Study of phase stabilization and oxide–ion conductivity in BICUMGVOX solid electrolyte

Solid State Ionics 261 (2014) 125–130

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Study of phase stabilization and oxide–ion conductivity in BICUMGVOX solid electrolyte Saba Beg a,⁎, Shehla Hafeez a, Niyazi A.S. Al-Areqi b a b

Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India Department of Chemistry, Faculty of Applied Sciences, Taiz University, Taiz, Republic of Yemen

a r t i c l e

i n f o

Article history: Received 17 August 2013 Received in revised form 28 February 2014 Accepted 28 February 2014 Available online 24 May 2014 Keywords: BIMEVOX Phase stability Ionic conductivity AC impedance

a b s t r a c t Samples of Cu(0.10 − x)–Mgx doubly substituted bismuth vanadate, BICUMGVOX (Bi2Cu0.10 − xMgxV0.9O5.35) were synthesized by the citrate sol–gel method. The influence of Cu-Mg substitution on phase stability and oxide-ion performance have been investigated using X-ray powder diffraction (XRPD), FT–IR spectroscopy, differential thermal analysis (DTA) and AC impedance spectroscopy. The highly conductive γ –phase was effectively stabilized at room temperature for compositions with x ≥ 0.06 whose thermal stability increases with Mg content. The complex plane plots of impedance for polycrystalline samples showed the variation of equivalent circuit models with composition at isothermal conditions. It was found that the highest value of conductivity 1.5 × 10−3 S.cm−1 at 300 °C is obtained from the composition x = 0.06 whose conductivity at 600 °C also reaches its maximum value of 4.9 × 10−2 S.cm−1. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Oxide–ion conductors have received much attention due to their promising application in the field of solid oxide fuel cells (SOFCs), oxygen sensors, oxygen membrane catalysts and oxygen pumps [1–5]. SOFCs are one of the most attractive energy conversion devices due to their high power generation efficiency and very low greenhouse gas emission [6,7]. Recent developments in the SOFCs require oxide–ion electrolytes with high oxide ionic conductivity at intermediate temperatures (IT: 400–700) [8]. It has been found that Bi4V2O11 is the parent compound for a novel family of oxide – ion conductors known as BIMEVOX (BI =bismuth, ME = metal, V = vanadium, OX = oxide) [9,10]. This compound has a layered perovskite – type structure (or Aurivillius structure [11]) and is formulated as (Bi2O2)2 + (VO3.5❑0.5)2 −, where ❑ stands for an oxide – ion vacancy. Three principal phases (α, β and γ) were characterized on increasing the temperature with two obvious successive phase transitions, namely α ↔ β and β ↔ γ, occurring around 440 and 560 °C, respectively. Both α– and β– polymorphs are relatively ordered phases, while the γ – phase is fully disordered possessing a high ionic conductivity at moderate temperatures (~10−2 S.cm−1 at 600 °C). Many attempts have been successfully made to develop new BIMEVOXes based on stabilization of the γ–form via the substitution of a large variety of iso–or aliovalent cations [12–18] for vanadium.

⁎ Corresponding author at: Department of Chemistry, Aligarh Muslim University Aligarh-202002, India.

http://dx.doi.org/10.1016/j.ssi.2014.02.020 0167-2738/© 2014 Elsevier B.V. All rights reserved.

This paper reports the study of phase stability and conductivity performance of Cu(0.10 x)–Mgx doubly substituted bismuth vanadate system, BICUMGVOX (Bi2Cu0.10 xMgxV0.9O5.35) synthesized by the citrate sol–gel method. 2. Experimental 2.1. Synthesis procedure BICUMGVOX (Bi2Cu0.10 − xMgxV0.9O5.35) solid solutions were prepared by taking analytical grade Bi(NO3 )3 ·.5H2 O, NH4VO 3, CuNO 3 and MgNO3 as starting materials. Stock solutions of the starting materials (1 M) were prepared by dissolving an accurately weighed amount of corresponding material in deionized water. Stock solution of citric acid (2 M) used as complexing agent was also prepared. Another solution containing NH3 (2 M) was used to adjust the pH of the sol. Solutions of the starting materials were mixed in a molar ratio of Bi3 +: Cu 2 +:Mg2 +: V5 + = 2: 0.10 –x: x: 0.90. Citric acid solution was then added to each mixture for complexation. The ratio of citric acid to total metal ions was set at 1.5:1. The pH of resulting solutions was then adjusted to ~ 7 by the addition of ammonia solution. The solution was heated at 80 °C with vigorous stirring for one hour to form a transparent gel. The xerogel (precursor metal complex) was then obtained by drying the resulting gel in air at 90 °C for 24 h. The xerogels were thoroughly mixed in an agate mortar for further homogenization and were calcined in a muffle furnace at 650 °C for 3 h. After the completion of calcination, the samples were slow cooled in air to room temperature. The resultant BICUMGVOX powders were

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2.2. Structural characterization

o

700 C X–ray powder diffraction analysis was employed to characterize the phase structures in the BICUMGVOX system using a Philips PW 1050/30 X–ray diffractometer with Ni– filtered CuKα radiation (λ = 1.5418 Å). The diffraction beams were collected in flat plate θ/2θ geometry in the range 10° ≤ 2θ ≤ 80° with an increment of 0.018° at scantime of 31.16 s/increment. The unit cell parameters were calculated by the Reitveld refinement method using the X'Pert Plus software program. BICDVOX compositions [18] were used as starting models for the Rietveld analysis. Differential thermal analysis (DTA) measurements were carried out on a Schimadzu SC–TA 60 thermal analyzer. Approximately weighed 20 mg of the dry powder sample was placed in the alumina cell. The experiments were run in N2 atmosphere. The flow rate of N2 was maintained at 30 ml min−1 with a heating rate of 10 °C min−1 from ambient to 900 °C.

o

Intensity (a.u.)

400 C

o

300 C

o

200 C

20

25

30

35

40

45

50

55

2.3. Electrical measurements AC impedance measurements were carried out on a Waynekerr 4100 LCR meter operated in the frequency range of 20 Hz–1 MHz with an AC single of ~50 mV. The sintered pellets were made conducting by applying chemically pure silver paste on both surfaces. The experiments were run in air in the temperature range of 90–700 °C with an increment of 50 °C. Impedance spectra were subjected to non-linear leastsquares fitting using the Z view software program [19].

60

2θ (o)

3. Results and discussion

Fig. 1. PXRD patterns of the BICUMGVOX xerogel for x = 0.04 calcined at different temperatures for 5 h.

3.1. X– ray crystallography thereafter pelletized into a cylindrical shape with constant dimensions (13 mm in diameter × 1 mm in thickness) under isostatic pressure of 510 MPa (Spectralab SL–98). The pellets were sintered at 800 °C for 5 h and then slow cooled in air.

Fig. 1 shows the variation in PXRD patterns of the xerogel with x = 0.06 as a function of calcination temperature. The sample obtained after calcination at 400 °C for 5 h was still amorphous and was completely converted into a crystalline phase upon increasing calcination

Counts

5000

0 1000 0 -1000 10

20

30

50

40

60

70

80

o

2Theta

Fig. 2. XRPD Rietveld refinement of the sample x = 0.04 calcined at 700 °C for 5 h. Unit cell dimensions: a = 3.92354(7) Å, c = 15.3476(3) Å, V = 236.263(4) Å3; Z = 2; Crystallographic density ρ = 7.687 g cm−3; R–factors: Rp = 9.24%, Rwp = 6.52%.

S. Beg et al. / Solid State Ionics 261 (2014) 125–130

127

x=0.02

x=0.04

Transmittance

x=0.08

Intensity (a.u.)

116 213

200

104

112 006

044

110

x=0.06

x=0.06

x=0.08

x=0.04

2000

1600

1200

800

400

113

ν , cm-1

220 336

204

313

602 006

x=0.02

115

202 200

Fig. 4. FT–IR spectra of the BICUMGVOX system.

x=0.02

o

20

25

30

40

026 606 363

024

35

604

x=0.00

315 006

022 220

490 C

45

50

55

x=0.04

60

2θ(o)

o

510 C

Heat flow(uV)

Fig. 3. Room temperature PXRD patterns of the BICUMGVOX system.

temperature due to the partial decomposition of citrate precursor. Generally, the increase in calcination temperature results in the formation of well–crystallized product. The observed, calculated and difference profiles of the XRPD Rietveld refinement for the x = 0.06 sample calcined at 700 °C for 5 h are shown in Fig. 2. The existence of a characteristic singlet diffraction peak 110 around 2θ ~ 32.5° is a clear evidence for the stabilization of ionic conducting γ′–tetragonal phase with 14I4/mmm symmetry. Room temperature PXRD patterns of the BICUMGVOX samples in the composition range of 0.02 ≤ x ≤ 0.08, are presented in Fig. 3. The diffraction data have been graphed in the 2θ range of 20–60° for the sake of visibility of the important sublattice peak convergence. It is noticed that for compositions with x ≤ 0.04, the singlet indexed as 220 is clear evidence for the stabilization of metastable orthorhombic β–phase to room temperature. However, the stabilization of tetragonal γ′-phase occur for x ≥ 0.06 as reflected in the convergence of sub lattice doublet at 2θ ≈ 32.19° into a singlet indexed in the tetragonal cell as (110) [20–22]. For x ≥ 0.08, a weak additional peak is clearly observed at 2θ ≈ 28.10° to the right of the most intense peak, indicating

x=0.06 o

480 C

x=0.08 o

450 C

100

200

300

400

500

Temperature

600

700

800

900

(oC)

Fig. 5. DTA thermograms of the BICUMGVOX system versus composition.

Table 1 Refined unit cell parameters, phase stabilizations, average crystallite sizes and crystallographic densities of prepared samples of the BICUMGVOX samples after calcinations at 700 °C for 5 h. x

Unit cell parameters

0.00 0.02 0.04 0.06a 0.08a a

Phase stabilization

a (Å)

b (Å)

c (Å)

V(Å3)

Phase

Space group

5.5534 5.5512 5.5493 5.5487 5.5466

5.6168 5.6153 5.6121 – –

15.3537 15.3511 15.3497 15.3476 15.3452

478.918 478.519 478.039 475.785 472.092

β β β γ′ γ′

Acam Acam Acam I4/mmm I4/mmm

Average crystallite size (μm)

Density, dXRD (g cm−3)

5.42 5.98 6.23 6.46 6.71

7.713 7.721 7.792 7.687 7.628

For the sake of comparison, the tetragonal lattice parameter (aγ) was converted into the mean orthorhombic dimension (aβ) using the relation, aβ ¼ aγpffiffi2 .

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the increase of BiVO4 impurities and the separation of MgVO3.5 phase with increasing Mg content. The compositional dependences of unit cell parameters for the BICUMGVOX samples are illustrated in Table 1. Although, ionic radii of Cu2 + and Mg2 + are approximately equal, the decrease of unit cell parameters as a function of composition may be attributed to the lowering in the lattice contribution of Cu 3d orbital polarization with Cu(0.10 – x)–Mgx substitution trend. This is in perfect agreement with XRD and DTA results. It can be seen that the broadening of the peaks gradually decreases with increasing temperature of the calcination, signifying the increase in crystalline size with increasing temperature [23].

3.3. Differential thermal analysis (DTA) The temperature ranges of stability of the polymorphic modifications of the solid solutions were refined by using DTA analysis (Fig. 5). The relatively large endothermic peaks seen for the composition x ≤ 0.04, are assigned to β → γ transitions, while the occurrence of incommensurate → commensurate, γ′ → γ transitions are detected in the composition range of 0.04 b x ≤ 0.08. It is interesting to note that except for x = 0.02, the transition temperature and the heat flow per unit mass at the onset temperature generally go on decreasing as the Mg substitution for Cu increases. This implies that the oxygen vacancy disordering is very much dependent on the composition but is independent of the nature of dopants [26].

3.2. FT–IR spectroscopy The XRD results were also confirmed by FT–IR spectra of the BICUMGVOX system illustrated in Fig. 4. It is noticed that with increase in Mg concentration the vibration position of vanadate tetrahedra shifts to lower frequency (1003–800 cm−1). This shift in vanadate tetrahedra can be attributed to the lowering in the average reduced atomic masses of V/Cu/Mg and oxygen as Mg substitution for Cu increases. The unchanged vibration position of Bi\O bond is clear evidence for the substitution of Cu and Mg predominantly in the V– sites of perovskite vanadate layers. Moreover the disappearance of fine structure at x = 0.06 in the vanadate anion region reveals the presence of crystallographic disordering in the structure of perovskite layers in the tetragonal phase [24,25].

3.4. AC impedance spectroscopy The complex plane plots of impedance for the investigated BICUMGVOX samples show a behavior typical for the oxygen– conducting BIMEVOX family (Fig. 6). It is interesting to note that the variation of double substitution composition results in various impedance regimes. For compositions with x ≤ 0.04, two separate semicircles are seen at high–and low– frequency regions, which are respectively assigned to grain and grain boundary contributions. The impedance of the electrode–electrolyte interface is clearly represented by the inclined spikes appearing at very low frequencies [27–29]. While the complex plane plot of impedance for the composition x = 0.06 is typical of a

25 35 30

20

20

-Z''(kΩ)

-Z''(kΩ)

25

15 10 5

x=0.02

15

10 x=0.04

5

0 -5 300

330

360

390

420

450

0 300

480

310

Z'(kΩ)

320

330

340

350

360

Z'(kΩ)

15

12 10

12

-Z''(kΩ)

-Z''(kΩ)

8 9

6

6 4

3

2

x=0.06

0 300

305

310

Z'(kΩ)

315

320

325

0

x=0.08

306

309

Z'(kΩ)

Fig. 6. Complex plane plots of impedance measured at 280 °C for the BICUMGVOX system.

312

315

S. Beg et al. / Solid State Ionics 261 (2014) 125–130

(a)

R2

129

5 4

R1

CPE2

(b)

R2

W2

log σ T, S cm-1K

3 2 1 0

x=0.02 x=0.04 x=0.06 x=0.08

R3 -1

R1

-2 -3

CPE2

1.0

1.2

1.4

1.6

1.8

2.0

2.2

1000/T, K

CPE3

Fig. 8. Arrhenius plots of conductivity for the BICUMGVOX system.

(c)

R1

R2

R3

CPE1

CPE2

CPE3

Fig. 7. Equivalent electrical circuits models used for fitting impedance plots of the BICUMGVOX samples measured at 280 °C.

porous oxygen conducting ceramics apparently with two overlapping semicircles and inclined spike. Upon the further increase of Mg dopant concentration, the semicircular arcs start to disappear and only the electrode–electrolyte interface impedance is evident as clearly observed for x = 0.08. These impedance plots were well–modeled by the equivalent electrical circuits reported for the BINBVOX system [30]. The equivalent electrical circuit used for fitting the impedance plot for sintered sample x = 0.08 (Fig. 7a) consists of the following elements: R1, corresponding to the total resistance of the grains and grain boundaries, connected in parallel to a complex of R2/CPE2 (constant phase element) and W2 (Warburg element). The constant phase element, CPE is composed of two parts— CPE–T (Ohm) and CPE–P—which is equal to the order of magnitude. It describes complex processes and can stand for resistance or capacitance depending on the value of the CPE–P element. The

Warburg element in the equivalent circuits stands for complex processes that take place at the electrode–electrolyte interface. It consists of three components, which can be assigned to overall resistance (Ws–R) and complex capacitance (Ws–T, Ws–P) of these processes. For composition x = 0.06, the circuit (Fig. 7b) is represented by a series connection of one resistance and two Voight elements (R/CPE). In this case the R1 and R2/CPE2 elements correspond to the grain and grain boundary resistance of the electrolyte material, respectively. The equivalent electrical circuit used for fitting the impedance plots (Fig. 7c) for samples with x = 0.02 and x = 0.04 consists of three Voight elements (R/CPE), corresponding to three idealized impedance components. The equivalent circuit parameters deduced from the complex plane plots are summarized in Table 2. The validation of fitting procedure was found to be in the ranges of 1.43 × 10−3 ≤ χ2 ≤ 1.82 × 10−3 and 0.121 ≤ WSS ≤ 0.147. 3.5. Temperature dependence of conductivity The total electrical conductivity below 400 °C, was calculated from the sum of grain and grain boundary resistances, using the relation σ = 1 / (R1 + R2) × (L/A). where (L/A) is the ratio of thickness to section area of the sintered pellet. The temperature dependences of conductivity were adapted by the Arrhenius dependence, σT = Aexp(− ΔE/kT) (Fig. 8). The first point of interest to be observed here is the discontinuity in conductivity data around the transition temperatures that clearly discriminate two linear regions of the Arrhenius plot with quite different slopes (activation energies; ΔELT and ΔEHT fitted at two temperature

Table 2 Values of the equivalent circuit parameters estimated from complex impedance plane plots of selected compositions at 260 °C. Equivalent circuit parameters

R1,kΩ R2,kΩ R3,kΩ CPE1–T,F CPE2–T,F CPE3–T,F CPE1–P CPE2–P CPE3–P Ws2–R, kΩ Ws2–T, F Ws2–P χ2 × 10−3 WSS

Composition (Cu0.10 – xMgx) x = 0.02

x = 0.04

x = 0.06

x = 0.08

396.12 57.41 4.73 × 104 7.15 × 10−11 4.78 × 10−7 8.45 × 10−4 0.45 0.57 0.26 – – – 1.43 0.126

328.52 16.17 2.34 × 103 5.22 × 10−11 4.05 × 10−7 6.12 × 10−4 0.51 0.72 0.32 – – – 1.48 0.121

306.75 7.43 1.27 × 103 2.94 × 10−11 4.33 × 10−7 5.25 × 10−4 0.62 0.83 0.56 – – – 1.82 0.147

310.08 127.67 – 1.06 × 10−11 3.55 × 10−7 – 0.40 0.35 – 50.84 3.67 × 10−5 0.23 1.63 0.132

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ranges of LT: 200–460 °C and HT: 540–800 °C, respectively). These two linear regions are associated with the phase stability of low and high temperature phases. It can also be observed that a subtle discontinuity in the Arrhenius plots for the compositions x ≤ 0.04 and x ≥ 0.06 is associated with the thermal stability of β → γ and γ′ → γ transitions, respectively. These results are in a reasonable agreement with those obtained from DTA discussed above. It is found that the highest value of conductivity 1.5 × 10− 3 S.cm− 1 at 300 °C is obtained from the composition x = 0.06 whose conductivity at 600 °C also reaches its maximum value of 4.9 × 10−2 S.cm− 1. This can be attributed to the optimal concentration of fully disordered vacancies located on equatorial lines of the perovskite layers (VO3.5❑0.5)2− available for oxide ion migration [31–35]. 4. Conclusion Double substitution of 10 mol% V by Cu(0.10 – x)–Mgx in the monoclinic α– Bi4V2O11 phase leads to the stabilization of the tetragonal γ′– phase for x = 0.06 and x = 0.08 while β phase is observed for compositions x = 0.02 and x = 0.04. It is shown that, like single substitution, double substitution can also enhance the phase stabilization and relevant oxide–ion performance of the BIMEVOX family. Acknowledgments This work was financially supported by Universities Grants Commission (UGC), Grant no. 39-798/2010 (SR), New Delhi — India. The authors are also grateful to Department of Chemistry, Aligarh Muslim University (AMU), Aligarh. References [1] H. Yahiro, T. Ohuchi, K. Eguchi, J. Mater. Sci. 23 (1988) 1036. [2] N.Q. Minh, J. Am. Ceram. Soc. 563 (1993) 76.

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