Structural, optical and magnetic properties of Sr and Ni co-doped YFeO₃ nanoparticles prepared by simple co-precipitation method

Abstract

YFeO_3+δ, Y_0.8Sr_0.2FeO_3+δ, YFe_0.8Ni_0.2O_3+δ, and Y_0.8Sr_0.2Fe_0.8Ni_0.2O_3+δ nanoparticles have been successfully synthesized by a simple co-precipitation technique. Results obtained using thermogravimetry and differential scanning calorimetry, powder X-ray diffraction, transmission electron microscopy indicate that YFeO₃, Sr-doped YFeO₃, Ni-doped YFeO₃, and Sr and Ni co-doped YFeO₃ nanoparticles with orthorhombic structure were fabricated at 800 °C for 1 h. The obtained materials have the crystallite sizes below 30 nm and particle sizes below 40 nm. Sr and/or Ni doping led to the distortion of the YFeO₃ crystal structure and thus altered the magnetic properties of the corresponding materials. The Sr-doped YFeO₃, Sr and Ni co-doped YFeO₃, and especially Ni-doped YFeO₃ samples have significantly higher absorbance in the visible light region (~ 400–800 nm) and lower band gap than those of pure YFeO₃ sample. Magnetic hysteresis loop analyses illustrate that ferromagnetic behavior of the YFeO₃ nanopowders can be strongly enhanced with the addition of Sr and/or Ni. The coercivity and remanent magnetization of Sr and Ni co-doped YFeO_3+δ are, respectively, around 80 and 104 times higher than those of the pure YFeO_3+δ sample. The excellent optical and magnetic properties of Sr and Ni co-doped YFeO_3+δ nanomaterials suggest great potential for applications related to optics and magnetism.

1 Introduction

Yttrium iron oxide (YFeO₃) is a LnFeO₃ perovskite orthoferrite having an orthorhombic structure [1,2,3]. Orthoferrite yttrium (o-YFeO₃) has been attracting much interest in both fundamental research and applied science [4, 5]. The structure and properties of o-YFeO₃ materials depend on many factors such as particle shape and size, crystallite size, preparation method and especially, on the nature and concentration of the dopants in o-YFeO₃ crystal lattice [6,7,8]. The partial doping at the site of Y³⁺ or Fe³⁺ in the lattice of YFeO₃ can lead to multiple oxidation states and crystal defects, and thus, to changes in the properties of the materials [9,10,11,12]. The perovskite-type structure can be discriminated by the Goldschmidt tolerance factor (t), which can be calculated following the Eq. (1) [13,14,15,16,17].

$$t = \frac{{r_{A} + r_{O} }}{{\sqrt 2 (r_{B} + r_{O} )}},$$

(1)

where $r_{A}$,$r_{B}$ and $r_{O}$ are the radii of large cation A, small cation B and anion O²⁻, respectively. For t = 1, perovskite ABO₃ has an ideal cubic structure. In cases where 0.75 < t < 0.9, the structure of ABO₃ is orthorhombic. For o-YFeO₃ with the radii of Y³⁺ = 0.094, Fe³⁺ = 0.065, and O²⁻ = 0.126 nm, we have t = 0.82 [8]. In all cases of t ≠ 1, the crystal lattice is distorted, the B–O–B bond angle is no longer 180° but bent, and the B–O bond lengths are different in different directions. The ABO₃ structure is modified, resulting in the alteration of its electrical, magnetic, and optical properties [18,19,20,21].

Besides, according to Goldschmidt’s rules, in order to form a homogenous solid solution at a temperature largely different from the melting points of the components, the difference between the ionic radii of the cation and its dopant (Δr) should not exceed 15%, i.e., Δr/r ≤ 15% (r is the radius of the smaller cation) [10]. Indeed, according to [10], Ba-doped YFeO₃ nanomaterials were successfully synthesized by the sol–gel method, followed by calcination of the precursor at 750 °C for 1 h. The results showed that the substitution of Ba²⁺ in the crystal lattice of YFeO₃ only happened with x = 0.1. For x > 0.1, besides the orthorhombic phase YFeO₃, XRD peaks of Y₂O₃, BaO, and BaY₂O₄ were also observed. The doping of Ba²⁺ in YFeO₃ increased the crystallite size from 30 ± 2 nm (for YFeO₃) to 55 ± 5 nm (for Y_0.9Ba_0.1FeO₃). Barium-doped yttrium orthoferrite with x = 0.1 underwent qualitative transformation from a soft to a hard magnetic material, with H_c increased from 70 to 1000 Oe [22]. The structural defects, leading to changes in the magnetic properties of Ba-doped YFeO₃ nanomaterials, can be explained by the large difference between the radii of the cations Y³⁺ (0.094 nm) and Ba²⁺ (0.134 nm) (Δr = 0.04 nm). Even with a small Δr (Δr ≠ 0), Mn-doped YFeO₃ orthoferrite also underwent significant changes in the structure and properties [11]. The YFe_1-xMn_xO₃ series (x = 0, 0.05, 0.1, 0.15, and 0.2) orthorhombic nanomaterials were prepared by the citric acid sol–gel method and annealing of the precursor at 800 °C for 2 h [11], showing the domination of YFeO₃ hexagonal structure when the calcination temperature (T°) < 800 °C or x > 0.2. When the Mn concentration in the YFeO₃ orthorhombic crystal lattice increased, particle size increased from 50 to 90 nm and saturation magnetization (M_s) increased from 0.015 to 0.08 emu g⁻¹ for YFeO₃ and YFe_0.8Mn_0.2O₃, respectively. The partial replacement of Y³⁺ or Fe³⁺ by another metal cation also affected the optical properties of the YFeO₃ material. Indeed, the doping of Sm³⁺ in YFeO₃ orthorhombic structure decreased H_c and absorbance in the visible range (~ 400–600 nm), while increased M_s (emu g⁻¹) and E_g (eV) [12]. In this work, Y_1-xSm_xFeO₃ (x = 0, 0.05, 0.1, and 0.15) orthorhombic materials of 300–1200 µm were synthesized by the citric acid sol–gel method and annealing of the precursor at 900 °C for 8 h.

In our previous publication [23], YFe_1-xNi_xO₃ nanomaterials were prepared by a simple co-precipitation method with KOH 5% solution as the precipitating agent. Single phase orthorhombics YFe_1-xNi_xO₃ (x = 0, 0.1, 0.15, 0.2, and 0.25) were formed after annealing the precipitates at 800 °C for 1 h. NiO and Y₂O₃ appeared as impurities when T° > 800 °C or x > 0.25. The substitution of Ni with an yttrium iron oxide lattice also increased the crystallite size and magnetic characteristics of the materials. However, to the best of our knowledge, the simultaneous doping of two different metal cations (specifically Sr²⁺ and Ni²⁺) to both Y³⁺ and Fe³⁺ positions in YFeO₃ crystal lattice has not yet been reported.

Thus, the aim of this work is to study of how the doping of strontium and nickel, individually and simultaneously, affected the structure, the optical and magnetic properties of YFeO₃ orthorhombic nanomaterials. Based on previous works [10,11,12, 23, 24], the concentration of dopants was 20% molar ratio, corresponding to the empirical formulae of YFeO₃, Y_0.8Sr_0.2FeO₃, YFe_0.8Ni_0.2O₃, and Y_0.8Sr_0.2Fe_0.8Ni_0.2O₃.

2 Experimental

2.1 Materials and methods

Y(NO₃)₃·6H₂O (99.9% purity, Sigma-Aldrich), Ni(NO₃)₂·6H₂O (99.8% purity, Acros Organic), Fe(NO₃)₃·9H₂O (99.6% purity, Sigma-Aldrich), Sr(NO₃)₂ (99.9% purity, Sigma-Aldrich), Na₂CO₃·10H₂O (99.7% purity, Merck), and double distilled water were employed as the starting materials.

Y_1-xSr_xFe_1-yNi_yO₃ nanoparticles were synthesized by a simple co-precipitation method based on the preparation of YFe_1-xNi_xO₃ [23]. Instead of KOH 5%, Na₂CO₃ was used as the precipitating agent because Sr²⁺ cations cannot be precipitated by OH⁻ anions [25].

50 mL of the aqueous solution containing the mixture of Y(NO₃)₃, Sr(NO₃)₂, Fe(NO₃)₃ and Ni(NO₃)₂ with the molar ratio of Y³⁺:Sr²⁺:Fe³⁺:Ni²⁺  = (1 − x):x:(1 − y):y (for 4 series of materials: (1) YFeO₃ (x = y = 0), (2) Y_0.8Sr_0.2FeO₃ (x = 0.2, y = 0), (3) YFe_0.8Ni_0.2O₃ (x = 0, y = 0.2), and (4) Y_0.8Sr_0.2Fe_0.8Ni_0.2O₃ (x = y = 0.2) was added dropwise to 400 mL of boiling double distilled water on a heating stirrer (T° > 95 °C). The mixture was then boiled for another 10 min before being cooled down to room temperature (~ 30 °C). Next, 50 mL of Na₂CO₃ 5% solution was added dropwise to the system while continuously stirring. The amount of Na₂CO₃ 5% solution was enough to precipitate all the cations according to the chemical Eqs. (2, 3, 4, 5) [25, 26].

$$2{\text{Y}}\left( {{\text{NO}}_{3} } \right)_{3} + 3{\text{Na}}_{2} {\text{CO}}_{3} \to {\text{Y}}_{2} \left( {{\text{CO}}_{3} } \right)_{3} \downarrow + 6{\text{NaNO}}_{3}$$

(2)

$${\text{Sr}}\left( {{\text{NO}}_{3} } \right)_{2} + {\text{Na}}_{2} {\text{CO}}_{3} \to {\text{SrCO}}_{3} \downarrow + 2{\text{NaNO}}_{3}$$

(3)

$$2{\text{Fe}}\left( {{\text{NO}}_{3} } \right)_{3} + 3{\text{Na}}_{2} {\text{CO}}_{3} + 3{\text{H}}_{2} {\text{O}} \to 2{\text{Fe}}\left( {{\text{OH}}} \right)_{3} \downarrow + 6{\text{NaNO}}_{3} + 3{\text{CO}}_{2} \uparrow$$

(4)

$${\text{Ni}}\left( {{\text{NO}}_{3} } \right)_{2} + {\text{Na}}_{2} {\text{CO}}_{3} \to {\text{NiCO}}_{3} + 2{\text{NaNO}}_{3}$$

(5)

After the addition of the Na₂CO₃ 5% solution, the system was the stirred for 1 h to homogeneously disperse the precipitates. The precipitates were subsided for 30 min then filtered and washed by double distilled water until pH 7 (the filtrate was checked by pH paper). The obtained solid was left to dry at room temperature for 3–5 days then ground, resulting in brownish yellow powder (precursor for Y_1-xSr_xFe_1-yNi_yO₃ nanoparticles).

2.2 Characterization

Thermogravimetry and differential scanning calorimetry (TG–DSC) curves of the precursor for the synthesis of YFeO₃ nanoparticles were recorded under dry air at the heating rate of 10 K·min⁻¹, maximum temperature of 1000 °C, platinum crucibles, using a Labsys Evo (France).

Powder X-ray diffraction analysis (PXRD) of the obtained Y_1-xSr_xFe_1-yNi_yO₃ samples was carried out using a D8-ADVANCE X-ray diffractometer (Germany) (CuK_α radiation, λ = 1.5406 Å, angle range of 2θ = 10°–80°, scanning step of 0.19° s⁻¹). The crystallite sizes of Y_1-xSr_xFe_1-yNi_yO₃ (D_XRD, nm) were determined based on Scherrer’s equation [4, 27].

$$D{ = }\frac{0.89\lambda }{{\left( {{\text{FWHM}}} \right)\cos \theta }},$$

(6)

where FWHM is the full-width at half maximum, and θ is the diffraction angle of the maximum reflection.

Lattice constants (a; b; c, Å) and unit cell volume (V, ų) were calculated using the formulas presented in Ref [27]:

$$\frac{1}{{d^{2} }} = \frac{{h^{2} }}{{a^{2} }} + \frac{{k^{2} }}{{b^{2} }} + \frac{{l^{2} }}{{c^{2} }},$$

(7)

$$V = a \cdot b \cdot c.$$

(8)

The morphology and particle size were investigated by transmissions electron microscopy (TEM; Jeol-1400, Japan).

The quantitative and qualitative composition of the samples was determined by energy dispersive X-ray spectroscopy (EDX-analysis) using an FE-SEM S-4800 scanning electron microscope (Japan). The quantitative elemental composition was determined as the average of the values obtained at five different points of each sample.

The UV–Vis absorption spectra of the Y_1-xSr_xFe_1-yNi_yO₃ nanopowders were studied on a UV–visible spectrophotometer (UV–Vis; JASCO V–550, Japan). The optical energy gap (E_g, eV) was determined by fitting the absorption data to the direct transition as Eq. (9):

$$Ahv = \sqrt {\alpha \left( {h\nu - E_{{\text{g}}} } \right)} ,$$

(9)

where A is the optical absorption coefficient, hν is the photon energy, E_g is the direct band gap and α is a constant [28]. The extrapolation of the linear portions of the curves toward absorption equal to zero (y = 0) gives E_g for direct transitions.

The hysteresis loop and magnetic properties including coercive force (H_c, Oe), remanent magnetization (M_r, emu g⁻¹) and saturation magnetization (M_s, emu·g⁻¹) were recorded at 300 K on a vibrating sample magnetometer (VSM, MICROSENE EV11, Japan) under a magnetic field in the range of − 16 000 Oe to + 16 000 Oe.

3 Result and discussion

As the radii of Fe³⁺ (0.065 nm) and Ni³⁺ (0.061 nm) are approximate while those of Y³⁺ (0.094 nm) and Sr²⁺ (0.112 nm) have a big difference (Δr = 0.018 nm) [25], the two precursors for YFeO₃ and Y_0.8Sr_0.2FeO₃ nanoparticles were chosen for TG–DSC analysis. The results are shown in Fig. 1.

Fig. 1

The TG–DSC curves of the precipitate sample corresponding to YFeO₃ nanoparticles (A) and Y_0.8Sr_0.2FeO₃ nanoparticles (B)

The TG curves of two samples (YFeO₃ and Y_0.8Sr_0.2FeO₃) from room temperature to 1000 °C are quite similar. The mass loss percentage are 32.81% for YFeO₃ and 31.25% for Y_0.8Sr_0.2FeO₃, and become negligible at around 750 °C for both samples (according to TG curves). Those results can be derived from their resemblance to the average atomic masses of yttrium and strontium, and the formation of stable carbonate precipitates of both Y³⁺ and Sr²⁺ cations (Y = 88.91, Sr = 87.62) [25]. The mass losses of the samples when heated were from the dehydration of moisture, dehydration by pyrolysis of iron precipitates, and carbon dioxide loss by base–carbonate pyrolysis of yttrium and strontium carbonate according to the chemical Eqs. (10, 11, 12, 13, 14, 15, 16) [30,31,32,33].

$${\text{Fe}}\left( {{\text{OH}}} \right)_{3} \to {\text{FeO}}\left( {{\text{OH}}} \right) + {\text{H}}_{2} {\text{O}}$$

(10)

$${\text{2FeO}}\left( {{\text{OH}}} \right) \to \gamma {\text{ - Fe}}_{2} {\text{O}}_{3} {\text{ + H}}_{2} {\text{O}}$$

(11)

$$\gamma {\text{ - Fe}}_{2} {\text{O}}_{3} \to \alpha {\text{ - Fe}}_{2} {\text{O}}_{3}$$

(12)

$${\text{Y}}_{2} \left( {{\text{CO}}_{3} } \right)_{x} \left( {{\text{OH}}} \right)_{{2\left( {3 - x} \right)}} \cdot n{\text{H}}_{2} {\text{O}} \to {\text{Y}}_{2} \left( {{\text{CO}}_{3} } \right)_{x} \left( {{\text{OH}}} \right)_{{2\left( {3 - x} \right)}} { + }n{\text{H}}_{2} {\text{O}}$$

(13)

$${\text{Y}}_{2} \left( {{\text{CO}}_{3} } \right)_{x} \left( {{\text{OH}}} \right)_{{2\left( {3 - x} \right)}} \to {\text{Y}}_{2} {\text{O}}_{3 - x} \left( {{\text{CO}}_{3} } \right)_{x} { + }\left( {3 - x} \right){\text{H}}_{2} {\text{O}}$$

(14)

$${\text{Y}}_{2} {\text{O}}_{3 - x} \left( {{\text{CO}}_{3} } \right)_{x} \to {\text{Y}}_{2} {\text{O}}_{3} + x{\text{CO}}_{2}$$

(15)

$${\text{SrCO}}_{3} \to {\text{SrO}} + {\text{CO}}_{2}$$

(16)

The thermal behaviors of YFeO₃ and Y_0.8Sr_0.2FeO₃ samples also have similarity in their endothermic peaks at 128.86 °C/133.33 °C and exothermic peaks at 302.13 °C/321.90 °C on the DSC curves, respectively. The endothermic effects at 70–200 °C come from the removal of moisture, dehydration by pyrolysis of iron (III) and of yttrium and strontium precipitates [25, 30, 31, 33]. The exothermic peaks at 200–400 °C correspond to the γ-Fe₂O₃ → α-Fe₂O₃ phase transition according to Eq. (12) [30, 32]. The only difference in the thermal behaviors of YFeO₃ and Y_0.8Sr_0.2FeO₃ is the absence of the endothermic peak at 450.17 °C for the YFeO₃ sample. This difference originates from the carbonate hydrolysis according to Eq. (16), resulting in a stronger endothermic effect. Finally, exothermic peaks at 700–800 °C were observable for both samples, which indicates the perovskite phase formation as shown in Eqs. (17) and (18).

$${\text{Y}}_{2} {\text{O}}_{3} + {\text{Fe}}_{2} {\text{O}}_{3} \to 2{\text{YFeO}}_{3}$$

(17)

$$0.8{\text{Y}}_{2} {\text{O}}_{3} + 0.2{\text{SrO}} + {\text{Fe}}_{2} {\text{O}}_{3} \to 2{\text{Y}}_{0.8} {\text{Sr}}_{0.2} {\text{FeO}}_{3 - \delta }$$

(18)

From the above TG–DSC analysis and previous synthesis of YFe_1-xMn_xO₃ [11], Y_1-xSm_xFeO₃ [12], and YFe_1-xNi_xO₃ [23] nanoparticles, the precipitates YFeO₃ and Y_1-xSr_xFe_1-yNi_yO₃ were annealed at 800 °C for 1 h, followed by a structural analysis with PXRD.

The PXRD pattern of the YFeO₃ sample (Fig. 2A) has the specific peaks of the YFeO₃ orthorhombic phase, Pnma (62) space group (JCPDS: 00-039-1489). The good crystallinity of the YFeO₃ sample is proven by its high peak intensity and flat, fine baseline, and no minor impurity peaks were present.

Fig. 2

PXRD patterns of YFeO₃ (A) and Y_1-xSr_xFe_1-yNi_yO₃ (B) nanopowders annealed at 800 °C for 1 h

The PXRD patterns of the doped and co-doped nanopowders indicate the same characteristic shape as that of the pristine YFeO₃ pattern, with a slight shift in the positions of the peaks (Fig. 2B and Table 1). This is a normal situation for the YFeO₃ perovskite, involving the difference in the radii of the cations, according to Goldschmidt [24]. Table 1 shows that, compared to pure YFeO₃, 2θ values underwent a left-shift by doping Sr²⁺ with a bigger radius than that of Y³⁺, while the doping of Ni³⁺ with smaller radius than that of Fe³⁺ resulted in a right-shift of 2θ positions. For Sr and Ni co-doped YFeO₃ samples, 2θ positions also had a right-shift. Similar results were reported for Gd and Co co-doped YFeO₃ nanopowders [28]. Thus, it is evident that Sr²⁺ and Ni³⁺ replaced the Y³⁺ and Fe³⁺ ions in YFeO₃, respectively. The shift of 2θ values is also consistent with the changes in the crystallite size (D_XRD, nm) and lattice cell volume (V, ų) (Table 1).

Table 1 Structural characteristics of Y_1-xSr_xFe_1-yNi_yO_3±δ nanocrystals

Table 1 shows the structural characteristics of Y_1-xSr_xFe_1-yNi_yO₃ nanocrystals. The crystallite size (D_XRD, nm) and unit cell volume (V, ų) of YFeO_3±δ increased with an increase in Sr content and decreased with the addition of Ni because $r_{{Sr^{2 + } }} > r_{{Y^{3 + } }}$, while $r_{{Ni^{3 + } }} < r_{{Fe^{3 + } }}$. This is another indication of the successful substitution of Y³⁺ and Fe³⁺ ions in the YFeO₃ lattice by Sr²⁺ and Ni²⁺ ions. The smallest values of D_XRD and V correspond to Sr and Ni co-doped YFeO₃ samples, because the simultaneous substitution of Y³⁺ and Fe³⁺ by Sr²⁺ and Ni³⁺ led to an increase in the amount of oxidized Fe³⁺ to form Fe⁴⁺ compared to Y_0.8Sr_0.2FeO_3±δ and YFe_0.8Ni_0.2O₃ ($r_{{Fe^{4 + } }} < r_{{Fe^{3 + } }}$), resulting in the decrease in D_XRD and V according to Vegard’s law [14, 20, 34, 35]. This result is also in good agreement with the TEM images of Y_1-xSr_xFe_1-yNi_yO₃ samples in Fig. 3.

Fig. 3

TEM images of nanostructured YFeO₃ (a, b, c), Y_0.8Sr_0.2FeO₃ (d), YFe_0.8Ni_0.2O₃ (e), and Y_0.8Sr_0.2Fe_0.8Ni_0.2O₃ (f) perovskites after annealing at 800 °C for 1 h

The TEM images in Fig. 3 show that the obtained Y_1-xSr_xFe_1-yNi_yO₃ samples are composed of spherical or slightly angular-shaped particles. The average particle sizes of the test samples of YFeO₃, Y_0.8Sr_0.2O₃, YFe_0.8Ni_0.2O₃, and Y_0.8Sr_0.2Fe_0.8Ni_0.2O₃ were approximately 35.17, 39.86, 34.85, and 32.43 nm, respectively. However, aggregations and clusters are still observable, maybe due to the strong magnetic interaction between the particles.

Elemental composition analysis from EDX shows no impurity elements apart from Y, Sr, Fe, Ni, and O which were detected in the Y_1-xSr_xFe_1-yNi_yO₃ nanomaterials (Fig. 4). The percentage of atoms of each element is also in accordance with their ratio in the nominal compositions (Table 2). Table 2 also shows that the oxygen content in Y_1-xSr_xFe_1-yNi_yO₃ materials is much greater than that in the nominal composition, proving the successful substitution of Y³⁺ and Fe³⁺ by Sr²⁺ and Ni³⁺ in the YFeO₃ lattice. When Y³⁺ was replaced by Sr²⁺ and Fe³⁺ was replaced by Ni²⁺ (Ni²⁺ in Ni(NO₃)₂·6H₂O), some Fe³⁺ ions would be oxidized to Fe⁴⁺ and Ni²⁺ to Ni³⁺ to balance the local charge, resulting in the increase of oxygen content in Y_1-xSr_xFe_1-yNi_yO₃ [14, 20, 34, 35]. For Sr and Ni co-doped YFeO₃ sample, not all Ni²⁺ ions were oxidized to Ni³⁺, this sample may have the highest amount of Fe³⁺ oxidized to Fe⁴⁺, and thus it has the highest oxygen content (Table 2). Besides, oxygen adsorption on the surface of Y_1-xSr_xFe_1-yNi_yO₃ orthoferrite also led to an increased oxygen percentage in the Y_1-xSr_xFe_1-yNi_yO₃ structure [34, 36], hence the empirical formula of this material series should be Y_1-xSr_xFe_1-yNi_yO_3+δ.

Fig. 4

EDX patterns of Y_1-xSr_xFe_1-yNi_yO₃ nanoparticles annealed at 800 °C for 1 h

Table 2 EDX results of Y_1-xSr_xFe_1-yNi_yO₃ nanopowders annealed at 800 °C for 1 h

The substitution of Sr²⁺ and/or Ni³⁺ in the YFeO_3+δ crystal lattice affected not only the structure but also the optical and magnetic properties of the materials (Figs. 5, 6 and Table 3). The UV–Vis absorption spectra of the Sr-doped, Ni-doped, and Sr, Ni co-doped YFeO₃ nanoparticles showed strong absorption in the ultraviolet (~ 250–400 nm) and visible light regions (~ 400–800 nm) (Fig. 5A). While the absorption spectra of pristine and Sr²⁺ and/or Ni³⁺ doped YFeO_3+δ show no significant difference in the UV region, doped YFeO_3+δ samples exhibited much higher absorption in the visible range compared to pure YFeO_3+δ, with the highest absorption in the case of Ni-doped YFeO_3+δ and Sr, Ni co-doped YFeO_3+δ samples. This strong transition corresponds to the electronic transition from the 2p valence orbital of oxygen to the 3d conduction band of iron or nickel [12, 28]. There is a bathochromic shift (toward the longer wavelength) in the absorption edge of Sr-doped, Ni-doped, and Sr, Ni co-doped YFeO_3+δ nanoparticles, indicating a decrease in their band gaps. The optical energy gaps (E_g, eV) of the Y_1-xSr_xFe_1-xNi_yO_3+δ nanoparticles were calculated using the Tauc-plot and are shown in Table 3 and Fig. 5B. The optical band gap for the YFeO_3+δ sample was calculated to be 1.87 eV, which is lower than previously reported values [12, 28]. For the Sr-doped YFeO_3+δ sample, and especially the Ni-doped YFeO_3+δ and Sr and Ni co-doped YFeO_3+δ, there is a sudden drop in the band gap values (Fig. 5B and Table 3). The band gap of obtained Y_1-xSr_xFe_1-xNi_yO_3+δ nanoparticles (1.87–0.60 eV) is much narrower than those of Y_1-xSm_xFeO₃ and Y_1-xGd_xFe_0.95Co_0.05O₃ nanomaterials (see Table 3) [12, 28]. Besides the intrinsic nature of the materials, according to references [37, 38], reduced particle size leads to a narrow energy gap. Indeed, the size of the obtained YFeO_3+δ nanoparticles is remarkably smaller than other YFeO_3+δ synthesized by solid-state reaction [28] and sol–gel methods [12]. Furthermore, the size of Y_0.8Sr_0.2FeO_3+δ > Y_0.8Sr_0.2Fe_0.8Ni_0.2O_3+δ ~ YFe_0.8Ni_0.2O_3+δ, hence the band gap values also decrease accordingly (see Tables 1, 3 and Fig. 3). With a narrow band gap, the Y_1-xSr_xFe_1-xNi_yO_3+δ nanoparticles synthesized in this work can be used as a photocatalyst in the visible region for organic decomposition processes.

Fig. 5

A Room-temperature optical absorbance spectra of the Y_1-xSr_xFe_1-yNi_yO_3+δ samples; B Tauc-plot of (ahν)² as a function of photon energy for Y_1-xSr_xFe_1-yNi_yO_3+δ nanoparticles

Fig. 6

Room-temperature magnetic hysteresis loops of as-prepared Y_1-xSr_xFe_1-yNi_yO_3+δ nanoparticles

Table 3 Optical band gap and magnetic parameters of Y_1-xSr_xFe_1-yNi_yO_3+δ nanoparticles at 300 K in this work and from the published literature for comparison

Figure 6 shows the magnetic properties of the Y_1-xSr_xFe_1-yNi_yO_3+δ nanopowders at room temperature. The doping of Sr and/or Ni into the YFeO₃ crystal lattice clearly changed the magnetic nature of the base YFeO₃ material. The pure YFeO₃ sample, with a narrow hysteresis loop and a very low M_r (0.021 emu g⁻¹) and H_c (50.25 Oe) (Table 3), is a typical weak ferromagnetic material [22]. For Sr and/or Ni-doped YFeO₃, a large open region was seen at the center of the hysteresis loops, suggesting ferromagnetic behavior, especially in cases of Sr and Ni co-doped YFeO₃ samples (Fig. 6). The H_c (Oe) and M_r (emu g⁻¹) values of the Y_0.8Sr_0.2O_3+δ and YFe_0.8Ni_0.2O_3+δ samples are 40 and 12–21 times greater than those of the pure YFeO_3+δ sample, respectively. For Sr and Ni co-doped YFeO₃, the values of H_c and M_r are even much higher (80 times higher for H_c, and 104 times for M_r) (Table 3). The H_c value of Y_0.8Sr_0.2Fe_0.8Ni_0.2O_3+δ at 15,000 Oe was approximately that of Y_0.8Gd_0.2Fe_0.95Co_0.05O₃ at 60 000 Oe [28] (Table 3). Moreover, the Sr and Ni co-doped YFeO₃ sample did not reach saturation at 15,000 Oe, while pure YFeO₃, Sr-doped YFeO_3, and Ni-doped YFeO₃ samples were already saturated at 15,000 Oe. The reasons for the observed improvement in the magnetization can be explained as follows. (1) When the particle size is small, uncompensated surface spins of Fe³⁺ ions are created, leading to a strong magnetic enhancement [39, 40]. In fact, particle and crystallite sizes are in the order of Y_0.8Sr_0.2Fe_0.8Ni_0.2O_3+δ < Y_0.8Fe_0.8Ni_0.2O_3+δ < Y_0.8Sr_0.2Fe_0.8O_3+δ (See Table 1 and Fig. 3). (2) The distorted structure is affected by the doping and co-doping effects. The doping of Sr and/or Ni into the YFeO₃ structure increased the crystalline magnetic anisotropy, resulting in the large increase in H_c and M_r [39, 40]. (3) Finally, the substitution of Y³⁺ and/or Fe³⁺ by Sr²⁺ and/or Ni²⁺/Ni³⁺ in YFeO₃ enhanced the oxidation process, Fe³⁺ → Fe⁴⁺ + e, which led to the increase of super-exchange interactions Fe³⁺–O²⁻–Fe⁴⁺, Y³⁺–O²⁻–Fe³⁺/Fe⁴⁺, Sr²⁺–O²⁻–Fe³⁺/Fe⁴⁺, Ni²⁺/Ni³⁺–O²⁻–Fe³⁺/Fe⁴⁺, Ni²⁺–O²⁻–Ni³⁺, Y³⁺–O²⁻–Ni²⁺/Ni³⁺, Sr²⁺–O²⁻–Ni²⁺/Ni³⁺ compared to the super-exchange interactions Fe³⁺–O²⁻–Fe³⁺, Y³⁺–O²⁻–Y³⁺ va Y²⁺–O²⁻–Fe³⁺ in pure YFeO₃ [41].

In this manner, YFeO₃ nanopowder, with its weak ferromagnetic behavior, can be used as a material operating under external fields, such as the cores of transformers, electromagnets, or magnetic conductors. Owing to their high H_c and M_r, Y_0.8Sr_0.2Fe_0.8O_3+δ, Y_0.8Fe_0.8Ni_0.2O_3+δ, and Y_0.8Sr_0.2Fe_0.8Ni_0.2O_3+δ can be applied to permanent magnets, magnetic recording materials in hard drives, or magnetic tapes, etc. [22].

4 Conclusions

The single-phase nanostructured YFeO_3+δ, Y_0.8Sr_0.2Fe_0.8O_3+δ, Y_0.8Fe_0.8Ni_0.2O_3+δ, and Y_0.8Sr_0.2Fe_0.8Ni_0.2O_3+δ perovskites have been synthesized by the simple co-precipitation method. The hydrolysis of Y³⁺, Sr²⁺, Fe³⁺, and Ni²⁺ cations were carried out in boiling water (T° ≥ 95 °C) with Na₂CO₃ 5% as a precipitant. Sr and/or Ni doping distorted the crystal structure of the base material YFeO₃, thus correspondingly modified its optical and magnetic properties. The Sr-doped YFeO_3+δ, Ni-doped YFeO_3+δ, and Sr and Ni co-doped YFeO_3+δ nanopowders have narrow band gaps (of 1.31 eV, 0.60 eV, and 0.76 eV, respectively) and can be used as photocatalysts for the decomposition of organic compounds in the visible light region, which can be recovered easily after their use by rare-earth magnets owing to their high M_r (of 0.261, 0.459, and 2.182 emu g⁻¹, respectively) and H_c (of 2029.37, 2117.32, and 3957.06 Oe, respectively).