Study of Structural, Magnetic, and Mossbauer Properties of Dy 2 Fe 16 Ga 1 − x Nb x (0.0 ≤ x ≤ 1.0) Prepared via Arc Melting Process

: Intermetallic compounds of Dy 2 Fe 16 Ga 1 − x Nb x ( x = 0.0 to 1.00) were synthesized by arc melting. Samples were investigated for structural, magnetic, and hyperﬁne properties using X-ray diffraction, vibration sample magnetometer, and Mossbauer spectrometer, respectively. The Rietveld analysis of room temperature X-ray diffraction data shows that all the samples were crystallized in Th 2 Fe 17 structure. The unit cell volume of alloys increased linearly with an increase in Nb content. The maximum Curie temperature Tc ~523 K for x = 0.6 sample is higher than Tc = 153 K of Dy 2 Fe 17 . The saturation magnetization decreased linearly with increasing Nb content from 61.57 emu/g for x = 0.0 to 42.46 emu/g for x = 1.0. The Mössbauer spectra and Rietveld analysis showed a small amount of DyFe 3 and NbFe 2 secondary phases at x = 1.0. The hyperﬁne ﬁeld of Dy 2 Fe 16 Ga 1 − x Nb x decreased while the isomer shift values increased with the Nb content. The observed increase in isomer shift may have resulted from the decrease in s electron density due to the unit cell volume expansion. The substantial increase in Tc of thus prepared intermetallic compounds is expected to have implications in magnets used for high-temperature applications.


Introduction
Intermetallic compounds based on rare-earth elements (R) and 3d-transition elements (T) are important from a fundamental and a technological point of view as they possess outstanding magnetic properties because of their high saturation magnetization, Ms. R 2 Fe 17 (2:17) alloy has the most Fe-content among all R-iron intermetallic and hence they have the highest Ms in the class of intermetallic magnets. However, these compounds have relatively low Curie temperature, Tc. For example, Tc~473 K for Gd 2 Fe 17 and 370 K for Dy 2 Fe 17, along with low magnetic anisotropies [1]. Various strategies have been employed to address issues related to improving magnetic anisotropy, magnetization, and Curie temperature of R 2 Fe 17 compounds. Efforts in this direction include insertion of metalloids, hydrogen, nitrogen, and carbon in the R 2 Fe 17 matrix [2][3][4][5]. An improvement in the Tc value of Ce 2 Fe 17 [6], Gd 2 Fe 17 [7,8], Dy 2 Fe 17 [9,10], Pr 2 Fe 17 [11], Nd 2 Fe 17 [12], Er 2 Fe 17 [13] , Tm 2 Fe 17 [14], Lu 2 Fe 17 [15,16], Ho 2 Fe 17 [17], Sm 2 Fe 17 [18] have been reported by adding metallic atoms like Si, Cr, Mn and metalloids like Ga on Fe sites. The substitution of non-magnetic atoms at Fe sites has been reported to increase the ferromagnetic coupling, leading to an increase in the Tc value [19,20] and magneto-crystalline anisotropy [9] of the compound. Improvements in the Tc of R 2 Fe 17 with substitution of a non-magnetic atom Ga have been reported for Ce 2 Fe 17−x Ga x [21], Sm 2 Fe 17−x Ga x [22] compounds, where the maximum in Tc~462 K was observed for Dy 2 Fe 16 Ga 1 [9].
Low substitution of Ga content has been identified to increase the Tc of rare-earth intermetallic compound [9]. Additional phases are reported for a higher content of Ga Figure 1 represents the XRD patterns of Dy 2 Fe 16 Ga 1−x Nb x (0.0 ≤ x ≤ 1.0) samples. The spectra show that the samples had Th 2 Ni 17 structure (hexagonal, space group, P6 3 /mmc) without any impurity; however, a small additional peak of NbFe 2 and DyFe 3 was detected for x = 1.0. The Rietveld [25] refined XRD patterns are shown in Figure 1b. The refined profile shows an excellent match between observed and calculated profiles. During the refinement process, all structural and lattice parameters, peak shift, background profile function, thermal parameters, etc., were refined until the observed profile functions matched with the calculated profile. The initial crystal structure parameter used for the refinement was adapted from Laio et al. [26]. The lattice parameters a and c of Dy 2 Fe 16 Ga 1−x Nb x (0.0 ≤ x ≤ 1.0) are shown in Figure 2, and the corresponding data are listed in Table 1. It is observed from Figure 2 that the unit cell volume increases with the increase in the Nb content, x. The increase in the unit cell volume is due to Nb ionic radius (~0.86 Å) is greater than the ionic radius of Ga (0.62 Å) [27]. The linear increase in the c/a ratio indicates that the unit cell's expansion along the c-axis is greater than that along the a-axis. Considering the fact that the 6c(4f ) dumbbell site is located along the c-axis, the c-axis expansion could affect the Tc value of the 2:17 compound [1]. The c/a ratio deviates at higher Nb concentrations (x = 1.0) due to the formation of secondary phases. In addition, it is observed from Table 1 Figure  4. From Figure 4, it is observed that the Ms of Dy2Fe16Ga1−xNbx decreases at the rate of −3.7 emu/g per Nb atom. The decrease in the magnetization of substituted R2Fe17 compounds can be understood based on 3d band model. Inherently R2Fe17 compounds are weak ferromagnets because both spin-up and spin-down bands are filled incompletely. The reduction in the Fe magnetic moment upon non-magnetic atom substitution results from the transfer of valence electrons of the substituted atom to the 3d band of Fe [28,29], which progressively fills the 3d band and moves the Fermi level up. A decrease in Fe magnetic moment is due to the electronic hybridization effect in R2Fe17Mx with M = Ga, Al, and Si is well reported in the literature [30,31]. This decrease in Fe moment with the substitution is due to Fe ([Ar]3d 6 4s 2 )-Ga([Ar]4s 2 4p 1 ), Fe-Al ([Ne]3s 2 3p 1 ) and Fe-Si ([Ne]3s 2 3p 2 ) electronic hybridizations. The decrease in the magnetic moment in Nb substituted Dy2Fe16Ga1-xNbx compound is due to indirect (3d-4s-4d) hybridization between Fe ([Ar]3d 6 4s 2 ), Ga([Ar]4s 2 4p 1 ), and Nb ([Kr]4d 4 5s 1 ), which reduces the spin polarization of Fe-3d states, resulting in a net decrease in the magnetic moment. In a study reported by Lekdadri et al.;    Figure 4. From Figure 4, it is observed that the Ms of Dy 2 Fe 16 Ga 1−x Nb x decreases at the rate of −3.7 emu/g per Nb atom. The decrease in the magnetization of substituted R 2 Fe 17 compounds can be understood based on 3d band model. Inherently R 2 Fe 17 compounds are weak ferromagnets because both spin-up and spin-down bands are filled incompletely. The reduction in the Fe magnetic moment upon non-magnetic atom substitution results from the transfer of valence electrons of the substituted atom to the 3d band of Fe [28,29], which progressively fills the 3d band and moves the Fermi level up. A decrease in Fe magnetic moment is due to the electronic hybridization effect in R 2 Fe 17 M x with M = Ga, Al, and Si is well reported in the literature [30,31]. This decrease in Fe moment with the substitution is due to Fe ([Ar]3d 6 4s 2 )-Ga([Ar]4s 2 4p 1 ), Fe-Al ([Ne]3s 2 3p 1 ) and Fe-Si ([Ne]3s 2 3p 2 ) electronic hybridizations. The decrease in the magnetic moment in Nb substituted Dy 2 Fe 16 Ga 1x Nb x compound is due to indirect (3d-4s-4d) hybridization between Fe ([Ar]3d 6 4s 2 ), Ga([Ar]4s 2 4p 1 ), and Nb ([Kr]4d 4 5s 1 ), which reduces the spin polarization of Fe-3d states, resulting in a net decrease in the magnetic moment. In a study reported by Lekdadri et al.; on Co 1−x Nb x alloy, cobalt moment was observed to decrease with increasing Nb content in the alloys due to a similar hybridization effect [32].
Magnetochemistry 2021, 7, x FOR PEER REVIEW 5 of 10 on Co1−xNbx alloy, cobalt moment was observed to decrease with increasing Nb content in the alloys due to a similar hybridization effect [32].  The Curie temperature, Tc, of Dy2Fe16Ga1−xNbx as a function of Nb content is shown in Figure 5. It is observed that Curie temperature of Dy2Fe16Ga1−xNbx alloys increases with increase in Nb concentration from 488 K (x = 0.00) to a maximum of 523 K (x = 0.6) and then decreases to 460 K (x = 1.00). The achieved Tc for Dy2Fe16Ga0.4Nb0.6 is 35 K higher than that of Dy2Fe17Ga1 and 153 K higher than that of Dy2Fe17 [10]. In general, Tc in rare-earth intermetallic compound is due to three kinds of exchange interactions, namely the 3d-3d exchange interactions, i.e., between the magnetic moment of the Fe sub-lattice (JFeFe), 4f-4f exchange interaction, i.e., the interaction between the magnetic moment within the R sublattice (JRR), and the inter sub-lattice 3d-4f exchange interaction (JRFe). It is reported that the Tc increases with an increase in the JFeFe [33]. The interactions between the rare-earth spins Saturation Magnetization (emu/g) on Co1−xNbx alloy, cobalt moment was observed to decrease with increasing Nb content in the alloys due to a similar hybridization effect [32].  The Curie temperature, Tc, of Dy2Fe16Ga1−xNbx as a function of Nb content is shown in Figure 5. It is observed that Curie temperature of Dy2Fe16Ga1−xNbx alloys increases with increase in Nb concentration from 488 K (x = 0.00) to a maximum of 523 K (x = 0.6) and then decreases to 460 K (x = 1.00). The achieved Tc for Dy2Fe16Ga0.4Nb0.6 is 35 K higher than that of Dy2Fe17Ga1 and 153 K higher than that of Dy2Fe17 [10]. In general, Tc in rare-earth intermetallic compound is due to three kinds of exchange interactions, namely the 3d-3d exchange interactions, i.e., between the magnetic moment of the Fe sub-lattice (JFeFe), 4f-4f exchange interaction, i.e., the interaction between the magnetic moment within the R sublattice (JRR), and the inter sub-lattice 3d-4f exchange interaction (JRFe). It is reported that the Tc increases with an increase in the JFeFe [33]. The interactions between the rare-earth spins Saturation Magnetization (emu/g) The Curie temperature, Tc, of Dy 2 Fe 16 Ga 1−x Nb x as a function of Nb content is shown in Figure 5. It is observed that Curie temperature of Dy 2 Fe 16 Ga 1−x Nb x alloys increases with increase in Nb concentration from 488 K (x = 0.00) to a maximum of 523 K (x = 0.6) and then decreases to 460 K (x = 1.00). The achieved Tc for Dy 2 Fe 16 Ga 0.4 Nb 0.6 is 35 K higher than that of Dy 2 Fe 17 Ga 1 and 153 K higher than that of Dy 2 Fe 17 [10]. In general, Tc in rare-earth intermetallic compound is due to three kinds of exchange interactions, namely the 3d-3d exchange interactions, i.e., between the magnetic moment of the Fe sub-lattice (J FeFe ), 4f-4f exchange interaction, i.e., the interaction between the magnetic moment within the R sub-lattice (J RR ), and the inter sub-lattice 3d-4f exchange interaction (J RFe ). It is reported that the Tc increases with an increase in the J FeFe [33]. The interactions between the rare-earth spins (4f -4f ) are assumed to be weak and negligible compared to the other two types of interactions. Thus, the Tc in the R 2 Fe 17 intermetallic compound is mainly dictated by J FeFe . The strength of Fe-Fe exchange interaction highly depends on interatomic Fe-Fe distance [34,35]. Accordingly, the exchange interactions between iron atoms situated at distances smaller (greater) than 2.45-2.50 Å are negative (positive). In the R 2 Fe 17 , the majority of Fe-Fe distances favor the negative interaction [9]. The negative exchange interaction can be reduced either by volume expansion or by reducing the number of Fe-Fe pairs with negative exchange interactions. The low T C observed in the parent Dy 2 Fe 17 compound is believed to result from the antiferromagnetic coupling of Fe-Fe moments at the 6c(4f ) sites [36]. Their Fe-Fe distance separation is less than 2.45 Å needed for the ferromagnetic ordering Table 1, [37]. An increase in Tc has been reported earlier with Ga substitution (x = 1) [9] but with a concomitant decrease in magnetization due to the magnetic dilution effect. However, the simultaneous substation of non-magnetic Ga and Nb atoms enhances the intermetallic Curie temperature without significantly lowering the saturation magnetization. It is noted from Table 1 that 4f -4f distances steadily increase with the Nb content, thus making J FEFe more positive with a steady improvement in the Tc value.
Magnetochemistry 2021, 7, x FOR PEER REVIEW 6 of 10 (4f-4f) are assumed to be weak and negligible compared to the other two types of interactions. Thus, the Tc in the R2Fe17 intermetallic compound is mainly dictated by JFeFe. The strength of Fe-Fe exchange interaction highly depends on interatomic Fe-Fe distance [34,35]. Accordingly, the exchange interactions between iron atoms situated at distances smaller (greater) than 2.45-2.50 Å are negative (positive). In the R2Fe17, the majority of Fe-Fe distances favor the negative interaction [9]. The negative exchange interaction can be reduced either by volume expansion or by reducing the number of Fe-Fe pairs with negative exchange interactions. The low TC observed in the parent Dy2Fe17 compound is believed to result from the antiferromagnetic coupling of Fe-Fe moments at the 6c(4f) sites [36]. Their Fe-Fe distance separation is less than 2.45 Å needed for the ferromagnetic ordering Table 1, [37]. An increase in Tc has been reported earlier with Ga substitution (x = 1) [9] but with a concomitant decrease in magnetization due to the magnetic dilution effect. However, the simultaneous substation of non-magnetic Ga and Nb atoms enhances the intermetallic Curie temperature without significantly lowering the saturation magnetization. It is noted from Table 1 that 4f-4f distances steadily increase with the Nb content, thus making JFEFe more positive with a steady improvement in the Tc value.  Figure 6 shows the room temperature fitted Mössbauer spectra for Dy2Fe16Ga1−xNbx. The hyperfine parameters viz. hyperfine field, HF, and isomer shift, IS, extracted from the fits are plotted in Figure 7 and listed in Table 2. Dy2Fe17 compounds have a basal magnetization that needs eight magnetic sextets to fit their Mössbauer spectra [38]. The fitting of spectra, shown in Figure 7, was carried out with eight magnetic sextets assigned to 4f, 6g, 12j, and 12k sites in Dy2Fe17 [21,[39][40][41]. The Mössbauer spectral analysis was carried out with magnetic sextets assigned to the 4f, 6g, 12j, and 12k sites in Dy2Fe17−xNbx. The 12j and 12k sites were further split into two, corresponding to the site occupancies of Fe atoms in the crystal structure of R2Fe17 with planar anisotropy. The intensities of the six absorption lines of each sextet were assumed to follow the 3:2:1 intensity ratio expected for randomly oriented powder samples in zero magnetic fields, and a single common linewidth was assumed for all the seven sextets. The isomer shifts (δ) for the magnetically inequivalent sites were constrained to be the same, whereas the hyperfine fields were expected to vary at pairs of magnetically inequivalent sites due to variations in the dipolar and orbital contributions to the magnetic hyperfine fields [42]. Doublets were used for additional phases for x = 1.0 to include paramagnetic phases DyFe3 and NbFe2 during Mössbauer fitting. It was found that the additional doublet covered 8.86% of the area. This is in confirmation of the additional phases observed in the x-ray diffraction pattern. From Table 2, it is evident that the HF follows a 4f(6c) > 6g(9d) > 12j(18f) > 12k(18h) sequence, which is similar to the sequence observed in other similar RE2Fe17 compounds [43,44].  Figure 6 shows the room temperature fitted Mössbauer spectra for Dy 2 Fe 16 Ga 1−x Nb x . The hyperfine parameters viz. hyperfine field, HF, and isomer shift, IS, extracted from the fits are plotted in Figure 7 and listed in Table 2. Dy 2 Fe 17 compounds have a basal magnetization that needs eight magnetic sextets to fit their Mössbauer spectra [38]. The fitting of spectra, shown in Figure 7, was carried out with eight magnetic sextets assigned to 4f, 6g, 12j, and 12k sites in Dy 2 Fe 17 [21,[39][40][41]. The Mössbauer spectral analysis was carried out with magnetic sextets assigned to the 4f, 6g, 12j, and 12k sites in Dy 2 Fe 17−x Nb x . The 12j and 12k sites were further split into two, corresponding to the site occupancies of Fe atoms in the crystal structure of R 2 Fe 17 with planar anisotropy. The intensities of the six absorption lines of each sextet were assumed to follow the 3:2:1 intensity ratio expected for randomly oriented powder samples in zero magnetic fields, and a single common linewidth was assumed for all the seven sextets. The isomer shifts (δ) for the magnetically inequivalent sites were constrained to be the same, whereas the hyperfine fields were expected to vary at pairs of magnetically inequivalent sites due to variations in the dipolar and orbital contributions to the magnetic hyperfine fields [42]. Doublets were used for additional phases for x = 1.0 to include paramagnetic phases DyFe 3 and NbFe 2 during Mössbauer fitting. It was found that the additional doublet covered 8.86% of the area. This is in confirmation of the additional phases observed in the x-ray diffraction pattern. From Table 2, it is evident that the HF follows a 4f (6c) > 6g(9d) > 12j(18f ) > 12k(18h) sequence, which is similar to the sequence observed in other similar RE 2 Fe 17 compounds [43,44]. Magnetochemistry 2021, 7, x FOR PEER REVIEW 7 of 10   From Figure 7, it is observed that the HF decreases with an increase in Nb content which is in agreement with the reduction in the magnetic moment. On the other hand, the isomer shift value increases with the Nb content. The isomer shift value reflects s-electron charge density at the Fe nucleus. With the unit cell volume expansion upon Nb substitution, the s-electron charge density at the Fe nucleus decreases, resulting in a concomitant increase in the isomer shift value.

Conclusions
In the present study, single phase Dy 2 Fe 16 Ga 1−x Nb x intermetallics were successfully synthesized using arc melting. The X-ray powder diffraction (XRD) results and the Rietveld refinements show that the samples have a Th 2 Ni 17 -type structure (space group, P6 3 /mmc). The XRD patterns show the presence of impurities phases DyFe 3 and NbFe 2 at higher Nb content (x > 0.80) only, which is also confirmed by Mossbauer spectral analysis. Rietveld analysis shows a linear unit-cell volume expansion with increasing Nb content. The reduction in Ms of Dy 2 Fe 16 Ga 1−x Nb x at 300 K with an increase in Nb content is attributed to the hybridization effect. For Dy 2 Fe 16 Ga 0.4 Nb 0.6 , maximum Tc was observed to 523 K, which is 35 K higher than the Dy 2 Fe 16 Ga compound and 153 K more elevated than its parent compound Dy 2 Fe 17 . The hyperfine fields, HF, of Dy 2 Fe 16 Ga 1−x Nb x decreased upon Nb substitution, reflecting moment reduction. The enhancement in the Curie temperature of thus prepared Dy 2 Fe 16 Ga 1−x Nb x compound with a judicious choice of Ga and Nb content can be helpful in areas demanding the high-temperature operation of magnets.