Structure and dielectric properties of $$\hbox {Ba}_{2}\hbox {Cu}_{x}\hbox {Y}_{1-x} \hbox {TaO}_{6-y}$$ double perovskite

F. S. Oliveira; C. A. M. dos Santos; A. J. S. Machado; P. Banerjee; A. Franco

doi:10.1007/s42452-019-1479-z

SN Applied Sciences

November 2019, 1:1447 | Cite as

Structure and dielectric properties of $\hbox {Ba}_{2}\hbox {Cu}_{x}\hbox {Y}_{1-x} \hbox {TaO}_{6-y}$ double perovskite

Authors
Authors and affiliations

F. S. Oliveira
C. A. M. dos Santos
A. J. S. Machado
P. Banerjee
A. FrancoJr.

Research Article

First Online: 21 October 2019

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4. Materials (general)

Abstract

In this paper, we reported the effect of Cu doping on the structural and dielectric properties of $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ (0.00 $\le x \le$ 0.50) ceramics at room temperature. The copper for yttrium substitution reduces the sintering temperature and leads to structural changes in the $\hbox {Ba}_{2}\hbox {YTaO}_{6}$ rock-salt crystalline structure. Dielectric permittivity and complex impedance spectroscopy measurements suggested enhancement of the dielectric constant and occurrence of interfacial Maxwell–Wagner polarization.

Keywords

Dielectric Perovskite Complex impedance spectroscopy

1 Introduction

Systematic analysis of $\hbox {YBa}_{2}\hbox {Cu}_{3}\hbox {O}_{7-\delta }$ (Y123) superconductor shows that $\hbox {Ta}_{2}\hbox {O}_{5}$ helps to achieve large-grains [1, 2] for the magnetic levitation and magnets applications [3, 4, 5]. The grain growth is attributed to the decrease in the peritectic temperature transformation [2], and the homogeneous segregation of $\hbox {Ba}_{2}\hbox {YTaO}_{6}$ (BYT) secondary phase in the Y123 superconducting matrix acts as vortex pinning centers, which are non-superconducting regions that confine the quantum magnetic flux, and consequently, optimizes the critical current density parameter [1, 6]. Furthermore, BYT phase also induces an unusual paramagnetic Meissner effect in Y123 superconductor [7].

Fig. 1
SEM images for a $x = 0.00$ and b $x = 0.40$ sample

Fig. 2
a XRD pattern of $x = 0.00$ sample sintered at 1450 $^{\circ }$ C for 120 h. b XRD pattern of $x = 0.50$ sample sintered at $1250\,^{\circ }\hbox {C}$ for 15 h. The inset shows the Vegard’s law dependence between lattice parameter and copper content

Fig. 3
Williamson–Hall plot of $x = 0.10$ and $x = 0.50$, where $\beta$ is the full width at half maximum of the reflection peaks

Fig. 4
Ordering degree (left-axis) and ratio between (111) and (220) peak intensities (right-axis). The dashed lines are just guides to the eyes

Table 1

Rietveld refinement results as function as x in $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$. Refinement agreement factors: $R_{exp}$ (expected) $R_{p}$ (profile), $R_{wp}$ (pondered profile), and goodness of fit ($\chi ^{2}$). Lattice parameter (a), crystallite size (D), strain ($\epsilon$), ordering degree ($\eta$), and the ratio between (111) and (220) peak intensities

$\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$
x	0.00	0.10	0.20	0.30	0.40	0.50
$R_{exp}\,(\%)$	5.11	5.69	5.13	6.48	4.62	5.13
$R_{p}\,(\%)$	8.58	9.01	9.45	6.13	4.31	7.3
$R_{wp}\,(\%)$	17.55	15.96	18.36	7.42	5.63	14
$\chi ^{2}$	3.42	2.81	3.58	1.14	1.22	2.73
a (Å)	8.423(1)	8.433(1)	8.403(1)	8.379(1)	8.343(1)	8.342(3)
D (Å)	973(229)	830(96)	1041(236)	1031(201)	4338(2590)	4795(2723)
$\epsilon \,(\%)$	0.05(4)	0.09(4)	0.04(2)	0.04(2)	0.09(3)	0.11(2)
$\eta \,(\%)$	99.9	99.4	96.7	96.1	97.2	95.5
$I_{(111)}:I_{(220)}$	0.132	0.176	0.113	0.119	0.095	0.090

Table 2

Electrical parameters of the equivalent electrical circuit obtained from complex impedance spectrum fits using Eq. 3 for $\hbox {BaY}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ samples

x	$\hbox {R}_{b}$ ($k\Omega$)	$\hbox {R}_{gb}$ ($\hbox {M}\Omega$)	$\hbox {C}_{gb}$ (pF)	P $\left( nFs^{n-1}\right)$	n
0.00	4.06	2.82	3.157	2.81	0.484
0.10	263.5	92.21	3.18	0.378	0.587
0.20	63.5	58.76	2.743	0.363	0.571
0.30	71.76	10.22	6.01	0.12	0.642
0.40	4.74	0.47	9.672	12.2	0.435
0.50	12.85	3.75	7.22	2.264	0.491

Single crystals of BYT were grown by Galasso et al. using $\hbox {B}_{2}\hbox {O}_{3}$ flux in 1960s [8]. Later, the dielectric properties of BYT ceramic were studied in the microwave frequency range [9], and a structural phase transition [10] from cubic to tetragonal space group around 260 K was observed by diffractometry, calorimetry, transmission electronic microscopy, and Raman scattering [9, 11, 12]. This transition is quite similar to the $\hbox {SrTiO}_{3}$ antiferrodistortive case [13].

Fig. 5
Variation of a Real (${\epsilon '}$) and b Imaginary (${\epsilon ''}$) parts of permittivity with the variation of frequency at room temperature, the solid lines show the fitting with Maxwell–Wagner model using Eq. 2 and the inset shows ${\epsilon ''}$ as function as ${1/\omega }$

Fig. 6
a Dependence of phase angle with frequency at room temperature and b dependence of real part of impedance ($Z^{\prime }$) with frequency at room temperature

Fig. 7
a Nyquist plots for $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ ceramics with $0.00\le x\le 0.50$ (markers) and solid lines are fits using Equation 3. b Equivalent circuit used for fitting

BYT belongs to the family of complex perovskite [14] oxides. This family group has attracted a lot of attention due to the presence of a large number of oxide materials which can be formed [15]. The long-range order of the crystal lattice is responsible for several properties in these complex perovskites [14, 15] such as the high dielectric permittivity [16, 17]. On the other hand, the extrinsic giant dielectric permittivity frequency observed in related perovskites such as $\hbox {CaCu}_{3}\hbox {Ti}_{4}\hbox {O}_{12}$ (CCTO) [18] is a consequence of semiconducting grains limited by insulating grain boundaries which act as a kind of barrier to the free carriers motion inside the grains [19, 20, 21]. The dielectric relaxation at these barriers of charge is well described by the Maxwell–Wagner (M–W) polarization model [22, 23, 24].

Since BYT appears as a secondary phase in Ta-doped Y123 [1], it suggests to one that Copper should have some solubility in the BYT complex perovskite, such as the $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {WO}_{6-y}$ system already reported [25]. This paper presents the first investigation of the physical properties of $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ system for 0.00 $\le x \le$ 0.50. Our results demonstrate that the sintering process, crystal structure, and dielectric relaxation change are dependent of the sample composition. We also show that copper enhances the dielectric constant of BYT ceramics and leads to interfacial Maxwell–Wagner polarization at the grain boundaries. The extrinsic effects induced by Cu turn $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ ceramics new candidates for some applications in electronic devices.

2 Experimental procedure

Ceramics of $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ (0.00 $\le$ x $\le$ 0.50) were prepared by the standard solid state reaction method using BaCO$_{3}$ (Sigma-Aldrich, 99.95%) $\hbox {Y}_{2}\hbox {O}_{3}$ (Sigma-Aldrich, 99.99%) $\hbox {Ta}_{2}\hbox {O}_{5}$ (Cerac, 99.99%) and CuO (Sigma-Aldrich, 99.99%) powders. For each stoichiometry, powders were ball milled for about 12 h, then calcined at $950\,^{\circ }\hbox {C}$ in the air during 96 h. Approximately 10% in mass of polyvinyl alcohol (PVA) binder additive was added to the powders, and disk pellets with about 8 mm in diameter and 2 mm in thickness were pressed uniaxially. These pellets were annealed at $500\,^{\circ }\hbox {C}$ for 1 hour to decompose the organic PVA [26], and thereafter sintered in air at 1250 to $1400\,^{\circ }\hbox {C}$ for up to 120 h. The microstructure of ceramics was analyzed by scanning electron microscopy (SEM).

The crystalline structure of all samples was analyzed by XRD technique using Empyrean PANalytical diffractometer with $\hbox {CuK}\alpha$ radiation ($\lambda = 1.5406\text{ angstrons}$) and Ni filter. The diffractometry measurements were carried out with $0.01^{\circ }$ step in $10^{\circ }\le 2\theta \le 90^{\circ }$ range. Model refinement profiles with Pseudo–Voigt function by Rietveld method were performed in HighScore Plus program using information from inorganic crystal structure database (ICSD) [27]. Both subdomain size and microstrain were obtained from Williamson–Hall plots [28, 29, 30]. The dielectric properties of the ceramic samples were studied using a computer controlled Agilent 4980A LCR meter. An alternating voltage of 1.0 V was applied on the ceramic pellets with silver painted faces over 20 Hz to 2 MHz frequency. Nyquist plots were analyzed using EIS Spectrum Analyzer program [31].

3 Results and discussion

The pure $\hbox {Ba}_{2}\hbox {YTaO}_{6}$ was formed only after long sintering time, 120 h at 1400 $^{\circ }$ C, while the Cu-doped $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ ($x=0.40$) was formed after much shorter sintering time, 15 h at $1250\,^{\circ }\hbox {C}$. Thus, it was evident that Cu for Y substitution lowers both sintering temperature and sintering time. Figure 1 shows that $x = 0.40$ sample (Fig. 1b) has signatures of liquid phase and more defects when compared with $x = 0.00$ sample (Fig. 1a). It is an evidence that the Cu concentration contributes significantly to the microstructure.

Figure 2 exhibits the XRD patterns for $x = 0.00$ and $x = 0.50$ samples. Our analysis suggests that the sample with the highest Cu doping level possesses a single cubic perovskite crystallographic phase. The lattice parameter of the undoped sample is in good agreement with those reported in literature [8, 9, 11, 12], and the decrease in the lattice parameter (see inset of Fig. 2b) can be attributed to the differences in the ionic radii of $\hbox {Cu}^{2+}\, (0.73 \text{ angstrom}$) and $\hbox {Y}^{3+} (0.9\text{ angstrom}$) ions [32].

The Williamson–Hall plot shown in Fig. 3 gives the strain and the sub-cell domain information from the slope and the reciprocal of the intercept, respectively [28]. The Rietveld refinement results obtained for all samples are shown in Table 1. The Cu for Y substitution that produces compensating oxygen vacancies may induce strain in the crystal lattice. Additionally, XRD refinement also suggests that substitution increases the crystallite size in $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ system.

In a double perovskite, the quantity of the long-range ordering degree ($\eta$) is given by [33]:

$$\begin{aligned} \eta =2\left| M_{0}-0.5\right| \end{aligned}$$

(1)

where $M_{0}$ is the refined occupation of Ta in the 4a (0,0,0) site or the refined occupation of Y or Cu in the 4b (1/2,1/2,1/2) site [28]. Beyond that, superlattice reflection peaks are sharper in high-ordered perovskites and the ordering degree is also related to the ratio between superlattice (odd,odd,odd) and sub-cell (even,even,even) peaks of the XRD pattern [33]. The results based upon both structure refinement and peak intensities of the XRD pattern suggest that the Cu for Y substitution may reduce the long-range ordering in BYT crystalline structure, as is shown in both Table 1 and Fig. 4.

The high dielectric permittivity observed in non-ordered double perovskites is understood in terms of randomic cation/valence long-range order distribution [14, 16]. In the $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ system reported here, the ordering degree decreasing affects the distribution of both compensating oxygen vacancies and hole carriers within the grains [19]. It also offers considerable contributions for the dielectric relaxation [34, 35, 36, 37].

Figure 5a shows the frequency dependence of real part of permittivity ($\epsilon '$) for $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$. It is evident that the low-frequency dielectric permittivity value for $x = 0.40$ ceramics at room temperature is higher than for $x = 0.00$. The initial high value of the real part of the dielectric permittivity could be due to the drop of applied voltage across the thin grain boundary widths, and space charge polarization is generated in $x = 0.40$ and $x = 0.50$ samples, which enhances the dielectric constant at the lower-frequency region.

The initial high value of dielectric constant in Cu-doped samples indicates the presence of dc conductivity due to Maxwell–Wagner (M–W) relaxation process [23], which can be better visualized in the imaginary part ($\epsilon ''$) of the dielectric permittivity, as is shown in Fig. 5b. In accordance with Maxwell–Wagner (M–W) model, the imaginary part of dielectric permittivity can be written with the following expression [24],

$$\begin{aligned} \epsilon ^{\prime \prime }=\frac{1}{\omega C_{0}\left( R_{1}+R_{2}\right) }+\Delta \epsilon '\frac{\omega \tau }{1+\omega ^{2}\tau ^{2}}, \end{aligned}$$

(2)

where $\sigma =1/C_{0}\left( R_{1}+R_{2}\right)$ term is known as Ohmic conductivity ($\sigma$), where $C_{0}$ is a geometric factor, and $R_{1}$ and $R_{2}$ are the resistances of the real and imaginary dielectric components, respectively [23, 24]. The magnitude of the Ohmic conductivity can be determined from the slope of $\epsilon ^{\prime \prime }$ versus $1/\omega$ graph, as is shown in Fig. 5b inset where one can see that $x = 0.40$ has the higher conductivity. Equation 2 fits well with the experimental data which indicate the presence of M–W polarization. Hence, the oxygen vacancies generated due to the doping with Cu replacing Y increased the Ohmic conductivity for $x=0.40$ and $x=0.50$ samples as they diffused into the grain boundary regions, which enhances their dielectric constants at lower frequencies.

To understand the contribution of the interfacial polarization, complex impedance spectroscopy (CIS) was performed at room temperature. The phase angle of samples is shown in Fig. 6a.

In general, the approach of phase angle toward $90^{\circ }$ represents the ideal poling state [38]. So it can be observed that slight addition of Cu enhances the poling condition $\sim \,87^{\circ }$ in the $x = 0.10$ ceramic samples. But the magnitude of phase angle for $x = 0.40$ was found to be $\sim \,76^{\circ }$. It suggests that sufficient amount of Cu in the Y site of $\hbox {Ba}_{2}\hbox {YTaO}_{6}$ crystalline structure leads to changes in poling state and domain switching.

The dependence of the impedance with frequency is shown in Fig. 6b on a double logarithmic scale. It can be observed that the magnitude of $Z^{\prime }$ decreases gradually for $x = 0.10, x = 0.20$, and $x = 0.30$ ceramics with the increase of ac frequency [39]. But for $x = 0.00, x = 0.40$, and $x = 0.50$ ceramics, the magnitude of $\vert Z \vert$ decreases gradually after 10 kHz frequency. The decrease of the real part of impedance at higher frequency domain and thereafter gradual merger suggests a possible release of space charge [40] from the ceramics.

Figure 7a shows the Nyquist plots for $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ ceramics. Since the observed semicircles are non-centered, non-Debye type relaxation, i.e., Maxwell–Wagner relaxation, exists in these ceramics due to M–W relaxation [41].

An equivalent circuit shown in Fig. 7b may be represented by a bulk resistance ($R_{\mathrm{b}}$) in series with a grain boundary resistance ($R_{\mathrm{gb}}$), grain boundary capacitance ($C_{\mathrm{gb}}$), and a constant phase element impedance ($Z_{\mathrm{CPE}}$) in parallel as:

$$\begin{aligned} Z(\omega )=R_{b}+\left[ \frac{1}{R_{gb}}+\frac{1}{\frac{1}{C_{gb}(j\omega )}} +\frac{1}{\frac{1}{P(j\omega )^{n}}}\right] ^{-1} \end{aligned}$$

(3)

In, $Z_{CPE}=1/P(j\omega )^{n}$ , P is the CPE parameter and n is the CPE element which behaves like a double-layer capacitor. The CPE is identical to a capacitance when $n=1$ and to a simple resistance when $n=0$ [42]. The experimental data fitted well with Eq. 3, establishing the validity of the equivalent circuit model. The single semicircle for each type of composition indicates the single conductivity mechanism in the ceramics. Table 2 shows the values of the fitting parameters for the ceramics samples.

It can be observed from the magnitude of Table 2 that for all samples, the CPE behaves like a parallel capacitor–resistor in the equivalent circuit [35]. With the addition of Cu in $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$, both resistance and capacitance of grain boundaries increase gradually which contribute to the barrier to the motion of charge carriers within large domain bulks of electrical resistance orders of magnitude lower than the boundaries resistance. Then, we conclude that it builds up a space charge polarization across the boundary regions which were represented by M–W model.

4 Conclusions

Ceramics samples of $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ with $0.00\le x\le 0.50$ were studied by X-ray diffractometry, electronic scanning microscopy, dielectric permittivity measurements, and complex impedance spectroscopy. SEM images show liquid phase and defects induced by copper. The Rietveld refinement of the XRD patterns reveals systematic changes in the crystalline structure, ordering degree, and domain sizes with the Cu content. The complex dielectric permittivity measurement demonstrated that the dielectric relaxation of $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ ceramics is described by the Maxwell–Wagner model. The complex impedance spectroscopy suggests that sufficient Cu for Y substitution in $\hbox {Ba}_{2}\hbox {YTaO}_{6}$ ceramics leads to changes in poling state and domain switching. The study also confirmed that the x value ($0.00\le x\le 0.50$) in $\hbox {Ba}_{2}\hbox {Y}_{1-x}\hbox {Cu}_{x}\hbox {TaO}_{6-y}$ ceramics affected resistance and capacitance of grain boundaries which contributed to the barrier in motion of charge and build up a space charge polarization across the boundary regions.

Notes

Acknowledgements

F. S. Oliveira acknowledges the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) — Finance Code 001. P. Banerjee acknowledges UGC, India, for Grant No. F.30-457/2018 (BSR).

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

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Authors and Affiliations

F. S. Oliveira
- 1
Email authorView author's OrcID profile
C. A. M. dos Santos
- 1
A. J. S. Machado
- 1
P. Banerjee
- 2
A. FrancoJr.
- 3

1.Escola de Engenharia de LorenaUniversidade de São PauloLorenaBrazil
2.Department of PhysicsGandhi Institute of Technology and Management (GITAM) UniversityBengaluruIndia
3.Instituto de FísicaUniversidade Federal de GoiásGoiâniaBrazil

Structure and dielectric properties of \(\hbox {Ba}_{2}\hbox {Cu}_{x}\hbox {Y}_{1-x} \hbox {TaO}_{6-y}\) double perovskite