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Dependence on sintering temperature of structure, optical and magnetic properties of La0.625Ca0.315Sr0.06MnO3 perovskite nanoparticles
Ceramics International 45 (2019) 17467–17475
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Dependence on sintering temperature of structure, optical and magnetic properties of La0.625Ca0.315Sr0.06MnO3 perovskite nanoparticles
T
Yang Liu, Tao Sun, Gang Dong, Shuai Zhang, Kaili Chu, Xingrui Pu, Hongjiang Li, Xiang Liu∗ School of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, Yunnan, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sintering temperature Nanoparticles Perovskite manganites Optical properties Oxygen vacancies
In this study, La0.625Ca0.315Sr0.06MnO3 (LCSMO) nanoparticles were prepared by facile sol-gel method at low crystallization temperatures. Various test methods were used to characterize structure, optical and magnetic properties of LCSMO nanoparticles. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) suggested complete crystallization of LCSMO nanoparticles sintered at 700 °C. In addition, unit cell volume and grain size increased with sintering temperature. Besides, X-ray photoemission spectroscopy (XPS) fitting results of Mn2p core level peaks confirmed the increase in Mn3+ ion concentration with sintering temperature, mainly attributed to formation of more oxygen vacancies. Raman microscopy and Fourier transform infrared spectrometry (FTIR) jointly depicted the existence of Mn–O bond, indicating that sintering temperature definitely impacted vibration mode of Mn–O and affected both crystal structure and performance. UV–vis optical band gap width of LCSMO nanoparticles sintered at 700 °C, 1000 °C, and 1500 °C decreased from 1.2 to 0.75 eV as sintering temperature increased, suggesting the semiconducting properties of nanoscale LCSMO particles. Magnetization dependent temperature (M-T) and magnetic field (M-H) measurements revealed degradation in magnetic properties of the specimens with temperature. Overall, LCSMO nanoparticles sintered at different sintering temperatures provided novel insights into properties of rare earth doped perovskite manganites.
1. Introduction Rare earth doped perovskite manganites have received increasing attention during the last several decades [1–8]. Manganite materials have widely been used in different electronic and magnetic devices due to their complex physical phenomena, such as colossal magnetoresistance (CMR) effect, ferromagnetic (FM) metallic phase, paramagnetic (PM) insulating phase, and metal-insulator (MI) transition [9–13]. These behaviors have made rare earth manganites fascinating and challenging materials. The ferromagnetic and metallic manganites states of manganite materials are often related to valence state of manganese ion (ratio of Mn3+ to Mn4+) [9,14], which can be dominated by oxygen stoichiometry. The double exchange (DE) effect between Mn3+ and Mn4+ plays an important role in transport and magnetic properties of manganites [15]. On the one hand, the crystalline size affects both transport and magnetic properties of manganites [16,17]. The grain size can be controlled by the sintering temperature, and increase in sintering temperature should promote grain growth [18,19]. Nowadays, studies dealing with perovskite manganites with nanoscale particle size has
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increased due to their interesting characteristics, such as large surface to volume ratios and surface effect [14,20]. For instance, nanoscale La1xCaxMnO3 (LCMO) and La1-xSrxMnO3 (LSMO) showed novel properties caused by finite size effect, which compare well to those of bulk materials [12,16,21]. However, despite many studies involving LCMO and LSMO systems, only a few have reported systems with coexisting Ca and Sr. On the other hand, the structural, optical and magnetic properties of manganite nanoparticles are related to both magnetic interactions and ion distributions of A-site (trivalent alkali metal) and B-site (divalent alkali metal) [22]. These properties are critical to better understand the behavior and application of manganite materials. So far, only few previously published studies have been focused on the optical properties of manganite nanoparticles since these materials are mostly metal conductors [14]. However, nanoscale LCSMO systems are expected to have different performances when co-doped with Ca and Sr through adjusted sintering temperatures. Various methods could be used to synthesize perovskite manganite nanoparticles, including the sol-gel route [23], solid state reaction [24], hydrothermal method [25], and molten salt synthesis [20]. In this
Corresponding author. E-mail address: [email protected] (X. Liu).
https://doi.org/10.1016/j.ceramint.2019.05.308 Received 24 May 2019; Received in revised form 27 May 2019; Accepted 28 May 2019 Available online 30 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. (a–b) XRD spectra of LCSMO ceramics sintered from 500 to 1500 °C. Rietveld refinement plots of LCSMO sample sintered at 700 °C (c) and 1000 °C (d). Table 1 The structure and refined parameters of the LCSMO ceramics sintered at 500–1500 °C. Temp.
500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C 1200 °C 1300 °C 1400 °C 1500 °C
Space group
Pnma Pnma Pnma Pnma Pnma Pnma Pnma Pnma Pnma Pnma Pnma
Lattice constant (Å) a
b
c
5.379 5.388 5.415 5.416 5.432 5.437 5.438 5.439 5.439 5.439 5.440
7.629 7.624 7.648 7.653 7.682 7.686 7.685 7.683 7.684 7.684 7.676
5.503 5.505 5.489 5.488 5.465 5.464 5.452 5.451 5.453 5.452 5.457
Cell volume (Å3)
dMn-O1(Å)
dMn-O2(Å)
θMn-O2-Mn (°)
θMn-O2-Mn (°)
Re (%)
Rp (%)
Rb (%)
χ
225.82 226.14 227.32 227.47 228.02 228.35 227.86 227.75 227.87 227.84 227.88
1.938 1.937 1.943 1.944 1.951 1.952 1.952 1.951 1.951 1.951 1.950
1.946 1.948 1.952 1.952 1.950 1.950 1.948 1.948 1.948 1.948 1.949
159.43 159.41 159.53 159.55 159.71 159.72 159.76 159.76 159.75 159.76 159.72
161.90 161.91 161.89 161.88 161.83 161.83 161.82 161.82 161.83 161.83 161.84
10.1 10.1 10.2 10.2 9.9 10.3 10.3 10.3 11.1 10.9 10.7
4.8 4.5 4.0 3.8 3.2 3.3 3.3 3.2 3.7 3.3 3.2
2.8 2.2 2.1 2.7 1.7 1.2 2.0 2.2 2.3 2.4 1.2
0.42 0.40 0.30 0.29 0.21 0.19 0.21 0.19 0.20 0.17 0.15
work, La0.625Ca0.315Sr0.06MnO3 (LCSMO) nanoparticles were synthesized by the sol-gel method due to its simplicity, low cost and moderate sintered temperature to obtain crystallized nanoparticles. To better understand the effect of temperature on LCSMO properties, the nanoparticles were sintered at various temperatures. The structural, optical and magnetic properties of LCSMO were then systematically characterized by different analytical instruments. The optical test results suggested LCSMO nanoparticles to own typical characteristics.
2. Materials and methods LCSMO nanoparticles were synthesized by the sol-gel process using analytical grade La(NO3)3·nH2O, Ca(NO3)2·4H2O, Sr(NO3)2 and Mn (NO3)2·4H2O as reactants. Briefly, the reactants were dissolved in 200 ml deionized water and then 40% molar citric acid and 15 ml ethylene glycol were added as chelating agent and polymerizing agent, respectively. Next, the obtained mixture was kept under magnetic stirring at 363 K until formation of a homogeneous brown gel, which then was evaporated in vacuum oven at 413 K for 12 h. The obtained
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Fig. 2. SEM graphs of LCSMO nanoparticles sintered at 500 °C (a), 700 °C (b), 1000 °C (c), 1300 °C (d) and 1500 °C (e).
Fig. 3. EDS mapping graphs and spectra of LCSMO nanoparticles sintered at 1000 °C.
xerogel was ground to yield a powder followed by calcination at 773 K for 10 h. The resulting pre-powder was ground again and sintered at various temperatures for 12 h to obtain nanoparticles. The obtained LCSMO specimens were analyzed by several analytical methods to determine the structure, optical, and magnetic properties. X-ray diffraction (XRD, BDX3200) with Cu Kα radiation was used for crystal phase identification of LCSMO nanoparticles, and Rietveld structure refinement was used to analyze XRD patterns. The microstructures, compositions and morphologies were characterized by scanning electron microscopy (SEM, SU8010), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM, FEI Tecnai G2 F20), respectively. The X-ray photoemission spectroscopy (XPS,
Thermo Fisher Scientific K-Alpha) was employed to reveal the chemical composition and Mn oxide states. The optical properties were evaluated by Fourier transform infrared spectrometry (FTIR, TENSOR 27), Raman microscopy (RENISHAW inVia), and UV–vis spectrophotometry (HITACHI U-4100). The magnetizations dependent on field and temperature were determined by vibrating sample magnetometer (VSM, Quantum).
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Fig. 4. TEM and HRTEM images of LCSMO nanoparticles sintered at 700 °C (a–b), 1000 °C (c–d) and 1500 °C (e–f). Particle size statistic of TEM images for 700 °C (g) and 1000 °C (h).
3. Results and analysis 3.1. Structural analysis Fig. 1(a and b) show the XRD patterns of LCSMO nanoparticles sintered at temperatures ranging from 500 to 1500 °C. The full width at half maximum (FWHM) of XRD diffraction main peak (200) decreased with Ts, indicating optimization of crystallinity by improving Ts. At Ts below 700 °C, the characteristic peak of LCSMO nanoparticles appeared extremely weak but became sharp above 700 °C. These features suggested well crystallization of LCSMO nanoparticles at low temperature of 700 °C. The Rietveld refinement plots of LCSMO nanoparticles sintered at 700 °C and 1000 °C are presented in Fig. 1(c and d), and both crystal structure and refinement parameters of all specimens are listed in Table 1. As Ts rose, lattice constant a, b and unit cell volume (V) all increased and consistent with reported literature [26,27]. This was due
to slight distortion inside cell unit caused by the increased temperature. From refinement plots and parameters (viz. Re, Rp, Rb, and χ), the diffraction peaks of LCSMO nanoparticles matched well with standard PDF card no. 46–0513, further suggesting that all LCSMO specimens were crystallized in orthorhombic structure with space group of Pnma. Fig. 2(a–e) illustrate the SEM micrographs of LCSMO nanoparticles sintered at 500 °C, 700 °C, 1000 °C, 1300 °C, and 1500 °C. As sintering temperature increased, the grain size gradually increased from nanometer to micrometer. The particles in Fig. 2(a) looked agglomerated together without obvious morphology due to poor crystallinity of the grains at low sintering temperature (500 °C). At 700 °C, the crystal grains became completely crystallized, forming uniform and independent nanoparticles (Fig. 2(b)). By comparison, LCSMO specimen sintered at 1500 °C illustrated larger and inhomogeneous grains (Fig. 2(e)), indicating over-sintering. The crystallization results from SEM agreed well with those from XRD.
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Fig. 5. (a) XPS for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. High resolution core spectra of (b) O1s region, (c) Mn2p region for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. Fitting curves of Mn2p region for LCSMO nanoparticles sintered at 700 °C (d), 1000 °C (e) and 1500 °C (f).
To gain a better understanding of the distribution of surface individual elements, EDS analyses of LCSMO nanoparticles sintered at 1000 °C were collected and the results are gathered in Fig. 3. The elemental mappings confirmed the presence of La, Ca, Sr, Mn and O elements on the surface with even distribution in the nanoparticles. The elemental mass percentages of La, Ca and Sr determined from the energy spectra were 42.8%, 7.5% and 2.8%, respectively. These values
were consistent with the theoretical 41.8%, 6.1% and 2.5%, respectively. Hence, the amounts of La, Ca and Sr elements obeyed to the stoichiometric proportions used during the experimental design. To further assess the degree of aggregation and morphology, TEM and HRTEM images of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C were obtained. In Fig. 4(a–f), the particles sintered at 700 °C looked well separated from other particles and with uniform size
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Table 2 The fitting parameters of Mn2p region for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. Tempt.
700 °C 1000 °C 1500 °C
Mn2p3/2
Mn3+ concentration
Mn2p1/2
Mn3+
Mn4+
Mn3+
Mn4+
642.13 eV 641.93 eV 641.73 eV
643.88 eV 643.58 eV 643.88 eV
653.73 eV 653.73 eV 653.38 eV
655.43 eV 655.83 eV 655.43 eV
the corresponding lattice planes were compared with standard PDF card (Fig. 4(b). The inner-planar spacings were calculated as 0.272 nm, 0.273 nm and 0.256 nm, and attributed to (121), (002) and (210) planes, respectively. 3.2. Spectral analysis
62.8% 65.8% 67.7%
Fig. 6. Raman spectra of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C.
Fig. 7. FTTR spectra of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C.
similar to those sintered at 1000 °C. The particle sizes of the specimens sintered at 700 °C and 1000 °C were counted using a Nano Measurer and the columnar distribution is shown in Fig. 4(g and h). The average particle size obviously increased from ∼14 nm (700 °C) to ∼36 nm (1000 °C). The TEM images further demonstrated that particle size rose with sintering temperature, further confirming the oversizing of particles sintered at 1500 °C. The HRTEM images showed clear lattice fringes. The inner-planar spacings of lattice fringes were measured and
To investigate the electronic structures, XPS valence spectra were obtained and the results are shown in Fig. 5(a–c). The survey scans of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C confirmed the presence of La, Ca, Sr, Mn and O elements in LCSMO nanoparticles (Fig. 5(a)). The O1s core level peaks are depicted in Fig. 5(b). The peak at 528.7 eV was ascribed to presence of crystal lattice oxygen and that at 530.7 eV was associated with contribution of adsorbed oxygen [14]. The XPS peak intensity of crystal lattice oxygen obviously decreased as Ts rose, suggesting reduction in lattice oxygen and formation of more oxygen vacancies. The lattice oxygen reduction may be due to the elevated sintering temperature, leading the oxygen on the surface of the crystal matrix was oxidized to O− [28]. The Mn2p spectra exhibited two distinct peaks at 641.2 eV for Mn2p3/2 and 652.4 eV for Mn2p1/2 (Fig. 5(c)). The fittings of Mn2p sintered at 700 °C, 1000 °C and 1500 °C that could provide the oxidation states of Mn atom are displayed in Fig. 5(d–f). Table 2 collected the binding energies of deconvoluted peaks at Mn2p3/2 and Mn2p1/2, relating to Mn3+ and Mn4+, respectively [28,29]. From the fitting parameters of Mn2p (Table 2), Mn3+ ions concentration obviously increased from 62.8% (700 °C) to 67.7% (1500 °C) with Ts, and attributed to formation of increased numbers of oxygen vacancies. The electrical neutrality suggested that increase in oxygen vacancies induced a decrease in negative ions levels, thereby increasing the concentration of Mn3+ ions. To gain a better understanding of lattice distortion, the Raman spectra of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C were investigated. Numerous studies showed 24 active Raman modes (7Ag + 7B1g + 5B2g + 5B3g) for orthorhombic manganites [14,30,31]. As shown in Fig. 6, a main symmetrical feature peak was observed at 650 cm−1, which was assigned to B2g (1) mode in LCSMO nanoparticles sintered at 700 °C and 1000 °C. This peak was linked to symmetric stretching vibration of Mn–O bond in MnO6 octahedron [30,32–34]. The uniformity of the Raman peaks for specimens sintered at 700 °C and 1000 °C indicated slight lattice distortion, consistent with the XRD data. As sintering temperature rose to 1500 °C, the peak for the symmetric stretching vibration of Mn–O bond vanished. Hence, excess sintering temperature weakened the activity of MnO6 octahedron vibrations [35]. To investigate the phase composition and purity, FTIR spectra of LCSMO nanoparticles were recorded and the data are plotted in Fig. 7. A total of five characteristic peaks were shown in the infrared spectrum. The intensities of these peaks became weaker as sintering temperature increased. The stretching mode of Mn–O bond could clearly be identified at 621 cm−1 [30,31,36]. The decrease in intensity of Mn–O characteristic peak with Ts may be due to elongation of Mn–O bond length, which can be seen from the structural refinement parameters (Table 1). The peak at 872 cm−1 was described to out-of-plane bending mode of C–O bond [37,38]. The extra intensive absorption peak at 1398 cm−1 was associated with asymmetric stretching vibrations of C]C bond [14], formed during pre-sintering. The band at 1641 cm−1 was assigned to the symmetric stretching mode of C]O bond [14,39]. The broad peak at 3418 cm−1 was related to stretching vibration of hydroxyl groups [39,40]. All bending and stretching modes appeared in FTIR spectra, suggesting formation of perovskite structure in LCSMO nanoparticles, and further confirming the XRD results. To study the optical band gap of LCSMO nanoparticles, UV–vis absorption spectra were recorded and the results are shown in Fig. 8(a). The specimens sintered at 700 °C and 1000 °C depicted sharp absorption peak around 370 nm. By contrast, broad absorption peak was observed around 660 nm for the specimen sintered at 1500 °C. The optical
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Fig. 8. (a) Ultraviolet–visible spectra and (b)Variation of (αhν)2 versus photon energy (hν) for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C.
3.3. Magnetic properties
Fig. 9. Field cooled temperature dependence of magnetizations for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. Inset shows the –dM/dT versus temperature.
Fig. 9 summarizes the magnetization dependent temperature (M-T) of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C measured from 400 to 2 K under a magnetic field of 100 Oe. The M-T curves of all specimens showed strong ferromagnetic-paramagnetic (FM-PM) transitions. The magnetic moments at 2 K of specimens sintered at 700 °C, 1000 °C and 1500 °C were estimated to 14.46 emu/g, 13.71 emu/g and 9.06 emu/g, respectively. The inset illustrates the variation of –dM/dT as a function of temperature. The corresponding Curie temperature (TC) values were recorded as 275 K, 295 K and 290 K. The changes in magnetic moment and TC indicated the influence of particle size on magnetic moments and Curie temperature. The magnetization dependent magnetic field (M-H) of LCSMO nanoparticles and the corresponding local enlarged M-H hysteresis loops are gathered in Fig. 10(a–d). All M-H hysteresis loops measured by VSM at 2 K from −7 to 7 T exhibited ferromagnetic behaviors. The characteristic parameters obtained from the local enlarged M-H hysteresis loops included the saturation magnetization (Ms), remanent magnetization (Mr), and coercive field (Hc). LCSMO nanoparticles sintered at (700 °C, 1000 °C, and 1500 °C) exhibited Ms values of (95.0 emu/g, 92.1 emu/g, and 65.5 emu/g), Mr values of (22.9 emu/g, 17.6 emu/g, and 7.0 emu/g) and Hc of (314 Oe, 285 Oe, and 110 Oe), respectively. Hence, increase in particle size led to reduced magnetic response.
absorption edge was analyzed according to Eq. (1):
αhν ∝ A(hν − Eg )n
(1)
where α is the absorption coefficient, h is the Planck constant, ν represents the frequency of incident light, A is an optical constant, and n equals to 1/2 or 2 for direct or indirect transition process, respectively. Fig. 8(b) depicts the relationship between (αhν)2 and hν of LCSMO nanoparticles sintered at 700 °C, 1000 °C, and 1500 °C. These properties agreed well with direct transition type materials [14,31,41,42]. The band gaps (viz. intercepts of tangent line on hν axis) of these specimens were recorded as 1.2 eV, 1.0 eV and 0.75 eV, respectively. This suggested transition of the materials to conductor type as Ts increased. The decrease in band gap and broadening of absorption peaks with Ts were ascribed to enlarged particle size, as confirmed by SEM and TEM results.
4. Conclusions The phase formations, elemental distributions and crystal structures of LCSMO nanoparticles synthesized by sol-gel method were analyzed by various analytical methods. XRD and Rietveld refinement showed crystallization of all prepared LCSMO nanoparticles in the orthorhombic structure (Pnma space group). No structural phase transition was observed as Ts increased. SEM and TEM confirmed the nanocrystal structures and indicated that particle size increased with sintering temperature. Both EDS and XPS illustrated the presence of La, Ca, Sr, Mn and O elements in the prepared specimens. The XPS patterns exhibited presence of Mn atom as trivalent and tetravalent, and the number of oxygen vacancies increased with sintering temperature. This, in turn, led to incremented Mn3+ ion concentrations. The Raman spectra of LCSMO nanoparticles confirmed the symmetric oxygen stretching mode (B2g (1)) in MnO6 octahedron. FTIR absorption further
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Fig. 10. (a) M-H hysteresis loops of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. (b), (c) and (d) are the local enlarged M-H hysteresis loops for 700 °C, 1000 °C and 1500 °C.
confirmed the existence of stretching vibration mode of Mn–O bond around 621 cm−1, suggesting formation of perovskite structure in LCSMO nanoparticles. The UV–vis measurements showed that the increase in particle size with Ts had decreased the band gap (1.2–0.75 eV) and broadened the absorption peaks. Strong ferromagnetic-paramagnetic (FM-PM) transition and ferromagnetic behaviors were confirmed by magnetic measurements. The increase in sintering temperature led to enhanced TC (275–290 K) and decrease in Ms (95.0–65.5 emu/g), Mr (22.9–7.0 emu/g) and Hc (314–110 Oe). Therefore, the sintering temperature significantly impacted the magnetic properties of obtained LCSMO nanoparticles.
[10]
[11]
[12]
[13]
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Acknowledgements [15]
This work was financially supported by the National Natural Science Foundation of China (Grant no. 11674135), and the Analysis and Testing Foundation of Kunming University of Science and Technology (Grant no. 2018M20172130004).
[16]
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Dependence on sintering temperature of structure, optical and magnetic properties of La0.625Ca0.315Sr0.06MnO3 perovskite nanoparticles
T
Yang Liu, Tao Sun, Gang Dong, Shuai Zhang, Kaili Chu, Xingrui Pu, Hongjiang Li, Xiang Liu∗ School of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, Yunnan, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sintering temperature Nanoparticles Perovskite manganites Optical properties Oxygen vacancies
In this study, La0.625Ca0.315Sr0.06MnO3 (LCSMO) nanoparticles were prepared by facile sol-gel method at low crystallization temperatures. Various test methods were used to characterize structure, optical and magnetic properties of LCSMO nanoparticles. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) suggested complete crystallization of LCSMO nanoparticles sintered at 700 °C. In addition, unit cell volume and grain size increased with sintering temperature. Besides, X-ray photoemission spectroscopy (XPS) fitting results of Mn2p core level peaks confirmed the increase in Mn3+ ion concentration with sintering temperature, mainly attributed to formation of more oxygen vacancies. Raman microscopy and Fourier transform infrared spectrometry (FTIR) jointly depicted the existence of Mn–O bond, indicating that sintering temperature definitely impacted vibration mode of Mn–O and affected both crystal structure and performance. UV–vis optical band gap width of LCSMO nanoparticles sintered at 700 °C, 1000 °C, and 1500 °C decreased from 1.2 to 0.75 eV as sintering temperature increased, suggesting the semiconducting properties of nanoscale LCSMO particles. Magnetization dependent temperature (M-T) and magnetic field (M-H) measurements revealed degradation in magnetic properties of the specimens with temperature. Overall, LCSMO nanoparticles sintered at different sintering temperatures provided novel insights into properties of rare earth doped perovskite manganites.
1. Introduction Rare earth doped perovskite manganites have received increasing attention during the last several decades [1–8]. Manganite materials have widely been used in different electronic and magnetic devices due to their complex physical phenomena, such as colossal magnetoresistance (CMR) effect, ferromagnetic (FM) metallic phase, paramagnetic (PM) insulating phase, and metal-insulator (MI) transition [9–13]. These behaviors have made rare earth manganites fascinating and challenging materials. The ferromagnetic and metallic manganites states of manganite materials are often related to valence state of manganese ion (ratio of Mn3+ to Mn4+) [9,14], which can be dominated by oxygen stoichiometry. The double exchange (DE) effect between Mn3+ and Mn4+ plays an important role in transport and magnetic properties of manganites [15]. On the one hand, the crystalline size affects both transport and magnetic properties of manganites [16,17]. The grain size can be controlled by the sintering temperature, and increase in sintering temperature should promote grain growth [18,19]. Nowadays, studies dealing with perovskite manganites with nanoscale particle size has
∗
increased due to their interesting characteristics, such as large surface to volume ratios and surface effect [14,20]. For instance, nanoscale La1xCaxMnO3 (LCMO) and La1-xSrxMnO3 (LSMO) showed novel properties caused by finite size effect, which compare well to those of bulk materials [12,16,21]. However, despite many studies involving LCMO and LSMO systems, only a few have reported systems with coexisting Ca and Sr. On the other hand, the structural, optical and magnetic properties of manganite nanoparticles are related to both magnetic interactions and ion distributions of A-site (trivalent alkali metal) and B-site (divalent alkali metal) [22]. These properties are critical to better understand the behavior and application of manganite materials. So far, only few previously published studies have been focused on the optical properties of manganite nanoparticles since these materials are mostly metal conductors [14]. However, nanoscale LCSMO systems are expected to have different performances when co-doped with Ca and Sr through adjusted sintering temperatures. Various methods could be used to synthesize perovskite manganite nanoparticles, including the sol-gel route [23], solid state reaction [24], hydrothermal method [25], and molten salt synthesis [20]. In this
Corresponding author. E-mail address: [email protected] (X. Liu).
https://doi.org/10.1016/j.ceramint.2019.05.308 Received 24 May 2019; Received in revised form 27 May 2019; Accepted 28 May 2019 Available online 30 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. (a–b) XRD spectra of LCSMO ceramics sintered from 500 to 1500 °C. Rietveld refinement plots of LCSMO sample sintered at 700 °C (c) and 1000 °C (d). Table 1 The structure and refined parameters of the LCSMO ceramics sintered at 500–1500 °C. Temp.
500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C 1200 °C 1300 °C 1400 °C 1500 °C
Space group
Pnma Pnma Pnma Pnma Pnma Pnma Pnma Pnma Pnma Pnma Pnma
Lattice constant (Å) a
b
c
5.379 5.388 5.415 5.416 5.432 5.437 5.438 5.439 5.439 5.439 5.440
7.629 7.624 7.648 7.653 7.682 7.686 7.685 7.683 7.684 7.684 7.676
5.503 5.505 5.489 5.488 5.465 5.464 5.452 5.451 5.453 5.452 5.457
Cell volume (Å3)
dMn-O1(Å)
dMn-O2(Å)
θMn-O2-Mn (°)
θMn-O2-Mn (°)
Re (%)
Rp (%)
Rb (%)
χ
225.82 226.14 227.32 227.47 228.02 228.35 227.86 227.75 227.87 227.84 227.88
1.938 1.937 1.943 1.944 1.951 1.952 1.952 1.951 1.951 1.951 1.950
1.946 1.948 1.952 1.952 1.950 1.950 1.948 1.948 1.948 1.948 1.949
159.43 159.41 159.53 159.55 159.71 159.72 159.76 159.76 159.75 159.76 159.72
161.90 161.91 161.89 161.88 161.83 161.83 161.82 161.82 161.83 161.83 161.84
10.1 10.1 10.2 10.2 9.9 10.3 10.3 10.3 11.1 10.9 10.7
4.8 4.5 4.0 3.8 3.2 3.3 3.3 3.2 3.7 3.3 3.2
2.8 2.2 2.1 2.7 1.7 1.2 2.0 2.2 2.3 2.4 1.2
0.42 0.40 0.30 0.29 0.21 0.19 0.21 0.19 0.20 0.17 0.15
work, La0.625Ca0.315Sr0.06MnO3 (LCSMO) nanoparticles were synthesized by the sol-gel method due to its simplicity, low cost and moderate sintered temperature to obtain crystallized nanoparticles. To better understand the effect of temperature on LCSMO properties, the nanoparticles were sintered at various temperatures. The structural, optical and magnetic properties of LCSMO were then systematically characterized by different analytical instruments. The optical test results suggested LCSMO nanoparticles to own typical characteristics.
2. Materials and methods LCSMO nanoparticles were synthesized by the sol-gel process using analytical grade La(NO3)3·nH2O, Ca(NO3)2·4H2O, Sr(NO3)2 and Mn (NO3)2·4H2O as reactants. Briefly, the reactants were dissolved in 200 ml deionized water and then 40% molar citric acid and 15 ml ethylene glycol were added as chelating agent and polymerizing agent, respectively. Next, the obtained mixture was kept under magnetic stirring at 363 K until formation of a homogeneous brown gel, which then was evaporated in vacuum oven at 413 K for 12 h. The obtained
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Fig. 2. SEM graphs of LCSMO nanoparticles sintered at 500 °C (a), 700 °C (b), 1000 °C (c), 1300 °C (d) and 1500 °C (e).
Fig. 3. EDS mapping graphs and spectra of LCSMO nanoparticles sintered at 1000 °C.
xerogel was ground to yield a powder followed by calcination at 773 K for 10 h. The resulting pre-powder was ground again and sintered at various temperatures for 12 h to obtain nanoparticles. The obtained LCSMO specimens were analyzed by several analytical methods to determine the structure, optical, and magnetic properties. X-ray diffraction (XRD, BDX3200) with Cu Kα radiation was used for crystal phase identification of LCSMO nanoparticles, and Rietveld structure refinement was used to analyze XRD patterns. The microstructures, compositions and morphologies were characterized by scanning electron microscopy (SEM, SU8010), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM, FEI Tecnai G2 F20), respectively. The X-ray photoemission spectroscopy (XPS,
Thermo Fisher Scientific K-Alpha) was employed to reveal the chemical composition and Mn oxide states. The optical properties were evaluated by Fourier transform infrared spectrometry (FTIR, TENSOR 27), Raman microscopy (RENISHAW inVia), and UV–vis spectrophotometry (HITACHI U-4100). The magnetizations dependent on field and temperature were determined by vibrating sample magnetometer (VSM, Quantum).
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Fig. 4. TEM and HRTEM images of LCSMO nanoparticles sintered at 700 °C (a–b), 1000 °C (c–d) and 1500 °C (e–f). Particle size statistic of TEM images for 700 °C (g) and 1000 °C (h).
3. Results and analysis 3.1. Structural analysis Fig. 1(a and b) show the XRD patterns of LCSMO nanoparticles sintered at temperatures ranging from 500 to 1500 °C. The full width at half maximum (FWHM) of XRD diffraction main peak (200) decreased with Ts, indicating optimization of crystallinity by improving Ts. At Ts below 700 °C, the characteristic peak of LCSMO nanoparticles appeared extremely weak but became sharp above 700 °C. These features suggested well crystallization of LCSMO nanoparticles at low temperature of 700 °C. The Rietveld refinement plots of LCSMO nanoparticles sintered at 700 °C and 1000 °C are presented in Fig. 1(c and d), and both crystal structure and refinement parameters of all specimens are listed in Table 1. As Ts rose, lattice constant a, b and unit cell volume (V) all increased and consistent with reported literature [26,27]. This was due
to slight distortion inside cell unit caused by the increased temperature. From refinement plots and parameters (viz. Re, Rp, Rb, and χ), the diffraction peaks of LCSMO nanoparticles matched well with standard PDF card no. 46–0513, further suggesting that all LCSMO specimens were crystallized in orthorhombic structure with space group of Pnma. Fig. 2(a–e) illustrate the SEM micrographs of LCSMO nanoparticles sintered at 500 °C, 700 °C, 1000 °C, 1300 °C, and 1500 °C. As sintering temperature increased, the grain size gradually increased from nanometer to micrometer. The particles in Fig. 2(a) looked agglomerated together without obvious morphology due to poor crystallinity of the grains at low sintering temperature (500 °C). At 700 °C, the crystal grains became completely crystallized, forming uniform and independent nanoparticles (Fig. 2(b)). By comparison, LCSMO specimen sintered at 1500 °C illustrated larger and inhomogeneous grains (Fig. 2(e)), indicating over-sintering. The crystallization results from SEM agreed well with those from XRD.
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Fig. 5. (a) XPS for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. High resolution core spectra of (b) O1s region, (c) Mn2p region for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. Fitting curves of Mn2p region for LCSMO nanoparticles sintered at 700 °C (d), 1000 °C (e) and 1500 °C (f).
To gain a better understanding of the distribution of surface individual elements, EDS analyses of LCSMO nanoparticles sintered at 1000 °C were collected and the results are gathered in Fig. 3. The elemental mappings confirmed the presence of La, Ca, Sr, Mn and O elements on the surface with even distribution in the nanoparticles. The elemental mass percentages of La, Ca and Sr determined from the energy spectra were 42.8%, 7.5% and 2.8%, respectively. These values
were consistent with the theoretical 41.8%, 6.1% and 2.5%, respectively. Hence, the amounts of La, Ca and Sr elements obeyed to the stoichiometric proportions used during the experimental design. To further assess the degree of aggregation and morphology, TEM and HRTEM images of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C were obtained. In Fig. 4(a–f), the particles sintered at 700 °C looked well separated from other particles and with uniform size
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Table 2 The fitting parameters of Mn2p region for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. Tempt.
700 °C 1000 °C 1500 °C
Mn2p3/2
Mn3+ concentration
Mn2p1/2
Mn3+
Mn4+
Mn3+
Mn4+
642.13 eV 641.93 eV 641.73 eV
643.88 eV 643.58 eV 643.88 eV
653.73 eV 653.73 eV 653.38 eV
655.43 eV 655.83 eV 655.43 eV
the corresponding lattice planes were compared with standard PDF card (Fig. 4(b). The inner-planar spacings were calculated as 0.272 nm, 0.273 nm and 0.256 nm, and attributed to (121), (002) and (210) planes, respectively. 3.2. Spectral analysis
62.8% 65.8% 67.7%
Fig. 6. Raman spectra of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C.
Fig. 7. FTTR spectra of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C.
similar to those sintered at 1000 °C. The particle sizes of the specimens sintered at 700 °C and 1000 °C were counted using a Nano Measurer and the columnar distribution is shown in Fig. 4(g and h). The average particle size obviously increased from ∼14 nm (700 °C) to ∼36 nm (1000 °C). The TEM images further demonstrated that particle size rose with sintering temperature, further confirming the oversizing of particles sintered at 1500 °C. The HRTEM images showed clear lattice fringes. The inner-planar spacings of lattice fringes were measured and
To investigate the electronic structures, XPS valence spectra were obtained and the results are shown in Fig. 5(a–c). The survey scans of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C confirmed the presence of La, Ca, Sr, Mn and O elements in LCSMO nanoparticles (Fig. 5(a)). The O1s core level peaks are depicted in Fig. 5(b). The peak at 528.7 eV was ascribed to presence of crystal lattice oxygen and that at 530.7 eV was associated with contribution of adsorbed oxygen [14]. The XPS peak intensity of crystal lattice oxygen obviously decreased as Ts rose, suggesting reduction in lattice oxygen and formation of more oxygen vacancies. The lattice oxygen reduction may be due to the elevated sintering temperature, leading the oxygen on the surface of the crystal matrix was oxidized to O− [28]. The Mn2p spectra exhibited two distinct peaks at 641.2 eV for Mn2p3/2 and 652.4 eV for Mn2p1/2 (Fig. 5(c)). The fittings of Mn2p sintered at 700 °C, 1000 °C and 1500 °C that could provide the oxidation states of Mn atom are displayed in Fig. 5(d–f). Table 2 collected the binding energies of deconvoluted peaks at Mn2p3/2 and Mn2p1/2, relating to Mn3+ and Mn4+, respectively [28,29]. From the fitting parameters of Mn2p (Table 2), Mn3+ ions concentration obviously increased from 62.8% (700 °C) to 67.7% (1500 °C) with Ts, and attributed to formation of increased numbers of oxygen vacancies. The electrical neutrality suggested that increase in oxygen vacancies induced a decrease in negative ions levels, thereby increasing the concentration of Mn3+ ions. To gain a better understanding of lattice distortion, the Raman spectra of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C were investigated. Numerous studies showed 24 active Raman modes (7Ag + 7B1g + 5B2g + 5B3g) for orthorhombic manganites [14,30,31]. As shown in Fig. 6, a main symmetrical feature peak was observed at 650 cm−1, which was assigned to B2g (1) mode in LCSMO nanoparticles sintered at 700 °C and 1000 °C. This peak was linked to symmetric stretching vibration of Mn–O bond in MnO6 octahedron [30,32–34]. The uniformity of the Raman peaks for specimens sintered at 700 °C and 1000 °C indicated slight lattice distortion, consistent with the XRD data. As sintering temperature rose to 1500 °C, the peak for the symmetric stretching vibration of Mn–O bond vanished. Hence, excess sintering temperature weakened the activity of MnO6 octahedron vibrations [35]. To investigate the phase composition and purity, FTIR spectra of LCSMO nanoparticles were recorded and the data are plotted in Fig. 7. A total of five characteristic peaks were shown in the infrared spectrum. The intensities of these peaks became weaker as sintering temperature increased. The stretching mode of Mn–O bond could clearly be identified at 621 cm−1 [30,31,36]. The decrease in intensity of Mn–O characteristic peak with Ts may be due to elongation of Mn–O bond length, which can be seen from the structural refinement parameters (Table 1). The peak at 872 cm−1 was described to out-of-plane bending mode of C–O bond [37,38]. The extra intensive absorption peak at 1398 cm−1 was associated with asymmetric stretching vibrations of C]C bond [14], formed during pre-sintering. The band at 1641 cm−1 was assigned to the symmetric stretching mode of C]O bond [14,39]. The broad peak at 3418 cm−1 was related to stretching vibration of hydroxyl groups [39,40]. All bending and stretching modes appeared in FTIR spectra, suggesting formation of perovskite structure in LCSMO nanoparticles, and further confirming the XRD results. To study the optical band gap of LCSMO nanoparticles, UV–vis absorption spectra were recorded and the results are shown in Fig. 8(a). The specimens sintered at 700 °C and 1000 °C depicted sharp absorption peak around 370 nm. By contrast, broad absorption peak was observed around 660 nm for the specimen sintered at 1500 °C. The optical
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Fig. 8. (a) Ultraviolet–visible spectra and (b)Variation of (αhν)2 versus photon energy (hν) for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C.
3.3. Magnetic properties
Fig. 9. Field cooled temperature dependence of magnetizations for LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. Inset shows the –dM/dT versus temperature.
Fig. 9 summarizes the magnetization dependent temperature (M-T) of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C measured from 400 to 2 K under a magnetic field of 100 Oe. The M-T curves of all specimens showed strong ferromagnetic-paramagnetic (FM-PM) transitions. The magnetic moments at 2 K of specimens sintered at 700 °C, 1000 °C and 1500 °C were estimated to 14.46 emu/g, 13.71 emu/g and 9.06 emu/g, respectively. The inset illustrates the variation of –dM/dT as a function of temperature. The corresponding Curie temperature (TC) values were recorded as 275 K, 295 K and 290 K. The changes in magnetic moment and TC indicated the influence of particle size on magnetic moments and Curie temperature. The magnetization dependent magnetic field (M-H) of LCSMO nanoparticles and the corresponding local enlarged M-H hysteresis loops are gathered in Fig. 10(a–d). All M-H hysteresis loops measured by VSM at 2 K from −7 to 7 T exhibited ferromagnetic behaviors. The characteristic parameters obtained from the local enlarged M-H hysteresis loops included the saturation magnetization (Ms), remanent magnetization (Mr), and coercive field (Hc). LCSMO nanoparticles sintered at (700 °C, 1000 °C, and 1500 °C) exhibited Ms values of (95.0 emu/g, 92.1 emu/g, and 65.5 emu/g), Mr values of (22.9 emu/g, 17.6 emu/g, and 7.0 emu/g) and Hc of (314 Oe, 285 Oe, and 110 Oe), respectively. Hence, increase in particle size led to reduced magnetic response.
absorption edge was analyzed according to Eq. (1):
αhν ∝ A(hν − Eg )n
(1)
where α is the absorption coefficient, h is the Planck constant, ν represents the frequency of incident light, A is an optical constant, and n equals to 1/2 or 2 for direct or indirect transition process, respectively. Fig. 8(b) depicts the relationship between (αhν)2 and hν of LCSMO nanoparticles sintered at 700 °C, 1000 °C, and 1500 °C. These properties agreed well with direct transition type materials [14,31,41,42]. The band gaps (viz. intercepts of tangent line on hν axis) of these specimens were recorded as 1.2 eV, 1.0 eV and 0.75 eV, respectively. This suggested transition of the materials to conductor type as Ts increased. The decrease in band gap and broadening of absorption peaks with Ts were ascribed to enlarged particle size, as confirmed by SEM and TEM results.
4. Conclusions The phase formations, elemental distributions and crystal structures of LCSMO nanoparticles synthesized by sol-gel method were analyzed by various analytical methods. XRD and Rietveld refinement showed crystallization of all prepared LCSMO nanoparticles in the orthorhombic structure (Pnma space group). No structural phase transition was observed as Ts increased. SEM and TEM confirmed the nanocrystal structures and indicated that particle size increased with sintering temperature. Both EDS and XPS illustrated the presence of La, Ca, Sr, Mn and O elements in the prepared specimens. The XPS patterns exhibited presence of Mn atom as trivalent and tetravalent, and the number of oxygen vacancies increased with sintering temperature. This, in turn, led to incremented Mn3+ ion concentrations. The Raman spectra of LCSMO nanoparticles confirmed the symmetric oxygen stretching mode (B2g (1)) in MnO6 octahedron. FTIR absorption further
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Fig. 10. (a) M-H hysteresis loops of LCSMO nanoparticles sintered at 700 °C, 1000 °C and 1500 °C. (b), (c) and (d) are the local enlarged M-H hysteresis loops for 700 °C, 1000 °C and 1500 °C.
confirmed the existence of stretching vibration mode of Mn–O bond around 621 cm−1, suggesting formation of perovskite structure in LCSMO nanoparticles. The UV–vis measurements showed that the increase in particle size with Ts had decreased the band gap (1.2–0.75 eV) and broadened the absorption peaks. Strong ferromagnetic-paramagnetic (FM-PM) transition and ferromagnetic behaviors were confirmed by magnetic measurements. The increase in sintering temperature led to enhanced TC (275–290 K) and decrease in Ms (95.0–65.5 emu/g), Mr (22.9–7.0 emu/g) and Hc (314–110 Oe). Therefore, the sintering temperature significantly impacted the magnetic properties of obtained LCSMO nanoparticles.
[10]
[11]
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[13]
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Acknowledgements [15]
This work was financially supported by the National Natural Science Foundation of China (Grant no. 11674135), and the Analysis and Testing Foundation of Kunming University of Science and Technology (Grant no. 2018M20172130004).
[16]
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