Mechanosynthesis, Crystal Structure and Magnetic Characterization of Neodymium Orthoferrite

Neodymium-based materials are used in such applications and products as magnetic suspensions 1 , photoluminescence 2, measurement of the grain size distribution in thin films 3 , magnetic molecules 4, optical ring cores 5, coloring agents for glasses 6, and hard or soft magnets, depending on the crystal structure (a hexagonal or garnet crystal structure for hard magnets and a spinel crystal structure with an applied external magnetic field for temporary, soft magnets). When the external field decreases or disappears, the saturation magnetic value in a soft magnetic material is reduced. In these cases, different material groups exist. Ferrites 7 , such as neodymium orthoferrite NdFeO3, are one such group. NdFeO3 is a soft material with an orthorhombically distorted structure derived from a cubic perovskite structure and was recently applied as an anode in solid oxide fuel cells 8,9. Although there have been various reports of mechanical alloy-based syntheses of NdFeO3, the products were generated under different conditions (generating sub-products) and using different precursors (T. Alonso, Yinong Liu, and P:G:McComirc), such as Ar and Fe3O4 10. Alternative synthesis methods were studied in 2011 and 2013 11,12, whereas the ceramic method has been traditionally used for the synthesis of this material. In this report, the mechanosynthesis of NdFeO3 is investigated as an alternative room-temperature method of driving a reaction using impact energy. The energy consumption of this method is markedly lower than that of the traditional synthesis approach, which utilizes temperatures of approximately 1000-1300 °C for periods of 12 or 24 h 10,13,14. In some cases, the detected nanoscale particles exhibited improved magnetic properties and electric conductivities relative to the microscale particles 15. The morphology change observed for the materials obtained by mechanosynthesis is possible due to the use of such precursor materials as oxides and carbonates, which allows the reaction to be achieved via oxygen reduction and avoids contamination with acids or toxic gas vapors 16,17. The process is easy when the precursors are reactive and their atomic diameters and structures are similar. In this paper, we present the crystal structure, magnetic saturation and particle morphology of NdFeO3 powders obtained by mechanosynthesis using a high-energy shaker/mixer mill at room temperature from Fe2O3 and Nd2O3 precursors under dry conditions without the use of a noble gas.


Introduction
Neodymium-based materials are used in such applications and products as magnetic suspensions 1 , photoluminescence 2 , measurement of the grain size distribution in thin films 3 , magnetic molecules 4 , optical ring cores 5 , coloring agents for glasses 6 , and hard or soft magnets, depending on the crystal structure (a hexagonal or garnet crystal structure for hard magnets and a spinel crystal structure with an applied external magnetic field for temporary, soft magnets). When the external field decreases or disappears, the saturation magnetic value in a soft magnetic material is reduced. In these cases, different material groups exist. Ferrites 7 , such as neodymium orthoferrite NdFeO 3 , are one such group. NdFeO 3 is a soft material with an orthorhombically distorted structure derived from a cubic perovskite structure and was recently applied as an anode in solid oxide fuel cells 8,9 . Although there have been various reports of mechanical alloy-based syntheses of NdFeO 3 , the products were generated under different conditions (generating sub-products) and using different precursors (T. Alonso, Yinong Liu, and P:G:McComirc), such as Ar and Fe 3 O 4 10 . Alternative synthesis methods were studied in 2011 and 2013 11,12 , whereas the ceramic method has been traditionally used for the synthesis of this material. In this report, the mechanosynthesis of NdFeO 3 is investigated as an alternative room-temperature method of driving a reaction using impact energy. The energy consumption of this method is markedly lower than that of the traditional synthesis approach, which utilizes temperatures of approximately 1000-1300 °C for periods of 12 or 24 h 10,13,14 . In some cases, the detected nanoscale particles exhibited improved magnetic properties and electric conductivities relative to the microscale particles 15 . The morphology change observed for the materials obtained by mechanosynthesis is possible due to the use of such precursor materials as oxides and carbonates, which allows the reaction to be achieved via oxygen reduction and avoids contamination with acids or toxic gas vapors 16,17 . The process is easy when the precursors are reactive and their atomic diameters and structures are similar.
In this paper, we present the crystal structure, magnetic saturation and particle morphology of NdFeO 3 powders obtained by mechanosynthesis using a high-energy shaker/mixer mill at room temperature from Fe 2 O 3 and Nd 2 O 3 precursors under dry conditions without the use of a noble gas.

Experimental
Iron oxide III (Sigma-Aldrich) and neodymium oxide III (Sigma-Aldrich) precursor oxides with a purity of higher than 99% were used in the milling process. The ratio was stoichiometric, as shown in equation 1. The volume of the stainless steel vials was 50 cm 3 , and the milling times

Pedro Vera Serna a *, Cecilio García Campos a , Felix Sánchez De Jesús b , Ana María Bolarín
The crystal structure was evaluated using a Bruker D8 Advance theta-theta X-ray diffractometer (XRD), with 2.2 kW Cu (λ= 0.14051 Å) radiation. The milling product was measured by diffraction in the 5-90° 2θ range with a step size of 0.02º. A LynxEye XE detector was used to provide improved fluorescence filtering and thereby improve the peak intensity. DIFFRAC.TOPAS software, version 4.2, was used for the Rietveld refinement, and the results were then compared with those in the ISCD structural database. The morphology was observed using a JEOL scanning electron microscope at 15 kV, and the magnetic saturation was evaluated using a MicroSense EV7 vibrating sample magnetometer (VSM-EV7) with a maximum field of 18000 Oe.

Crystal structure characterization
The mixture of Nd 2 O 3 and Fe 2 O 3 powders was structurally characterized with XRD using the LynxEye XE detector (minimizing fluorescence contamination), which provides easier peak identification relative to traditional devices using Cu radiation. Figure 1 shows the peaks at 0 hours as a baseline. The identified Nd 2 O 3 peaks correspond to the ICSD database PDF 00-041-1089 hexagonal structure, those of Nd 2 O 2 CO 3 correspond to PDF 00-037-0806, and those of Fe 2 O 3 correspond to the PDF 04-003-2900 cubic structure. Other peaks are also present due to the absorption of hydrogen and oxygen by neodymium as Nd(OH) 3 , identified in PDF 01-070-0214. The difference in the ionic radii of Fe and Nd is small, allowing the diffusion of Nd in Fe 2 O 3 , as shown in Figure 2. The synthesized NdFeO 3 (PDF 04-010-3990) was detected at 4.5 hours, a similar time to that reported by Alonso et al 10 . Examination of the XRD pattern shows that Nd(OH) 3 peaks disappear due to the diffusion of Nd. With additional milling, the structure became orthorhombic, and the maximum quantity of NdFeO 3 was synthesized at 13.5 hours, as shown in Figure 3. It is possible to observe only the orthorhombic phase of neodymium ferrite. The hexagonal structure of NdO 3 was transformed to distorted perovskite. The results indicate that the low activation energy allowed the chemical reaction to start at the ambient room temperature within only a few hours, thus minimizing contamination or residual impurities from other materials.
The XRD pattern shown in Figure 4 corresponds to 15 hours of milling; in this pattern, it was possible to identify peaks between 2θ =24º and 2θ =25º, corresponding to Fe 2 O 3 (PDF 00-033-0664). This observation was most likely caused by wear and tear on the iron milling balls. Energy dispersive spectrometry (EDS) was used to find possible metallic elements derived from the balls; the results presented in Figure 5 show that iron, oxygen and   Figure 7 shows the particle morphology images. We found that the morphology was irregular during the synthesis process. In the image taken at 0 hours, the particle sizes of the powders are similar, and at 4.5 hours, some particles with dimensions of approximately 10 micrometers were observed. This finding can be attributed to the milling process, in which the particles are deformed and the newly exposed surfaces facilitate the growth of agglomerates and particles. The NdFeO 3 phase identified by XRD indicated that the chemical reaction was facilitated by the exposure of new surfaces. As the milling continued, different particle sizes were detected at 4.5 hours, while the particles observed at 9 hours had similar morphologies. Finally, 13.5 hours, the particles were small with a narrow size distribution.

Particle morphology
The images presented in Figure 8 suggest that the NdFeO 3 particles obtained after 13.5 hours of milling were smaller than those obtained using the mixture of Nd 2 O 3 and Fe 2 O 3 , as the particles in the center of the right-hand image are smaller than those in the left-hand image. The increased interaction between the reactants due to the milling process and the exposure of new surfaces favored the chemical reaction.

Magnetic saturation characterization
The magnetic saturation data for powders obtained at different milling times are shown in Figure 9. It is well known that mechanosynthesis changes the crystal structure of materials, thereby modifying their magnetic behavior. The magnetic saturation for NdFeO 3 obtained herein (1.15 emu/g) was superior to the value obtained by Hu et al. (0.8 emu/g) and similar to the value reported for NdFeO 3 with cubic morphology by Wang et al. 18,19 . At 15 hours of milling, corresponding to the time at which carbon was detected, the magnetic saturation was inferior to that obtained after 13.5 hours of milling. Interestingly, when the phase transformation began (4.5 hours), the magnetic saturation was 1.54 emu/g, superior to the values reported for other ferrites.  carbon were detected, of which the carbon originated from the polymer used to fix the sample. The results by weight percent for the powders obtained from the milling process were calculated using Topas software ( Figure 6). The optimal mechanosynthesis time for NdFeO 3 was 13.5 h. Table 1 shows the weight percentages obtained by Rietveld refinement, and Table 2 shows the lattice parameters.

Conclusion
The mechanosynthesis of NdFeO 3 from the stoichiometric ratio of Nd 2 O 3 and Fe 2 O 3 powders was possible at room temperature, avoiding the high temperature (1000 °C) required when using the calcination method. The synthesis yield was close to 100% at 13.5 hours of milling, and the NdFeO 3 phase was detected after 4.5 hours of milling, as indicated by the formation of new surfaces after a few hours. The mechanosynthesis modified the crystal structure, reduced the particle size and increased the magnetic saturation above that of other orthoferrites. Our report provides new information about the morphology and magnetic saturation of this material and unprecedentedly high-resolution XRD results obtained using a LynxEye XE detector with Cu radiation.