Rietveld refinement and dielectric relaxation of a new rare earth based double perovskite oxide: BaPrCoNbO6

Journal of Solid State Chemistry 210 (2014) 219–223

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Rietveld refinement and dielectric relaxation of a new rare earth based double perovskite oxide: BaPrCoNbO6 Chandrahas Bharti a,n, Mrinmoy K. Das a, A. Sen a, Sadhan Chanda b, T.P. Sinha b a b

Sensor and Actuator Division, CSIR-Central Glass and Ceramic Research Institute, 196, Raja SC Mullick Road, Kolkata 700032, India Department of Physics, Bose Institute, 93/1, Acharya Prafulla Chandra Road, Kolkata-700009, India

art ic l e i nf o

a b s t r a c t

Article history: Received 12 September 2013 Received in revised form 24 October 2013 Accepted 17 November 2013 Available online 23 November 2013

A new rare earth based double perovskite oxide barium praseodymium cobalt niobate, BaPrCoNbO6 (BPCN) is synthesized by solid-state reaction technique. Rietveld analysis of X-ray diffraction (XRD) data shows that the compound crystallizes in a perovskite like tetragonal structure which belongs to the I4/mmm space group with lattice parameters a ¼b ¼ 5.6828(9) Å, c¼ 8.063(2) Å. Structural analysis reveals 1:1 ordered arrangement for the Co2 þ and Nb5 þ cations over the six-coordinate B-sites of BPCN. The superlattice line (1 0 1) at 2θ¼ 19.101 arising from the alternate ordering of Co2 þ and Nb5 þ sites is observed in the XRD pattern which confirms the presence of cation ordering in BPCN. Fourier transform infrared spectrum shows two phonon modes of the sample due to the antisymmetric NbO6 stretching vibration. The relaxation dynamics of the conductive process in BPCN is investigated in the temperature range 303 to 503 K and in the frequency range 100 Hz to 1 MHz using impedance spectroscopy. The relaxation mechanism of the sample in the framework of electric modulus formalism is modeled by Davidson–Cole model (DCM). The values of α (distribution of relaxation time) for the DCM varies from 0.1 to 0.3 which suggests the asymmetric distribution of relaxation time for BPCN. The activation energy of the sample, calculated from both conductivity and modulus spectra, are found to be almost the same  0.4 eV, which indicates that the conduction mechanism for BPCN is polaron hopping. The scaling behaviour of the imaginary part of electric modulus suggests that the relaxation follows the same mechanism at various temperatures. & 2013 Elsevier Inc. All rights reserved.

Keywords: Rare earth Perovskite Dielectric properties Ac conductivity Praseodymium

1. Introduction Double perovskite oxides (DPOs) having general formula A2B′B″O6/ A′A″B′B″O6 (A′, A″ – rare earth or alkaline earth ions and B′, B″ – transition metal ions or metalloids) have recently received great attention because some of them exhibit interesting physico-chemical properties such as colossal magnetoresistance, half-metallicity, second order Jahn–Teller (SOJT) distortions, ionic conductivity and multiferroicity [1–8]. Very recently, an emphasis has been given to the synthesis and study of the various physico-chemical properties such as structural, electrical and magnetic of a number of barium based rare earth double perovskite oxides [3–8]. Koshy et al. have synthesized a series of double perovskite oxide: REBa2NbO6 (RE¼ La and Dy) for their use as thin film substrates [3]. Barium based DPOs Ba2YbNbO6, Ba2ErNbO6 and Ba2HoNbO6 (BHN) have been synthesized and characterized by Nair et al. for technological applications [4]. Tellez et al. have also synthesized BHN in cubic phase as a substrate for the

n Corresponding author. Tel.: þ 91 33 24735829, Mobile: þ 91 9903447948; fax: þ91 33 24730957. E-mail addresses: [email protected], [email protected] (C. Bharti).

0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.11.024

fabrication of LaBaCaCu3O7 δ superconducting films [5]. Karunadasa et al. have proposed that rare earth double perovskites are excellent model systems for geometric magnetic frustration [6]. The impedance spectroscopic studies of various barium based rare earth double perovskite oxides have been performed by Korchagina et al. [7]. In our earlier communication, we have investigated structural and electrical properties of a barium based rare earth compound Ba2CeNbO6 using impedance spectroscopy (IS) [8]. IS is a very convenient and powerful experimental tool to investigate the contribution of various electroactive regions to the relaxation process of perovskite oxides. This method describes the electrical processes occurring in a system on applying an ac signal as an input perturbation. This technique has been applied to analyze the relaxation spectrum of many bulk materials [7–10]. Although a handful of A2B′B″O6/A′A″B′B″O6 perovskites with simultaneous A- and B-site cation ordering have been discovered, there is a little structural and electrical information available on these compounds. Moreover, to the best of our knowledge, there are no reports on structural and electrical properties of BaPrCoNbO6 (BPCN) till date. In particular, the presence of both SOJT distortions of d0 cations and the presence of magnetic ions make BPCN potentially attractive candidate for various technological

3. Results and discussion 3.1. Structural studies The Rietveld refinement of BPCN has been carried out in the tetragonal I4/mmm space group. The excellent fit to the observed XRD pattern of BPCN is depicted in Fig. 1 by solid line. All the indices (h k l) in the XRD profile conform the body-centering condition: hþ kþ l¼ 2n. The degree of the B′/B″-site ordering for tetragonal lattice can be estimated from the diffracted intensity ratio I(1 0 1)/{I(2 0 0)þI(1 1 2)} [12]. The intensity ratio as observed in Fig. 1 is 0.06 which indicates the ordering in the Co/Nb site in BPCN. A schematic presentation of the BPCN cell is shown in the inset of Fig. 1 with the distribution of ions at crystallographic positions of 4d for Ba2 þ and Pr3 þ ions, 2b for Co2 þ ions, 2a for Nb5 þ ions and 8h and 4e for O2 ions as given in Table 1. Each Co2 þ and Nb5 þ ions are surrounded by six O2 ions, constituting CoO6 and NbO6 octahedra, respectively. It has been found that the inter-atomic bond length values are 1.99 and 2.03 Å for Nb–O and Co–O, respectively, which suggest that the Nb-ions are in higher oxidation state (since higher oxidation states usually possess smaller average inter-atomic bond lengths). The structural parameters along with

30

40

50 60 2θ (degree)

70

404

116

224

204

004

20

301 222

The conventional solid-state reaction technique is used for the synthesis of BPCN. The starting materials were reagent grade BaCO3 (99.9%, Loba Chemie), Pr2O3 (99.99%, Sigma-Aldrich), CoO (99.99%, Sigma-Aldrich) and Nb2O5 (99.9% Loba Chemie). These oxides were taken in stoichiometric ratio and mixed in presence of acetone in an agate mortar and a pestle for 4–5 h and calcined at 1300 1C for 10 h. The grinding and re-calcinations were done once more so that a single phase BPCN was obtained. The calcined sample was crushed thoroughly and pelletized (diameter 8 mm and thickness 1.2 mm) using polyvinyl alcohol (PVA) as a binder. The pellets were sintered at 1350 1C for 5 h and cooled to room temperature at a cooling rate of 100 1C/h. The X-ray diffraction (XRD) pattern of the powder sample of BPCN was taken at room temperature using a Bruker XRD System (D8 Advance Davinci Bruker, Germany) over a scanning range of Bragg angles 101r2θr901 at the rate of 0.021 per step. The Rietveld refinement of the XRD profile was performed using the Fullprof program [11]. The background was fitted with six-coefficients polynomial function, while the peak shapes were described by pseudoVoigt profiles. Throughout the refinement, scale factor, lattice parameters, positional coordinates (x, y, z) and thermal parameters were varied and the occupancy parameters of all the ions were kept fixed. Fourier transform infrared (FT-IR) spectrum was recorded in transmittance mode at room temperature between 350 to 4000 cm  1 with a Fourier transform infrared spectrometer (Perkin Elmer Spectrum 1000) using KBr pellet technique [10]. In order to study the electrical properties, both the flat surfaces of the pellets were electroded with silver paint and were kept at 500 1C for 1 h prior to conduct the experiment. Impedance measurement was performed as a function of frequency (102–106 Hz) and temperature (303–503 K) using a computer controlled LCR meter (Hioki-3532, Japan). An Eurotherm (2214e) temperature controller (UK) was used to control the temperature.

103 202

2. Experiment

Observed calculated Bragg-position Difference

101 002

applications such as microelectronics, telecommunications, memory devices, and charge storage devices [1–10]. Also the possible ordering of B-cations namely Co2 þ and Nb5 þ should make this compound more interesting for investigating its structural detail. In this paper, we have investigated the structural and electrical properties of the new compound BPCN synthesized by solid-state reaction technique.

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Intensity (arb. unit)

220

80

Fig. 1. Rietveld refinement plot for BPCN. Inset shows the schematic presentation of the BPCN tetragonal unit cell. The Co2 þ atoms are located at the centers of the CoO6 (blue) octahedra. The Nb5 þ atoms are located at the centers of the NbO6 (green) octahedra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

bond distances and bond angles associated with CoO6 and NbO6 octahedra of BPCN are listed in Table 1. It is observed that I4/mmm involves some deformations of the octahedral units but neither tilts nor rotations of the octahedra are observed. The stability of perovskites is determined to a first approximation by the ratio of A–O to B–O bond length, expressed as the tolerance factor (Tf) [13]: RðBa þ PrÞ=2 þ RO T f ¼ pffiffiffi 2½ðRCo þ RNb Þ=2 þ RO 

ð1Þ

where RBa, RPr, RCo, RNb and RO are the ionic radii of Ba, Pr, Co, Nb and O ions, respectively [14]. The value of Tf is found to be 0.98 for BPCN. This value of Tf also suggests that BPCN may have tetragonal phase with perovskite-like structure. The FT-IR spectrum of BPCN is shown in Fig. 2. All the peaks and bands are the characteristic of the material except one hump 2000–4000 cm  1, which is due to the presence of adsorbed moisture in KBr. The first two peaks at 351 and 355 cm  1 correspond to IR-active vibrational perovskite mode of BPCN. The small intensity peaks found in the range 360–380 cm  1 likely correspond to the overtones of the fundamental vibrations in BPCN. The presence of the medium intensity peak at 473 cm  1 can eventually be assigned to the antisymmetric stretching vibration. The strong high-energy band centered at about 945 cm  1 can be assigned to the symmetric stretching mode of the NbO6-octahedra due to the higher charge of this cation. The splitting of this absorption band is probably arises as a consequence of the non cubic symmetry of BPCN. 3.2. Ac electrical conductivity analysis In order to understand ac electrical conduction mechanism in BPCN, the frequency (angular) dependence of ac conductivity sac (¼ εoωε″) was characterized over the temperature range 303 to 503 K. The ac electric response is a superposition of different contributions: the dielectric response of the bound charges (dipolar response), hopping of the localized charge carriers and the response produced by the deformation of the structures following the diffusion of charge carriers along a percolation path. Fig. 3 shows the log–log plot of frequency dependence of the ac conductivity for BPCN at various temperatures. As shown in Fig. 3, BPCN shows a dispersion that shifts to the high-frequency side with increasing temperature. A plateau is observed in the low frequency region

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Table 1 Structural parameters obtained from Rietveld refinement of the XRD data of BaPrCoNbO6 double perovskite oxide. Space group ¼ I4/mmm Lattice parameter: a ¼b ¼ 5.6828(9) Å, c ¼8.063(2) Å Reliability factors: Rp ¼14.4, Rwp ¼19.4, Rexp ¼16.45, χ2 ¼ 1.4 Atom

Wyckoff site

x

Y

Z

B

Bond length (Å)

Bond angle (deg)

Ba/Pr Co Nb O1 O2

4d 2a 2b 8h 4e

0.5 0.0 0.0 0.228(5) 0.0

0.0 0.0 0.0 0.228(5) 0.0

0.25 0.0 0.5 0.0 0.27(1)

0.077 0.32 0.75 0.74 0.93

Co–O1(  4) ¼ 1.83 Co–O2(  2) ¼ 2.23 Nb–O1 (  4) ¼2.18 Nb–O2 (  2) ¼1.80

Co–O1–Nb ¼180 Co–O2-Nb ¼ 180

103/T (K-1) 2.5

Antisymmetric NbO6

3.0

-2

-2 0.7 kHz Ea = 0.398 eV

overtones

-3

IR vibrational mode

Antisymmetric NbO6

390

380 370 360 Wavenumber (cm-1)

log σdc(Sm-1)

streatching vibration

400

-3 0.1 kHz Ea = 0.399 eV

350

symmetric stretching vibration of NbO6

4000 3600 3200 2800 2400 2000 1600 1200

800

400

-4 -2

-4 -2 5.3 kHz Ea = 0.395 eV

-1

Wavenumber (cm ) Fig. 2. FT-IR spectrum of BPCN.

-3

-1.95

303 K 343 K 353 K 363 K 373 K 393 K 403 K 423 K 433 K 443 K 453 K 473 K Solid lines: power law fit

log σac(Sm-1)

-2.60

-3.25

-3.90 2.6

3.9

5.2

6.5

7.8

-4 2.0

-3

2.1 kHz Ea = 0.396 eV

-4 2.4

2.8 103/T (K-1)

3.2

Fig. 4. sdc plots at various frequencies. Solid lines are the linear fits to the data.

9.1

log ω (rad s-1) Fig. 3. Frequency (angular) dependence of the ac conductivity at various temperatures for BPCN. The solid lines are the fitting of the experimental data with the power law.

(below 10 kHz), i.e., a region where sac is frequency independent. This plateau region extends to higher frequencies with increasing temperature, which is the region of dc conductivity (sdc). The value of sdc is found to increase with the increase of temperature showing the semiconducting behaviour of BPCN. At frequencies above 10 kHz, sac(ω) shows dispersion increasing in a power law fashion. The real part of conductivity spectra sac(ω) in such a situation can be expressed as [15] sac ¼ sdc ½1 þ ðω=ωH Þn 

log σdc(Sm-1)

Transmitance (arb. unit)

Transmitance (arb. unit)

2.0

streatching vibration

moisture in KBr

ð2Þ

where ωH is the hopping frequency of the charge carriers and ‘n’ is the dimensionless frequency exponent. The experimental conductivity spectra of BPCN are fitted to above Eq. (2) with sdc and ωH as variables keeping in mind that the values of the parameter ‘n’ are weakly temperature dependent. The solid lines in Fig. 3 show the fitting of the Jonscher's power law [15] with the experimental data and the values of exponent ‘n’ range from 0.54 to 0.39 for BPCN with the increase of temperature from 303 to 473 K. The sdc plots for BPCN are shown for various frequencies in Fig. 4. The sdc follows Arrhenius law for all the plotted frequencies below 10 kHz as shown in Fig. 4 with the activation energy (Ea)  0.4 eV. Such a value of Ea is attributed to polaron hopping within the system. In order to further confirm the conduction mechanism in BPCN, we have used Summerfield scaling (SS) [16,17]. The SS uses the directly measurable quantities sdc and T as the scaling parameter at different temperatures. Fig. 5 shows log(sac/sdc) versus log(ω/ sdcT) plot, so-called SS for BPCN. From Fig. 5, it is seen that a satisfying overlap of the data at different temperatures on a single master curve illustrates well the dynamic processes occurring at different frequencies need almost the same thermal activation energy. Another indication of the scaled master curve is that all temperature dependence of conductivity is embedded in the dc conductivity term. The basic fact about ac conductivity in BPCN is that sac increases as a function of frequency (any hopping model has this feature). In a hopping model it is possible to distinguish

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0.4

0.03

log(σ /σdc)

0.2

0.02

1.0 M" / M"m

0.3

M'(Ω)

343 K 353 K 363 K 373 K 403 K 423 K 433 K 443 K 473 K

303 K 343 K 363 K 383 K 403 K 423 K 453 K 483 K

333 K 343 K 353 K 363 K 373 K 383 K

0.5

0.0

0.01

0.4

0.6

0.8

1.0

1.2

log(ω /ωm)

0.00

0.1

6.72

Data Points Linear Fit

log ωm (rad s-1)

0.012

MEa = 0.42 eV

6.44

2

3

4 5 6 log(ω /σdcT) [(rad/s)/(S/m)K]

7

8

M"(Ω)

0.0

0.008

6.16

303 K 343 K 363 K 383 K 403 K 423 K 453 K 483 K

5.88

0.004

Fig. 5. Summerfield scaling for conductivity spectra at different temperatures.

2.6

2.8

3.0

103 / T (K-1)

different characteristic regions of frequency. At low frequencies where conductivity is constant, the carrier transport takes place on infinite paths. For a region of frequencies where conductivity increases markedly with frequency, the carriers can hop only between two sites and a total response is produced by the sum of the individual responses of pairs of sites randomly distributed throughout the material. In order to clarify the observed behaviour of sac as mentioned above, we have used the electric modulus formalism to study the relaxation of the electric field in BPCN in more detail. 3.3. Complex electric modulus (CEM) analysis The dielectric relaxation mechanism of BPCN has been demonstrated using electric modulus (EM) formalism. The EM corresponds to the relaxation of the electric field in the materials when the electric displacement remains constant. Therefore, the modulus represents the real dielectric relaxation process. The CEM is defined as [18,19]:     Z 1 dϕðtÞ M n ðωÞ ¼ M′ðωÞ þ jM″ðωÞ ¼ M 1 1  dt ð3Þ e  jωt dt 0 where the function ϕðtÞ gives the time evaluation of electric field within the material. CEM formalism is a very important tool to determine, analyze and interpret the dynamical aspects of electrical transport phenomena, i.e., parameters such as carrier/ion hopping rate and conductivity relaxation time. In Fig. 6(a) and (b), we have plotted the real (M′) and imaginary (M″) parts of complex electric modulus (Mn) as a function of frequency (angular) at several temperatures. The value of M′(ω) [Fig. 6(a)] increases with the increase in frequency and reaches a constant value, M1 (the asymptotic value of M′(ω) at high frequencies). This indicates that BPCN is very capacitive in nature. Peaks in the values of M″(ω) are found supporting the dielectric relaxation process for BPCN. The relaxation peak for M″(ω) that moves towards higher frequencies with the increase of temperature shows that the relaxation rate for this process increases with increasing temperature. The frequency region below peak maximum (Mm″) determines the range in which charge carriers are mobile on long distances. At frequency above peak maximum the carriers are confined to potential wells and mobile on short distances only. As a convenient measure of the characteristic relaxation time one can choose the inverse of frequency of the maximum peak position, i.e., τm ¼ωm  1. Thus, we can determine the temperature dependence of the characteristic relaxation time as shown in the inset of Fig. 6(a) which satisfies the Arrhenius law. From the numerical fitting analysis, we have obtained the value of Ea ¼0.42 eV. This value of Ea is very close to that of the value of Ea obtained from the sdc plots. Dielectric

0.000

Davidson-Cole Fitting

2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8 5.1 5.4 5.7 6.0 6.3 6.6 6.9

log ω (rad s-1) Fig. 6. Frequency dependence of the M′ (a) and M″ (b) of BPCN at various temperatures. Solid lines are the fitting to the data using Davidson–Cole equation. Arrhenius plot corresponding to M″ and the scaling behaviour of imaginary part of electric modulus (M″) for BPCN are shown in the insets of (a) and (b), respectively.

Table 2 Various electrical parameters obtained from Davidson–Cole fitting of BaPrCoNbO6 double perovskite oxide. T (K)

Ms

M1

γ (α ¼ 1 γ)

303 313 323 333 343 363 383 403 423

3.1E  7 3.0E  7 2.9E  7 2.9E  7 2.8E  7 2.2E  7 1E  7 6E  8 4E  8

0.032 0.032 0.032 0.0325 0.0335 0.035 0.035 0.04 0.04

0.863 0.866 0.865 0.86 0.842 0.82 0.835 0.835 0.835

relaxation can be studied in detail by Debye [20], Cole–Cole [21], Davidson–Cole [22] or Havriliak–Negami [23] model. For the present system the nature of dielectric relaxation could be modeled by Davidson–Cole model (DCM) [22]. The DCM accounts for an asymmetric distribution of relaxation times resulting from a dielectric dispersion within a system by introducing an exponential parameter γ in the Debye dielectric function [20]. The Davidson–Cole equation is defined as [22] M′ ¼

M 1 M s ½ðM s þ M 1  M s Þð cos ϕÞγ cos γϕ M s þ ðM 1  M s Þð cos ϕÞγ ½2M s cos γϕ þðM 1  M s Þð cos ϕÞγ  2

ð4Þ M″ ¼

M 1 M s ½ðM 1  M s Þð cos ϕÞγ sin γϕ

; M s þ ðM 1  M s Þð cos ϕÞγ ½2M s cos γϕ þ ðM 1  M s Þð cos ϕÞγ  2

ð5Þ where 0 oγ r1; tan ϕ¼ωτ; ωmτ¼ tan (π=2=γ þ 1) We have fitted the experimental data using Eq. (5). A good agreement between the experimental data and the data obtained by using Eq. (5) is observed as shown by solid line in Fig. 6(b) for various temperatures with the value of Ms, M1 and γ having their usual meanings [22]. The values of Ms, M1 and γ obtained for various temperatures by using Eq. (5) are summarized in Table 2.

C. Bharti et al. / Journal of Solid State Chemistry 210 (2014) 219–223

The scaling behaviour of M″ is shown in the inset of Fig. 6(b), where we have scaled each M″ by M″m and each frequency by ωm (ωm corresponds to the frequency of the peak position of M″ in the M″ vs. log ω plots). The perfect overlap of the curves at different temperatures into a single master curve indicates that the relaxation describes the same mechanism at various temperatures for BPCN. 4. Conclusions A rare earth based new double perovskite oxide BaPrCoNbO6 has been synthesized by the solid-state reaction technique. The structural analysis showed that this compound has a perovskitelike structure with tetragonal phase having the I4/mmm space group and 1:1 ordered arrangement of the Co2 þ and Nb5 þ cations over the six-coordinate B-sites. The alternate ordering of Co2 þ and Nb5 þ sites is observed in the XRD pattern. The dielectric relaxation in BPCN is investigated in the temperature range 303 to 503 K and in the frequency range 100 Hz to 1 MHz using impedance spectroscopy. The frequency dependent ac conductivity follows Jonscher's power law. The sdc follows Arrhenius law. Summerfield scaling of conductivity data suggests that the dynamic processes occurring at different frequencies need almost the same thermal activation energy for BPCN. Davidson–Cole model is found to be the most suitable model to explain the dielectric relaxation mechanism in BPCN. The activation energy of the sample, calculated from both ac conductivity and electric modulus spectra, is found to be  0.4 eV which indicates that the conduction mechanism for BPCN is polaron hopping. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation follows the same mechanism at various temperatures. Acknowledgments Dr. C. Bharti is grateful to CSIR, New Delhi for providing Nehru PDF (CSIR offer letter No. HRDG/CSIR-Nehru PDF-EN-ES-PS/EMR-I/ 01/2012).

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