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Structural and conductivity properties of K doped Ba4Ca2Nb2O11 (BCN) complex perovskite for energy applications
Journal of Alloys and Compounds 686 (2016) 930e937
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Structural and conductivity properties of K doped Ba4Ca2Nb2O11 (BCN) complex perovskite for energy applications K. Kavitha a, T. Vijayaraghavan a, Chandrasekhar Kumbhar b, P.K. Ojha b, Anuradha Ashok a, * a b
Functional Materials Laboratory, PSG Institute of Advanced Studies, Coimbatore 641 004, T.N., India Naval Materials Research Laboratory, Ambernath 421 506, Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2016 Received in revised form 10 June 2016 Accepted 21 June 2016 Available online 23 June 2016
Developing new proton and oxide ion conducting material with better stability is of great importance for the performance enhancement of devices for energy related applications. In this work Ba1xKx(Ca0.5Nb0.5)O2.75d (x ¼ 0, 0.25, 0.50, 0.75 and 1) complex perovskites were synthesised through solid state reaction route. The XRD patterns of calcined samples showed the formation of single phase double perovskite structure having favourable crystal lattice for oxygen vacancies for achieving enhanced proton and oxide ion conductivity. The investigated K doped BCN perovskite with 25% dopant concentration exhibited both higher total conductivity and stability when compared to the undoped BCN perovskite. This enhancement in total conductivity is suggested to be due to increase in number of oxygen vacancies on doping. These results are encouraging considering the stability and higher oxide ion conductivity. © 2016 Elsevier B.V. All rights reserved.
Keywords: Perovskite Electrode/electrolyte Energy devices Impedance spectroscopy
1. Introduction Perovskites with general formula ABO3 is a class of complex oxides that can incorporate large number of point defects, such as oxygen vacancies and protons that compensate dopants or part of inherent off-stoichiometry. In perovskite structure both A and B sub-lattices can adopt cations with different valence and size because of their high tolerance resulting in the formation of various types of superstructures [1]. Generally, lower valent dopants are introduced to the perovskite structure in order to produce vacancy defects in anion sublattice which help in the migration of oxygen ions through the crystal [2,3]. This type of oxygen deficient perovskite compounds are given more importance among solid state proton and oxide ion conductors due to their potential applications in fuel cell electrolytes, air separation membranes, oxygen sensor, H2 pumps etc [2]. Du and Nowick first introduced complex perovskites of type A3(B1þxB0 2x)O93x/2 where A, B ¼ divalent alkaline earth metals and B0 ¼ pentavalent Nb, Ta as important class of model materials for proton and oxide ion conduction [4,7]. When x > 0, it is reported that they exhibit high oxide ion conductivity at higher
* Corresponding author. E-mail address: [email protected] (A. Ashok). http://dx.doi.org/10.1016/j.jallcom.2016.06.198 0925-8388/© 2016 Elsevier B.V. All rights reserved.
temperatures and proton conductivity at lower temperatures in hydrated atmospheres [4,5]. When x ¼ 0, these materials are in the form of A3B1B0 2O9 with 1:2 ordered (based on the ratio of B cations) trigonal crystal structure as shown in Fig. 1a with no structural oxygen vacancies. The only possibility of creating vacancies in this composition is by introducing lower valent cationic dopants. Whereas when x ¼ 0.5, the composition A3(B1.5B0 1.5)O8.25 or A4(B2B0 2)O11 with 1:1 ordered B cation sublattice exhibits a face centered cubic structure as shown in Fig. 1b. Though this unit cell can accommodate 24 oxygen atoms, the 1:1 ordered A4(B2B0 2)O11 with two formula units per unit cell can provide only 22 oxygen atoms. This results in a unit cell with two inherent structural oxygen vacancies [4]. Introducing a lower valent dopant in the A site of this particular composition further increases the number of structural oxygen vacancies which act as active sites for ion conductivity [3]. Du and Nowick also showed that the 1:1 ordered face centered cubic crystal structure with random or locally ordered vacancies is more favourable for protonic/oxide ion conduction than 1:2 ordered trigonal structure [4]. The major requirement of any electrolyte material in solid oxide fuel cell is high ionic conductivity and stability [6]. Ba3Ca1þxNb2xO93x/2 is one such perovskite which was found to show high proton and oxide ion conductivity for x ¼ 0.5 and 0.18 [1,5,7e10]. In another study, S.S.Bhella et al. [11,12,17] have
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Fig. 1. a.1:2 ordered trigonal unit cell and b.1:1 ordered face centered cubic unit cell.
investigated the electrical conductivity of Ba3xKxCaNb2O9d (x ¼ 0.5, 0.75, 1, 1.25) composition. In this study they have reported that the base composition has 1:2 ordered B site cations with no structural oxygen vacancies. However, on substitution of Ba2þ ions by Kþ ions in the A site, they observed increased ionic conductivity and stability with increasing dopant concentration [12]. In the present study, Ba2þ is partially substituted by Kþ in the 1:1 ordered Ba8(Ca4Nb4)O22 structure, i.e. Ba1xKx(Ca0.5Nb0.5) O2.75d. Complex perovskites with different degree of substitution (x ¼ 0, 0.25, 0.50, 0.75 & 1) are synthesised by solid state reaction method. The reason of choosing 1:1 ordered composition is that it already has two inherent structural oxygen vacancies, further substituting Ba2þ by Kþ will increase the number of oxygen vacancies thereby increasing the number of active sites for ionic conduction. As proven by Bhella et al. [12] it is also expected that this substitution will enhance the stability of the perovskite. In this study, several compositions are synthesised and investigated to evaluate the structural properties and variation of total conductivity with different dopant concentrations. The nominal compositions Ba8(Ca4Nb4)O22 (BCN), Ba6K2(Ca4Nb4)O21 (BKCN25), Ba4K4(Ca4Nb4)O20 (BKCN50), Ba2K6(Ca4Nb4)O19 (BKCN75) and Fig. 3. XRD patterns of BKCN series calcined at 800 C 2 h.
K8(Ca4Nb4)O18 (BKCN100) when/if synthesised as single complex perovskite phase (as in Fig. 1b) are expected to have 2, 3, 4 and 5 oxygen vacancies per cubic double perovskite unit cell respectively.
2. Experimental
Fig. 2. Thermogravimetry curves of 20 h ball milled precursors of BKCN perovskites.
Complex perovskite oxides Ba8(Ca4Nb4)O22 (BCN), Ba6K2(Ca4Nb4)O21 (BKCN25), Ba4K4 (Ca4Nb4)O20 (BKCN50), Ba2K6(Ca4Nb4) O19 (BKCN75) and K8(Ca4Nb4)O18 (BKCN100) were prepared by solid state method. High purity (99.999%) powders of BaCO3, K2CO3, CaCO3 and Nb2O5 imported from Sigma Aldrich, USA were used as precursors for synthesis of these perovskite oxides. Stoichiometric amounts of preheated (at 500 C) precursor powders corresponding to each composition were milled using Planetary ball milling (Pulverisette P5, Fritch, Germany) for 20 h. Milling was carried out in tungsten carbide bowl with ball to total powder weight ratio 10:1 and toluene as the process control agent. The ball milled mixtures were subjected to thermogravimetry (TG) analysis using NETZSCH STA 449 F3 Jupiter at a temperature range from room temperature
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Fig. 4. XRD patterns and corresponding unit cells of BKCN perovskite series sintered at 1200 C for 5 h a. BCN, b. BKCN 25, c. BKCN 50, d. BKCN 75 and e. BKCN 100.
to 1200 C and at a heating rate of 20 C/min in dry air atmosphere. The ball milled samples were calcined at 800 C for 2 h using a high
temperature muffle furnace in clean alumina crucibles. Later the calcined samples were subjected to XRD analysis using X’Pert
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Table 1 Parameters obtained from structural analyses of BKCN series. XRD analysis Composition
HRTEM analysis PCPDF no.
Space group
Bravais lattice
Refined cell parameters (Å)
Bravais lattice
Cell parameters (Å)
Ba8 Ca4Nb4O22 (BCN)
PDF#500075
Fm3m (225)
Face centered cubic
a ¼ b ¼ c ¼ 8.3400(0)
Primitive cubic
a ¼ b ¼ c ¼ 8.04
Ba6K2 Ca4Nb4O21 (BKCN 25)
PDF#500075
Face centered cubic
a ¼ b ¼ c ¼ 8.3402(0)
Primitive cubic
a ¼ b ¼ c ¼ 8.04
Ba4K4 Ca4Nb4O20 (BKCN 50)
PDF#470691
Fm3m (225) P4bm (100)
Primitive Tetragonal
Primitive tetragonal
a ¼ b ¼ 8.1, c ¼ 7.8
Ba2K6 Ca4Nb4O19 (BKCN 75)
PDF#470691
P4bm (100)
Primitive Tetragonal
Primitive tetragonal
a ¼ b ¼ 8.1, c ¼ 7.8
K8 Ca4Nb4O18 (BKCN 100)
Ref [18,19]
I4mmm (139)
Body centered Tetragonal
a ¼ b ¼ 12.4751(5) c ¼ 3.9673(0) a ¼ b ¼ 12.4700(0) c ¼ 3.9600(0) a ¼ b ¼ 8.0999(9) c ¼ 15.7000(0)
Body centered Tetragonal
a ¼ b ¼ 8.5, c ¼ 15.77
PANalytical diffractometer to confirm the phase purity. The XRD data was collected at room temperature using Cu Ka radiation with a step size of 0.05 in the 2q range from 20 to 80 . On confirmation of phase purity, the samples were pressed into pellets of 10 mm diameter and 2 mm thickness using PVA (PolyVinyl Alcohol) as binder in a hydraulic press with a pressure of 10 MPa. The pressed pellets were sintered at 1200 C for 5 h. XRD patterns of sintered pellets were analysed to evaluate the thermal stability of phases formed on calcination. For High Resolution Transmission Electron Microscopic (HRTEM) analysis, the sintered pellets were crushed and dispersed in ethanol and sonicated for 3 min at 50 Hz. Later a drop containing the particles was loaded onto a carbon coated copper grid and subsequently dried before viewed under electron microscope. The particle size and morphology of the powder was examined by using High Resolution Transmission Electron Microscope (HRTEM) (JEOL JEM 2100), fitted with an OXFORD Energy dispersive X-ray spectrometer (EDS). The samples were viewed at an accelerating voltage of 200 KV. The Selected Area Electron Diffraction (SAED) patterns were processed and analysed using the software Digital Micrograph-1.85.1535 version. The total conductivity measurement of samples were carried out using impedance analyzer, AUTOLAB, ECO CHEMIE, Netherlands from 10 Hz to 10 MHz in the temperature range 100 C to 800 C in air atmosphere. The data analysis was performed using the Autolab software Frequency Response analyzer (FRA) 4.9 version. 3. Results and discussions 3.1. Thermogravimetric analysis (TGA) Fig. 2 shows the thermal decomposition of 20 h ball milled precursors of BKCN perovskite series. The TGA curves exhibit weight loss in several stages. The first stage between room temperature to 200 C results due to the loss of the adsorbed moisture and structural water from the starting materials. The second stage of weight loss around 650 C is due to the removal of carbonaceous species present in the materials. The third stage of weight loss around 800 C is attributed to the final phase formation. TG curves of BCN and BKCN 25 are stable above 800 C indicating constant mass, whereas the TG curves of BKCN 50, BKCN 75 and BKCN 100 are stable above 1100 C. This thermal analysis data helps us in developing the two stage calcination protocol, one at 800 C and another at 1100 C. First, the samples are calcined at lower temperature of 800 C for 2 h and checked for single phase formation. 3.2. XRD analysis Fig. 3 shows the XRD patterns of 20 h ball milled precursors of
BKCN perovskites calcined at 800 C for 2 h. The diffraction peaks were indexed according to a face centered cubic crystal structure (Fm3m (225)) with cell parameter ac z 8.407 Å. This is in congruence with the reported structural data for similar perovskites [12]. Absence of any peaks corresponding to additional phases and modification of crystal structure indicates that the dopant K atoms have been incorporated and distributed in the parent crystal (BCN perovskite) in a random manner. Perovskite with maximum dopant concentration (x ¼ 1) BKCN 100 shows doubling of ‘some of the XRD peaks possibly due to the formation of tetragonal phase with a z ac and c z 2ac [18,19]. This could happen due to the ordering of either the dopants or oxygen vacancies [12]. XRD patterns of crushed powders of sintered pellets of BKCN series and the respective unit cells based on which the diffractograms are interpreted are shown in Fig. 4aee and ieiii. The diffractograms indicate no change in the crystal structure for BCN, BKCN 25 and BKCN 100 after sintering. Moreover sharper diffraction peaks in BKCN 25 show an increase in the size of single crystallites of the sample. In the case of BKCN 50 and BKCN 75, a change in crystal structure from face centered cubic (Fm3m) to primitive tetragonal (P4bm) is observed. This change in structure may be due to the effect of dopants at higher temperature. The details of structural analyses (through Rietveld refinement also shown as supporting data) of all the compositions are listed in Table 1. Any specific modification of the crystal unit cell symmetry based on the ordering of the dopants or oxygen vacancies cannot be seen through these XRD data. All unit cells shown in Fig. 4ieiii give all possible oxygen and A site cation positions indicated as O2 and Baþ2 respectively. Depending on their nominal compositions the dopant K atoms can be present in any of the Ba sites and vacancies can be present in any of the oxygen sites. In the case of BKCN 100, all the divalent Ba positions are occupied by monovalent K atoms and each unit cell nominally has 12 vacancies distributed among 48 oxygen positions. 3.3. HRTEM imaging Fig. 5 shows the results obtained from HRTEM analysis of sintered BKCN perovskites. The images taken at lower magnification show flaky morphology for all compositions. Selected Area Electron Diffraction Patterns (SAED) of all compositions showed spot type patterns indicating that the flakes are single crystalline. The crystal structures based on which SAED pattern from each composition is indexed are given in Table 1. In general SAED patterns give the local crystal structure whereas XRD data will give the average crystallographic information of the material. SAED and XRD data taken from the same material may differ if the local and average crystal structures are different. These differences can appear due to the local ordering of some of the constituents in the crystal over a small volume of the sample. In the present study local and average crystal
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Fig. 5. TEM image, SAED pattern and high resolution images of a. BCN, b. BKCN 25, c. BKCN 50, d. BKCN 75 and e. BKCN 100.
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Fig. 6. Cole-Cole plots of a. BCN, b. BKCN 25, c. BKCN 50, d. BKCN 75 and e. BKCN 100.
structures are the same for compositions BKCN 50, BKCN 75 and BKCN 100. But for BCN and BKCN 25, though the average crystal is cryolite type double perovskite structure (Fm3m with a z8.34 Å), on a local scale the crystal symmetry is primitive cubic (Pm3m) having the same cell dimension. The high resolution images are interpreted based on the crystal structures exhibited by the electron diffraction patterns. 3.4. Impedance spectral analysis Impedance analysis is generally performed by applying an AC signal across the pellets and recording the corresponding electrical response of the system after the sinusoidal perturbation. Impedance is calculated as a function of the frequency of this perturbation [13]. Complex impedance (Z*) is given by, Z* ¼ Z0 jZ^00 ¼ RS 1/juCS
(1)
where u ¼ 2pf is the angular frequency, Z0 is the real part of impedance, Z00 is the imaginary part of impedance, RS and CS are the resistance and capacitance in series respectively [14]. Fig. 6 shows Cole-Cole plots of BKCN perovskite pellets with different compositions sintered at 1200 C for 5 h. Shape of each impedance plot was reproducible. A single semicircle was observed in the Cole-Cole plots at the measured temperature range for BCN perovskite (Fig. 6a). Absence of intermediate frequency impedance arc that corresponds to grain boundary contribution confirms that this semicircle is related to the bulk contribution to the conductivity. For K- doped BCN perovskites one semicircle along with intermediate frequency impedance arc corresponding to grain boundary contribution is observed. Generally the grain boundary effect arises due to the presence of high resistance and the blocking
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decrease due to the grain boundary effect [15]. Similar semicircle with negligible contributions due to grain-boundary and electrode effects was reported by S.S.Bhella [12] for K-doped Ba3CaNb2O9. The total conductivity of the samples is determined from the intercepts of the semicircle on the real axis by using the relationship:
s¼
Fig. 7. Arrhenius plots of BKCN perovskite series.
l Rb A
(2)
where l/A is the geometrical factor, l is the thickness, A is the area of the sample and Rb is the bulk resistance of the electrode which is indicated by the intercepts of the semicircle. The average thickness of the pellets was 0.22 cm and area was 0.634 cm. In those compositions which showed two semicircles in the Cole-Cole plot, total conductivity is calculated by considering only the first semicircle corresponding to bulk conductivity assuming that the grain boundary contribution might be due to electrons. Fig. 7 shows the Arrhenius plots of total electrical conductivity of BKCN series with varying dopant concentration. Here the conductivity values are plotted against 1000/T factor and the slope is calculated by linearly fitting the curve using origin pro software. From the obtained slope the activation energy is calculated using the following equation:
s ¼ s0 exp (Ea/kT)
Fig. 8. Variation of total conductivity with x in Ba1xKx(Ca0.5Nb0.5)O2.75d at different temperatures.
layer near the grain boundaries. Our results indicate that there is no significant resistance offered by the grain boundaries to the carriers. It was reported that the oxide ion conductivity of materials
(3)
where s0 is the pre-exponential factor, Ea is the activation energy, k is the Boltzman constant and T is the absolute temperature. The variation in total conductivity with respect to dopant concentration at various measured temperatures is depicted in Fig. 8. An overall increment in total conductivity can be observed due to doping of K in BCN. As mentioned in the introduction, the structural oxygen vacancies also increase with the increase in dopant concentration. From Figs. 7 and 8 it can also be seen that total conductivity increases with increasing dopant concentration for most of the compositions. However the sintered pellets of BKCN 50, BKCN 75 and BKCN 100 disintegrated in a very short interval of time, whereas sintered pellets of BCN and BKCN 25 did not disintegrate and remained intact and stable for a longer duration of time on exposure to atmosphere. The higher total conductivity in the case of BKCN 50, BKCN 75 and BKCN 100 when compared to BKCN 25 may be due to the increased surface activity in the form of electronic contribution to the total conductivity. This might also be another reason for the disintegration of their sintered pellets. In addition, after sintering, BCN and BKCN 25 exhibited the crystal structure that was reported to be favourable for higher conductivity due to disordered vacancies [4,9,10]. Among these two
Table 2 Conductivity values of BKCN perovskite series in comparison to reported compositions. Composition
Temperature ( C)
s (S cm1) (Atmosphere)
Ea (eV)
Ba3Ca1.18Nb1.82O9d Ba1.75K1.25CaNb2O9d Ba1.75K1.25CaNb2O9d Ba1.75K1.25CaNb2O9d Ba8 Ca4Nb4O22 (BCN)
200 700 700 700 800 700 800 700 800 700 800 700 800 700
5.3 105 (wet Ar/O2) 1.7 105 (wet H2O/N2) 2.4 104 (air) 2.8 104 (wet H2O/H2) 9.38 108 (air) 2.56 108 (air) 6.93 106 (air) 1.10 106 (air) 3.79 105 (air) 1.51 105 (air) 1.30 105 (air) 3.11 106 (air) 1.02 105 (air) 1.17 106 (air)
0.55 [16] 0.89 [12] 1.0 [12] 1.17 [12] 1.26 (Present work)
Ba6K2Ca4Nb4O21 (BKCN 25) Ba4K4Ca4Nb4O20 (BKCN 50) Ba2K6Ca4Nb4O19 (BKCN 75) K8Ca4Nb4O18 (BKCN 100)
1.71 (Present work) 1.12 (Present work) 1.29 (Present work) 1.41 (Present work)
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compositions, by virtue of having larger number active sites BKCN 25 exhibited higher value of total conductivity. The calculated activation energies and the conductivity values of BKCN series are listed in Table 2. For comparison some of the previously reported results are also mentioned in this Table. The conductivity values listed in Table 2 obtained in the present work are comparable to the previously investigated K doped Ba3CaNb2O9 [11,17]. As shown in Fig. 8 an enhancement in total conductivity of the undoped composition BCN can be observed with increase in K doping. The higher total conductivity value of K doped compositions when compared to undoped BCN in this study could be due to the additional active sites for ionic conductivity along with inherent structural oxygen vacancies in the base composition created by doping lower valent cation.
4. Conclusion This study presented the effect of various concentrations of monovalent potassium in the position of divalent barium on 1:1 ordered Ba4Ca2Nb2O11 synthesised by solid state reaction route. Thermal analysis was used to perform the heat treatment as one step calcination process to obtain the required final perovskite phase. Structural studies indicated a modification of the original crystal structure after sintering for compositions with higher dopant concentrations. An overall increment in the total conductivity in air atmosphere was observed after doping and it was found to increase with increase in potassium concentration. However the stability of the material decreased for higher dopant concentration. This study showed that BKCN 25 (25% of Ba replaced by K) has both higher total conductivity and stability and hence a prominent material for energy applications such as SOFC. Further increase in dopant concentration decreased the stability. It is the formation of extra oxygen vacancies in addition to the already existing inherent structural vacancies suggested to be responsible for the enhancement of total conductivity.
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Acknowledgements The authors acknowledge Naval Research Board (Grant number: DNRDO/05/4003/NRB/287) for financial support and PSG Institute of Advanced studies and Naval Materials Research Laboratory (NMRL) for their infrastructure and intellectual support.
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.06.198.
References [1] I. Animitsa, A. Neiman, N. Kochetova, B. Melekhk, A. Sharafutdinov, Solid State Ionics 162e163 (2003) 63e71. [2] K.R. Kendall, C. Navas, J.K. Thomas, H.C. Loye, Solid State Ionics 82 (1995) 215e223. [3] Deepthi N. Rajendran, K. Ravindran Nair, P. Prabhakar Rao, K.S. Sibi, Peter Koshy, V.K. Vaidyan, Mater. Chem. Phys. 109 (2008) 189e193. [4] Y. Du, A.S. Nowick, J. Am. Ceram. Soc. 78 (1995) 3033. [5] K.C. Liang, Y. Du, A.S. Nowick, Solid State Ionics 69 (1994) 117e120. [6] N. Tarasova, I. Animitsa, Solid State Ionics 264 (2014) 69e75. [7] A.S. Nowick, Yang Du, K.C. Liang, Solid State Ionics 125 (1999) 303e311. [8] K.C. Liang, A.S. Nowick, Solid State Ionics 61 (1993) 77e81. [9] A.S. Nowick, Y. Du, Solid State Ionics 77 (1995) 137e146. [10] I. Animitsa, A. Neiman, N. Kochetova, D. Korona, A. Sharafutdinov, Solid State Ionics 177 (2006) 2363e2368. [11] S.S. Bhella, V. Thangadurai, Solid State Ionics 186 (2009) 311e319. [12] S.S. Bhella, V. Thangadurai, Solid State Ionics 192 (2011) 229e234. [13] Jian Ye, Chang Chen, Liesbet Lagae, Guido Maes, Gustaaf Borghs, Pol Van Dorpe, Phys. Chem. 12 (2010) 11251e11262. [14] Shrabanee Sen, R.N.P. Choudhary, Mater. Chem. Phys. 87 (2004) 256e263. [15] D.N. Rajendran, K. Ravindran Nair, P. Prabhakar Rao, K.S. Sibi, Peter Koshy, V.K. Vaidyan, Mater. Chem. Phys. 109 (2008) 189e193. [16] H.G. Bohn, T. Schober, T. Mono, W. Schilling, Solid State Ionics 117 (1999) 219. [17] V. Thangadurai, W.H. Kan, B. Mirfakhraei, S.S. Bhella, T.T. Trinh, ECS Trans. 35 (1) (2011) 483e492. [18] Anuradha M. Ashok, Camilla Haavik, Poul Norby, Truls Norby, Arne Olsen, Micron 62 (2014) 11e27. [19] Anuradha M. Ashok, Reidar Haugsrud, Truls Norby, Ame Olsen, Mater. Charact. 102 (2015) 71e78.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Structural and conductivity properties of K doped Ba4Ca2Nb2O11 (BCN) complex perovskite for energy applications K. Kavitha a, T. Vijayaraghavan a, Chandrasekhar Kumbhar b, P.K. Ojha b, Anuradha Ashok a, * a b
Functional Materials Laboratory, PSG Institute of Advanced Studies, Coimbatore 641 004, T.N., India Naval Materials Research Laboratory, Ambernath 421 506, Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2016 Received in revised form 10 June 2016 Accepted 21 June 2016 Available online 23 June 2016
Developing new proton and oxide ion conducting material with better stability is of great importance for the performance enhancement of devices for energy related applications. In this work Ba1xKx(Ca0.5Nb0.5)O2.75d (x ¼ 0, 0.25, 0.50, 0.75 and 1) complex perovskites were synthesised through solid state reaction route. The XRD patterns of calcined samples showed the formation of single phase double perovskite structure having favourable crystal lattice for oxygen vacancies for achieving enhanced proton and oxide ion conductivity. The investigated K doped BCN perovskite with 25% dopant concentration exhibited both higher total conductivity and stability when compared to the undoped BCN perovskite. This enhancement in total conductivity is suggested to be due to increase in number of oxygen vacancies on doping. These results are encouraging considering the stability and higher oxide ion conductivity. © 2016 Elsevier B.V. All rights reserved.
Keywords: Perovskite Electrode/electrolyte Energy devices Impedance spectroscopy
1. Introduction Perovskites with general formula ABO3 is a class of complex oxides that can incorporate large number of point defects, such as oxygen vacancies and protons that compensate dopants or part of inherent off-stoichiometry. In perovskite structure both A and B sub-lattices can adopt cations with different valence and size because of their high tolerance resulting in the formation of various types of superstructures [1]. Generally, lower valent dopants are introduced to the perovskite structure in order to produce vacancy defects in anion sublattice which help in the migration of oxygen ions through the crystal [2,3]. This type of oxygen deficient perovskite compounds are given more importance among solid state proton and oxide ion conductors due to their potential applications in fuel cell electrolytes, air separation membranes, oxygen sensor, H2 pumps etc [2]. Du and Nowick first introduced complex perovskites of type A3(B1þxB0 2x)O93x/2 where A, B ¼ divalent alkaline earth metals and B0 ¼ pentavalent Nb, Ta as important class of model materials for proton and oxide ion conduction [4,7]. When x > 0, it is reported that they exhibit high oxide ion conductivity at higher
* Corresponding author. E-mail address: [email protected] (A. Ashok). http://dx.doi.org/10.1016/j.jallcom.2016.06.198 0925-8388/© 2016 Elsevier B.V. All rights reserved.
temperatures and proton conductivity at lower temperatures in hydrated atmospheres [4,5]. When x ¼ 0, these materials are in the form of A3B1B0 2O9 with 1:2 ordered (based on the ratio of B cations) trigonal crystal structure as shown in Fig. 1a with no structural oxygen vacancies. The only possibility of creating vacancies in this composition is by introducing lower valent cationic dopants. Whereas when x ¼ 0.5, the composition A3(B1.5B0 1.5)O8.25 or A4(B2B0 2)O11 with 1:1 ordered B cation sublattice exhibits a face centered cubic structure as shown in Fig. 1b. Though this unit cell can accommodate 24 oxygen atoms, the 1:1 ordered A4(B2B0 2)O11 with two formula units per unit cell can provide only 22 oxygen atoms. This results in a unit cell with two inherent structural oxygen vacancies [4]. Introducing a lower valent dopant in the A site of this particular composition further increases the number of structural oxygen vacancies which act as active sites for ion conductivity [3]. Du and Nowick also showed that the 1:1 ordered face centered cubic crystal structure with random or locally ordered vacancies is more favourable for protonic/oxide ion conduction than 1:2 ordered trigonal structure [4]. The major requirement of any electrolyte material in solid oxide fuel cell is high ionic conductivity and stability [6]. Ba3Ca1þxNb2xO93x/2 is one such perovskite which was found to show high proton and oxide ion conductivity for x ¼ 0.5 and 0.18 [1,5,7e10]. In another study, S.S.Bhella et al. [11,12,17] have
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Fig. 1. a.1:2 ordered trigonal unit cell and b.1:1 ordered face centered cubic unit cell.
investigated the electrical conductivity of Ba3xKxCaNb2O9d (x ¼ 0.5, 0.75, 1, 1.25) composition. In this study they have reported that the base composition has 1:2 ordered B site cations with no structural oxygen vacancies. However, on substitution of Ba2þ ions by Kþ ions in the A site, they observed increased ionic conductivity and stability with increasing dopant concentration [12]. In the present study, Ba2þ is partially substituted by Kþ in the 1:1 ordered Ba8(Ca4Nb4)O22 structure, i.e. Ba1xKx(Ca0.5Nb0.5) O2.75d. Complex perovskites with different degree of substitution (x ¼ 0, 0.25, 0.50, 0.75 & 1) are synthesised by solid state reaction method. The reason of choosing 1:1 ordered composition is that it already has two inherent structural oxygen vacancies, further substituting Ba2þ by Kþ will increase the number of oxygen vacancies thereby increasing the number of active sites for ionic conduction. As proven by Bhella et al. [12] it is also expected that this substitution will enhance the stability of the perovskite. In this study, several compositions are synthesised and investigated to evaluate the structural properties and variation of total conductivity with different dopant concentrations. The nominal compositions Ba8(Ca4Nb4)O22 (BCN), Ba6K2(Ca4Nb4)O21 (BKCN25), Ba4K4(Ca4Nb4)O20 (BKCN50), Ba2K6(Ca4Nb4)O19 (BKCN75) and Fig. 3. XRD patterns of BKCN series calcined at 800 C 2 h.
K8(Ca4Nb4)O18 (BKCN100) when/if synthesised as single complex perovskite phase (as in Fig. 1b) are expected to have 2, 3, 4 and 5 oxygen vacancies per cubic double perovskite unit cell respectively.
2. Experimental
Fig. 2. Thermogravimetry curves of 20 h ball milled precursors of BKCN perovskites.
Complex perovskite oxides Ba8(Ca4Nb4)O22 (BCN), Ba6K2(Ca4Nb4)O21 (BKCN25), Ba4K4 (Ca4Nb4)O20 (BKCN50), Ba2K6(Ca4Nb4) O19 (BKCN75) and K8(Ca4Nb4)O18 (BKCN100) were prepared by solid state method. High purity (99.999%) powders of BaCO3, K2CO3, CaCO3 and Nb2O5 imported from Sigma Aldrich, USA were used as precursors for synthesis of these perovskite oxides. Stoichiometric amounts of preheated (at 500 C) precursor powders corresponding to each composition were milled using Planetary ball milling (Pulverisette P5, Fritch, Germany) for 20 h. Milling was carried out in tungsten carbide bowl with ball to total powder weight ratio 10:1 and toluene as the process control agent. The ball milled mixtures were subjected to thermogravimetry (TG) analysis using NETZSCH STA 449 F3 Jupiter at a temperature range from room temperature
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Fig. 4. XRD patterns and corresponding unit cells of BKCN perovskite series sintered at 1200 C for 5 h a. BCN, b. BKCN 25, c. BKCN 50, d. BKCN 75 and e. BKCN 100.
to 1200 C and at a heating rate of 20 C/min in dry air atmosphere. The ball milled samples were calcined at 800 C for 2 h using a high
temperature muffle furnace in clean alumina crucibles. Later the calcined samples were subjected to XRD analysis using X’Pert
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Table 1 Parameters obtained from structural analyses of BKCN series. XRD analysis Composition
HRTEM analysis PCPDF no.
Space group
Bravais lattice
Refined cell parameters (Å)
Bravais lattice
Cell parameters (Å)
Ba8 Ca4Nb4O22 (BCN)
PDF#500075
Fm3m (225)
Face centered cubic
a ¼ b ¼ c ¼ 8.3400(0)
Primitive cubic
a ¼ b ¼ c ¼ 8.04
Ba6K2 Ca4Nb4O21 (BKCN 25)
PDF#500075
Face centered cubic
a ¼ b ¼ c ¼ 8.3402(0)
Primitive cubic
a ¼ b ¼ c ¼ 8.04
Ba4K4 Ca4Nb4O20 (BKCN 50)
PDF#470691
Fm3m (225) P4bm (100)
Primitive Tetragonal
Primitive tetragonal
a ¼ b ¼ 8.1, c ¼ 7.8
Ba2K6 Ca4Nb4O19 (BKCN 75)
PDF#470691
P4bm (100)
Primitive Tetragonal
Primitive tetragonal
a ¼ b ¼ 8.1, c ¼ 7.8
K8 Ca4Nb4O18 (BKCN 100)
Ref [18,19]
I4mmm (139)
Body centered Tetragonal
a ¼ b ¼ 12.4751(5) c ¼ 3.9673(0) a ¼ b ¼ 12.4700(0) c ¼ 3.9600(0) a ¼ b ¼ 8.0999(9) c ¼ 15.7000(0)
Body centered Tetragonal
a ¼ b ¼ 8.5, c ¼ 15.77
PANalytical diffractometer to confirm the phase purity. The XRD data was collected at room temperature using Cu Ka radiation with a step size of 0.05 in the 2q range from 20 to 80 . On confirmation of phase purity, the samples were pressed into pellets of 10 mm diameter and 2 mm thickness using PVA (PolyVinyl Alcohol) as binder in a hydraulic press with a pressure of 10 MPa. The pressed pellets were sintered at 1200 C for 5 h. XRD patterns of sintered pellets were analysed to evaluate the thermal stability of phases formed on calcination. For High Resolution Transmission Electron Microscopic (HRTEM) analysis, the sintered pellets were crushed and dispersed in ethanol and sonicated for 3 min at 50 Hz. Later a drop containing the particles was loaded onto a carbon coated copper grid and subsequently dried before viewed under electron microscope. The particle size and morphology of the powder was examined by using High Resolution Transmission Electron Microscope (HRTEM) (JEOL JEM 2100), fitted with an OXFORD Energy dispersive X-ray spectrometer (EDS). The samples were viewed at an accelerating voltage of 200 KV. The Selected Area Electron Diffraction (SAED) patterns were processed and analysed using the software Digital Micrograph-1.85.1535 version. The total conductivity measurement of samples were carried out using impedance analyzer, AUTOLAB, ECO CHEMIE, Netherlands from 10 Hz to 10 MHz in the temperature range 100 C to 800 C in air atmosphere. The data analysis was performed using the Autolab software Frequency Response analyzer (FRA) 4.9 version. 3. Results and discussions 3.1. Thermogravimetric analysis (TGA) Fig. 2 shows the thermal decomposition of 20 h ball milled precursors of BKCN perovskite series. The TGA curves exhibit weight loss in several stages. The first stage between room temperature to 200 C results due to the loss of the adsorbed moisture and structural water from the starting materials. The second stage of weight loss around 650 C is due to the removal of carbonaceous species present in the materials. The third stage of weight loss around 800 C is attributed to the final phase formation. TG curves of BCN and BKCN 25 are stable above 800 C indicating constant mass, whereas the TG curves of BKCN 50, BKCN 75 and BKCN 100 are stable above 1100 C. This thermal analysis data helps us in developing the two stage calcination protocol, one at 800 C and another at 1100 C. First, the samples are calcined at lower temperature of 800 C for 2 h and checked for single phase formation. 3.2. XRD analysis Fig. 3 shows the XRD patterns of 20 h ball milled precursors of
BKCN perovskites calcined at 800 C for 2 h. The diffraction peaks were indexed according to a face centered cubic crystal structure (Fm3m (225)) with cell parameter ac z 8.407 Å. This is in congruence with the reported structural data for similar perovskites [12]. Absence of any peaks corresponding to additional phases and modification of crystal structure indicates that the dopant K atoms have been incorporated and distributed in the parent crystal (BCN perovskite) in a random manner. Perovskite with maximum dopant concentration (x ¼ 1) BKCN 100 shows doubling of ‘some of the XRD peaks possibly due to the formation of tetragonal phase with a z ac and c z 2ac [18,19]. This could happen due to the ordering of either the dopants or oxygen vacancies [12]. XRD patterns of crushed powders of sintered pellets of BKCN series and the respective unit cells based on which the diffractograms are interpreted are shown in Fig. 4aee and ieiii. The diffractograms indicate no change in the crystal structure for BCN, BKCN 25 and BKCN 100 after sintering. Moreover sharper diffraction peaks in BKCN 25 show an increase in the size of single crystallites of the sample. In the case of BKCN 50 and BKCN 75, a change in crystal structure from face centered cubic (Fm3m) to primitive tetragonal (P4bm) is observed. This change in structure may be due to the effect of dopants at higher temperature. The details of structural analyses (through Rietveld refinement also shown as supporting data) of all the compositions are listed in Table 1. Any specific modification of the crystal unit cell symmetry based on the ordering of the dopants or oxygen vacancies cannot be seen through these XRD data. All unit cells shown in Fig. 4ieiii give all possible oxygen and A site cation positions indicated as O2 and Baþ2 respectively. Depending on their nominal compositions the dopant K atoms can be present in any of the Ba sites and vacancies can be present in any of the oxygen sites. In the case of BKCN 100, all the divalent Ba positions are occupied by monovalent K atoms and each unit cell nominally has 12 vacancies distributed among 48 oxygen positions. 3.3. HRTEM imaging Fig. 5 shows the results obtained from HRTEM analysis of sintered BKCN perovskites. The images taken at lower magnification show flaky morphology for all compositions. Selected Area Electron Diffraction Patterns (SAED) of all compositions showed spot type patterns indicating that the flakes are single crystalline. The crystal structures based on which SAED pattern from each composition is indexed are given in Table 1. In general SAED patterns give the local crystal structure whereas XRD data will give the average crystallographic information of the material. SAED and XRD data taken from the same material may differ if the local and average crystal structures are different. These differences can appear due to the local ordering of some of the constituents in the crystal over a small volume of the sample. In the present study local and average crystal
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Fig. 5. TEM image, SAED pattern and high resolution images of a. BCN, b. BKCN 25, c. BKCN 50, d. BKCN 75 and e. BKCN 100.
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Fig. 6. Cole-Cole plots of a. BCN, b. BKCN 25, c. BKCN 50, d. BKCN 75 and e. BKCN 100.
structures are the same for compositions BKCN 50, BKCN 75 and BKCN 100. But for BCN and BKCN 25, though the average crystal is cryolite type double perovskite structure (Fm3m with a z8.34 Å), on a local scale the crystal symmetry is primitive cubic (Pm3m) having the same cell dimension. The high resolution images are interpreted based on the crystal structures exhibited by the electron diffraction patterns. 3.4. Impedance spectral analysis Impedance analysis is generally performed by applying an AC signal across the pellets and recording the corresponding electrical response of the system after the sinusoidal perturbation. Impedance is calculated as a function of the frequency of this perturbation [13]. Complex impedance (Z*) is given by, Z* ¼ Z0 jZ^00 ¼ RS 1/juCS
(1)
where u ¼ 2pf is the angular frequency, Z0 is the real part of impedance, Z00 is the imaginary part of impedance, RS and CS are the resistance and capacitance in series respectively [14]. Fig. 6 shows Cole-Cole plots of BKCN perovskite pellets with different compositions sintered at 1200 C for 5 h. Shape of each impedance plot was reproducible. A single semicircle was observed in the Cole-Cole plots at the measured temperature range for BCN perovskite (Fig. 6a). Absence of intermediate frequency impedance arc that corresponds to grain boundary contribution confirms that this semicircle is related to the bulk contribution to the conductivity. For K- doped BCN perovskites one semicircle along with intermediate frequency impedance arc corresponding to grain boundary contribution is observed. Generally the grain boundary effect arises due to the presence of high resistance and the blocking
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decrease due to the grain boundary effect [15]. Similar semicircle with negligible contributions due to grain-boundary and electrode effects was reported by S.S.Bhella [12] for K-doped Ba3CaNb2O9. The total conductivity of the samples is determined from the intercepts of the semicircle on the real axis by using the relationship:
s¼
Fig. 7. Arrhenius plots of BKCN perovskite series.
l Rb A
(2)
where l/A is the geometrical factor, l is the thickness, A is the area of the sample and Rb is the bulk resistance of the electrode which is indicated by the intercepts of the semicircle. The average thickness of the pellets was 0.22 cm and area was 0.634 cm. In those compositions which showed two semicircles in the Cole-Cole plot, total conductivity is calculated by considering only the first semicircle corresponding to bulk conductivity assuming that the grain boundary contribution might be due to electrons. Fig. 7 shows the Arrhenius plots of total electrical conductivity of BKCN series with varying dopant concentration. Here the conductivity values are plotted against 1000/T factor and the slope is calculated by linearly fitting the curve using origin pro software. From the obtained slope the activation energy is calculated using the following equation:
s ¼ s0 exp (Ea/kT)
Fig. 8. Variation of total conductivity with x in Ba1xKx(Ca0.5Nb0.5)O2.75d at different temperatures.
layer near the grain boundaries. Our results indicate that there is no significant resistance offered by the grain boundaries to the carriers. It was reported that the oxide ion conductivity of materials
(3)
where s0 is the pre-exponential factor, Ea is the activation energy, k is the Boltzman constant and T is the absolute temperature. The variation in total conductivity with respect to dopant concentration at various measured temperatures is depicted in Fig. 8. An overall increment in total conductivity can be observed due to doping of K in BCN. As mentioned in the introduction, the structural oxygen vacancies also increase with the increase in dopant concentration. From Figs. 7 and 8 it can also be seen that total conductivity increases with increasing dopant concentration for most of the compositions. However the sintered pellets of BKCN 50, BKCN 75 and BKCN 100 disintegrated in a very short interval of time, whereas sintered pellets of BCN and BKCN 25 did not disintegrate and remained intact and stable for a longer duration of time on exposure to atmosphere. The higher total conductivity in the case of BKCN 50, BKCN 75 and BKCN 100 when compared to BKCN 25 may be due to the increased surface activity in the form of electronic contribution to the total conductivity. This might also be another reason for the disintegration of their sintered pellets. In addition, after sintering, BCN and BKCN 25 exhibited the crystal structure that was reported to be favourable for higher conductivity due to disordered vacancies [4,9,10]. Among these two
Table 2 Conductivity values of BKCN perovskite series in comparison to reported compositions. Composition
Temperature ( C)
s (S cm1) (Atmosphere)
Ea (eV)
Ba3Ca1.18Nb1.82O9d Ba1.75K1.25CaNb2O9d Ba1.75K1.25CaNb2O9d Ba1.75K1.25CaNb2O9d Ba8 Ca4Nb4O22 (BCN)
200 700 700 700 800 700 800 700 800 700 800 700 800 700
5.3 105 (wet Ar/O2) 1.7 105 (wet H2O/N2) 2.4 104 (air) 2.8 104 (wet H2O/H2) 9.38 108 (air) 2.56 108 (air) 6.93 106 (air) 1.10 106 (air) 3.79 105 (air) 1.51 105 (air) 1.30 105 (air) 3.11 106 (air) 1.02 105 (air) 1.17 106 (air)
0.55 [16] 0.89 [12] 1.0 [12] 1.17 [12] 1.26 (Present work)
Ba6K2Ca4Nb4O21 (BKCN 25) Ba4K4Ca4Nb4O20 (BKCN 50) Ba2K6Ca4Nb4O19 (BKCN 75) K8Ca4Nb4O18 (BKCN 100)
1.71 (Present work) 1.12 (Present work) 1.29 (Present work) 1.41 (Present work)
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compositions, by virtue of having larger number active sites BKCN 25 exhibited higher value of total conductivity. The calculated activation energies and the conductivity values of BKCN series are listed in Table 2. For comparison some of the previously reported results are also mentioned in this Table. The conductivity values listed in Table 2 obtained in the present work are comparable to the previously investigated K doped Ba3CaNb2O9 [11,17]. As shown in Fig. 8 an enhancement in total conductivity of the undoped composition BCN can be observed with increase in K doping. The higher total conductivity value of K doped compositions when compared to undoped BCN in this study could be due to the additional active sites for ionic conductivity along with inherent structural oxygen vacancies in the base composition created by doping lower valent cation.
4. Conclusion This study presented the effect of various concentrations of monovalent potassium in the position of divalent barium on 1:1 ordered Ba4Ca2Nb2O11 synthesised by solid state reaction route. Thermal analysis was used to perform the heat treatment as one step calcination process to obtain the required final perovskite phase. Structural studies indicated a modification of the original crystal structure after sintering for compositions with higher dopant concentrations. An overall increment in the total conductivity in air atmosphere was observed after doping and it was found to increase with increase in potassium concentration. However the stability of the material decreased for higher dopant concentration. This study showed that BKCN 25 (25% of Ba replaced by K) has both higher total conductivity and stability and hence a prominent material for energy applications such as SOFC. Further increase in dopant concentration decreased the stability. It is the formation of extra oxygen vacancies in addition to the already existing inherent structural vacancies suggested to be responsible for the enhancement of total conductivity.
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Acknowledgements The authors acknowledge Naval Research Board (Grant number: DNRDO/05/4003/NRB/287) for financial support and PSG Institute of Advanced studies and Naval Materials Research Laboratory (NMRL) for their infrastructure and intellectual support.
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.06.198.
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