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Table of Contents
Abstract
1. Introduction
2. Results And Discussions
2.1. Structural Studies
2.2. Microstructural Studies
2.3. Optical Studies
2.4. FTIR Studies
2.5. Magnetic Studies
2.6. Ferroelectric Studies
2.7. Ac Conductivity Studies
3. Conclusions
Acknowledgements
Abstract
Polycrystalline Bi1-xRxFeO3 (R–Ho and Y, x = 0.00, 0.05 and 0.10) compounds were prepared to perform a systematic investigation of the role of chemical nature and effect of internal chemical pressure on the structural, microstructural, magnetic, electric, ferroelectric and optical properties of the compounds. Structural analysis revealed that lattice distortions observed in Ho and Y-substituted compounds were not the same. The lattice parameters were larger in the case of the Y-substitution compared to the Ho-substituted counterpart. Scanning electron micrographs confirmed the formation of dense, well-connected grains exhibiting a reduction in size with increasing substitution. The magnetic properties of BiFeO3 were enhanced through the suppression of the spin structure with the substitution. The substitution of magnetic (Ho3+) ions led to an improvement in remanent magnetization and coercive field, whereas the substitution of non-magnetic (Y3+) ions resulted in enhanced maximum magnetization with negligible coercive field at room temperature. The ferroelectric measurements evidenced that both remanent polarization and coercive fields improved in substituted compounds, attributed to a decrease in charge carriers. Furthermore, the Y3+ ion substitution positively influenced ferroelectric properties by reducing leakage currents compared to the Ho3+ ion substitution. The optical absorption measurements indicated a decrease in the energy band gap of BiFeO3 with substitution, implying alterations in the material’s optical characteristics. The ac conductivity studies demonstrated a discernible reduction in the conductivity of the substituted compounds. Specifically, Ho-substitution exhibited a more pronounced magnitude in the decline in conductivity relative to Y-substitution. This outcome signifies the potential efficacy of Ho-substitution in exerting control over the insulating characteristics of the compounds.
Systematic investigation of the influence of magnetic and non-magnetic ion substitutions in BiFeO3 under similar internal chemical pressure Ch. Komala Lakshmi a, T. Durga Rao a,*, G. Bhavani a, M. Sudhadhar a, B. Sattibabu a, V. Satya Narayana Murthy b, T. Karthik c, Saket Asthana d a Department of Physics, School of Science, GITAM (Deemed to Be University), Visakhapatnam, Andhra Pradesh, 530045, India b Department of Physics, BITS Pilani Hyderabad Campus, Hyderabad, 500078, Telangana, India c Centre for Materials for Electronics Technology [C-MET], Ministry of Electronics & Information Technology (MeitY), Thrissur, Kerala, 680581, India d Advanced Functional Materials Laboratory, Department of Physics, Indian Institute of Technology Hyderabad, Telangana, 502284, India A R T I C L E I N F O Keywords: Multiferroics G-type antiferromagnetism ferroelectric Band energy gap A B S T R A C T Polycrystalline Bi1-xRxFeO3 (R–Ho and Y, x = 0.00, 0.05 and 0.10) compounds were prepared to perform a systematic investigation of the role of chemical nature and effect of internal chemical pressure on the structural, microstructural, magnetic, electric, ferroelectric and optical properties of the compounds. Structural analysis revealed that lattice distortions observed in Ho and Y-substituted compounds were not the same. The lattice parameters were larger in the case of the Y-substitution compared to the Ho-substituted counterpart. Scanning electron micrographs confirmed the formation of dense, well-connected grains exhibiting a reduction in size with increasing substitution. The magnetic properties of BiFeO3 were enhanced through the suppression of the spin structure with the substitution. The substitution of magnetic (Ho3+) ions led to an improvement in remanent magnetization and coercive field, whereas the substitution of non-magnetic (Y3+) ions resulted in enhanced maximum magnetization with negligible coercive field at room temperature. The ferroelectric measurements evidenced that both remanent polarization and coercive fields improved in substituted compounds, attributed to a decrease in charge carriers. Furthermore, the Y3+ ion substitution positively influenced ferroelectric properties by reducing leakage currents compared to the Ho3+ ion substitution. The optical absorption measurements indicated a decrease in the energy band gap of BiFeO3 with substitution, implying alterations in the material’s optical characteristics. The ac conductivity studies demonstrated a discernible reduction in the conductivity of the substituted compounds. Specifically, Ho-substitution exhibited a more pronounced magnitude in the decline in conductivity relative to Y-substitution. This outcome signifies the potential efficacy of Ho-substitution in exerting control over the insulating characteristics of the compounds.
1. Introduction
Multiferroics are materials characterized by the simultaneous presence of various ferroic orders, such as ferroelectric, (anti)ferromagnetic, and ferroelastic properties. These materials hold significant promise for technological applications in memory devices, microelectronics, and spintronics due to the concurrent existence of different ferroic orders [1–3]. One particularly intriguing material in this category is bismuth ferrite (BiFeO3 or BFO), which has been extensively studied for several decades. BFO exhibits a rhombohedral structure with the R3c space group [4] and possesses a high Curie temperature (TC = 1103 K) and a high Néel temperature (TN = 643 K) [5]. The ferroelectricity in BFO arises from the distortion induced by the stereochemically active 6s2 lone pair electrons of Bi3+, while the G-type canted antiferromagnetic ordering results from the indirect magnetic exchange interaction between Fe3+ ions through O2− ions. Despite its potential, synthesizing single-phase BFO poses a significant challenge. Issues such as the presence of oxygen vacancies, mixed valence states of Fe ions, and impurity phases like Bi2Fe4O9 and Bi25FeO40 can enhance electrical conductivity, presenting a drawback for device applications [6]. Numerous methodologies have been implemented to mitigate impurity phases and diminish leakage current in multiferroic materials. These strategies encompass diverse processing techniques [7,8], elemental substitutions at Bi/Fe-sites [9,10] and composite formations * Corresponding author. E-mail address: dtadiset@gitam.edu (T. Durga Rao). Contents lists available at ScienceDirect Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc https://doi.org/10.1016/j.jssc.2024.125019 Received 4 March 2024; Received in revised form 1 September 2024; Accepted 17 September 2024 Journal of Solid State Chemistry 340 (2024) 125019 Available online 18 September 2024 0022-4596/© 2024 Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies. [11,12]. Notably, the substitution of elements such as La3+, Nd3+, Sm3+, Y3+, Ba2+, Ca2+, etc., at the A-site and transition metal elements such as Mn3+, Cr3+, Ni2+, etc., at the B-site, has demonstrated efficacy in tuning the physical properties of BFO [9,10,13,14]. Nevertheless, the substitution of divalent ions at the A-site may induce an elevation in oxygen vacancies for getting charge neutrality in the compounds. This leads to an augmented leakage current, thereby detrimentally influencing the electrical characteristics of the compound. In most of the studies, the investigations predominantly focus on elucidating the influence of specific substituents at varying percentages of substitutions on the physical properties of BFO, thereby examining the material’s response to diverse internal chemical pressures induced by such substitutions. The properties of materials are investigated by substituting ions with varying internal chemical pressure. The trivalent elements Ho3+ (1.015 Å) and Y3+ (1.019 Å) ions have almost the same ionic sizes and are smaller than the ionic size of Bi3+ ions (1.17 Å). While the Y3+ ion is non-magnetic, the Ho3+ ion has a high magnetic moment of 10.6 μB. Further, the bond enthalpies of Ho− O and Y–O bonds are 606 kJ/mol and 708 kJ/mol, respectively, and are higher than that of Bi–O bonds (343 kJ/mol) [15]. There have been numerous studies discussing the impact of individual substitution of Ho/Y on the physical characteristics of BiFeO3. For example, S. G. Nair et al. synthesized Bi1− xHoxFeO3 (x = 0.2, 0.4, 0.6 and 0.8) compounds and studied the structural, dielectric, ferroelectric and ac conductivity studies [16]. P. Suresh et al. investigated the structural and magnetic properties of Bi1-xHoxFeO3, x = 0 to 0.20. They reported that BFO was still persistent in rhombohedral R3c for x < 0.15, transforming to orthorhombic Pnma structure for higher values of x, accompanied by improved magnetic properties with Ho-substitution [17]. Min Zhong et al. observed the suppression of impurity phases and enhancement of magnetic and dielectric properties in Y-substituted BFO compounds [18]. N. S. Abdul Satar et al. observed changes in the particle size and energy band gap in the Y-substituted compounds [19]. The substitution of Ho3⁺ and Y3⁺ions at the Bi3+-site of BFO will explore the following questions: (i) As Ho3⁺ and Y3⁺ ions have almost similar ionic sizes, their substitution leads to structural distortions in the R3c structure. Now, will the substitution with Ho3⁺ and Y3⁺ ions induce similar structural distortions in the crystal lattice of BiFeO₃, given their comparable ionic sizes?, (ii) It is reported that substitution of rare earth elements such as Nd, Eu, Dy etc., leads to enhancement of magnetocrystalline anisotropy [20] and affects the spiral spin structure of BFO. Now, under similar internal chemical pressures in the lattice, how do these ion substitutions influence the magnetic properties of BiFeO₃, considering the magnetic nature of Ho3⁺ ion compared to the non-magnetic Y3⁺ ion?, and (iii) how does the higher bond enthalpies of Ho–O and Y–O bonds help in improving the insulating behaviour in BiFeO₃? And (iv) Substitution may change in the optical band gap due to the formation of impurity energy levels in the forbidden energy gap [21]. How do these ion substitutions impact the optical properties of BFO? These questions are interesting in understanding the role of ion substitutions in tuning the physical properties of BiFeO₃ under equivalent internal chemical pressures. To the best of our knowledge, there have not been any reports on the influence of substitution under equal internal chemical pressures in BFO. A systematic exploration is imperative to discern the influence of these substitutions under equivalent internal chemical pressures within the lattice. This study contributes valuable insights for the research community in selecting the element to tune a particular physical property of BFO. In this work, we have chosen Ho3+ and Y3+ ions to substitute at Bi-site and study their influence on the structural, microstructural, magnetic, FTIR, ferroelectric, and optical properties. Experimental procedure: Polycrystalline compounds Bi1-xRxFeO3 (R–Ho and Y, x = 0, 0.05 and 0.10) compounds were synthesized using the conventional solid-state reaction technique. The starting materials Bi2O3 (Sigma-Aldrich, 99.999 %), Fe2O3 (Sigma-Aldrich, 99.99 %), Ho2O3 (Alfa aesar, 99.9 %) and Y2O3 (Alfa aesar, 99.9 %) oxides were employed, and the powders were mixed with their stoichiometric ratios and underwent thorough grinding for 2 h. A two-step calcination process followed, with the powders first treated at 780 ◦C for 2 h and then at 815 ◦C for 3 h. The calcined powder was pelletized into circular discs by incorporating freshly prepared polyvinyl alcohol (PVA) as a binder. The final sintering step occurred at 830 ◦C for 5 h, with a heating rate of 5 ◦C per minute. Phase analysis of the compounds was conducted using an Xray diffractometer (Panalytical X’pert Pro) with Cu Kα radiation (λ = 1.5406 Å). Microstructure analysis was carried out using field effect scanning electron microscopy (FESEM) with a Carl Zeiss Supra 40 instrument equipped with energy dispersive x-ray spectroscopy (EDS). Magnetic properties were investigated using a physical property measurement system (PPMS) with a VSM assembly (Quantum Design, USA). Electrical properties were conducted on the silver electrode compounds utilizing a Wayne Kerr 6500 B impedance analyzer across a frequency range of 100 Hz to 1 MHz. Ferroelectric hysteresis loops, i.e., polarization (P) -electric field (E) loops, were measured using aixACCT TF 2000 analyzer. The energy band gap was measured using a UV-VIS-NIR spectrometer (SHIMADZU) in the absorbance mode. Infrared spectra were recorded using Bruker (ALPHA-II) ATR-FTIR spectrophotometer to identify the various bands formation in Mid-IR region.
2. Results and discussions
2.1. Structural studies
Fig. 1(a–e) shows the X-ray diffraction (XRD) patterns of Bi1xRxFeO3, R–Ho and Y, x = 0 (BFO), 0.05 (BHFO5 for R–Ho and BYFO5 for R–Y), and 0.10 (BHFO10 for R–Ho and BYFO10 for R–Y) compounds. A nominal amount of impurity phases, such as Bi2Fe4O9 and Bi25FeO40, are observed in the XRD patterns, which are designated as # and * respectively. The appearance of these phases, along with the main phase, is quite common [22]. Structural phase analysis has been carried out by Rietveld refinement using Fullprof software. It is evident from the refinement that the compounds crystallise in a rhombohedral crystal structure with R3c space group. However, the shifting of Bragg’s peaks (104) and (110) towards a higher angle side with the increase of Ho/Y substitution is evident due to the substitution of smaller ionic sizes of Ho3+/Y3+ ( rHo3+ = 1.015 Å and rY3+ = 1.019 Å) ions at Bi3+- sites (rBi3+ = 1.17 Å). The shift and changes in the intensity of Bragg’s peaks infer that the substituents have entered the A-site and caused subsequent changes in the materials. The changes in the intensity of (104) and (110) peaks, partial overlapping, and evolution of a small reflection (111) near 2θ = 25.4o will indicate the evolution of orthorhombic structure in BHFO10 and BYFO10 compounds [23]. The substitution of the smaller ionic sizes of elements at the A-site generally exerts internal chemical pressure and hence, the structural distortions in the lattice. The internal chemical pressure can be quantified by the Goldschmidt tolerance factor t, defined as t=(〈rA〉+ rO) / ̅̅̅ 2 √ (rB + rO) 1 where < rA > is the average radius at A site and rB and rO are the radii of Fe3+ and O2− respectively. The decrease in t with the substitution leads to changes in Fe–O distances due to the compressional forces [24]. Although Ho3+ ionic size is slightly smaller than Y3+ ionic size, the lattice parameters are observed to be different and greater in the case of Y-substituted BFO. Therefore, it reveals that the chemical nature of the element also influences the lattice parameters in addition to the ionic size of the substituents. The modification of lattice parameters, Fe–O bond distances and Fe–O–Fe bond angles leads to distortions in FeO6 octahedra which in turn help in tuning the physical properties of the materials. It is evident from Table 1 that the distortion parameters are not the same with the substitution of Ho3+ and Y3+ ions at the Bi3+-site, although their corresponding ionic sizes are almost the same, indicating that the internal chemical pressures created are not the same and hence the influence of substitution on the properties. The lattice parameters and other structural parameters are given in Table 1. The average crystallite size D is estimated by using the Debye-Sherrer formula [25] D=Kλ/β cos θ 2 where K is the shape factor (which is ~0.9), λ is the wavelength of Cu Kα X-ray radiation, β is FWHM, i.e., full width at half maximum of diffraction peaks, θ is the Bragg angle. The average crystallite size, calculated using equation (2), was observed to decrease from around 100 nm to around 25 nm with the increase of Ho/Y content. The lattice strain η is calculated from the formula η= β cos θ/4 3 The observed values of η increase with the increase of Ho/Y content. The increase in η would affect the magnetic, and ferroelectric properties of BFO.
2.2. Microstructural studies
Fig. 2 shows the scanning electron micrographs (SEM) of Bi1-xRxFeO3 (R–Ho and Y, x = 0, 0.05 and 0.10) compounds. BFO displays larger grains with an average grain size of approximately 9 μm and a few discernible pores. The substitution of Ho3+/Y3+ on grain size is evident in terms of reduction, minimized porosity, and improved compound density. The grain sizes are non-uniform in the substituted compounds, and their average grain sizes are in the range of 1–2 μm as shown in Fig. 2(f). The Y-substitution results in a higher level of grain non-uniformity. The cause for the reduction could be due to the suppression of grain growth due to the substitution. In addition to this, the reduction could also be due to differences in the diffusion rate of the constituent elements according to the Krikendall effect [26]. The improved grain connectivity and reduced porosity lead to the improved density of the material, which will be helpful in improving the dielectric and ferroelectric properties. Elemental mapping has been carried out to observe the elemental distribution in all the compounds. The energy dispersive X-ray analysis (EDX) studies indicated that the elements Bi/Ho/Y:Fe:O are in a ratio close to 1:1:3, confirming the stoichiometry of the compounds.
2.3. Optical studies
The optical characteristics of Bi1-xRxFeO3 (R–Ho and Y, x = 0, 0.05 and 0.10) compounds were studied by analysing UV–Vis absorption spectra as shown in Fig. 3. The UV–visible absorption data has been used to understand the nature and to estimate of the band gap of Bi1-xRxFeO3 (R–Ho and Y, x = 0, 0.05 and 0.10) compounds using to the equation given below (αhv)n =A ( hv − Eg ) 4 where α,A, hv, and Eg are the coefficient of absorption, proportional constant, photon energy, and energy gap, respectively. The exponent gives information about the nature of the band gap. The band gap can be direct allowed, indirect allowed, direct forbidden and indirect forbidden transition, respectively if n = 2.00, 0.50, 3.00 and 1.50. The band gap Eg and n are extracted by fitting equation (4) and the obtained values are given in Table 2. The fitting gives n = 2.00 which indicates that the compounds fall under the category of direct band gap semiconductor. The optical band gap of BFO is reported to be 2.08 eV, which is consistent with our Eg value [27]. The value of Eg decreases in the substituted compounds, with a greater reduction observed in the BHFO10 compound. There have been several reasons for the decrease in the band gap with the substitutions. The optical band gap in BFO depends on the hybridization of 3d orbital of Fe3+ ions with 2p orbital of O2− ions and the structural parameters such as Fe–O bond distances and Fe–O–Fe bond angles. The decrease in the band gap could be due to the changes in the structural parameters. The bond angle Fe–O–Fe increases and bond distance (dFe− O) decreases with the increase of Ho3+/Y3+ ion content (refer to Table 1). The increase in bond angle/decrease in bond distance leads to the decrease in the band gap energy according to the relation [28] Eg =Δ − W 5 Where W ∝ cos ω d3.5Fe− O and ω = [π − (Fe − O − Fe)] 2 . 6 The higher Fe–O–Fe bond angle and shorter average Fe–O bond distance in the BYFO5 compound, compared to the BHFO5 compound, contribute to its lower bandgap energy Eg, as indicated by equations (5) and (6). However, with a 10 mol% substitution at the Bi site with Ho, there are more significant changes in the structural parameters, and hence, the lowest Eg value is observed in the BHFO10 compound compared to BYFO10. These variations in structural parameters with increasing substitution levels can be attributed to the evolution of the orthorhombic phase. As the Fe–O bond distance decreases in the orthorhombic phase, an increased presence of this phase enhances the W parameter, thereby reducing the bandgap energy [29], in accordance with equations (5) and (6). The substitution may also introduce donor impurity levels near the valence band maximum or acceptor impurity levels near the conduction band minimum within the forbidden energy gap, leading to bandgap narrowing. Another possible factor contributing to the reduction in bandgap energy could be the presence of non-uniform micro-strain within the crystallites, which arises from a reduction in grain size. This micro-strain can modulate the electronic energy levels and influence the position of the band edges, leading to a narrower bandgap [30,31]. The lower Eg values indicate that these compounds will be promising for photocatalytic and optical device applications [32]. The decrease in band gap has been observed in other BFO-based systems [33,34].
2.4. FTIR studies
Fourier transformation infrared (FTIR) spectra of Bi1-xRxFeO3 (R–Ho and Y, x = 0, 0.05 and 0.10) compounds are shown in Fig. 4. It is reported that the absorptive band in the range 400 cm− 1 to 600 cm− 1 is associated with Fe–O stretching and bending vibration and their appearance indicates the formation of FeO6 octahedra within the perovskite structure. It is essential to acknowledge that the presented data initiates from 500 cm-1 due to instrumental limitations. Moreover, the absorption band from 500 to 650 cm− 1 is attributed to Bi–O stretching vibration in the BiO6 octahedral configuration [35]. The variations in intensity, broadness and peak position with the substitution of Ho/Y are ascribed to substitutional effects, which also corroborate the XRD finding. This implies that the introduction of Ho3+/Y3+ ions into the BFO matrix induces discernible changes in the vibrational characteristics of the Bi–O/Fe–O bonds, influencing the structural and electronic properties of the material.
2.5. Magnetic studies
Fig. 5(a–e) illustrates the temperature variation of magnetization curves of Bi1-xRxFeO3 (R–Ho and Y, x = 0, 0.05 and 0.10) under a magnetic field of 1 kOe. In the case of BFO, as the temperature decreases from 300 K, the magnetization decreases up to 100 K, exhibiting the antiferromagnetic nature of the compound. A sudden enhancement in magnetization has been discerned, particularly evident at temperatures below 25 K, indicating the emergence of an incommensurate magnetic structure [36]. Almost similar behaviour has also been observed in Ho-substituted compounds, except the enhanced magnetization is found in the latter in both zero-field cooled (ZFC) and field-cooled (FC) curves. Conversely, the ZFC-FC magnetization curves of Y-substituted compounds demonstrate an elevation in magnetization as temperature decreases, surpassing the magnitudes observed in Ho-substituted compounds. The increase in magnetization with the temperature depicted in the ZFC-FC curves aligns with previously reported findings in the literature [18]. Fig. 5(f) shows the field variation of magnetization hysteresis loops of all compounds at 300 K. Inset shows the magnetization hysteresis loops of all compounds at 10 K. It is known that the magnetic structure of BFO is a G-type antiferromagnetic structure below its Néel temperature of 643 K, which is superimposed with incommensurately modulated spin cycloid. BFO shows a linear relation between magnetization and the applied magnetic field with negligible remanent magnetization. The substitution of smaller ionic sizes of Ho3+/Y3+ ions at the Bi3+- site introduces distortions in terms of changes in both Fe–O bond distances and Fe–O–Fe bond angles [37], consequently modifying the spin structure and hence the magnetic properties of the compounds. In the case of Ho substitution, the magnetization hysteresis loops show weak ferromagnetism with improved remanent magnetization and coercive field with the increase of Ho content. The increase in the coercive field in these compounds could be due to the suppression of space-modulated spin structure [38]. In addition to this, an increase in magnetic properties can also be due to the increase in anisotropy and/or due to the decrease in the grain size of the compounds with the Ho-substitution [39–41]. Further, the magnetization curves are not saturated because of the intrinsic antiferromagnetic nature of the compounds. In the case of Y-substituted compounds, the magnetization suddenly increases at a low applied magnetic field, and then it increases linearly, which is also observed in the literature [18]. The decrease in the coercive field in Y-substituted compounds could be due to the non-uniform distribution of grain, as shown in Fig. 2. It is reported that the coercive field decreases when the grain distribution changes from uniform to non-uniform [42]. Although the structural parameters Fe–O and Fe–O–Fe are a little different with both Ho and Y-substituted compounds, the magnetitic properties are different. The room temperature magnetization at the highest applied field, 5 T, is higher in Y-substituted compounds. However, at 10 K, it is higher in Ho-substituted compounds which can be explained as follows: In G-type AFM BFO, there exists a superexchange interaction between Fe3+-Fe3+ ions via O2− ions. The substitution of magnetic elements leads to additional interactions in the lattice (Fe3+-O2--Fe3+, Ho3+-O2--Fe3+, and Ho3+-O2--Ho3+ interactions in the present work), i.e., along with Fe3+-Fe3+ (strongest) exchange interactions, there exist Ho3+-Fe3+ (intermediate) and Ho3+-Ho3+ (weakest) exchange interactions. The exchange interaction exists between the 4d sub-shell of Ho3+ ions and the 2p subshell of Fe3+ ions via O2− ions in the 90o position. The superexchange interaction between Ho3+–Ho3+ ions dominates in the low-temperature region [39] and hence it can be reasonable to think that the role of Ho3+-Fe3+ and Ho3+-Ho3+interactions are important in addition to Fe3+-Fe3+ interactions in tuning the magnetic properties, i.e., the substitution of magnetic Ho3+ ions is quite effective in tuning the improved magnetic properties than that of non-magnetic Y3+ ions at low temperatures although they exert the same chemical pressure. The enhanced magnetization observed in these compounds is not ascribed to the existence of impurity phases such as Bi2Fe4O9 and Bi25FeO40. The magnetic influence of these impurity phases on the overall magnetization is deemed negligible, as these phases manifest paramagnetic behaviour at room temperature [43].
2.6. Ferroelectric studies
Polarization hysteresis loops of Bi1-xRxFeO3 (R–Ho and Y, x = 0, 0.05 and 0.10) compounds are shown in Fig. 6(a–e). All the hysteresis loops present non-saturated polarization hysteresis loops. The ferroelectric polarization in BFO is mainly due to the hybridization of 6s2 lone pair electrons of Bi3+ ions with the 2p orbital of O2− ions. BFO hysteresis loop shows the opening of the loop, which is a signature of leaky character in ferroelectric materials. The leakage currents originate due to the presence of mixed valence state ions (Fe2+/Fe3+), defects and defect complexes, which are likely to exist in ABO3-kind perovskite ferroelectric materials. It is known that the partial substitution of suitable elements will improve the ferroelectric properties of BFO due to the offcentring of Fe3+ ions relative to FeO6 octahedra. The partial substitution of suitable elements is manifestly associated with a discernible decrease in leakage current density [44], as evidenced by the substituted compounds in terms of closed and quality of ferroelectric hysteresis loops. The reduction of leakage current with the substitution is also corroborated by the ac conductivity studies given below. When Ho/Y is substituted at Bi-site, the leakage current density will reduce due to the higher bond enthalpy of R–O bonds. The substitution of Ho/Y at Bi-site restrains the formation of oxygen vacancies due to the higher bond enthalpies, and hence, the better polarization hysteresis loops are observed as shown in Fig. 6. The remnant polarization Pr and coercive field Ec for all the compounds are given in Table 2. In addition to this, a reduction in grain size is correlated with an augmentation of grain boundaries, which helps to reduce leakage currents and helps to obtain better ferroelectric loops. The substitution of Y3+-ions is observed to yield superior ferroelectric properties compared to Ho3+-ion substitution, attributable to the higher bond enthalpy of Y–O bonds relative to Ho–O bonds.
2.7. ac conductivity studies
Fig. 7 presents the frequency-dependent ac conductivity of BFO, BHFO10 and BYFO10 compounds at various temperatures, offering insights into the material’s response to the applied electric field (the ac conductivity of BHFO5 and BYFO5 compounds are not given here). The ac electrical conductivity σac is determined using the relation: σac = ϵrϵoω tan δ 7 Where ϵo is permittivity of free space, ϵr is the dielectric constant, ω is the angular frequency and tan δ is the dielectric loss tangent. The frequency dependence σac follows Joncher’s power law [45] and is articulated by the equation σac = σ(0) + Aωs 8 where σ(0) is the dc conductivity, A is the temperature-dependent factor, and s is an exponent. The ac conductivity data is fitted with the above equation. It is observed from the fitting that s decreases with the increase in temperature, and hence, the conduction model follows the correlated barrier hopping (CBH) model [46]. The ac conductivity of BFO depends on the bond enthalpy of the substituent elements with oxygen [47]. The bond enthalpy of Bi–O, Ho− O and Y–O bonds are 343 kJ/mol, 606 kJ/mol, and 708 kJ/mol respectively. The higher bond enthalpies associated with Ho–O and Y–O bonds play a crucial role in impeding the formation of oxygen vacancies, consequently contributing to a reduction in the conductivity of the compounds. It has been reported that the substitution of elements at the A-site and/or B-site of BFO reduces oxygen vacancies and, hence, the reduced conductivities in the BHFO10 and BYFO10 compounds. However, although the bond enthalpy is higher in the BYFO10 compound than in the BHFO10 compound, the reduction in conductivity is slightly higher in the BHFO10 compound compared to that in the BYFO10 compound. The substitution of elements of similar ion sizes results in equivalent internal chemical pressures within the lattice, thereby leading to comparable conductivity. Based on this, the ac conductivity is not solely contingent on internal chemical pressure and bond enthalpy, it is also influenced by the mass of the substituent. The greater mass of holmium (Ho) in comparison to yttrium (Y) ensures heightened electron-phonon coupling, consequently yielding lower conductivity and higher resistivity in the BHFO10 compound.
3. Conclusions
Polycrystalline Bi1-xRxFeO3 (R–Ho and Y, x = 0, 0.05 and 0.10) compounds were prepared by solid state reaction method. A systematic investigation was conducted on these compounds to investigate the role of chemical nature and the effect of internal chemical pressure on the physical properties of the compounds. The Y-substitution led to pronounced lattice distortions compared to the Ho-substituted counterpart although both substitutions stabilized the compounds in rhombohedral structures. The grain size of BiFeO3 was decreased with the substitution of Ho/Y at the Bi-site due to a decrease in grain growth with the substitution. The enhanced magnetic properties were observed in BiFeO3 due to the suppression of the spin structure with substitution. The substitution of magnetic and non-magnetic ions modified the remanent magnetization and coercive fields due to changes in anisotropy and grain sizes. The better ferroelectric properties by reducing leakage currents were found with Y3+ ion substitution compared to Ho3+ substitution, which could be due to the stronger bond enthalpy of Y–O bonds compared to Ho–O bonds. Notably, the energy band gap of BiFeO3 decreased with the substitution, indicating alterations in the optical characteristics of the material. CRediT authorship contribution statement Ch. Komala Lakshmi: Writing – original draft, Data curation, Conceptualization. T. Durga Rao: Writing – review & editing, Supervision, Data curation, Conceptualization. G. Bhavani: Formal analysis, Data curation. M. Sudhadhar: Formal analysis, Data curation. B. Sattibabu: Formal analysis, Data curation. V. Satya Narayana Murthy: Formal analysis, Data curation. T. Karthik: Formal analysis, Data curation. Saket Asthana: Formal analysis, Data curation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request.
Acknowledgements
TDR is grateful to UGC-DAE CRS, Mumbai Centre, India, for a research grant under a collaborative research scheme [CRS-M-313] and RSG [No. F. 2021/0070], GITAM (Deemed to be University), Visakhapatnam, to carry out this work.
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