Investigation of structural and electrical properties of synthesized Sr-doped lanthanum cobaltite (La_1−xSr_xCoO₃) perovskite oxide

1 Introduction

ABO₃ type LaCoO₃ (lanthanum cobaltite) perovskite oxides have two different sized cations of A-site with rare earth element and B-site with transition element surrounded by oxygen anions show great interest in diverse applications including optical (Farhadi and Sepahvand 2010), magnetic (Bellakki et al., 2010), thermoelectric (Kun et al., 2013), electro-catalysis (Jayapandi et al., 2018; Lu et al., 2019), photo-catalysis (Fu et al., 2013; Jayapandi et al., 2018), fuel cells (Rehman et al., 2018) and sensing (Wang et al., 2017) applications. LaCoO₃ perovskite oxides also exhibit good conductivity (Huang et al., 1998; Nakayama et al., 2003; Zhu et al., 2018). Thermally stable LaCoO₃ perovskite oxides have stable crystal structure and can be used as electrode material for high-temperature solid oxide fuel cells (SOFCs) (Rehman et al., 2018). Physico-chemical and catalytic properties of ABO₃ perovskites can be influenced efficiently with the substitution of A-site by other elements. The oxygen vacancy defects in perovskite also play an important role in the mobility of oxygen from the crystalline oxygen to the surface oxygen of perovskite. Sr-doped LaCoO₃ perovskite oxides (i.e. La_1−xSr_xCoO₃) are recently reported as excellent bifunctional electro-catalysts in oxygen reduction and oxygen evolution reactions (ORR/OER) due to high surface area, virtuous electronic conductivity and unique electronic structure (Cheng et al., 2015; Mefford et al., 2016). La_1–xSr_xCoO_3-δ perovskite oxides have also shown interesting crystal structure, magnetic behaviour, thermal and electrical properties (Iwasaki et al., 2008; Petrov et al., 1995). Thermal studies and crystal structure of nickel-substituted La_0.5Sr_0.5CoO_3−δ ceramics have been reported with enhanced electrical properties (Lu et al., 2020). Sr or Ce doped LaCoO₃ perovskites could be promising materials due to electronic configuration and electrical conductivity as also reported elsewhere (Hwang et al., 2017; Wang et al., 2010). La_1−xSr_xCoO₃ perovskite oxides have been used extensively as magnetic (Chennabasappa et al., 2020), thermoelectric (Viskadourakis et al., 2019), optical (Fang et al., 2016) and electro-catalytic (Lal et al., 2005) materials for various applications. Magneto-caloric effect (Long et al., 2018) and structural properties (Li et al., 2019) of La_1-xSr_xCoO₃ perovskite oxides were also reported for oxygen storage application. La_1-xSr_xCoO₃ perovskite oxides were also reported as good electrical conducting materials and used as cathode materials in SOFCs (Björketun et al., 2017; Wu et al., 2017). The electrical conductivity and the crystal structure of La_1-xSr_xCoO₃ depend on Sr-doping and temperature (Wu et al., 2017). Electrical and thermal properties could be affected by the structural change of La_1−xSr_xCoO₃ perovskite oxides (Liu et al., 2016; Viskadourakis et al., 2016). La_1-xSr_xCoO₃ perovskite materials were also used as sensor for acetone sensing at room temperature (Liu et al., 2017). An attractive and promising way has also been designed for white and green light emitting phosphorus materials (Khan et al., 2018, 2017). Recently, we have developed La₂MnNiO₆ (Ahmed et al., 2020), Na₂La₂P₄O₁₂ (Alhokbany et al., 2020) and NiMoO₄@rGO (Ahmed et al., 2021) electrode materials for energy conversion and energy storage applications. Herein, we report synthesis of La_1−xSr_xCoO₃ perovskite oxides at different compositions (x = 0.0, 0.20, 0.50, 0.80) using the sol–gel method at 600 °C. Structural and electrical properties of La_1−xSr_xCoO₃ perovskite oxides were successfully investigated in details.

2 Experimental

La_1−xSr_xCoO₃ perovskite oxides by varying the compositions were prepared using the modified sol–gel method (Ahmed et al., 2016). The starting reagents such as La(NO₃)₃·6H₂O (Sigma Aldrich, 99%), Co(NO₃)₂·6H₂O (Sigma Aldrich, 98%) and Sr(NO₃)₂ (Sigma Aldrich, 99.9%) were taken in an appropriate stoichiometric ratio with excess of citric acid (Sigma Aldrich, 99%) and transferred to the porcelain crucible containing de-ionized water followed by the formation of viscous gel. The evaporation of water was taken place for dryness and then calcined the materials at 400 °C/2h and 600 °C/10 h at constant heating rate of 10C/min. La_1−xSr_xCoO₃ perovskite oxides (x = 0.0, 0.20, 0.50, 0.80) were obtained using the above process at 600 °C. The proposed reaction in the synthesis of La_1−xSr_xCoO₃ perovskite oxides is given as:

La(NO₃)₃·6H₂O + Sr(NO₃)₂ + Co(NO₃)₂·6H₂O + C₆H₈O₇ → La_1-xSr_xCoO₃ + nCO₂ + nN₂ + nH₂O

The synthesized La_1−xSr_xCoO₃ perovskite oxides were characterized using powder x-ray diffraction (XRD) followed by Rietveld refinement using the fullprof program (Reis et al., 2017; Rodríguez-Carvajal, 1993). XRD data was recorded on Bruker D-8 Advance diffractometer in 2θ range between 10 and 100° at the rate of 0.01°/second. XRD diffractometer contains Ni filtered CuKα radiation. The cell parameters were refinement by fullprof software, match software (personal licenses- version 3.6.2.121- Crystal Impact, Bonn, Germany) based on the Rietveld refinement method. Match software was used to carry out search and match technique with an international database i.e. crystallography open database (COD) to identify the unknown phases. The peak position and relative intensities of the experimental (observe) data and reference (calculate) data were used in fitting to obtain the best matching between them. Fourier-transform infrared spectroscopy (FTIR) data was collected on Bruker TENSOR 27 Spectrometer. The samples for FTIR measurements were prepared with KBR followed by the palletization at room temperature. Scanning electron microscopic (SEM) studies of La_1−xSr_xCoO₃ perovskite oxides were carried out with JEOL JSM-7600F machine for surface morphology. The oxidation states of the elements were investigated by x-ray photoelectron spectroscopy (XPS) using a Thermo Scientific Theta Probe Angle-Resolved XPS system (USA). Details of the calibration of XPS were reported elsewhere (Dobler et al., 2002). The instrument was run with a standard Al-Kα source radiation with the photon energy of 1486.6 eV and operated at 9 kV and 20 mA. Pass energy values of 200 eV and 50 eV were employed for survey scans and detailed scans, respectively. The data were corrected for any drift by setting the binding energies were referenced to C1s peak at 284.8 eV. The electrical conductivity and dielectric measurements were recorded with Wayne Kerr 6440B Precision Component Analyzer at different frequency range from 20 Hz to 3 MHz.

3 Results and discussion

The lattice parameters of the prepared materials are resulted from the fitting of the experimental diffraction patterns by Rietveld refinement. The powder XRD patterns of LaCoO₃ are shown in Fig. 1a. Rietveld refinement method displays smaller values of R_wp (i.e. 22.2%) and R_p, which imply the best fit as shown in Fig. 1b. The resulting fitting patterns confirm the formation of pure phase material. The XRD patterns of LaCoO₃ show single-phase and could be indexed with rhombohedral (space group of R-3c), and consistent with standard JCPDS file # 04–013-6817 (Haron et al., 2017). These results are also in good agreement with previous reports (Cheng et al., 2015; Haron et al., 2017). Major diffraction peaks were detected at two theta (2θ) of 23.25°, 32.89°, 33.27°, 40.73°, 41.38°, 47.48°, 53.36°, 53.78°, 58.93°, 59.71°, 68.97°, 70.04° 78.74°, and 79.42°correspond to the lattice planes such as (1 0 1), (1 1 0), (0 1 2), (0 2 1), (0 0 3), (2 0 2), (2 1 1), (1 1 3), (1 2 2), (1 0 4), (2 2 0), (0 2 4), (3 1 2) and (2 1 4) respectively, which detected the high degree of La and Co incorporated in the perovskite lattice structure. The characteristic doublet highest intensity peaks at (2θ) of 32.89° and 33.27° indicate that the calcination temperature had a significant effect on the formation of the perovskite phase. The sharp diffraction peaks of LaCoO₃ reveal its high crystalline nature with no impurities such as La₂O₃ and Co₃O₄ phase. Rietveld refinement of XRD patterns shows good profile fitting patterns of experimental and calculated profile data, which can be identified with rhombohedral symmetry (space group of R-3c). The refined unit cell parameters of rhombohedral structure were found to be “a”= 5.4461 Å, “b”= 5.4461 Å, “c”= 13.1077 Å, “α”=90.000°, “β”=90.064° and “γ”=180.000°. Fig. 2 shows the crystal structure of the unit cell of LaCoO₃ along with a-axis. In this structure, Co⁺³ and La⁺³ cations are 6-fold and 9-fold coordinated with oxygen anion, respectively. This trigonal symmetry involves an alternating rotation of the octahedral CoO₆ subunits connected through corner-shared oxygen. In the axes sittings, the structure has been described by the lattice constants of rhombohedral angle (α) and oxygen coordinated (x). The oxygen atoms are fully occupied 18e (x,0,1/4) sites in the unit cell of LaCoO₃. Fig. 3 shows the XRD patterns of the La_1-xSr_xCoO₃ (x = 0.0, 0.20, 0.50, 0.80). From the XRD studies, we observed clearly a phase transition from the rhombohedral structure (space group of R-3c) of La_1-xSr_xCoO₃ (x = 0.0, 0.20) to the cubic structure (space group of Pm-3 m) of La_1-xSr_xCoO₃ (x = 0.50, 0.80). We have observed that the diffraction angle of the prepared materials gradually shifted toward small diffraction angles with increasing Sr²⁺ contents in perovskite unit cell. This can be ascribed to the successive replacement of smaller La³⁺ cation (rLa³⁺=0.136 nm) by bigger Sr²⁺ cation (rSr²⁺= 0.144 nm), leading to the expansion of perovskite unit cell. Due to the high tolerance of the perovskite structure to distortion with metallic elements of different sizes and oxidation states, substitution of La³⁺ with a higher ionic radius of Sr²⁺ into the A site of LaCoO₃ maintained the rhombohedral structure at low strontium substitution levels (x ≤ 0.50) with doublet peak at 2ϴ of 33–34°. By increasing the strontium substitution (i.e. x = 0.80), the doublet peak suggested that rhombohedral structure became weaker and slowly changed to singlet peak which indicates the formation of cubic symmetry. This is noteworthy that the crystal symmetry changes from rhombohedral to cubic at higher strontium substitution (x = 0.80) due to different ionic radii between La³⁺and Sr²⁺ and the diffraction lines of La_{1- x}Sr_xCoO₃ gradually shifted to lower 2ϴ side.

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Fig. 1

(a) X-ray diffraction patterns and (b) Rietveld refinement of LaCoO₃ perovskite oxide.

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Fig. 2

Crystal structure of rhombohedral unit cell of LaCoO₃ perovskite oxide.

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Fig. 3

XRD patterns of La_1-xSr_xCoO₃ perovskite oxides showing shift to higher 2θ degrees for samples with Sr compared with un-doped LaCoO₃.

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Fig. 4 shows Rietveld refinement of XRD patterns of La_{1- x}Sr_xCoO₃ perovskite oxides (x = 0.20, 0.50, 0.80). Fig. 5 shows the projection of the cubic crystal structure of La_0.2Sr_0.8CoO₃ perovskite oxide along with ‘ac’ plane. The basic units of the composites are (La/Sr)-O₁₂ and CoO₆ octahedral. The (La/Sr)-O₁₂ sharing oxygen atoms with the CoO₆ by corner-sharing and La/Sr atoms are fully occupied in the Wyckoff position 1a. An interatomic distance of La/Sr-O bond shows bond length of 2.76 Å. In CoO₆ octahedral, Co atoms are locating in Wyckoff position 1b at the position of (0.5,0.5,0.5). Each CoO₆ shows the bond length of the Co-O bond is of 1.90 Å with bond angle Co-O-Co of 180°. This is noteworthy that present XRD studies of La_1-xSr_xCoO₃ perovskite oxides are in good agreement with previous reports (Cheng et al., 2015; Mefford et al., 2016). Table 1 present the refinement agreement factors, cell parameters, atomic coordination, and selected interatomic distance. According to XRD results, the substitution of La³⁺ by the larger Sr²⁺ cations aligned the Co-O-Co atoms (from 161.6° of LaCoO₃ to ideal 180° of La_0.2Sr_0.8CoO₃), resulting in a structural change from a rhombohedral perovskite to a cubic structure. The unit cell also decreased and mainly attributed to the transition of Co³⁺, with an ionic radius of (0.61 Å) to the smaller ion Co⁴⁺ (0.53 Å).

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Fig. 4

Rietveld refinement of La_1-xSr_xCoO₃ perovskite oxides (x = 0.20, 0.50, 0.80).

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Fig. 5

Crystal structure of the cubic unite cell of La_0.2Sr_0.8CoO₃ perovskite oxide.

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Electron microscopic studies of La_1-xSr_xCoO₃ (x = 0.0, 0.20, 0.50, 0.80) perovskite oxides were also conducted for surface morphology and particles size analysis. SEM studies of of La_1-xSr_xCoO₃ perovskite oxides reveal the formation of uniformed particles in the particle size range from 100 to 150 nm as also seen in Fig. 6 (a-c). This is clearly visible that the average particle size increases with substitution Sr⁺² cations for La⁺³ cations. Therefore, sub-micron sized particles of La_1-xSr_xCoO₃ (x = 0.80) were detected with high concentration of Sr⁺² (Fig. 6d). Elemental studies of La_1-xSr_xCoO₃ perovskite oxides were carried out by energy dispersive studies (EDS) equipped with SEM. EDS data confirmed that the initial loaded compositions were matched with the resulted composition as summarized in Table 2. Fig. 7a shows FT-Infrared spectra of La_1-xSr_xCoO₃ perovskite oxides. The O-Co-O bending and Co-O stretching vibrations of octahedral coordinated CoO₆ were detected at ∼420 cm⁻¹ and ∼600 cm⁻¹, respectively. The appeared band at ∼600 cm⁻¹ resembles to both doped and undoped perovskite materials due to La-O vibration. FT-IR band at ∼852 cm⁻¹ was attributed to Sr-O vibrations in doped samples. XPS studies of La_1-xSr_xCoO₃ perovskites were also conducted for elemental and surface composition analysis. XPS spectra of La_1-xSr_xCoO₃ perovskites clearly refer to La3d, Co2p, Sr3d and O1s as also shown in Fig. 7b. XPS peaks at ∼780 eV and ∼796 resemble to Co2p_3/2 and Co2p_1/2 respectively of Co³⁺. A satellite peak was appeared at ∼786 eV, which could be suggested a small amount of Co⁺² on the surface of the materials. XPS peaks at ∼834 and ∼854 eV can be associated with La3d_5/2 and La3d_3/2 respectively and existed in tri-valence state (La⁺³). The binding energy of ∼134 eV can be referred to Sr 3d_5/2 which is existed in divalence state (Sr⁺²). After Sr⁺² doping, the signal at ∼531 eV increases which indicates the improvement of oxygen adsorption after partial substitution of Sr⁺² for La⁺³. XPS peak at ∼528 eV could be associated with O1s of surface lattice oxygen (O^-2_latt), whereas XPS peaks at ∼531 eV can be assigned to adsorbed oxygen (O₂^-2) species. It is confirmed by the surface O_ads/O^-2_latt ratio that increases from 0.77 to 1.05 after doping of Sr⁺² to LaCoO₃ lattice.

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Fig. 6

SEM micrographs of La_1-xSr_xCoO₃ perovskite oxides.

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Fig. 7

(a) FTIR spectra and (b) XPS survey of La_1-xSr_xCoO₃ perovskite oxides.

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Variation in the electrical conductivity of La_1-xSr_xCoO₃ perovskite oxides (x = 0.0, 0.20, 0.50, 0.80) was measured as a function of Sr doping with frequency at room temperature (Fig. 8a). Electrical conductivity relaxation has been examined using four-point DC method. The geometry of the samples was kept as rectangular with the area of 1.0 cm × 5.0 cm and thickness of 1–2 mm. The following equation was used to calculate the conductivity of the perovskite oxides: σ = L / R*A; where ‘R’ is dc resistance, ‘L’ is thickness of pellet, and A is cross-sectional area of the flat surface of the pellet. It was observed that the values of electrical conductivity (σ) increasing with increase in the Sr content at La site. Electrical conductivity (σ) was increased four orders of magnitude as the Sr concentration increased. LaCoO₃ perovskite oxide act as a charge–transfer insulator and doping of Sr²⁺ into La³⁺ site introduces hole carriers in La_1–xSr_xCoO₃ perovskite oxides (Lu et al., 2020; Mineshige et al., 1996). Therefore, high electrical conductivity may be attributed to increase the hole concentration with Sr-substitution in LaCoO₃ perovskite oxide. The dielectric properties of the materials depend on the applied frequency, temperature and grain boundaries. Frequency dependent dielectric measurements of La_1–xSr_xCoO₃ perovskite oxide materials were carried out at room temperature. Fig. 8(a) and (b) show the variation of dielectric constant and dielectric loss of La_1–xSr_xCoO₃ perovskite oxide materials as a function of frequency of applied field (20 Hz–3 MHz) at room temperature. Fig. 8b illustrates that the dielectric constant (έ) decreases with increasing frequency and becomes almost frequency independent at high frequency. This could be ascribed to the normal behaviour of the dielectric materials due to the relaxation behaviour of the system. Larger value of dielectric constant at lower frequency could be due to the accumulation of ions at the interface of conducting regions, which may cause the dielectric constant to increase. Consequently, the polarization as well as dielectric constant decreases at high frequency and reaches to a constant value because the ions do not have enough time to accumulate at the interface. Fig. 8c shows frequency effect on dielectric loss (tanδ) at room temperature. It is clear from the results that the loss tangent decreases with increasing frequency, which exhibit dispersion behaviour similar to dielectric constant. Relatively high conducting behaviour at low frequency could be related with polarization changes and charge exchange between cobalt ions. This dielectric behavior of La_1–xSr_xCoO₃ perovskite oxide materials could be attributed to Maxwell-Wagner interfacial polarization model with Koop’s phenomenological theory.

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Fig. 8

(a) Variation of DC conductivity with frequency for La_1-xSr_xCoO₃ perovskite oxides. (b) Plot of real part of dielectric permittivity against frequency. (c) Plot of dielectric loss against frequency for La_1-xSr_xCoO₃ perovskite oxides at room temperature with 20 Hz – 3 MHz frequency range.

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4 Conclusion

La_1−xSr_xCoO₃ perovskite oxides (x = 0.0, 0.20, 0.50, 0.80) were successfully synthesized using the sol–gel method. The crystal structure of La_1−xSr_xCoO₃ perovskite oxides was changed from rhombohedral to primitive cubic structure with Sr-doping as also confirmed by Rietveld refinement. Sr-doped dependent electrical properties of La_1−xSr_xCoO₃ perovskite oxides have been improved by Sr-substitution of La. Frequency dependent dielectric properties of La_1−xSr_xCoO₃ perovskite materials were constant at high frequency.