Structure and dielectric properties of Ba$_2$Cu$_x$Y$_{1-x}$TaO$_{6-y}$ double perovskite

TL;DR

This study investigates how Cu doping affects the structure and dielectric properties of Ba$_2$Y$_{1-x}$Cu$_x$TaO$_{6-y}$ ceramics, revealing structural modifications and enhanced dielectric behavior due to interfacial polarization.

Contribution

It presents new insights into Cu doping effects on the structural and dielectric properties of Ba$_2$YTaO$_6$ ceramics, including reduced sintering temperature and improved dielectric constant.

Findings

Cu doping reduces sintering temperature.

Structural changes in the crystalline structure.

Enhanced dielectric constant due to interfacial polarization.

Abstract

In this paper, we reported the effect of Cu doping on the structural and dielectric properties of Ba$_2$Y$_{1-x}$Cu$_x$TaO$_{6-y}$ (0.00 $\leq$ x $\leq$ 0.50) ceramics at room temperature. The Copper for Yttrium substitution reduces the sintering temperature and leads to structural changes in the Ba$_2$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.

Figures

Tables

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

Ba2Y1-xCuxTaO6-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)
$ϵ(\%)$	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
$I_{\left(111\right)}:I_{\left(220\right)}$	0.132	0.176	0.113	0.119	0.095	0.090

Table 2. Table 2: Electrical parameters of the equivalent electrical circuit obtained from complex impedance spectrum fits using Equation 3 for BaY 1-x Cu x TaO 6-y samples

x	Rb ($k\Omega$)	Rgb (M$\Omega$)	Cgb ($pF$)	P $\left(nF{s}^{n-1}\right)$	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

Equations

$$\eta =2\left|M_0-0.5\right|$$

$$ϵ"=\frac{1}{\omega C_0\left(R_1+R_2\right)}+\Delta {ϵ}^{\prime }\frac{\omega \tau }{1+{\omega }^2{\tau }^2},$$

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

Full text

In this paper, we reported the effect of Cu doping on the structural and dielectric properties of Ba2Y1-xCuxTaO6-y (0.00 $\leq$ x $\leq$ 0.50) ceramics at room temperature. The Copper for Yttrium substitution reduces the sintering temperature and leads to structural changes in the Ba2YTaO6 rock-salt crystalline structure. Dielectric permittivity and complex impedance spectroscopy measurements suggested enhancement of the dielectric constant and occurrence of interfacial Maxwell-Wagner polarization.

I Introduction

Systematic analysis of YBa2Cu3O7-δ (Y123) superconductor shows that Ta2O5 helps to achieve large-grains (oliveira2005, ; bortolozo2004, ) for the magnetic levitation and magnets applications (bondarenko2017, ; siems2004, ; yu1997, ). The grain growth is attributed to the decrease in the peritectic temperature transformation (bortolozo2004, ), and the homogeneous segregation of Ba2YTaO6 (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 (oliveira2005, ; coll2014, ). Futhermore, BYT phase also induces an unusual Paramagnetic Meissner Effect in Y123 superconductor (dias2014, ).

Single Crystals of BYT were grown by Galasso et al. using B2O3 flux in 1960’s (galasso1966, ). Later the dielectric properties of BYT ceramic were studied in the microwave frequency range (zurmuhlen1994, ), and a structural phase transition (gupta2013, ) from cubic to tetragonal space group around 260 K was observed by diffractometry, calorimetry, transmission electronic microscopy, and Raman scattering (zurmuhlen1994, ; lufaso2006, ; zhou2011, ). This transition is quite similar to the SrTiO3 antiferrodistortive case (de2015, ).

BYT belongs to the family of complex perovskite (king2010, ) 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 (vasala2015a2b, ). The long-range order of the crystal lattice are responsible for several properties in these complex perovskites (king2010, ; vasala2015a2b, ) such as the high dielectric permittivity (yanez2009, ; chandrasekhar2012, ). On the other hand the extrinsic giant dielectric permittivity frequency observed in related perovskites such as CaCu3Ti4O12 (CCTO) (subramanian2000, ) 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 (adams2002, ; tien1967, ; waser1991, ). The dielectric relaxation at these barriers of charge is well described by the Maxwell-Wagner (M-W) polarization model (wang2013, ; o2000, ; banerjee2017, ).

Since as BYT appears as a secondary phase in Ta-doped Y123 (oliveira2005, ), it suggests to one that Copper should have some solubility in the BYT complex perovskite, such as the Ba2Y1-xCuxWO6-y system already reported (garcia1992, ). This paper presents the first investigation of the physical properties of Ba2Y1-xCuxTaO6-y system for 0.00 $\leq$ x $\leq$ 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 turns Ba2Y1-xCuxTaO6-y ceramics new candidates for some applications in electronic devices.

II Experimental Procedure

Ceramics of Ba2Y1-xCuxTaO6-y (0.00 $\leq$ x $\leq$ 0.50) were prepared by the standard solid state reaction method using BaCO3 (Sigma-Aldrich, 99.95%) Y2O3 (Sigma-Aldrich, 99.99%) Ta2O5 (Cerac, 99.99%) and CuO (Sigma-Aldrich, 99.99%) powders. For each stoichiometry, powders were ball milled for about 12 hours, then calcined at 950*∘* C in the air during 96 hours. Approximately 10% in mass of polyvinyl alcohol (PVA) binder additive was added to the powders, and disc pellets with about 8 mm in diameter and 2 mm in thickness were pressed uniaxially. These pellets were annealed at 500*∘* C for 1 hour to decompose the organic PVA (banerjee2016, ), and thereafter sintered in air at 1250*∘* C to 1400*∘* C for up to 120 hours. The microstructure of ceramics were analyzed by Scanning electron microscopy (SEM).

The crystalline structure of all samples was analyzed by XRD technique using Empirian Pan Analytical diffractometer with CuK$\alpha$ radiation ($\lambda$ = 1.5406 Å) and Ni filter. The diffractometry measurements were carried out with 0.01*∘* step in $10^{\circ}\leq 2\theta\leq 90^{\circ}$ range. Model refinement profiles with Pseudo-Voigt function by Rietveld method were performed in HiScore Plus program using information from inorganic crystal structure database (ICSD) (belsky2002, ). Both subdomain size and microstrain were obtained from Williamson-Hall plots (woodward1994, ; alves2017, ; dhahri2018, ). 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 (bondarenko2005inverse, ).

III Results and Discussion

The pure Ba2YTaO6 was formed only after long sintering time, 120 hours at 1400*∘* C while the Cu-doped Ba2Y1-xCuxTaO6-y (x=0.40) was formed after much shorter sintering time, 15 hours at 1250*∘* C. Thus it was evident that Cu for Y substitution lower both sintering temperature and sintering time. The figure 1 shows that x = 0.40 sample (fig. 1(b)) has signatures of liquid phase and more defects when compared with x = 0.00 sample (fig. 1(a)). 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 suggest 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 the literature (galasso1966, ; zurmuhlen1994, ; lufaso2006, ; zhou2011, ) and the decrease in the lattice parameter (see inset of the figure 2(b) can be attributed to the differences in the ionic radii of Cu2+ (0.73 Å) and Y3+ (0.9 Å) ions (shannon1976, ).

The Williamson-Hall plot shown in Figure 3 gives the strain and the sub-cell domain information from the slope and the reciprocal of the intercept respectively (woodward1994, ). The Rietveld refinement results obtained for all samples are shown in the Table 1. The Cu for Y substitution produce compensating oxygen vacancies may induce strain in the crystal lattice. Additionally, XRD refinement also suggests that substitution increases the crystallite size in Ba2Y1-xCuxTaO6-y system.

In a double perovskite, the quantity of the long-range ordering degree ($\eta$) is given by (liu2003, ):

[TABLE]

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 (woodward1994, ). 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 (liu2003, ). The results based upon both structure refinement and peak intensities of the XRD pattern suggest that the Cu for Y substitution may reduces the long-range ordering in BYT crystalline structure, as is shown in both Table 1 and Figure 4.

The high dielectric permittivity observed in non-ordered double perovskites is understood in terms of randomic cation/valence long-range order distribution (king2010, ), (yanez2009, ). In the Ba2Y1-xCuxTaO6-y system reported here the ordering degree decreasing affects the distribution of both compensating oxygen vacancies and hole carriers within the grains (adams2002, ). It also offers considerable contributions for the dielectric relaxation (yang2010, ; mahato2017, ; rai2016, ; banerjee2019functional, ).

Figure 5(a) shows the frequency dependence of real part of permittivity ($\epsilon^{\prime}$) for Ba2Y1-xCuxTaO6-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 (o2000, ), which can be better visualized in the imaginary part ($\epsilon^{\prime\prime}$) of the dielectric permittivity shown in Figure 5b. In accordance with Maxwell-Wagner (M-W) model, the imaginary part of dielectric permittivity can be written with the following expression (banerjee2017, ),

[TABLE]

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 (banerjee2017, ; o2000, ). The magnitude of the Ohmic conductivity can be determined from the slope of $\epsilon"$ vs. $1/\omega$ graph, as is shown in the Figure 5(b) inset where one can see that x = 0.40 has the higher conductivity. The eq. 2 fits well with the experimental data which indicates 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 figure 6(a).

In general, the approach of phase angle towards 90*∘* represents the ideal poling state (izquierdo2014, ). So it can be observed that slight addition of Cu enhances the poling condition $\sim$ 87*∘* in the x = 0.10 ceramic samples. But the magnitude of phase angle for x = 0.40 was found to be $\sim$ 76*∘. It suggests that sufficient amount of Cu in the Y site of Ba2YTaO6* crystalline structure leads to changes in poling state and domain switching.

The dependence of the impedance with frequency is shown in figure 6(b) on a double logarithmic scale. It can be observed that the magnitude of Z’ decreases gradually for x = 0.10, x = 0.20, and x = 0.30 ceramics with the increase of ac frequency (kumari2015, ). But for x = 0.00, x = 0.40, and x = 0.50 ceramics the magnitude of $|Z|$ 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 (sutar2014, ) from the ceramics.

Figure 7(a) shows the Nyquist plots for Ba2Y1-xCuxTaO6-y ceramics. Since the observed semi circles are non-centered, Non-Debye type relaxation i.e. Maxwell-Wagner relaxation exists in these ceramics due to M-W relaxation (barsukov2012, ).

An equivalent circuit shown in figure 7(b) may be represented by a bulk resistance (Rb) in series with a grain boundary resistance (Rgb ), grain boundary capacitance (Cgb ), and a constant phase element impedance (ZCPE ) in parallel as:

[TABLE]

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 (yu2019effect, ). The experimental data fitted well with Equation 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 all samples the CPE behaving like a parallel capacitor-resistor in the equivalent circuit (mahato2017, ). With the addition of Cu in Ba2Y1-xCuxTaO6-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 was represented by M-W model.

IV Conclusions

Ceramics samples of Ba2Y1-xCuxTaO6-y with $0.00\leq x\leq 0.50$ were studied by X ray diffratometry, 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 reveal 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 Ba2Y1-xCuxTaO6-y ceramics is described by the Maxwell-Wagner model. The complex impedance spectroscopy suggests that sufficient Cu for Y substitution in Ba2YTaO6 ceramics leads to changes in poling state and domain switching. The study also confirmed that the $x$ value ($0.00\leq x\leq 0.50$) in Ba2Y1-xCuxTaO6-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.

Acknowledgments

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.