E ﬀ ect of Bi Substitution on Structural and AC Magnetic Susceptibility Properties of Nd 1 − x Bi x MnO 3

: This study synthesizes the neodymium-based manganites with Bi doping, Nd 1 − x Bi x MnO 3 (x = 0, 0.25 and 0.50) using the solid-state reaction route. The crystal structural, morphological and magnetic properties were determined using X-ray di ﬀ raction (XRD), fourier transform infrared spectroscopy (FTIR), ﬁeld emission scanning electron microscopy (FESEM) and AC magnetic susceptibility. The Rietveld reﬁnement conﬁrmed that the compounds were in the single-phase orthorhombic structure of the NdMnO 3 with Pbnm space group and lattice parameter b increased with doping from 5.5571 (x = 0) to 5.6787 (x = 0.5). FTIR spectra showed that absorption bands were located within the range of 550–600 cm − 1 , which corresponded to the Mn–O stretching vibration. FESEM exhibited homogenous compound. The AC magnetic susceptibility measurement studies showed a strong antiferromagnetic (AFM) to paramagnetic (PM) transition existed at 76 K, 77 K and 67 K for samples (x = 0, 0.25 and 0.50), respectively. Curie–Weiss (C–W) 𝜒 𝐶/(𝑇 − 𝜃), is Curie constant θ is Curie–Weiss The is the best fit for C–W law in the paramagnetic region. The inverse susceptibility curve follows the C–W law at the higher temperature and starts to deviate from the linear fitting of C–W law at the lower temperature above T C , which shows the existence of Griffiths phase (GP) in the samples [24,25]. The deviation shows that the C–W law is inapplicable in the temperature range between T C and Griffiths temperature, T G (T C < T < T G ). This anomaly of ferromagnetic cluster within the paramagnetic region in the the total magnetic susceptibility of the samples The temperature ranges of Griffiths phase normalized with T C were evaluated using the equation GP = (T G − T C )/T C found as the substitution of Bi increased. The increasing values were of inhomogeneity near T C substitution of Bi in Nd 1−x Bi x MnO 3 system Griffiths phase values T C , T , T G


Introduction
Existing studies on the family of manganites perovskites, RMnO 3 as a parent of mixed valence perovskites, where R is made up of rare earth elements, such as La, Pr and Nd, were found a few decades ago due to their exotic behavior in electrical and magnetic properties [1,2]. Manganites perovskites were the favorite amongst researchers compared with other families of oxides because of their various possible substitutions that may change their crystal structure and properties [3]. Since the rediscovery of colossal magnetoresistance (CMR), many researchers have been focusing on the doped compound manganites due to their fascinating basis in physics and significance in technological importance, such as magnetic sensors [4]. Few theories have surfaced to explain the CMR mechanisms, such as double exchange (DE), superexchange (SE), Jahn-Teller (JT) effect, polaronic effects and so on.
Primarily, undoped NdMnO 3 is an antiferromagnetic (AFM) insulator with Néel temperature 67.2 K due to the interplay between the ferromagnetic (FM) DE and AFM SE interactions present in the compound [5]. Chatterji et al. confirmed that NdMnO 3 has an A-type AFM structure of the Mn magnetic sublattice below Néel temperature, T N ≈ 81.7 K [6]. When doped with divalent cations on the Nd sites, for example in Nd 1−x Ca x MnO 3 , FM semiconducting behavior was observed with x ≤ 0.25. However, when x > 0.80, the compound exhibited AFM semiconducting at low temperatures [7]. For Nd 0.5 Ca 0.5 MnO 3 , the compound undergoes a charge ordering (CO) transition at T CO = 250 K and the AFM ordering transition at T N = 160 K, which is consistent with the study done by Wu et al. [7,8]. In this case, both authors suggested that the presence of CO was due to the JT distortion, resulting in the distortion of MnO 6 octahedral and localization of charge carrier, by decreasing its mobility, which seemed to weaken the DE interaction [7][8][9]. Meanwhile, when NdMnO 3 was doped with monovalent cations such as Na + and K + , it showed an FM insulator behavior and CO transition was observed for Nd 1−x Na x MnO 3 at −250 K when x > 0.1 [5,10]. However, a lack of knowledge still exists on Nd series doped with trivalent cations such as Nd 1−x Bi x MnO 3 .
Previous studies reported that although Bi 3+ (1.24 Å) and La 3+ (1.22 Å) have almost similar ionic radii, both systems exhibit different structures and properties [11]. LaMnO 3 exhibited an AFM insulator [12], while BiMnO 3 exhibited an FM insulator with T C of 105 K-110 K [11,13,14]. Recent studies on bismuth-doped manganites, such as La 0.7−x Bi x Ag 0.3 MnO 3 [12], La 1−x Bi x MnO 3 [13] and La 0.67−x Bi x Sr 0.33 MnO 3 [15], showed a decreased metal-insulator transition temperature, T MI , with an increase of Bi content. The author suggested that the behavior of the latter was related to possible hybridization of Bi 6s 2 lone pairs and e g electron of Mn 3+ , which could have affected the localization of carriers and weakened the DE interaction. Moreover, Bi substitution may also reduce the Mn-O-Mn angle, which can block the movement on e g electron from Mn 3+ to Mn 4+ , which decreases the mobility of conduction electron and weaken the DE process [2,12].
Several works were reported on Bi substitution, especially in La-based system, but a lack of study on Bi substitution in Nd-based system still exists, especially for optical and magnetic properties. In this study, we focus on reporting the structural, optical and magnetic properties of Nd 1−x Bi x MnO 3 to investigate the unique role of Bi in the system.

Materials and Methods
Polycrystalline samples for Nd 1−x Bi x MnO 3 (x = 0, 0.25 and 0.50) were synthesized by using conventional solid state reaction method. High purity (≥99.9%) neodynium oxide (Nd 2 O 3 ), bismuth oxide (Bi 2 O 3 ) and manganese oxide (Mn 2 O 3 ) powders were weighed in stoichiometric ratio using electronic balance and mixed together. Agate mortar and pestle were used to grind the powders for about 2 h before calcination in air at 700 • C for 24 h, repeating twice in order to eliminate volatile foreign particles. Finally, the obtained powders were then pressed into pellet under a pressure of 5 tons and were sintered at 1100 • C for 24 h.
The structure and phase purity of the samples were observed by using the X-ray diffraction technique (X'pert PRO MPD) at room temperature with Cu-K α (λ = 1.5418 Å) radiation by using a PANanalytical diffractometer. The XRD data were analyzed using the Rietveld refinement method using several software programs, such as General Structure Analysis System (GSAS) and Graphical User Interfaces (EXP-GUI) for the refinement and CMPR to convert data from ASC file to DAT file. Fourier transform infrared spectroscopy (FTIR) results were recorded using FTIR-Raman Drift Nicolet 6700 in the range of 450-2000 cm −1 to directly probe with the functional group present in perovskite samples. Samples were thoroughly mixed in KBr before the characterization. LEO model 982 Gemini Field Emission Scanning Electron Microscopy (FESEM) with energy-dispersive spectroscopy (EDX) were used to determine the surface morphology and homogeneity of the perovskite samples. AC susceptibility (χ') measurements were carried out using a Stanford research model SR-7265 lock-in amplifier and were performed in the temperature range from 30 K to 300 K.

Structural Analysis
This study obtained all the samples of NdMnO 3 doped with Bi as black and well-crystallized powders. Figure 1 shows the XRD patterns for manganites Nd 1−x Bi x MnO 3 (x = 0, 0.25 and 0.50) that were conducted at room temperature to validate the structure and sample purities and were then analyzed by using the Rietveld refinement technique ( Figure 2). All samples were indexed in an orthorhombic structure (a b c and α = β = γ = 90 • ) with Pbnm space group. The refined patterns were quite acceptable as the obtained value of agreement factor, χ 2 , showed a value of ∼1, Crystals 2020, 10, 521 3 of 10 which indicated a good fit. Rietveld refinement showed the samples x = 0 and x = 0.25 crystallized in a single phase and were chemically pure. However, a small amount of Bi 2 O 3 phase was detected in Nd 0.5 Bi 0.5 MnO 3 sample . The appearance of a secondary phase and redistribution of peak intensities in the Nd 0.5 Bi 0.5 MnO 3 sample was an indication of incomplete crystallization of the sample at the sintering temperature. The assumed structure for NdMnO 3 was consistent with most previous studies [6,10,16,17] that reported the same structure with Pbna or Pbnm space group (different setting), ICSD No. 153214, 247143. The sharpening of the peaks was due to the better crystallinity of the nanoparticles and only minimal impurities were detected in the XRD pattern of the x = 0.50 sample. In addition, the main diffraction peaks corresponded to (121) hkl plane and matched with the previous study [18]. The crystallite size (D) of the samples estimated by the Debye-Scherer equations, D = Kλ/(β(θ) cos θ), where D is the crystallite size (nm), K is constant with 0.94, λ is wavelength of XRD that is 0.1541 nm for CuK α radiation Å and β is full-width at half-maximum (FWHM) and θ is the angle of peak of XRD. The average crystallite size was found to be in the range 74-128 nm.
Crystals 2020, 10, x FOR PEER REVIEW 3 of 11 indicated a good fit. Rietveld refinement showed the samples x = 0 and x = 0.25 crystallized in a single phase and were chemically pure. However, a small amount of Bi2O3 phase was detected in Nd0.5Bi0.5MnO3 sample. The appearance of a secondary phase and redistribution of peak intensities in the Nd0.5Bi0.5MnO3 sample was an indication of incomplete crystallization of the sample at the sintering temperature. The assumed structure for NdMnO3 was consistent with most previous studies [6,10,16,17] that reported the same structure with Pbna or Pbnm space group (different setting), ICSD No. 153214, 247143. The sharpening of the peaks was due to the better crystallinity of the nanoparticles and only minimal impurities were detected in the XRD pattern of the x = 0.50 sample. In addition, the main diffraction peaks corresponded to (121) hkl plane and matched with the previous study [18]. The crystallite size (D) of the samples estimated by the Debye-Scherer equations, = /( ( ) cos ), where D is the crystallite size (nm), K is constant with 0.94, λ is wavelength of XRD that is 0.1541 nm for CuKα radiation Å and β is full-width at half-maximum (FWHM) and θ is the angle of peak of XRD. The average crystallite size was found to be in the range 74-128 nm.  Tick marks indicate the allowed Bragg reflections. Bi2O3 impurity phase was detected for Nd0.5Bi0.5MnO3 and indicated by green tick marks. Table 1 presents the corresponding data of lattice parameters obtained from Rietveld refinement technique. For the undoped sample, the lattice parameters obtained, a = 5.424 Å , b = 5.5570 Å and c = 7.6648 Å , were found to be quite consistent with the value reported by the previous study [10]. The structural parameter b of the samples increased as the Bi doping increased up to x = 0.50. However, for both lattice parameters, a and c showed no systematic trend due to Bi substitution. The unit cell volume of the samples also increased with the increase of the Bi 3+ substitution. The increases in unit cell volume were suggested due to the Bi substitution at Nd site, where the Bi 3+ had a slightly larger ionic radius compared with Nd 3+ and could replace it, which led  Table 1 presents the corresponding data of lattice parameters obtained from Rietveld refinement technique. For the undoped sample, the lattice parameters obtained, a = 5.424 Å, b = 5.5570 Å and c = 7.6648 Å, were found to be quite consistent with the value reported by the previous study [10]. The structural parameter b of the samples increased as the Bi doping increased up to x = 0.50. However, for both lattice parameters, a and c showed no systematic trend due to Bi substitution. The unit cell volume of the samples also increased with the increase of the Bi 3+ substitution. The increases in unit cell volume were suggested due to the Bi substitution at Nd site, where the Bi 3+ had a slightly larger ionic radius compared with Nd 3+ and could replace it, which led to the expansion of cell volume [12,13]. Both authors stated that the radius of Bi 3+ may depend on the character of 6s 2 lone pairs; it can be seen that when Bi 6s 2 lone pair is dominant, the ionic radius of Bi 3+ is 1.24 Å, while when not, the ionic radius of Bi 3+ is approximately 1.16 Å [12]. Hence, the increase in structural parameter, b, and cell volume, V, with Bi substitution suggests that the Bi 6s 2 lone pair is dominant in the Nd 1−x Bi x MnO 3 system. From Table 2, the Mn-O bond length decreased and the average ionic radius, r A , increased as Bi concentration level increased. However, Mn-O-Mn bond angle showed no trend with the increased Bi-doped. The tolerance factor (τ) was calculated using formula τ = (<r A > + <r O >)/ √ 2 ((<r B > + <r O >), where <r A >, <r B > and <r O > represent the average ionic radii of A, B and O sites, respectively [9]. The values of τ showed an increase from 0.515 (x = 0) to 0.529 (x = 0.50), as seen in Table 2. The increase in τ indicates that a slight reduction of MnO 6 distortion exists, thus the lattice reduced the mismatch between A-O and B-O layer as the Bi concentration increased [18,19].        [3,19]. The absorption peak band at the range of 600 cm −1 -550 cm −1 seem to shift towards lower wave number values for a structure having lower symmetry with decreasing Bi 3+ ion content, according to the increase of e g electrons in the anti-bonding orbital. Hence, the bond order would decrease with an increase in the number of e g electrons. The lower frequency band can be related to a higher deformation mode of the MnO 6 octahedron [20,21]. The FTIR spectra confirm the formation of the NdBiMnO 3 sample. perovskite-type ABO3 [3,19]. The absorption peak band at the range of 600 cm −1 -550 cm −1 seem to shift towards lower wave number values for a structure having lower symmetry with decreasing Bi 3+ ion content, according to the increase of eg electrons in the anti-bonding orbital. Hence, the bond order would decrease with an increase in the number of eg electrons. The lower frequency band can be related to a higher deformation mode of the MnO6 octahedron [20,21]. The FTIR spectra confirm the formation of the NdBiMnO3 sample.

Morphology
To understand the morphology of the samples, we performed FESEM with EDX and obtained the results, as shown in Figure 5. The images in Figure 5 reveal that the surface of the sample is non-homogeneous and the grains are agglomerated. The particles are also observed as non-spherical in shape. According to the image, the grain size of all samples are in the range of ~3.7 μm to ~5.6 μm, respectively. The grain sizes observed by FESEM are larger than those calculated by the Debye-Scherer formula. This can be explained by the fact that each particle observed by FESEM is

Morphology
To understand the morphology of the samples, we performed FESEM with EDX and obtained the results, as shown in Figure 5 . The images in Figure 5 reveal that the surface of the sample is Crystals 2020, 10, 521 6 of 10 non-homogeneous and the grains are agglomerated. The particles are also observed as non-spherical in shape. According to the image, the grain size of all samples are in the range of~3.7 µm to~5.6 µm, respectively. The grain sizes observed by FESEM are larger than those calculated by the Debye-Scherer formula. This can be explained by the fact that each particle observed by FESEM is formed by several crystallized grains. The results of EDX plot matches with a standard peak position of Nd, Bi, Mn and O from the previous study [19]. The EDX spectrum confirms the homogeneity of the samples.  Figure 6 shows the temperature dependence of the AC susceptibility of the Nd1−xBixMnO3 system on the real part χ' with the inset dχ'/dT versus the temperature range of 30-300 K. For x = 0 and x = 0.25, a definite and sharp peak was observed at 70 K and 72 K, respectively, associated to the ordering temperature of Mn spins [22]. The observed cusp depicts the PM to AFM transition, which is consistent with the results from previous studies [5,8,23]. For x = 0.50, two sharp peaks were observed, which were at 42 K and 57 K, respectively. The existence of two peaks indicates the magnetic inhomogeneity that may be induced by Bi substitution [12]. The TC was determined using   Figure 6 shows the temperature dependence of the AC susceptibility of the Nd 1−x Bi x MnO 3 system on the real part χ' with the inset dχ'/dT versus the temperature range of 30-300 K. For x = 0 and x = 0.25, a definite and sharp peak was observed at 70 K and 72 K, respectively, associated to the ordering temperature of Mn spins [22]. The observed cusp depicts the PM to AFM transition, which is consistent with the results from previous studies [5,8,23]. For x = 0.50, two sharp peaks were observed, which were at 42 K and 57 K, respectively. The existence of two peaks indicates the magnetic inhomogeneity that may be induced by Bi substitution [12]. The T C was determined using the minimum, while T N was determined using the maximum point of differentiation, as shown in the inset of the graph in Figure 6. In the figure, all the three compounds exhibit a strong AFM with a competing weak FM, where T N and T C decreased with Bi substitution. The decreasing value of T C shows the weakening of the FM behavior due to the Bi substitution. Temperature dependence of inverse susceptibility, 1/χ', for all samples is plotted in Figure 7. The experimental data were fitted by using Curie-Weiss (C-W) equation, χ = C/(T − θ), where C is Curie constant and θ is Curie-Weiss temperature. The red line is the best fit for C-W law in the paramagnetic region. The inverse susceptibility curve follows the C-W law at the higher temperature and starts to deviate from the linear fitting of C-W law at the lower temperature above T C , which shows the existence of Griffiths phase (GP) in the samples [24,25]. The deviation shows that the C-W law is inapplicable in the temperature range between T C and Griffiths temperature, T G (T C < T < T G ). This anomaly indicates the possible existence of ferromagnetic cluster within the paramagnetic region in the system which would enhance the total magnetic susceptibility of the samples [24][25][26][27]. The temperature ranges of Griffiths phase normalized with T C were evaluated using the equation GP = (T G − T C )/T C and were found to increase as the substitution of Bi increased. The increasing values were due to an increase of magnetic inhomogeneity near T C . Therefore, the substitution of Bi in Nd 1−x Bi x MnO 3 system could enhance the appearance of Griffiths phase [24]. The values of T C , T N , T G and GP are summarized in Table 3. inverse susceptibility, 1/χ', for all samples is plotted in Figure 7. The experimental data were fitted by using Curie-Weiss (C-W) equation, = /( − ), where C is Curie constant and θ is Curie-Weiss temperature. The red line is the best fit for C-W law in the paramagnetic region. The inverse susceptibility curve follows the C-W law at the higher temperature and starts to deviate from the linear fitting of C-W law at the lower temperature above TC, which shows the existence of Griffiths phase (GP) in the samples [24,25]. The deviation shows that the C-W law is inapplicable in the temperature range between TC and Griffiths temperature, TG (TC < T < TG). This anomaly indicates the possible existence of ferromagnetic cluster within the paramagnetic region in the system which would enhance the total magnetic susceptibility of the samples [24][25][26][27]. The temperature ranges of Griffiths phase normalized with TC were evaluated using the equation GP = (TG − TC)/TC and were found to increase as the substitution of Bi increased. The increasing values were due to an increase of magnetic inhomogeneity near TC. Therefore, the substitution of Bi in Nd1−xBixMnO3 system could enhance the appearance of Griffiths phase [24]. The values of TC, TN, TG and GP are summarized in Table 3.

Conclusions
In summary, this study investigates the effect of Bi 3+ substitution on structural, optical, morphological and magnetic properties of Nd1−xBixMnO3 samples. The XRD analysis and Rietveld refinement show the pattern corresponding to the perovskite-type NdMnO3, which crystallizes in the orthorhombic system with main peak at (121) hkl plane with the Pbnm space group. The structural parameter and cell volume of the samples increase as the Bi 3+ content increases. In addition, the average ionic radii of A site, < rA >, increased, hence τ also increased. The FTIR spectra show that active vibrational bands present at 600 cm −1 -550 cm −1 , which correspond to the metal oxide Mn-O stretching vibration of perovkite NdMnO3. FESEM images show that the sample is non-spherical and somewhat agglomerated. The samples show a strong AFM behavior at low temperature with decreasing TC due to the Bi substitution. Deviation of temperature dependence of inverse susceptibility curves shows the existence of Griffiths phase in these materials.

Conclusions
In summary, this study investigates the effect of Bi 3+ substitution on structural, optical, morphological and magnetic properties of Nd 1−x Bi x MnO 3 samples. The XRD analysis and Rietveld refinement show the pattern corresponding to the perovskite-type NdMnO 3 , which crystallizes in the orthorhombic system with main peak at (121) hkl plane with the Pbnm space group. The structural parameter and cell volume of the samples increase as the Bi 3+ content increases. In addition, the average ionic radii of A site, <r A >, increased, hence τ also increased. The FTIR spectra show that active vibrational bands present at 600 cm −1 -550 cm −1 , which correspond to the metal oxide Mn-O stretching vibration of perovkite NdMnO 3 . FESEM images show that the sample is non-spherical and somewhat agglomerated. The samples show a strong AFM behavior at low temperature with decreasing T C due to the Bi substitution. Deviation of temperature dependence of inverse susceptibility curves shows the existence of Griffiths phase in these materials.