2TbxCu3Ti4−xTbxO12 ceramics

Author’s Accepted Manuscript Mixed-valent structure, dielectric properties and defect chemistry of Ca1−3x/2TbxCu3Ti4−xTbxO12 ceramics Da-Yong Lu, Xin-Yu Yu, Jun-Wei Liu www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)30585-0 http://dx.doi.org/10.1016/j.ceramint.2017.03.191 CERI14960

To appear in: Ceramics International Received date: 7 February 2017 Revised date: 23 March 2017 Accepted date: 30 March 2017 Cite this article as: Da-Yong Lu, Xin-Yu Yu and Jun-Wei Liu, Mixed-valent structure, dielectric properties and defect chemistry of Ca1−3x/2TbxCu3Ti4−xTbxO12 ceramics, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.191 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mixed-valent structure, dielectric properties and defect chemistry of Ca1−3x/2TbxCu3Ti4−xTbxO12 ceramics Da-Yong Lu a,b,*, Xin-Yu Yu a,c, Jun-Wei Liu a,d a

Research Center for Materials Science and Engineering, Jilin Institute of Chemical Technology, Jilin 132022, China b

Key Laboratory for Special Functional Materials in Jilin Provincial Universities, Jilin 132022, China c d

College of Chemistry, Jilin University, Changchun 130012, China

State Key Laboratory for Superhard Materials, Jilin University, Changchun 130012, China

[email protected] [email protected] *

Corresponding author. Tel: +86 0432 62815308.

Abstract Single-phase Ca1−3x/2TbxCu3Ti4−xTbxO12 (0.025 ≤ x ≤ 0.075) (CTCTT) ceramics with a cubic perovskite-like structure and a fine-grained microstructure (1.6‒2.3 μm) were prepared using a mixed oxides method. The results revealed that mixed valence states of Cu2+/Cu+, Ti4+/Ti3+, and Tb3+/Tb4+ coexisted in CTCTT. A multiphonon phenomenon in the Raman scattering at 1148, 1323, and 1502 cm-1 was reported for undoped and doped CTTO. Tb was mainly incorporated in the interior of the CTCTT grains rather than on the surface. The dielectric permittivity of CTCTT (εr'RT = 3590‒5200) decreased relative to CCTO (εr'RT = 10240) at f = 1 kHz, but the dielectric loss of CTCTT (the minimum value of tan δ = 0.12 at RT) increased as a result of Tb doping. The defect chemistry of CTCTT is discussed. The internal barrier layers capacitance (IBLC) model was adopted for impedance spectroscopy (IS) analysis. The activation energies of the grain boundaries (Egb) and semi-conductive grains (Eg) for CTCTT were determined to be 0.52 eV and 104 meV, respectively. The IS and defect chemistry analyses confirmed that the decrease in the dielectric permittivity was mainly due to a decrease in conductivity in the semiconducting CTCTT grains caused by the acceptor effect of Tb4+ at the Ti site, which resulted in a decrease in the IBLC effect.

Key-words: Ceramics; Dielectric properties; Electrical properties; Electron Paramagnetic Resonance; X-ray photoelectron spectroscopy

1. Introduction 1

The perovskitelike compound CaCu3Ti4O12 (CCTO) was first reported in 1967 [1], and a derivative of the compound, [ACu3](B4)O12, was described in 1979 [2,3]. CCTO has a body-centered cubic primitive cell that contains 20 atoms and belongs to the centrosymmetric Th (Im3) space group (Z = 2). CCTO remains cubic and centric down to 35 K without any indication of a structural phase transition [3]. The dielectric behavior of this complex perovskite CCTO compound was not investigated in the thirty years following its discovery because of the non-ferroelectric nature of its centrosymmetric structure. It was not until the year 2000 that Subramanian et. al. first investigated the dielectric properties of CCTO and discovered that it has an extraordinarily high dielectric permittivity (ε') of > 10000 at room temperature; furthermore , ε' was found to be nearly independent of frequency in the range of 103 to 106 Hz and independent of temperature at 1 kHz in the range of 100 to 450 K [3,4]. Subsequently, CCTO attracted considerable attention from researchers due to its abnormal dielectric behavior. Many models have been proposed to interpret the origin of CCTO’s permittivity , such as the ten mechanisms that were summarized by Thomas et al [5]. Among these models, the internal barrier layers capacitance (IBLC) model, which consists of large semiconducting grains and insulating thin grain boundaries, is widely accepted [6]. To improve the dielectric properties of CCTO, different elements have been introduced as doping ions to modify the structure of CCTO, such as Zr- [7] and Sr-doped CCTO [8,9]. Rare earth (RE) ions are hailed as the “vitamin of the modern chemical industry”. The dielectric response of RE-modified CCTO has been widely studied for compounds doped with La [9–17], Ce [18,19], Nd [20], Sm [21], Eu [22,23], Gd [23,24], Tb [25], Dy [26], Er [27], Yb [28,29], Lu [30], and Y [23,31] Within these compounds the RE ions were mainly substituted at the Ca site of CCTO, thereby forming compositions Ca1−3x/2RExCu3Ti4O12 [9–31]. Substitution of RE ions into the Ca sites generally caused a decrease in ε' and a rise in tan δ. However, no

2

attempt was made to incorporate RE ions into both the Ca and the Ti sites of CCTO. In this work, Tb ions were simultaneously incorporated into the Ca and Ti sites of CCTO using a solid-state reaction method to give compositions Ca1−3x/2TbxCu3Ti4−xTbxO12 (CTCTT), where Tb exhibited amphoteric behavior with mixed valence states of Ca-site Tb3+ and Ti-site Tb4+. Tb was selected based on the following reasons: (1) unlike other rare earth elements, Tb was recently discovered to have the advantage of self-adjustable site occupations between Ba-site Tb3+ and Ti-site Tb4+ ions in another high-permittivity perovskite system, BaTiO3, and the dielectric loss was reduced due to an acceptor effect of Tb4+ [32]. This finding lays the foundation for the fabrication of CTCTT; (2) Thongbai et. al. prepared Tb-doped CaCu3Ti4O12 ceramics with Tb3+ in the Ca site (Ca1−3x/2TbxCu3Ti4O12) and investigated their nonlinear current–voltage properties [25]. However, information on mixed-valent compounds and on the electrical properties of Tb-doped CaCu3Ti4O12 is lacking. For this reason, novel CTCTT ceramics were prepared and investigated in detail; and (3) the IBLC model can be examined using CTCTT. In this work, CTCTT was investigated in detail by X-ray diffraction (XRD), Raman spectroscopy (RS), Scanning electron microscopy (SEM), backscattered electron (BSE), energy dispersive spectroscopy (EDS), Electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), impedance spectroscopy, temperature and frequency dependencies of the dielectric permittivity and dielectric loss, and the electrical properties related to electrical conductivity and modulus. The structure, microstructure, mixed valence states, dielectric properties, electrical properties, and their interconnection were studied. A multiphonon phenomenon in Raman scattering over the wavenumber range of 900–1600 cm–1 was reported for undoped and Tb-doped CCTO. Several forms of the Arrhenius law were applied to determine the activation energy. The study of many correlations for CTCTT provides sufficient support for the IBLC model.

2. Experimental 3

2.1．Synthesis The ceramic raw materials were reagent-grade CaCO3, CuO, TiO2, and Tb4O7 powders. The ceramics were prepared according to the nominal compositions Ca1−3x/2TbxCu3Ti4−xTbxO12 (x = 0, 0.025, 0.05, 0.075) (CTCTT) using a solid-state reaction method. The sample with x = 0 is CaCu3Ti4O12 (CTCO). No ball milling was required because the raw materials were nano-sized. After being ground in an agate mortar for 1 h, the mixture was calcinated in air at 950 °C for 5 h for decarbonation. After being ground again for 30 min, the calcinated mixture with a PVA binder was uniaxially pressed at 200 MPa for 2 min into pellets (ф12 mm). The pellets were sintered at 1100 °C for 24 h in air with a heating rate of 100 °C/h and a cooling rate of –200 °C/h to 700 °C followed by furnace cooling to room temperature. The pellets were densified into crack-free ceramics with lower relative densities (see Table 1). 2.2．Characterization 2.2.1. X-ray diffraction (XRD) and lattice parameter determination Powder XRD measurements were made at room temperature using a DX-2700 X-ray diffractometer (Dandong Haoyuan Inc.). All XRD data of CCTO and CTCTT were collected between 20° ≤ 2θ ≤ 85° in steps of 0.02° and a collecting time of 3 s per step. The X-ray supply voltage and filament current were set to 35 kV and 30 mA, respectively. Lattice parameters and unit cell volume were calculated with a MS Modeling software package (Accelrys, Inc.) using Rietveld refinement in Reflex Package and Cu Kα1 radiation (λ = 1.540562 Å). 2.2.2. Room-temperature and temperature-dependent Raman spectroscopy Because the intensity of the Raman signals for CCTO and CTCTT ceramic powders was very low even at Filter = 50 %, unpolarized Raman spectra were measured for the ceramic bulks at room temperature using a LabRAM XploRA Raman spectrometer (Horiba Jobin Yvon) with excitation from the 532 nm line

4

of a Nd:YAG laser focused on a spot approximately 3–5 μm in diameter. A Linkam-600 heating and cooling stage was utilized for temperature-dependent Raman measurements. The laser power level was adjusted to 50 % (Filter) of the normal output of 25 mW because of a weak spectral intensity. The accumulation time and resolution were 2 s and 2.7 cm-1, respectively. 2.2.3. Scanning electron microscopy (SEM), backscattered electron (BSE), and energy dispersive spectroscopy (EDS) The microstructure was observed using an EVOMA 10 scanning electron microscope (Zeiss) operated at 15 keV. The average grain size (GS) was obtained using Fullman’s method [33]. To observe potential secondary phases, SEM investigations in backscattered electron (BSE) mode were performed. An Aztec 2.3 energy dispersive spectrometer (Oxford Ins.) was attached to the SEM for compositional analyses. 2.2.4. X-ray photoelectron spectra (XPS) To investigate the valence states of the cations in the ceramics, XPS measurements were performed at room temperature using an ESCALAB 250 X-ray photoelectron spectrometer (Thermo Electron Co.). Raw XPS data were processed by smoothing multiple times. The core-level binding energy was calibrated using the C 1s peak located at 285 eV. The binding energy peaks corresponding to different valence state ions were fitted with a program (XPSpeak 4.1) using Gaussian-Lorentzian lines. The ratio of the ionic concentrations between two ionic valence states of an element was determined via the area under each fitted line. 2.2.5. Room-temperature and temperature-dependent Electron paramagnetic resonance (EPR) EPR spectra were measured using an A300-10/12 X-band spectrometer (Bruker) operated at 9.84 GHz for room-temperature measurements and at 9.435 GHz for temperature-dependent measurements. 5 mg of powder for each sample was heat-treated at 500 °C for 2 h to remove absorbed gas for the EPR

5

measurements. The g-factor of each signal was calculated according to the relationship hν 0 = gβH, where h is Planck’s constant (h = 6.626 × 10 −34 J s), ν 0 is the microwave frequency, β is the Bohr magnetron (β = 9.262 × 10 −24 J/T), and H is the magnetic field strength. 2.2.6. Dielectric measurements The ceramics were polished into disks (Φ10.2 mm in diameter, 0.8 mm in thickness). Both surfaces of samples were made into electrodes with sputtered Au atoms and silver paste, then heat-treated at 500 °C for 30 min. The dielectric and electrical properties of the bulk ceramics were investigated at 1 kHz using a Turkey Concept 41 Dielectric/Impedance spectrometer (Novocontrol) with an applied ac voltage signal of 1 V in a temperature range of −100 to 200 °C and a frequency range of 1 Hz to 15 MHz. The accuracy of the measurements of dielectric permittivity, dielectric loss, resistivity, and temperature control were less than 5 %, 3 × 10−5, 3 × 10−5, and ±0.3 °C, respectively. 2.2.7. Impedance spectroscopy (IS) and electrical measurements Impedance spectra of the ceramics were obtained in terms of separated real and imaginary parts of the complex impedance (Z'−Z'') over a frequency range of f = 1 Hz to 15 MHz between −100 to 200 °C using the above Dielectric/Impedance spectrometer. The complex data obtained in terms of Z'−Z'' were converted into the conductivity σ'−σ'' and modulus M'−M'' notations using the standard conversions [34,35].

3. Results

3.1．Structural evolution Room temperature

(RT) powder

XRD

patterns of

CaCu3Ti4O12

(x

=

0;

CCTO)

and

Ca1−3x/2TbxCu3Ti4−xTbxO12 (x = 0.025, 0.05, 0.075; CTCTT) ceramics are given in Fig. 1. The XRD pattern of CCTO is the same as the reported cubic CCTO phase (JCPDS Cards No. 75-2188). Similar to CCTO,

6

CTCTT with x ≤ 0.075 exhibited a single-phase perovskite-like structure, indicating that the Tb ions were completely incorporated into the lattice. No secondary phases of CaTiO3 [36], CuO [37,38], Tb4O7 (JCPDS Cards No. 13-387), or Tb2Ti2O7 (JCPDS Cards No. 41-363), which are often present in similar compounds, were

observed.

Our

experiments

indicate

that

the

solid

solution

limit

of

Tb

ions

in

Ca1−3x/2TbxCu3Ti4−xTbxO12 is greater than x = 0.1 (its XRD pattern is not presented here). This value is twice that of Ca1−3x/2TbxCu3Ti4O12 (x = 0.1) due to the difference in the Ca/Ti ratio [25]. The lattice parameters of CCTO and CTCTT are displayed in Table 1. The unit-cell volume (V0) decreased with increasing x. 3.2．Room-temperature and temperature-dependent Raman scattering and multiphonon phenomenon The perovskite-like compound CCTO has a body-centered cubic primitive cell that contains 20 atoms and belongs to the centrosymmetric Th (Im3) space group (Z = 2) [2]. Its structure yields a total of 24 Γ-point phonons, eight of which are Raman active (2Ag + 2Eg + 4Fg) [39]. CCTO exhibited a weak Raman scatter and only five of the eight predicted modes were observed at approximately 293 (Fg (1)), 445 (Ag (1)), 511 (Ag (2)), 576 (Fg (3)), and 761 cm-1 (Fg (4)) [40–48]. Among these five modes, three modes at 445, 511, and 576 cm-1 were relatively strong and the other two modes at 293 and 761 cm-1 were very weak. Raman spectra of the CCTO and CTCTT ceramic bulks at RT are shown in Fig. 2 (a). The above-mentioned five bands appeared in the spectra of all samples. No Raman signals from CaTiO3 [49] or CuO [50–53] were detected. Therefore, long-term sintering was concluded to form CTCTT with a Im3 structure. Three additional broad bands appeared between 1000 and 1600 cm−1, and their spectral intensity was insensitive to the content of Tb ions (x). Thus, these bands do not originate from the Raman charge effect of Tb3+ ions in the Ca2+ site [54,55]. These three additional bands can be attributed to multiphonon Raman modes involving 2 or 3 phonons based on the multiphonon phenomenon in other systems [56–60],

7

i.e., the band at 1148 cm-1 involves two Fg (3) phonons (2 × 574 cm−1), one at 1323 cm−1 involves three Ag (1) phonons (3 × 447 cm−1), and one at 1502 cm−1 involves three Ag (2) phonons (3 × 510 cm−1). Higher order combination modes above 1600 cm−1 were not detected for CCTO and CTCTT. Raman scattering was too weak to be observed for the CCTO and CTCTT ceramic powders, but their ceramic bulks exhibited distinct Raman spectra. Multiphonon Raman scattering has been reported to be readily detectable in bulk semiconductors [58–60]. The presence of three multiphonon modes in CCTO and CTCTT therefore suggests semiconducting properties of the grains in these ceramics. However, the multiphonon modes in CCTO and CTCTT were significantly weaker in intensity and broadened relative to their intrinsic one-phonon modes. This broad feature is similar to the multiphonon bands observed for germanium crystals, in which Raman modes involving just two phonons are extremely broad [61]. The temperature-dependent Raman spectra of the x = 0.075 CTCTT ceramic bulk are shown in Fig. 2 (b). Upon increasing the measuring temperature (T) from −100 to 150 °C, the intensity of both the intrinsic Th Raman signals and the multiphonon bands decreased continuously, implying a more ordered atomic arrangement for CTCTT at low temperatures. However, the spectral intensity of the multiphonon modes decreased rapidly from T = −50 to 0 °C, which is consistent with the EPR measurements that are discussed below. The reason for this anomaly is unclear. 3.3．Microstructure evolution The SEM images of the CCTO and CTCTT surface topography are depicted in Fig. 3. The ceramic relative density (ρr) (the ratio of the volumetric mass density to the theoretical density) and average grain size (GS) are listed in Table 1. CCTO exhibited a bimodal grain size microstructure consisting of large grains (~20 μm) and fine grains (3~5 μm). No terrace-ledge or bump domain morphologies [62] were observed on the grain surfaces of either CCTO or CTCTT. Tb doping in CCTO led to a significant decrease

8

in GS, which indicates that Tb inhibited grain growth of CTCTT. As x increased, the GS distribution of CTCTT became more uniform with a value of 1.6‒2.3 μm. However, both CCTO and CTCTT exhibited a high porosity, and the incorporation of Tb in CCTO did not enhance the ρr value. Although ρr increased slightly with increasing x, Tb doping via long-time sintering in the presence of a sufficient oxygen supply did not make CTCTT denser. The background electron (EDS) images of a coarse grain, a fine grain at a triple-point site, and a grain boundary junction in an SEM image of CTCTT with x = 0.05 are given in Fig. 4. The observed peaks correspond to peak positions of Ca, Cu, Ti, O, Tb, and Au. The Au composition originated from the Au atoms sputtered on the surfaces of the samples for the SEM observations. No impurity peaks were detected. Although the current EDS technique cannot provide accurate quantitative information on the composition, it permits a semi-quantitative comparison based on the assumption that the concentration of any element is proportional to the intensity of its EDS peak. There was nearly no change in the concentration of Ti and Ca. The higher O content on the grain boundary compared to on the coarse and fine grains was attributed to absorbed oxygen (surface oxygen) and not the lattice oxygen, which is consistent with the XPS analysis discussed below. Cu accompanied the higher O content at the grain boundary junctions, but no clear secondary phase was segregated along the grain boundary region. The distribution of Tb in the CTCTT grains was inhomogeneous. The concentration of Tb decreased in the order coarse grain, fine grain at a triple-point site, and grain boundary (Fig. 4). This fact illustrates that Tb was mainly incorporated into the interior of the CTCTT grains rather than on the surface. Fabrication of CCTO is commonly accompanied by Cu, CuO, Cu2O, Cu-rich intergranular phase, and Cu-rich intergranular liquid phase by segregation in the grain boundaries [62–68]. Backscattered electron (BSE) images from SEM were used for further phase analyses in addition to XRD, SEM, and EDS. A BSE

9

image of CTCTT with x = 0.025 is depicted in the inset of Fig. 4. All of the grains in this sample were detected as dark regions in the BSE image. No grains from a secondary phase were visible. 3.4．XPS investigations and mixed valence states X-ray photoelectron spectroscopy (XPS) is a powerful probe for determining both valence band and core levels. The mixed valence states of Cu2+/Cu+, Ti4+/Ti3+, and Tb3+/Tb4+ for CCTO and CTCTT were confirmed by XPS, as illustrated in Fig. 5. The data for the binding energy of the core levels of these ions and oxygen are listed in Table 2. Ti exhibited clear orbit-spin splitting of 2p1/2 and 2p3/2. The XPS peaks at ~943 eV do not originate from Cu 2p1/2 (at ~953 eV), but from a satellite of Cu2+ 2p3/2 (3d9) [69]. The 2p peaks for Cu and Ti could be resolved into contributions from Cu2+/Cu+ (Fig. 5 (a)) and Ti4+/Ti3+ (Fig. 5 (b)) by fitting the spectra with two Gaussian-Lorentzian curves. This result is consistent with reports on doped CCTO [20,21,28,70–73] and oxide ceramics with structures similar to CaCu3Ti4O12 [74–76]. The O 1s peak exhibited overlap of two peaks (Fig. 5 (d)), which arose from absorbed oxygen OA (surface oxygen) and lattice oxygen OL [28,70,71]. The area ratios between XPS spectra of two different ions for CCTO and CTCTT are given in Table 3. The area ratio equals the ratio of Cu+/Cu2+, Ti3+/Ti4+, and OA/OL. As x increased from 0.025 to 0.075, the ratio of Cu+/Cu2+ and Ti3+/Ti4+ increased from 0.42 to 0.81 and from 0.38 to 0.63, respectively, indicating that Tb doping in CCTO led to gradual reduction of Cu2+ to Cu+ and Ti4+ to Ti3+. This reduction is the main reason that there was no CuO phase segregated in the grain boundaries. The OA on the surface of CTCTT increased slightly due to Tb doping. Tb 4d core-level lines generally show multiplet structures due to the dn−1 configuration [77–79]. This spectral structure was attributed to the final state effect in the valence bands of rare earth ions [78]. The two Tb 4d peaks at ~146.5 and ~155 eV for CTCTT were attributed to the contributions from multiplet structures of Tb3+ [78–82] and Tb4+ [82], respectively. Although the core-level binding energies for Tb 4d5/2

10

were accurately and unambiguously distinguished by Sarma and Rao, 148.9 eV for Tb4+ 4d5/2 from TbO2 and ~148.4 eV for Tb3+ 4d5/2 from Tb2O3 [77], the designation of Tb 4d is still ambiguous to date because of the absence of Tb 4d3/2. Our fitting data of Tb4+ 4d5/2 and Tb3+ 4d5/2 in Table 2 showed excellent agreement with Sarma and Rao’s data [77]. Another peak at ~153 eV was fitted with contributions from two overlapping peaks at ~153.2 and ~152.7 eV. We attributed these two peaks to Tb4+ 4d3/2 and Tb3+ 4d3/2 based on the following three reasons: (1) the peak at ~153 eV was the main peak of Tb 4d. It is impossible that this peak originated from a multiplet structures because the intensity of the multiplet is generally weaker; (2) the core-level line of Tb4+ 4d3/2 should have a higher core-level binding energy than Tb4+ 4d5/2; and (3) the difference between Tb 2p3/2 and Tb4+ 4d5/2 orbit-spin splitting binding energies for CTCTT was ~4.2 eV. This value is nearly the same as that of Yb-doped CCTO [28]. Thus, the multiplet peak at ~155 eV may be considered to be an indicator of Tb4+ [82]. All of the cations in CTCTT coexisted in the forms of mixed valence Cu2+/Cu+, Ti4+/Ti3+, and Tb4+/Tb3+. As x was increased from 0.025 to 0.075, the area ratio of Tb4+/Tb3+ increased slightly from 0.80 to 0.87 (Table 3), suggesting that approximately 45 % of the Tb ions in the Ti sites exist as Tb4+ and 5 % exist as Tb3+ in CTCTT. 3.5．Room-temperature and temperature-dependent EPR investigations EPR spectra of CCTO and CTCTT at RT are shown in Fig. 6. A singlet signal with g = 2.15 appeared in the EPR spectrum of all the samples. This signal is attributed to the EPR caused by Cu2+ Kramers ions with square planar coordination [83–88]. Cu2+ (3d9, I = 3/2) in compounds can theoretically give four EPR lines due to hyperfine interactions. The singlet signal in the ceramics, which can be explained by overlap of the four hyperfine lines and further broadening of the signal, arose from the high concentration of Cu2+ in CCTO and CTCTT. Non-Kramers Cu+ (3d10) is EPR silent. The Tb4+ (4f7)-related signal at g = ~6.5 [32,82] was not detected by EPR for CTCTT because the Tb4+ signal is concealed by the strong Cu2+ signal.

11

Several researchers have associated oxygen vacancies (VO) with the large permittivity of CCTO [67,86–88]. They thought that the broadening of the EPR line-width of CCTO when annealed in argon or vacuum was caused by VO. However, Capsoni et al [64] and Thomas et al [5] claimed that the presence of VO in CCTO ceramics can be ruled out. There is still no convincing EPR evidence of the presence of VO to date. Temperature-dependent EPR spectra over a temperature (T) range of −100 to 200 °C for CCTO and CTCTT are given in Fig. 7. As T increased, the g value shifted slightly from g = 2.148 to 2.151, and the intensity of the Cu2+ signal from CCTO and CTCTT decreased continuously, but a sudden drop occurred at between 0 and 50 °C for CTCTT with x = 0.075. The reason is unclear. At the same T, the intensity of the Cu2+ signal became weaker for CTCTT with increasing x, implying increased reduction of Cu2+ to Cu+, which is consistent with the XPS conclusion. The line-width (ΔH) of the Cu2+ EPR signal as a function of T for CCTO and CTCTT is displayed in Fig. 8. The ΔH of both CCTO and CTCTT increased continuously with increasing T from −50 to 150 °C, but the value of ΔH (32–45 G) was far less than that of the air-sintered CCTO ceramic (ΔH = 134 G) reported by Luo et. al. [88]. Because the Cu2+ EPR line-width is strongly dependent on the concentration of VO [86–88], the content of VO in our CCTO and CTCTT can be inferred to be negligible. As x increased, ΔH decreased at room temperature (Fig. 8 inset). This illustrates that Tb doping in CCTO further reduced the amount of negligible VO. Recently, we succeeded in detecting VO and Ti3+ in doped BaTiO3 at low temperatures [89]. To detect signals from VO and Ti3+, we measured an EPR spectrum of the precious perovskite-type compound Ba1−xCaxTiO3 (x = 0.03) (BC3T) at T = −188 °C and compared this spectrum with the EPR spectrum of CTCTT with x = 0.05, as illustrated in Fig. 9. The EPR spectra of these two samples were measured with a receiver gain value of 105 and 10, respectively. Ideally, the same gain value would have been used for the

12

comparison. However, with an increase in gain, the EPR signal of CTCTT deformed and broadened significantly due to the high intensity of the Cu2+ signal. Thus, we had to use different gain values for the EPR measurements of these two samples. Numerous experiments verify that Ca vacancies are easily 

induced by the substitution of RE3+ for Ca2+ because of the relation 2RE Ca  VCa in the formula ''

Ca1−3x/2RExCu3Ti4O12 [9–31]. It can be seen in Fig. 9 that the intensity of the Cu2+ signal is 104 times larger than that of the other signals. No signals from VO, Ti3+, Tb4+, or Ca vacancies were detected. The main reason is that the strong Cu2+ signal concealed these signals. Thus, the EPR experiment at low temperature still cannot provide evidence for the presence of VO. 3.6．Temperature and frequency dependences of dielectric permittivity The temperature dependences of the dielectric permittivity (εr') and dielectric loss (tan δ) at 1 kHz for CCTO and CTCTT are depicted in Fig. 10. The dielectric data at 1 kHz are listed in Table. 1. The εr' of CCTO (~10000) was nearly constant over the T range of −75 to 100 °C. Tb doping reduced the εr' of CCTO, resulting in a lower room-temperature permittivity of εr'RT = 3500~5200 and formation of a relaxation peak over the T range of 100 to 175 °C, which shifted toward lower temperatures with increasing x for CTCTT. The tan δ of CTCTT showed a strong temperature dependence relative to CCTO. Compared to CTCTT with x = 0.075, the lower GS for CTCTT with x = 0.05 was accompanied by a lower εr'RT and tan δ (Table. 1). The frequency (f) dependences of εr' and tan δ for CCTO and CTCTT at RT from f = 102 Hz to 15 MHz are illustrated in Fig. 11. According to the IBLC model, the resistance and capacitance are associated with semiconducting grains and insulating grain boundaries, respectively [6]. An equivalent circuit in the inset in Fig. 11 consists of two parallel RC elements connected in series that represent different regions of a heterogeneous electroceramic. The εr' of CCTO decreased slowly below f = 1 MHz but dropped

13

dramatically in the MHz region, while that of CTCTT dropped rapidly above 105 Hz. The dramatic decrease in εr' for CCTO and CTCTT indicates the presence of conductive grains. The variation in tan δ with f at RT for CTCTT indicates minimum values of 0.12 at 2.14 × 104 Hz for x = 0.025, 0.129 at 7.48 × 103 Hz for x = 0.05, and 0.147 at 3.61 × 104 Hz for x = 0.075. AC conductivity (σ'ac) as a function of frequency (f) at T = 150 °C for CTTO and CTCTT is shown in Fig. 12. The σ'ac of all the samples remained nearly unchanged below f = 104 Hz and increased rapidly above f = 105 Hz. 3.7．Impedance spectroscopy, AC conductivity, electric modulus, and activation energy CTCTT with x = 0.05 was chosen for subsequent detailed investigations. An impedance complex plane plot, Z* at different temperatures (T = 110–190 °C), is displayed in Fig. 13. Two circular arcs were observed at all temperatures in this plot, and the diameter of the arc shrank with increasing T. According to the IBLC model, the smaller arc in the high frequency region represents the contribution from grains while the arc in the low frequency region represents the contribution from grain boundaries, which are often modeled by an equivalent circuit consisting of two parallel RC elements connected in series (Fig. 11 inset). Z* can be calculated from the following equation:

Z *  Z   iZ  

1 iC0 *

(1)

where Z' and Z" are the real and imaginary parts of Z* and ε* = ε' − iε". C0 = ε0S/d is the empty cell capacitance where S is the sample area and d is the sample thickness. According to the IBLC equivalent circuit, Z' and Z" can be expressed as [11,90]

Z 

Rg 1  (Rg Cg )

2



Rgb 1  (RgbCgb ) 2

(2)

14

 Rg Cg   RgbCgb  Z   Rg   Rgb  2 2 1  (Rg Cg )  1  (RgbCgb ) 

(3)

where Rg, Rgb and Cg, Cgb are the resistances and capacitances of the grains and grain boundaries, respectively, and ω is the angular frequency. The non-zero intercept of the arc passing through the origin on the Z' axis gives the value of Rg in the high frequency region while that of the other arc in the low frequency region gives the value of Rgb [6,91]. Both Rg and Rgb were found to increase with decreasing T (Fig. 13 inset) and obey the Arrhenius law. The activation energy (Eg and Egb) required for conduction of charge carriers inside the grains and on the grain boundaries can be calculated as follows:.

Rg  R0e



Rgb  R0e

Eg k BT



(4)

E gb k BT

(5)

where T is absolute temperature (K), kB is the Boltzmann constant, and R0 is a pre-exponential term. The values of Eg and Egb were determined to be 104 and 525 meV, respectively. Note that the value of Eg is equal to the activation energy of dielectric relaxation in the high frequency range, suggesting that relaxation is related to the behavior of grains. Electric modulus (M* = M' + iM" = 1/ε* = iωCZ*) is an important means of investigating the electrical transport process in ceramics and reflecting the real dielectric relaxation process. The grain boundary response peaks were observed at several selected temperatures. The temperature-dependent relaxation time was found to obey the Arrhenius law and the activation energy was 517 meV (Fig. 14), which is equal to Egb. The frequency dependence of the AC conductivity (σ*ac) is also used to understand the conduction mechanism. The real part of the AC conductivity (σ'ac) decreased with decreasing f and became f-independent below f = 103 Hz, as illustrated in Fig. 15. The f-independent value of σ'ac is considered to be 15

dc conductivity (σdc). The variation of σdc with temperature can be described by the Arrhenius equation

 dc   0e



E gb k BT

(6)

E gb

   0e k

BT

(7)

where τ is the relaxation time (s) and σ0 and τ0 are pre-exponential terms. The activation energy was found to be 518 meV, which is in good agreement with the value calculated from the modulus. The frequency dependences and scaling behavior of Z" at different temperatures (110–190 °C) are shown in Fig. 16. Thermally activated relaxation was observed (Fig. 16 (a)). The spectrum of Z" at each T showed a relaxation peak, and the peak position (fm) in the Z"-f curve shifted toward higher frequencies with increasing T. A plot of Z"/Z"m versus ω/ωm reveals that all of the curves at different temperatures converge to one single curve (Fig. 16 (b)). The overlapping of the curves for all the temperatures into a single master curve indicates that the dynamic processes are nearly temperature independent. The frequency dependences of the normalized peaks Z"/Z"m and M"/M"m at 70 and 130 °C are shown in Fig. 17. Comparison of the impedance and electrical modulus data revealed that the two peaks of Z"/Z"m and M"/M"m separate slightly at higher temperatures, which indicates that both long- and short-range movements of charge carriers co-exist in CTCTT.

4. Discussion

4.1. Defect chemistry of Ca1−3x/2TbxCu3Ti4−xTbxO12 (CTCTT) For CaCu3Ti4O12 (CCTO), the mixed-valent structure of Cu2+/Cu+ and Ti4+/Ti3+ is a feature of the rigid CCTO structure with tilted of TiO6 octahedrons and CuO4 square planes, as confirmed by XPS [20,21,28,70–73] (Fig. 5). For single-phase Ca1−3x/2TbxCu3Ti4−xTbxO12 (CTCTT) (Fig. 1), Tb in CCTO also exhibited a mixed-valent structure of Tb3+/Tb4+ (Fig. 5) in which Tb3+ is present in the Ca site and Tb4+ is

16

predominantly present in the Ti site (Tb4+/Tb = ~45 %). The distribution of Tb in the CTCTT grains is inhomogeneous, and Tb is understood to exist mainly in the coarse grains rather than on the grain boundaries (Fig. 4). In other words, Tb is mainly incorporated in the interior of the CTCTT grains rather than on the surface. EPR and temperature-dependent EPR experiments confirmed that Cu2+ is present (Fig. 6), more Cu2+ is reduced to Cu+ (Fig. 7), and VO in CTCTT is negligible (Fig. 8). Based on the strong reduction of Cu2+ to Cu+ and Ti4+ to Ti3+ (Table 3), it is impossible that the large number of electrons required for this reduction were provided by the loss of lattice oxygens during sintering at a relatively low temperature of 1100 °C a small number of Ti4+ ions were substituted at the Cu sites [18] because of the large difference in the valence state and ionic radii between Cu2+ (0.57 Å) and Ti4+ (0.605 Å) [92]. Long-term sintering for 24 h in air is responsible for this strong reduction due to a sufficient oxygen supply. The defect notation proposed by Kröger and Vink [93] was adopted.

CuCu  e  Cu 'Cu

(8)

TiTi  e  Ti'Ti

(9)

A small number of Ti-site Tb4+ ions (~5 %) in the interior of the CTCTT grains may act as acceptors because of their amphoteric behavior in the Ti site [82].

TbTi  e  Tb'Ti

(10)

In the mixed-valent structure of Cu2+/Cu+, Ti4+/Ti3+, and Tb3+/Tb4+ in CTCTT, Cu, Ti, and Tb ions can maintain a dynamic equilibrium.

TbTi  Cu 'Cu  Tb'Ti  CuCu

(11)

TbTi  Ti'Ti  Tb'Ti  TiTi

(12)

At the same time, half of the Tb3+ ions enter the Ca sites to induce Ca vacancies.

2TbCa  VCa

(13)

17

The study of the point defect chemistry determined that the point defects in CTCTT are composed of six 

'



'

types: Ca-site Tb3+ ( TbCa ), Ca vacancies ( VCa ); Cu-site Cu+ ( Cu Cu ); Ti-site Ti3+ ( TiTi ), Tb4+ ( TbTi ), '



'

and Tb3+ ( TbTi ). Oxygen vacancies ( VO ) are negligible. TbTi is dominant (~45 %) and TbTi is minimal (~5 %). The formula of CTCTT can be described as Ca1−3x/2TbxCu3Ti4−xTbxO12. 4.2. Effect of Tb doping on the unit-cell volume of CTCTT Several researchers have investigated CCTO doped with M = Fe3+ [71], Mo4+ [94], Sn4+ [95], Ta5+ [73], and Nb5+ [71,96], yet they rarely discussed the change in the unit-cell volume (V0) with x for CaCu3Ti4−xMxO12. In general, V0 decreased with x for 0.03 ≤ x ≤ 0.10 [95,96]. Sulaiman et. al. attributed this decrease to a significant amount of oxygen vacancies; however, their samples were not single-phase and no evidence of oxygen vacancies was provided [96]. The V0 of compositions Ca1−3x/2RExCu3Ti4O12 (CRCT)in which RE ions are substituted at the Ca sites of CCTO showed a slight shrinkage or expansion compared to CCTO (a = 7.391 Å) (JCPDS Cards No. 75-2188) due to different preparation conditions. For example, for RE = La with x = 0.10, a = 7.36 [13], 7.3838 [16], and 7.394 Å [9]. The change in the concentration of Ca vacancies can be concluded to have little effect on V0 based on comparison of CRCT compounds with RE = Sm (a = 7.392 Å) [21] with Sm2/3Cu3Ti4O12 (a = 7.394 Å) [97]. Thus, the RE ions do not play a decisive role in the change in V0 of CRCT. For CTCTT, the value of a decreased slightly from 7.3961 Å3 at x = 0 to 7.3930 Å3 at x = 0.075 (Table 1). The study of the defect chemistry indicated that oxygen vacancies are negligible in CTCTT. Tb3+ ions in the Ca sites also have little impact on V0 of CTCTT. Similar to the changing trend in the above CaCu3Ti4−xMxO12 compounds, the decrease in V0 with x for CTCTT is mainly caused by an increase in the concentration of Tb4+/Tb3+ ions in the Ti sites. The mixed-valent feature in CTCTT alters the tilted TiO6

18

groups in the CCTO structure and is responsible for the slight decrease in V0 with increasing Tb doping concentration (x) in CTCTT. 4.3. Effect of Tb doping on the grain growth of CCTO The SEM and EDS results reveal that Tb doping suppressed grain growth of CCTO ceramics (Fig. 3) and that Tb ions are mainly incorporated in the interior of the CTCTT grains (Fig. 4). The suppression of grain growth in CTCTT is attributed to the solute drag mechanism that was proposed for Sm- or Yb-doped CCTO ceramics [21,29]. Grain growth in a ceramic is primarily driven by grain boundary (GB) mobility. During the sintering process, a liquid phase is present in CCTO, and the initial segregation of Tb ions in the GBs of CCTO reduces the driving force for migration of GBs by dragging on the GBs via the capillary force produced by the liquid phase, which contributes to the solidification process. Thus, Tb doping can suppress grain growth of CCTO ceramics. Tb4O7 in the initial materials consists of two Tb3+ ions and two Tb4+ ions. During sintering in air, Tb ions are capable of entering Ca sites as Tb3+ and Ti sites as Tb4+ [32]. The ability to alter the lattice that is caused by the mixed-valent structure can reduce the lattice distortion and enhance the incorporation of Tb ions in the CTCTT grains. Compared to a coarse grain, a fine grain is less capable of enduring lattice distortion. Thus, the concentration of Tb ions in coarse grains is higher and that in a fine grain at a triple-point site is lower (Fig. 4). 4.4. Effect of Tb doping on the dielectric properties of CTCTT The dielectric permittvity of CTCTT (εr'RT = 3590‒5200) decreased relative to CCTO (εr'RT = 10240) at f = 1 kHz, and the dielectric loss of CTCTT (tan δ = 0.12 at RT) increased as a result of Tb doping. According to the IBLC model (Fig. 11 inset), Egb and Eg of CCTO were determined to be 0.60 eV and 96 meV, respectively [98], and the values for CTCTT were 0.52 eV and 104 meV, respectively (Figs. 13‒15).

19

Ti3+ (3d1) is generally considered to mainly act as a charge carrier and create semi-conductive grains in CCTO. To a certain extent, the acceptor effect of Tb4+ (Eq. (10)) may suppress the short- and long-range movement of charge carriers caused by hopping of electrons between Cu2+ and Cu+ or Ti4+ and Ti3+ , as shown in Eqs. (10) and (11). Tb doping in CCTO therefore results in an increase in ac conductivity (σ'ac) (Fig. 12) and a decrease in the activation energy of charge carriers in the semi-conducting grains (Eg), which is supported by the Tb4+/Tb ratio of ~45 % (Table 3). Dielectric loss is correlated to the insulating grain boundaries. A low concentration of Tb ions was observed in the GBs of CTCTT (Fig. 4). Similar to the role of La3+ doping in CCTO [11], Tb3+ substitution in the Ca site results in an increase in grain resistance and a decrease in grain-boundary resistance (a decrease in Egb), which reduces the IBLC effect. Thus, the dielectric permittivity (εr') of CTCTT is lower than that of CCTO, and the dielectric loss (tan δ) of CTCTT is far higher than that of CCTO (Fig. 11). The combined effect of Ca-site Tb3+ and Ti-site Tb4+ contributes to the reduction of εr' and the increase in tan δ for CTCTT.

5. Conclusions

Ca1−3x/2TbxCu3Ti4−xTbxO12 (0 ≤ x ≤ 0.075) (CTCTT) ceramics were prepared at 1100 °C using a mixed oxides method. CTCTT exhibited a single-phase cubic perovskite-like structure. Tb doping in CCTO inhibited grain growth of CTCTT, forming a fine-grained microstructure with an average grain size of 1.6‒2.3 μm. The XPS results revealed the coexistence of mixed valence states of Cu2+/Cu+, Ti4+/Ti3+, and Tb3+/Tb4+. Tb is mainly incorporated in the interior of the CTCTT grains, rather than on the surface. A multiphonon phenomenon in the Raman scattering over the wavenumber range of 1000–1600 cm–1 was present for undoped and doped CTTO. The dielectric permittvity of CTCTT (εr'RT = 3590‒5200) decreased

20

relative to CCTO (εr'RT = 10240) at f = 1 kHz, but the dielectric loss of CTCTT (the minimum value of tan δ = 0.12 at RT) increased as a result of Tb doping. The point defects in CTCTT are composed of Ca-site Tb3+, Ca vacancies; Cu-site Cu+; Ti-site Ti3+, Tb4+, and Tb3+. The formula of CTCTT can be described as Ca1−3x/2TbxCu3Ti4−xTbxO12. The internal barrier layers capacitance (IBLC) model was adopted for the impedance spectroscopy (IS) analysis. The activation energies of the semi-conductive grains (Egb) and grain boundaries (Eg) for CTCTT were determined to be 0.52 eV and 104 meV, respectively. The IS and defect chemistry analyses confirmed that the decrease in the dielectric permittivity was mainly due to the decrease of conductivity in the CTCTT grains caused by the acceptor effect of Tb4+ in the Ti site, which resulted in a decrease in the IBLC effect.

Acknowledgements

This work was supported by the National Natural Science Foundations of China (Grant No. 21271084) and of Jilin Province (20160101290JC), and by the Changbai Mountain Scholar Distinguished Professor (2015047).

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30

Figure captions Fig. 1. XRD patterns of CaCu3Ti4O12 (x = 0; CCTO) and Ca1−3x/2TbxCu3Ti4−xTbxO12 (x = 0.025, 0.05, 0.075; CTCTT) ceramics measured at room temperature. Fig. 2. (a) Raman spectra of CCTO and CTCTT ceramics measured at room temperature, (b) Raman spectra of CTCTT ceramic bulk with x = 0.075 measured at different temperatures. Fig. 3. SEM images of the surface morphologies of CCTO with (a) x = 0, and CTCTT with (b) x = 0.025, (c) x = 0.05, (d) x = 0.075. The inset in (b) shows the BSE image of CTCTT with x = 0.025. Fig. 4. (a) SEM image, EDS at (b) a grain boundary junction (c) a fine grain at a triple-point site, and (d) a coarse grain for CTCTT with x = 0.05. Fig. 5. Room temperature XPS spectra smoothed for CCTO and CTCTT: (a) Cu 2p, (b) Ti 2p, (c) Tb 4d, and (d) O 1s. Fig. 6. EPR spectra of CCTO and CTCTT measured at room temperature. Fig. 7. Temperature-dependent EPR spectra of CCTO and CTCTT. Fig. 8. Line-width (ΔH) of the Cu2+ EPR signal as a function of the measuring temperature (T) for CCTO and CTCTT. The inset depicts ΔH as a function of x at room temperature. Fig. 9. EPR spectra of the Ba1−xCaxTiO3 (x = 0.03) ceramic prepared at Ts = 1500 °C [86] and CTCTT with x = 0.05 measured at T = −188 °C. Fig. 10. Temperature dependence of (a) the dielectric permittivity (εr') and (b) the dielectric loss (tan δ) for CTTO and CTCTT, measured at 1 kHz. Fig. 11. Frequency dependences of (a) εr' and (b) tan δ for CCTO and CTCTT measured at room temperature from f = 100 Hz to 15 MHz. An equivalent circuit consisting of two parallel RC elements connected in series represents the different regions of a heterogeneous electroceramic. Fig. 12. AC conductivity (σ'ac) as a function of frequency (f) at T = 150 °C for CTTO and CTCTT. Fig. 13. Impedance complex plane plot Z* at different temperatures (110–190 °C) for CTCTT with x = 0.05. Fig. 14. Frequency dependence of (a) the real part (M') and (b) the imaginary part (M") of the electric modulus in a temperature range of 70~190 °C for CTCTT with x = 0.05. The inset gives the activation energy of the grain boundary. Fig. 15. Variations in log σ'ac with f at higher temperatures (110–190 °C) for CTCTT with x = 0.05. The inset gives the Arrhenius plot of the temperature dependence of the GB conductivity (σ'ac). Fig. 16. (a) Frequency dependences of the imaginary part (Z") of impedance; (b) the inset depicts the scaling behavior of Z" of the impedance at different temperatures (110–190 °C) for CTCTT with x = 0.05. Fig. 17. Frequency dependences of normalized peaks Z"/Z"m and M"/M"m for CTCTT with x = 0.05 at (a) 70 °C and (b) 130 °C.

31

Table 3 Area ratio determined from the XPS data for CCTO and CTCTT. x 0 0.025 0.050 0.075

Cu+/ Cu2+ 0.42 0.58 0.66 0.81

Ti3+/Ti4+ 0.38 0.44 0.50 0.63

OA/OL

Tb4+/Tb3+

1.06 1.27 1.33 1.12

0.80 0.83 0.87

Table 2 Binding energy (eV) of core levels for CCTO and CTCTT. x

0 0.025 0.050 0.075

Cu2+ 2p3/2 936.2 935.0 935.3 935.4

Cu+ 2p3/2 934.7 934.1 934.3 934.3

Cu2+ 2p1/2

Cu+ 2p1/2

Ti4+ 2p3/2

Ti3+ 2p3/2

Ti4+ 2p1/2

Ti3+ 2p1/2

942.5 943.8 943.8 943.8

945.0 941.4 941.5 941.4

458.6 458.7 458.9 458.8

457.9 457.9 458.1 458.0

464.5 464.6 464.7 464.4

463.5 463.5 463.8 463.7

The 4dm peaks originated from the multiplet structure of Tb 4d.

O2O2Tb4+ 1s (OL) 1s (OA) 4dm 529.8 530.0 530.1 530.0

Tb4+ Tb3+ Tb4+ Tb3+ Tb3+ 4d3/2 4d3/2 4d5/2 4d5/2 4dm

531.3 531.7 155.1 153.2 152.7 149.0 148.4 146.1 531.8 155.2 153.4 152.2 149.4 148.9 146.5 531.7 155.0 153.2 152.8 149.6 148.9 146.5

Table 1 Effect of Tb addition on the lattice parameters, ceramic relative density (ρr), average grain size (GS), room-temperature dielectric permittivity (εr'RT) and loss (tan δ), maximum permittivity (εr'm), dielectric-peak temperature (Tm) at 1 kHz, and dc conductivity (σdc). x

a (Å)

V0 (Å3)

ρr (%)

0 0.025 0.050 0.075

7.3961 7.3943 7.3937 7.3930

404.58 404.29 404.19 404.07

62 64 66 67

GS (μm) 10.0 2.3 1.8 2.0

εr'RT

tan δ

εr'm

Tm

σdc (S/cm)

10240 5200 3590 4950

0.036 0.385 0.341 0.644

11630 6880 7360

140 140 110

1.90×10−7 1.02×10−6 6.25×10−7 1.63×10−6

1.5

1.5

1.0

0.5

0.5

0.0

0.0

2

(b)

4

6

8

1.5

o

130 C

1.0

1.0

0.5

0.5

0.0

0.0

0

M"/M"max

1.0

0 1.5

Z"/Z"max

70 C

2

4

6

log ω (rad/s)

M"/M"max

Z"/Z"max

(a)

o

8

Fig. 17. Frequency dependences of normalized peaks Z"/Z"m and M"/M"m for CTCTT with x = 0.05 at (a) 70 °C and (b) 130 °C.

Word-PDF transfer of the EES system

4000

(a) x = 0.05 110 oC 130 oC 150 oC 170 oC 190 oC

3000

Z''

Original figure with the format jpg

2000 1000 0 102

Z"/Z" m

1.2

103

104

105

106

107

f (Hz)

(b)

0.8

0.4

0.0 10-4

10-3

10-2

10-1

100

101

102

103

ω /ω max Fig. 16. (a) Frequency dependences of the imaginary part (Z") of the impedance; (b) the inset depicts the scaling behavior of Z" of impedance at different temperatures (110–190 °C) for CTCTT with x = 0.05.

Ln σ'dc (S/cm)

Log σ'ac (S/cm)

-2

-7

Egb = 518 meV

-8

x = 0.05

-9

-10

-3

2.2

2.4

-1

2.6

110oC 130oC 150oC 170oC 190oC

1000/T (K )

-4

102

103

104

105

106

f (Hz) Fig. 15. Variations in log σ'ac with f at higher temperatures (110–190 °C) for CTCTT with x = 0.05. The inset gives the Arrhenius plot of the temperature dependence of the GB conductivity (σ'ac).

M' (×10-4)

6

(a)

x = 0.05

4 70oC 90oC 110oC 130ooC 150 C 170oC 190oC

2

0 102

(b)

M'' (× 10-4)

Ln ( τ )

3

103

2

-10

104

105

106

107

Egb = 517 meV

-12 -14 2.2

2.4

2.6

2.8

1000/T (K)-1

3.0 70oC 90oC 110oC 130oC 150oC 170oC 190oC

1

0 102

103

104

105

106

107

f (Hz) Fig. 14. Frequency dependence of (a) the real part (M') and (b) the imaginary part (M") of the electric modulus in a temperature range of 70~190 °C for CTCTT with x = 0.05. The inset gives the activation energy of the grain boundary.

10 8 6 4 2

Ln (Rg, Rgb)

8000

Z " (Ω )

6000

Egb = 525 meV

x = 0.05

Rgb Rg

Eg = 104 meV 2.2

2.4

110 oC 130 oC 150 oC 170 oC 190 oC

2.6

1000/T (K-1)

4000

2000

0 0

2000

4000

6000

8000

10000

Z ' (Ω ) Fig. 13. Impedance complex plane plot Z* at different temperatures (110–190 °C) for CTCTT with x = 0.05.

σ'ac (S/cm)

6

4

x x x x

=0 = 0.025 = 0.05 = 0.075

T = 150oC

2

0 102

103

104

105

106

f (Hz) Fig. 12. AC conductivity (σ'ac) as a function of frequency (f) at T = 150 °C for CTTO and CTCTT.

Word-PDF transfer of the EES system

15000

T = 25 oC

(a)

x x x x

ε′r

10000

=0 = 0.025 = 0.05 = 0.075

5000

102 3.0

103

(b)

104

105

Rg

2.0

tan δ

107

Rgb

Cg Grain bulk

106

Cgb Grain boundaries

1.0

0.0 102

103

104

105

106

107

f (Hz) Fig. 11. Frequency dependences of (a) εr' and (b) tan δ for CCTO and CTCTT measured at room temperature from f = 100 Hz to 15 MHz. An equivalent circuit consisting of two parallel RC elements connected in series represents the different regions of a heterogeneous electroceramic.

Original figure with the format jpg

20000

(a)

ε′r

15000

x x x x

=0 = 0.025 = 0.05 = 0.075

f = 1 kHz

10000 5000 0 6

-50

0

50

100

150

200

50

100

150

200

(b)

tanδ

4

2

0 -50

0

T (oC) Fig. 10. Temperature dependence of (a) the dielectric permittivity (εr') and (b) the dielectric loss (tan δ) for CTTO and CTCTT, measured at 1 kHz.

g = 2.004 (V Ti )

T = -188 oC 3+

g = 1.932 (Ti )

Ba1-x CaxTiO 3 (x = 0.03)

Gain = 10 5

CTCTT ( T = 0.05) g = 1.974 (VBa)

g = 1.955 (VO)

Gain = 10

2+

g = 2.15 (Cu )

3000

3200

3400

3600

3800

H (G) Fig. 9. EPR spectra of the Ba1−xCaxTiO3 (x = 0.03) ceramic prepared at Ts = 1500 °C [86] and CTCTT with x = 0.05 measured at T = −188 °C.

Word-PDF transfer of the EES system

55 ΔH (G)

50

45 g = 2.15 (Cu

2+

)

35 30

Δ H (G)

Cu2+ EPR line-width

T = 25 o C

40

Original figure with the format jpg

45

0.000

0.025

x

0.050

0.075

40 x x x x

35

=0 = 0.025 = 0.05 = 0.075

30 -100

-50

0

50

100

150

200

T (oC) Fig. 8. Line-width (ΔH) of the Cu2+ EPR signal as a function of the measuring temperature (T) for CCTO and CTCTT. The inset depicts ΔH as a function of x at room temperature.

(a) x = 0

2000

(c) x = 0.05

(b) x = 0.025 o

4000

H (G)

(d) x = 0.075

200 C

200 C

200 C

200 oC

150 oC

150 oC

150 oC

150 oC

100 oC

100 oC

100 oC

100 oC

50 oC

50 oC

50 oC

50 oC

0 oC

0 oC

0 oC

0 oC

-50 oC

-50 oC

-50 oC

-50 oC

-100 oC

-100 oC

-100 oC

-100 oC

6000

o

2000

4000

H (G)

6000

o

2000

4000

6000

H (G)

Fig. 7. Temperature-dependent EPR spectra of CCTO and CTCTT.

2000

4000

H (G)

6000

o

T = 25 C

g = 2.15 (Cu2+ )

x = 0.075

x = 0.050

x = 0.025

x=0

1000

2000

3000

4000

5000

6000

H (G) Fig. 6. EPR spectra of CCTO and CTCTT measured at room temperature.

Fig. 5. Room temperature XPS spectra smoothed for CCTO and CTCTT: (a) Cu 2p, (b) Ti 2p, (c) Tb 4d, and (d) O 1s.

(a)

x = 0.05 (b) Grain boundary

Cu O

Ti

Au Tb

Tb

Ca Au Cu

Tb Cu

Ti Au

Cu

Au

Cu

Au

Ti

O

(c) Triple-point grain Tb Tb Ca

Au

Tb

Ti

Cu

Cu Au

Ti

O

(d) Coarse grain Tb Ca

Au

2

Cu

Ti

4

Tb Cu

6

8

Au

10

Energy (keV) Fig. 4. (a) SEM image, EDS at (b) a grain boundary junction (c) a fine grain at a triple-point site, and (d) a coarse grain for CTCTT with x = 0.05.

Fig. 3. SEM images of the surface morphologies of CCTO with (a) x = 0, and CTCTT with (b) x = 0.025, (c) x = 0.05, (d) x = 0.075. The inset in (b) shows the BSE image of CTCTT with x = 0.025.

o

1502

1315

1148

Bulk at T = 25 C Fg (4)

Ag (1) Ag (2) Fg (3)

Fg (1)

(a)

x = 0.075 bulk

(b)

T = 150 oC (CT CT T )

T = 100 oC

x = 0.075 T = 50 oC T = 0 oC

T = -50 oC

1502

1323

1148

x = 0.025 760

447 512 574

298

x = 0.050

(CCT O) x=0

400

800

1200

Raman shift (cm-1)

1600

T = -100 oC

2000

500

1000

1500

2000

Raman shift (cm-1)

Fig. 2. (a) Raman spectra of CCTO and CTCTT ceramics measured at room temperature, (b) Raman spectra of CTCTT ceramic bulk with x = 0.075 measured at different temperatures.

(220)

(CTCTT)

Ca 1-3x/2Tb xCu 3Ti 4-xTb xO12

(620)

(440)

(422)

(400)

(321)

(222)

(211)

(013)

x = 0.075

x = 0.050

20

30

60

(015)

(332)

(411) (024)

50

(125) (440) (433) (600) (611) (620)

(CCTO) x=0

(422)

(400)

40

(321)

(222)

(013)

(110)

(211)

(220)

x = 0.025

70

80

2 θ (o) Fig. 1. XRD patterns of CaCu3Ti4O12 (x = 0; CCTO) and Ca1−3x/2TbxCu3Ti4−xTbxO12 (x = 0.025, 0.05, 0.075; CTCTT) ceramics measured at room temperature.