Structural and Magnetic Properties of Perovskite Functional Nanomaterials La1−xRxFeO3 (R = Co, Al, Nd, Sm) Obtained Using Sol-Gel

Perovskite is the largest mineral on earth and has a variety of excellent physical and chemical properties. La1−xRxFeO3 (R = Co, Al, Nd, Sm) were synthesized using the sol-gel method and analyzed by XRD, TG-DTA, and VSM. With the increase in the Co2+ doping content, the diffraction peak drifted in the direction of a larger angle. The grain size of La1−xRxFeO3(R = Co) is mainly concentrated between 50.7 and 133.5 nm. As the concentration of Co2+ increased, the magnetic loop area and magnetization increased. La1−xRxFeO3(R = Al) is an orthorhombic perovskite structure, the grain size decreased with the increase in Al3+ doping concentration, and the minimum crystallite is 17.9 nm. The magnetic loop area and magnetization increased with the increase in Al3+ ion concentration. The enclosed area of the M-H curve of the sample decreased, and the ferromagnetic order gradually weakened and tended to be antiferromagnetic, which may be due to the increase in sintering temperature, decrease in the iron oxide composition, and changes in the magnetic properties. Proper doping can improve the magnetization of La1−xRxFeO3(R = Nd), refine the particles, and obtain better magnetic performance. As the Nd3+ ion concentration increased, the magnetic properties of the samples increased. Ms of La0.85Co0.15FeO3 prepared by different calcination time increases with the increase in calcination time. As the Sm3+ ion concentration increased, the magnetic properties of the samples increased. Proper doping can improve the magnetization of La1−xRxFeO3(R = Sm), refine the particles, and generate better magnetic performance.


Introduction
Perovskite is the largest mineral on earth and has a variety of excellent physical and chemical properties, such as magnetoelectric effect, magnetostriction, variable magnetic phase transition, catalytic activity, and piezoelectric effect. It is one of the new functional materials with great development prospects [1,2]. LaFeO 3 oxides with perovskite structure have a unique crystal structure; a slight change in its structure, especially defects in the crystal structure or changes in the performance caused by doping, will lead to new properties [3,4]. In the perovskite structure oxide ABO 3 , the A-site is generally a rare earth element with large radius (i.e., La, Pr, and Gd). The interaction between A-site ions and oxygen ions plays a decisive role in the mutual evolution of perovskite structures [5,6]. Lili Liu et al. [7] studied synthesis and characterization of Al 3+ doped LaFeO 3 compounds. Sr-doped porous LaFeO 3 samples were fabricated via the sol-gel method [8]. Nabasmita Saikia et al. [9] prepared (La 0.75 Gd 0.25 )FeO 3 successfully via the typical solid casting route method. Chethana Aranthady et al. [10] studied the Ca substituted LaFeO 3 . Yongfang Rao et al. [11] prepared Cu-doped LaFeO 3 samples and studied heterogeneous catalysts of PMS for the degradation of pharmaceuticals. Wankassama Haron et al. [12] studied the structural characteristics Figure 1 is the XRD diffraction pattern of La 1−x Co x FeO 3 (x = 0~0. 25). The XRD diagram can analyze the structural changes caused by atomic doping. When the doping amount x = 0.20, the samples showed a CoFe 2 O 4 mixed phase, indicating that with the increase in Co 2+ doped, Co 2+ does not fully enter the La 3+ node, and part of Co 2+ and Fe 3+ form a CoFe 2 O 4 compound, which will decrease the magnetic properties of the samples. method. Chethana Aranthady et al. [10] studied the Ca substituted LaFeO3. Yongfang Rao et al. [11] prepared Cu-doped LaFeO3 samples and studied heterogeneous catalysts of PMS for the degradation of pharmaceuticals. Wankassama Haron et al. [12] studied the structural characteristics and dielectric properties of La1−xCoxFeO3 and LaFe1−xCoxO3 samples using the thermal decomposition method. Xiutao Ge et al. [3] studied the gas sensitivity of LaFe1−yCoyO3 via the co-precipitation method. Yutana Janbutrach et al. [5] synthesized La1−xAlxFeO3 nanocrystals and studied their magnetic and optical properties. The saturation magnetization, the coercivity, and the remanent magnetization of the samples increase with the increase in the concentration of Al 3+ ion. There are few studies that investigate the Al 3+ -doped LaFeO3 nanoparticles synthesized at low temperature using the citric acid sol-gel method and their structure and magnetic properties. LI Fa-tang et al. [1] synthesized La1−xNdxFeO3 via the citric acid complexation method and studied its morphology. Enrico traversa et al. [2] synthesized La1−xSmxFeO3 sample via the thermal decomposition method and studied its structure and composition. In this paper, La1−xRxFeO3 (R = Co, Al, Nd, Sm) powder was synthesized using the citric acid sol-gel method [13][14][15][16][17][18][19][20] and the influences of different Co, Al, Nd, and Sm doping ratios and calcination temperatures on the structure, morphology, and magnetic properties of the samples were studied Figure 1 is the XRD diffraction pattern of La1−xCoxFeO3 (x = 0~0. 25). The XRD diagram can analyze the structural changes caused by atomic doping. When the doping amount x = 0.20, the samples showed a CoFe2O4 mixed phase, indicating that with the increase in Co 2+ doped, Co 2+ does not fully enter the La 3+ node, and part of Co 2+ and Fe 3+ form a CoFe2O4 compound, which will decrease the magnetic properties of the samples. The diffraction peak (121) intensity is decreased with the increase in Co 2+ doping concentration. The diffraction peak (121) is wider. The crystallinity of the sample is reduced [21][22][23][24]. The drift pattern shows that with the increase in the Co 2+ doping amount, the diffraction peak (121) drifted in the direction of a larger angle. Wankassama Haron [12] came to a similar conclusion. Wankassama Haron et al. [12] explained that the substitution of small radius Co 2+ (ionic radius of 0.074 nm) for larger radius La 3+ (ionic radius 0.274 nm) will reduce the c/a. Therefore, the lattice spacing d decreased. According to the Bragg diffraction condition 2dsinθ = nλ, it is known that diffraction peak drifted in the direction of 2θ increased. This explains that according to the equation below, the smaller radius of The diffraction peak (121) intensity is decreased with the increase in Co 2+ doping concentration. The diffraction peak (121) is wider. The crystallinity of the sample is reduced [21][22][23][24]. The drift pattern shows that with the increase in the Co 2+ doping amount, the diffraction peak (121) drifted in the direction of a larger angle. Wankassama Haron [12] came to a similar conclusion. Wankassama Haron et al. [12] explained that the substitution of small radius Co 2+ (ionic radius of 0.074 nm) for larger radius La 3+ (ionic radius 0.274 nm) will reduce the c/a. Therefore, the lattice spacing d decreased. According to the Bragg diffraction condition 2dsinθ = nλ, it is known that diffraction peak drifted in the direction of 2θ increased. This explains that according to the equation below, the smaller radius of Co 3+ (ionic radius of 0.167 nm) replaced the larger radius of La 3+ (ionic radius of 0.274 nm) [14][15][16]:

XRD Analysis of La1−xRxFeO3 (R = Co)
Molecules 2023, 28, 5745 3 of 17 Figure 2 shows the change trend of the average grain size obtained via jade software. After doping Co 3+ , the diffraction peak was wide, and the grain size decreased. When the doping amount x > 0.05, the average grain size changed irregularly with the increase in x value. This may be related to the second phase CoFe 2 O 4 [3]. Figure 3 is an XRD diffraction pattern of the La 0.85 Co 0.15 FeO 3 sample. When the calcination temperature is 1000 • C, XRD detects impurities CoFe 2 O 4 . Rare earth transition metal composite oxide LaFeO 3 material is a typical perovskite orthorhombic structure and a p-type semiconductor [5,6]. The rare earth ion La 3+ with relatively large ion radius is more likely to occupy position A, which is located in the hole composed of FeO 6 octahedron, whereas the transition metal ion Fe 3+ with relatively small ion radius is more likely to occupy position B [7,12]. Fe 3+ and the surrounding six O 2™ form a FeO 6 octahedron structure [17].
Co 3+ (ionic radius of 0.167 nm) replaced the larger radius of La 3+ (ionic radius of 0.274 nm) [14][15][16]: (1) Figure 2 shows the change trend of the average grain size obtained via jade software. After doping Co 3+ , the diffraction peak was wide, and the grain size decreased. When the doping amount x > 0.05, the average grain size changed irregularly with the increase in x value. This may be related to the second phase CoFe2O4 [3]. Figure 3 is an XRD diffraction pattern of the La0.85Co0.15FeO3 sample. When the calcination temperature is 1000 °C, XRD detects impurities CoFe2O4. Rare earth transition metal composite oxide LaFeO3 material is a typical perovskite orthorhombic structure and a p-type semiconductor [5,6]. The rare earth ion La 3+ with relatively large ion radius is more likely to occupy position A, which is located in the hole composed of FeO6 octahedron, whereas the transition metal ion Fe 3+ with relatively small ion radius is more likely to occupy position B [7,12]. Fe 3+ and the surrounding six O 2-form a FeO6 octahedron structure [17].   Figure 4 is the XRD pattern of the La1−xAlxFeO3 (x = 0~0.10) sample. This may be due to the fact that the Al 3+ ionic radius (0.0535 nm) is similar to the B-site Fe 3+ ionic radius (0.078 nm), resulting in the formation of the Fe2O3 impurity phase. Co 3+ (ionic radius of 0.167 nm) replaced the larger radius of La 3+ (ionic radius of 0.274 nm [14][15][16]: Figure 2 shows the change trend of the average grain size obtained via jade software After doping Co 3+ , the diffraction peak was wide, and the grain size decreased. When th doping amount x > 0.05, the average grain size changed irregularly with the increase in value. This may be related to the second phase CoFe2O4 [3]. Figure 3 is an XRD diffraction pattern of the La0.85Co0.15FeO3 sample. When the calcination temperature is 1000 °C, XRD detects impurities CoFe2O4. Rare earth transition metal composite oxide LaFeO3 materia is a typical perovskite orthorhombic structure and a p-type semiconductor [5,6]. The rar earth ion La 3+ with relatively large ion radius is more likely to occupy position A, which is located in the hole composed of FeO6 octahedron, whereas the transition metal ion Fe 3 with relatively small ion radius is more likely to occupy position B [7,12]. Fe 3+ and th surrounding six O 2-form a FeO6 octahedron structure [17].   Figure 4 is the XRD pattern of the La1−xAlxFeO3 (x = 0~0.10) sample. This may be du to the fact that the Al 3+ ionic radius (0.0535 nm) is similar to the B-site Fe 3+ ionic radiu (0.078 nm), resulting in the formation of the Fe2O3 impurity phase. Table 1 shows that th   Table 1 shows that the lattice parameters (a, b, c), crystal cell volume and particle size showed a trend of being smaller with the increase in Al 3+ doping concentration because La 3+ ion radiuses (0.274 nm) are replaced by a smaller Al 3+ ion radiuses [25].

XRD and TG-DTA Analysis of La1−xRxFeO3 (R = Al)
Molecules 2023, 28, x FOR PEER REVIEW 4 of 17 lattice parameters (a, b, c), crystal cell volume and particle size showed a trend of being smaller with the increase in Al 3+ doping concentration because La 3+ ion radiuses (0.274 nm) are replaced by a smaller Al 3 + ion radiuses [25].   Table 1 shows that as the Al 3+ ion doping amount increased, the cell volume decreased. It was estimated using Scherrer's formula: The XRD pattern clearly shows that the diffraction peak drifted in the direction of a larger angle, which can also indicate that the lattice parameters and the unit cell volume have a tendency to decrease. Figure 5 is the XRD diffraction pattern of La0.9Al0.1FeO3 samples. When the calcination temperature was 800 °C, Fe2O3 peaks began to appear. Table 2 presents the lattice parameters of the La0.9Al0.1FeO3 sample calcined at 600 °C, 800 °C, and 1000 °C. This explains that according to the equation below, the smaller radius of Al 3+ (ionic radius of 0.182 nm) replaced the larger radius of La 3+ (ionic radius of 0.274 nm). At the same time, to maintain the charge balance, Fe 3+ was oxidized to Fe 4+ or oxygen vacancies appeared, resulting in lattice distortion.    Table 1 shows that as the Al 3+ ion doping amount increased, the cell volume decreased. It was estimated using Scherrer's formula: The XRD pattern clearly shows that the diffraction peak drifted in the direction of a larger angle, which can also indicate that the lattice parameters and the unit cell volume have a tendency to decrease. Figure 5 is the XRD diffraction pattern of La 0.9 Al 0.1 FeO 3 samples. When the calcination temperature was 800 • C, Fe 2 O 3 peaks began to appear. Table 2 presents the lattice parameters of the La 0.9 Al 0.1 FeO 3 sample calcined at 600 • C, 800 • C, and 1000 • C. This explains that according to the equation below, the smaller radius of Al 3+ (ionic radius of 0.182 nm) replaced the larger radius of La 3+ (ionic radius of 0.274 nm). At the same time, to maintain the charge balance, Fe 3+ was oxidized to Fe 4+ or oxygen vacancies appeared, resulting in lattice distortion. Figure 6 is the TG and DTA curves of La 0.9 Al 0.1 FeO 3 xerogel. When the temperature rose from 30 • C to 140 • C, the TG curve showed that the weight loss rate was about 10%. Then, at 90 • C, the DTA curve shows a weak endothermic peak due to water evaporation from the wet gel and the expulsion of water inside the dry gel (adsorbed water, crystallization water, and water vapor generated during the reaction). When the temperature increases from 140 • C to 207 • C, there is a weight loss of about 70 percent and a sharp exothermic peak corresponding to the DTA curves at about 207 • C. When the temperature is higher than 207 • C, the weight loss rate was less than 5%, forming La 1−x Al x FeO 3 [7].  Figure 6 is the TG and DTA curves of La0.9Al0.1FeO3 xerogel. When the temperature rose from 30 °C to 140 °C, the TG curve showed that the weight loss rate was about 10%. Then, at 90 °C, the DTA curve shows a weak endothermic peak due to water evaporation from the wet gel and the expulsion of water inside the dry gel (adsorbed water, crystallization water, and water vapor generated during the reaction). When the temperature increases from 140 °C to 207 °C, there is a weight loss of about 70 percent and a sharp exothermic peak corresponding to the DTA curves at about 207 °C. When the temperature is higher than 207 °C, the weight loss rate was less than 5%, forming La1−xAlxFeO3 [7].  Figure 7 shows the XRD diffraction pattern of La1−xNdxFeO3 (x = 0~0.25) samples calcined at 600 °C for 2 h and (121) peak drift pattern. According to Figure 7b, the (121) diffraction peak slightly increased with the increase in Nd 3+ doped concentration and drifted in the direction of 2θ [24]. The substitution of small radius Nd 3+ (ionic radius of 0.127 nm) for larger radius La 3+ (ionic radius of 0.274 nm) leads to lattice distortion [26]. The lattice parameters of corresponding samples are as shown in Table 3. As the Nd 3+ content increased, the crystal lattice parameters (a, b, and c) changed accordingly [27], and the cell volume of the samples decreased. The reason is that the La ion was gradually replaced by Nd ion, which reduced the average ionic radius of A-bit and, in turn, caused cell shrinkage, resulting in a decrease in cell volume [28]. The grain size of the samples can be estimated    Figure 6 is the TG and DTA curves of La0.9Al0.1FeO3 xerogel. When the temperature rose from 30 °C to 140 °C, the TG curve showed that the weight loss rate was about 10%. Then, at 90 °C, the DTA curve shows a weak endothermic peak due to water evaporation from the wet gel and the expulsion of water inside the dry gel (adsorbed water, crystallization water, and water vapor generated during the reaction). When the temperature increases from 140 °C to 207 °C, there is a weight loss of about 70 percent and a sharp exothermic peak corresponding to the DTA curves at about 207 °C. When the temperature is higher than 207 °C, the weight loss rate was less than 5%, forming La1−xAlxFeO3 [7].  Figure 7 shows the XRD diffraction pattern of La1−xNdxFeO3 (x = 0~0.25) samples calcined at 600 °C for 2 h and (121) peak drift pattern. According to Figure 7b, the (121) diffraction peak slightly increased with the increase in Nd 3+ doped concentration and drifted in the direction of 2θ [24]. The substitution of small radius Nd 3+ (ionic radius of 0.127 nm) for larger radius La 3+ (ionic radius of 0.274 nm) leads to lattice distortion [26]. The lattice parameters of corresponding samples are as shown in Table 3. As the Nd 3+ content increased, the crystal lattice parameters (a, b, and c) changed accordingly [27], and the cell volume of the samples decreased. The reason is that the La ion was gradually replaced by Nd ion, which reduced the average ionic radius of A-bit and, in turn, caused cell shrinkage resulting in a decrease in cell volume [28]. The grain size of the samples can be estimated    Figure 7b, the (121) diffraction peak slightly increased with the increase in Nd 3+ doped concentration and drifted in the direction of 2θ [24]. The substitution of small radius Nd 3+ (ionic radius of 0.127 nm) for larger radius La 3+ (ionic radius of 0.274 nm) leads to lattice distortion [26]. The lattice parameters of corresponding samples are as shown in Table 3. As the Nd 3+ content increased, the crystal lattice parameters (a, b, and c) changed accordingly [27], and the cell volume of the samples decreased. The reason is that the La ion was gradually replaced by Nd ion, which reduced the average ionic radius of A-bit and, in turn, caused cell shrinkage, resulting in a decrease in cell volume [28]. The grain size of the samples can be estimated according to Scherrer's formula. When x ≤ 0.05, the average grain size decreased. When x > 0.05, as the doping amount increased, the grain size increased. It is possible that the ion size of A-bit changed with its nearest neighbor, and the size of the neighboring oxygen atoms depends on the grain size [29,30].

XRD and TG-DTA Analysis of La1−xRxFeO3 (R = Nd)
Molecules 2023, 28, x FOR PEER REVIEW 6 of 17 according to Scherrer's formula. When x ≤ 0.05, the average grain size decreased. When x > 0.05, as the doping amount increased, the grain size increased. It is possible that the ion size of A-bit changed with its nearest neighbor, and the size of the neighboring oxygen atoms depends on the grain size [29,30].  The XRD diffraction pattern in Figure 8 shows that the main diffraction peak is consistent with the standard sample LaFeO3 card (JCPDS No. . No other phases were generated, and the space group is Pnma.   The XRD diffraction pattern in Figure 8 shows that the main diffraction peak is consistent with the standard sample LaFeO 3 card (JCPDS No. . No other phases were generated, and the space group is Pnma. according to Scherrer's formula. When x ≤ 0.05, the average grain size decreased. When x > 0.05, as the doping amount increased, the grain size increased. It is possible that the ion size of A-bit changed with its nearest neighbor, and the size of the neighboring oxygen atoms depends on the grain size [29,30].  The XRD diffraction pattern in Figure 8 shows that the main diffraction peak is consistent with the standard sample LaFeO3 card (JCPDS No. . No other phases were generated, and the space group is Pnma.   The results show that under four calcination conditions, Nd 3+ is better immersed in the crystal lattice of the perovskite, and all characteristic peaks have been indexed according to the orthorhombic structure [1]. As the calcination temperature rose, the diffraction peak became sharper, the half-height width decreased, and the average grain size estimated by Scherrer's formula gradually increased. The results show that the calcination temperature had a direct effect on the grain size of the powder when the samples were synthesized using the sol-gel method. The higher the calcination temperature, the higher the energy, the larger the grain size, and the larger the size of the powder. 2.4. XRD and TG-DTA Analysis of La 1−x R x FeO 3 (R = Sm) Figure 9 shows an XRD diffraction pattern of the La 1−x Sm x FeO 3 (x = 0~0.5) samples. According to the tolerance factor t = r A + r O /1.414 (r B + r O ), r A , r B , and r O are A-bit ionic radius, B-bit ionic radius, and O ionic radius, respectively. For part of the A-bit doped samples, r A = (A ionic radius) (1 − x) + (radius of doped ions) x; when t values were between 0.75 and 1.00, the perovskite structure was stable. According to the ionic radius records [31,32], La 3+ ionic radius was 0.274 nm, Sm 3+ ionic radius was 0.124 nm, Fe 3+ ionic radius was 0.0645 nm, O ionic radius was 0.132 nm, and the t value of the series samples was between 0.75 and 1.00, which had a stable perovskite structure [33,34]. As the Sm 3+ doping concentration increased, the intensity of diffraction peaks decreased. The drift pattern shows that as the Sm 3+ doping content increased, the diffraction peak drifted in the direction of the larger angle. This explains that according to the equation below, the smaller radius of Sm 3+ (ionic radius of 0.124 nm) replaced the larger radius of La 3+ (ionic radius of 0.274 nm). The results show that under four calcination conditions, Nd 3+ is better immersed in the crystal lattice of the perovskite, and all characteristic peaks have been indexed according to the orthorhombic structure [1]. As the calcination temperature rose, the diffraction peak became sharper, the half-height width decreased, and the average grain size estimated by Scherrer's formula gradually increased. The results show that the calcination temperature had a direct effect on the grain size of the powder when the samples were synthesized using the sol-gel method. The higher the calcination temperature, the higher the energy, the larger the grain size, and the larger the size of the powder. Figure 9 shows an XRD diffraction pattern of the La1−xSmxFeO3 (x = 0~0.5) samples. According to the tolerance factor t = rA + rO/1.414 (rB + rO), rA, rB, and rO are A-bit ionic radius, B-bit ionic radius, and O ionic radius, respectively. For part of the A-bit doped samples, rA = (A′ ionic radius) (1 − x) + (radius of doped ions) x; when t values were between 0.75 and 1.00, the perovskite structure was stable. According to the ionic radius records [31,32], La 3+ ionic radius was 0.274 nm, Sm 3+ ionic radius was 0.124 nm, Fe 3+ ionic radius was 0.0645 nm, O ionic radius was 0.132 nm, and the t value of the series samples was between 0.75 and 1.00, which had a stable perovskite structure [33,34]. As the Sm 3+ doping concentration increased, the intensity of diffraction peaks decreased. The drift pattern shows that as the Sm 3+ doping content increased, the diffraction peak drifted in the direction of the larger angle. This explains that according to the equation below, the smaller radius of Sm 3+ (ionic radius of 0.124 nm) replaced the larger radius of La 3+ (ionic radius of 0.274 nm).  Table 4 shows that as the Sm 3+ ion doping amount increased, the cell volume decreased. Figure 10 shows the XRD diffraction pattern of uncalcined La0.8Sm0.2FeO3 samples, and then calcined at 700 °C and 800 °C for 2 h. The sample diffraction peaks were sharp, suggesting that the samples crystallized well after sintering at a certain temperature. The half-width of the diffraction peak of the samples was 0.371, 0.333, and 0.378, respectively, corresponding to the aforementioned calcination temperatures, with a first decreasing then increasing change trend [1,2]. The average grain size of the samples was 22.7, 25.9, and 21.1 nm, respectively, corresponding to the aforementioned calcination temperatures, with a first increasing then decreasing change trend [6,7].  Table 4 shows that as the Sm 3+ ion doping amount increased, the cell volume decreased. Figure 10 shows the XRD diffraction pattern of uncalcined La 0.8 Sm 0.2 FeO 3 samples, and then calcined at 700 • C and 800 • C for 2 h. The sample diffraction peaks were sharp, suggesting that the samples crystallized well after sintering at a certain temperature. The half-width of the diffraction peak of the samples was 0.371, 0.333, and 0.378, respectively, corresponding to the aforementioned calcination temperatures, with a first decreasing then increasing change trend [1,2]. The average grain size of the samples was 22.7, 25.9, and 21.1 nm, respectively, corresponding to the aforementioned calcination temperatures, with a first increasing then decreasing change trend [6,7].    Figure 11 shows the TG and DTA curves of La0.8Sm0.2FeO3 xerogel. When the temperature rose from 30 °C to 200 °C, the TG curve showed that the weight loss rate was about 13%. Then, the DTA curve showed a weak endothermic peak at 98.5 °C due to the water evaporation from the wet gel. The weak endothermic peaks at 133 °C and 171 °C were due to the expulsion of water from inside the dry gel. When the temperature rose from 200 °C to 226 °C, the weight loss was about 63%, showing a sharp exothermic peak corresponding to the DTA curves at about 226 °C, which may be due to emissions of nitrates and organic substances, including NOx, COx, and H2O. When the temperature was higher than 226 °C, the weight loss rate was less than 3%, indicating the formation of La1−xSmxFeO3.  Figure 11 shows the TG and DTA curves of La 0.8 Sm 0.2 FeO 3 xerogel. When the temperature rose from 30 • C to 200 • C, the TG curve showed that the weight loss rate was about 13%. Then, the DTA curve showed a weak endothermic peak at 98.5 • C due to the water evaporation from the wet gel. The weak endothermic peaks at 133 • C and 171 • C were due to the expulsion of water from inside the dry gel. When the temperature rose from 200 • C to 226 • C, the weight loss was about 63%, showing a sharp exothermic peak corresponding to the DTA curves at about 226 • C, which may be due to emissions of nitrates and organic substances, including NO x , CO x , and H 2 O. When the temperature was higher than 226 • C, the weight loss rate was less than 3%, indicating the formation of La 1−x Sm x FeO 3 .   Figure 11 shows the TG and DTA curves of La0.8Sm0.2FeO3 xerogel. When the temperature rose from 30 °C to 200 °C, the TG curve showed that the weight loss rate was about 13%. Then, the DTA curve showed a weak endothermic peak at 98.5 °C due to the water evaporation from the wet gel. The weak endothermic peaks at 133 °C and 171 °C were due to the expulsion of water from inside the dry gel. When the temperature rose from 200 °C to 226 °C, the weight loss was about 63%, showing a sharp exothermic peak corresponding to the DTA curves at about 226 °C, which may be due to emissions of nitrates and organic substances, including NOx, COx, and H2O. When the temperature was higher than 226 °C, the weight loss rate was less than 3%, indicating the formation of La1−xSmxFeO3.   Figure 12 shows the hysteresis loop of the La 1−x Co x FeO 3 (x = 0~0.25) sample. When the applied magnetic field is 8000 Oe, the magnetization reaches a saturation state [35]. As shown in Table 5, with the increase in Co 2+ concentration, the magnetic loop area and magnetization increased. The change of valence state of the A-site ions will directly affect the state of the oxygen ions, which will eventually lead to the generation of oxygen vacancies.

Magnetic Analysis of La 1−x R x FeO 3 (R = Co)
Molecules 2023, 28, x FOR PEER REVIEW 9 of 17 Figure 12 shows the hysteresis loop of the La1−xCoxFeO3 (x = 0~0.25) sample. When the applied magnetic field is 8000 Oe, the magnetization reaches a saturation state [35]. As shown in Table 5, with the increase in Co 2+ concentration, the magnetic loop area and magnetization increased. The change of valence state of the A-site ions will directly affect the state of the oxygen ions, which will eventually lead to the generation of oxygen vacancies.   Figure 13 shows the hysteresis loop of La0.85Co0.15FeO3 sample calcined at temperatures between 600 and 1000 °C for 6 h. When the applied magnetic field is 8000 Oe, the magnetization reaches a saturation state. Figure 13 and Table 6 show that when the samples calcined temperature rose from 600 °C to 700 °C, the loop area of the M-H curve changed significantly. With the increase in calcination temperature, the saturation magnetization, remanent magnetization, and coercivity of the samples increased, which may be affected by the mixed-phase CoFe2O4 samples.   Figure 13 shows the hysteresis loop of La 0.85 Co 0.15 FeO 3 sample calcined at temperatures between 600 and 1000 • C for 6 h. When the applied magnetic field is 8000 Oe, the magnetization reaches a saturation state. Figure 13 and Table 6 show that when the samples calcined temperature rose from 600 • C to 700 • C, the loop area of the M-H curve changed significantly. With the increase in calcination temperature, the saturation magnetization, remanent magnetization, and coercivity of the samples increased, which may be affected by the mixed-phase CoFe 2 O 4 samples.

Magnetic Analysis of La1−xRxFeO3 (R = Co)
In Table 7, Co 2+ doping improves the coercive force of a sample. It is caused by the relation of coercive force Hc and magnetocrystalline anisotropy [18,19]. The magnetic moments of the sublattice array composed of iron ions are oppositely aligned in the same straight line, so the samples show antiferromagnetic properties on a macroscopic scale. The magnetic ion Co has a 3d 7 electronic configuration and stronger spin-orbit coupling. When Co 2+ replaced A-bit non-magnetic ion La 3+ , the La 1−x Co x FeO 3 samples had a stronger magnetocrystalline anisotropy constant. When the doping amount x ≥ 0.10, the changes in the coercivity were irregular. There may be a relationship between coercivity and grain size [36]. When the calcination temperature was 600 • C, no CoFe 2 O 4 mixed phase was detected. When it rose to 700 • C, we found the second phase CoFe 2 O 4 , and when it rose to 1000 • C, the magnetization of the sample was the largest and the coercive force of sample was 1213 Oe. Figure 14 shows the hysteresis loop of La 0.85 CO 0.15 FeO 3 samples. The calcination time can regulate the magnetic properties of the sample, as shown in Table 7. From 2 h to 6 h, the magnetic properties reduced significantly.
tures between 600 and 1000 °C for 6 h. When the applied magnetic field is 8000 Oe, the magnetization reaches a saturation state. Figure 13 and Table 6 show that when the samples calcined temperature rose from 600 °C to 700 °C, the loop area of the M-H curve changed significantly. With the increase in calcination temperature, the saturation magnetization, remanent magnetization, and coercivity of the samples increased, which may be affected by the mixed-phase CoFe2O4 samples.    In Table 7, Co 2+ doping improves the coercive force of a sample. It is caused by the relation of coercive force Hc and magnetocrystalline anisotropy [18,19]. The magnetic moments of the sublattice array composed of iron ions are oppositely aligned in the same straight line, so the samples show antiferromagnetic properties on a macroscopic scale. The magnetic ion Co has a 3d 7 electronic configuration and stronger spin-orbit coupling. When Co 2+ replaced A-bit non-magnetic ion La 3+ , the La1−xCoxFeO3 samples had a stronger magnetocrystalline anisotropy constant. When the doping amount x ≥ 0.10, the changes in the coercivity were irregular. There may be a relationship between coercivity and grain size [36]. When the calcination temperature was 600 °C, no CoFe2O4 mixed phase was detected. When it rose to 700 °C, we found the second phase CoFe2O4, and when it rose to 1000 °C, the magnetization of the sample was the largest and the coercive force of sample was 1213 Oe. Figure 14 shows the hysteresis loop of La0.85CO0.15FeO3 samples. The calcination time can regulate the magnetic properties of the sample, as shown in Table 7. From 2 h to 6 h, the magnetic properties reduced significantly.   Figure 15 is hysteresis loop of La1−xAlxFeO3 (x = 0~0.10) samples, the magnetic loop area and magnetization increased with the increase in Al 3+ ion concentration and the weak ferromagnetic properties are shown in Table 8. In fact, in the perovskite structure, Fe 3+ was in a distorted octahedral B-site location, and the octahedron distortion was inclined along the c-axis direction, and the inclination depended on the size of the adjacent A-site ions,   Table 8. In fact, in the perovskite structure, Fe 3+ was in a distorted octahedral B-site location, and the octahedron distortion was inclined along the c-axis direction, and the inclination depended on the size of the adjacent A-site ions, which ultimately determined the Fe-O-Fe super-exchange angle. The reason this may enhance the magnetic properties is as follows: First, when Al 3+ ions substituted La 3+ ions, the effective size of the A-site of octahedral reduced, changing the Fe-O-Fe superexchange angle and promoting super-exchange interaction. Second, when the sample was doped, only a small fraction of La 3+ was substituted by Al 3+ ions, which tend to occupy Fe 3+ ion positions at octahedral B-bit. When Al 3+ ion substituted the Fe 3+ ion, Fe 3+ was squeezed into the La 3+ ion position of the regular tetrahedron A-bit. This time, the Fe 3+ ion spin was not compensated, and the magnetic properties of the sample can be improved. In addition, Fe 2 O 3 impurity phase was detected by XRD, which may also enhance the magnetic properties of the La 1−x Al x FeO 3 samples [21,22]. Figure 16 shows the hysteresis loop of La 0.9 Al 0.1 FeO 3 samples. It can be seen from the figure that as the calcination temperature increased, the M-H curve of the sample enclosed area reduced, the ferromagnetic order gradually weakened and tended to be of the antiferromagnetic order, which may be due to with the increase in sintering temperature, decrease in iron oxide composition, and changes in the magnetic properties [14].
Molecules 2023, 28, x FOR PEER REVIEW 11 of 17 which ultimately determined the Fe-O-Fe super-exchange angle. The reason this may enhance the magnetic properties is as follows: First, when Al 3+ ions substituted La 3+ ions, the effective size of the A-site of octahedral reduced, changing the Fe-O-Fe super-exchange angle and promoting super-exchange interaction. Second, when the sample was doped, only a small fraction of La 3+ was substituted by Al 3+ ions, which tend to occupy Fe 3+ ion positions at octahedral B-bit. When Al 3+ ion substituted the Fe 3+ ion, Fe 3+ was squeezed into the La 3+ ion position of the regular tetrahedron A-bit. This time, the Fe 3+ ion spin was not compensated, and the magnetic properties of the sample can be improved. In addition, Fe2O3 impurity phase was detected by XRD, which may also enhance the magnetic properties of the La1−xAlxFeO3 samples [21,22]. Figure 16 shows the hysteresis loop of La0.9Al0.1FeO3 samples. It can be seen from the figure that as the calcination temperature increased, the M-H curve of the sample enclosed area reduced, the ferromagnetic order gradually weakened and tended to be of the antiferromagnetic order, which may be due to with the increase in sintering temperature, decrease in iron oxide composition, and changes in the magnetic properties [14].    which ultimately determined the Fe-O-Fe super-exchange angle. The reason this may enhance the magnetic properties is as follows: First, when Al 3+ ions substituted La 3+ ions, the effective size of the A-site of octahedral reduced, changing the Fe-O-Fe super-exchange angle and promoting super-exchange interaction. Second, when the sample was doped, only a small fraction of La 3+ was substituted by Al 3+ ions, which tend to occupy Fe 3+ ion positions at octahedral B-bit. When Al 3+ ion substituted the Fe 3+ ion, Fe 3+ was squeezed into the La 3+ ion position of the regular tetrahedron A-bit. This time, the Fe 3+ ion spin was not compensated, and the magnetic properties of the sample can be improved. In addition, Fe2O3 impurity phase was detected by XRD, which may also enhance the magnetic properties of the La1−xAlxFeO3 samples [21,22]. Figure 16 shows the hysteresis loop of La0.9Al0.1FeO3 samples. It can be seen from the figure that as the calcination temperature increased, the M-H curve of the sample enclosed area reduced, the ferromagnetic order gradually weakened and tended to be of the antiferromagnetic order, which may be due to with the increase in sintering temperature, decrease in iron oxide composition, and changes in the magnetic properties [14].    Table 9 shows that the calcination temperature has the same effect on Ms, Mr, and Hc; all of which showed a declining trend as T increased. Figure 17 shows the hysteresis loop of La 0.9 Al 0.1 FeO 3 samples. The calcination time can regulate the magnetic properties of the sample, as shown in Table 10. From 2 h to 6 h, the magnetic properties reduced significantly. When the calcination time increased from 6 h to 10 h, the sintering time showed little impact on the changes in the magnetic properties.  Table 9 shows that the calcination temperature has the same effect on Ms, Mr, and Hc; all of which showed a declining trend as T increased. Figure 17 shows the hysteresis loop of La0.9Al0.1FeO3 samples. The calcination time can regulate the magnetic properties of the sample, as shown in Table 10. From 2 h to 6 h, the magnetic properties reduced significantly. When the calcination time increased from 6 h to 10 h, the sintering time showed little impact on the changes in the magnetic properties.    Figure 18 is the hysteresis loop of the La1−xNdxFeO3 (x = 0~0.25) samples calcined at 600 °C for 2 h, which was measured at room temperature, and has an applied magnetic field of 0.6 T. It can be seen that the hysteresis loop had a narrow shape, and all samples did not reach a saturation state, showing weak ferromagnetic properties. When x ≤ 0.20, as the Nd 3+ doped concentration increased, the magnetization of sample increased. This may be due to the fact that the Nd atomic magnetic moment is not zero. The introduction of Nd 3+ leads to an unbalanced distribution of magnetic moment of the whole structure, thereby increasing the magnetization of the samples. When Nd 3+ was doped, the magnetization of sample began to decrease.   Figure 18 is the hysteresis loop of the La 1−x Nd x FeO 3 (x = 0~0.25) samples calcined at 600 • C for 2 h, which was measured at room temperature, and has an applied magnetic field of 0.6 T. It can be seen that the hysteresis loop had a narrow shape, and all samples did not reach a saturation state, showing weak ferromagnetic properties. When x ≤ 0.20, as the Nd 3+ doped concentration increased, the magnetization of sample increased. This may be due to the fact that the Nd atomic magnetic moment is not zero. The introduction of Nd 3+ leads to an unbalanced distribution of magnetic moment of the whole structure, thereby increasing the magnetization of the samples. When Nd 3+ was doped, the magnetization of sample began to decrease. The Nd 3+ ion concentration reached a certain amount, the Fe-O bond length decreased, and the inclination of the octahedron decreased, thereby strengthening antiferromagnetic exchange interaction and weakening the interaction between weak ferromagnetics. Eventually, the magnetization of the samples was weakened. Table 11 shows the magnetic parameters of the samples. As the Nd 3+ content increased, the remanent magnetization and saturation magnetization of the samples first increased then decreased. The magnetic moment and ionic radius of rare earth elements ions are the main factors affecting the saturation magnetization of the doping samples [37]. The coercivity of the samples shows an upward trend, and the main factors affecting the coercivity of the samples include magnetic anisotropy, grain size, micro strain, stress, crystal symmetry and spin-orbit coupling effect, magnetic single domain size, and impurities [31]. From Figure 19, it can be seen that the magnetization of the samples hardly changed when the calcination temperature rose from 400 °C to 600 °C. As the calcination temperature decreased, the saturation magnetization (Ms) of the samples decreased. The magnetization of the samples changed little when the temperature continued to rise to 1000 °C. As the calcination temperature increased, the magnetization of the samples decreased. It can be seen from Figure 20 that as the calcination time extended, the saturation magnetization of the samples first decreased then increased. When the calcination time was extended, the magnetization changed significantly, and the optimum calcination time was 2 h. The main factors affecting the coercivity of the samples include magnetic anisotropy, grain size, micro strain, stress, crystal symmetry and spin-orbit coupling effect, magnetic single domain size, impurities, and calcination temperature. The Nd 3+ ion concentration reached a certain amount, the Fe-O bond length decreased, and the inclination of the octahedron decreased, thereby strengthening antiferromagnetic exchange interaction and weakening the interaction between weak ferromagnetics. Eventually, the magnetization of the samples was weakened. Table 11 shows the magnetic parameters of the samples. As the Nd 3+ content increased, the remanent magnetization and saturation magnetization of the samples first increased then decreased. The magnetic moment and ionic radius of rare earth elements ions are the main factors affecting the saturation magnetization of the doping samples [37]. The coercivity of the samples shows an upward trend, and the main factors affecting the coercivity of the samples include magnetic anisotropy, grain size, micro strain, stress, crystal symmetry and spin-orbit coupling effect, magnetic single domain size, and impurities [31]. From Figure 19, it can be seen that the magnetization of the samples hardly changed when the calcination temperature rose from 400 • C to 600 • C. As the calcination temperature decreased, the saturation magnetization (Ms) of the samples decreased. The magnetization of the samples changed little when the temperature continued to rise to 1000 • C. As the calcination temperature increased, the magnetization of the samples decreased. It can be seen from Figure 20 that as the calcination time extended, the saturation magnetization of the samples first decreased then increased. When the calcination time was extended, the magnetization changed significantly, and the optimum calcination time was 2 h. The main factors affecting the coercivity of the samples include magnetic anisotropy, grain size, micro strain, stress, crystal symmetry and spin-orbit coupling effect, magnetic single domain size, impurities, and calcination temperature.    Figure 21 shows the hysteresis loop of the uncalcined La1−xSmxFeO3 (x = 0~0.5) samples. It can be seen that the hysteresis loop had a narrow shape, and all samples did not reach a saturation state, showing weak ferromagnetic properties. Table 12 shows the distribution of magnetic parameters of the sample. This may be because the Sm 3+ ions are magnetic in nature [37], and together with Fe 3+ and O 2− , generate a super-exchange interaction, which affected the net magnetic moment size of the crystals, ultimately affecting the magnetization of the samples.    Figure 21 shows the hysteresis loop of the uncalcined La1−xSmxFeO3 (x = 0~0.5) samples. It can be seen that the hysteresis loop had a narrow shape, and all samples did not reach a saturation state, showing weak ferromagnetic properties. Table 12 shows the distribution of magnetic parameters of the sample. This may be because the Sm 3+ ions are magnetic in nature [37], and together with Fe 3+ and O 2− , generate a super-exchange interaction, which affected the net magnetic moment size of the crystals, ultimately affecting the magnetization of the samples.    Figure 21 shows the hysteresis loop of the uncalcined La 1−x Sm x FeO 3 (x = 0~0.5) samples. It can be seen that the hysteresis loop had a narrow shape, and all samples did not reach a saturation state, showing weak ferromagnetic properties. Table 12 shows the distribution of magnetic parameters of the sample. This may be because the Sm 3+ ions are magnetic in nature [37], and together with Fe 3+ and O 2− , generate a super-exchange interaction, which affected the net magnetic moment size of the crystals, ultimately affecting the magnetization of the samples.

Experimental
Compared with other preparation methods, the advantages of the sol-gel method are as follows [13,14]: (1) The uniformity of components in the reaction process is good, and