Microstructural, Magnetic, and Optical Properties of Pr-Doped Perovskite Manganite La0.67Ca0.33MnO3 Nanoparticles Synthesized via Sol-Gel Process

We report on microstructural, magnetic, and optical properties of Pr-doped perovskite manganite (La1 − xPrx)0.67Ca0.33MnO3 (LPCMO, x = 0.0–0.5) nanoparticles synthesized via sol-gel process. Structural characterizations (X-ray and electron diffraction patterns, (high resolution) TEM images) provide information regarding the phase formation and the single-crystalline nature of the LPCMO systems. X-ray and electron diffraction patterns reveal that all the LPCMO samples crystallize in perovskite crystallography with an orthorhombic structure (Pnma space group), where the MnO6 octahedron is elongated along the b axis due to the Jahn-Teller effect. That is confirmed by Raman spectra. Crystallite sizes and grain sizes were calculated from XRD and TEM respectively, and the lattice fringes resolved in the high-resolution TEM images of individual LPCMO nanoparticle confirmed its single-crystalline nature. FTIR spectra identify the characteristic Mn–O bond stretching vibration mode near 600 cm− 1, which shifts towards high wavenumbers with increasing post-annealing temperature or Pr-doping concentration, resulting in further distortion of the MnO6 octahedron. XPS revealed dual oxidation states of Mn3+ and Mn4+ in the LPCMO nanoparticles. UV-vis absorption spectra confirm the semiconducting nature of the LPCMO nanoparticles with optical bandgaps of 2.55–2.71 eV. Magnetic measurements as a function of temperature and magnetic field at field cooling and zero-field cooling modes, provided a Curie temperature around 230 K, saturation magnetization of about 81 emu/g, and coercive field of 390 Oe at 10 K. Such magnetic properties and the semiconducting nature of the LPCMO nanoparticles will make them as suitable candidate for magnetic semiconductor spintronics.


Background
Perovskite manganites R 1 − x A x MnO 3 (R = La, Pr, and other rare earth elements, A = Ca, Sr, Ba, and other alkalineearth elements) have received considerable interests during the past decade due to their colossal magnetoresistance (CMR) and potential applications in magnetic storage devices, magnetic sensors, and so on [1][2][3]. These materials exhibit interesting physical properties of concurrent ferromagnetism and metallic conductivity in the intermediate composition [2], which are ascribed to the complex interactions of the charge, orbital, spin, and lattice degrees of freedom [4][5][6][7]. La 1 − x Ca x MnO 3 (LCMO) manganite, as a prototypical system of perovskite manganites, has been of great interest because of its magnetic behavior and rich phase diagram [8,9]. In the past decade, various synthesized methods such as sol-gel process [10,11], polymeric precursor route [12], mechanical milling method [13], molten salt method [14] have been used to synthesize perovskite LCMO nanoparticles, and the effect of particle size on the structural, transport, and optical properties are also investigated [15][16][17][18]. Simultaneously, Ca-doped PrMnO 3 (Pr 1 − x Ca x MnO 3 : PCMO) also have some unusual electrical, magnetic, and optical properties, which are dependent on the Ca-doped concentration [19,20]. As one typical representative of the partially substituted compounds in the Pr-doping La 1 − x Ca x MnO 3 (LPCMO) system, smaller cation Pr 3+ replacing the larger cation La 3+ has led to more interesting phenomena such as magnetocaloric effect and transverse Kerr effect [21,22]. And also the LPCMO system is one of the most convenient ones for studying the phase separation behavior [23]. For example, TEM Uehara et al. [24] observed sub-micrometer sized phase separation involving ferromagnetic and chargeordered antiferromagnetic domains with a typical size of about 0.2 μm in La 0.625 − y Pr y Ca 0.375 MnO 3 . Furthermore, in the nanostructured LPCMO narrow strips (spatial confined system), several new transport features such as giant resistance jumps [25][26][27], reentrant M-I transitions [28], negative differential resistances, and intrinsic tunneling magnetoresistance [29,30] were observed, which were absent in the counterparts of thin films and bulks. Hwang et al. [31] performed detailed studies on the transport and magnetic properties of Pr-doped manganite La 0.7 − x Pr x Ca 0.
3 MnO 3 (x = 0.0-0.7), and they found that the transition temperature (T C ) from paramagnetism to ferromagnetism phase was decreased monotonically and the magnitude of the magnetoresistance was enhanced dramatically. Cao et al. [32] studied the magnetic properties of La 0.67 − x Pr x Ca 0. 33 MnO 3 (x = 0-0.67) synthesized by a conventional solidstate reaction, and found that the compounds underwent a ferromagnetic transition (T C ) when the Pr-doping concentration (x) was below 0.4. Recently, Kumar et al. [33] performed studies on the structural, transport, and optical properties of the (La 0.6 Pr 0.4 ) 0.65 Ca 0.35 MnO 3 nanoparticles post-annealed at different temperatures. The optical bandgaps of the (La 0.6 Pr 0.4 ) 0.65 Ca 0.35 MnO 3 nanoparticles were deduced from their UV-vis absorption spectra, which were found to be ∼ 3.5 eV.
Up to now, the magnetic and transport properties of perovskite manganites have been widely investigated, whereas their optical properties are rarely reported since these systems exhibit either insulator behavior (with larger bandgaps over 4 eV) or metallic behavior (no bandgap). In this work, we report on the microstructural, magnetic, and optical properties of Pr-doped La 0.67 Ca 0.33 MnO 3 nanoparticles [(La 1 − x Pr x ) 0.67 Ca 0.33 MnO 3 : LPCMO with x = 0.0-0.5] synthesized via a sol-gel process. The effects of the Pr-doping concentration and the post-annealed temperature on the structural, transport, and optical properties of perovskite LCMO nanoparticles are investigated systematically.
67 Ca 0.33 MnO 3 nanoparticles were first synthesized via sol-gel process and post-annealed at 700, 800, 900, and 1000°C. And then, perovskite (La 1 − x Pr x ) 0.67 Ca 0.33 MnO 3 nanoparticles with x = 0.0-0.5 were synthesized by the same method and post-annealed at 800°C. The starting materials were analytical grade La 2 O 3 , Pr 6 O 11 , CaCO 3 , and Mn(NO 3 ) 2 ·4H 2 O. First, at room temperature, analytical grade La 2 O 3 powders and CaCO 3 powders were dissolved in nitric acid with stirring. Simultaneously, analytical grade Pr 6 O 11 powders were also dissolved in nitric acid with stirring and heating. Then, Mn(NO 3 ) 2 ·4H 2 O were added to the mixed solution of the above two solutions to form a solution of metal nitrates. To obtain the desired precursor solution, the solution of analytical grade citric acid and ethylene glycol prepared before were added to the former solution. Citric acid, ethylene glycol, and metal nitrates were prepared with a molar ratio of 4: 3: 1. The citric acid solution was used as a chelating agent while ethylene glycol was used as a gelification agent. After stirring for 10 min, the homogeneous precursor solution was dried in oven at 200°C for 12 h to form the xerogel. The swelled xerogel was ground into powders and then was postannealed at the temperature as mentioned above for 5 h with a heating rate of 5°C/min. After heat treatment, the samples were cooled naturally to room temperature.
Phase identification of the LPCMO samples was performed by X-ray powder diffraction (XRD) at room temperature. The XRD data were collected from a Rigaku D/Max-RA diffractometer with Cu Kα radiation. A typical scan rate was 0.01 o /s, and the 2θ range was 15 o -85 o . The average crystallite size (D) of the LPCMO samples was evaluated by using the Debye-Scherrer's equation: D = 0.9λ/(βcosθ), where λ is the wavelength of Cu Kα radiation (λ = 1.5406 Å), β is the full width at half maximum intensity (FWHM) of the strongest XRD peak, and θ is the corresponding diffraction angle. The morphology and microstructure of the LPCMO samples were examined by analytical TEM (Tecnai G2S-Twin, FEI), and their compositions were determined by X-ray energy dispersive spectroscopy (EDS) (EX-250 spectroscopy, HORIBA Corporation). The specimens for TEM observations were prepared by drying droplets of the LPCMO powders from ethanol dispersion onto a holey carbon grid. Fourier transform infrared spectroscopy (FTIR) was performed with a FTIR Spectrometer (NEXUS870, Thermo Nicolet Corporation, USA) in the range of 400-4000 cm − 1 with a resolution of 1 cm − 1 . The samples were mixed with KBr, and the pellets were prepared from the mixture. Raman spectroscopy measurements were carried out by using a Raman Spectrometer (LabRAM HR Evol, HORIBA Scientific, Japan) with visible laser light (wavelength 514. 5 nm) as the excitation source. The slits were adjusted so that the resolution was 1 cm − 1 . Room temperature XPS measurements were performed by a XPS Spectrometer (PHI 5000 Versa Probe, UlVAC-PHI, Japan). A MgKα anode was operated at 250 W, providing the excitation. The XPS spectra obtained were referenced to the referenced C ls peak (binding energy 284.60 eV). The absorption optical spectra of the LPCMO nanoparticles were measured in the range of 100-1000 nm by UV-vis spectrophotometer (UV2550, SHIMADZU, Japan) by using BaSO 4 as a reference. The field and temperature dependence of magnetizations of the LCMO nanoparticles were measured by a SQUID magnetometer (Quantum design, America). First, the temperature was dropped from 300 to 2 K. The ZFC mode measurement data were collected with the temperature increasing from 2 to 300 K. And then, adding 0.01 T external magnetic field, the FC mode measurement data were collected with the temperature decreasing from 300 to 2 K.

Results and Discussion
Phase Identification of the LPCMO Nanoparticles That indicates all LPCMO samples have a single phase and there is no detectable secondary phase present. In fact, all the LPCMO samples crystallized in a single phase orthorhombic perovskite structure with space group Pnma. The lattice parameters and unit cell volumes of the LPCMO samples calculated from the XRD patterns are presented in Table 1. It was found that the lattice parameter a was generally increased with increasing the post-annealed temperature, which was confirmed by the left-shifting of the (200) diffraction peak, as shown in Fig. 1b. Meanwhile, the unit cell volumes of the LPCMO nanoparticles were also generally increased with increasing the post-annealed temperature. From the lattice parameters listed in Table 1, it is noticed that the lattice parameters (a, b, and c) satisfy a relationship of a ≈ c ≈ b/√2, indicating an orthorhombic distortion in perovskite crystallography [34]. The average crystallite sizes were determined by Scherrer's equation, which were found to be 21, 32, 40, and 47 nm for the LPCMO nanoparticles post-annealed at 700, 800, 900, and 1000°C, as listed in Table 1.  Table 2. The lattice parameters are also found to satisfy the relationship of a ≈ c ≈ b/√2, indicating a typical orthorhombic structural distortion in perovskite crystallography, where the MnO 6 octahedron was elongated along the b axis due to the Jahn-Teller distortion in MnO 6 octahedron [34]. It is also found that the lattice parameter a and the unit cell volumes of the samples have a slight decrease as increasing the Pr-doping concentrations. That is mainly attributed to the ionic radius of Pr 3+ (99.0 pm) smaller than that of La 3+ (103.2 pm). With increasing Pr-doping concentrations, the lattice parameter a was slightly decreased, leading to the right-shift of the (200) diffraction peak, as observed in Fig. 2b.

Spectra Analysis of the LPCMO Nanoparticles
Fourier transform infrared (FTIR) spectroscopy is used to investigate the lattice vibration in the present LPCMO system. Figure 5a shows the FTIR spectra of the (La 0. 6     Raman spectroscopy is also used to study the lattice distortions in the LPCMO nanoparticles. Figure 6  in Fig. 6a, b, respectively. The Raman peak around 224 cm − 1 can be assigned as A g (2), which is related to the tilting of MnO 6 octahedron, whereas the Raman peak around 425 cm − 1 is related to the Jahn-Teller type modes of the MnO 6 octahedron [33]. The Raman peak around 680 cm − 1 can be assigned as B 2g (1), which is related to the symmetric stretching vibration of oxygen in MnO 6 octahedron [33]. With increasing the Pr-doping concentration (x) up to x = 0.4, the Raman peak around 680 cm − 1 disappeared, as shown in Fig. 6c, d. That was ascribed to the increased orthorhombic distortion in the LPCMO nanoparticles with high Pr-doping concentrations, leading to the much weak symmetric stretching vibration of oxygen in MnO 6 octahedron.
In order to evaluate the ionic valence states in the LPCMO nanoparticles, especially Mn ions, XPS measurements were performed on the (La 0.6 Pr 0.4 ) 0.67 Ca 0.33 MnO 3 nanoparticles post-annealed at 800°C for 5 h, and the results are shown in Fig. 7. As shown in Fig. 7a, a survey XPS scan of the (La 0.6 Pr 0.4 ) 0.67 Ca 0.33 MnO 3 nanoparticles reveals the La 3d, Pr 3d, Ca 2p, Mn 2p, and O 1s XPS peaks, indicating the existence of La, Pr, Ca, Mn, and O elements in the LPCMO nanoparticles. The observed C 1s XPS peak in this spectrum is probably due to the surface contamination in air. The narrow-scan XPS spectrum for Ca 2p from the LPCMO nanoparticle is shown in Fig. 7b, where two XPS peaks are located at 345.38 and 348.88 eV, which are assigned as Ca 2p 3/2 and Ca 2p 1/2 , respectively due to the spin-orbit splitting of 3.5 eV. That indicates that Ca exists in + 2 oxidation state. Figure 7c shows the narrow-scan XPS spectrum of Mn 2p from the LPCMO nanoparticle, where two XPS peaks located at 641.13 and 652.88 eV are assigned as Mn 2p 3/2 and Mn 2p 1/2 , respectively. These two XPS peaks are further analyzed by XPSpeak-differentiation-imitating method. The deconvoluted peaks of the Mn 2p 3/2 and Mn 2p 1/2 XPS peaks are shown in Fig. 7d. Clearly, the Mn 2p 3/2 XPS peak is deconvoluted into two peaks at 640.80 and 642.72 eV, corresponding to the Mn 3+ and Mn 4+ ions, respectively. Similarly, the Mn 2p 3/2 XPS peak is also deconvoluted into two peaks at 652.40 and 654.00 eV, corresponding to the Mn 3+ and Mn 4+ , respectively. As a consequence, the deconvoluted Mn 2p XPS peaks reveal the existence of dual two oxidation states of the Mn 3+ and Mn 4+ ions. The Mn 2p 3/2 and Mn 2p 1/2 core levels are split into two peaks due to two valences of manganese upon Ca 2+ doping, which forms the basis of the double exchange interaction. In addition, the content ratio of the Mn 3+ to Mn 4+ ions estimated from the deconvoluted XPS peak areas was approximately 2:1. Ultraviolet-visible (UV-vis) absorption spectra were measured to estimate the optical bandgaps of the (La 1 − x Pr x ) 0.67 Ca 0.33 MnO 3 (x = 0.0-0.5) nanoparticles post-annealed at 800°C for 5 h. The optical absorption edges can be analyzed as follows [36]: where α is the absorption coefficient, depending upon the optical absorbance and thickness of the samples [36]. n can be equal to 1/2 (for direct transition process) or 2 (for indirect transition process). The plots of (αhν) 2 versus the energy of photon (hν) for the LPCMO nanoparticles are shown in Fig. 8. A linear relationship between (αhν) 2 and hν in a wide range is observed, suggesting a direct transition process taking place in the present system. The intercepts of these plots on the hν axis provide the optical bandgaps of the LPCMO nanoparticles, which are measured in the range of 2.55-2.71 eV (in the region of wide bandgap semiconductors), indicating the semiconducting nature of the LPCMO nanoparticles. The observed bandgaps of the LPCMO nanoparticles are smaller than that reported previously for the (La 0.6 Pr 0.4 ) 0.65 Ca 0.35 MnO 3 nanoparticles (~3.5 eV) by S. Kumar et al. [33]. The possible origins may be their different La/Ca ratios in the perovskite manganites and their different particle sizes.  [38,39]. Figure 10 shows the magnetic field dependence of the magnetizations of the (La 0.6 Pr 0.4 ) 0.67 Ca 0.33 MnO 3 samples post-annealed at 800 and 1000°C, which are measured at different temperatures. Clearly, the M-H hysteresis loops demonstrate that all the samples exhibit ferromagnetic behavior at low temperatures below the T C (e.g., 2 and 10 K), whereas a paramagnetic behavior is observed at   Fig. 12b, b d, f, and h, respectively), which are presented in Table 3. Based on the above structural data obtained from XRD patterns, the unit cell volumes of the (La 1 − x Pr x ) 0.67 Ca 0.33 MnO 3 (x = 0.1-0.4) nanoparticles are found to be decreased with increasing the Pr-doping concentration, as demonstrated in Table 2. That is ascribed to the ionic radius of Pr 3+ (99.0 pm) being smaller than that of La 3+ (103.2 pm) ion. As a consequence, the volume of the MnO 6 octahedron in an orthorhombic perovskite structure is reduced with increasing the Pr-doping concentration. Thus, the Mn-O bond length in the MnO 6 octahedron becomes shorter, which leads to the blue-shift of the stretching vibration mode frequency in the FITR spectra. From the magnetic data, it is observed that the T c values of the (La 1 − x Pr x ) 0.67 Ca 0.33 MnO 3 (x = 0.1-0.4) nanoparticles are decreased with increasing the Pr-doping concentration, which is similar to the system of the La 0.7 − x Pr x Ca 0.3 MnO 3 (x = 0.0-0.45) [37]. It is reported that the e g electron bandwidth became narrow as the Pr-doping concentration was increased in the La 0.7 − x Pr x Ca 0.3 MnO 3 (x = 0.0-0.45) system, and the electron-phonon interaction was increased, resulting in a reduction of the mobility of e g electrons [37]. Therefore, the double-exchange interactions in the (La 1 − x Pr x ) 0.67 Ca 0.33 MnO 3 (x = 0.1-0.4) nanoparticles became weakened due to the narrower bandwidth and the reduced mobility of e g electrons as the Pr-doping concentration was increased. This leads to a decrease of T c as increasing Pr-doping concentration. It is also noticed that as the Pr-doping concentration is increased, the saturation magnetization (M s ) is generally decreased whereas the coercive field (H c ) is increased.

Conclusions
In summary, structural measurements based on X-ray diffraction, TEM, HRTEM, and SAED patterns provide information regarding the phase formation and the single- The M-H hysteresis loops measured at different temperatures demonstrate that all the samples exhibit ferromagnetic behavior at 2 and 10 K, whereas paramagnetic behavior is observed at 300 K. The magnetic measurements provide a Curie temperature around 230 K, saturation magnetization (M s ) of about 81 emu/g, and coercive field of 390 Oe at 10 K. These strong magnetic behaviors as well as their semiconducting nature will enable the LPCMO nanoparticles to be a suitable candidate used for magnetic semiconductor devices.

Funding
This work is partially supported by the National Natural Science Foundation of China (grant nos. 11674161 and 11174122); six big talent peak project from Jiangsu Province (grant no. XCL-004).

Availability of Data and Materials
The data will not be shared due to private and confidential for the purpose of patent filling.

Authors' Contributions
WX had performed the experimental works and wrote this manuscript. XZ designed the experiments and modified the manuscript. HW and PX helped in data analysis. All authors read and approved the final manuscript.