Dielectric properties of CaCu3Ti4O12 ceramics doped with aluminium and fluorine

Article abstract of DOI:10.1016/j.jallcom.2021.159861

The ceramics СaСu₃Тi_4–xAl_xО_{12–0.5x–y}F_y were prepared using solid-state reactions technique and pre-synthesized CaTiO₃. It was found that the resistances of both the grain boundary R_GB and the interface between sample and electrode R_SE increase at the combined introduction of Al³⁺ and F^– ions. At 1100 °С, a decrease in dielectric properties after 12 h of sintering time due to copper losses was observed. The samples with x/y = 0.04/0.04 sintered for 10 h at 1 kHz are characterized by the following dielectric parameters: ε′ ≈ 72,000 and tan δ ≈ 0.047. In the sample x/y = 0.04/0, the lowest dielectric loss (tan δ ≈ 0.044) were found.

Full text of DOI:10.1016/j.jallcom.2021.159861

Journal of Alloys and Compounds 874 (2021) 159861
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Dielectric properties of CaCu3Ti4O12 ceramics doped with aluminium and
fluorine
O.Z. Yanchevskii, O.I. V’yunov⁎, A.G. Belous, L.L. Kovalenko
V.I.Vernadsky Institute of General and Inorganic Chemistry NAS of Ukraine, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
The ceramics СaСu3Тi4–xAlxО12–0.5x–yFy were prepared using solid-state reactions technique and pre-syn-
thesized CaTiO3. It was found that the resistances of both the grain boundary RGB and the interface between
sample and electrode RSE increase at the combined introduction of Al3+ and F– ions. At 1100 °С, a decrease in
dielectric properties after 12 h of sintering time due to copper losses was observed. The samples with x/
y = 0.04/0.04 sintered for 10 h at 1 kHz are characterized by the following dielectric parameters: ε′ ≈ 72,000
and tan δ ≈ 0.047. In the sample x/y = 0.04/0, the lowest dielectric loss (tan δ ≈ 0.044) were found.
© 2021 Elsevier B.V. All rights reserved.
Received 7 October 2020
Received in revised form 22 March 2021
Accepted 5 April 2021
Available online 10 April 2021
Keywords:
CaCu3Ti4O12 ceramic
Complex dopant
Aluminium
Fluorine
Dielectric properties
1. Introduction
oxygen octahedra. In a real structure, the local disorder of Cu2+ and
Ca2+ ions and the nanoscale disorder phases are possible [13], which
Miniaturization of electronic devices is one of the tasks of modern
microelectronics. In this regard, the development of materials with high
(“colossal”) dielectric constant (ε′ > 104) and low dielectric loss (tan
δ < 0.05) in a wide frequency and temperature range are of interest [1,2].
CaCu3Ti4O12 (CCTO) is known to have a high dielectric constant (ε′ ≈ 104
in ceramics and thin films, and ~105 in single crystals) in a wide tem-
perature (100–600 K) and frequency (1 Hz–10 MHz) ranges [3–5]. Var-
ious models explain the nature of high ε′. The internal barrier layers
capacitance (IBLC) model assumes that the CCTO ceramics consists of
high-conductive grains and non-conductive grain boundaries [4,6,7]. The
surface barrier layers capacitance (SBLC) model takes into account the
formation of the space charge regions near the electrodes [8,9]. Fang and
Liu [10] directly observed internal domains and domain walls in
CaСu3Тi4О12 by electron microscopy. Earlier Subramanian et al. dis-
covered the phenomenon of CCTO crystals twinning [11]. These facts
allow the development of the nanoscale barrier layers capacitance
(NBLC) model in the bulk of grains, where polarons exist within domains
limited by plane defects [12].
increases the dynamic dielectric response. The disorder in single-
crystal CCTO decreases with decreasing temperature and disappears
at 100 K [14]. At the same temperature, the dielectric constant drop,
and the large dielectric dispersion was observed [3].
The grain resistance CCTO depends on the number of weak-
trapped electrons, which associated with the following n-type, or p-
type transitions:
Cu2+ + e′ ↔ Cu1+; Ti4+ + e′ ↔ Ti3+; or Cu3+ + e′ ↔ Cu2+
In the temperature range of 150–350 K, the polaron electronic
defects, in particular, the complex Ti4+O5·V×O + e- (Ti4+O5·VO• or
Ti3+O5·V×O) contribute to the conductivity. The conductivity arises due
to electron transfer between TiO5·V•O–Cu′Cu and TiO5·V×O–CuC×u ad-
jacent structures [12].
Copper ions have high thermal activity and at the sintering of CCTO
ceramics leave their positions in the crystal lattice. This process changes
the oxidative state of the remaining Cu2+ and Ti4+ ions [15–20]. The
following mechanisms of defect formation are possible [17]:
CaCu3Ti4O12 has the body-centred cubic perovskite structure
4+
2+
4+
3Cu2+ → 2Cu1+ + Ti + Cu↑: Ca(Cu
Cu1+Ti4v+)Ti O12
(1)
(2)
ABO3 with space group Im3
m [3]. The Cu2+ and Ca2+ ions belong to A
Cu
3−3v 2v
4+
4−v
sublattice, but occupy crystallographic positions with different co-
3+
2+
Cu+ +Ti4+ → Cu2+ +Ti : Ca(Cu Ti4v+)Ti
Ti3+O12
Ti
3−v
4−2v 2v
ordination (4 for Cu2+ and 12 for Ca2+) due to the inclination of TiO6
If sintering is prolonged, the following defect formation me-
chanism becomes dominant:
⁎
2+
3Cu2+ → V″Cu + 2Cu3+ + Cu↑: Ca(Cu
Cu32+vV″v)Ti4O12
(3)
Corresponding author.
E-mail address: vyunov@ionc.kiev.ua (O.I. V’yunov).
3−3v
https://doi.org/10.1016/j.jallcom.2021.159861
0925-8388/© 2021 Elsevier B.V. All rights reserved.

O.Z. Yanchevskii, O.I. V’yunov, A.G. Belous et al.
Journal of Alloys and Compounds 874 (2021) 159861
During sintering at high temperatures, O2– anions can also re-
lease from the crystal lattice, form neutral oxygen vacancies, and
reduce the cations Cu2+, Ti4+ [18]:
3. Results and discussion
Earlier we proposed the separate synthesis of CCTO components
(CaTiO3 and CuTiO3) to avoid the formation of undesirable phases
[45]. CuTiO3 is not formed as a result of the eutectic reaction be-
tween CuO and TiO2 from 500 to 1020 °С [46]. CuTiO3 synthesized by
the sol-gel method was no single phase [47]. We found that heat
treatment of an equimolar mixture of CuC2O4 and ТiO2 at
600–900 °С for up to 12 h leads only phases of rutile and CuO. All
these facts imply the thermodynamic instability of CuTiO3.
2+
O2O− + 2Cu2+ → V˙O˙ + 2Cu1+ +1/2O2↑: Ca(Cu Cuu1+)Ti44+O12−u/2
(4)
(5)
3−u
O2O− + 2Ti4+ → V˙O˙ + 2Ti3+ +1/2O2↑: CaCu32+(Ti Ti3w+)O12−w/2
4+
4−w
The decrease in the electrical resistance of grain of air-quenched
CCTO ceramics confirms the significant impact of the oxygen va-
cancies in the conductivity formation [21].
Cu ions diffuse into the boundary layer results in the formation of
Cu-rich area (СuxO-containing phases (CuO, Cu4O3, Cu2O) or posi-
tively charged Cu ions) at intercrystalline grain boundaries and Cu
vacancies at the domain boundaries. Cu vacancies at the domain
boundaries form potential barriers for electron moving [19,22].
Thus, CCTO-based materials are sensitive to heat treatment
conditions (temperatures, duration, sintering atmosphere) and to
deviation from stoichiometry [17,23–25]. In order to improve CCTO
characteristics, cation and anion substitutions were used [26–32].
The additions of Al3+ [33–39] and/or F– [40–42] can affect the elec-
trical characteristics of CCTO ceramics, but their role is not clearly
understood. In this paper, we consider the effect of separate and
combined additions of aluminium and fluorine on the dielectric
properties of CCTO ceramics.
XRD shows that after double (with intermediate grinding) heat
treatments of the powder at 920–960 °С for at least 12 h, the single-
phase solid solutions СaСu3Тi4−xAlxО12–0.5x–yFy are formed. Ceramic
samples sintered at 1100 °С for 10 h matched the ICDD card 75–1149
and had space group Im m (Fig. 1a). For Al-doped samples, the
3
traces of copper oxides (Cu2O, Cu4O3) arise due to the precipitation
of the Cu-O secondary phase at grain boundaries of the ceramics
surface layer [19,37].
The unit cell volume measured using the Le Bail method (Fig. 1b)
at y = 0 (without fluorine), x = 0 (without aluminium), x = y (with an
equal amount of aluminium and fluorine simultaneously) are shown
in Fig. 2. A linear decrease in unit cell volume (V) from 404.08 Å3 at
x = 0 to 403.78 Å3 at x = 0.08 (Fig. 2) indicates the formation of the
СaСu3Тi4−xAlxО12–0.5x solid solution. The unit cell volume decreases
since Ti4+ ion (R = 0.605 Å) is replaced with a smaller Al3+ ion
(R = 0.535 Å) [48]. In addition, to satisfy the electroneutrality con-
dition, a decrease in the charge of B sublattice of ABO3 perovskite
structure should be compensated by an increase in the charge of the
A-sublattice due to a decrease in the number of Cu1+ ions. When an
Al-containing compound was added to pre-synthesized CCTO
2. Experimental
Solid solutions with nominal composition СaСu3Тi4–xAlxО12–0.5x–yFy
(where x/y = 0/0; 0.04/0; 0/0.04; 0.08/0; 0/0.08; 0.04/0.04; 0.08/0.08)
were synthesized by solid-state reaction technique. Analytical grade
CuC2O4, CaCO3, CaF2, Al(NO3)3, and extra pure grade TiO2 (anatase) were
used. At the first stage, CaTiO3 was prepared from the stoichiometric
mixture of CaCO3 and TiO2. Both reagents were previously dried for 4 h
at 500 and 600 °C, respectively. Then they were milled in a vibrating mill
using ZrO2 balls and deionized water for 6 h. After drying at 100 °С, the
mixture of CaCO3 and TiO2 heat-treated at 1100 °С for 6 h. At the second
stage, stoichiometric mixtures of synthesised CaTiO3, CuC2O4, TiO2, CaF2,
and 1 M water solution of Al(NO3)3 were prepared. These mixtures were
ground in a Retch PM100 planetary mill using ZrO2 balls and isopropyl
alcohol for 4 h at 300 rpm. Then, the mixtures were dried at 100 °С and
double heat treated at 920 °С/12 h and 960 °С/12 h with intermediate
grinding in an agate mortar with a pestle. The synthesized powders
were mixed with a 5% aqueous solution of polyvinyl alcohol and passed
through a 150 mesh nylon sieve. Disc-shaped specimens with a diameter
of 8.5 mm and a thickness of 2 mm were pressed under a pressure of
120 MPa. Ceramics were sintered at 1100 °C for 10–12 h.
The phase composition of the products was determined by X-ray
phase analysis (XRD) on a DRON-4-07 diffractometer (40 kV, 20 mA)
using CuKα radiation and Ni filter. As certified standards, NIST
SRM640e-SiO2 (2Θ standard) and NIST SRM1976-Al2O3 (intensity
standard) were used. The relative X-ray impulse counting error did
not exceed 0.5%. The unit cell parameters of the samples were de-
termined using FullProf software by the whole-pattern profile-
matching Le Bail procedure [43]. The crystallite sizes of etched
ceramic samples were studied using scanning electron microscope
JEOL JEM-12301 and the average grain diameters were measured by
at least 50 grains from different areas using ImageJ software [44].
The density of ceramics was determined by Archimedes principle.
To deposit the silver electrodes on polished ceramic samples, Ag-
containing paste was burned at 600 °C for 0.5 h. The complex im-
pedance of the samples with a diameter of 7.25 mm and a thickness
of 1.6 mm was investigated using a 1260 Impedance/Gain-Phase
Analyzer (Solartron Analytical). The equivalent circuit and the value
of its components were determined using the ZView® software
(Scribner Associates Inc., USA). Measurement error in the frequency
range 103–105 Hz does not exceed 500 for ε′ and 0.002 for tan δ.
Fig. 1. (a) XRPD patterns of ceramic samples СaСu3Тi4–xAlxО12–0.5x–yFy, sintered at
1100 °C for 10 h; (b) experimental (dots) and calculated (line) room-temperature
XRPD pattern of ceramic x/y = 0.08/0.08. Bars show the peak positions.
2

O.Z. Yanchevskii, O.I. V’yunov, A.G. Belous et al.
Journal of Alloys and Compounds 874 (2021) 159861
404.6
structure promotes grain growth. The simultaneous introduction of
aluminium and fluorine leads to an abnormal grain growth, which
indicates a synergistic effect. The reason for the abnormal grain
growth in СaСu3Тi4−xAlxО12–0.5x–yFy ceramics may be a presence of
liquid phase at the grain boundaries. The liquid phase at the grain
boundaries promotes interface migration. In this case, the size dis-
tribution of ceramic grains is bimodal [50]. A liquid phase is probably
formed as a eutectic in a complex system Cu–Al–Ti–F–O. This as-
sumption is supported by the fact that a liquid phase occurs in the
known oxide systems at temperatures below 1100 °C, namely: 919 °C
(CuO-TiO2) [46] and 740–750 °C (CuF2-Cu2O-CuO) [51]. The release of
copper from grains and segregation at grain boundaries with the
formation of low-melting eutectics is also confirmed by a decrease in
the unit cell volume for the sample with x/y = 0.08/0.08 (Fig. 2).
Fig. 5 shows Cole-Cole diagrams of the complex impedance of the
ceramic samples СaСu3Тi4−xAlxО12–0.5x–yFy at room temperature.
The diagram for sample х/y = 0/0 consists of two spaced in frequency
semicircles that indicate two relaxation mechanisms in electrically
inhomogeneous phases. The diagrams for other samples contains the
distorted semicircles, the size of which increases in the series: х/
y = 0/0 < x/у= 0.04/0.04 < x/у= 0/0.08 < х/y = 0.04/0 < х/y = 0.08/0 < x/
у = 0.08/0.08. The equivalent circuit for the complex impedance of
СaСu3Тi4−xAlxО12–0.5x–yFy ceramics (Fig. 5, insert) is composed of
resistance RG and two series linked subcircuits with a parallel-con-
nected resistor and constant phase element (CPE) [9,52,53]. Sub-
scripts show elements belonging to grain (G), grain boundary (GB),
and the interface between sample and electrode (SE). The CPE con-
stant phase element describes the electrical inhomogeneity of
phases (due to chemical or size inhomogeneity) according to the
following formula [53]:
Al (x) = var
F (y) = var
404.4
Al+F (x+y) = var
404.2
404.0
403.8
0.00
0.04
0.08
Degree of Ti or O substitution, x or y
Fig. 2. Concentration dependences of the unit cell volume for ceramic samples
СaСu3Тi4–xAlxО12–0.5x-yFy, sintered at 1100 °C for 10 h.
particles, a core-shell structure with Al2O3 outside was formed [36].
Therefore, substitutions Al3+→Ti4+ in CCTO occur only at the synth-
esis stage: at 700–750 ℃ chemically inert Al2O3 react with CuO to
form the intermediate compound CuAl2O4 [49].
In СaСu3Тi4О12−yFy system, the substitution of O2– (R = 1.40 Å) by
F– ions (R=1.33 Å) [48] results in V increase (Fig. 2) from 404.08 Å3
(y = 0) to 404.46 Å3 (y = 0.08). This fact can be explained by the ap-
1+
2+
pearance of large ions Cu1+ (RCu = 0.60 Å > RCu = 0.57 Å) and Ti3+
(RTi3+ = 0.67 Å > RTi4+ = 0.605 Å) due to the reduction of the Cu2+ and/
or Ti4+ [48]. The difference in ionic radii of O2- and F- is insignificant
and the appearance of Cu1+ and Ti3+ ions is the key factor increasing
the unit cell volume. In this case, solid solutions can be written as Сa
(Сu Сu1y+)Тi4О12−yFy or СaСu23+(Тi Ti3y+)О12−yFy respectively.
2+
4+
3−y
4−y
In the system СaСu3Тi4–xAlxО12–0.5x–yFy (Fig. 2), the change in V
additively depends on the contributions of Al3+ and F– additives (x
and y values respectively). At low degrees of doping (x/y = 0.04/
0.04), the unit cell volume increases as expected. At a high degree of
doping (x/y = 0.08/0.08), a decrease in the unit cell volume to
403.99 Å3 indicates a decrease in the content of Cu1+ and Ti3+ ions in
the grains. Substitutions at Ti4+ and O2– positions differently affect
the relative density of the ρexper/ρtheor of СaСu3Тi4–xAlxО12–0.5x–yFy
ceramic. In Al-containing system, the ceramic density monotonically
decreases from 95.5% to 96% (x = 0) to 91–90% (x = 0.08) with the
degree of substitution and increases with the sintering time (Fig. 3a).
In the F-containing system, the ceramic density slightly decreases
both with the degree of substitution and with the sintering time
from 95% to 94% (Fig. 2b). An increase in the sintering time or
temperature results in similar phenomena, namely, additional
copper losses and an increase in the number of voids.
ZCPE = A-1 (iω)-n
(6)
where A is the proportionality factor, the dimension of which de-
pends on the exponent n; i is the imaginary unit, ω is the angular
frequency of the alternating current. At n = 1 and in the vicinity, the
CPE is identical to the distributed capacitor and at n = 0.5 and in the
vicinity, the CPE is identical to the Warburg element.
The parameters of the equivalent circuit of the samples were de-
termined by fitting the experimental dependence Z″ = f(Z′) (Table 1).
Table 1 shows that the addition of aluminium and fluorine respectively
increases and slightly decreases the electrical resistance of the grain RG.
The monotonic increase in RG of the ceramics of the system
СaСu3Тi4−xAlxО12–0.5x with the aluminium content (Table 1) in-
dicates the number of Cu1+ and Ti3+ forming due to Eqs. (1), (2)
decreases. However, the significant increase in RG cannot be ex-
plained only by the proportional decrease in the number of reduced
cations, because the concentration of Al3+ ions in the Ti4+ positions is
≤ 2%. At the same time, the formation of solid solution
СaСu3Тi4−xAlxО12–0.5x is accompanied by the appearance of oxygen
vacancies. These oxygen vacancies form Al3+O5·VO× clusters that
Fig.
4 shows the microstructure of СaСu3Тi4–xAlxО12–0.5x–yFy
ceramics. The average ceramic grain size is 1.5(5) μm at x/y = 0/0
(Fig. 4a); 4(1) μm at x/y = 0.08/0 (Fig. 4b). A bimodal structure with
small, 2.5(5) μm and large, 10(2) μm grains is observed at x/y = 0/0.08
(Fig. 4c). The largest grains of 125(25) μm are observed at x/у = 0.08/
0.08 (Fig. 4d). Obviously, the addition of both Al3+ and F– ions in CCTO
96
93
96
93
(a)
(b)
10 h
12 h
10 h
90
90
87
12 h
87
0.00
0.04
0.08
0.00
0.04
0.08
Degree of Ti substitution, x
Degree of O substitution, y
Fig. 3. The relative density of ceramics СaСu3Тi4–xAlxО12–0.5x (a) and СaСu3Тi4О12−yFy (b), sintered at 1100 °C.
3

O.Z. Yanchevskii, O.I. V’yunov, A.G. Belous et al.
Journal of Alloys and Compounds 874 (2021) 159861
Fig. 4. The microstructure of ceramic samples СaСu3Тi4–xAlxО12–0.5x–yFy, sintered at 1100 °C for 10 h: х/y = 0/0 (a); х/y = 0.08/0 (b); x/у= 0/0.08 (c); x/у= 0.08/0.08 (d).
cannot participate in polaron charge transfer but have a competitive
advantage over the formation of conducting polarons Ti3+O5·VO× [12].
This fact can explain the significant increase in RG with the increase
of Al content.
decreases insignificantly. This fact can be explained by an increase in
fluorine segregation with an increase in y and the formation of low-
melting (740–750 °C) CuF2 – Cu2O – CuO eutectics [51]. Therefore,
this part of fluorine does not participate in the formation of a solid
solution and does not contribute to the reduction of Cu2+ and Ti4+
cations.
In contrast to aluminium-doped CCTO, in fluorine-doped one, the
ions in lower oxidation states are formed not only at the sintering
stage, but already at the synthesis stage. The reasons for Cu2+ and
The addition of aluminium and fluorine significantly, by order of
magnitude and more, increases the resistance of the grain boundary
RGB and of the interface between sample and electrode RSE (Table 1).
The resistance of grain RG increases due to an increase in the alu-
minium content for the reasons described above. The increase in the
RGB and RSE can be explained by the segregation of copper-con-
taining phases (CuxO, x ≤ 1) in these regions. The electrical resistance
of these phases is several orders of magnitude higher than CCTO
[54]. In addition, due to abnormal grain growth in aluminium- and
fluorine-doped ceramic, the surface of grain boundaries decreases
sharply, and the surface density of CuxO phases increases.
The increase in the duration of ceramics sintering from 10 to 12 h
increases the values of RG, RGB, and RSE (Table 1). This can be ex-
plained by the fact that the dominant mechanism of defect forma-
tion changes with time from (1) to (3). In this case, mutual
compensation of Cu+/Ti3+and Cu3+ ions occurs:
Ti4+ reduction in СaСu3Тi4О12−yFy is the charge compensation re-
2+
quirements for the formation of solid solutions Сa(Сu Сuy1+
)
3−y
Тi44+ 12−yFy and СaСu23+(Тi Tiy3+)О12−yFy. Thus, it can be expected
О
4+
4−y
that the resistance of the grain will increase in order
RG(СaСu3Тi4О12−yFy) < RG(СaСu3Тi4О12) < RG(СaСu3Тi4–xAlxО12–0.5x).
However, really the resistance of the grains of СaСu3Тi4О12−yFy
RG
RGB
RSE
CPEGB
CPESE
-4.0x107
-2.0x107
0.0
-4x106
x/y = 0/0
x/y = 0.04/0
-2x106
x/y = 0.08/0
x/y = 0/0.08
x/y = 0.04/0.04
x/y = 0.08/0.08
0
2x106 4x106
0
Cu3+ + Cu1+→ 2Cu2+
Cu3+ + Ti3+→ Cu2+ + Ti4+
(7)
(8)
2x107
4x107
0
Real part of impedance (Ohm)
Eqs. (7), (8) shows that with long sintering, the number of charge
carriers decreases, and RG increases. An increase in sintering time is
accompanied by further segregation of copper and the formation of
copper-containing phases at grain boundaries, which increases RGB
Fig. 5. Complex impedance diagrams at room temperature and an equivalent circuit
(insert) for ceramic samples СaСu3Тi4–xAlxО12–0.5x–yFy, sintered at 1100 °C for 10 h. R
and CPE are resistance and a constant phase element. Subscripts show grain (G), grain
boundary (GB) and the interface between sample and electrode (SE).
and RSE
.
4

O.Z. Yanchevskii, O.I. V’yunov, A.G. Belous et al.
Journal of Alloys and Compounds 874 (2021) 159861
Table 1
Equivalent circuit elements parameters for ceramic samples СaСu3Тi4–xAlxО12–0.5x–yFy, sintered at 1100 °C for 10 and 12 h.
x/y
Sinter. time, h
Grain
RG,
Grain boundary
RGB,
Interface sample/electrode
Ω
Ω
С
GB, F
nGB
RSE,
Ω
CSE, F
nSE
0/0
10
12
10
12
10
12
10
12
10
12
10
12
22(4)
24(2)
37(2)
46(3)
58(7)
70(7)
19(4)
22(5)
26(5)
32(3)
42(4)
47(5)
3.8(2) × 105
5.5(3) × 105
2.3(1) × 106
5.4(3) × 106
3.1(2) × 107
3.9(2) × 107
1.2(2) × 106
7.1(1) × 106
2.6(3) × 106
4.9(4) × 106
4.3(3) × 107
9.8(5) × 107
8.3(3) × 10-7
1.2(5) × 10-8
3.2(1) × 10-8
2.4(4) × 10-8
4.6(5) × 10-8
3.4(2) × 10-8
5.7(6) × 10-7
2.2(1) × 10-8
3.3(5) × 10-7
2.7(7) × 10-7
1.6(5) × 10-8
2.4(2) × 10-8
0.802(4)
0.791(2)
0.968(6)
0.930(5)
0.803(2)
0.862(3)
0.818(9)
0.793(7)
0.978(5)
0.932(5)
0.924(4)
0.908(5)
3.1(1) × 105
6.4(3) × 105
3.2(1) × 105
5.6(2) × 105
5.4(2) × 106
1.2(2) × 107
5.0(3) × 106
1.1(1) × 107
8.0(2) × 105
1.8(2) × 106
2.4(2) × 106
7.2(2) × 106
3.3(1) × 10-8
2.8(5) × 10-8
8.1(5) × 10-8
6.6(7) × 10-8
4.2(1) × 10-8
2.2(1) × 10-8
2.3(1) × 10-8
3.6(1) × 10-8
3.1(1) × 10-7
6.2(5) × 10-7
2.1(5) × 10-8
4.6(7) × 10-8
0.905(3)
0.917(2)
0.885(3)
0.898(4)
0.980(9)
0.993(5)
0.935(3)
0.971(2)
0.903(2)
0.919(4)
0.876(5)
0.891(3)
0.04/0
0.08/0
0/0.08
0.04/0.04
0.08/0.08
Fig. 6 and Table 2 show the frequency dependences of ε′ and tan δ
at room temperature of the ceramic СaСu3Тi4–xAlxО12–0.5x–yFy sin-
tered at 1100 °С/10 h. All samples except х/у = 0.08/0 have high
permittivity ε′ > 30,000 in a wide frequency range (Fig. 6a). It should
be noted that only aluminium at a low degree of substitution
(x ≤ 0.04) increases the dielectric constant of the solid solution as
compared to CCTO. The value ε′ increases only with joint doping
when x/y = 0.04/0.04. The dielectric properties of pure CCTO cera-
mics differ markedly from the doped ones (Fig. 6). At ultra-low
frequencies, strong relaxation and high values of ε′ > 106 are ob-
served. Low-frequency relaxation is probably observed because of
the interfacial polarization contribution of electrically in-
homogeneous phases. Both aluminium and fluorine dopants sup-
press the contribution of interfacial polarization of CCTO ceramic at
ultra-low frequencies and extend the range of low dielectric losses
towards low frequencies. In addition, dielectric losses are sig-
nificantly decreased (Fig. 6b). Despite the increase in the density of
СaСu3Тi4−xAlxО12–0.5x–yFy ceramics with an increase in sintering
Table 2
Dielectric characteristics of ceramic samples СaСu3Тi4–xAlxО12–0.5x–yFy, sintered at
1100 °C for 10 and 12 h.
x/y
Sintering
time, h
Permittivity, ε′
Dielectric loss, tan δ
at 1 kHz
at 100 kHz
at 1 kHz
min value
0/0
10
12
10
12
10
12
10
12
10
12
10
12
57,000
46,000
63,500
50,000
36,000
32,000
36,000
23,500
72,000
66,800
50,000
43,000
47,000
38,000
47,000
40,000
24,000
25,000
31,000
19,500
53,000
48,300
32,600
26,200
0.088
0.098
0.047
0.066
0.055
0.065
0.046
0.058
0.047
0.057
0.060
0.077
0.070
0.068
0.044
0.066
0.048
0.056
0.046
0.058
0.047
0.055
0.057
0.072
0.04/0
0.08/0
0/0.08
0.04/0.04
0.08/0.08
time from 10 h to 12 h (Fig. 3), its dielectric constant decreases and
dielectric losses increase (Table 2). This may be due to excess copper
losses and, accordingly, a decrease in the number of Cu1+ and Ti3+
ions in the grains. The composition with x/y = 0.04/0 at room tem-
perature and 1 kHz has the lowest dielectric loss (tan δ ≈ 0.044). The
composition with x/y = 0.04/0.04 has the best combination of di-
electric characteristics: ε′ ≈ 72,000, tan δ ≈ 0.047. These character-
istics are comparable or higher than known ones for Al2O3-modified
CCTO ceramics (ε′ ≈ 43,000, tan δ ≈ 0.06 [37], ε′ ≈ 81,000, tan δ ≈ 0.08
[36]) or F-modified CCTO ceramics (ε′ ≈ 81,000, tan δ ≈ 0.077 [41]).
107
x/y=0/0
x/y=0/0.04
106
x/y=0/0.08
x/y=0/0.08
x/y=0.04/0.04
x/y=0.08/0.08
105
104
(a)
4. Conclusions
103
Ceramics of the system СaСu3Тi4–xAlxО12−yFy were prepared by
solid-state reactions technique that involves the preliminary
synthesis of CaTiO3. The ability to use CaF2 as a fluorine source was
shown. The combined introduction of aluminium and fluorine ions
results in synergetic abnormal grain growth due to the formation of
a liquid phase at grain boundaries. The liquid phase is supposed to
form as a eutectic in a complex system Cu–Al–Ti–F–O.
In the system СaСu3Тi4−xAlxО12–0.5x, the resistance of grains mono-
tonically increases with x. In the system СaСu3Тi4О12−yFy, the difference
between the grain resistance of doped and undoped samples in the
system СaСu3Тi4О12−yFy is small. The ceramic sample СaСu3Тi4−x
AlxО12−yFy with x/y = 0.04/0.04 after sintering for 10 h has the following
dielectric parameters: ε′ ≈ 71,000 (1 kHz) and tan δ ≈ 0.047.
10-1
101
103
105
Frequency (Hz)
0.08
0.07
0.06
0.05
6
4
2
102 103 104
(b)
10-1
101
103
105
CRediT authorship contribution statement
Frequency (Hz)
Yanchevskii O.Z. – Methodology, Resources, Writing - original
draft. V’yunov O.I. – Conceptualization, Investigation, Writing -
original draft. Belous A.G. – Supervision, Validation. Kovalenko L.L.
– Investigation, Data curation.
Fig. 6. Frequency dependences of permittivity ε′ (a) and dielectric loss tan δ (b) at
room temperature for ceramic samples СaСu3Тi4–xAlxО12–0.5x−yFy, sintered at 1100 °C
for 10 h.
5

O.Z. Yanchevskii, O.I. V’yunov, A.G. Belous et al.
Journal of Alloys and Compounds 874 (2021) 159861
Declaration of Competing Interest
[22] T.-T. Fang, K.-T. Lee, New insights into understanding the defect structures and
relationship of frequency dependences of dielectric permittivity and ac con-
ductivity of CaCu3Ti4O12, J. Appl. Phys. 125 (2019) 215106, https://doi.org/10.
1063/1.5086328
[23] J. Zhao, H. Zhao, Z. Zhu, Influence of sintering conditions and CuO loss on di-
electric properties of CaCu3Ti4O12 ceramics, Mater. Res. Bull. 113 (2019) 97–101,
https://doi.org/10.1016/j.materresbull.2019.01.014
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
[24] S. Kumar, N. Ahlawat, N. Ahlawat, Effect of heating rate on microstructure and
electrical properties of microwave sintered CaCu3Ti4O12 ceramics, Adv. Mater.
Lett. 8 (2017) 605–613, https://doi.org/10.5185/amlett.2017.6398
Acknowledgments
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This work was supported by the Research program of the
Ukrainian National Academy of Sciences “New functional substances
and materials for chemical production” (Fine Chemicals) [grant
number 0119U101351].
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