Sol-gel synthesis, crystal structure and magnetic properties of nanocrystalline praseodymium orthoferrite

In this work, nano-sized crystalline praseodymium orthoferrite was successfully synthesized via sol-gel method using water – methanol co-solvent. Single-phase PrFeO3 nanoparticles were formed after annealing the precursors at 650, 750, 850, and 950 °C during 60 min. The crystal size, lattice volume and coercivity (Hc) of nanocrystalline PrFeO3 increase with the annealing temperature. The obtained praseodymium orthoferrite exhibited paramagnetic properties with Hc = 28 – 34 Oe.

A mixture of Pr(NO 3 ) 3 · 6H 2 O and Fe(NO 3 ) 3 · 9H 2 O (1:1 mol to mol ratio) was dissolved in 50 mL solvent of H 2 O -CH 3 OH (1:1, V/V). The mixture solution was then added dropwise to a roundbottom flask containing 150 mL boiling H 2 O -CH 3 OH co-solvent (T ~ 85 °C). The slow addition of Pr (III) and Fe (III) mixture to the co-solvent at 85 °C would increase the hydrolysis process, thus hinder and control the particle size of orthoferrite PrFeO 3 . Details of optimized conditions can be found in previous reports on the synthesis of LnFeO 3 (Ln = Y, La, Nd) orthoferrites [17][18][19]. The system continued to be refluxed for an additional 30 minutes before cooling down to ~ 30 °C, resulted in a brownish-yellow mixture. By refluxing, the solvent volume was maintained and the diffusion of toxic CH 3 OH vapor to the environment could be minimized. Next, NH 3 5 % solution was added dropwise to the system until pH ~ 9÷10 (tested by pH paper). The system was stirred for 30 minutes, then vacuum filtered. After removing all the filtrate, the residue was dry at 50 °C during 3 hours and grounded to obtain brownish-yellow powder (precursor for the synthesis of o-PrFeO 3 ).
The content and surface distribution of the elements (Pr, Fe, O) were studied by energy-dispersive X-ray spectroscopy (EDX and EDX-mapping), FE-SEM S-4800 (Japan). The quantitative elemental composition were taken as the average of 5 different positions of each sample.
Crystal size and morphology of PrFeO 3 samples were characterized by transmission electron spectroscopy (TEM), Joel JEM-1400 (Japan). The crystal size distribution of were determined by IMAGE J.
Hysteresis loop and magnetic properties including coercive force (H c , Oe), remanent magnetization (M r , emu/g) and saturation magnetization (M s , emu/g) were recorded on vibrating sample magnetometer (VSM, MICROSENE EV11) under the magnetic field in the range of -21 000 Oe to +21 000 Oe. Fig. 1 shows the TG-DSC curves of precursors for the synthesis of o-PrFeO 3 nanomaterial. The total mass loss from room temperature to 950 °C was 23.67 %. This result proves the formation of bonds between Pr(III) and Fe(III) cation with CH 3 -group in the precipitate [21]. Indeed, if this precipitate had only included Pr(OH) 3 ↓ and Fe(OH) 3 ↓, mass loss deduced from equation (1) would have been 18.07 %.

Results and discussion
The mass loss by decomposition of M 3+ -CH 3 (M = Pr, Fe) bonds corresponds to the exothermic peak at 270.56 °C on the DSC curve (Fig. 1). The endothermic peaks at 113.37 and 358.52 °C are the dehydrate and decomposition of praseodymium (III) and iron (III) hydroxides. Similar results were also observed in previous works [13,19] for HoFeO 3 and NdFeO 3 orthoferrite. The exothermic peak at 617.31 °C correspond to the phase formation of PrFeO 3 orthoferrite from Pr 2 O 3 and Fe 2 O 3 according to equation (2). This inference is in good agreement with the mass change on the TG curve (there were no observable changes in the sample's mass from ~650 °C). From the TG-DSC results, the sample was annealed at 650, 750, 850, and 950 °C for 60 min to characterize the structural properties of PrFeO 3 crystals by XRD.
XRD patterns of praseodymium orthoferrite precursor after annealing at different temperatures for 60 min were shown in Fig. 2. The results give single phase orthorhombic PrFeO 3 . All obtained peaks match well with the standard peaks of PrFeO 3 (JCPDS: 74-1472), without any observable oxide peaks such as Pr 2 O 3 , Pr 6 O 11 or Fe 2 O 3 . The degree of crystallinity and crystal phase content of PrFeO 3 samples increased with the annealing temperature, however, this increment was not linear ( Table 1). The crystallinity of the sample annealed at 750 °C (592.04 cts) and that annealed at 950 °C (614.66 cts) were approximate, but the PrFeO 3 crystal phase content of the sample  (Table 1). Thus, it can be assumed that 750 °C for 60 min is the appropriate conditions for the formation of single phase praseodymium orthoferrite (o-PrFeO 3 ) by sol-gel method using water-methanol co-solvent.
From the EDX and EDX-mapping analysis of PrFeO 3 sample annealed at 750 °C, only  praseodymium, iron and oxygen peaks were observable without any other signals of impurity elements (Fig. 3). The averages of weight percentage and atomic percentage of the elements Pr, Fe, O from five different positions are shown in Table 2. The obtained results are consistent with expected chemical composition ( Table 2). Particle size and morphology of PrFeO 3 powder annealed at 750 °C are shown in the TEM image (Fig. 4a). The obtained particles have slightly angular spherical shape with the size mostly in the range of 20-60 nm (Fig. 4b). Average size calculated by IMAGE J was 46.28 nm. This result is rather close to the crystal size by Debye-Scherrer equation (D XRD = 48.6 nm) ( Table 1).
Field dependence of the magnetization of PrFeO 3 nanomaterials at 300K are shown in Fig. 5. The coercive force (H c = 20.8 ÷ 30.7 Oe) and saturation magnetization (M r = 0.13 ÷ 0.76 emu/g) ( Table 1) of all three PrFeO 3 samples in this work are much lower than those of PrFeO 3 prepared by co-precipitation method reported by Sudandararaj T. S. A. et. al. [22] (H c = 505.45 Oe, M r = 27.63 emu/g). The low value of H c and   M r could be originated from the homogeneity in shape and size of the PrFeO 3 nanoparticles with clear particle boundaries (see Fig. 4) while in the TEM image of the corresponding PrFeO 3 in [22], the particle boundaries are not observable with severe aggregation that joined the entire area of the material despite the particle size of 36.0 nm (by Image J). Most interestingly, the magnetic parameters of PrFeO 3 nanomaterials changed irregularly with the annealing temperature ( Table 1). The PrFeO 3 sample annealed at 750 °C has the lowest M r (0.13 emu/g) while its M s (1.10 emu/g) has the highest value. This can be ascribed by the highest crystallinity and crystal phase content of the PrFeO 3 sample annealed at 750 °C (see Table 1) which decreased the magnetocrystalline anisotropy of the material, leading to the rise in M s and the decreased in M r [23 -24].
Thus, with low H c , M r and high M s , obtained PrFeO 3 nanomaterials are soft magnetics that can be applied as material working under the external field as transformer cores, electromagnet cores, and conductive cores [24].

Conclusions
In this study, nanocrystalline praseodymium orthoferrite (o-PrFeO 3 ) was successfully synthesized by sol-gel method using watermethanol co-solvent. The PrFeO 3 nanocrystal formed after annealing the precursor at different temperatures (650, 750, 850, and 950 °C) for 1 hour. The crystal size of PrFeO 3 samples are in the range of 45÷70 nm (XRD, TEM). The PrFeO 3 sample annealed at 750 °C had the highest crystallinity (592.04 cts) and crystal phase content (93.4 %) (XRD) with smallest particle size (46.28 nm, TEM). The obtained PrFeO 3 nanomaterials are soft magnetic materials with low coercive force and remanent magnetization, high saturation magnetization.