Structure-magnetic and Morphology Properties of La 1-x Sr x MnO 3 Nanomaterials by sol-gel

: In this paper, perovskite La 1 -x Sr x MnO 3 powder was successfully prepared by Sol-gel method. The effects of different doping amount, different pH value , different calcination temperature and different calcination time on the structure and magnetic properties of La 1 -x Sr x MnO 3 material were studied by XRD, SEM, FT-IR and VSM. When the doping amount is x ≥0.3, the XRD shows that the sample is the perovskite material with cubic structure, and the doping amount is x= 0.3, the SEM shows that the sample has a better particle morphology. The analysis of different pH values shows that the sample La 0.7 Sr 0.3 MnO 3 has the maximum specific saturation magnetization of 62.42 emu/g and the maximum coercivity of 66.87 Oe at pH = 7. The analysis of different calcination temperatures shows that the sample La 0.7 Sr 0.3 MnO 3 has the maximum specific saturation magnetization of 62.42 emu/g and the maximum coercivity of 66.87 Oe, the calcination temperature is 1000℃. Different calcination time has little effect on the specific saturation magnetization and coercivity of the sample La 0.7 Sr 0.3 MnO 3 , but it shows a decreasing trend. When the calcination time is 2h, it has the largest specific saturation magnetization and coercivity.


Introduction
In 1839 , researchers first discovered an inorganic material (CaTiO3) in the metamorphic rocks of the Ural Mountains in Russia , and named it after the Russian mineralogist A. Von Perovski.Since then, the perovskite material with this structural feature has been and its structure began to enter the stage of scientific research [1][2] .The basic chemical formula of the perovskite material is ABO3 , and its space group is Pm3m [3][4][5] .Perovskite nanomaterials (ABO3)exhibit their unique advantages in various aspects due to their unique physical and chemical properties, among which LaMnO3 oxides have a typical ABO3 structure and an antiferromagnetic structure at room temperature [6][7] .With LaMnO3 as the parent body , with some divalent metals ( Mg, Ca, Sr, Ba, etc. ), monovalent metals ( K, Na, etc. ), transition metals, The mixed valence of +3 and +4 of Mn ions will appear, which leads to the emergence of ferromagnetic metallic states, accompanied by a huge magnetoresistance effect, which is a hot issue in the research of condensed matter physics in recent years [8][9] .In recent years , there have been many studies on the structure and properties of La1-xSrxMnO3 materials [3], bcause different preparation conditions have a great influence on the microstructure and magnetic properties of the synthesized samples.The effects of preparation conditions on the microstructure and magnetic properties of La1-xSrxMnO3 material samples are useful for further in-depth exploration of the intrinsic mechanism of La1-xSrxMnO3 -based oxide materials , as well as for novel functionalities.The application and development of materials and nanoelectronic devices are of great significance.In this paper, La1-xSrxMnO3 materials were prepared by Sol-gel method under different process conditions.The effects of different pH, impurity ratio of Sr element, calcination temperature and calcination time on La1-xSrxMnO3 were studied.The influence of the material's microstructure and magnetic properties is of great significance for further exploring the internal mechanism of A-site doped LaMnO3 composite oxide materials , improving the application value of the material in the field of magnetism, and laying a foundation for the development of new materials.

Preparation of Materials
Experimeriment 3: (different calcination temperature) Step 1: according to the molar ratio of La0.7Sr0.3MnO3, weigh the total amount of La(NO3)3 and Sr(NO3)2 , and 0.01 mol of Mn(NO3)2 (50%) solution , according to the mol ratio of the complexing agent and the metal ion to be 1:1 , take the citric acid monohydrate that the amount of the material is 0.02mol, and the weighed nitrate and the complex are respectively deionized water.Dissolving and mixing to obtain a precursor solution; Step 2: the precursor solution is placed in a water bath at 50℃ and stirred , the pH of the solution is adjusted to 7 with ammonia water , and a small amount of ethylene glycol is added dropwise as a dispersant, and stirring and evaporation are continued until the solution is evaporated to dryness to obtain wet condensation glue.
Step 3: put the wet gel in a digital display drying box , dry it at 100℃ for 24 hours to obtain a dry gel , use alcohol self-propagating combustion to obtain a black precursor powder , fully grind the powder and put it in a muffle furnace , Then calcined at 400~1000℃ for 2 hours to obtain black and shiny sample powder.
Experiment 4: (different calcination time) Step 1: according to the molar ratio of La0.7Sr0.3MnO3, weigh the total amount of La(NO3)3 and Sr(NO3)2 , and 0.01 mol of Mn(NO3)2 (50%)solution , according to the mol ratio of the complexing agent and the metal ion to be 1:1 , take the citric acid monohydrate that the amount of the material is 0.02mol, and the weighed nitrate and the complex are respectively deionized water.Dissolving and mixing to obtain a precursor solution; Step 2: the precursor solution is placed in a water bath at 50℃ and stirred , the pH of the solution is adjusted to 7 with ammonia water , and a small amount of ethylene glycol is added dropwise as a dispersant, and stirring and evaporation are continued until the solution is evaporated to dryness to obtain wet condensation glue.
Step 3: put the wet gel in a digital display drying box , dry it at 100℃ for 24 hours to obtain a dry gel , use alcohol self-propagating combustion to obtain a black precursor powder , fully grind the powder and put it in a muffle furnace , The samples were calcined at 1000℃ for different time periods of 2 to 14 h to obtain black and shiny sample powders.

Characterization Method
In this paper, the X-ray diffractometer (D/max-2500v/pc) of Rigaku Company in Japan was used to characterize and analyze the phase structure of the sample.The FT-IR spectrometer of PerkinElmer Company was used to analyze the possible functional groups and internal chemical bonds in the sample.The EVO18 electron scanning microscope of ZEISS company was used to test the microscopic morphology of the sample.The VSM-100 vibrating sample magnetometer of Yingpu Magnetics was used to test the hysteresis loop of the sample at room temperature.

XRD Analysis
The XRD pattern of La1-xSrxMnO3 (x=0 ~ 0.6) calcined at 1000℃ for 2 hours.It can be seen from the figure 1.The crystal structures of all samples conform to the perovskite structure.The ideal perovskite has a cubic structure with space group Pm3m, but the experimentally prepared samples with Sr doping ratio x ≤ 0.2 have a certain degree of distortion in their crystal structures, which are rhombohedral (R-3C) structure, the lattice parameters of which are in perfect agreement with the standard card (JCPDS: 70-3942) values , which may be due to the higher activation energy provided under the experimental conditions with higher calcination temperature, which promotes the distortion of the crystal structure.When the doping ratio x>0.2 of Sr, the main peak changes from double peaks to single peaks, and the corresponding other peak positions also change from double peaks to single peaks.The diffraction peaks correspond to the perovskite materials with cubic structure , indicating that with the increase of the doping ratio of Sr, it is more conducive to the formation of the cubic phase structure.The reason for this change may be related to the lattice distortion caused by different Sr doping ratio.According to the calculation formula of tolerance factor t=rA+ro/√2(rB+ro) and the ion mismatch degree formula σ 2 =∑ixiri 2 -＜rA＞ 2 .The parameters of the t-factor and A-site ion mismatch σ 2 of the group samples are shown in Table 1.The calculation results show that with the increase of the doping ratio of Sr, t increases from 0.841 to 0.902 , indicating that with the doping of Sr As the ratio increases, the smaller the lattice distortion , the easier it is to form a stable perovskite structure.Taking LaMnO3 as an example, as shown in Figure 2 , La 3+ with larger ionic radius is located at the top corner of the simple cube , Mn 3+ with smaller ionic radius is located in the center of the cube , and O 2-is located at the corner of the cube.Face-centered , the main feature of this structure is that Mn-O ions form MnO6 octahedral structure , which is connected with La 3+ at the top corners to form a spatial network.Table 1   It can be seen from the peak shift diagram in Figure 1, with the increase of doping ratio , when x≤ 0.2 , the main peak shifts to low angle first , and its lattice parameters also change correspondingly, according to the Bragg equation 2dsinθ= nλ is known , λ is constant , the smaller the θ, the larger the d, and the larger the unit cell parameter , corresponding to the increasing trend of the volume in Table 1 , indicating that after the doped Sr atoms replace the La atoms into the LaMnO3 lattice.The lattice distortion causes the size of the unit cell to become larger; and x>0.2 , the position of the peak shifts to a high angle with the increase of the doping ratio of Sr.According to the Bragg equation 2dsinθ=nλ , λ is constant , and the larger the θ, the higher the angle.The smaller the d, the smaller the unit cell parameters, corresponding to the decreasing trend of the volume in Table 1.The reason for the decrease may be that the substitution of Sr will cause the change of the size of the A site and the change of the valence state of Mn , although the Sr 2+ ionic radius (0.118nm) is close to La 3+ ionic radius (0.1032nm) , with the increase of Sr substitution amount x, in order to maintain the electrical neutrality of the whole compound [2] , B-site Mn 3+ (ionic radius 0.0645nm) is oxidized Mn 4+ (ionic radius 0.0530nm) is formed, and the content of Mn 4+ increases, the covalency between Mn and O ions increases, and the atomic orbitals between Mn -O overlap more, resulting in a decrease in the unit cell volume [4] .Another factor that affects the unit cell volume is the oxygen content in the compound.With the increase of the Mn substitution amount x, the perovskite structure will generate oxygen vacancies, which may also lead to changes in the unit cell volume.In addition , the average grain size D of the sample can be calculated from the full width at half maximum (FWHM) of the main diffraction peak using the Scherrer formula: D=0.94λ/β Cos θ , where λ is the x-ray wavelength and θ is the diffraction angle, the average grain sizes of the crystals were calculated to be 35.7019nm,23.9441nm, 24.6239nm, 29.1053nm, 31.4387nm,35.7270nm, 25.1403nm, the samples were nanoparticles.

also gives the lattice parameter table of samples
Figure 3 is the XRD diffraction pattern of the sample La0.7Sr0.3MnO3calcined at 400℃, 500℃, 600℃, 700℃, 800℃, 900℃, and 1000℃ for 2 h.When the calcination temperature is 400℃, the sample begins to generate corresponding diffraction peaks and impurity phases are generated.As the calcination temperature increases, the diffraction peaks tend to be complete and the impurity phases decrease.When the temperature rises to 700℃, no impurity phase disappears, and the diffraction peaks are very sharp, forming a unique perovskite structure.After comparing with the standard card ( ICSD NO. 75-0440), these diffraction peaks correspond to the perovskite with cubic structure respectively.With the continuous increase of calcination temperature, the main diffraction peak is further enhanced.By Scherrer's formula: D=0.94λ/βCosθ, the average grain size D of the sample can be calculated from the full width at half maximum ( FWHM)of the main diffraction peak, where λ is the x-ray wavelength and θ is the diffraction angle.As shown in Table 2, The average grain sizes of the obtained crystals were 17.5234 nm, 18.8254 nm, 18.2343 nm, 20.4325 nm, 23.2957 nm, 25.5735 nm, 29.6014 nm, the samples were nanoparticles, and with the increase of calcination temperature, the average grain size shows an increasing trend, indicating that the crystallinity of the sample is more complete.

SEM Analysis
Figure 4 shows the SEM images of (1) La0.9Sr0.1MnO3(2) La0.7Sr0.3MnO3(3) La0.5Sr0.5MnO3samples.The different ratios of Sr doping were used to prepare the La1-xSrxMnO3 samples have different characteristics, when the doping ratio x=0.1, the sample has more serious particle agglomeration and blurred grain boundaries, and as the doping ratio increases, the agglomeration between particles decreases, when the doping ratio x=0.5, the agglomeration between the particles is the least, and the boundary is clear, but the particle size of the samples is inconsistent, which may be related to the preparation process conditions and the eg electrons of the Mn atoms make the octahedron formed by oxygen.It is related to the occurrence of Jahn-Teller distortion.When the doping ratio is small, the tolerance factor is small, and it is easy to distort the lattice, resulting in strong interaction between grains, resulting in agglomeration.In addition, with the doping ratio of Sr As the Sr ion radius is larger than the La ion radius, the A-site ion mismatch degree of the sample doped with a high proportion further increases, causing the mismatch between adjacent layers, which is easy to distort the lattice, resulting in The structure changes, the macroscopic particle shape becomes irregular, and the particle size is inconsistent.According to the statistics of the particle distribution analysis software, the particle size of the sample is mainly concentrated in the range of 28-68.8nm,and the Scherrer formula to calculate the average grain size to be 24.9065nm,indicating that the sample prepared in the experiment is the polycrystalline material.

Infrared Analysis
Figure 5 presents the FT-IR images of La1-xSrxMnO3 , with KBr as the background, the diagrams indicate the absorption at the low-frequency wavenumber stage in the main band around 450-600 cm -1 peaks, which can be attributed to the Mn-O stretching vibration and the O-Mn-O bending vibration in the MnO6 regular octahedron caused by the Jahn-teller distortion, including the changes of its bond length and bond angle, which are related to its vibrational state [1,10] .There is a narrow vibrational peak at the wavenumber of about 1384 cm -1 , which is the asymmetric stretching vibration of the metal carbonate [1,11] , a small vibrational peak at the wavenumber of about 1056 cm -1 reflects the stretching vibration of the carboxyl radical [2,5] , a very small peak near 2900 cm -1 may be due to the impurity SrCO3 , or the combined effect of CH3 , CH2 and OH [6,8] .There is a broad peak at about 3436 cm -1 corresponding to the stretching vibration of water molecule OH [9,10] , and a strong absorption peak at 1630 cm -1 is attributed to the bending vibration of surface water molecule HOH [11,12] .In addition , it can be seen from the XRD pattern that the sample has produced corresponding perovskite diffraction peaks at 400℃.Combined with the FT-IR pattern, it can be found that the CH and CC framework vibration peaks in the sample due to organic matter have disappeared [13] .All in all, the stretching vibration peaks appearing from the spectra indicate that all the as-prepared samples form perovskite structures, which are consistent with our conclusions obtained from XRD diffraction.

Magnetic Analysis
The experimental phenomena of the preparation of La0.7Sr0.3MnO3materials at different PH.It can be seen from Table 3, with PH=1 , the precursor solution is a colorless solution, and with PH=3 and PH=5, both a stable gel cannot be formed , and finally a white blocky dry gel is obtained; with PH=7 or PH=9 , the sol is a yellow clear solution , and a reddish-brown gel is obtained.4 shows magnetic parameters of La0.7Sr0.3MnO3calcined at 1000℃ for 2 hours at different pH.It can be seen from Table 4 that with the increase of pH value, the specific saturation magnetization of the sample first increases and then decreases.When PH=7, the sample has the maximum specific saturation magnetization of 62.42 emu/g.In addition , it can be seen from Table 4 that with the increase of pH value, the coercive force also increases first and then decreases.When PH=7, the sample has the maximum coercive force of 66.87 Oe.  6 shows the hysteresis loop of La1-xSrxMnO3 at room temperature.The pure LaMnO3 powder has antiferromagnetic properties.The hysteresis loops are all closed curves, showing obvious ferromagnetism at room temperature, and the magnetization basically reaches saturation around 0.2T , which is a soft magnetic material with small coercivity.Table 5 shows La1-xSrxMnO3 calcined at 1000℃ for 2 hours, the specific saturation magnetization of the samples are 32.03 emu/g, 56.78 emu/g, 62.42 emu/g, 51.38 emu/g, 23.52 emu/g, 12.50 emu/g.With the increase of doping ratio, the specific saturation magnetization first increases and then decreases, when the doping ratio x=0.3, the La1-xSrxMnO3 has the largest specific saturation magnetization, which has the value of 62.42 emu/g.When the A site of LaMnO3 is replaced by a part of cations, in order to maintain the balance of the whole valence, the manganese ions in it generate mixed valence states of Mn 3+ and Mn 4+ .When the bond angle of the Mn-O-Mn coupling is 180°, its energy distribution is shown in Figure7, the electronic configuration of Mn 4+ is 3d 3 (t2g 3 eg), since the eg orbital is closely related to the the energy of the 2p orbital of O 2-, and it is easy to transfer an electron on the 2p orbit of O 2-to the empty orbit of Mn 4+ , as shown in Figure 7.While the adjacent Mn 3+ the electrons on the eg orbital of the ion are filled to the 2p orbital of O 2-, which is the double exchange effect, and the traveling electron eg is freely exchanged between Mn 3+ and Mn 4+ to form the conductance.At the same time, it is constrained by Hund's law to keep parallel alignment with the t2g electrons, which leads to the appearance of ferromagnetism.During the entire exchange process, the energy of the system remains unchanged.When an external magnetic field is applied, the resistivity decreases due to the orientational ordering of the localized spins, resulting in a negative CMR effect.According to the double exchange model, when the ratio of the number of Mn 4+ to Mn 3+ ions is 1:2, eg electrons can be continuously transferred between adjacent Mn 4+ and Mn 3+ ions, and the double exchange effect is the strongest at this time.On the other hand, the ratio of Mn 4+ /Mn 3+ of 1:2 is also favorable for the formation of an ideal cubic perovskite structure, and any deviation from the ideal cubic perovskite structure will lead to lattice distortion and reduce Mn-O-Mn bond angle, weakening the double exchange.Therefore, in the La1-xSrxMnO3 system, the ratio of the number of Mn 4+ to Mn 3+ ions of La0.7Sr0.3MnO3 is close to 1:2 , and has a higher specific saturation magnetization.
Figure 8 shows the magnetic images of La0.7Sr0.3MnO3calcined at different temperatures for 2 hours.The sample exhibits obvious ferromagnetism with the increase of temperature.When the temperature is 1000℃, the The specific saturation magnetization is the largest; and the calcination temperature exceeds 1000℃ , the specific saturation magnetization of the sample begins to decrease again, which may be because the crystal structure tends to be complete with the increase of the calcination temperature.However, when the temperature increases to 1000℃, the specific saturation magnetization tends to be saturated.Continuing to increase the calcination temperature will easily cause the crystals to agglomerate, thereby reducing the specific saturation magnetization.Table 6 gives the magnetic parameters of La0.7Sr0.3MnO3calcined at different temperatures for 2 hours, it can be seen from the Table 6 that with the increase of the calcination temperature of the sample, the specific saturation magnetization first increases and then decreases.The sample has the largest specific saturation magnetization of 64.24 emu/g.9 shows the magnetic images of La0.7Sr0.3MnO3calcined at 1000℃ for different times.Al the samples show ferromagnetism.Table 7 shows that La0.7Sr0.3MnO3calcined at 1000℃ for different times.It can be seen from the Table 7 that different calcination time has little effect on the specific saturation magnetization, remanent magnetization and coercivity of the samples, but the magnetic parameters show the decreasing trend.

Conclusion
Through the research in this paper, it is found that different doping amounts have a great influence on the grain structure and magnetic properties of the samples.XRD shows that with the increase of the doping ratio of Sr, the smaller the lattice distortion, the easier it is to form stable calcium titanite structure, when the doping amount x≥0.3, the sample is a perovskite material with a cubic structure.SEM shows that the samples with different doping amounts have different morphological characteristics.When the doping amount x=0.3, the SEM shows that the morphology of the sample is better.The VSM shows that with the increase of doping amount, the saturation magnetization and coercivity first increase and then decrease.When the doping amount is x=0.3, the sample has saturation magnetization and coercivity.Therefore, the optimal doping amount is 0.3.The influence of different pH values on the magnetic properties of the samples.When the pH value is 7, the sample La0.7Sr0.3MnO3has the maximum specific saturation magnetization of 62.42 emu/g and the maximum coercivity of 66.87 Oe.Therefore, the optimal pH is 7.The influence of different calcination temperatures on the magnetic properties of the samples, VSM shows that with the increase of calcination temperature, the saturation magnetization and coercivity first increase and then decrease.The specific saturation magnetization is 62.42 emu/g, and the maximum coercivity is 66.87 Oe.Therefore, the optimal calcination temperature is 1000℃.The influence of different calcination time on the magnetic properties of the sample, VSM shows that with the increase of calcination time, the saturation magnetization and coercivity have little change but show a decreasing trend.When the calcination time is 2h, the sample La0.7Sr0.3MnO3has the largest specific saturation magnetization of 62.42 emu/g, and has the largest coercivity of 66.87 Oe.Therefore, the optimal calcination time is 2h.The optimal experimental conditions determined in this paper can provide experimental reference and reference for other researches.

Figure 7 :
Figure 7: Schematic diagram of double exchange between manganese ions

Table 3 :
The experimental phenomena in the process of preparing La0.7Sr0.

Table 6 :
Magnetic parameters of La0.7Sr0.3MnO3calcined at different temperatures for 2 hours