTED MANUSCRIPT
Acknowledgments
This work was supported by the State Grid Science and Technology Projects (China,
22722180007), the National Key Research and Development Program of China
2017YFB0406301), the National Natural Science Foundation of China (U1632146),
5
(
the Key Basic Research Program of Shaanxi Province (2017GY-129), and the
Fundamental Research Funds for the Central University. The authors would like to
thank the administrators in IR beamline workstation (BL01B) of National
Synchrotron Radiation Laboratory (NSRL) for their help in the IR measurement and
fitting. The SEM work was done at the International Center for Dielectric Research
(
ICDR), Xi'an Jiaotong University, Xi'an, China and the authors thank Ms. Yan-Zhu
Dai for her help in using SEM.

ACCEPTED MANUSCRIPT
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structure,
and
microwave
dielectric
properties
of
water-insulation
(
[
1−x)LaNbO -LaVO (0 ≤ x ≤0.9), Inor. Chem. 56 (2017) 9321-9329.
4
4
27] R.D. Shannon, Dielectric polarizabilities of ions in oxides and fluorides, J. Appl.
Phys. 73 (1993) 348–366.

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Table 1 Crystallographic parameters of the 0.9La(Nb V )O -0.1CaMoO and the
0
.9 0.1
4
4
0
.8La(Nb V )O -0.2CaMoO ceramics (R = 9.94 %, R = 12.9 %, and R
=
exp
0
.9 0.1
4
4
p
wp
7
.06 %).
x
Atom
x
y
z
Occupancy
Mult
Space group: I2/a
Ca
La1
Nb1
V1
0.25000
0.25000
0.25000
0.25000
0.25000
0.01472
0.91465
0.12327
0.12327
0.63214
0.63214
0.63214
0.71352
0.45835
0.00000
0.00000
0.00000
0.00000
0.00000
0.16672
0.24286
0.050
0.450
0.405
0.045
0.050
1.000
1.000
4
4
4
4
4
8
8
0
.1
Mo1
O1
O2
Space group: I4 /a
1
Ca
La1
Nb1
V1
0.00000
0.25000
0.25000
0.25000
0.25000
0.25000
0.00500
0.62500
0.050
0.200
0.180
0.020
0.050
1.000
4
4
0.00000
0.00000
0.00000
0.00000
0.15684
0.62500
0.12500
0.12500
0.12500
0.20917
4
0
.2
4
Mo1
O1
4
16

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Figure Captions
Fig. 1. XRD patterns of the (1−x)La(Nb V )O -xCaMoO (0 ≤ x ≤ 1) ceramics
0
.9 0.1
4
4
sintered at their optimal temperatures.
Fig. 2. Lattice parameters variation in the (1−x)La(Nb V )O -xCaMoO (0 ≤ x ≤ 1)
0
.9 0.1
4
4
ceramics.
Fig. 3. Observed XRD (red points) and calculated pattern (black solid line) along with
difference plot (at the bottom) of (a) the 0.9La(Nb V )O -0.1CaMoO ceramics and
0
.9 0.1
4
4
(
b) the 0.8La(Nb V )O -0.2CaMoO ceramics. Allowed Bragg reflections are
0.9 0.1 4 4
indicated by vertical bars and goodness-of-fit shown in inset.
Fig. 4. SEM images of the (1−x)La(Nb V )O -xCaMoO (a) x = 0.05, (b) x = 0.10,
0
.9 0.1
4
4
(c) x = 0.20, (d) x = 0.35, (e) x = 0.50 ceramics sintered at 1160 o C for 2h.
Fig. 5 Variation of density of the (1−x)La(Nb V )O -xCaMoO (0.05 ≤ x ≤ 0.5)
0
.9 0.1
4
4
ceramics as a function of sintering temperature.
Fig. Variation of permittivity (a) and Q×f values (b) of the
1−x)La(Nb V )O -xCaMoO (0.05 ≤ x ≤ 0.50) ceramics as a function of sintering
6
(
0
.9 0.1
4
4
temperature.
Fig. εcal and εr (a), Q×f value (b), and TCF (c) of the
1−x)La(Nb V )O -xCaMoO (0 ≤ x ≤ 1) ceramics as a function of x value.
7
(
0
.9 0.1
4
4

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1
2
3
, Fergusonite solid solutions were formed in (1−x)La(Nb V )O -xCaMoO ;
0
.9 0.1
4
4
, A pseudo phase diagram is given in (1−x)La(Nb V )O -xCaMoO ;
0
.9 0.1
4
4
,
High Q×f ~ 76,310 GHz was obtained in 0.8La(Nb V )O -0.2CaMoO
0.9 0.1 4 4
ceramic.