Microsoft Word - 36-3443_s_ETASR_V10_N2_pp5538-5546 Engineering, Technology & Applied Science Research Vol. 10, No. 2, 2020, 5538-5546 5538 www.etasr.com Irmak: Structural and Electrical Properties of Ca 2+ Doped LaFeO3: The Effect of A-site Cation Size … Structural and Electrical Properties of Ca2+ Doped LaFeO3: The Effect of A-site Cation Size Mismatch Ali Ekber Irmak Physics Department Faculty of Science Mugla Sitki Kocman University Mugla, Turkey ekberirmak@gmail.com Abstract—In this study, nanosized La1-xCaxFeO3 (0.00≤≤≤≤x≤≤≤≤0.40) compounds prepared via sol-gel method followed by heat treatment at 1100oC for 24 hours are studied. Crystal structure, microstructure, surface morphology and temperature-dependent resistivity of the samples are investigated. TEM investigation reveals nanoparticles with an average size of 35nm produced from the sol-gel process. The crystal structure of the compounds belongs to an orthorhombically distorted perovskite structure with Pbnm space group. Lattice distortion and cation size mismatch increase with an increase in Ca and particle and grain growth are suppressed by Ca doping. Electrical conduction is explained via thermally activated hopping of small polarons. Unit cell volume, charge ordering temperature, and activation energy for small polarons decrease linearly with an increase in cation size mismatch. Room temperature resistivity decreases with Ca doping and gets its minimum value for 30% Ca at which the orthorhombic distortion is maximum. Keywords-cation size mismatch; lattice distortion; particle size; grain size; charge order I. INTRODUCTION Rare-earth (Re) transition metal perovskite oxides attract attention due to their electronic and magnetic properties, such as insulator to metal and paramagnetic to ferromagnetic phase transitions, magnetoresistance, electroresistance, magnetocaloric effect and charge order properties. LaFeO3 is a semiconductor/insulator perovskite oxide and its derived compounds by replacement of some of La atoms by other Re or alkaline earth (Ae) elements give rise to significant physical and chemical properties with promising applications in advanced technologies as in electronics, magnetics, fuel cells, gas sensors, and catalysts [1]. By partially exchanging La in the parent compound with other Re or Ae (La1-xAxFeO3, A: Re or Ae) lead to structurally distorted compounds, and induce different lattice parameters and unit cell volumes [2-4]. In addition, Ae doping has an influence on particle size (PS) and grain size (RG). The former is calculated using XRD data and the later is calculated using surface images taken by Scanning Electron Microscope (SEM) or Atomic Force Microscope (AFM). PS has an effect on electronic, magnetic and chemical properties and is an important parameter in determining the performance of the material. Lack of holes or a trace of holes due to nonstoichiometric oxygen undoped compound has high resistivity at room temperature. Ae doping increases the number of holes in the compound and the polyvalent nature of the Fe compensates the created charge imbalance by oxidation. Ae with a valance state of + oxidizes two Fe+3 to two Fe+4 and Ae with a valance state of 2+ oxidizes one Fe+3 to Fe+4. Therefore, the charge imbalance created by Ae doping is removed, and this process also increases holes in the compound leading to lower resistivity. Besides, oxygen content plays an important role in charge compensation. Oxygen deficiency leads to decrease the number of Fe4+, and excess oxygen leads to increase the number of Fe4+. Furthermore, doping at La site and O vacancy, have a direct impact on the local environment of Fe ions [4] and thereby resistivity. In addition, the difference of ionic radii of La-site ions in the compound and the difference of ionic radii of Fe+3 and Fe+4 have an influence on lattice and electrical properties. The measure of distortion due to La-site ionic radius difference is the cation size mismatch, �� = ∑ �����−〈� 〉�� , where xi and ri are the concentration and ionic radius of the i th A-site ion and 〈� 〉 is the average A-site ionic radius. The electrical transport in doped LFO compounds usually takes place by the “polaron hopping”. An electron or a hole is trapped by a potential well resulting from the ionic displacements it created. Moreover, oxygen vacancy, which is a defect site, traps an extra electron and leads to polaron like bound states [5]. Charge Order (CO) is another property in some of these compounds which takes place in a material by a localization of charge carriers below a certain temperature, TCO, giving rise to long-range order throughout the crystal structure leading a sudden increase in resistivity. The electrical transport and the structural properties of La1- xAxFeO3 have been studied extensively. In ReFeO3 oxides having an orthorhombic perovskite structure with Pbnm space group, the structural distortion increases with decrease in Re3+ ionic radii [6]. It was shown that a decrease in ionic radii of Re3+ leads to decrease in lattice parameters and unit cell volume. Authors in [7] reported that doping the LFO with Li+ destroys the perfection of crystallinity and lead to decrease of PS. Authors in [8], contrary to other studies, reported that high- temperature synthesis of K+ doped LFO doping with an ion having larger ionic radii than that of La, leads to a decrease of Corresponding author: Ali Ekber Irmak Engineering, Technology & Applied Science Research Vol. 10, No. 2, 2020, 5538-5546 5539 www.etasr.com Irmak: Structural and Electrical Properties of Ca 2+ Doped LaFeO3: The Effect of A-site Cation Size … unit cell volume. An increase in Mg2+ doping in LFO leads to lowered PS and doping with Mg2+ prevents PS from growing up during high temperature treatment [2]. However, despite the smaller ionic size of Mg2+ there was no considerable change in unit cell volume up to 70% doping. A study on Ba2+ doped LFO revealed that an increase in doping decreased the PS and contrary to the larger ionic size of Ba2+ the unit cell volume decreased and appropriate Ba-doping restrained the growth of grains [9]. Moreover, the effect of Pb2+ doping on the structural properties of LFO was conducted in [10] with no change in the unit cell volume and a decrease in the PS with an increase in Pb2+ content [10]. An increase in Ca2+ in LFO leads to decrease in PS [11, 12] and in grain size [13]. Structural and electrical properties of simultaneously doping LFO with Ca2+ and Pb2+ [14] showed that doping with larger Pb2+ ions up to 20% did not change the principal phase, but lattice parameters, unit cell volume and PS increased. However, another study reported that Pb2+ doping lead PS to decrease [15]. Electrical transport properties of LFO can be tuned by alkaline ion substitution. The incorporation of Ca2+ ions into the La sub-lattice decreased the lanthanum ferrite’s electrical resistivity [16, 17]. Moreover, doping the parent compound LFO with another divalent ion, Mg2+, led to increase the resistance of LFO due to secondary phases and nonstoichiometric oxygen in the structure [2]. Although there are studies involving the effect of divalent ion doping on the structural properties of the LaFeO3 compound, a detailed and combined study on the effect of divalent ion doping on structural parameters, microstructure, and electrical properties is required. Therefore, in this study, the effect of cation size mismatch on the unit cell volume, charge order temperature, compound activation energy, microstructure, particle size, and grain size evaluation are discussed in detail. Using divalent Ca2+ as a dopant ion, crystal structure, microstructure, and the electrical properties of LaxCa1-xFeO3 (x=0.10, 0.20, 0.30, 0.40,) perovskites prepared by standard sol-gel route are considered. II. EXPERIMENATAL PROCEDURE La1-xCaxFeO3 (0.0≤x≤0.4) samples have been prepared by standard sol-gel method, using appropriate amounts of La(NO3)3.6H2O, Ca(NO3)2.4H2O, Fe(NO3)3.9H2O with the desired stoichiometry. Pellets pressed using the fine powders were sintered at 1100oC for 24h and cooled down to room temperature in the furnace. Close cycle cryostat was used to perform temperature dependent resistivity measurement using four-point probe from 320K down to 140K. Details of the experimental procedure can be found in [18]. III. RESULTS AND DISCUSSION A. Crystal Structure Analysis The room temperature structures of the La1-xCaxFeO3 (0.0≤x≤0.4) samples were investigated using their respective X-ray diffraction profiles. To explore the effect of Ca doping in detail, the structural parameters of all the samples were refined by Rietveld’s profile-fitting using freely available FullProf Suit software [19]. The given structural model was refined with the peak shapes of the experimental patterns were assumed to be a Thompson-Cox-Hastings (TCH) pseudo-Voigt profile functions. Lattice parameters, particle size and lattice strain were refined to fit the peak shifting and broadening of the experimental data. The values of goodness of fit (GoF) for all of the refinement lie between 0.87 and 1.3. GoF was calculated by the ratio Re/Rwp, where Re and Rwp are the expected weighted and weighted profile factors. The experimental and calculated profiles with their differences were figured out along with Bragg positions (see Figure 1). Rietveld refinement reveals that the samples crystallize in orthorhombically distorted perovskite structure with Pbnm space group. Inspecting on main peak, in Figure 1 (XRD pattern of LCFO40) and in Figure 2, there emerges another peak (marked with a ∗) in the pattern of LCFO40. Fig. 1. The experimental and calculated XRD patterns of all samples. Open circles indicate the experimental data and the calculated data are shown in the continuous line overlapping them. The curve (at the bottom) shows the difference between the experimental and calculated patterns. The vertical bars indicate the expected reflection positions for the main (blue-upperbars) and Fe2O3 impurity (red-lower bars) phases. Further investigation revealed that this peak was due to the trace of the rhombohedrally distorted perovskite Fe2O3 (hematite) with �3�� space group (Crystallographic Open Data (COD) base card # 96-901-0140). Refined lattice parameters, unit cell volumes, average A-site radii, 〈� 〉, average distortion ratio (D), cation size disorder (σ2), and Rietveld refinement parameters are summarized in Table I. Inspection on the XRD patterns of the samples makes clear that following Ca doping, main-peak of parent compound LFO at 32.17o (Figure 2) is rendered broader and shifted to higher angles. It is known that the former is due to decrease in particle size and the latter is a change of the lattice constants. Engineering, Technology & Applied Science Research Vol. 10, No. 2, 2020, 5538-5546 5540 www.etasr.com Irmak: Structural and Electrical Properties of Ca 2+ Doped LaFeO3: The Effect of A-site Cation Size … Fig. 2. Main peaks of LCF, LCFO10, LCFO20, LCFO30, and LCFO40 samples with * indicating the main peak of the Fe2O3 phase Calculated unit cell volumes for all samples after Reitveld refinement are given in Table I. Unit cell volume calculated for the parent compound LFO is 242.72Å3. A 10% increase in Ca, results to smaller unit cell with volume 239.85Å3. Further increase in Ca leads to decreased unit cell volumes: 238,04Å3, 235.26Å3, and 235.12Å3 for 20%, 30%, and 40% Ca respectively. Therefore, an increase in Ca amount decreases the unit cell volume which agrees with the replacement of La3+ having 12 coordinated ionic radii of 1.36Å with the smaller Ca+2 ion with 12 coordinated ionic radii of 1.34Å. Doping the parent compound with smaller ion decreased 〈� 〉 thus unit cell volume decreases. Moreover, there is another contribution on shrinkage of the unit cell which can be explained by parent compound doping, LFO, with Ca2+ ion resulting to oxidation of Fe3+ to Fe4+. An increase in doping leads to a decrease in concentration of Fe3+ (0.645Å) with larger ionic radii, and an increase of concentration of Fe4+ (0.585Å) with smaller ionic radii. Hereby, there is a decrease in size around Fe-site which may also contribute to a decrease of unit cell volume [20]. TABLE I. LATTICE PARAMETERS x (%) a (Å) b (Å) c (Å) V (Å3) 〈��〉 (Å) D (%) σσσσ2 (10-4ÅÅÅÅ2) Rp (%) Rwp (%) χχχχ2 GoF 0.00 5.5598 5.5565 7.8569 242.72 1.360 0.06 0.000 24.2 13.4 1.66 1.3 0.10 5.5399 5.5353 7.8216 239.85 1.358 0.11 0.360 34.5 18.4 1.45 1.2 0.20 5.5075 5.5162 7.8024 238.04 1.356 0.15 0.640 22.4 13.5 1.13 1.1 0.30 5.5063 5.4973 7.7721 235.26 1.354 0.16 0.840 34.7 17.4 1.02 1.0 0.40 5.5045 5.4935 7.7755 235.12 1.352 0.14 0.960 23.9 14.1 0.75 0.87 Fig. 3. Change of unit cell volume with respect to cation size mismatch. The line represents the fitting result. Even expressing the decrease in unit cell volume with Ca doping, it will be better to express the change in the unit cell volume with respect to A-site cation size mismatch (σ2) which is an inherent parameter for doped perovskite structures and it measures the size disorder of the structure [21]. To represent the change of unit cell volume, V is plotted with respect to �� which is given in Figure 3. It is clear from the Figure that as the cation size mismatch increases the unit cell volume decreases linearly. Therefore, fitting the data with following linear equation in σ2 was performed: ����� = �� − ��� (1) where V0 represents the volume of the sample at which �� is zero (namely V0 is the volume of the parent compound, LFO) and m is the slope of the line. Fitting results to χ2=0.98, V0 and m, 242.81±0.41Å3 and -82373±6268 Å respectively. The calculated unit cell volume for the parent compound using linear fitting is almost equal to the value (242.72Å3) from Rietveld refinement. It is seen from Table I and Figure 3 that there is no considerable change in the volume of LCFO40 as it is expected due to 10% more Ca with respect to LCFO30. For LCFO40, the amount of Ca in the structure may be lower than the expected. This can be explained with the appearance of Fe2O3 as an impurity phase in LCFO40 (Figure 1). If some Fe atoms are not in the main phase, then there should be the compounds or the oxides of Ca and/or La. However, there is no trace of the compound or the oxide of La and/or Ca in the XRD pattern. An amount of impurity phase which should be dispersed in the main perovskite phase and the precipitates with low concentration usually exhibit particle sizes in the range of a few nm, invisible to the XRD [22]. Doping an orthorhombic perovskite component at the A- site with an element having different ionic radii results to deviation from the cubic symmetry. The measure of deviation is an average distortion ratio, D, which is defined as [23]: � = �� ∑ � ����〈�〉� �� ����� (2) where 〈 〉 = ! "� √2⁄ & � �⁄ , � = , � = ", � = � √2⁄ , and a, b, and c are calculated lattice parameters. The calculated values of D for all of the compounds are given in Table I. The average distortion ratio D, exhibits a drastic increase from 0.6% to 0.11% for 10% and 20% Ca respectively which implies a larger orthorhombic distortion from sample LCFO10 to LCFO20 and it increases monotonically with increase in Ca. However, there is a distortion decrease for the sample doped with 40% Ca compared to the 30% sample. One can argue that this is compatible with Fe2O3 as an impurity in the structure, which Engineering, Technology & Applied Science Research Vol. 10, No. 2, 2020, 5538-5546 5541 www.etasr.com Irmak: Structural and Electrical Properties of Ca 2+ Doped LaFeO3: The Effect of A-site Cation Size … may indicate that the Ca amount in the compound is less than 40%. B. Microstructural Analysis: Particle Size and Strain Particle size and lattice strain (s) are parameters which can be extracted from the peak width analysis. Particle size generally has been calculated using Scherrer equation and Williamson-Hall (W-H) method. The full width at half maximum of the most intense peak for a given pattern is used in Scherrer’s equation (� = )* +�,-./ 0. Without correction for the lattice strain, this gives a rough particle size. One can say that, Scherrer’s equation gives only an approximate estimate of the mean particle size. But, the peak broadening due to size and strain is expressed in Williamson-Hall method as: +123�,-. = 45 6 7 49-:;. (3) where +123 is the full with at half maximum of the corresponding reflections in XRD pattern, K is the shape factor, λ is the X-Ray wavelength, D is particle size and ε is strain. Plotting βhklcosθ values with respect to 4sinθ is called a Williamson-Hall plot. Fitting the data with a linear equation gives the strain and particle size from the slope and y-intercept of the fitted line respectively [24]. The FullProf Suite program was used to calculate the microstructural parameters of the sample. The microstructural effects were handled using the Voigt approximation: both instrumental and sample intrinsic profile are supposed to be described approximately by a convolution of Lorentzian and Gaussian components [19]. TABLE II. AVERAGE PARTICLE SIZE, AND AVERAGE GRAIN SIZE Ca (%) 〈<=〉(nm) s RG (nm) 0 77.18±0.08 1.95±0.02 940 10 56.43±0.01 18.25±0.03 670 20 51.43±0.05 25.17±0.01 620 30 33.12±0.05 17.39±0.00 450 40 24.06±0.02 6.48±0.01 540 The parent compound LFO has the largest particles with average size of 77.18±0.08nm. The doping parent compound with 10% Ca, had an average PS decrease to 56.43±0.01nm. PS gets its minimum value for LCFO40 at 24.06±0.02nm. As doping increases, the average PS decreases. The average strain for the parent compound is 1.95 which is the smallest, while 10% Ca doping increased strain to 18.25. Further increase in Ca with 10% leads strain to attain its maximum for LCFO20. Doping more Ca decreases strain. The average strain is calculated as 6.48 for the LCFO40 sample with 40% Ca. The calculated particle size and strain for the samples are given in Table II, and the plot representing the change in these parameters with respect to Ca is given in Figure 4. It is clear from the plot that as Ca doping increases, PS size decreases. The particle size of the doped oxide becomes smaller than the pure host oxide, as a consequence of the restriction of the motion of particles at the boundaries between host and dopant particles [25]. The results obtained in this study are consistent with findings in [2, 8-12, 26], even if these studies have concluded to a decrease in average PS with an increase in doping. The method used in these studies to calculate average PS is only taking the contribution of the main peak in the XRD pattern. It is well established that, as the particle size decreases, surface and interface between the particles increase. And, surfaces are the place of broken bonds and oxygen vacancies which increase stress and strain on the surface and interface of the particles [27]. C. Atomic Force Microscopy Analyss Surface scanning of the samples was performed by AFM in non-contact mode with a resolution of 5µm×5µm. Scanned images of all samples are given in Figure 5. These images reveal that larger grains are surrounded by smaller ones and they are tightly connected with each other. This introduces that the grain growth process is not finalized yet. The average grain size calculated for the undoped sample is 940nm. Grain size decreases from 940nm, as nominal Ca increases in the samples up to 30% Ca for which grain size is 450nm. The grain size of the doped sample becomes smaller than the pure host sample [9, 13, 28], and the sample with 40% Ca has an average grain size of 540nm. A decreasing trend in grain size up to 40% Ca is consistent with the decreasing trend of particle size with Ca content. However, the average grain size of the sample LCFO40 was calculated to be 540nm. This increase in grain size (from 450nm to 540nm) may result from a Ca deficiency in the sample LCFO40 and impurity phase La2FeO3. The change in RG with Ca content is given in Figure 4. The largest particles calculated for the parent compound are 77.18±0.08nm. The sample with the largest average PS has the largest average grain size (940nm). Average PS is smaller than the average grain size calculated from AFM images (Table II). Fig. 4. Variation of particle and grain size as a function of nominal Ca. Inset represents the change of strain with nominal Ca. This suggests that, the grains viewed by AFM images are agglomerates of almost a few thousands (>? @A 0 � of particles [29]. Grain size calculated using the AFM image is a local average over tens of grains, but particle size calculated using XRD data is an average namely over 1010 particles. One can say that particle size calculated using XRD pattern with W-H analysis is a more accurate method. The change of microstructural strain with respect to Ca is given in Figure 4 as an inset. Since the sign of the strain is positive, it stands for compressive strain. Generally, there is a trend of increasing and decreasing strain with Ca. The samples undergo increasing compressive strain up to 20% Ca and then decrease up to 40% Ca. Engineering, Technology & Applied Science Research Vol. 10, No. 2, 2020, 5538-5546 5542 www.etasr.com Irmak: Structural and Electrical Properties of Ca 2+ Doped LaFeO3: The Effect of A-site Cation Size … Fig. 5. 5µm×5µm AFM images of the samples, (a) LFO, (b) LCFO10, (c) LCFO20, (d) LCFO30, (e) LCFO40 D. High-resolution TEM Analysis Surface morphology, particle size distribution and structure of the nanoparticles were studied by High-Resolution Transmission Electron Microscopy (HRTEM) using a JEOL- JEM 2010 operated at 200kV. Powder specimens were dispersed into acetone and underwent an ultrasonic bath for 20 minutes. A drop of solution was taken on a carbon grid to carry out HRTEM analysis. TEM investigation was performed to clarify the nanosized particle distribution and perfect crystallinity of LCFO20 precursor after calcination of the sol- gel route for 12hs at 600oC. A general view of the particle distribution of LCFO20 precursor is given in Figure 6(a). It is clear from the image that nanoparticle distribution with size between 20nm and 50nm was produced from the sol-gel process. Though TEM images are not necessarily representative of the whole sample, the average value of particle size calculated from XRD analysis (Table II) was closely represented by them. For further structural investigation one of the nanoparticle’s surface was focused (Figure 6(b)) and HRTEM technique was applied. Inverse Fast Fourier Transform (IFFT) of the real image indicated by a square in Figure 6(b) was taken in order to filter the unwanted noise (Figure 6(c)). The perfect crystalline structure of LCFO20 precursor before heat treatment is clear. FFT diffraction pattern extracted by applying a rectangular Region of Interest (R-ROI) is given in the upper left inset of Figure 7(a). IFFT of the R-ROI of real image is given in the upper right inset of Figure 7(a). Five spots were identified which correspond to the distance between different lattice planes as 0.5929, 0.4122, 0.3124, and 0.2048nm. These planes, having plane distances of 0.2601, 0.3124 and 0.2048nm, labeled as 1, 2 and 3 respectively, were indexed using lattice parameters of LCFO20 calculated from XRD data. The planes were indexed as follows: plane 1 was indexed as (0 2 0), (1 1 2), (2 0 0), plane 2 was indexed as (1 1 1) and plane 3 was indexed as (0 0 4) and (2 2 0). Theoretical calculation using lattice parameters of the sample LCFO20 gives an angle of 5.86o between the planes (2 2 0) and (1 1 1), and 50.89o between the planes (2 0 0) and (1 1 1). The two planes shown in Figure 7(a) form an angle of 50.06o which was calculated by IFFT analysis. This result suggests that high-quality crystalline structure was obtained in this study. The line profile is given in Figure 7(b) to emphasize interatomic distances. The average distance between the atoms was found to be 5.1Å which closely represents the lattice parameters a and/or b of the LCFO20 (Table I). Fig. 6. (a) TEM image of the prepared LCFO20 taken after burning the dried jel at 600oC, (b) HRTEM image of the particles with size 20nm, (c) IFFFT of the image in (b). Fig. 7. High resolution TEM image (HRTEM) of LCFO20 after heath treatment at 1100 oC. (a) FFT diffraction pattern extracted by applying rectangular R-ROI left inset, IFFT of the R-ROI of real image is given in the upper right inset. (b) The intensity profile of the atomic layers. Engineering, Technology & Applied Science Research Vol. 10, No. 2, 2020, 5538-5546 5543 www.etasr.com Irmak: Structural and Electrical Properties of Ca 2+ Doped LaFeO3: The Effect of A-site Cation Size … E. Electrical Properties Temperature dependence of the samples’ resistance is given in Figure 8. The change of the resistance of the samples shows the same trend with temperature which exhibits insulating properties for the studied temperature range (140-320K). Moreover, all samples exhibit strong insulating behavior at low temperatures. Due to the compliance limit of the measuring device, the resistance of the parent compound LFO could not be measured. At high temperature, the resistance of the sample changes slightly and increases as the temperature decreases. However, at low temperatures, resistance increases abruptly. This abrupt increase in resistivity may indicate that the samples are charge ordered (CO) insulators. This phenomenon is another property of mixed valence perovskite oxide. The onset of CO phase can be triggered by temperature and causes a sudden increase in the transport properties of the perovskite and lattice degrees of freedom [30]. The charge order temperature, TCO, is a temperature at which the onset of CO is observed. TCO can be calculated by taking the derivative of ln(ρ/T) against the 1/T curve (d(lnρ)/d(1/T)) and plotting the derivative as a function of temperature (Figure 9). The temperature at which a sharp peak within the derivative plot observed is the TCO. The calculated values of TCO for all the samples are shown in Table III and the plot of the variation of TCO with σ 2 is given in Figure 9. For the samples LCFO10, LCFO20, LCFO30, and LCFO40 the calculated values are 197K, 177K, 161K, and 164K. There is a decreasing trend in TCO with increase in Ca except for LCFO40. Ca deficiency and Fe2O3 as an impurity phase in LCFO40 may lead to an increase of TCO for this sample. Fig. 8. Plot of ln(ρ/T) vs 1/T. The continuous lines represent the linear fitting. The temperature dependent resistances of the samples are given in the inset. TABLE III. DEBYE TEMPERATURE, ACTIVATION ENERGY AND ρ300K Ca (%) θθθθD (K) Ea (meV) ρρρρ0 (µΩcm) ρ300Κ (Ωcm) 10 400 332.57±0.36 3.59±0.06 405.16 20 368 312.45±0.41 1.14±0.02 52.12 30 332 287.65±0.54 0.49±0.04 10.47 40 320 276.57±0.46 2.11±0.05 29.37 The resistivity of all samples increased rapidly after CO took place, due to the localization of electronic states which characterize the CO state. Changing the number of charge carriers by hole doping (Ca2+ amount in the sample) has an effect on TCO, but hole doping alone cannot describe the change of CO temperature. The structural factors including structural distortion or cation size disorder influence the electrical transport properties of these compounds. Electrical properties can be modified by changing the type and the concentration of trivalent Re, divalent or monovalent Ae cations, which led distorted crystal structure due to ionic size differences [31]. In our study, La3+ and Ca2+ have different ionic radii coordinated by 12 nearest O ions in the perovskite structure (〈rCD〉=1.36Å and 〈rED〉=1.34 Å). Fig. 9. Plot of TCO vs σ 2. The line represents the fitting result. Inset, the plot of the derivative of ln(ρ/T) curve with respect to T. The concentration of the cations and their ionic radii differences determine the distortions of crystal structure [31, 32]. The plot of TCO with σ 2 is used to express the influence of a measure of this distortion at the A−site on TCO (Figure 9). A linear decrease of TCO with σ 2 is clear. Linear variation of TCO versus σ2 was analyzed with [33]: FGH���� = FGH�0� − ��� (4) where FGH�0� represents the estimated value of the TCO of a parent compound in the given series and m is the proportionality factor which was obtained from the slope of the corresponding figure. Fitting gives FGH�0�=224.20±1.53K, and slope m=-18.69×10−4K/Å2 with reduced chi-square of 0.97. Due to the compliance of the measurement devices ρ(T) for the parent compound, LFO was not performed. But, for the parent compound, TCO was estimated to be 224.20K with the help of (4) and fitting results. 224.20K is the estimated highest possible CO temperature of the parent compound with no mismatch, which is consistent with the results of [34]. With a linear decrease in TCO with σ 2 we may conclude that upon increasing A-site disorder, i.e. an increase in σ2 from 0.0 to 1.0×10-4Å2, the CO is gradually suppressed. Since ionic radii of Ca2+ are close to ionic radii of La3+ (〈rCD〉=1.36Å and 〈rED〉=1.34Å, the change of σ2 even at high doping with Ca2+ will be small. However, using dopant elements with larger ionic size may give high disorder at A-site, which may lead to collapse of charge order for a given composition. Typical plots of temperature variations of electrical resistivity for all samples are shown in Figure 8. The resistivity axis is represented with Engineering, Technology & Applied Science Research Vol. 10, No. 2, 2020, 5538-5546 5544 www.etasr.com Irmak: Structural and Electrical Properties of Ca 2+ Doped LaFeO3: The Effect of A-site Cation Size … ln(ρ/T) and temperature axes with 1/T scale. All samples exhibit insulating behavior. Thermally activated hopping of small polarons is a model which describes the resistivity of perovskite oxides [35, 36]. Therefore, the resistivity was analyzed with the use of the Small Polaron Hopping (SPH) model [37]. According to the SPH model, the expression for the temperature dependence of the electrical resistivity is given by [38]: K = KLFM�N( OPQRS0 (5) where ρ0 is a pre-exponential factor, T is the absolute temperature, kB is the Boltzmann constant and Ea represents the activation energy for small polarons. Calculated Debye temperatures are given in Table III (the temperature at which resistivity deviates from linearity is θD/2). In the high temperature region, linear increase of ln(ρ/T) with 1/T is clear for all samples (Figure 8). Therefore, resistivity data in the given temperature range, (temperatures higher than θD/2) fit to a linear equation. From the fitting results of the best fitted lines, the slope gives Ea/kB and the intercept gives ρ0. The results of Ea for all samples are given in Table III. An increase in Ca leads to a decrease in Ea from 332.57±0.36meV, to 312.45±0.41meV for samples LCFO10 and LCFO20 respectively. The calculated activation energies for samples LCFO30 and LCFO40 are 287.65±0.54meV and 276.57±0.46meV. As the doping increases, Ea decreases for all the samples, in accordance to [2, 16, 17]. The perovskite oxides are classified in terms of their conduction mechanism based on the values of Ea. If Ea>1eV then that material is oxide ion conductor [39, 40]. Whereas, it is p-type polaronic insulator if 900meV>Ea>200meV and it is an n-type polaronic insulator if Ea≤200meV [41, 42]. Therefore, the activation energies calculated for samples in this study reveal that they are p-type polaronic insulators. To explore the effect of cation size mismatch on activation energy, Ea is plotted against cation size mismatch (Figure 10). From the plot, the inverse linear relation with Ea and σ 2 is clear. The highest value of Ea occurs at the lowest σ 2 and vice versa. As the cation size mismatch increases, activation energy decreases. It is well known that the electronic properties of perovskite oxides are often crucially connected with structural distortions around the Re site and/or transition metal sites [43, 44]. Linear fitting was performed to represent the variation of Ea with σ 2 and the following relation was written from the fitting result: T����� = T��0� − ��� (6) where Ea(0) represents the activation energy at which σ 2=0, or the activation energy of the parent compound LFO which was not calculated in this study due to the compliance limit of the measuring devices, and m is the slope of the best fitted line. Fitting results are: Ea(0)=367.87±5.49meV and m=934502.54K/Å2. Ea(0) is the activation energy of the undoped sample LFO which was estimated with the use of (6) and the fitting results. It is clearly evident (Figure 10) that room temperature resistivity decreases with increasing Ca and gets its minimum value at 10.47Ωcm for sample LCFO30. As Ca2+ increases, the p-type carriers in a doped compound increase. This results to an increase in conductivity or equivalently a decrease in resistivity. Moreover, increasing p-type carriers causes Ea to decrease, leading to a decrease in the energy gap of the doped compound. Furthermore, the decrease in resistivity possibly suggests that the Fermi level shifts to the nearer edge of the conduction band [45]. Fig. 10. Variation of Ea as a function of σ2. Chance of r0 at 300K with nominal Ca (inset) The room temperature resistivity of LCFO40 increased to a value of 29.47Ωcm. This may be the result of oxygen vacancy and appearance of impurity phases in that compound. Oxygen vacancies can provide electrons which compensate some of the holes created with Ca doping. This results to filling the d band of Fe4+ ions and transforming them to Fe3+. In addition, Ca deficiency and impurity phase La2FeO3 in LCFO40 leads to decrease of the Fe4+ ions. As a result, the number of holes contributing to conduction decreases and therefore resistivity increases. IV. CONCLUSION The structural and electrical properties of nanosized La1- xCaxFeO3 compounds prepared via sol-gel method have been studied. Rietveld refinement of the XRD pattern of the samples indicates that all the samples have distorted orthorhombic structure with Pbnm space group, except the sample doped with 40% Ca which is single phase. Increase of Ca doping leads to increase of the average distortion ratio. There is a drastic increase of average distortion ratio from parent compound LFO to LCFO10 and to LCFO20. Moreover, particle and grain growth are suppressed with Ca. There is an inverse linear relation between unit cell volume and σ2. It was shown that electrical conduction in these compounds is thermally hooping of small polarons. The studied compounds are p-type polaronic insulators considering the conduction mechanism based on the values of Ea. 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