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Structural, Magnetic, and Mo1 ssbauer Study of U2Fe12Al5 A. P. Gonc¸ alves,† H. Noe¨l,‡ J. C. Waerenborgh,† and M. Almeida*,† Departamento de Quı´mica, Instituto Tecnolo´ gico e Nuclear, P-2686-953 Sacave´ m, Portugal, and Laboratoire de Chimie du Solide et Inorganique Mole´ culaire, UMR CNRS 6511, Universite´ de Rennes 1, Avenue de Ge´ ne´ ral Leclerc, 35042 Rennes, France Received March 8, 2002. Revised Manuscript Received July 4, 2002
The U2Fe12Al5 compound was prepared by arc melting, followed by annealing at 850 °C. This compound crystallizes in a structure derived from the Th2Ni17-type, (space group P63/ mmc, a ) 8.5631(7) and c ) 8.438(1) Å, R ) 0.050), with a partial delocalization of the uranium along the z-axis and with the iron atoms mainly located in the 6g and 12k positions. Magnetization measurements versus temperature indicate a ferromagnetic-type behavior below 295 K and a much less pronounced anomaly at 168 K, which was related with the existence of UFe2-based impurities. Measurements on magnetically oriented powder reveal that U2Fe12Al5 has a uniaxial anisotropy, with c as easy axis. At 2 K the anisotropy constant values are K1 ) 203 kJm-3, K2 ) 515 kJm-3, and K3 ) -132 kJm-3, and the spontaneous magnetization takes the 16.6 µB/fu value, significantly lower than the observed in R2Fe12Al5 with the nonmagnetic RdCe and Y. The 57Fe Mo¨ssbauer study shows that the hyperfine fields are smaller than in the similar R2Fe12Al5 compounds (R ) Ce, Nd, Sm, and Tm). The magnetic and Mo¨ssbauer results are discussed in terms of the interatomic distances and iron site preferences.
Introduction Binary R2Fe17 rare-earth compounds crystallize with the hexagonal Th2Ni17 and/or with the rhombohedral Th2Zn17-type structure,1 two closely related structures,2 the former being observed predominantly for the heavy and the later for the light lanthanides. In these compounds the iron atoms are located in four different crystallographic sites, labeled as 4f(6c), 6g(9d), 12j(18f), and 12k(18h) for the hexagonal (rhombohedral) phases, respectively. The iron moments are usually large, with a value close to the observed in iron metal.3 However, the R2Fe17 compounds have an easy-plane magnetic anisotropy at room temperature and the Curie temperatures are very low when compared with the iron metal or with the R2Co17 compounds.4 For many years these phases were considered not attractive for applications as permanent magnetic materials, but the discovery that the addition of interstitial atoms could enhance the intrinsic magnetic properties of the R2Fe175,6 has renewed the interest in the study of this class of compounds and their derivatives. * To whom correspondence should be addressed. Tel: (+351)-219946171. Fax: (+351)-21-9941455. E-mail:
[email protected]. † Departamento de Quı´mica. ‡ Laboratoire de Chimie du Solide et Inorganique Mole ´ culaire. (1) Iandelli, A.; Palenzona, A. Handbook on the Physics and Chemistry of Rare Earths; North-Holland Publishing Company: Amsterdam, 1979; Vol. 2, p 1. (2) Parthe´, E.; Chabot, B. Handbook on the Physics and Chemistry of Rare Earths; North-Holland Publishing Company: Amsterdam, 1984; Vol. 6, p 113. (3) Buschow, K. H. J. Rep. Prog. Phys. 1991, 54, 1123. (4) Kirchmayr, H. R.; Poldy, C. A. Handbook on the Physics and Chemistry of Rare Earths, North-Holland Publishing Company: Amsterdam, 1979; Vol. 2, p 55. (5) Mooij, D. B. de; Buschow, K. H. J. J. Less-Common Met. 1988, 142, 349. (6) Coey, J. M. D.; Sun, H. J. Magn. Magn. Mater. 1990, 87, L251.
A primary reason for the increase of the Curie temperature was proposed to be the expansion of the lattice parameters caused by the interstitial atoms,7 as the iron exchange interactions are usually strongly influenced by the Fe-Fe distances.8 An alternative route for expanding the lattice parameters is to partially substitute the iron by other elements with a larger metallic radius.9 Numerous R2Fe17-xMx (M ) Al, Si, and Ga) solid solutions have been extensively studied before.10-15 As expected, substituting aluminum and gallium for iron increases the Curie temperature, up to a maximum for x ) 3-4. However, albeit its smaller radii and a consequent contraction of the cell parameters, it was also observed that the introduction of silicon can have the same effect. This provides direct evidence that the expansion of the lattice parameters is not the only factor involved in the improving of the (7) Qi, Q.; Sun, H.; Skomski, R.; Coey, J. M. D. Phys. Rev. B 1992, 45, 12 278. (8) Givord, D.; Lemaine, R.; James, W. J.; Moreau, J. M.; Shah, J. S. IEEE Trans. Magn. 1971, MAG7, 657. (9) Jacobs, T. H.; Buschow, K. H. J.; Zhou, G.-F.; Li, X.; Boer, F. R. de. J. Magn. Magn. Mater. 1992, 116, 220. (10) Long, G. J.; Pringle, O. A.; Ezekwenna, P. C.; Mishra, S. R.; Hautot, D.; Grandjean, F. J. Magn. Magn. Mater. 1998, 186, 10. (11) Mishra, S. R.; Long, G. J.; Pringle, O. A.; Middleton, D. P.; Hu, Z.; Yelon, W. B.; Grandjean, F.; Buschow, K. H. J. J. Appl. Phys. 1996, 79, 3145. (12) Long, G. J.; Marasinghe, G. K.; Mishra, S.; Pringle, O. A.; Hu, Z.; Yelon, W. B.; Middleton, D. P.; Buschow, K. H. J.; Grandjean, F. J. Appl. Phys. 1994, 79, 5383. (13) Middleton, D. P.; Mishra, S. R.; Long, G. J.; Pringle, O. A.; Hu, Z.; Yelon, W. B.; Grandjean, F.; Buschow, K. H. J. J. Appl. Phys. 1995, 78, 5568. (14) Long, G. J.; Mishra, S. R.; Pringle, O. A.; Hu, Z.; Yelon, W. B.; Grandjean, F.; Middleton, D. P.; Buschow, K. H. J. J. Magn. Magn. Mater. 1997, 176, 217. (15) Hu, Z.; Yelon, W. B.; Mishra, S.; Long, G. J.; Pringle, O. A.; Middleton, D. P.; Buschow, K. H. J.; Grandjean, F. J. Appl. Phys. 1994, 76, 443.
10.1021/cm020253m CCC: $22.00 © 2002 American Chemical Society Published on Web 08/29/2002
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magnetic properties of the R2Fe17-based compounds. The mechanism of the magnetic changes due to these substitutions is not yet well understood, and a more complete investigation of such substituted compounds is necessary for a future production of new high-quality hard-magnetic materials. The U2Fe17 binary compound does not exists. However, it is possible to stabilize the Th2Ni17-type structure by alloying uranium and iron with a p element like silicon, germanium, and aluminum.16-21 From the ternary U2Fe17-xMx solid solutions with M ) Si, Ge, and Al, the aluminum-containing phase U2Fe12Al5 was briefly reported as the one having the highest cell parameters but the lowest Curie temperature (TC ) 298(5) K).21 The understanding of the reason for the decrease of the Curie temperature in this phase is dependent on a more detailed study of its structural and magnetic properties. In this work we report on the crystal structure determination, from powder and singlecrystal X-ray diffraction data, and on the magnetic properties, studied by magnetization and Mo¨ssbauer spectroscopy measurements, of the U2Fe12Al5 phase. Experimental Details Polycrystalline samples with U2Fe12Al5 nominal composition, weighting ∼0.5 g, were prepared by arc-melting the calculated amounts of the elements, with purity better than 99.9%, on a water-cooled copper crucible under Ti-gettered high-purity argon atmosphere. The samples were melted four times in order to ensure good homogeneity, with weight losses 0; a conical alignment when K1 < 0 and K1 + 2K2 > 0; an easy plane when K1 > 0 and K1 + K2 < 0, or K1 < 0 and K1 + 2K2 < 0.46 The calculated K1 and K2 values confirm the easy axis anisotropy type for U2Fe12Al5 at all temperatures up to TC. The anisotropy field (Ha) can be defined as being the minimum field needed to rotate the magnetic moments from the easy to the hard direction. According to this definition, for a compound with an easy axis anisotropy Ha can be calculated from the anisotropy constants and the spontaneous magnetization by the expression49
Ha ) (2K1 + 4K2 + 6K3)/MS
(3)
The comparison between the calculated values and the experimental data is also a good tool to determine the correctness of the anisotropy constant magnitudes. The experimental determination of Ha can be done directly from the dM2/dH2 curve as a function of the field:50 at the anisotropy field a singularity appears in the curve, as can be seen in Figure 10 where dM2/dH2 is plotted as a function of the field at 2 K. µ0Ha at 2 K is 3.1 T, a value higher than the observed in the Y2Fe12M5 (M ) Al or Ga) compounds,46,49 pointing to the direct influence of uranium in the magnetic properties of U2Fe12M5. In Figure 11 it is shown the temperature dependence of the experimental and calculated anisotropy fields as a function of temperature. There is a decrease of Ha with the increasing temperature down to a µ0Ha value of ∼1 T at 260 K, with an excellent agreement between the experimental data and calculated values at all temperatures, confirming the reliability of the estimated values for K1, K2, and K3. Mo1 ssbauer Measurements. The Mo¨ssbauer spectra (Figure 12) show that at 297 K all the iron atoms are paramagnetic and between this temperature and 272 K magnetic ordering is established, confirming the previous supposition of the iron magnetic ordering at the Curie temperature. Just by looking at the 10 K spectrum of UFe12Al5 (Figure 12) and considering the velocity range where (50) Kirchmayr, H. Supermagnets, Hard Magnetic Materials; NATO ASI Series C; Kluwer Academic Publishers: Dordrecht, 1991; Vol. 331, p 449.
U2Fe12Al5
Figure 11. Temperature dependence of the U2Fe12Al5 experimental and calculated anisotropy fields.
Figure 12. 57Fe Mo¨ssbauer spectra of U2Fe12Al5, obtained at different temperatures. The calculated curve plotted on the experimental points is the sum of 15 sextets corresponding to iron on the four crystallographic sites and with different numbers of iron nearest neighbors, as explained in the text. The individual sextets are shown slightly shifted.
most of the total absorption area is observed, it becomes obvious that the magnetic hyperfine fields, Bhf, corresponding to the saturated iron magnetic moments in UFe12Al5 are significantly lower than those observed in the rare-earth-based compounds with a similar degree of substitution of iron by aluminum, namely, R2Fe17-xAlx where x ) 5 and R ) Nd, Tb, or Ce51-53; x ) 4 and R ) Sm54; or x ) 2 and R ) Y.55 This is in agreement with the decrease of the iron magnetic moments as the reason for the reduction of the U2Fe12Al5 spontaneous magnetization, when compared with those observed in the rare-earth compounds. Considering that the magnetically aligned powder X-ray diffractogram and the magnetization measurements indicate uniaxial anisotropy, no splitting of the hyperfine patterns of iron on the same crystallographic (51) Long, G. J.; Marasinghe, G. K.; Mishra, S.; Pringle, O. A.; Hu, Z.; Yelon, W. B.; Middleton, D. P.; Buschow, K. H. J.; Grandjean, F. J. Appl. Phys. 1994, 76, 5383. (52) Mishra, S.; Long, G. J.; Pringle, O. A.; Marasinghe, G. K.; Middleton, D. P.; Buschow, K. H. J.; Grandjean, F. J. Magn. Magn. Mater. 1996, 162, 167. (53) Mishra, S.; Long, G. J.; Pringle, O. A.; Middleton, D. P.; Hu, Z.; Yelon, W. B.; Grandjean, F.; Buschow, K. H. J. J. Appl. Phys. 1996, 79, 3145. (54) Long, G. J.; Pringle, O. A.; Ezekwenna, P. C.; Mishra, S. R.; Hautot, D.; Grandjean, F. J. Magn. Magn. Mater. 1998, 186, 10. (55) Morariu, M.; Rogalski, M. S.; Plugaru, N.; Valeanu, M.; Lazar, D. P. Solid State Commun. 1994, 92, 889.
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site is expected due to the anisotropy of the dipolar and orbital contributions to Bhf or to the different directions of Bhf relative to the electric quadrupole axes.56 Nevertheless, in UFe12Al5, the iron atoms on the same crystallographic position may have different Bhf corresponding to different numbers of aluminum nearest neighbors as generally observed in Fe-Al containing intermetallics. The dependence of Bhf on the number of aluminum nearest neighbors has been observed not only in the above referred R2Fe17-xAlx analogues, but also in other ternary intermetallics such as those crystallizing in the ThMn12-type structure27 or in the Fe-Al binaries.57 As explained by Long et al.54 and Waerenborgh et al.27 in the cases of R2Fe17-xAlx and UFexAl12-x, respectively, by assuming that aluminum substitutes iron randomly on the 4f, 12j, and 12k sites of U2Fe12Al5 with the site occupation factors estimated from diffraction data (Table 2), it is possible, using the binomial distribution function, to calculate for each crystallographic site, s, the probability of finding m iron atoms in a shell of n nearest neighbor sites. From these probabilities the fraction Ps(z) of iron atoms on each site s and with z iron nearest neighbors may be obtained as explained before for the case of the ThMn12-type compounds.27 When analyzing the Mo¨ssbauer spectrum, the iron atoms on a specific crystallographic site are represented by several magnetic sextets corresponding to iron atoms with a different z. The relative areas of each sextet, I, should be constant and equal to Ps(z). In the U2Fe12Al5 case, where there is a strong overlapping of the sextets, the contributions of subspectra with I < 3.5% were not individually considered. Their I was summed to that of the more intense sextet corresponding to the iron atoms on the same site and with z differing by one. As a consequence of this simplification, only one sextet could be considered for the 4f sites as a whole, since they only contain 8.5% of the total iron in the compound. As a further simplification, and in agreement with the models used for the analyses of the spectra of R2Fe17-xAlx54 and AFe12-xAlx with A ) U or Y,27,58 the quadrupole shifts, , of all the Fe atoms on the same site were assumed to be equal. Since each configuration of nearest neighbors should give rise to different electric field gradients, each sextet therefore represents a small distribution of magnetic splittings with different and it is not surprising that the widths of the peaks are larger than those observed in the calibration spectrum obtained with an iron foil. The widths of the innerpeak pairs of all sextets (final values Γ3,4 ) 0.37) were kept equal during refinement except for the 4f sextet which, as explained above, represents a distribution not only of but also of Bhf and of isomer shifts, δ. In this case Γ3,4 ) 0.43 was even larger. The ratio of the line widths of the outer lines relative to the inner lines are larger than in the other cases as well, Γ1,6/Γ3,4 ≈ 1.5 > 1.2 and Γ2,5/Γ3,4 ≈ 1.2 > 1.1. δ and Bhf for the iron atoms with the largest Ps(z) on each site as well as the change in Bhf when one (56) Gubbens, P. C. M.; Buschow, K. H. J. J. Phys. C6 1974, 35, 617. (57) Dubiel, S. M.; Zinn, W. J. Magn. Magn. Mat. 1984, 45, 298. (58) Waerenborgh, J. C.; Salamakha, P.; Sologub, O.; Gonc¸ alves, A. P.; Se´rio, S.; Godinho, M.; Almeida, M. J. Alloys Compd. 2001, 317318, 44.
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Chem. Mater., Vol. 14, No. 10, 2002 Table 6. Magnetic Parameters of the U2Fe12Al5
T (K)
MS (µB)
K1 (kJm-3)
2 50 98 154 191 244 262 273
16.6 16.2 15.6 14.0 12.7 10.5 9.4 8.7
203 177 164 139 108 72 48 39
K2 (kJm-3)
K3 (kJm-3)
µ0Haexp (T)
µ0Hacalc (T)
515 475 396 254 194 85 71 50
-132 -112 -81 -29 -34 -3 -
3.1 2.9 2.7 2.3 1.9 1.1 0.9 -
2.92 2.84 2.64 2.32 1.80 1.28 1.02 -
Table 7. Estimated Parameters from the Mo1 ssbauer Spectrum of UFe12Al5 Taken at 5 K; E ) (e2VZZQ/4) (3cos2 θ - 1), Calculated from (O1 + O6 - O2 - O5)/2 where On Is the Shift of the nth Line of the Magnetic Sextet Due to Quadrupole Coupling site
z
I (%)
δ (mm/s)
12k
g8 7 6 5 e4 g8 7 6 5 e4 g8 7 6 e5 9
5.0 9.4 11.1 7.8 4.0 4.2 5.8 6.7 5.2 4.2 9.9 8.1 6.1 3.9 8.5
0.05(2) 0.06(2) 0.07(2) 0.08(2) 0.09(2) 0.08(2) 0.09(2) 0.10(2) 0.11(2) 0.12(2) 0.05(2) 0.06(2) 0.07(2) 0.08(2) 0.04(2)
6g
12j
4f
(mm/s)
Bhf (T)
0.13(2)
12.6(2) 11.6(2) 10.6(2) 9.6(2) 8.6(2) 9.2(2) 7.7(2) 6.2(2) 4.7(2) 3.2(2) 14.4(2) 13.2(2) 12.0(2) 10.8(2) 14.6(2)
0.09(2)
0.17(2)
-0.31(2)
aluminum replaces one iron atom in the near-neighbor environment of the site were allowed to vary together with the total area of the spectrum. The best fits to the spectrum were always obtained if the estimated δ values for the different crystallographic sites were similar. On the other hand if the δ value for each site was allowed to vary, as the Bhf, by a fixed increment when one aluminum replaces one iron atom in the nearest neighbor environment, the quality of the fitting further improved. This suggests that δ of iron in U2Fe15Al2 is sensitive to the number of iron and aluminum nearest neighbors as in the Fe-Al binaries or on the 8f sites of the AFexAl12-x compounds. The best fit was obtained for the parameters summarized in Table 7. The Bhf values increase with z, and for iron atoms on different sites with the same z, they appear to increase with the average Fe-Fe interatomic distances. δ, as referred above, seem to increase with the number of aluminum nearest neighbors. Although most of the differences found are smaller than the experimental error (Table 7) this trend was expected because, as observed in the other Fe-Al containing intermetallics, there is a mixing of the 3d-band electrons of iron with the aluminum 3p valence band, and the consequent 3d-4s intratomic iron electronic redistribution is reflected in an additional screening of the 4s electrons. In the case of the U2Fe12Al5 spectrum, where the average values of Bhf are lower than in the R2Fe17-xAlx spectra, a much stronger degree of overlapping of the magnetic hyperfine splittings occurs. The detailed analysis performed in the present study is therefore less reliable than those performed for the R2Fe17-xAlx intermetallics, in the sense that it is more likely to find alternative models that may fit the U2Fe12Al5 spectrum equally well and which are also physically reasonable.
The present Mo¨ssbauer study, however, clearly shows that the Bhf values in all the crystallographic sites of U2Fe12Al5 are lower than the corresponding Bhf in R2Fe17-xAlx with x ≈ 5 and R ) Ce, Nd, Sm, and Tb, which agrees with the lower iron magnetic moments deduced from magnetization data. A stronger hybridization between iron and uranium as compared to iron and yttrium or the rare earths may explain the lower Bhf in U2Fe12Al5. The U-Fe interatomic distances, particularly in the case of the iron atoms on the 4f and 12j sites, e 3.0 Å (Table 5), are sufficiently low to justify this hybridization. In the ThMn12-type UFexAl12-x compounds with 4 e x e 5.6 the Fe-U interatomic distances are g3.1 Å.59 Although a reduction in Bhf is not observed between RFe4Al8 (RdY, Tm, or Lu) and UFe4Al8, the Bhf of iron on the 8j sites or on the 8f sites with three iron nearest neighbors are significantly lower in UFe4.2Al7.8 than that in RFe4.2Al7.8.27,60 Conclusion The U2Fe12Al5 compound was prepared by arc melting, followed by annealing at 850 °C. It crystallizes in a structure derived from the Th2Ni17-type, with a partial delocalization of the 2b uranium along the z-axis. The lack of vertical constraints and the presence of nearest equatorial atoms are probably the reasons for the U2 (2b) delocalization. Magnetization versus temperature measurements indicate a ferromagnetic-type behavior below TC ) 295 K, significantly lower than the TC observed in other R2Fe12Al5 compounds. A comparison between the iron occupation factors points to a decisive influence of the number of the magnetic nearest neighbors on the Curie temperature of this family of compounds. The UFe12Al5 spontaneous magnetization is also significantly lower than that observed for R2Fe12Al5 (R ) Y and Ce), due to the decrease of the iron magnetic moments, which was confirmed by Mo¨ssbauer spectroscopy. A stronger hybridization between iron and uranium, as compared to iron and yttrium or the rare earths, may explain the lower Bhf in U2Fe12Al5. The Bhf values increase with the number of iron neighbors, and for iron atoms on different sites with the same number of iron neighbors, they appear to increase with the average Fe-Fe interatomic distances. U2Fe12Al5 has a uniaxial anisotropy, with c as easy axis. The need of high order anisotropy constants for the correct description of the magnetization points to an important uranium contribution to the magnetism and to a coupling between the iron and the uranium sublattices up to temperatures close to TC, which can influence the Curie temperature. Acknowledgment. This work was partially supported by the exchange Program ICCTI/CNRS. CM020253M (59) Gonc¸ alves, A. P.; Estrela, P.; Waerenborgh, J. C.; Paixa˜o, J. A.; Bonnet, M.; Spirlet, J. C.; Godinho, M.; Almeida, M. J. Magn. Magn. Mater. 1998, 189, 283. (60) Waerenborgh, J. C.; Salamakha, P.; Sologub, O.; Gonc¸ alves, A. P.; Cardoso, C.; Se´rio, S.; Godinho, M.; Almeida, M. Chem. Mater. 2000, 12, 1743.
