SYNTHESIS OF HOLMIUM ORTHOFERRITE NANOPARTICLES BY THE CO-PRECIPITATION METHOD AT HIGH TEMPERATURE

25.05.2021 Abstract Holmium orthoferrite HoFeO 3 nanoparticles were synthesized by a simple co-precipitation method via the hydrolysis of Ho (III) and Fe (III) cations in boiling water with 5% aqueous ammonia solution. After annealing the precipitate at 750 and 850 °C for 1 hour, the single-phase HoFeO 3 product formed with particle size < 50 nm. The synthesized nanopowders are paramagnetic materials with remanent magnetization M r < 0.01 emu·g -1 , the coercive force H c = 20÷21 Oe, and magnetization M s ~ 2.73 emu·g -1 at 300 K in a maximum field of 16,000

In the study [8], holmium orthoferrite nanocrystals (HoFeO3) with an average size of 27-40 nm (according to the XRD results) obtained by a combustion method using glycine were used to decompose methyl orange in visible light due to its narrow energy gap (Eg = 2.12-2.14 eV). HoFeO3 orthoferrite with a particle size of 149.30 nm (SEM), obtained by the ceramic method from the corresponding holmium and iron (III) oxides, was characterized by saturation magnetization Ms = 25.5 emu.g -1 , remanent magnetization Mr = 4.08 emu.g -1 , and a very high coercive force Hc = 2659 Oe at 10 K in a maximum field of 60,000 Oe [10]. The HoFeO3 obtained in this study is a magnetically hard material (Hc >> 100 Oe) [14]; it can be used for the production of electromagnets, magnetic tapes, and magnetic recording materials. However, the magnetic characteristics of this material not only depend on the nature of the compound and its crystalline structure but also depend on the size of particles, their morphology, impurity content, and the synthesis method [10,[14][15][16][17][18].
Various methods have been developed for the formation of nanocrystalline holmium orthoferrite (HoFeO3). These methods included a mechanochemical method with a high annealing temperature (usually > 1200 °C) [10,15], hydrothermal synthesis with a long heating time (usually 12-48 h) [11], sol-gel technology in compliance with several influencing factors [8,12] and even the formation under the influence of gamma radiation with modern equipment [19][20].
Several studies [21][22][23][24][25][26] described the features of the formation of RFeO3 orthoferrite nanoparticles (R = La, Y, Nd) and doped with metal cations (for example, Mn, Ni, Co) by a simple co-deposition method via the hydrolysis of cations in boiling water (t° > 90 °C), followed by the addition of the corresponding precipitators. This kind of strategy has been supposed to stabilize the obtained precipitate. Thus, it results in the controllable growth of crystals better than the co-precipitation at room temperature [21][22]. HoFeO3 nanoparticles have not been synthesized yet by similar methods. HoFeO3 was also prepared by co-precipitation, using ethanol [27]. However, ethanol is volatile, pollutant, and flammable solvent compared to water solvent. Furthermore, the water solvent is cheaper.
This study aimed to synthesize holmium orthoferrite nanoparticles with narrow values of coercive force, remanent magnetization, and a high value of saturation magnetization by co-precipitation method in the water solvent and at high temperature.

Experimental
As starting materials, we used Ho(NO3)3·5H2O, Fe(NO3)3·9H2O, 25% an ammonia solution with a density of d = 0.901 g.mL -1 (all reagents were of CP grade), and distilled water. Solutions of holmium (III) and iron (III) nitrate were prepared by dissolving the corresponding salts in distilled water at room temperature (300 K). Boiling water (400 mL) was slowly added to the equimolar mixture (50 mL) of 0.1 M Ho(NO3)3 and 0.1 M Fe(NO3)3 solutions under continuously stirring with a magnetic stirrer (4000 rpm). After addition, the mixture of salt solutions was boiled for 10 min; then, under continuous stirring with a magnetic stirrer (4000 rpm), a 5% ammonia aqueous solution was dropped appropriately as a precipitant to complete precipitation of Ho (III) and Fe (III) cations (phenolphthalein test). Continuously, the mixture solution was stirred for more than 60 min. The precipitates were collected on a vacuum filter, washed with distilled water to a pH value of ~ 7.0, and dried at room temperature to constant weight (for approximately three days). The dried precipitate was homogenized and subjected to complex thermal analysis (Labsys Evo TG-DSC 1600 °C) with a heating rate of 10 deg.min -1 in an atmosphere of dry air up to 1000 °C in order to establish the optimal annealing regime, providing the formation of single-phase HoFeO3.
The following methods were used to comprehensively study the characteristics of the obtained nanopowders: the phase composition and crystal structure were studied by X-ray diffraction (XRD; D8-ADVANCE diffractometer) with CuKα radiation (λ = 1.540 Å) and Raman spectrometry (Horiba XploRA ONE); qualitative and quantitative elemental composition were studied by local X-ray microanalysis (XRM; scanning electron microscope FESEM S-4800), morphology and particle size were investigated by transmission electron microscopy (TEM; JEOL-1400); the average crystal size was determined according to the Debye Scherrer equation; parameters a, b, c and unit cell volume V of the were determined using the Rietveld method, implemented in the X'pert High Score Plus 2.2b software package, saturation magnetization in the maximal field, remanent magnetization and coercive force were determined using VSM MICROSENE EV11 magnetometer.

Results and discussion
The complex thermal analysis of the HoFeO3 sample obtained by the coprecipitation of holmium (III) and iron (III) hydroxides showed (Fig. 1a) that the mass loss was ~ 36.50%, which is much higher than the one (16.73%) calculated according to the reaction equation (1):

Ho(OH)3 + Fe(OH)3 → HoFeO3 + 3H2O 1
This difference is caused by the result of co-precipitation consisting not only hydroxides ((Fe2O3·xH2O [28] and HoO(OH)·yH2O [28]) but also carbonates (Ho2(CO3)3·8H2O or Ho2O(CO3)2·1.4H2O) [29][30]. The presence of carbonates in the sample was probably associated with the dissolution of carbon dioxide (air) in an ammonia solution. A similar observation was reported in our previous paper for systems based on YFeO3 and LaFeO3 [21-22, 24, 26]. The most significant mass loss (about 31.50%) observed in the range of 60-500 °С was accompanied with two endothermic peaks at 115.82 and 297.57 °С (Fig. 1a), which is assigned for water evaporation, and decomposition of holmium (III), iron (III) hydroxides. The dehydration started at 50 o C and finished at around 100 o C. On the other hand, the pyrolysis of iron (III) hydroxide and holmium hydroxide terminated at around 330 o C, 500 o C, respectively [28,32]. Similarly, in our previous study, the pyrolysis of lanthanum hydroxide was also observed at approximately 500 o C [21].

Fig. 2. XRD patterns of HoFeO3 formed at 750 °С and 850 °С for 1 h.
The second mass loss observed in the range of 500-800 °С was related to the decomposition of holmium carbonates (Ho2(CO3)3·8H2O or Ho2O(CO3)2·1.4H2O). We haven't observed any endothermic peak related to this phenomenon. Simultaneously, the crystallization of holmium orthoferrite nanoparticles (HoFeO3) occurred at the temperature range of 750 o C -800 o C, resulting in an obviously exothermic peak on the DSC diagram at 757 o C. Probably, the absence of an endothermic peak on the DSC thermogram in the temperature range of 500 -800 o C corresponding to the decomposition of holmium carbonates is due to the more significant crystallization of holmium orthoferrite nanoparticles. The crystallization was confirmed by the diffraction pattern of the sample after annealing at 750 °С, which was a single-phase product, HoFeO3 (Fig. 2).
Briefly, the formation of single-phase of orthoferrite holmium (HoFeO3) can be described as below equations [28][29][30]32]. 2  XRD diagram indicated that synthesized samples were a single-phase product with the structure of holmium orthoferrite HoFeO3 (JCPDS no. 00-046-0115, space group Pbnm (62); a = 5.282 Å, b = 5.592 Å, c = 7.608 Å) (Fig. 2). The increase in the annealing temperature led to an increase in the degree of crystallization, size of crystal particles, unit cell parameters, and a slight decrease in unit cell volume (Table 1). These results are consistent with the reported paper [27].  (Fig. 3a). The Raman active modes of the HoFeO3 were assigned based on the method recently proposed by Gupta et al. [31] for RFeO3 (R = Tb, Dy, Ho, Er, Tm) compounds. The strongest peak at 146.8 cm -1 was attributed to the Ho-O vibration modes. The Raman bands above 200 cm -1 correspond to oxygen ions. The high-frequency mode in the RFeO3 crystal may be assigned to the internal vibration related to the mutual Fe-O motion within the oxygen octahedron [20,31], which is present in this study at wavenumbers of 514.1 and 598.4 cm -1 .
TEM images indicated that HoFeO3 nanoparticles were uniform (if considering individual particles). The mean sizes of particles after annealing at 750 and 850 o C were 20-30 nm and 20-40 nm, respectively (Fig. 3b, 3c, and Fig. 4). The particles are strongly aggregated at higher temperatures. According to the results of local X-ray microanalysis, the HoFeO3 sample contained only three elements: Ho, Fe, and O (Fig. 1b). The calculation of the quantitative composition demonstrated that the actual content of each element was quite close to the nominal composition (Table 2).  The study of the HoFeO3 samples (annealing at 750 and 850 °С for 1 h) on a vibration magnetometer at 100 K and 300 K showed that in a maximum field of 16,000 Oe, according to the shape of the hysteresis loop, the nanopowders of HoFeO3 are paramagnets (Fig. 5). The magnetic hysteresis curves of HoFeO3 annealed at 750 are similar to that of HoFeO3 annealed at 850 °C. This observation is due to the same morphology (see Fig.3b-c) and insignificant changes in the crystalline size (see Table 1). Interestingly, the synthesized HoFeO3 nanoparticles presented lower Hc, and Mr values Hc ~ 20 Oe, Mr < 0.01 emu/g), but higher Ms compared to the orthoferrite nanoparticles of other rare-earth elements, such as LaFeO3, YFeO3, and NdFeO3 in similar conditions [22,26,32] and compared to the data of other authors for similar objects [1,17,33] (Table  3). Typically, the Ms value of obtained HoFeO3 was much higher than that of HoFeO3 prepared by the co-precipitation method using ethanol [27] (see Table 3). At 5 kOe, the magnetization of the HoFeO3 sample annealed at 850 °C increases when the temperature of the measurement decreases (Fig. 5a). The HoFeO3 orthoferrite samples with low coercive force, excessive magnetization, and high magnetization were investigated in this study. This material is paramagnetic and applicable in physics and biomedical, regarding quick responses to the external magnetic field [34].

Conclusions
In this study, HoFeO3 nanoparticles were synthesized by the simple coprecipitation method via a simple process. The first stage is the hydrolysis of Ho (III) and Fe (III) cations in boiling water with an aqueous ammonia solution. The second stage is the annealing at 750 and 850 °С for 1 h. The synthesized HoFeO3 orthoferrite nanopowders exhibited a narrow hysteresis loop, small values of remanent magnetization and coercive force, but high magnetization, which is potential use as paramagnetic material in physics and biomedical.