Phase evolution, magnetic study and evidence of spin-two phonon coupling in Ca modified Bi0.80La0.20FeO3 ceramics

Journal Pre-proof Phase evolution, magnetic study and evidence of spin-two phonon coupling in Ca modified Bi0.80La0.20FeO3 ceramics Subhash Sharma, Manish Kumar, J.M. Siqueiros, Oscar Raymond Herrera PII:

S0925-8388(20)30586-7

DOI:

https://doi.org/10.1016/j.jallcom.2020.154223

Reference:

JALCOM 154223

To appear in:

Journal of Alloys and Compounds

Received Date: 26 November 2019 Revised Date:

4 February 2020

Accepted Date: 5 February 2020

Please cite this article as: S. Sharma, M. Kumar, J.M. Siqueiros, O.R. Herrera, Phase evolution, magnetic study and evidence of spin-two phonon coupling in Ca modified Bi0.80La0.20FeO3 ceramics, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154223. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Subhash Sharma: Designed, performed and writing the manuscript. Manish Kumar: Helping in analysis in UV section. J. M. Siqueiros: Helping in the review and analysis of results. Oscar Raymond Herrera: Helping in the review and analysis of results.

The present studies are focused on the effect of divalent Ca2+ ion on the structural, magnetic, and optical properties of Bi0.80La0.20FeO3 (BLFO) ceramic samples, synthesized via conventional solid-state reaction route. The addition of Ca2+ ion into BLFO substituting to Bi, creates distortion in the structure and leads a phase transition from rhombohedral (R3c) to orthorhombic (Pbnm) crystal symmetry, confirmed by Rietveld analysis and further supported by Raman spectroscopy, attributed to a decrease in tilt angle for anti-phase rotation of BO6 octahedra. Weak ferromagnetic ordering was observed for all Bi0.80-xCaxLa0.20FeO3 samples with Ca at.% of 0.03 (BCLFO3), 0.06 (BCLFO6) and 0.12 (BCLFO12), associated to changes in Fe-O-Fe angle. The remnant magnetization has been found to increase from 1.2 x10-3 emu/g for BLFO to 9.1 x10-3 emu/g for BCLFO12. Similar trends were found in the deconvoluted ferromagnetic contribution with Ca2+ ion doping. Moreover, Arrott plot analysis suggests a second order meta-magnetic transition for all samples. Room temperature dielectric properties were improved with Ca2+ ion substitutions. The optical band gap significantly reduced from 2.10 eV for BLFO to 1.93 eV for BCLFO12, indicating the distortion induced by the Ca2+ ion.

Phase evolution, magnetic study and evidence of spin-two phonon coupling in Ca modified Bi0.80La0.20FeO3 ceramics Subhash Sharma1,2, Manish Kumar3, J. M. Siqueiros2, Oscar Raymond Herrera2 1

CONACyT- Centro de Nanociencias y Nanotecnología. Universidad Nacional Autónoma de México, Km 107 Carretera Tijuana-Ensenada s/n, Ensenada, B.C., C.P. 22800, México 2 Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Km 107 Carretera Tijuana-Ensenada, AP 14, Ensenada 22860, B.C., México 3 Experimental Research Laboratory, Department of Physics, ARSD College, University of Delhi, New Delhi 110021, India

Corresponding Author: [email protected]

Abstract The present studies are focused on the effect of divalent Ca2+ ion on the structural, magnetic, and optical properties of Bi0.80La0.20FeO3 (BLFO) ceramic samples, synthesized via conventional solid-state reaction route. The addition of Ca2+ ion into BLFO substituting to Bi, creates distortion in the structure and leads a phase transition from rhombohedral (R3c) to orthorhombic (Pbnm) crystal symmetry, confirmed by Rietveld analysis and further supported by Raman spectroscopy, attributed to a decrease in tilt angle for anti-phase rotation of BO6 octahedra. Weak ferromagnetic ordering was observed for all Bi0.80-xCaxLa0.20FeO3 samples with Ca at.% of 0.03 (BCLFO3), 0.06 (BCLFO6) and 0.12 (BCLFO12), associated to changes in Fe-O-Fe angle. The remnant magnetization has been found to increase from 1.2 x10-3 emu/g for BLFO to 9.1 x10-3 emu/g for BCLFO12. Similar trends were found in the deconvoluted ferromagnetic contribution with Ca2+ ion doping. Moreover, Arrott plot analysis suggests a second order meta-magnetic transition for all samples. Room temperature dielectric properties were improved with Ca2+ ion substitutions. The optical band gap significantly reduced from 2.10 eV for BLFO to 1.93 eV for BCLFO12, indicating the distortion induced by the Ca2+ ion.

1

1. Introduction Multiferroic materials drew worldwide attention due to their immense application potential in data storage, Spintronics, and microelectronics devices [1-4]. Notably, these materials show the simultaneous existence of more than two ferroic orders such as ferroelectric, ferromagnetic and ferroelastic in the same phase [5]. However, there is a scarcity of single phase multiferroic materials due to the intriguing physics behind the ferroelectric and ferromagnetic ordering. Till now, the most studied multiferroic material is bismuth ferrite (BiFeO3) due to its room temperature multiferroic properties (ferroelectric Tc ~ 1123 K and ferromagnetic TN ~ 643 K) [6-7]. There is still controversy in the literature for the adoption of different synthesis methods for high quality single phase BiFeO3 (BFO) [8-9]. As per the structure point of view, BFO has rhombohedrally distorted perovskite structure with space group R3c, lattice parameters in rhombohedral fitting are ar = 5.63 Å, αr = 59.35o whereas in hexagonal fitting a = 5.58 Å, c = 13.87 Å [1]. Irrespective of room temperature multiferroic properties, BFO shows several downsides such as low magneto-electric coupling, high leakage current, difficult single phase synthesis, poor ferroelectric and weak ferromagnetic orderings [10-13]. Therefore, in order to optimize BFO and prepare it for suitable applications, it must be workout to improve its magneto-electric properties. In recent times, research studies on doped BFO materials demonstrate an improvement in the physical properties [14-18]. The doped materials having a composition near to phase boundary. The enhanced properties of these materials have been linked with structural instability near the phase boundary. Additionally, these instabilities are found to be highly sensitive to external stimuli such as electric/magnetic field etc. [14, 19]. In order to understand the physics behind structural changes and enhancement in physical properties of the materials, it is mandatory to do careful crystal structure evolution and find the related parameters associated to the improved physical properties. In earlier studies, several research groups have made attempts to dope with 3+ valence lanthanide ions as well as transition metal ions such as Mn, Ti and Cr in BFO [20-25]. They observed that below 10 at.% La doping BFO maintains the R3c symmetry, however, at 20 at.% and more La doping, the structure changes to the orthorhombic and/or tetragonal structures [23]. Moreover, it is observed that La doping notably enhances the ferromagnetic ordering with a decrease in leakage current caused by the structural distortion. It is a well known fact that the perovskite 2

phase may stabilize and enhances its physical properties by adding dopants at A/B sites. The doping helps to prevent the evaporation of Bi and a decrease in oxygen vacancies. Even though intense research work has been done, but there are very few reports on the complete structural evolution using Rietveld analysis and magnetic properties for the Bi0.80xCaxLa0.20FeO3

ceramic systems. The present investigation is focused on the detailed analysis

on structural, magnetic and optical properties after the doping of La/Ca doped BFO at the Bi sites.

2. Experimental procedure The conventional solid state ceramic route is used to synthesize the Bi0.80-xCaxLa0.20FeO3 ceramics with x = 0.0, 0.03, 0.06 and 0.12, labelled BLFO, BCLFO3, BCLFO6 and BCLFO12, respectively. The appropriate amounts in a stoichiometric ratio were taken and mixed with acetone (4 h) in an agate mortar. The calcination process was done in a furnace at 830 ºC for 2 h. The powder was pressed into cylindrical pellets using polyvinyl alcohol (PVA) as binder followed by sintering at 850 ºC for 2 h. Powder X-ray diffraction (XRD) measurements to analyse the crystal structure and structural phase evolution were done using a Shimadzu XRD-6000 difractometer with CuKα radiation. Rietveld refinement analysis was carried out using the FullProf Program. Renishaw Via Reflex Raman spectrometer was used for Raman analysis. Magnetization as function of the applied magnetic field curves (M−H) were obtained using a superconducting quantum interference device magnetometer (SQUID, Quantum Design). Infrared transmission spectra of pure BFO and Ca doped BLFO samples embedded in KBr powder media have been recorded in the 380 - 800 nm wavelength range.

3. Results and discussion 3.1. Structural Properties To determine structural changes, XRD patterns for all Bi0.80-xCaxLa0.20FeO3 samples with x = 0.0, 0.03, 0.06 and 0.12, were recorded and are shown in Fig. 1. It is clearly seen that the small amount of impurity (identified as Bi2Fe4O9) associated to peak near 2 = 27.54º in the BLFO sample pattern, disappears for Ca doped BLFO compositions, i.e., BCLFO3, BCLFO6 and BCLFO12. The disappearance of the impurity peak for the doped samples suggests that doping is a good way to get a stable structure for the BFO based material [14-16]. The splitting of the peak near 32º [see Fig. 1 (b)] and the presence of a super-lattice reflection 3

(35.55º) confirms the typical rhombohedral structure (R3c) of BLFO and Ca doped BLFO compositions. However, for BCLFO12, it is observed that the peaks near 32º merge and become a broad peak and the super-lattice reflection vanishes, indicating a structural phase transition as obtained by many researchers [25, 14]. The Rietveld analysis has been performed using Full-Prof software for all samples and the results are shown in Fig. 2. In this refinement, pseudo-Voigt function for peak shape and linear interpolation between points for background has been considered. It is clearly seen from Fig. 2 that the XRD pattern of BLFO was well fitted with the R3c model with realistic refinement factors (see Table 1). Furthermore, for higher Ca content up to 6 at.%, the R3c model still fitted well with reasonable refinement factors. As discussed above, for x = 0.12, near 32º there is a single broad peak and the disappearance of the super-lattice plane motivated us to change structural model. Many structural models obtained from a literature survey were tested to check the exact phase [25]. After applying different structural models, the results showed the orthorhombic phase (Pbnm) fits closely to the experimental data. Also, from Rietveld refinement lattice constants, the fitting parameters (RF, RBragg and χ2) and bond angle (Fe-O-Fe) have been determined and enlisted in Table 2. Table 2 displays all the obtained structural parameters such as Fe-O-Fe bond angles, cubic like distortions and particle size. With an increase in doping concentration, parameters like lattice volume, lattice parameters and bond angle (Fe-O-Fe) were found to differ. For an ideal octahedron, the FeO-Fe angle must be 180º however, for BLFO, BCLFO3, BCLFO6 and BCLFO12 samples, the Fe-O-Fe angle turns out to be 158.03º, 158.53º, 157.40º and 154.30º, respectively, indicating a distorted R3c stable structure. Furthermore, for sample x = 0.15 there is an anomalous increase (see Table-1) in factors like rhombohedral distortion parameters such as ac =

√

, Cc=

√

–



, cubic

in comparison to those of pure BLFO. Such structural

distortions could be associated to a distortion of the FeO6 octahedra which occurs due to doping and, in turn, affecting the magnetic properties of these samples [29]. In addition, Table 2 displays the average crystallite size for all samples determined from XRD peak broadening using the Debye Scherrer’s formula. Lattice parameters have been found to decrease with the incorporation of Ca ion into the structure a fact that can be inferred from the high angle shift in 2θ values as shown in Fig. 1(b). This behavior can be understood as follows: when a smaller Ca2+ (1.12 Å) ion goes to occupy the A site it can’t fill the empty space but it provokes tilting and shrinking of the 4

octahedra. The lattice distortion induced by the tilting of the octahedral leads to suppression of the rhombohedral phase. This results leads to a lower symmetry orthorhombic phase. Moreover, the distortion is provoked by mismatching of the host and dopant ions with the ensuing tilting of the BO6 octahedra. As is well known the measure for how well an ion that fits into a unit cell is defined by the Goldschmidt Tolerance Factor (t) as given below:

=

(

√ (

)

)



(1)

Where rA, rB and ro are the average ionic radius of A, B and oxygen, respectively. Basically it gives an idea about the structure stability of a unit cell. The ‘t’ value of the ideal perovskite structure without distortion is 1. Smaller values (t <1) implies a compressive strain on Bi-O and Fe-O bonds leading to the evolution of crystal symmetries. Table 2 tabulates the tolerance factor for all the samples. The tolerance factor value less than one is noted for BLFO samples decreasing further with Ca concentration causing buckling of oxygen octahedra. Furthermore, in the case of complex perovskites, Reaney et al. suggested three more tilt regimes that depend on the tolerance factor as follows: i. without tilt, Goldschmidt factor lies in the 0.985 < t < 1.06 range; ii. for anti-phase tilting 0.964 < t < 0.985, iii. for in plane and anti-phase titling t < 0.964. In the present scenario, only anti-phase tilting is present up to BCLFO6 as shown in Table 2. However, for BCLFO12, there could be both in-plane and anti-phase tilting present. The super-lattice reflection seen in anti-phase tilted R3c structure is due to the doubling of all the three pseudo cubic cell parameters. Table 1 presents the coordinates in hexagonal setting for rhombohedral R3c structure. These coordinates are as follows: A3+(0,0, 0.25 + s), B3+(0,0,r) and O2- (1/6 - 2p - 2d, 1/3 - 4d,1/12). The ‘s’ and ‘r’ parameters account for the displacement of cations A and B, respectively, with respect to their mean position. The relation between the tilting angle ω of the BO6 octahedra with the ‘p’ factor that denotes the displacement of oxygen from their ideal position is expressed as:

ω = tan-1(4×√3×p)

(2)

Moreover, the symbol ‘d’ represents the distortion of the BO6 octahedra parallel to the [111]rh axis. Table 3 shows these parameters as determined from the Rietveld refinement. It is noted from the Table 3 for the BLFO sample, the tilt angle for anti-phase is 10.05º showing a decrease (~0.53º) with increasing Ca (6 at.%). Kumar et. al., studies shows similar results regarding tilt angle and “p” factor relation and our results are consistent with their studies [ 25]. 5

3.2. Raman Spectroscopy As it is observed in the XRD section there is a phase transition occurring in BLFO and Ca doped BLFO samples. It is well known that Raman spectroscopy is very sensitive to atomic displacements. Therefore, to confirm the phase transition with Ca substitution, Raman spectroscopy was performed in all samples. The spectra was fitted with multi-Lorentzian functions for quantitative analysis in the 100-700 cm-1 range. Two phonon Raman modes were shown in Fig. 3 after being deconvoluted. The assignment of Raman modes in these samples was done on the basis of group theory which states that there are 13 active (LRaman = 4A1+9E) Raman active modes present in pure BFO in rhombohedral (R3c) symmetry while the five A2 modes are silent [26]. The irreducible representation for the Raman modes is shown in following equation: Γ

=4

( ,

,

,

)+5

(−) + 9#( , ,

−

,

,

,

)

(3)

The A1-1, A2-2 modes are directly related to displacement of Bi, which is brought about by the stereochemical activity of lone pair of Bi (6s2). As observed in the present case i.e. Bi0.80La0.20FeO3 there are only eight Raman active modes possibly due to distortion created by La3+ in crystal structure. After de-convoluting the Raman spectra, the exact peak position and name of Raman modes is shown in Fig 3(b) for typical sample x = 0.0. As observed the E(TO1), E(TO2) and A(TO1) modes correspond to the displacement of Bi due to the activation of the Bi3+ ion lone pair electrons [27]. With increasing Ca content we observed a significant decrease in intensity of A modes and the absence of some A modes for higher Ca concentration attributed to structural changes in these samples [28, 16]. The two phonon Raman spectra for all samples was also fitted with Lorentzians in the 900 cm-1-1200 cm-1range, and are shown in Fig. 3(c). Three peaks (2A-4,2E-8 and 2E-9) are observed in this spectra, corresponding to second overtone of A-4, E-8 and E-9 modes seen in the 450 cm-1 to 700 cm-1 interval. These peaks in the two phonon spectra are a direct consequence of strong spin lattice coupling arising from the interaction between the adjacent magnetic sublattices. The phonon 2E8 and 2E9 modes correspond to the Fe-O1 and Fe-O2 bonding, respectively, where O1 and O2 are axial and equatorial O-ions, respectively [27]. The structural distortion due to the substitution Bi by La and Ca ions must be reflected in the evolution of Raman modes if spin- phonon coupling is present. Firstly, for the x = 0.3 6

sample, the intensity of the 2E9 mode is reduced in comparison to that of the BLFO sample and further decreases for higher percentage of Ca, demonstrating the change in rotation of the oxygen octahedron in BFO from critical to weak ferromagnetism via superexchange interaction (see in next section). The spin-two phonon coupling in these samples confirms the variation of intensity of the 2E9 overtones, consistent with the magnetic behavior.

3.3. Magnetic Properties The magnetization as function of the applied magnetic field curves (M-H) were carried for all samples at room temperature as shown in Fig. 4a. A clear deviation from linearity is observed for the BLFO sample as shown in Fig. 4a, suggesting a weak ferromagnetic (w-FM) behaviour for La doped BFO. A decrease in ferromagnetic behaviour has been found with increasing Ca content of the BLFO sample. However, further increasing of Ca content leads to an increase in the remanent magnetization. The graphs of the magnetic properties of these samples show unsaturated loops and low values of the remnant magnetization, indicating contributions from antiferromagnetic (AFM) and/or paramagnetic (PM) behaviour in the M-H data. Therefore, in order to extract the FM behaviour contribution from the M-H loops, the following equation has been employed and contribution from AFM and/or PM parts were calculated and listed in Table 4 [29].

$ (%) = &2

+ ()*

,

2±2 4 ,(9 tan 1 )* 5 + 5:; 2 4 ()*

-./ 01

+ <%

(4)

The first term of equation (4), indicates the FM part and second one depicts the linear > contribution from AFM and/or PM part. Further, $=( is FM saturation magnetization, $=(

is remnant magnetization, Hci is intrinsic coercivity, and < is the magnetic susceptibility. The

fitted M-H data with the FM and PM contributions has been plotted and shown in Fig.4(b-c) and the characteristic parameters are tabulated in Table 4. It is observed that the χ values obtained from the linear part increase up to 6 at.% of Ca but decreases for 12 at.%, attributed to structural change. The remanent magnetization (Mr) values, obtained from the FM contributions, are consistent with the experimental data, i.e., decreases for 3 at.% and, afterwards, increases up to 12 at.% of Ca. The same trend has been found for theoretical saturation magnetization values (MS). This analysis indicates the FM contribution is maximum for x = 0.12. The origin of w-FM induced by Ca doping can be understood in terms of partial suppression of the spiral spin structure due to structural distortion (indicated by structural analysis) and exchange interaction between Ca2+ - Fe3+ and Ca2+-Ca2+ ions. 7

Moreover, dopant cation changes the interatomic distance in a way that leads to ferromagnetic exchange energy becoming positive and resulting in the alignment of neighbouring atoms [30]. The oxygen vacancies or Fe4+ distribution cannot be related to ferromagnetic coupling since Fe3+ –O –Fe3+ and Fe3+ –O –Fe4+ leads to AFM ordering [3132]. However, canting of AFM sublattices and Ca2+ –O –Fe3+ and Ca2+ –O –Ca2+ may be responsible for w-FM ordering. Additionally, the anti-symmetric Dzyaloshinskii–Moriya (DM) exchange interaction between neighbouring spins induced by the spin–orbit interaction also supports the w-FM. Moreover, it is accepted that the direction of the DM vector is controlled by rotation of oxygen octahedra (tilting ω) [31-33]. So, except in the present case the change in ω will increase the DM interaction; modify the canting of the magnetic sublattices, supress the spin cycloid arrangements of Fe3+ ions and in turn enhance the net magnetization. Furthermore, as observed the change in bond angle and bond lengths with Ca concentration also contributed in the FM ordering in oxide systems. The addition of Ca in the Bi site leads to dilution of 6s2 lone-pair, which changes the BiO6 octahedral environment and gets distorted. The distortion in the BiO6 octahedra changes the canting angle, which leads to the enhancement of the magnetic property. Therefore, the apparent rise in Mr accompanied by reducing particle size is due to the Fe-O-Fe bond angle and crystal structural distortions. Magneto-crystalline and magneto-elastic anisotropy are the major factors for the increase in coercivity (Hc) with Ca doping. Morover, it is clearly seen from Fig.4(a-b) that the observed hysteresis loops look have wasp-waisted shape, a fact that can be attributed to the broken exchange bonds as well asto the highly anisotropic layer on the surface. Also, it can be relates to the loss of long-range order in the surface of magnetic particle in the samples. In order to confirm the w-FM ordering in these samples and the magnetic transition order, Arrot-Below-Kouvel (ABK) plot (M2 vs. H/M curve) has been done as shown in Fig. 5. The curvature and slope in high field region of the M2 vs. H/M isotherm indicates the magnetic ordering of the sample. It is observed a positive slope in the high field region for all samples which suggests the presence of the weak ferromagnetic ordering and a second-order meta-magnetic transition in all samples [31]. Figure 6 demonstrates the dM/dH vs H plots for the precise coercivity values validation. The maxima in the dM/dH vs H plot represents the critical magnetic field (HC) and is symptomatic of dynamical magnetic alterations about Ca doped BLFO samples. The absolute value of the magnetic field parallel to maxima of dM/dH, i.e., HC for both positive

8

and negative (slightly slender half width) cycle diverges with variable Ca content with a shift near higher magnetic field (absolute value) with respect to increased Ca content.

3.4. Dielectric Properties In order to get insight into the dielectric behavior, we performed dielectric measurements as functions of frequency (between 20 Hz and 1 MHz) at room temperature for all samples and the results are shown in Fig. 7. It is observed in all samples, that the dielectric constant (εr) and dielectric loss show frequency dependent behaviors in the low frequency region, which is followed by a frequency independent behavior at higher frequency ranges. Moreover, it can be noted that the dielectric constant exhibits a decrease of its values with frequency increase and stays constant at higher frequencies, while the dielectric loss (representing the energy dissipation in the dielectric system) shows a characteristic change at intermediate frequencies above 100 Hz, suggesting the dipole moment contribution to polarization at lower frequencies. This response can be understood in terms of a dipole relaxation process, i.e. dipoles are able to follow the frequency of the applied field at the low frequency range but unable to follow the frequency of the applied field at the high frequency range [34]. It is noticed an increase in dielectric constant and a decrease in dielectric loss with the increase of Ca concentration which can be ascribed to the reduction in oxygen and bismuth vacancies [35]. Moreover, the reduction in unit cell volume with Ca concentration leads to an increase in packing fraction as described by the Bottcher’s formula resulting in changes in dielectric polarization and finally in dielectric constant [34-35].

3.5. Optical Properties To investigate the optical properties of Ca doped BLFO samples, UV–visible diffused absorption spectra were used to collect the data at room temperature, shown in Fig. 8. The samples are showing the absorption band edges in the visible spectrum range indicating the potential application in the area of photocatalysis. From Fig. 8, it is observed that a band is cantered on 1.93 eV and it correlates to the 6A1g– 4T2g (4G) excitation which is induced because of the d-d crystal filed excitations of Fe3+ ions. The absorption near, above 2.0 eV, is found to be significantly increased up to 2.45 eV and thereafter starts decreasing in the given range. Additionally, above 2.0 eV, it consists of three dipole-allowed p-d charge transfer (CT) transitions. The assignment of these three transitions is reported in our previous report [35]. Additionally, a shift in d-d and C-T transitions has been found suggesting distortion in 9

BO6 octahedra induced by Ca doping. The corresponding values of direct optical band gap energy have been determined by typical Tau’c relation as given by the following equation:

α = A(hυ − E g ) m / hυ

(5)

Where, A is constant, hv is the photon energy, Eg is optical bandgap energy, and α is the absorption coefficient. The exponent m = 1/2 for direct allowed transitions while m = 2 for indirect allowed transition [36-39]. Therefore, bandgap energy can be found out using the (αhν)2 vs. hν plots extrapolating the linear portion of (αhν)2 to the energy (hν) axis at α = 0, as shown in Fig. 9. The values of the direct band gap were found to be 2.10, 2.04, 1.95 and 1.93 eV for x = 0.0, 0.0, 0.06 and 0.12, respectively, demonstrating the red shift in bandgap energy induced by Ca doping. One can explain the red shift to changes seen in Fe–O bond length and Fe–O–Fe bond angle in the Rietveld analysis. There is a direct relation between one–electron bandwidth (W) and the band gap of BFO which further can be modified due to a critical role of cation doping [37]. The one electron bandwidth (W) depends on both bond length and bond angle related to each other by the formula,

W ∝ cos ω /d3.5Fe-O

(6)

where ω represents ½ [π–(Fe–O–Fe)] and dFe–O represents the Fe–O bond length [30]. Moreover, the band gap is related to W by:

Eg = ∆–W

(7)

where ∆ represents the charge–transfer energy [38]. In the present case the Fe-O-Fe angle increases with Ca% resulting in a decrease of electron bandwidth. Due to all these structural changes there may be a net decrease in one electron bandwidth (W). So, substituting Ca ion by a larger A site cation leads to a decrease in band gap. Figure 10 demonstrates the room temperature FTIR spectra for the Ca doped BLFO samples in the range 400 – 1000 cm-1. The two absorption bands (seen in Fig. 11) at ~ 450 cm-1 (E(TO8) and ~ 560 cm-1 (E(TO9) in the spectra were attributed to overlapping of bending and stretching modes of Bi-O and Fe-O bonds [39-40]. Moreover, these bands are the characteristics of the octahedral FeO6 and BiO6 groups in the perovskite compounds. Additionally, a small hump clearly seen in Fig. 9 around 635 cm-1 is attributed to the bending vibration mode of Bi2O3 which is consistent with previously reported data [39]. The band at 450 cm-1 shifts to the higher wave number side due to lower atomic weight of Ca/La as 10

compared to Bi, however, the band at 550 cm-1 is found to shift to the lower wavenumber side. The band at around 668 cm-1 is attributed to absorption by water molecules.

4. Conclusion In summary, lead free Ca-modified BLFO samples were successfully prepared by solid state ceramic route. Rietveld analysis and Raman spectroscopy demonstrate that the structural phase transition from rhombohedral (R3c) to orthorhombic (Pbnm) crystal symmetry, indicating distortion in BO6 inducing by Ca into BLFO, further supported by the fact that a decrease in tilt angle for anti-phase rotation of BO6 octahedra with Ca doping. The presence of spin-two phonon coupling confirmed by Raman analysis in these samples. Weak ferromagnetic ordering was observed for BLFO, BCLFO3, BCLFO6 and BCLFO12 samples, attributed to structural distortion (change in Fe-O-Fe angle) induced by the divalent Ca ion. The remnant magnetization increases with Ca concentration from 1.2x10-3 emu/g for BLFO to 9.1x10-3 emu/g for BCLFO12. A similar trend was found regarding the ferromagnetic (FM) contribution with Ca doping. Second order meta-magnetic transition observed for BLFO, BCLFO3, BCLFO6 and BCLFO12 samples, confirmed by Arrott plots analysis. Room temperature dielectric properties found to be better with addition of Ca ion. The optical band gap lies in visible region (2.10 eV- 1.93 eV), suggesting the distortion induced by the Ca ion.

Acknowledgements Subhash Sharma, acknowledges support from Conacyt Catedra Programs through to Project 352-2018; Department of Physics (BHU) for VSM facility. This work was partially supported by PAPIIT-DGAPA-UNAM Grants IN107918 and IN104320. The authors thank P. Casillas for his technical assistance.

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Figure captions Fig. 1. (a) X-ray diffraction patterns of the Bi0.80-xCaxLa0.20FeO3 system with 0.0 ≤ x ≤ 0.12. The peak labelled with the symbol # correspond with the impurity phase of Bi2Fe4O9. (b) Expanded patterns in the 29 o - 35o range. Fig. 2. Rietveld refinement profiles of X-ray diffraction data of the Bi0.80-xCaxLa0.20FeO3. The red dots represent the observed data, whereas the solid black line through dots is the calculated profile, and vertical tics below curves represent allowed Bragg reflections. The blue curve is the difference pattern between the experimental and fitted data. Fig. 3. (a) Raman spectra and (b) deconvulated Raman spectra (c) Two phonon spectra and their deconvoluted curves for Bi0.80-xCaxLa0.20FeO3 system with 0.0 ≤ x ≤ 0.12. Fig. 4. Magnetic characterization of Bi0.80-xCaxLa0.20FeO3 system with 0.0 ≤x ≤0.12. (a) Magnetization as function of the applied magnetic field (M-H) hysteresis loops obtained at room temperature. (b) Deconvoluted M-H loops of the FM contribution. Insets the values at low H. (c) Fitted M-H experimental data for x = 0.12; the inset at left shows the experimental data and the FM contribution , and at right the AFM and/or PM contribution. Fig. 5. Arrot (M2 vs H/M) plots for Bi0.80-xCaxLa0.20FeO3 system with 0.0 ≤x ≤0.12. Fig. 6. dM/dH vs H plots for Bi0.80-xCaxLa0.20FeO3 system with 0.0 ≤x ≤0.12. Fig. 7. Room temperature dielectric response (εr and tanδ vs. log (f) plots) for Bi0.80xCaxLa0.20FeO3 system with 0.0 ≤ x ≤ 0.12. Fig. 8. (αhv)2 vs. hv plots for Bi0.80-xCaxLa0.20FeO3 system with 0.0 ≤x ≤0.12. Fig. 9. Tac’s plots for Bi0.80-xCaxLa0.20FeO3 system with 0.0 ≤x ≤0.12. Fig. 10. Room temperature FTIR spectra for Bi0.80-xCaxLa0.20FeO3 system with 0.0 ≤ x ≤0.12.

Table captions Table 1. Atomic fractional coordinates for Bi0.80-xCaxLa0.20FeO3 with composition range 0.0 ≤x ≤0.12 used for R3c and Pbnm space group. Table 2. Refined structural parameters in Bi0.80-xCaxLa0.20FeO3 system composition range 0.0 ≤x ≤0.12 using R3c and Pbnm models. Table 3. Values of A and B site displacement parameters (s and t) and FeO6 octahedra tilt angle (ω) for R3c phase extracted from Rietveld method using the rhombohedral lattice parameters for Bi0.80-xCaxLa0.20FeO3 compositions with x = 0.00, 0.03, and 0.06 at %.

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Table 4. Magnetic parameters obtained after the theoretical treatment, from the FM contribution (Mr, MS, and HC) and the linear contribution (<), for Bi0.80-xCaxLa0.20FeO3 with 0.0 ≤x ≤0.12.

17

Fig. 1

Fig. 2

18

Fig. 3

Fig. 4

19

Fig. 5

Fig. 6

20

Fig. 7

Fig. 8

21

Fig. 9

Fig. 10

22

Table 1.

Element Bi/La/Ca Fe O1 O2

R3c X 0 0 0.239

site 6a 6a 18b

Y 0 0 0.354

Pbnm site X 4c 0.0003 4a 0 4c -0.055 8d 0.2177

Z 0.2971 (0.25+s) 0.0197 (t) 0.0833

Y 0.519 0 -0.008 0.2802

Table 2. Structural model → Parameters a (Å) b (Å) c (Å) Volume (Å3) t (tolerance factor) RF RBragg Crystallite size (nm) Rp Rwp (cc – ac)/cc (%) Fe-O-Fe χ2

R3c (hexagonal axis) x = 0.03 x = 0 .06 5.5696 (Å) 5.5690(Å) 5.5696 (Å) 5.5690(Å) 13.8215(Å) 13.8178 371.3104 371.1255 0.9647 0.9641 3.88 4.36 5.19 5.62 35 29 14.8 16.6 12.6 13.9 1.577 1.633 158.53 157.40 2.56 2.40

x = 0.0 5.5734(Å) 5.5734(Å) 13.8259 371.9355 0.9654 4.03 5.43 40 16.4 13.3 1.511 158.03 2.92

Pbnm x = 0.12 5.5793 5.5526 7.8619 243.5633 0.9629 3.73 4.80 25 14.5 12.9 153.30 2.20

Table 3. BLCFO

arh

αrh (o)

s

r

ω

x = 0.00 x = 0.03 x = 0.06

5.620 5.618 5.617

59.257 59.428 59.435

0.0403 0.0400 0.0445

0.0134 0.0129 0.0167

10.8 9.72 9.60

Table 4. Ca doped BLFO

x = 0.00 x = 0.03 x = 0.06 x = 0.12

Magnetic parameters for Exp. data after theoretical treatment Mr (10-3) (emu/g)

MS(10-3) (emu/g)

HC (Oe)

χ (10-6)

1.2 4.0 5.4 9.1

32 25 32 78

97 52 67 110

5.582 6.60 6.71 6.072

23

Z 0.25 0 0.25 0.0286

XRD and Rietveld refinement confirms structural phase transitions. Evidence of spin two phonon coupling has been found. Weak ferromagnetic ordering provoked by Ca doping. Dielectric properties improved by Ca doping has been observed. Optical bandgap energy meaningfully reduced with Ca doping.

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: