Pravin Kumara,
Nitish Kumar Singha,
Govind Guptab and
Prabhakar Singh*a
aDepartment of Physics, Indian Institute of Technology (BHU), Varanasi-221005, India. E-mail: psingh.app@iitbhu.ac.in
bNational Physical Laboratory, Dr K. S. Krishnan Road, New Delhi-110012, India
First published on 9th February 2016
To study the effect of La3+ doping on structural and electrical properties, a few compositions of the system Sr2−xLaxNiMoO6−δ (x = 0.02, 0.04, 0.06, 0.08, and 0.10) were synthesized via the citrate–nitrate route. Thermally activated electrical conductivity was observed in this system. The reduction of Mo6+ to Mo5+ was confirmed not only in the thermal analysis but also in the detailed XPS investigation of the samples. The structural behavior was probed by phase formation of the compositions confirmed in X-ray diffraction Rietveld analysis. The SEM images of the compositions indicated that a grain size of the order of a few micrometers was observed in this system. The highest conductivity was obtained for the composition with x = 0.04. The electrical behaviour of the system was explained in terms of the lattice strain, the impure phase present in the system and also in terms of the enhanced charge density . The XPS and XRD analysis support the variation of conductivity behaviour with composition.
Although a few materials of double perovskite structure have been investigated for renewable energy techniques.5 In renewable energy applications, most of molybdenum based double perovskite materials are promoted for advanced solid oxide fuel cells.6 Many of them might play key role in development of electrode materials for SOFC in terms of long-term reliability and advanced ceramic materials.7,8 In view of their remarkable electrical performance, some specific Mo based double perovskite ceramics are being appreciated as a potential candidate for anode materials in SOFCs.9 The electrical properties of Mo based double perovskite can be modified with different elements of A and B′. Many researchers have tried for the improvement of the electrical conductivity with substitution of isovalent and aliovalent cations at A, B′ and B′′ sites of double perovskite AB′B′′O6 systems.10 It is reported that Fe doping increases the electrical conductivity of Sr2MgMoO6−δ.11 Doping of Ni on Fe site enhances the electrical conductivity and also improves the catalytic activity with excellent redox stability.12,13 Doping of Mn on Mg site enhances the catalytic activity of Sr2MgMoO6−δ with increase of Mn content.14 In Sr2MgMoO6 system, the electrical conductivity increases with Al doping on Sr site.15 The Sr-site deficiency in Sr2−xMgMoO6−δ system improved the electrical conductivity with increasing deficiency of Sr due to increased oxygen vacancies and reduced polarization resistance.16 Some of the trivalent (La3+, Sm3+, Al3+) doping on Sr-site improved the electro-catalytic activity which contributed to better fuel oxidation and electrochemical performance.15–18 Owing to these properties, some secondary phases are commonly observed in addition to the main phase in such type of materials.7,10,19 These additional phases have its merits and demerits, which affect the structural and electrical properties of such type of materials. Mainly, SrMoO4 phase is observed in Sr2MMoO6−δ (M = Mg, Mn, Fe, Co, Ni, Zn) systems.4,10,19,20 Phase impurity of SrMoO4 in Sr2MgMoO6 increases the electrical conductivity after reduction of SrMoO4 into SrMoO3.4 The main advantage of SrMoO4 is that it goes to stabilize the evaporation of Mo during high temperature synthesis of Sr2MgMoO6.19
Recently, another promising double perovskite Sr2NiMoO6 has been explored due to number of attractive features to use such as good electronic conductivity, high catalytic activity and thermal stability with other cell components.1 The Sr2NiMoO6 system has also been investigated for the stable electrical conductivity with matched thermal expansion coefficient of LSGM based electrolyte materials.21 In Sr2Ni0.75Mg0.25MoO6 system, electrical conductivity was increased with increasing temperature.20 Ti substitution in Sr2NiMoO6 optimized the stability in reducing atmosphere and exhibits good thermal expansion with respect to LSGM.22 In particular system, ferromagnetic and ferroelectric properties have been investigated as multiferroic to be a focus for great interest in essential physics and chemistry.23 At room temperature, Sr2NiMoO6 has tetragonal (s.g. I4/m) crystal geometry and gets phase transition at temperature ranging from 270–300 °C into cubic phase (s.g. Fmm).23–25
In this work, we have studied the effect of La doping on structural and electrical properties of the system Sr2−xLaxNiMoO6 (0.02 ≤ x ≤ 0.10). This system has been synthesized by a citrate–nitrate auto combustion method. The structural and electrical properties have been characterized by using XRD (X-ray powder diffraction), TGA/DSC (thermo gravimetric analysis/differential scanning calorimetry), SEM (scanning electron microscopy), XPS (X-ray Photoelectron Spectroscopy) and impedance spectroscopy, respectively. The compositions with x = 0.02, 0.04, 0.06, 0.08 and 0.10 in the system Sr2−xLaxNiMoO6 are abbreviated as SLNM02, SLNM04, SLNM06, SLNM08, and SLNM10, respectively throughout the manuscript.
All the samples of the system SLNM-x show the peak corresponding to a minor phase at ∼27.67°. This phase was identified as scheelite type SrMoO4 with the help of standard JCPDS card no. 85-0586.
The determined average crystallite size and experimental density of each composition was observed from Archimedes principle and are shown in Table 1. Also, Fig. 1(a)–(e) shows the Rietveld refinement of XRD data of the system SLNM-x using Fullprof software. In the Rietveld refinement, the peak profile was refined by the Thompson–Cox–Hastings pseudo-Voigt function while the background was refined with six coefficients polynomial function. The Rietveld refinement also confirms the presence of SrMoO4 phase (indicated with asterisk symbol). The refined parameters are also given in Table 1. From this table, one can observe that the cell parameters for all the compositions are almost constant except the composition SLNM04 for which it is minimum. The average crystallite size and lattice strain for all powder samples was also determined. We also observe that for x = 0.04 composition the volume fraction of the SrMoO4 phase is maximum and it has the lowest lattice strain. However, considering the doping of La3+ ion, which has smaller ionic radii (1.032 Å) compared to that of Sr2+ (1.18 Å), it is expected that the lattice parameter should decrease with the increase of dopant's concentration. In present case, no such trend is observed. It appears that the lattice strain and the impure phase together govern the structural behaviour of the system. The increase in the lattice constant due to La3+ doping is compensated by the lattice strain and impure phase.26
Samples | Lattice parameters | Crystallite size (nm) | Experimental density (g cm−3) | %Porosity | χ2 | % of volume fraction of SrMoO4 | Average lattice strain (× 10−4) | ||
---|---|---|---|---|---|---|---|---|---|
a (Å) | b = c (Å) | Cell volume (Å)3 | |||||||
SLNM02 | 5.549 | 7.897 | 243.220 | 59.78 | 5.7 | 50.35 | 2.66 | 3.45 | 10.35 |
SLNM04 | 5.539 | 7.882 | 241.803 | 52.33 | 5.8 | 49.52 | 4.18 | 14.31 | 6.13 |
SLNM06 | 5.553 | 7.902 | 243.629 | 58.92 | 5.8 | 49.70 | 2.71 | 4.86 | 12.13 |
SLNM08 | 5.548 | 7.896 | 243.039 | 72.13 | 5.7 | 50.61 | 3.06 | 9.53 | 11.99 |
SLNM10 | 5.555 | 7.906 | 243.962 | 65.87 | 5.7 | 50.95 | 6.66 | 8.76 | 10.19 |
Fig. 3 Field emission scanning electron micrograph of chemically etched various compositions (a) SLNM02, (b) SLNM04, (c) SLNM06 and (d) SLNM08 and (e) SLNM10. |
This segregation further inhibits the grain growth and results a decrease in the grain size.28,29
Fig. 4 XPS spectra for compositions SLNM02, SLNM04, SLNM06, SLNM08 and SLNM10: (a) Mo-3d, and (b) O-1s. |
Compositions | Parameters | Mo6+ | Mo5+ | Mo5+/Mo6+ | Adsorbed oxygen (%) | ||
---|---|---|---|---|---|---|---|
3d5/2 | 3d3/2 | 3d5/2 | 3d3/2 | ||||
SLNM02 | Binding energy (eV) | 232.39 | 235.82 | 231.40 | 234.98 | 0.59 | 22.24 |
Area | 6206.23 | 1561.42 | 2742.49 | 1866.49 | |||
SLNM04 | Binding energy (eV) | 231.99 | 235.43 | 230.91 | 234.78 | 0.71 | 31.78 |
Area | 4972.60 | 648.29 | 1885.44 | 2142.79 | |||
SLNM06 | Binding energy (eV) | 232.39 | 235.43 | 231.50 | 235.08 | 0.63 | 26.26 |
Area | 3101.15 | 4529.71 | 3026.51 | 1829.29 | |||
SLNM08 | Binding energy (eV) | 232.26 | 235.62 | 231.24 | 234.78 | 0.68 | 27.62 |
Area | 4756.73 | 1507.74 | 2943.58 | 1308.85 | |||
SLNM10 | Binding energy (eV) | 232.29 | 235.88 | 231.25 | 234.98 | 0.69 | 29.64 |
Area | 4843.54 | 788.14 | 2060.20 | 1838.64 |
Fig. 4(b) illustrates the O-1s core-level spectra. The peaks in these spectra are broader and asymmetrical in nature for all the compositions. Hence, the peak for all the samples could be splitted into two peaks and attributed to two different types of oxygen species: lattice oxygen and adsorbed oxygen. The percentage of adsorbed oxygen for all the samples is given in Table 2. Sample SLNM04 has highest percentage of adsorbed oxygen as compared to other samples. Thus the composition SLNM04 will show better catalytic activity towards the oxygen association/dissociation process.9
Fig. 5 The complex plane impedance plots for the composition SLNM04: (a) 200 °C, (b) 275 °C, (c) 350 °C, (d) 475 °C, (e) 525 °C and (f) 600 °C. |
But, there may be another possibility that the observed semicircular arc is due to combined contribution of grains and grain-boundaries. Thus, to confirm whether this is only due to grain-boundaries or grains and grain-boundaries both, the analysis of data by modulus spectroscopy is also needed. The modulus plots for the sample SLNM04 at different temperatures are shown in Fig. 6(a)–(d). In these plots, a part of depressed semicircular arc with a tail at one end was also observed. The capacitance corresponding to observed depressed semicircular arc was also calculated by the relation 2πfRC = 1, which holds good at the peak of the corresponding depressed semicircular arc. The capacitance is found to be order of pF. Primarily, it indicates that the observed semicircular arc represents the contribution of grains. But, in the impedance and modulus plots the observed semicircular arc passes through origin. This indicates that the observed depressed semicircular arc in impedance plots should correspond to grains while the calculated capacitance indicates towards the grain-boundaries. In the same manner, in modulus plots, the observed depressed semicircular arc should correspond to grain-boundaries while the calculated capacitance indicates the grains contribution. From above discussions, it can be concluded that in both the plots, observed depressed semicircular arc are same and have contributions from grains as well as from grain-boundaries. The observed difference in capacitances of both the plots is due to dominance of most resistive part i.e. grain-boundaries in impedance and least capacitive part i.e. grains in modulus. Here the separation of grains and grain-boundaries contributions is not observed, because of the comparable relaxation times of both the contributions.
Fig. 6 The modulus spectra for the composition SLNM04 at: (a) 200 °C, (b) 250 °C, (c) 300 and (d) 350 °C. |
To show the variation of impedance for all compositions, typical complex plane impedance plots with nonlinear curve fitting are shown in Fig. 7(a)–(f) at 375 °C. Where, Fig. 7(a)–(e) represent separate plots for each composition at 375 °C and Fig. 7(f) represents the combined plots for all compositions. The above figure clearly indicates that the sample SLNM04 has the lowest impedance.
Fig. 7 The complex plane impedance plots for compositions: (a) SLNM02 (b) SLNM04, (c) SLNM06 and (d) SLNM08 and (e) SLNM10 (f) combined plot for SLNM-x (0.02 ≤ x ≤ 0.10): at 375 °C. |
Here, the total resistance, Rt corresponding to both contributions can be obtained from the intercept of the depressed semicircular arc on the real axis while the total conductivity of the samples can be calculated applying the formula
(1) |
Fig. 8 shows the Arrhenius plot of logσT vs. 1000/T for all compositions of the system SLNM-x. The inset depicts the variation of logarithms of conductivity with composition at 300 °C. The inset also includes the conductivity data of the undoped system (Sr2NiMoO6).33
Fig. 8 Arrhenius logσtT vs. 1000/T plots of various samples of the system SLNM-x (0.02 ≤ x ≤ 0.10). The inset depicts the variation of logarithms of conductivity with composition at 300 °C. |
The activation energy of conduction for all the compositions was calculated from the slope of the plot logσT vs. 1000/T applying the Arrhenius relationship
(2) |
(3) |
Mo×Mo + e′ → Mo′Mo | (4) |
Compositions | Activation energy, Ea (eV) | Pre-exponential factor σ0 |
---|---|---|
SLNM02 | Ea = 1.1 | 1.11 × 106 |
SLNM04 | Ea = 0.84 | 1.72 × 106 |
SLNM06 | Ea = 0.91 | 4.25 × 105 |
SLNM08 | Ea = 0.86 | 2 × 105 |
SLNM10 | Ea = 0.87 | 2.2 × 105 |
Fig. 9 Variation of pre-exponential factor σ0 and activation energy with respect to doping concentration x of the system SLNM-x (0.02 ≤ x 0.10). |
The electro-neutrality of the system can be described as follows
(5) |
Thus the doping of La at the place of Sr balances charge by the increase of Mo′Mo concentration.
The former will increase the electronic conductivity while the latter will decrease the oxide ion conductivity. From the Table 3 it is clear that the value of activation energy Ea for the samples SLNM04, SLNM06, SLNM08 and SLNM010 is lower than the sample SLNM02 due to easy hopping of electronic charge between Mo6+ and Mo5+ and also, the low contribution of oxide ion conduction.11 But on further doping there is no appreciable change in activation energy of the samples. Relative to SLNM02, all the other samples have high value of conductivity. Sample SLNM04 shows the highest value of conductivity. Huang et al.1 have reported that in Sr2MMoO6 (M = Co, Ni) system, SrMoO4 may block the catalytic pathways for reforming the fuels and thus its presence is not desirable for SOFC application. In their work, the formation of SrMoO4 was taken place after exposure to CH4. But in our system, SrMoO4 is already present along with main phase of Sr2NiMoO6. Kubo et al.34 has reported that this phase has catalytic activity for hydrocarbon oxidation, which could be advantageous in SOFC anode use. Also Vasala et al.19 reported that under reducing atmosphere the SrMoO4 is reduced to SrMoO3, which actually increases the electrical conductivity. However, in the present case conductivity measurement was done in air. Yuan et al.35 has mentioned that presence of SrMoO4 phase can alter the physical properties of the system. Thus increase and/or decrease of conductivity of the system will also depend on the volume fraction of this additional phase. Therefore, in the present case, the role of SrMoO4 phase in the Sr2−xLaxNiMoO6 cannot be denied. From the Rietveld analysis (Table 1) it is clear that for the system SLNM04 having the maximum impurity shows the highest conductivity. Further, the lattice strain analysis (Table 1) indicates that SLNM04 bears the minimum lattice strain, and all other samples show almost double lattice strain in comparison to this composition. The low lattice strain may cause less charge polarization leading to high value of conductivity in the system SLNM04.36
The other reason behind this highest conductivity in SLNM04 could also be due to enhancement of electronic charges (Mo′Mo) as per eqn (4). The XPS data showing the ratio of M05+ and Mo6+ (Table 2) also supports the reduction process as mentioned in eqn (4). The XPS data indicates that the ratio of Mo5+/Mo6+ is highest for SLNM04 due to more oxygen vacancies corresponding to high percentage of adsorbed oxygen as compared to other samples. The variation of percentage of adsorbed oxygen in XPS data corroborate with the variation of the Mo5+/Mo6+ ratio (Table 2). At the same time, the decrease in conductivity on doping higher than 4% may be due to formation of associate pairs between Mo′Mo and .15 Also it can be seen from Table 3 that with increasing concentration of La3+, pre-exponential factor σ0 (i.e. mobile charge carriers) increases up to 4% thereafter shows a decreasing trend and slight increase for 10% doping. But the activation energy decreases with the increasing pre-exponential factor σ0 up to 4% and shows a decreasing trend with decrease in pre-exponential factor σ0. The decrease in pre-exponential factor σ0 at higher doping may be due to the formation of associate pairs between Mo′Mo and which increases the activation energy and lowers the effective number of charge carriers contributing to the conduction process. The variation of σ0 with composition supports the formation of associate pairs at higher doping, which act as a lowering factor for the conductivity as discussed above.
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