Tailoring optical, magnetic and electric behavior of lanthanum strontium manganite La_1−xSr_xMnO₃ (LSM) nanopowders prepared via a co-precipitation method with different Sr²⁺ ion contents

Abstract

Lanthanum strontium manganite (LSM) nanopowders La_1−xSr_xMnO₃ (x = 0.2, 0.5, 0.8) have been synthesized using the co-precipitation method based on methyl amine as a base at a pH value 12. The effect of Sr²⁺ ion concentration on the crystal structure, microstructure, optical, magnetic and the electrical properties was investigated. Typically, in all of the formed LSM powders, XRD revealed that a pure single perovskite LSM phase was obtained after annealing at 1000 °C for 2 h. This temperature was relatively low compared to what has been reported elsewhere. The microstructure of the produced LSM nanopowders depends on the Sr²⁺ concentration. The observations of these powders confirmed their rough surface. Meanwhile, the transmittance of the sample was around 40% for LSM with an Sr²⁺ ratio of 0.8. Furthermore, the band gap energy of this powder was 2.6 eV. The refractive index was decreased with an enhanced Sr²⁺ ion content. Indeed, the saturation magnetization of the LSM powders was increased on increasing the Sr²⁺ ion concentration. Electrochemical impedance spectroscopy (EIS) evinced that the electrode specific polarization resistance of the LSM samples was increased by increasing the Sr²⁺ ion concentration from 3.10 to 8.56 Ω cm².

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A. Introduction

Strontium doped lanthanum manganite La_1−xSr_xMnO₃ (LSM) perovskites are the principal component of the neoteric materials for solid oxide fuel cell (SOFC) cathodes. LSM-based compositions have shown interesting electrochemical properties for oxygen activation at high temperatures while they are thermo-chemically compatible with the widely used YSZ electrolyte material.¹ The La_0.8Sr_0.2MnO₃ gives a good combination of electronic conductivity and expansion coefficient matching, and it is now available commercially for SOFC applications. Higher conductivity can be obtained at higher dopant levels, but the expansion coefficient then becomes overly high.^2–7 However, the polarization resistance of LSM dramatically increases with the decreasing operation temperature (1 Ω cm² at 1000 °C to 2000 Ω cm² at 500 °C), which seriously limits the overall cathode performance at the intermediate temperature.⁸ Besides, it has also reported that the electrochemical reactions of cathode materials, such as the LSM have proved to be very poor oxide ion conductors, but their electronic conductivity is high enough to make them attractive SOFC cathode material, which is particularly interesting when the strontium content is in the range of 0.1 to 0.3.

Otherwise, strontium doped lanthanum manganite La_1−xSr_xMnO₃ has been widely used as magnetic sensors, read heads, information storage and actuations, automotive exhaust control, elimination of CO pollutant and as cathodes in solid oxide fuel cells (SOFC).^9,10 Such extensive applications are attributed to their fascinating magneto-transport properties like colossal magneto resistance (CMR) coupled with excellent electrical, thermal, chemical and mechanical characteristics.¹¹ The microstructure and the homogeneity characteristics of LSM depend to a large extent on the processing conditions and are hard to control in the conventional ceramic synthesis process, which is based on the diffusion of the components in the solid state at high temperatures. For this reason alternative preparation routes for LSM synthesis based on wet chemical methods have been proposed.

A characterization of perovskite powders for the cathode and oxygen membranes prepared by different chemical routes was made by Sfeir et al.,¹² indeed, there are several routes to synthesize perovskite structured materials. These routes include solid-state reaction,¹³ co-precipitation,¹⁴ sol–gel,¹⁵ pyrolysis,¹⁶ combustion and Pechini method.^17–19 In this context, conventional solid-state reaction route requires high calcination temperatures and long annealing times which often results in agglomerated coarse particles and subsequently problems for achieving uniform distribution of particles for particular applications such as SOFCs cathodes.¹⁵ In comparison, wet chemical routes often results in almost pure phase with uniform distribution as well as high surface area without any consequently grinding as observed in solid state reaction strategy. Among the chemical routes, the chemical co-precipitation method ensures proper distribution of the various metals ions resulting to stoichiometric and smaller particles size product, compared to some of the other procedures. Moreover, the chemical co-precipitation method is a low-cost technique suitable for mass production. Besides, it is even found that the rate of addition of precipitating agent could have a significant effect on the shape and size of the final powders.^20,21 Herein, the aim of this article is to describe a versatile and cheap co-precipitation strategy to obtain La_1−xSr_xMnO₃ nanoparticles at different Sr²⁺ ion contents using methyl amine as a base for the first time. The impact of Sr²⁺ ion molar ratio on the structural, microstructure as well as optical, magnetic, and electrical properties, of the newly synthesized system was further investigated in details using different physical approaches.

B. Materials and methods

1. Materials

All the chemicals used in this study such as lanthanum nitrate hexahydrate La(NO₃)₃·6H₂O (Fluka) Analytical, anhydrous strontium nitrate Sr(NO₃)₂ (Sigma-Aldrich), manganese acetate tetrahydrate C₄H₆MnO₄ (AppliChem), and methylamine as a base were used as received. Moreover, deionized water was used in the whole work.

2. Procedure

Lanthanum strontium manganite (LSM) nanopowders were synthesized through co-precipitation strategy by mixing aqueous solutions of lanthanum nitrate hexahydrate La(NO₃)₃·6H₂O, anhydrous strontium nitrate Sr(NO₃)₂ and manganese acetate tetrahydrate C₄H₆MnO₄. Then, methyl amine solution as a base was gradually added to the mixture. Dark brown precipitate was formed. The formed precipitated precursors were filtered, washed with deionized water and ethanol. Thereafter, it was dried at 60 °C temperature for 24 h to form the hydroxide precursors.

Finally, the powder samples were annealed at 1000 °C at rate of 10 °C min⁻¹ in static air atmosphere and maintained at the temperature for annealed time of 2 h.

3. Physical characterization

X-ray powder diffraction (XRD) was carried out on a model Bruker AXS diffractometer (D8-ADVANCE Germany) with Cu Kα (λ = 1.54056 Å) radiation, operating at 40 kV and 40 mA. The diffraction data were recorded for 2θ values between 10 and 80°. Field emission scanning electron microscopy was performed by a FE-SEM (JEOL-JSM-5410 Japan). The UV-Vis absorption spectra were measured by UV-Vis-NIR-scanning spectrophotometer (JASCO V-570 spectrophotometer, Japan; Fig. S1†). The Dynamic Light Scattering (DLS) was measured using HORIBA LB-500 Particle Size Distribution Analyzer in the range from 3 to 6000 nm in a fixed angle 90° quartz cells. The magnetic properties of the prepared cobalt ferrite were measured at room temperature using vibrating sample magnetometer (7400-1 VSM, Lake Shore Co., Ltd., USA) with Integrated IDEAS VSM software at room temperature in a maximum applied field of 20 kOe. From the obtained hysteresis loops, the saturation magnetization (M_s), remanence magnetization (M_r) and coercivity (H_c) were determined. Electrochemical impedance spectrums (EIS) were recorded as a function of frequency with 6 points with an AC perturbation signal of 50 mA using a PARSTAT 4000 Potentiostat/Galvanostat/EIS Analyzer.

C. Results and discussion

1. Crystal structure

The XRD after thermal treatments at 1000 °C for LSM with different Sr²⁺ ion contents are depicted in Fig. 1. A low crystallized size powder was obtained at low strontium ion molar ratio. The crystallinity of the LSM samples was improved as the Sr²⁺ ion content was increased. Distinctly, all the XRD patterns showed that no excess impurity phases of La₂O₃ or La(OH)₃ was detected. Evidently, the nanoparticles were indexed as a single-phase perovskite LSM structure at different Sr²⁺ ion concentrations. Obviously, the data are in agreement with the Joint Committee on Powder Diffraction Standards card for LSM (JCPDS # 49-0595). There is no difference in the XRD patterns for the different LSM powders. The crystallite size of the formed powders was estimated from the most intense peak (024) plane of XRD patterns based on Scherrer formula. Apparently, the crystallite size was Sr²⁺ ion dependent. It was increased with further increasing of Sr²⁺ ion content. The substitution of La³⁺ cation (1.36 Å) by a larger one Sr²⁺ (1.44 Å) led to the progressive increase in the number of holes from Mn³⁺ to Mn⁴⁺. Subsequently, the substitution of La 3p by Sr 2p increases the content of Mn⁴⁺ ions to keep the charge neutrality.^17,18,22

Fig. 1 XRD patterns of the LSM powder synthesized using co-precipitation method using methyl amine as a base at pH 2 then annealed at 1000 °C for 2 h with different Sr²⁺ ion concentrations.

2. Microstructure

Fig. 2 shows the morphologies of LSM samples prepared via co-precipitation method annealed at 1000 °C for 2 h with different Sr²⁺ ion contents. At low Sr²⁺ content (Fig. 2a), it can be seen that the particles were exhibited as tablet like structure with homogeneous small size and hard agglomerated. However, non-spherical faceted powders were displayed with Sr²⁺ ion molar ratio of 0.5 as depicted in Fig. 2b. Further increasing the Sr²⁺ ion ratio to 0.8 was found to form agglomerated faceted powders. Meanwhile, the microstructure observation performed for the samples indicated that the grain size was increased with increasing the Sr²⁺ ion concentrations. The grain size was increased from 45 nm at Sr²⁺ molar ratio of 0.2 to 113 nm at Sr²⁺ content 0.8. These results have been confirmed by DLS measurement (Fig. S2†).

Fig. 2 Different magnifications of SEM images for lanthanum strontium manganite synthesized using co-precipitation method with different Sr²⁺ ion molar ratios and annealed at 1000 °C for 2 h.

3. Optical properties

Transmittance and absorbance spectra of the synthesized lanthanum strontium manganite nanopowders are shown in Fig. 3 and 4. The transmittance curves exhibited typical induced interference fringes, a quite rapid fall-off at the lower wavelengths and good transparency response through the examined spectral range. The highest average transparency was associated to high Sr²⁺ ion concentration. A blue-shift of the transmittance-curve edge can be observed as Sr²⁺ ion content increases from 0.2 to 0.8. This explanation could be misleading due to spectra overlapping around the transmittance edge (below 400 nm) and occurrence of a knee-feature (at 200–300 nm).^23,24 The transmittance percentage is halved (nearly 40%) by increasing the Sr²⁺ ion molar ratios. However, it is still good in the visible range and consistent with other reports available in literature.^24–26 This transmittance was found to be very appropriate for OLED applications.

Fig. 3 Transmittance spectrum of LSM nanopowders synthesized using co-precipitation method using methyl amine as a base at pH 12 with varied the Sr²⁺ ion ratios (x = 0.2, 0.5 and 0.8) annealed at 1000 °C for 2 h.

Fig. 4 Absorbance spectrum of LSM nanopowders synthesized using co-precipitation method using methyl amine as a base at pH 12 with varied the Sr²⁺ ion ratios (x = 0.2, 0.5 and 0.8) annealed at 1000 °C for 2 h.

The steep shape of the optical absorption spectra indicates that the visible light adsorption could be not caused by a transition from the impurity levels but was caused by the band-gap transition. The absorption band contains a tail extending right-wards until about 700 nm. This may result from the crystal defects formed during the growth of the LSM nanopowders. It is flat in the 400–800 nm range. It is noted that the lower UV cutoff of all samples appears at 300 nm and there is no remarkable absorption in the entire region of the spectrum. The spectrum clearly shows that the powders possess good optical absorbance 0.8 from 200 to 300 nm which is useful for second harmonic generation. By increasing the wavelength region to 400 nm, the absorbance was increased to 1.0 for all the prepared samples.²⁷

4. Band gap energy measurement

Here, the influence of the Sr²⁺ ion molar ratios on the band-gap energy of lanthanum strontium manganite has been investigated. hν is the photon energy and E_g is the energy of the optical transition corresponding to the optical band gap. Value of the exponent n depends on the nature of the optical transition and takes the value of 1/2, 2, 3/2, or 3 for the direct-allowed, indirect-allowed, direct-forbidden or indirect forbidden transitions, respectively. The [F(R_∞)hν]^1/n vs. hν plot shows the linear nature near the absorption edge for n = 2 as shown in Fig. 5. The band-gap energy (E_g) was evaluated by Wood–Tauc theory^28,29 as plotted in Fig. 5. The best linear relation of αhν ∝ (hν − E_g)ⁿ was determined as for n value of 2, indicating that this is an indirect allowed electronic transition. The band gap of the LSM nanopowders is calculated to be about 2.2 eV for La_0.8Sr_0.2MnO₃, 2.4 eV for La_0.5Sr_0.5MnO₃ and 2.6 eV for La_0.2Sr_0.8MnO₃ as shown in Fig. 5. Such a value of the band gap energy suggests that these compounds may be a potential candidate for new high frequency optoelectronic devices, since the materials having the band gap energy greater than 2 eV is of particular interest in the UV region of the spectrum.^30,31 These values were nearly similar to the previous published by Tanguturi et al.³² for Nd_0.7Sr_0.3MnO₃ thin film in which they were found to be 2.64 and 2.98 eV for deposited and annealed film. Cesaria et al.²⁷ have reported that the direct band gap energy of La_0.7Sr_0.3MnO₃ thin film deposited by pulsed laser deposition under different oxygen pressures was in the range between 2.44 to 3.5 eV. The change in band gap energy has been attributed to the change of Mn⁴⁺/Mn³⁺ ratio as the function of La/Sr ratio according to the chemical formula La_(1−x)³⁺Sr_x²⁺Mn_(1−x)³⁺Mn_x⁴⁺O₃. Moreover, the Mn valence also depend on the oxygen concentration, based on the requirement of charge neutrality within the LSMO unit cell La_(1−x)³⁺Sr_x²⁺Mn_(1−x+2δ)³⁺Mn_(x−2δ)⁴⁺O_(3−δ)²⁻.

Fig. 5 The optical band gap energy value of the LSM nanopowders synthesized using co-precipitation method with different Sr²⁺ ion molar ratios and annealed at 1000 °C for 2 h.

The results can be expressed on the basis of the band-structure states involved in LSMO optical response are the spin-unpolarized O 2p-states and the Mn d-like states which split into low-lying narrower t_2g and high-lying broader e_g sub bands. Each of these sub bands further split into up- and down-spin bands due to the exchange-interaction. The Mn e_g bands (i.e. the doublet e¹_g and e²_g) and the O 2p levels are involved in the so-called charge transfer transitions: the dipole allowed d–d transitions between different Mn sites (the inter-atomic charge transfer transitions Mn³⁺ (e¹_g) → Mn³⁺ (e²_g) and Mn³⁺ (e¹_g) → Mn⁴⁺ (e_g)) and the dipole allowed transition between an O 2p-like state and a Mn d-like (O 2p → Mn³⁺ (e²_g) and O 2p → Mn⁴⁺ (e_g)).²⁷

The energy-dependent absorbance spectra of LSM nanopowders prepared by co-precipitation method at calcination temperature 1000 °C for 2 h with different Sr²⁺ ion molar ratios obtained experimentally is presented in Fig. 6. The Sr²⁺ ions substitution of La are ionic size difference and electronegativity difference. Since the A site ions in ABO₃ structure are fully ionized, the electronegativity difference is of minor importance. Although local structural distortions may be sensitive to local ionic radii and their variations. Finally, possible effects due to integral charge states (Mn³⁺, Mn⁴⁺) will be averaged in E_g calculations. In the highly conducting regime where our treatment will be most realistic, fluctuations between the absorption spectra of LSM shows a peak at around 5.67 eV, which arises due to the transitions from the oxygen states at the uppermost valence band to Sr²⁺ states at the lower most conduction band. The rising trend of the experimental data for both LSM at the higher-energy side may show a tendency to give another absorption peak corresponding to the electronic transition from the semi-core states in the valence band to conduction band.

Fig. 6 Absorption spectra as a function of photon energy for LSM nanopowders prepared via co-precipitation method at calcination temperature of 1000 °C with different Sr²⁺ ion molar ratios (0.2, 0.5, and 0.8).

5. The refractive index

The refractive index of the samples is calculated using Moss relation³³

E_gn⁴ = 95 eV (1)

where E_g is the energy band gap. The refractive index of the samples is also calculated using a different relation presented by Herve and Vandamme as:³⁴where A and B are the numerical constants with values of 13.6 eV and 3.4 eV respectively. The refractive index was slightly decreased with increasing the Sr²⁺ ion content. The results can be discussed on the basis of the oxygen deficiency with enhanced of Sr²⁺ ion ratio. However, the values obtained were nearly similar to the given values of LSMO films grown on the various substrates in which the values were in the range of 2.06 to 2.46.³⁵

Otherwise, both static and high frequency dielectric constants are determined for all the samples. The high frequency dielectric constant (ε_α) is calculated using the relation:³³

ε_α = n² (3)

where ‘n’ is the refractive index.

The static dielectric constant (ε₀) of the samples is calculated using the relation:³³

ε₀ = 18.52 − 3.08E_g (4)

where E_g is the band gap energy. The calculated values of E_g, n, ε_α and ε₀ for different samples are presented in Table 1. Apparently, the values of ε_α and ε₀ were decreased with increasing the Sr²⁺ ion content. The results can be discussed on basis of polaron hopping mechanism, charge transported by the Mn³⁺–O–Mn⁴⁺ network. The partial replacement of La³⁺ by Sr²⁺ in LSM created polarons due to the conversion of Mn³⁺ to Mn⁴⁺ so as to achieve the electroneutrality. The concentration of polaron was increased with an increase in Sr content in LSM thereby the dielectric constant was decreased.³⁶

Table 1 Values of E_g, n, ε_α and ε₀ of lanthanum strontium manganite nanopowders prepared by the co-precipitation method at different Sr²⁺ ion molar ratios

Sample ID	Eg	n	εα	ε0	Crystallite size (nm)
LSM2	2.42	2.50	6.25	11.06	25
LSM5	2.62	2.45	6.00	10.45	38
LSM8	2.80	2.41	5.80	9.89	42

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6. Magnetic properties

The magnetic properties of the lanthanum strontium manganite La_1−xSr_xMnO₃ synthesized through co-precipitation method with different strontium ion contents (x = 0.2, 0.5 and 0.8) are presented in Fig. 7. From the hysteresis loops, the magnetic parameters such as saturation magnetization (M_s), remanent magnetization (M_r) and coercive field (H_c) are recorded in Table 2. Evidently, S-shaped loop was observed for low Sr²⁺ ion concentration which has the low saturation magnetization (M_s = 1.42 A m² kg⁻¹) and the high coercivity (H_c = 150.02 A m⁻¹). Otherwise, further increasing of the Sr²⁺ ion content to 0.5 was found to increase the saturation magnetization (M_s = 3.76 A m² kg⁻¹) and decrease the coercivity to 69.31 A m⁻¹. Meanwhile at high Sr²⁺ ion molar ratio, high saturation magnetization (5.2 A m² kg⁻¹) was accomplished whereas middle coercivity 104.2 A m⁻¹ was fulfilled. Plainly, the weak ferromagnetic as well as multiphase (ferromagnetic and paramagnetic) nature was observed for all tested samples. The results were agreement with the previous published result by Zhou et al.³⁷

Fig. 7 The ferromagnetic hysteresis curves for LSM nanopowders synthesized using co-precipitation method with different Sr²⁺ ion contents and annealed at 1000 °C for 2 h.

Table 2 Room temperature saturation magnetization (M_s), remnant magnetization, coercivity (H_c) and Bohr magneton for LSM nanopowders prepared by co-precipitation method

Sample ID	Ms (A m^2 kg^−1)	Mr (A m^2 kg^−1)	Hc (A m^−1)	Bohr magneton (A m^2 kg^−1)
La0.8Sr0.2MnO3	1.4252	0.0629	150.02	0.059
La0.5Sr0.5MnO3	3.7619	0.0216	69.318	0.145
La0.2Sr0.8MnO3	5.2772	0.0429	104.20	0.189

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However, the saturation magnetization was particle size dependent. It was found that the saturation magnetization was increased with increasing the particle size as the further increasing of Sr²⁺ ion content. For instance, the surface magnetic anisotropy originating from a magnetically disordered surface layer known as a dead or passivating layer existed in the nanoparticles is usually the evidence of the phenomenon. With decreasing particle sizes, the thickness of the passivating layer and the number of disordered spins increases, which are adverse to the ferromagnetic order, thus leading to the reduction of magnetization and the increase of H_c and M_r. However, the reduction of magnetic coercivity significantly depends on their magneto-crystalline anisotropy, micro-strain, inter-particle interaction, size and shape.^38–40

The magnetic moment per molecule (nB) was obtained as summarized in Table 2 using the empirical formula⁴⁰

where M_w is the molecular weight, M_s is the saturation magnetization and 5585 is the magnetic factor. It was demonstrated that the calculated nB values were increasing with increment of Sr²⁺ ion molar ratio.

7. Electrochemical impedance spectroscopy measurements

Electrochemical impedance spectroscopy (EIS) was also performed under open circuit voltage (OCV) conditions, in order to analyze the different limiting steps contributing to the cell polarization. An inductive loop is usually observed at low frequencies and can be attributed to the activation of a passive layer at the electrode surface.⁴¹ The analysis of the Nyquist diagrams is extremely complex.⁴² Impedance spectra of LSM electrodes correspond to at least two overlapping depressed arcs, one associated with bulk or surface diffusion and the other associated with surface exchange of oxygen and charge transfer.^43,44

Fig. 8 shows the Nyquist AC impedance plots of the LSM cell measured with different molar ratios of strontium. The highest frequency intercept on the real axis of Nyquist EIS spectra represented the total specific ohmic resistance (R_Ω), and the distance between the lowest and the highest frequency intercepts corresponded to the total area specific polarization resistance (R_p) from both anode and cathode. The LSM cell showed drastically increased R_p values from 3.10 to 5.21, and 8.56 Ω cm² at 0.2, 0.5, and 0.8 molar ratios of strontium ions. This suggests that the total area specific polarization resistance (R_p) from both anode and cathode was largely increased by increasing the molar ratio of strontium ions. Furthermore, this suggests a better interfacial contact between the electrolyte and cathode. The impedance spectra plotted in Fig. 7 were analyzed by fitting the data with the equivalent circuit shown in Scheme 1. In this scheme, L corresponds to an inductance, which is usually associated with the platinum current/voltage probes, or to the high-frequency phase shift of the electrochemical equipment; R_e is the ohmic resistance of the electrolyte which interpreted as the sum of grain and grain boundary resistance; (R₁, CPE₁) and (R₂, CPE₂) correspond to the high- and low-frequency arcs, respectively. The circuit may be described by the following equation: R_e(R₁Q₁)(R₂Q₂) where R is a resistance and Q the constant phase element (CPE).⁴¹

Fig. 8 Nyquist plots (Z′ vs. Z′′) of LSM prepared by the co-precipitation method and annealed at 1000 °C for 2 h with different Sr²⁺ ion molar ratios (0.2, 0.5 and 08).

Scheme 1 Schematic representation of the equivalent circuit employed to fit the impedance data.

D. Conclusions

La_xSr_1−xMnO₃ nanopowders with x = 0.2, 0.5 and 0.8 have been synthesized using co-precipitation method based on methyl amine as base for the first time. This technique gives more homogeneous and uniform structure than that obtained by the other methods as well as this method is simple, low cost and time saving. Obviously, the XRD profiles revealed that the pure monoclinic La_xSr_1−xMnO₃ phase could be obtained from the precipitated precursors at pH 12 by annealing at 1000 °C for 2 h. The Sr²⁺ ion content has significant effect on the surface morphology. SEM images indicated that tablet like structure was observed at low Sr²⁺ ion content. Clearly, the obtained powder was highly agglomerated and exhibits grains in nanometric size range. Meanwhile, the band gap energy was increased from 2.42 to 2.80 eV when increasing the Sr²⁺ ion content from 0.2 to 0.8. Of note, the refractive index n was slightly decreased from 2.50 to 2.41 with increasing the Sr²⁺ ion molar ratios. Prominently, the high frequency dielectric constant (ε_α) and the static dielectric constant (ε₀) were decreased with increasing the Sr²⁺ ion molar ratios. On other hand, the saturation magnetization (M_s) and the Bohr magneton were enhanced with further increasing the Sr²⁺ ion concentration. Overall, these materials could have applications in different fields such as cathode materials in solid oxide fuel cell.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27461c

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