Effect of Al Substitution on Structural , Magnetic , and Magnetocaloric Properties of Er 6 Fe 23 − x Al x ( x = 0 and 3 ) Intermetallic Compounds

The structural, magnetic, and magnetocaloric properties of Er6Fe23−xAlx (x = 0 and 3) intermetallic compounds have been studied systematically. Samples were prepared using the arc furnace by annealing at 1073 K for one week. Rietveld analysis of XRD shows the formation of pure crystalline phase with cubic Fm-3m structure. Refinement results show that the unit cell volume decreases with increasing Al content. The Curie temperature Tc of the prepared samples was found to be strongly dependent on the aluminum content. This reduces magnetization and the ferrimagnetic phase transition temperature (Tc) from 481 K (for x = 0) to 380 K (for x = 3), is due to the substitution of magnetic element (Fe) by non-magnetic atoms (Al). With the increase of the Al content, a decrease in the values of magnetic entropy is observed. The magnitude of the isothermal magnetic entropy (|∆SM|) at the Tc decreases from 1.8 J/kg·K for x = 0 to 0.58 J/kg·K for x = 3 for a field change 14 kOe. Respectively, the relative cooling power (RCP) decreases with increasing Al content reaching 42 Jkg−1 for x = 0 to 28 Jkg−1 for x = 3.

Investigations on the structure and the magnetic properties of R 6 M 23 (M = Mn, Fe) have been reported in the literature [10].It was found that R 6 Fe 23 compounds crystallize in Th 6 Mn 23 type structure and with a space group of Fm-3m.
Rare earth 4f-Transition metals 3d intermetallic compounds of 6:23 stoichiometry show very interesting magnetic properties [11].The magnetic characterization of the rare earth 5d states is of special significance for the magnetic properties of the R-M materials as they mediate the magnetic interaction between the R and M sublattices [12].
Er 6 Fe 23 is of great interest among the compounds in the Er-Fe system, because it contains a large amount of iron in the unit cell.There are 116 atoms per unit cell occupying five non-equivalent crystal sites (24e, 4b, 24d, 32f 1 , and 32f 2 ).The magnetic properties of Er 6 Fe 23 were studied by Boltich et al. [10].This compound presents compensation temperature at 95 K and a ferrimagnetic behavior at 493 K.However, there have been few studies on Er 6 Fe 23 since then.This seems to be due to the difficulty entailed in preparing the Er 6 Fe 23 intermetallic compound.
Neutronic study on Dy 6 Mn 23 and Er 6 Mn 23 [13] shows that this collinear ferromagnetic arrangement of the Mn sublattice persists at low temperature when the R sublattice is ordered.Both neutron diffraction study and magnetization measurements on single crystals [13][14][15] provide that the R sublattice then adopts a complex non-collinear magnetic order.More importantly, the R-Mn exchange coupling is stronger with the 24d and 4b sites than with the 32f 1 and 32f 2 sites.This yields a behavior in apparent contradiction with the usual sign of the R-Mn coupling in intermetallic: in R 6 Mn 23 the low temperature magnetization is higher with the heavy lanthanides than with the light ones [16][17][18][19].
In the present work, we focused on the preparation, structure, magnetic properties, and magnetocaloric of the Er 6 Fe 23−x Al x (x = 0 and 3) compounds and the influence of Al on the magnetic properties.

Experimental
Polycrystalline samples of Er 6 Fe 23−x Al x (x = 0 and 3) were synthesized by arc-melting stoichiometric amounts of the constituent elements: Fe, Er, and Al (with purity 99.99%) under protective argon atmosphere.The pellets were turned over and remelted several times, in order to ensure a good homogeneity.The samples were wrapped in a tantalum sheet and annealed at 1073 K for one week to improve the atomic diffusion kinetics and to ensure a good crystallinity.After arc-melting, these alloys were finished by direct water quenching of the pellets from the furnace.The intermetallics were ground to powder by means of a mortar to prepare them for measurements.The powders were sieved using a 40 µm pore size metallic sieve.
The final product was characterized using powder X-ray diffraction (XRD) on a Brucker diffractometer (Brucker, Thiais, France) using copper radiation (λKα1 = 1.540600Å, λKα2 = 1.544390Å) to determine the crystallographic structure and identify the present phase.Forty milligrams of powder was loaded on a sample holder as a randomly oriented powder.Scans of 2θ between 20 • and 80 • with steps 0.015 • and counting times of 13.5 s per point were measured at 300 K.In this latter procedure, the Bragg peak breadth is a combination of both instrument and sample dependent effects.To remove these aberrations, it is necessary to assemble a diffraction pattern from the line broadening of a standard material with a good crystallinity such as silicon, to determine the instrumental broadening.Silicon is added as an internal standard, the pattern is established by the Rietveld process [20] using Fullprof code as a single-phase pattern.The Si unit-cell parameter fixed at a = 12.4312 Å and U = 0.007754, V = −0.010764,and W = 0.005756.The data of Er 6 Fe 20 Al 3 were analyzed and refined with the same technique of refinement using for silicon.U, V, W-half-width parameters normally characterizethe instrumental resolution function.
The magnetic and magnetocaloric measurements of the sample were performed using the Physical Properties Measurement System PPMS9 (Quantum Design), at low temperature, under an applied field up to 5 T in a temperature range from 10 K to 300 K and by the magnetometer SQUID: (Superconducting QUantum Interference Device) at high temperature, between 300 K and 500 K (for magnetic and magnetocaloric measurement).

X-ray Diffraction Results
Erbium ternary compounds of the Er-Fe-Al system contain a large number of intermetallic compounds were constructed at 1073 K in our laboratory [21].We extended our investigation in a systematic search of new interesting intermetallic compounds in this ternary diagram, and confirmed a Crystals 2017, 7, 156 3 of 10 significant extension by the X-ray analysis.This result showed that the samples contained single phase of Er 6 Fe 23−x Al x (x = 0 and 3).A comparison of the X-ray diffraction patterns of the Er 6 Fe 23−x Al x (x = 0 and 3) compounds revealed that there is no change in the patterns, except the shift in position of Bragg reflections, towards higher angle sides, indicating variation in the lattice parameters.The index results of X-ray diffraction patterns indicated that all the alloys crystallizes in the cubic Th 6 Mn 23 -type crystal structure, Th 6 Mn 23 -type structure with space group Fm-3m and lattice parameters a = 11.991-12.068Å.The variation of the unit cell parameters shows that the unit-cell parameters decrease with the increase of Al contents.There are 116 atoms per unit cell occupying five non equivalent crystal sites (24e, 4a, 24d, 32f 1 , and 32f 2 ).The 24e positions are accessible only to the rare earth atoms, whereas iron atoms occupy the other positions.
The analysis of the XRD patterns of the Er 6 Fe 20 Al 3 compound shows that this system crystallizes in the cubic Th 6 Mn 23 -type structure and Fm-3m space group.The lattice parameter is: a = 12.0082(2) Å.This value is slightly smaller than that of Er 6 Fe 23 (a = 12.01(3) Å) [22]; in agreement with a simple steric effect of Al (r Fe = 1.26 Å) substitution by a bigger atom (r Al = 1.43 Å).The representative Rietveld refinement results for ErFe 20 Al 3 are shown in Figure 1 and listed in Table 1.The observed and the calculated diffraction patterns for Er 6 Fe 20 Al 3 at room temperature match well and show that the sample contains the single Th 6 Mn 23 phase.
Crystals 2017, 7, 156 3 of 10 the Er6Fe23−xAlx (x = 0 and 3) compounds revealed that there is no change in the patterns, except the shift in position of Bragg reflections, towards higher angle sides, indicating variation in the lattice parameters.The index results of X-ray diffraction patterns indicated that all the alloys crystallizes in the cubic Th6Mn23-type crystal structure, Th6Mn23-type structure with space group Fm-3m and lattice parameters a = 11.991-12.068Å.The variation of the unit cell parameters shows that the unit-cell parameters decreasewith the increase of Al contents.There are 116 atoms per unit cell occupying five non equivalent crystal sites (24e, 4a, 24d, 32f1, and 32f2).The 24e positions are accessible only to the rare earth atoms, whereas iron atoms occupy the other positions.The analysis of the XRD patternsof the Er6Fe20Al3 compoundshows that this system crystallizes in the cubicTh6Mn23-type structure and Fm-3m space group.The lattice parameter is: a = 12.0082(2) Å.This value is slightly smaller than that of Er6Fe23 (a = 12.01(3) Å) [22]; in agreement with a simple steric effect of Al (rFe = 1.26 Å) substitution by a bigger atom (rAl = 1.43 Å).The representative Rietveld refinement results for ErFe20Al3 are shown in Figure 1 and listed in Table 1.The observed and the calculated diffraction patterns for Er6Fe20Al3 at room temperature match well and show that the sample contains the single Th6Mn23phase.
We have determined the distribution of Al atoms over the three possible Fe crystallographic sites.According to the Rietveld refinement and especially to the occupation refinement, we demonstrated and determined that the Al atoms are distributed in three crystallographic sites: 32f1, 32f2, and 24d.The best agreement factors RF and RB ofthe Er6Fe20Al3 compound have been obtained with Al located in the 32f1, 32f2, and 24d (Table 2).In fact, in our groups, we have provenin the Er6Fe17.66Al5.34C0.65 compound [23] that only one of the two 32f sites exhibited a significant deviation We have determined the distribution of Al atoms over the three possible Fe crystallographic sites.According to the Rietveld refinement and especially to the occupation refinement, we demonstrated and determined that the Al atoms are distributed in three crystallographic sites: 32f 1 , 32f 2 , and 24d.The best agreement factors R F and R B of the Er 6 Fe 20 Al 3 compound have been obtained with Al located Crystals 2017, 7, 156 4 of 10 in the 32f 1 , 32f 2 , and 24d (Table 2).In fact, in our groups, we have proven in the Er 6 Fe 17.66 Al 5.34 C 0.65 compound [23] that only one of the two 32f sites exhibited a significant deviation from the full occupancy with Fe atoms, indicating a substitution with Al atoms.Carbon atoms in this compound localized a new octahedral site.However, with the same structure type, Lemoine et al. [24] did not demonstrate the distribution of Co in the Gd 6 (Mn 1−x Co x ) 23 system.
In the structure of Er6Fe20Al3 Figure 2, all metal atoms have high coordination numbers, as is usually the case for intermetallic phases.The Er atoms are coordinated by 12 Fe/Al, one Fe, and four Er atoms.The Er-Er distances of 3.445(2) Å are rather large, considering the metallic radius of erbium for the coordination number (CN) 17.Nevertheless, if one discounts these erbium neighbors, the coordination polyhedron of the erbium atom would look rather incomplete.The Er-Fe/Al or Er-Fe distances cover the range between 2.963(3) Å and 3.588(2) Å.

Magnetic Properties
The Curie temperature is a direct measure of the exchange interaction, which is the origin of ferromagnetism.This interaction depends on the interatomic distance.In fact, T C is determined by three kinds of exchange interactions: 3d-3d exchange interaction between magnetic Fe moment sublattice, 4f-4f exchange interaction between the magnetic moments of rare earth sublattice and the inter-sublattice 3d-4f and 3d-5d exchange interaction, which probably favor a ferrimagnetic collinear structure and the anisotropies of the Er, Al, and Fe siteshad to be considered.
Figure 3 shows the thermomagnetic curve of Er 6 Fe 23−x Al x with x = 0 and 3.The value of T C (which are flagged with arrows) has been estimated from the minimum of the temperature of the magnetization, dM = dT, vs. temperature which has presented in the set of Figure 3.

Magnetic Properties
The Curie temperature is a direct measure of the exchange interaction, which is the origin of ferromagnetism.This interaction depends on the interatomic distance.In fact, TC is determined by three kinds of exchange interactions: 3d-3d exchange interaction between magnetic Fe moment sublattice, 4f-4f exchange interaction between the magnetic moments of rare earth sublattice and the inter-sublattice 3d-4f and 3d-5dexchange interaction, which probably favor a ferrimagnetic collinear structure and the anisotropies of the Er, Al, and Fe siteshad to be considered.
Figure 3 shows the thermomagnetic curve of Er6Fe23−xAlx with x = 0 and 3.The value of TC (which are flagged with arrows) has been estimated from the minimum of the temperature derivative of the magnetization, dM = dT, vs. temperature which has presented in the set of Figure 3.For x = 0, the TC is equal to 481 K.The magneticresultis in good agreement with those already reported [10,25].In the compound Er6Fe23, moments of erbium and iron couples antiparallel and the Curie temperature depends on the interactions Fe-Fe, Er-Fe and Er-Er [25].The large exchange interaction depends on the Fe-Fe distance.It is obvious that the Fe-Fe interactions play the most significant role in the magnetic order of Er6Fe23 compound.
The magnetic transition TC decreases from 481 K, for x = 0, to 380 K, for x = 3.We can concludein the case of ternary Er6Fe20Al3 that the interatomic-distance Fe-Fe decreases with increasing the content of aluminum.When Fe is substituted by non-magnetic Al, Fe-Fe interactions become weaker, leading to a Curie temperature decrease and a tendency magnetic disorder.According to the dilution law [26], when iron atoms are substituted by aluminum atoms, the magnetization of the sublattice decreases and, consequently, the molecular magnetic moment of the compound increases because the magnetization of the erbium sublattice in Er6Fe23 is larger than the iron sublattice.This decrease of Curie temperature is comparable with a system having a same type of structure (Th6Mn23), in the magnetic properties of Gd6(Mn1−xFex)23 alloys (x < 0.2) [27], the substitution of Mn by Fe leads to a decrease ofthe TC down to 175 K for x = 0.2 [27][28][29][30]31].
Figure 4 shows that the Er6Fe20Al3 compound has a minimum magnetization at 75 K.Indeed, the net moment at low temperature points in the direction of the Er moment, while in others, it points in the direction of the Fe moment, as evidenced by the presence of the compensation temperature at Tcomp = 75 K.Before this temperature, the Er free ion moment coupled antiparallel to the Fe moments.The magnetic properties of the Er6Fe20Al3 sample can readily be understood in reference to the Er6Fe23 compound from a model in which the Er moments are coupled antiparallel to the iron moments.This behavior is similar to the Er6Fe23 magnetic behavior (Tcomp = 95 K).This For x = 0, the T C is equal to 481 K.The magnetic result is in good agreement with those already reported [10,25].In the compound Er 6 Fe 23 , moments of erbium and iron couples antiparallel and the Curie temperature depends on the interactions Fe-Fe, Er-Fe and Er-Er [25].The large exchange interaction depends on the Fe-Fe distance.It is obvious that the Fe-Fe interactions play the most significant role in the magnetic order of Er 6 Fe 23 compound.
The magnetic transition T C decreases from 481 K, for x = 0, to 380 K, for x = 3.We can conclude in the case of ternary Er 6 Fe 20 Al 3 that the interatomic-distance Fe-Fe decreases with increasing the content of aluminum.When Fe is substituted by non-magnetic Al, Fe-Fe interactions become weaker, leading to a Curie temperature decrease and a tendency magnetic disorder.According to the dilution law [26], when iron atoms are substituted by aluminum atoms, the magnetization of the sublattice decreases and, consequently, the molecular magnetic moment of the compound increases because the magnetization of the erbium sublattice in Er 6 Fe 23 is larger than the iron sublattice.This decrease of Curie temperature is comparable with a system having a same type of structure (Th 6 Mn 23 ), in the magnetic properties of Gd 6 (Mn 1−x Fe x ) 23 alloys (x < 0.2) [27], the substitution of Mn by Fe leads to a decrease of the T C down to 175 K for x = 0.2 [27][28][29][30][31].
Figure 4 shows that the Er 6 Fe 20 Al 3 compound has a minimum magnetization at 75 K.Indeed, the net moment at low temperature points in the direction of the Er moment, while in others, it points in the direction of the Fe moment, as evidenced by the presence of the compensation temperature at T comp = 75 K.Before this temperature, the Er free ion moment coupled antiparallel to the Fe moments.The magnetic properties of the Er 6 Fe 20 Al 3 sample can readily be understood in reference to the Er 6 Fe 23 compound from a model in which the Er moments are coupled antiparallel to the iron moments.This behavior is similar to the Er 6 Fe 23 magnetic behavior (T comp = 95 K).This decrease is due to substitution by the Al atoms, and is comparable with the Er 6−x Y x Fe 23 [32].The magnetic saturations at 300 K of Er 6 Fe 23−x Al x (x = 0 and 3) are about 14.74 and 10.24 µ B , respectively.

Magnetocaloric Effect
In order to understand the influence of Al substitution in the magnetocaloric properties, we plotted the magnetization vs. the applied magnetic field (from 0 to 14 KOe) obtained at various temperatures for the Er6Fe23−xAlx (x = 0 and 3) samples in Figure 5.These curves provide evidence of a close relationship between the magnetization M and the applied magnetic field H. Below TC, the magnetization M increases sharply with magnetic applied field for (H < 0.5 kOe) and then saturates above 1 kOe.The saturation magnetization shifts to higher values with thedecreasing temperature.When the temperature approaches TC, the magnetization change becomes large.To explain with precision the Curie temperature and the transition nature, the Arrott plots obtained from the magnetic field dependence of isothermal magnetization, are shown in Figure 6.The Banerjee criterion [33]has been used to determine the nature of the magnetic phase transition in intermetallics.The positive or negative slope of M 2 vsμ0H/M (Arrott plot) curves indicates whether the magnetic phase transition is second or first order, respectively.In this compound, the positive slope of the Arrott plots confirms a characteristic of the second-ordermagnetictransition.

Magnetocaloric Effect
In order to understand the influence of Al substitution in the magnetocaloric properties, we plotted the magnetization vs. the applied magnetic field (from 0 to 14 KOe) obtained at various temperatures for the Er 6 Fe 23−x Al x (x = 0 and 3) samples in Figure 5.These curves provide evidence of a close relationship between the magnetization M and the applied magnetic field H. Below T C , the magnetization M increases sharply with magnetic applied field for (H < 0.5 kOe) and then saturates above 1 kOe.The saturation magnetization shifts to higher values with the decreasing temperature.When the temperature approaches T C , the magnetization change becomes large.decrease is due to substitution by the Al atoms, and is comparable with the Er6−xYxFe23 [32].The magnetic saturations at 300 K of Er6Fe23−xAlx (x = 0 and 3) are about 14.74 and 10.24 μB, respectively.

Magnetocaloric Effect
In order to understand the influence of Al substitution in the magnetocaloric properties, we plotted the magnetization vs. the applied magnetic field (from 0 to 14 KOe) obtained at various temperatures for the Er6Fe23−xAlx (x = 0 and 3) samples in Figure 5.These curves provide evidence of a close relationship between the magnetization M and the applied magnetic field H. Below TC, the magnetization M increases sharply with magnetic applied field for (H < 0.5 kOe) and then saturates above 1 kOe.The saturation magnetization shifts to higher values with thedecreasing temperature.When the temperature approaches TC, the magnetization change becomes large.To explain with precision the Curie temperature and the transition nature, the Arrott plots obtained from the magnetic field dependence of isothermal magnetization, are shown in Figure 6.The Banerjee criterion [33]has been used to determine the nature of the magnetic phase transition in intermetallics.The positive or negative slope of M 2 vsμ0H/M (Arrott plot) curves indicates whether the magnetic phase transition is second or first order, respectively.In this compound, the positive slope of the Arrott plots confirms a characteristic of the second-ordermagnetictransition.To explain with precision the Curie temperature and the transition nature, the Arrott plots obtained from the magnetic field dependence of isothermal magnetization, are shown in Figure 6.The Banerjee criterion [33] has been used to determine the nature of the magnetic phase transition in intermetallics.The positive or negative slope of M 2 vsµ 0 H/M (Arrott plot) curves indicates whether the magnetic phase transition is second or first order, respectively.In this compound, the positive slope of the Arrott plots confirms a characteristic of the second-order magnetic transition.The magnetocaloric properties of Er6Fe23−xAlx have been calculated from magnetization vs. applied magnetic field (M(H)) curves, measured temperatures within a 50 K interval centered around the TC of each sample.The desired entropy variation ΔS(T) is then investigated, following the Maxwell relation [34,35] (Equation( 4)) From Maxwell's thermodynamic relation (Equation ( 5)) One can obtain the following expression (Equation ( 6)) where M is the magnetization, T is the temperature, and Hmax is the maximum external field, with  = 1 (c.g.s).
The Figure 7 presents the magnetic entropy change ΔSM for both samples as a function of temperature under different magnetic fields.A remarkable feature is the exact correspondence between the maximal ΔSM and the recorded ferrimagnetic transition at TC, while ΔSM distributes over a relatively wide T range.The maximum entropy change of only 0.58 J/kg K and 1.8 J/kg K upon a filed variation of 14 kOewas obtained for Er6Fe20Al3 and Er6Fe23, respectively, indicating relatively small magnetic order fluctuations around the ferrimagnetic transitions.For the materials actually used in magnetic refrigeration, the most meaningful parameter is the relative cooling power (RCP).The RCP based on the magnetic entropy change ΔSM is defined as The magnetocaloric properties of Er 6 Fe 23−x Al x have been calculated from magnetization vs. applied magnetic field (M(H)) curves, measured for temperatures within a 50 K interval centered around the T C of each sample.The desired entropy variation ∆S(T) is then investigated, following the Maxwell relation [34,35] (Equation ( 1)) From Maxwell's thermodynamic relation (Equation ( 2)) One can obtain the following expression (Equation ( 3)) where M is the magnetization, T is the temperature,and µ 0 H max is the maximum external field, with µ 0 = 1 (c.g.s).The Figure 7 presents the magnetic entropy change ∆S M for both samples as a function of temperature under different magnetic fields.A remarkable feature is the exact correspondence between the maximal ∆S M and the recorded ferrimagnetic transition at T C , while ∆S M distributes over a relatively wide T range.The maximum entropy change of only 0.58 J/kg K and 1.8 J/kg K upon a filed variation of 14 kOe was obtained for Er 6 Fe 20 Al 3 and Er 6 Fe 23 , respectively, indicating relatively small magnetic order fluctuations around the ferrimagnetic transitions.
The Figure 7 presents the magnetic entropy change ΔSM for both samples as a function of temperature under different magnetic fields.A remarkable feature is the exact correspondence between the maximal ΔSM and the recorded ferrimagnetic transition at TC, while ΔSM distributes over a relatively wide T range.The maximum entropy change of only 0.58 J/kg K and 1.8 J/kg K upon a filed variation of 14 kOewas obtained for Er6Fe20Al3 and Er6Fe23, respectively, indicating relatively small magnetic order fluctuations around the ferrimagnetic transitions.For the materials actually used in magnetic refrigeration, the most meaningful parameter is the relative cooling power (RCP).The RCP based on the magnetic entropy change ΔSM is defined as [3,36,37]  For the materials actually used in magnetic refrigeration, the most meaningful parameter is the relative cooling power (RCP).The RCP based on the magnetic entropy change ∆S M is defined as [3,36,37] RCP = |−∆S M (T, H)| × δT FWHM (4) where δT FWHM is the full-width at half the maximum of the magnetic entropy change curve.The calculated RCP values for ∆H = 14 kOe are 28 and 42 J/kg K for Er 6 Fe 20 Al 3 and Er 6 Fe 23 , respectively.The RCP values show a difference between the two samples, due to the Fe/Al substitution.We list in Table 3 the T C , the ∆S M (max), and the RCP values for our materials with 14 kOe magnetic applied field in comparison with other results reported in the literature.The lower values of (∆S M (max)) compared with those of the Er 6 Fe 23−x Al x compounds are explained by the small value of the iron magnetic moment which is associated with a non-collinear Er magnetic structure.The magnetic entropy depends on the magnetic applied field and the nature of rare-earth.

Conclusions
In conclusion, we determined the effect of the Al substitution on the magnetic behavior, magnetic phase transition and magnetocaloric properties of Er 6 Fe 23−x Al x (x = 0 and 3) compounds.The structural study revealed that the samples with x = 0 and 3 were crystallized in the cubic Th 6 Mn 23 type structure.In addition, the Curie temperature decreased with increasing Al concentration.We have found that the samples under went a second-order magnetic phase transition.The decrease of T C was accompanied by a reduction of the isothermal entropy change.For a magnetic field change of 0-14 kOe, a maximum magnetic entropy values of 0.58 J/kg•K and 1.8 J/kg•K for Er 6 Fe 23−x Al x (x = 0 and 3) compounds are determined around T C .The values of this parameter are lower than those reported in Th 6 Mn 23 alloys, which can be attributed to the presence of the contribution of Er free moment antiparallel to the Fe moment in this system.
the structure of Er 6 Fe 20 Al 3 Figure 2, all metal atoms have high coordination numbers, as is usually the case for intermetallic phases.The Er atoms are coordinated by 12 Fe/Al, one Fe, and four Er atoms.The Er-Er distances of 3.445(2) Å are rather large, considering the metallic radius of erbium for the coordination number (CN) 17.Nevertheless, if one discounts these erbium neighbors, the coordination polyhedron of the erbium atom would look rather incomplete.The Er-Fe/Al or Er-Fe distances cover the range between 2.963(3) Å and 3.588(2) Å. Crystals 2017, 7, 156 4 of 10 from the full occupancy with Fe atoms, indicating a substitution with Al atoms.Carbon atoms in this compound localized a new octahedral site.However, with the same structure type, Lemoine et al.[24] did notdemonstrate the distribution of Co in the Gd6(Mn1−xCox)23 system.

Figure 2 .
Figure 2. Crystal structure of Er6Fe20Al3 compound and Er coordination.

Figure 2 .
Figure 2. Crystal structure of Er 6 Fe 20 Al 3 compound and Er coordination.

Figure 3 .
Figure 3. Temperature dependence of magnetization of Er6Fe23-xAlx with x = 0 and 3 under a low applied magnetic field μ0H = 0.1 T.

Figure 3 .
Figure 3. Temperature dependence of magnetization of Er 6 Fe 23-x Al x with x = 0 and 3 under a low applied magnetic field µ 0 H = 0.1 T.

Figure 4 .
Figure 4. Shows temperature dependences of magnetization for Er6Fe20Al3 compound under a magnetic field of0.05 T.

Figure 4 .
Figure 4. Shows temperature dependences of magnetization for Er 6 Fe 20 Al 3 compound under a magnetic field of 0.05 T.

Figure 4 .
Figure 4. Shows temperature dependences of magnetization for Er6Fe20Al3 compound under a magnetic field of0.05 T.

Figure 7 .
Figure 7. Isothermal magnetic entropy change around the Curie temperature for Er6Fe20Al3 (a) and Er6Fe23 (b) compounds.

Figure 7 .
Figure 7. Isothermal magnetic entropy change around the Curie temperature for Er6Fe20Al3 (a) and Er6Fe23 (b) compounds.

Figure 7 .
Figure 7. Isothermal magnetic entropy change around the Curie temperature for Er 6 Fe 20 Al 3 (a) and Er 6 Fe 23 (b) compounds.

Table 1 .
Details of structure refinement for Er6Fe20Al3.

Table 1 .
Details of structure refinement for Er 6 Fe 20 Al 3 .

Table 2 .
Atomic coordinates, occupations, and equivalent isotropic displacement parameters for Er6Fe20Al3

Table 3 .
Curie temperature T C , magnetic entropy change (∆S M ), and relative cooling power (RCP) for R 6 M