**Ferroelectric Diode E**ff**ect with Temperature Stability of Double Perovskite Bi2NiMnO6 Thin Films**

#### **Wen-Min Zhong, Qiu-Xiang Liu, Xin-Gui Tang \*, Yan-Ping Jiang, Wen-Hua Li, Wan-Peng Li and Tie-Dong Cheng**

School of Physics Optoelectric Engineering, Guangdong University of Technology, Guangzhou Higher Education Mega Center, Guangzhou 510006, China; zhongwen\_min@163.com (W.-M.Z.); liuqx@gdut.edu.cn (Q.-X.L.); ypjiang@gdut.edu.cn (Y.-P.J.); liwenhuat@gdut.edu.cn (W.-H.L.); liwanpeng361@163.com (W.-P.L.); chengtiedong@126.com (T.-D.C.) **\*** Correspondence: xgtang@gdut.edu.cn; Tel./Fax: +86-20-3932-2265

Received: 20 November 2019; Accepted: 10 December 2019; Published: 15 December 2019

**Abstract:** Double perovskite Bi2NiMnO6 (BNMO) thin films grown on p-Si (100) substrates with LaNiO3 (LNO) buffer layers were fabricated using chemical solution deposition. The crystal structure, surface topography, surface chemical state, ferroelectric, and current-voltage characteristics of BNMO thin films were investigated. The results show that the nanocrystalline BNMO thin films on p-Si substrates without and with LNO buffer layer are monoclinic phase, which have antiferroelectric-like properties. The composition and chemical state of BNMO thin films were characterized by X-ray photoelectron spectroscopy. In the whole electrical property testing process, when the BNMO/p-Si heterojunction changed into a BNMO/LNO/p-Si heterojunction, the diode behavior of a single diode changing into two tail to tail diodes was observed. The conduction mechanism and temperature stability were also discussed.

**Keywords:** Bi2NiMnO6; thin films; diode effect; oxygen defect; conduction mechanism

#### **1. Introduction**

In the past few decades, electronic devices prepared using a semiconductor have become an important research project in the field of materials science [1–4]. These devices have garnered attention for their practical applications, such as magnetoresistance, photodetectors, p-n diodes and thin film transistors [5,6]. Bi2NiMnO6 has been widely studied as a multiferroic material. The ferromagnetic and ferroelectric Bi2NiMnO6 was successfully prepared at 6 GPa as reported by Azuma et al. [7]. Low temperature (about 100 K) ferroelectric properties in pulsed laser-deposition drive Bi2NiMnO6 thin films on (001)-oriented SrTiO3 single crystal substrates were reported by Sakai et al. [8]. The phase transition temperature of epitaxial Bi2NiMnO6 thin films affected by single crystal substrates was studied using Raman spectroscopy [9]. The magnetodielectric effect was obtained in single-phase and epitaxial thin film of multiferroic Bi2NiMnO6, as reported by Padhan et al. and Rathi et al. [10–12]. The ferroelectric behavior and magnetic exchange interaction effect of Bi2NiMnO6 with the electric polarization 19.01 μC/cm<sup>2</sup> was reported by Zhao et al. [13]. Theoretical and experimental results confirm that Bi2NiMnO6 thin films are multiferroic materials [8,14–18]. The ferroelectric and current leakage characteristics of La-doped Bi2NiMnO6 and Bi2NiMnO6 thin film was reported by Li et al. [19].

However, the ferroelectric diode effect and temperature stability of Bi2NiMnO6 thin film has never been reported. Therefore, in this work, a thin film of Bi2NiMnO6 was growth on p-Si and LaNiO3/p-Si substrates using chemical solution deposition technology, the ferroelectric diode effect and temperature stability of Bi2NiMnO6 thin film was first investigated, as was the conduction mechanism.

#### **2. Materials and Methods**

The Bi2NiMnO6 (abbreviated as BNMO) precursor was prepared by dissolving nitrogen salt, bismuth, manganese acetate, and nickel acetate in a ratio of 2.2:1:1 in ethylene glycol solution [16]. The excess of 10% bismuth was to prevent evaporation of the film during the drying and annealing process. Three milliliters of acetic acid was added to the solution to prevent precipitation. The precursor BNMO concentration is 0.2 M. The LaNiO3 (LNO) precursor was prepared by dissolving nickel acetate and lanthanum nitrate in a ratio of 1:1 in ethylene glycol solution and the resulting concentration was 0.3 M. The 5 mL acetone acetate stabilizer was also added to the precursor. The precursors were then aged for 3 days. The wet LNO/p-Si thin film was synthesized by a spin-coating process at a rate of 2500 rpm for 15 s. The LNO/p-Si substrate was made using a drying process at 573 K for 5 min and an annealing process at 973 k for 30 min. The BNMO/p-Si and BNMO/LNO/p-Si heterojunctions are prepared using the spin coating process at 3000 rpm for 15 s, the dry process at 573 K for 5 min and annealing process at 973 K for 10 min by rapid thermal annealing (RTA) in air atmosphere. A gold electrode with a diameter of 0.3 mm was plated on the surface of the film sample by a small high vacuum coater to form a film capacitor structure.

The crystal structure analysis was measured using XRD (Bruker D8 Advance, AXS, Germany) and the chemical states were examined using X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Sussex, UK). The surface topography and elemental analysis were performed with the field emission scanning electron microscope (FE-SEM, SU8220, Hitachi, Japan). The ferroelectric properties of the BNMO thin films were measured with a ferroelectric test system (Radiant Technologies Precision Workstation, Albuquerque, NM, USA). The current-voltage characteristic was measured with a Keithley 2400 system.

#### **3. Results and Discussion**

The XRD patterns are shown in Figure 1. From Figure 1, there are six peaks at (100), (110), (111), (200), (210) and (211), the crystal structure of LNO grown on p-Si can be judged by PDF card of 33-0710 as a cubic phase. The crystal structure of BNMO film grown on p-Si substrate without and with LNO can be determined by diffraction angles of 23.62◦ and 31.18◦. It has the monoclinic structure reported by Azuma et al. [7]. The unit cell of BNMO was considered to be similar to BiMnO3. The three possible transition metal sites of M1, M2, and M3 were filled with Bi3<sup>+</sup>, Ni2<sup>+</sup> and Mn4<sup>+</sup> cations and the Mn4<sup>+</sup>–O2<sup>−</sup>–Ni2<sup>+</sup> chemical links and Bi3+–O2<sup>−</sup> links were the main chain segment. The diffraction angle of 27.58◦and 29.22◦ matches the results of Li et al. [19], and it is a monoclinic structure with *C*2 space group [20]. In theory, The NiO6 and MnO6 octahedral of BNMO are isotropic and do not cause distortion.

**Figure 1.** The XRD patterns of LNO, BNMO/LNO and BNMO thin films on p-Si substrates.

Figure 2 show the surface and cross-section topographies of BNMO thin film and BNMO/LNO thin film grown p-Si substrates, respectively. From Figure 2a,b it can be clearly observed from the surface topography that the grain size of BNMO/p-Si thin film is nearly 15 nm, but the BNMO/LNO/p-Si thin film is nearly 10 nm. The BNMO thin film was formed by the Mn4<sup>+</sup> cations and Ni2<sup>+</sup> cations, and the thin films was annealed at air atmosphere for only 10 min. Therefore, the growth of crystal grains is relatively difficult, resulting in a small grain size. Observed from the cross-section images, it can be obtained that the thickness of the BNMO layer growth on the p-Si substrata is nearly 100 nm (see Figure 2c), the BNMO layer on LNO/p-Si substrate is 140 nm (see Figure 2d). The different film thickness of the BNMO layers may be caused by the different adhesion of LNO and Si to the solution and the first layer, respectively.

**Figure 2.** The surface topography and cross-section images: (**a**,**c**) for BNMO/p-Si, (**b**,**d**) for BNMO/LNO/p-Si.

The XPS spectra of Ni 2p and Mn 2p are shown in Figure 3. The binding energy of Mn 2p3/<sup>2</sup> and Mn 2p1/<sup>2</sup> of the BNMO/p-Si heterojunctions was 641.35 eV and 653.2 eV [21]. The binding energy of Mn 2p3/<sup>2</sup> and Mn 2p1/<sup>2</sup> of the BNMO/LNO heterojunction was 641.6 eV and 653.45 eV. The binding energy of Ni 2p3/<sup>2</sup> and Ni 2p1/<sup>2</sup> of the BNMO/p-Si heterojunction was 872.45 eV and 861.3 eV [22]. The binding energy of Ni 2p3/<sup>2</sup> and Ni 2p1/<sup>2</sup> of the BNMO/LNO/p-Si heterojunction was 855.1 eV and 872.55 eV.

The XPS spectrum is subject to peak processing. The binding energy of 638.85 eV, 641.65 eV, and 644.25 eV of the BNMO/p-Si heterojunction and 638.25 eV, 641.5 eV, and 643.95 eV of the BNMO/LNO/p-Si heterojunction indicates the Mn2<sup>+</sup>, Mn4<sup>+</sup> and Mn6<sup>+</sup> cation. The binding energy of 855 eV and 857.65 eV of the BNMO/p-Si heterojunction and 855 eV and 857.2 eV of the BNMO/LNO/p-Si heterojunction shows the Ni2<sup>+</sup> and Ni3<sup>+</sup> cation. The ion ratio of Mn2+:Mn4+:Mn6<sup>+</sup> on BNMO/p-Si thin film is 0.15:1:0.3 and for BNMO/LNO/p-Si thin film is 0.16:1:0.27. The ratio of Ni2+:Ni3<sup>+</sup> on the BNMO/p-Si heterojunction and on the BNMO/LNO heterojunction is 1:0.22 and 1:0.24, respectively. By analyzing the XPS spectrum, a variety of Ni, Mn ions are found in the BNMO heterojunction device. The cubic crystal structure of NiO and BiMnO3 octahedral crystallites interferes with the formation of pure phase monoclinic crystals and causes crystal defects.

**Figure 3.** The fitted narrow-scan spectra for (**a**) Mn 2p and (**b**) Ni 2p.

The room temperature polarization-electric field (*P-E*) properties are shown in Figure 4. The results show that the two films have antiferroelectric-like properties. The saturated polarization (2*Ps*), remnant polarization (2*Pr*), and coercive field (*Ec*) of BNMO/LNO/Si thin film were 0.875 μC/cm2, 0.150 μC/cm2, and 40.0 kV/cm, and 1.03 μC/cm2, 0.202 μC/cm2, and 38.4 kV/cm, respectively for BNMO/Si and BNMO/LNO/Si thin films at 500 Hz. The ferroelectric polarization was enhanced by using LNO as a buffer layer. The room temperature ferroelectric polarization phenomenon could be due to the incompletely symmetric monoclinic structure preventing the ferroelectric domain from flipping. It is a pinning effect caused by the interaction of defect dipoles in the BNMO layer. The BNMO growth on the LNO/p-Si substrate is a nanocrystalline state, resulting in more lattice defects, preventing the deformation of the crystal [7].

**Figure 4.** The typical hysteresis loops of the BNMO/Si and BNMO/LNO/Si thin films measured at 500 Hz.

The current-voltage characteristics of BNMO/Si and BNMO/LNO/Si thin films are shown in Figure 5. From Figure 5a, we know that from 0 to −1.0 V and from 0 to 0.25 V, the current hardly changed with the increase of absolute voltage value, when the voltage increases from −1.0 to −1.5 V, the current increased, and when the voltage increased from 0.65 to 1.5 V, the current increased rapidly. The results show typical diode characteristics.

**Figure 5.** I-V characteristics of BNMO thin films without (**a**) and with (**b**) a LNO buffer layer on p-Si substrates. Insets show the schematics of the diode cell used for measurement.

The BNMO/p-Si heterojunction exhibited rectification effect behavior of p-n junction. It is a forward-conducting heterojunction device with an ON/OFF ratio (*R* = *I*on/*I*off) of 65. The n-type semiconductor properties of BNMO have not been reported. It can be inferred from the results, and the process of converting Mn2<sup>+</sup> ions into Mn4<sup>+</sup> ions can release electrons.

As the bottom electrodes of the LNO grew on the p-Si substrate, the rectification effect of the forward conduction was forcibly converted into a reverse conduction rectification effect, and the ON/OFF ratio is increased to 1.4. The result shows two tail to tail diodes. The cubic phase of LNO acted as an n-type semiconductor to release electrons, while the nano-crystallinity of the BNMO layer defects acted as a hole-absorbing electron [23–25].The Schottky emission mechanism is determined by the linear relationship of Ln(*I*) versus V1/<sup>2</sup> [26,27]. If the relationship is linear, this is due to the thermionic emission by holes, vacancies and defects [28–32], respectively. The restricted behavior of the interface and the hole trapping behavior are considered to be a case of the Schottky emission mechanism, as expected with linear relationship of Ln(*I*) versus V1/<sup>2</sup> for the BNMO/LNO/p-Si heterojunction (see the inset of Figure 5b). Therefore, the large ON/OFF ratio of the BNMO/LNO/p-Si heterojunction is caused by the hole of the BNMO layer being filled.

The voltage-current characteristics measured at different temperatures were shown in Figure 6. The conductivity of the BNMO/p-Si heterojunction and the BNMO/LNO/p-Si heterojunction was increased due to increased test temperature. The ON/OFF ratio of the BNMO/p-Si heterojunction is decreased from 65 to 3.8, and the ON/OFF ratio of the BNMO/LNO/p-Si heterojunction is increased from 1.4 to 13.4. The result can be interpreted with the thermion emission effect of Schottky diodes [33–36]. The electrons are excited by the lattice defects caused by the heat. Therefore, the conductivity of the film increases. At the same time, the space charge accumulated at the interface of the heterojunction is also subject to thermal radiation. The BNMO/p-Si heterojunction is excited by thermal radiation, causing the electrons of the defect to be excited, forming a space charge region at the interface of the heterojunction, and finally the rectification effect is improved. The BNMO/LNO/p-Si heterojunction is thermally radiated, and the electron trapping ability of the hole is weakened, resulting in the rectification effect being weakened.

**Figure 6.** The I-V characteristics of BNMO thin films (**a**) without and (**b**) with a LNO buffer layer on p-Si substrates measured at different temperatures.

#### **4. Conclusions**

In conclusion, the BNMO thin films on Si and LNO/Si substrates show a monoclinic phase with *C2* space group and the porosity of BNMO thin film with LNO layer is smaller than without, resulting in a larger ferroelectric polarization. The conduction mechanism of the BNMO/Si and BNMO/LNO/Si heterojunctions were dominated by Ohmic conduction and Schottky emission mechanisms, respectively. In the case of temperature rise, the rectification effect of BNMO/Si will decrease due to the energy of the hole trapping electrons being weakened and the rectification effect of the BNMO/LNO/p-Si heterojunction will increase due to the charge accumulation, respectively. XPS testing shows that BNMO synthesized under normal pressure is a material with the coexistence of different valence cations.

**Author Contributions:** Author X.-G.T. and Q.-X.L., conceptualized the idea. The test was helped by the W.-M.Z., W.-P.L. and T.-D.C. The draft of the manuscript was reviewed and revised by W.-M.Z. and X.-G.T. The work was supported and supervised by Q.-X.L., Y.-P.J. and W.-H.L. All authors read and approved the final manuscript.

**Funding:** This research was funded by "the National Natural Science Foundation of China (Grant Nos. 11574057 and 51604087)", "the Guangdong Provincial Natural Science Foundation of China (Grant No. 2016A030313718)", and "the Science and Technology Program of Guangdong Province of China (Grant Nos. 2016A010104018 and 2017A010104022)".

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Bi1**−**xEuxFeO3 Powders: Synthesis, Characterization, Magnetic and Photoluminescence Properties**

**Vasile-Adrian Surdu 1, Roxana Doina Trus, că 1, Bogdan S, tefan Vasile 1,\*, Ovidiu Cristian Oprea 2, Eugenia Tanasă 1, Lucian Diamandescu 3, Ecaterina Andronescu <sup>1</sup> and Adelina Carmen Ianculescu 1,\***


Received: 23 September 2019; Accepted: 12 October 2019; Published: 16 October 2019

**Abstract:** Europium substituted bismuth ferrite powders were synthesized by the sol-gel technique. The precursor xerogel was characterized by thermal analysis. Bi1−xEuxFeO3 (x = 0–0.20) powders obtained after thermal treatment of the xerogel at 600 ◦C for 30 min were investigated by X-ray diffraction (XRD), scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Raman spectroscopy, and Mössbauer spectroscopy. Magnetic behavior at room temperature was tested using vibrating sample magnetometry. The comparative results showed that europium has a beneficial effect on the stabilization of the perovskite structure and induced a weak ferromagnetism. The particle size decreases after the introduction of Eu3<sup>+</sup> from 167 nm for x = 0 to 51 nm for x = 0.20. Photoluminescence spectroscopy showed the enhancement of the characteristic emission peaks intensity with the increase of Eu3<sup>+</sup> concentration.

**Keywords:** bismuth ferrite; sol-gel process; magnetic properties; photoluminescence properties

#### **1. Introduction**

Among various multiferroic compounds, bismuth ferrite (BiFeO3) stands out because it is one of the few magnetic ferroelectrics at room temperature. Therefore, there has been intensive research in the past decades to make it useful in practical applications. There are certain issues that are still open in what concerns voltage-induced changes, the possibility of reading magnetic data or the mechanism of magnetoelectric coupling, and whether it may be controlled [1]. Besides, extensive studies search for the possibility of using BiFeO3 based materials for applications such as actuators, transducers, magnetic field sensors, information storage devices, optical imaging, photocatalysis, or gas sensors [2–6]. Recently, BiFeO3 nanopowders were found to exhibit catalytic activity for doxorubicine degradation [7].

The antiferromagnetic structure in BiFeO3 is quite complex, usually being considered as a G-type with a spiral spin arrangement (about 62 nm wavelength), due to the interplay between exchange and spin-orbit coupling interactions involving Fe ions. There are several strategies to enhance its magnetic properties, including chemical modifications, or control of morphology and structure. In terms of morphologies, BiFeO3-based nanostructures exhibit increased magnetization than the corresponding bulks, due to the perturbation of the helimagnetic order by structural peculiarities (e.g., local defects) or the specific size of nanoparticles [8–12].

Another way to modify the magnetic structure consists in replacing Bi by rare earth ions, based on the fact that in the perovskite-like structure, the superexchange interaction between the localized RE*4f* and Fe*3d* electrons may play an important role. The effect of several rare-earth dopants/solutes, as Ho [13,14], Sm [15,16], La [17,18], Dy [19], Gd [20], or Nd [21] on the properties of bismuth ferrite have been investigated. There are some works which described the effect of Eu3<sup>+</sup> used as *A* site solute on the characteristics of bismuth ferrite powders prepared by various non-conventional techniques, such as hydrothermal process [22], ball milling [23], or different variants of the sol-gel methods [24–26], etc. Even if the magnetic behavior of these powders was extensively analyzed, however no data regarding other properties as photoluminescence were reported.

The aim of this work is to study the influence of europium addition on the phase purity, crystal structure, morphology, magnetic behavior, and optical properties of Bi1−xEuxFeO3 powders (x = 0; 0.05; 0.10; 0.15; 0.20) prepared by the sol-gel route. In order to be able to assess only the contribution of Eu substitution on the *A*-site of the perovskite structure, all the processing parameters were constantly maintained.

#### **2. Materials and Methods**

Synthesis of Bi1−xEuxFeO3 powders (x = 0; 0.05; 0.10; 0.15; 0.20) was carried out through sol-gel route. All the solvents and chemicals were of analytical grade and used without further purification. The precursor solution was prepared by dissolution of Bi(NO3)3·5H2O (Sigma Aldrich, St. Louis, MO, USA ≥98%), Eu(NO3)3·5H2O (Sigma Aldrich, 99.9%) and Fe(NO3)3·9H2O (Aldrich, 99.99%) in stoichiometric ratios in acetic acid solution (Honeywell Fluka, Wabash, IN, USA ACS Reagent, ≥99.7%). A transparent brownish red sol resulted after the complete dissolution (≈ 1 h) of the nitrates. The sol was stabilized with 2-methoxyetanol which was added in a 1:1 volume ratio with respect to acetic acid. The amounts of precursors are summarized in Table 1. After 1 h mixing at 400 rpm, the temperature was set to 80 ◦C and the sol was kept under magnetic stirring at this temperature for 12 h until a gel was obtained. Gel drying was carried out in a forced convection oven (Memmert Universal Oven U, Schwabach, Germany) in air at 120 ◦C for 12 h to obtain the xerogels. The precursor powders were heat treated in air at 600 ◦C with a soaking time of 30 min, a heating rate of 5 ◦C/min and then were slowly cooled at the normal rate of the oven (CWF 1200, Carbolite Gero, Hope Valley, England).


**Table 1.** Amounts of precursors for Bi1−xEuxFeO3 sol-gel synthesis.

Thermal behavior of the precursor powders was investigated by differential scanning calorimetry–thermogravimetry (DSC-TG) analyses carried out with a TG 449C STA Jupiter (Netzsch, Selb, Germany) thermal analyzer. Samples were placed in alumina crucible and heated with 10 ◦C/min from room temperature to 900◦, under dried air flow of 20 mL/min.

Room temperature X-ray diffraction (XRD) measurements were performed to investigate the phase purity and structure of the (Bi,Eu)FeO3 powders. For this purpose, an Empyrean diffractometer (PANalytical, Almelo, The Netherlands), using Ni-filtered Cu-Kα radiation (λ = 1.5418 Å) with a scan step increment of 0.02◦ and a counting time of 255 s/step, for 2θ ranged between 20–80◦was used. Lattice parameters were refined by the Rietveld method [27], using the HighScore Plus 3.0e software (PANalytical, Almelo, The Netherlands). After removing the instrumental contribution, the full-width

at half-maximum (FWHM) of the diffraction peaks can be interpreted in terms of crystallite size and lattice strain. A pseudo-Voigt function was used to refine the shapes of the BiFeO3 peaks.

The local order and the cation coordination in the calcined powders were studied by Raman spectroscopy carried out at room temperature, using a LabRAm HR Evolution spectrometer (Horiba, Kyoto, Japan). Raman spectra were recorded using the 514 nm line of an argon ion laser, by focusing a 125 mW beam of a few micrometer sized spots on the samples under investigation.

Mössbauer spectroscopy ICE Oxford Mössbauer cryomagnetic system (WissEL, Mömbris, Germany) was used to analyze the state of iron ions in the perovskite lattice. The system was equipped with a 10 mCi 57Co(Rh) source and the velocity was calibrated using a α-Fe standard foil.

Morphology and crystallinity degree of the (Bi,Eu)FeO3 particles were investigated by scanning electron microscopy operated at 30 kV (Inspect F50, FEI, Hillsboro, OR, USA) and transmission electron microscopy operated at 300 kV (TecnaiTM G2 F30 S-TWIN, FEI, Hillsboro, OR, USA). The average particle size of the (Bi,Eu)FeO3 powders was estimated from the particle size distributions, which were determined using the OriginPro 9.0 software (OriginLab, Northampton, MA, USA) by taking into account size measurements on ~100 particles performed by means of the software of the electron microscopes (ImageJ 1.50b, National Institutes of Health and the Laboratory for Optical and Computational Instrumentation, Madison, WI, USA) in the case of SEM, and Digital Micrograph 1.8.0 (Gatan, Sarasota, FL, USA) in the case of transmission electron microscopy (TEM).

Vibrating sample magnetometry (7404-s VSM, LakeShore, Westerville, OH, USA) was used in order to investigate the magnetic behavior of the processed powders. Hysteresis loops were recorded at room temperature with an applied field up to 15 kOe, with increments of 200 Oe and a ramp rate of 20 Oe/s.

The fluorescence spectra were recorded with a LS 55 spectrometer (Perkin Elmer, Waltham, MA, USA) using an Xe lamp as a UV light source, at ambient temperature, in the range 350–650 nm, with all the samples in solid state. The measurements were made with a scan speed of 200 nm/min, excitation and emission slits of 10 nm, and a cut-off filter of 350 nm. An excitation wavelength of 320 nm was used.

#### **3. Results**

#### *3.1. Thermal Behavior of the Precursor Powders*

The TG-DSC curves of the Bi1−xEuxFeO3 xerogels are shown in Figure 1. The peaks corresponding to the exothermic effects and associated mass loss are illustrated in Table 2.

**Figure 1.** (**a**) Differential scanning calorimetry (DSC) and (**b**) thermogravimetry (TG) curves of Bi1−xEuxFeO3 xerogels.


**Table 2.** TG-DSC effects corresponding to Bi1−xEuxFeO3 xerogels.

Thermal analysis reveals four step decomposition in the case of (Bi,Eu)FeO3 powders with x = 0, x = 0.05, x = 0.10, and five step decomposition for the samples with x = 0.15 and x = 0.20. The first step decomposition at 103–108◦ was attributed to dehydration of the xerogels.

The second decomposition step (110–230 ◦C) associated with exothermic reactions with the highest mass loss, between 18.2% and 26.5% for the selected compositions, correspond to decarboxylation of acetic acid and decomposition of small groups such as NO3 −. For the powders with x = 0.15 and x = 0.20, this reaction takes place in two steps, one at 206.7 ◦C and 208.7 ◦C, respectively, and the other at 239.2 ◦C and 240.3 ◦C, respectively [28].

The exothermic effect at 270–282 ◦C could be assigned to the collapse of the gel network and combustion of most organic materials. A small weight loss ≈2.5% occurring up to 430 ◦C corresponds to the end of CO2 release [28].

#### *3.2. Phase Composition and Structure of the (Bi,Eu)FeO3 Powders*

The room-temperature XRD patterns of Bi1−xEuxFeO3 powders are illustrated in Figure 2. The profiles of the peaks indicate a high crystallinity. A rhombohedral perovskite structure with space group R3c was indexed for the powders with x ≤ 0.10 [23]. A small amount of Bi25FeO40 sillenite phase is also detected for these compositions. Upon increasing the substitution ratio, the secondary phase diminishes until vanishing, which proves the beneficial effect of Eu3<sup>+</sup> in what concerns the stabilization of the perovskite phase. All the reflections corresponding to the major perovskite phase are shifted to higher values of the diffraction angle when x is increased. Besides, in the case of the compositions with x ≥ 0.15 it may be observed that the (012) peak is split and a supplementary interference occurs at 2θ ≈ 34◦. These are arguments that suggest Eu3<sup>+</sup> ions have substituted Bi3<sup>+</sup> in the BiFeO3 lattice and that a phase transition from rhombohedral R3c (α phase) to orthorhombic Pnma (β phase) crystal symmetry has occurred [29,30].

**Figure 2.** *Cont*.

**Figure 2.** (**a**) X-ray diffraction (XRD) patterns of Bi1−xEuxFeO3 calcined powders, (**b**,**c**) Rietveld refined patterns for x = 0.05 and x = 0.20.

Rietveld refinement was performed in order to accurately determine the phase composition and structure of the powders. For the specimens with x ≥ 0.15, the best fit to data was obtained when using a mixture of rhombohedral R3c and orthorhombic Pnma polymorphs. The quality of the fits is indicated by the agreement indices obtained from Rietveld refinement (Table 3).


**Table 3.** Agreement indices from Rietveld refinement for Bi1−xEuxFeO3 calcined powders.

The phase composition evolution versus Eu3<sup>+</sup> substitution degree is shown in Figure 3. For x <sup>≥</sup> 0.15, the Bi25FeO40 secondary phase vanishes in the limit of detection of X-ray diffraction. Stabilization of the perovskite phase is also accompanied by rapid polymorph transition. When increasing x from 0.10 to 0.15, phase composition changes from 97.4% R3c bismuth ferrite and 2.6% sillenite in the secondary phase to 62% R3c bismuth ferrite polymorph and 40% Pnma bismuth ferrite polymorph, respectively. These results are in good agreement with those reported by Iorgu et al. [31] and Khomchenko et al. [32] who also found a second orthorhombic polymorph in their Eu-substituted bismuth ferrite, with x ≥ 0.10 obtained by combustion method and solid state reaction, respectively.

**Figure 3.** Phase composition evolution in Bi1−xEuxFeO3 calcined powders.

Unit cell parameters and cell volume (Figure 4) decrease with the increasing amount of Eu solute. This, together with the phase transition, is supported most likely by the smaller ionic radius of Eu3<sup>+</sup> (1.07 Å) than that of Bi3<sup>+</sup> (1.17 Å) [33].

**Figure 4.** (**a**) Unit cell parameters corresponding to R3c polymorph, and (**b**) unit cell volume for Bi1−xEuxFeO3 calcined powders.

As expected, the formation of (Bi,Eu)FeO3 solid solutions drives to the decrease of the crystallite size and the concurrent increase of the internal microstrains (Figure 5).

**Figure 5.** Average crystallite size and lattice microstrain for Bi1−xEuxFeO3 calcined powders.

Raman spectroscopy is a powerful technique, which is sensitive to structural phase transitions and it has been carried out to further support the Rietveld analysis of the XRD patterns. The active Raman modes of the BiFeO3 solid solutions with rhombohedral R3c structure may be summarized using the irreducible representation of ΓRaman, R3c = 4A1 + 9E [34–36].

In the present study, for the powders with lower Eu content (x ≤ 0.10), the modes A1-1 and A1-2, attributed to Bi-O bonds shift to higher-frequency region. This may be explained by the partial substitution of Bi3<sup>+</sup> with Eu3<sup>+</sup> because the frequency of the mode is inversely proportional to the mass, M, at *A*-site. Since the mass of Eu is about 27% lower than the mass of Bi, substitution will induce the shift in the frequency of vibration of the modes, which is consistent to the data presented in Figure 6. When x increases from 0.10 to 0.15, the most significant feature in the Raman spectra is that A1-1 and A1-2 modes almost vanish and severely broaden, while the E mode at <sup>≈</sup>290 cm−<sup>1</sup> shifts to a higher frequency and increases in intensity. Such peak has been reported for orthorhombic rare earth ferrites and can be assigned to Ag mode [37]. The further increase of x from 0.15 to 0.20 indicate a visible distortion of FeO6 octahedra, which is evidenced by the increase of intensity of the 500 and 600 cm−<sup>1</sup>

modes [38]. All the discussed features are arguments that Eu3<sup>+</sup> is incorporated on the Bi site of the perovskite lattice of BiFeO3 forming solid solutions, and that when the substitution degree exceeds the value of 0.15, using the processing parameters in the present work, it induces a structural phase transition from rhombohedral to orthorhombic symmetry.

**Figure 6.** Typical Raman scattering spectra of Bi1−xEuxFeO3 calcined powders.

The 57Fe Mössbauer spectra for the selected compositions with x = 0 and x = 0.20 were recorded at room temperature. Results show that the spectra corresponding to the investigated samples present hyperfine magnetic sextet (Figure 7).

**Figure 7.** Room temperature 57Fe Mössbauer spectra of powders with x = 0 and x = 0.20.

The refining of the spectra under the assumption of the Lorentzian shape of the Mössbauer line allowed obtaining of the characteristic parameters: isomeric shift (IS), quadrupole splitting (ΔEq) and hyperfine field (Hhf), which are presented in Table 4.


**Table 4.** Mössbauer parameters for x = 0 and x = 0.20 samples.

The values of IS and ΔEq prove that Fe occupies only the B-site in the perovskite structure and correspond to high-spin Fe3<sup>+</sup> ions.

Upon introduction of Eu3<sup>+</sup> in the lattice, ΔEq switches from positive values (0.179 mm/s) in the case of x = 0 to negative values (−0.054 mm/s) in the case of x = 0.20. This means that the electric field gradient is drastically changed by the substitution and may be assigned to structural phase transition from rhombohedral to orthorhombic symmetry, as seen for Bi1−xDyxFeO3 nanoparticles obtained by Qian et al. [39]

In what concerns the Hhf, the substitution of Bi3<sup>+</sup> with Eu3<sup>+</sup> does not affect the obtained values nor the charge density reflected in the IS parameter which remains constant. All Mössbauer parameters are in good agreement with those obtained by Prado-Gonjal et al. for microwave-assisted hydrothermal processed BiFeO3 powders [40].

#### *3.3. Morphology*

Scanning electron microscopy (FE-SEM) images depicting the morphology and the particle size distribution of the calcined (Bi,Eu)FeO3 powders are shown in Figure 8. A general view of two selected compositions, x = 0 (Figure 8a) and x = 0.10 (Figure 8b), illustrate porous networks with pores in the micrometer and submicrometer range, which were formed after heat treatment of the precursor gels. The walls of the pores are dense and consist of agglomerated particles as it may be seen in the detail in the images of Figure 8c,e,g,i,k. In each case, the particles exhibit polyhedral shapes and as x increases the particles tend to have a more rounded aspect. Moreover, for ternary compositions, a tendency toward coarsening was observed. In what concerns the particle size, one can see a decrease after the introduction of Eu3<sup>+</sup> as a substituent in the perovskite lattice from 167 nm for x = 0 to 85 nm for x = 0.05, which becomes even more evident for the compositions where the polymorphic transformation occurs (x = 0.15 and x = 0.20). In the latter case, the particle size decreases from 78 nm for x = 0.10 to 56 nm for x = 0.15. This kind of effect is consistent with other studies regarding substituted BiFeO3 particles prepared by various techniques [24,26,31,41]. Moreover, Dai et al. explained this in the case of (Eu, Ti) co-substituted ceramics as a result of suppression of oxygen vacancies by the solutes, which slows oxygen ion motion and, consequently, grain growth rate [42]. The particle size distribution is unimodal (Figure 8d,f,h,j,l) and becomes narrower as the solute concentration increases. Thus, in the BiFeO3 sample, the unimodal distribution show 20%–25% of nanoparticles in the size range of 140–180 nm. Besides, the influence of the addition of Eu3<sup>+</sup> on the size and particle size distribution should be noted. The introduction of 5% Eu3<sup>+</sup> results in the particle size distribution shown in Figure 8f. The entire particle size distribution is between 50 and 120 nm, with a maxima at 80–90 nm, which represents a proportion of 35%. Actually, all the nanoparticles present sizes below those characteristic to BiFeO3 (80–280 nm). The slowing particle growth effect of europium is better observed when its concentration in the perovskite solid solution increases. Thus, for x = 0.10, even if 30% of the nanoparticles correspond to the size range of 80–90 nm, the unimodal distribution is asymmetric due to the increase of the ratio of nanoparticles in the range size of 50–80 nm. More obvious contribution of the solute is shown in the case of x = 0.15 and x = 0.20, where one can observe that 35%–40% of the nanoparticles are in the size range of 50–60 nm and, respectively, 40–60 nm. The measurements of the sizes and the corresponding distributions from FE-SEM data illustrate a clear influence of the Eu3<sup>+</sup> solute on the reduction of the particles size, as well as on the narrowing of the particle size distribution with the increase of the substitution rate.

**Figure 8.** *Cont*.

**Figure 8.** FE-SEM images showing the morphology and corresponding histograms for particle size distribution of Bi1−xEuxFeO3 powders: (**a**,**b**) General view for x = 0 and x = 0.10, (**c**,**d**) x = 0: (**c**) detail, (**d**) particle size distribution, (**e**,**f**) x = 0.05: (**e**) detail, (**f**) particle size distribution, (**g**,**h**) x = 0.10: (**g**) detail, (**h**) particle size distribution, (**i**,**j**) x = 0.15: (**i**) detail, (**j**) particle size distribution, (**k**,**l**) x = 0.20: (**k**) detail, (**l**) particle size distribution.

TEM investigations sustain FE-SEM observations. The coarsening of the particles is observed better in Bright-field TEM images (Figure 9a,e,i,m,q) by means of necks at the particles limits. Particle size distributions (Figure 9b,f,j,n,r) are similar to those measured from FE-SEM images, as the small differences are in the limits of the standard deviation. Morphology evolution with increasing Eu3<sup>+</sup> solute degree is similar to that reported by Bahraoui et al. who synthetized Bi1−xEuxFeO3 powders by the sol-gel method with calcination treatment at 500 ◦C for 24 h, but the average particle size is almost four times higher [26]. This shows that although the time of heat treatment at 600 ◦C was relatively short (30 min), the temperature has a stronger influence on the particle size growth.

The powders show a high crystallinity degree as assessed from the selected area electron diffraction (SAED) patterns (Figure 9c,g,k,o,s), which consist of well-defined diffraction spots arranged in concentric diffraction rings. For the pure BiFeO3 powder (x = 0), the diffraction rings are less visible due to the fact that both crystallite size and particle size are situated in the submicrometer scale and because the coarsening process may induce some preferential orientations of the aggregated particles. In the case of the samples with higher Eu3<sup>+</sup> content (x = 0.15 and x = 0.20), the patterns are more complicated because of the coexistence of rhombohedral and orthorhombic polymorphs which are homogeneously distributed.

**Figure 9.** *Cont*.

**Figure 9.** *Cont*.

**Figure 9.** *Cont*.

**Figure 9.** (**a**,**e**,**i**,**m**,**q**) Bright field TEM images, (**b**,**f**,**j**,**n**,**r**) particle size distributions, (**c**,**g**,**k**,**o**,**s**) Selected area electron diffraction patterns, and (**d**,**h**,**l**,**p**,**t**) High resolution TEM images corresponding to Bi1−xEuxFeO3 powders for: x = 0 (**a**–**d**), 0.05 (**e**–**h**), 0.10 (**i**–**l**), 0.15 (**m**–**p**) and 0.20 (**q**–**t**), respectively.

High resolution transmission electron microscopy (HR-TEM) investigations reveal long-range highly ordered fringes with spacing at 2.28 Å and 1.77 Å corresponding to the (2 0 2) and (1 1 6) crystalline planes of the rhombohedrally-distorted perovskite structure in the case of x = 0. For x = 0.05 and x = 0.10, there were also identified the crystallographic planes specific to rhombohedral polymorphs (Figure 9f,h). In the case of x = 0.20, both polymorphs were identified in the same particles consisting of multiple crystallites. It is worth mentioning that the substitution also induces the forming of polycrystalline particles, which is also evidenced in the HR-TEM images.

In order to have a better understanding of the nature of the particles, in Figure 10 a comparison between average crystallite size determined from XRD data and average particle size measured on SEM and TEM images was depicted. In the case of unsubstituted BiFeO3 particles, the three values are almost equal. Slightly differences that occur are in the range of standard deviation. This means that in this case, the particles are single crystals. Interestingly, when comparing the values obtained for Eu-substituted BiFeO3 compositions, one can observe that the values for the average nanoparticle size determined from SEM and TEM investigations are very close, whereas the average crystallite size presents at most a half value of the average particle size, proving that for x ≥ 0.05, the particles are polycrystalline and consist of two or more crystallites, which sustains the HR-TEM observations.

**Figure 10.** Comparison between average crystallite size determined from XRD data and average particle size measured from SEM and TEM images.

#### *3.4. Magnetic Behavior*

Figure 11 and Table 5 show the room-temperature M = f(H) hysteresis loops up to 15,000 Oe, and Ms, Mr, and Hc parameters of the Bi1−xEuxFeO3 powders.

**Figure 11.** M-H hysteresis loops for Bi1−xEuxFeO3 powders: Inset showing the low-field M = f(H) dependence.

**Table 5.** Ms, Mr and Hc for Bi1−xEuxFeO3 powders.


It can be observed that the sample with x = 0 shows a continuous linear increase of magnetization versus magnetic field which suggests the presence of the antiferromagnetic phase, involving relative low exchange integrals in order to progressively reorient the spins along the field direction.

However, at very low fields, there is a much faster variation of saturation magnetization of 0.3529 emu/g and a coercive field of 51.290 Oe. Unlike this sample, in the case of the samples with 0.05 ≤ x ≤ 0.15, a weak ferromagnetic behavior, with a saturation magnetization of 1.6570 emu/g for x = 0.05, 1.1089 emu/g for x = 0.10 and 0.7113 emu/g for x = 0.15 is present. A possible conclusion of these aspects is that Eu content influences the spin spiral structure, most likely by a perturbation of the superexchange interactions between localized Eu*4f* and Fe*3d* electrons.

At the maximum substitution degree studied in this work, the magnetic behavior shows a decrease in Ms, Mr, and Hc compared to pristine BiFeO3 particles, suggesting that increasing Eu content above x = 0.15 does not improve the magnetic behavior of the particles. This suggests that the presence of the orthorhombic Pnma polymorph affects the magnetic order.

#### *3.5. Photoluminescence Properties*

BiFeO3 is an interesting optical material, which shows promising applications in photocatalysts and photoconductive devices. Thus, photoluminescence spectroscopy was used to study the optical property of Bi1−xEuxFeO3 nanoparticles.

During synthesis there are generated several deep and shallow oxygen vacancies and surface defects that introduce localized electronic levels in-band [43]. Therefore, the PL spectra (Figure 12) are complex and present more than a single peak from band-to-band transition.

**Figure 12.** Fluorescence spectra for Bi1−xEuxFeO3 powders.

The most intense blue emission peak at the wavelength of 455 nm (2.72 eV) originates from self-activated centers in the synthesized nanoparticles [44,45]. The emission peak is broad and asymmetric, with a clear overlap with the peak from 479 nm (2.58 eV). This indicate the existence of another transition below the conduction band, due to the presence of defects in grain boundaries or oxygen vacancies, usually referred as near-band edge (NBE) transition [45–47].

In the blue-green region there are further shoulders at the 511–524 nm range which can be attributed to oxygen vacancies and a small, but broad peak in the range of 569–584 nm which has an unknown origin [48]. These peaks are usually referred as defect-level emissions (DLE).

The intensity of emission peaks increases with the increase of the solute content from the Eu3<sup>+</sup>-doped BiFeO3 with x = 0.05 to x = 0.20. This behavior cannot be explained in terms of the difference in nanoparticles dimensions, taking into account that the 5% and 10%-doped samples, as well as the 15% and 20%-doped powders exhibit roughly similar sizes.

In the first instance, for 5% Eu3<sup>+</sup>-doped sample, the photo-generated electron-hole pairs present a lower recombination rate, which leads to lower intensity of emission peaks. For the next three samples the increase of luminescent emission with the europium amount could be related to a higher concentration of surface defects as new crystalline phase is formed. These defects can contribute to the capture of photo-generated electrons, to produce excitons, which will enhance the emission intensity. A similar behavior was reported for Sn4+/Gd3<sup>+</sup> or Mn2<sup>+</sup>-doped BiFeO3 samples [49,50].

In the 550–650 nm range there are no peaks that can be assigned to Eu3<sup>+</sup> ions emission spectrum, indicating either a masking effect from BiFeO3 luminescence or, simply, a quenching of europium fluorescence. This effect was also observed for other rare earth ions used as soluted for bismuth ferrite [49,51,52].

#### **4. Conclusions**

Bi1−xEuxFeO3 powders were prepared by the sol-gel method. XRD and Raman spectroscopy investigations indicated phase-pure particles and a structural phase transition for x ≥ 0.15 when using the processing parameters presented in the present work. Mössbauer spectroscopy showed only the presence of Fe3<sup>+</sup> and a hyperfine magnetic sextet. FE-SEM and TEM analysis evidenced obtaining submicron-sized single-crystal particles for pure BiFeO3 composition, and polycrystalline nanoparticles in the case of Eu3+-substituted powders. The most pronounced ferromagnetic behavior was observed for Bi0.95Eu0.05FeO3 composition, which exhibited a saturation magnetization of 1.65 emu/g and a coercitive field of 100 Oe, which occurs, most likely, by a perturbation of the superexchange interactions between localized Eu*4f* and Fe*3d* electrons. This work shows a possibility to tailor magnetic behavior of bismuth ferrite using rare earth metal solute on the *A*-site of the perovskite structure. The luminescence emission increases with the increase of the Eu3<sup>+</sup> content, but the quenching of the fluorescence specific to europium ions seems to be induced by a masking effect of BiFeO3, as in other rare-earth doped bismuth ferrite systems.

**Author Contributions:** The authors contributions are as follows: methodology, V.-A.S. and A.C.I.; investigation, O.C.O., E.T., and L.D.; data curation, V.-A.S., R.D.T., and B.S, .V.; formal analysis, A.C.I.; writing—original draft preparation V. -A.S.; writing—review and editing, E.A. and A.C.I.; visualization, B.S, .V. and A.C.I.

**Funding:** This research was funded by Romanian National Authority for Scientific Research, CNCS-UEFISCDI, Project No. PN-III-P4-ID-PCE-2016-0072.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

#### **References**


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