On Structural and Morphological Characterizations of Ho-BFO Multiferroics

E S Sreelakshmi1,a), Soumya G Nair1,b) and Jyotirmayee Satapathy1,c)

1Department of Physics, Amrita Vishwa Vidyapeetham, Amritapuri, India, 690525 a)Corresponding author : essreelakshmi98@gmail.com

b)soumyagnair@am.amrita.edu c)jyotirmayees@am.amrita.edu

Abstract. Bismuth Ferrites, popularly known as BFO, in its pure and doped form are much alluring multiferroics among the present-day material scientist’s world. The peculiar properties, those make this category of multiferroics different, arise mainly due to their structural anomalies. Further, structural perturbations with a dopant element has also shown even fascinating results. In this work, Holmium doped BFO are synthesized using solid state reaction route and their structural characterizations with the help of XRD and FTIR along with morphological studies have been discussed extensively to deduce the influence of doping on the structure of BFO.

INTRODUCTION

Multiferroicity and magneto electric coupling are the interesting properties of bismuth ferrites (BFO). Coexistence of both magnetic as well as electric behavior of multiferroics can be analyzed in various ways[1]. Magnetocaloric properties of these compounds are also studied recently[2,3]. A lot of investigations are done to enhance the utility of multiferroic materials like TbMnO3, YMnO3, BiMnO3, BiFeO3 etc[4]. Because of their interesting electrical and magnetic properties seen at low and room temperature, increases the demand of their usage of BiFeO3 in electronic industry. BFO have proven their suitability in applications such as high-density ferroelectric devices, non volatile memories, photodetectors and in tuneable devices [5].

Due to their interesting properties and wide scope in different applications, much research has been attracted on their matter physics part. The reason of BFO giving rise to such interesting properties, lies in their structural anomalies. The basic foundation statement is they possess a noncentro symmetry [6].Further, with suitable doping on Bi, Fe or both sites, the structural anomalies are perturbed enough to extend fascinating results in their properties[7].This depends on doping element, doping concentration and the doping site, apart from other influential entities like synthesis route and environmental conditions[8].Spectroscopic diagnosis of BFO gives some interesting results[9].

In the present work, holmium doped on Bi-site of BiFeO3 with sample formula BixHo1-xFeO3 (Ho-BFO) has been prepared using solid state reaction route. These samples are undergone structural analysis by XRD and FTIR characterizations. Along with this, morphological study is also carried out on these Ho-BFO through FESEM. These studies are meant to understand the influence of doing on the structural transition and formation of metal oxide bonds as well as the grain growth which will lead the modifications in their multiferroic properties.

EXPERIMENTAL ANALYSIS

Bi1-xHoxFeO3 nano powders with x=0.2,0.4,0.6,0.8 were successfully synthesized by conventional solid state reaction route. Stoichiometric amounts of high purity oxide powders such as Fe2O3, Bi2O3, and Ho2O3 are weighed as per the composition Bi1-xHoxFeO3.The appropriate amounts of reactants are taken as per the equation (1).

((1-x)/2) Bi2O3 + (x/2) Ho2O3 + (1/2) Fe2O3 Bi1-xHoxFeO3 ----(1) Four samples of reactants with different concentrations are taken and mixed so as to obtain Bi0.8Ho0.2FeO3 ,

Bi0.6Ho0.4FeO3, Bi0.4Ho0.6FeO3, and Bi0.2Ho0.8FeO3 as products. The high purity oxide powders are ground homogeneously in mortar at room temperature and calcined at 700K intermittently. Pellets were prepared out of these powders utilizing a hydraulic press and then the samples were sintered at 800K.

The crystal structure of prepared samples was examined by the X-ray diffractometer (XRD) (Rigaku make) using Cu-Kα radiation. Lattice parameters from XRD data were measured using Unit cell software. Also measured Fourier Transform InfraRed (FTIR) spectra using IR Affinity-1, in order to identify the bond formation in the samples. Further, the microstructure properties were examined by FESEM. These results are discussed in the following paragraphs.

RESULTS AND DISCUSSIONS

Structural analysis has been carried out with XRD and FTIR additionally. XRD results primarily identifies the structure or phase formation whereas FTIR gives an detailed depth on the bonds specifically.

X-ray Diffraction Analysis

From the literatures, it is clear that pure BFO has rhombohedral structure with R3c space group. While with doping, it shows a structural transition. XRD results of holmium doped BFO with doping concentration of x=0.2,0.4,0.6,0.8 are shown in figure 1. For the first sample with x=0.2, major peaks obtained, are (012), (104), (110), (006), (202), (024), (116), (214) which is consistent with rhombohedral nature of earlier reported ones [10]. But, as we increase the dopant concentration, peaks are shifted to higher 2theta values with a lower intensity and there are signs of distorted rhombohedral structure as well as multiphases are see from the existence of separated peaks and peak splitting for x=0.4 and x=0.6 samples. However, when the concentration is increased to x=0.8, crystal structure flips back to orthorhombic nature with diffraction peaks (110), (111), (020), (112), (200), (021), (022), (202), (113), (220), (023), (221), (301), (113), (132) and (312), which confirms this transition[10]. In other words, it explains a compositional driven phase transition from rhombohedral to orthorhombic phase.

On detailed analysis, Goldsmith tolerance factor (t), for pure BFO is 0.88 which signifies how much the crystal structure of the sample deviates from its perovskite (t=1) nature [11]. Due to Holmium doping on A-site, the tolerance factor has shown a decrease indicating the structural transition from the pure perovskite nature to a distorted one which is the reason of breaking symmetry and thus, ferroelectricity. The values obtained in case of Ho-BFO with their concentration shown in brackets, are t=0.8718 (x=0.2), 0.8625 (x=0.4) ,0.8531 (x=0.6), 0.8437 (x=0.8). The formula used for this calculation is shown in equation (2) where R is the respective ionic radius.

푡 = ( ) √ ( )

-(2)

FIGURE 1: XRD patterns of Bi1-xHoxFeO3 (x=0.2,0.4,0.6,0.8)

Unit cell parameters such lattice constant, bond length etc are measured using ‘unit-cell’ software. Lattice parameters exhibit a change with increase in holmium content and reduction in cell volume has been observed. The obtained values are listed in table 1 for all the samples. The value of c/a changes from 2.491(x=0.2) to 1.4639(x=0.8) which indicates how the distortion is implicated in the structure. Similarly, lattice parameters and lattice angles shows a change via doping, 20% holmium on bismuth site of BFO to 80% holmium as given in table 1. These changes in parameters leads a structural transition from rhombohedral to orthorhombic. In addition to all these, cell volume also shows a reduction from 366.329(20%Ho-BFO) to 229.0731(80%Ho BFO).This reduction in unit cell volume is reflected in the reduction of particle size. Crystallite size (D) of doped BFOs are measured using Scherrer formula as shown in equation (3).

퐷 = -------- (3) where k is the shape factor, χ wavelength, θ is the Bragg angle and 훽 is full width at half maximum(FWHM). Crystallite size of doped sample is observed to be lower than the parent compound because of the smaller ionic

radius of Ho3+ (104.1pm) than Bi3+(117pm). The values obtained here are 40.29nm (x=0.2), 47.29nm (x=0.4), 32.01nm (x=0.6), 42.13nm (x=0.8). Lattice strain (휂) and dislocation density (훿) are obtained using the formulae 휂 = and δ= , and are listed in table 2.

TABLE 1: Lattice parameters of Ho-BFO

Lattice Parameters Bi0.8Ho0.2FeO3 Bi0.2Ho0.8FeO3

A 5.5446 5.30678 B 5.4965 5.5579 C 13.8093 7.7687 Alpha 89.2118 90.8465 Beta 90.4819 90.9329 Gamm

a 119.4786 89.6741 cell

volume 366.329 229.0731

TABLE 2: Particle size, lattice stain and dislocation density of Ho-BFO Parameters Bi0.8Ho0.2FeO3 Bi0.6Ho0.4FeO3 Bi0.4Ho0.6FeO3 Bi0.2Ho0.8FeO3

2theta(degree) 32.131 32.175 32.96 33.055

Inter planar spacing d(angstrom) 2.7835 2.7798 2.7153 2.70776

FWHM(radian) 0.00344 0.002931 0.00433 0.00329

Crystallite size D(nm) 40.29nm 47.29nm 32.01nm 42.13nm

Strain 8.5991x10-4 8.2242x10-4 1.0824x10-3 7.3268x10-4

Dislocation Density(lines/m2) 2.48x1011 2.115x1011 3.124x1011 2.373x1011

tolerance factor 0.8718 0.8625 0.8531 0.8437

Morphological Analysis

FESEM images of doped BFO samples are shown in figure 2. Figures a, b, c, d represents holmium doped bismuth ferrites with 20%, 40%,60% and 80% holmium content on bismuth site respectively. Grain size measured using XRD data are well corroborated with the FESEM results. As it’s observed, with an increase in holmium content microstructure shows well developed grains. Besides, homogeneously distributed, large grains with square shape are observed. This concludes that although Ho doping reduces the particle size, on the contrary, it facilitates the grain growth process in a constant temperature and pressure condition.

FIGURE 2: FESEM images of a) Bi0.8Ho0.2FeO3 b) Bi0.6Ho0.4FeO3 c) Bi0.4Ho0.6FeO3 d) Bi0.2Ho0.8FeO3

FTIR Analysis

Infrared spectroscopy has been a powerful tool to characterize the materials for the bond formation. FTIR spectrum represents a finger-print of a sample containing absorption peaks which correspond to the frequencies of bonds vibration of the atoms present in the material. As every bond has unique combination of atoms, no two materials can have similar IR spectrum. Therefore, this method is essentially used to identify the bond formation between different atoms present in the material. Fourier transformed infrared (FTIR) spectra of Ho substituted samples for two wave number range, 350 to 600cm-1 and 500-1200cm-1 are shown in figure 3. Typical band characteristics of all the metal oxide bonds are observed at the frequency range of 420-670 cm-1. Two absorption peaks at 432 cm-1 and 549 cm-1 corresponds to Fe-O-Fe bending vibrations and Fe-O stretching vibrations of FeO6 group respectively in the perovskite structure which is in agreement with characteristic infrared absorption bands of BFO [11]. The gradual shift from 549 cm-1 to 553 cm-1 of Fe-O stretching modes with an increase in substituent concentration indicates that they have absorbed into the lattice site of BFO and there is a compositional driven structural transition due to ionic size mismatch between the substituent and host cations. Force constant shows an increment from 2.208(x=0.2) to 2.2398(x=0.8) and bond length shows a decrease from 0.19748nm to 0.19652nm as we increase holmium content from 0.2 to 0.8.

The absorption band at 450cm-1 is due to the occurrence of Bi-O bond in BiO6 octahedra. In our work, this wave number shows a gradual variation from 453 cm-1 (x=0.2) to 445 cm-1 (x=0.8). As a result, force constant has a reduction from 1.7713 Ncm-1 to 1.7286 Ncm-1 and bond length has an increase from 0.21251 nm to 0.21425 nm as shown in table 3. Interestingly these changes in force constant and bond length for Fe-O and Bi-O bonds are in reverse order in case of Ho-BFO.

(a) (b)

FIGURE 3: FTIR spectra in wavenumber range of (a) 350 to 600cm-1 (b) 500 to 1200cm-1

TABLE 3: Force constant and bond length of Fe-O bonds Parameters for Fe-O bond Bi0.8Ho0.2FeO3 Bi0.6Ho0.4FeO3 Bi0.4Ho0.6FeO3 Bi0.2Ho0.8FeO3

Wave number(cm-1) 549 550 552 553 Effective Mass(10-26kg) 2.0635 2.0635 2.0635 2.0635 Force constant K(Ncm-1) 2.208 2.2156 2.2317 2.2398 Bond length(A0) 1.9748 1.9724 1.9676 1.9652

TABLE 4: Force constant and bond length of Bi-O bonds Parameters for Bi O bond Bi0.8Ho0.2FeO3 Bi0.6Ho0.4FeO3 Bi0.4Ho0.6FeO3 Bi0.2Ho0.8FeO3

Wave number(cm-1) 453 451 449 445 Effective Mass(10-26kg) 2.4319 2.4418 2.4509 2.4593 Force constant K(Ncm-1) 1.7713 1.7629 1.7538 1.7286 Bond length(A0) 2.1251 2.1285 2.1322 2.1425

CONCLUSION

Holmium doped bismuth ferrites with four different compositions are synthesized using solid-state reaction method. XRD results show that samples exhibit rhombohedral to orthorhombic phase transition with doping as well as increased distortion. Lattice parameters show an increase to higher side with reduction in unit volume as well as suppressed particle size. Microstructure analysis though FESEM results shows Ho doped facilitates the grain growth but reduces the size. However, existence of homogeneous square shaped large grains with respect to increase in holmium content are observed. FTIR analysis, carried out to depict the deeper structural studies in terms of proper bond formations. The absorption peaks and bond lengths of metal oxides Bi-O and Fe-O are quite in agreement. This concludes our small yet extensive analysis to understand the influence of Holmium in modifying the BFO structure.

ACKNOWLEDGEMENTS

Authors would like to thank Material Science Lab, Department of Physics, St.Thomas College, Pala for extending their XRD facilities, and Department of Physics, CUSAT for FESEM and FTIR facilities.

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