- Email: [email protected]
Crystallographic and magnetic properties of HfFe6Ge6-type REFe6Sn4Ge2 compounds (RE = Y, Gd–Er)
Journal of Alloys and Compounds 400 (2005) 37–42
Crystallographic and magnetic properties of HfFe6Ge6-type REFe6Sn4Ge2 compounds (RE = Y, Gd–Er) G. Venturini Laboratoire de Chimie du Solide Min´eral, Universit´e Henri Poincar´e-Nancy I, associ´e au CNRS (UMR 7555), B.P. 239, 54506 Vandoeuvre les Nancy Cedex, France Received 18 March 2005; accepted 31 March 2005 Available online 29 April 2005
Abstract The REFe6 Sn4 Ge2 (RE = Y, Gd–Er) compounds have been synthesized and studied by powder X-ray diffraction and magnetisation measurements. These compounds crystallize in the hexagonal HfFe6 Ge6 structure although the parent ternary compounds REFe6 X6 (X = Ge, Sn) display more complicated orthorhombic crystal structure. This evolution is discussed and interpreted on the basis of the relaxation of some RE–X contacts in the quaternary compounds. The iron sublattice order antiferromagnetically above room temperature (554 ≤ TN ≤ 560 K) while the paramagnetic RE compounds display a second transition at low temperature (7.3 ≤ Tt ≤ 42.7 K). The magnetisation versus field curves display a metamagnetic behaviour at 4.2 K. The corresponding value of the magnetisation suggests a non-collinear ordering of the RE sublattice. © 2005 Elsevier B.V. All rights reserved. Keywords: Rare earth iron tin germanium; Crystal-chemistry; Antiferromagnetism
1. Introduction The crystallographic and magnetic properties of the REFe6 Ge6 and REFe6 Sn6 compounds have been extensively studied [1–5]. Most of the paramagnetic rare earth element compounds crystallize in rather complicated structures resulting from different locations of the RE element in the host CoSn-type sub-structure. Several studies have evidenced some problems regarding the crystallographic ordering of the RE sites and have shown partial disorder depending on the thermal history of the samples [6–10]. The magnetic studies suggested an independent magnetic ordering of the RE and Fe sublattices and, in most of the cases, a non-collinear arrangement of the RE moments with a simultaneous presence of ferro-, antiferro- and/or helimagnetic components [8–14]. These problems are complicated by the presence of several different crystallographic RE sites and by the atomic disorder on these sites.
E-mail address: [email protected]. 0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.03.058
Recent studies of ErFe6 Sn6−x Gax and REMn6 Sn4 Ge2 (RE = Nd, Sm) substituted compounds have shown that the partial substitution of Sn by the smaller atoms Ge or Ga enabled the stabilisation of the more simple hexagonal HfFe6 Ge6 structure [15,16]. This one, being characterized by only one crystallographic transition metal and rare earth site, should allow a more simple examination of the order of the two magnetic sublattices and of their interplay. Attempts to synthesize the REFe6 Sn4 Ge2 compounds under the HfFe6 Ge6 form succeeded for RE = Y, Gd–Er. The crystallographic properties and the bulk magnetisation measurements of these phases are presented. 2. Experimental The REFe6 Sn4 Ge2 compounds have been synthesized from the elements melted in an induction furnace. The resulting ingots are annealed at 1123 K for 1 month. After this first stage, it is found that the samples still contain large amounts of a CeNiSi2 -type REFex Sn2 phase [17]. The ingot is therefore finely ground, compacted again into pellets and annealed for
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42
38
one additional month. The samples are checked by powder X-ray (Guinier camera Co K␣1 ). It is observed that the main remaining impurities are elemental tin and RE2 O3 oxide. The powder diffraction data have been collected on a Philips Xpert Pro powder diffractometer (Cu K␣) and the crystal structure has been refined using the FULLPROF software [18]. The magnetisation measurements have been performed on a MANICS magneto-susceptometer in the temperature range 4.2–700 K and in fields up to 1.5 T. The N´eel point of the iron sublattice which is not clearly observed by magnetisation measurements has been checked using a Mettler DSC30 differential scanning calorimeter.
3. Results 3.1. Crystallographic properties The analysis of the powder diffraction data of the REFe6 Sn4 Ge2 compounds clearly evidences the typical (h k 3) lines of the hexagonal HfFe6 Ge6 structure [P6/mmm; Hf in 1(a) (0, 0, 0); Fe in 6(i) (1/2, 0, z); Ge1 in 2(d) (1/3, 2/3, 1/2); Ge2 in 2(c) (1/3, 2/3, 0); Ge3 in 2(e) (0, 0, z)] (Figs. 1 and 2). A summary of the crystallographic properties is given in Table 1. Different refined occupation factors were found for the three tin sites with a particular low value for the Sn2 site thus indicating the preferential location of the germanium atoms on the 2(c) site. Correlatively, the zFe coordinate significantly deviates from the idealized zFe ≈ 0.25 value usually measured in ternary HfFe6 Ge6 compounds. This feature indicates that the iron atoms move towards the 2(c) site occupied by the smaller germanium atom in fair accordance with the calculated inter˚ dFe–Ge(2c) = 2.555 A ˚ in atomic distances (dFe–Sn(2d) = 2.763 A; GdFe6 Sn4 Ge2 ). The results of microprobe analysis confirm the initial stoichiometry of the samples (Table 2). There is not any significant under-stoichiometry in the RE element as observed in several HfFe6 Ge6 -type REFe6 Sn6 compounds [19]. 3.2. Magnetic properties The thermomagnetic and isotherm curves are displayed in Figs. 3 and 4 and a summary of the magnetic properties is gathered in Table 3. A very weak N´eel point may be observed at 518 K in the thermomagnetic curve of YFe6 Sn4 Ge2
Fig. 1. Representation of the HfFe6 Ge6 -type ErFe6 Sn4 Ge2 compound.
Fig. 2. Observed and calculated X-ray pattern of ErFe6 Sn4 Ge2 (upper tik: HfFe6 Ge6 -type lines, medium tik: Sn lines, lower tik: Er2 O3 lines).
but this transition is less detectable in the curves of the other RE compounds due to the large paramagnetic contribution of the RE element. However, this transition displays a small endothermic peak upon heating in differential thermal analysis and the N´eel temperature ranging between 553 and 560 K, has been measured by this way. As previously reported for the corresponding ternary germanides and stannides [12], there is no strong influence of the RE element on the N´eel
Table 1 Refined cell parameters, atomic coordinates and occupation factors of REFe6 Sn≈4 Ge≈2 compounds (RE = Y, Gd, Tb, Dy, Ho, Er) RE
˚ a (A)
˚ c (A)
zSn
zFe
mSn1
mSn2
mSn3
χ2
RBragg
Rf
Y Gd Tb Dy Ho Er
5.2936 (1) 5.3041 (1) 5.2876 (1) 5.2879 (1) 5.2875 (1) 5.2806 (1)
8.6649 (2) 8.6910 (2) 8.6510 (2) 8.6579 (2) 8.6623 (1) 8.6554 (2)
0.3385 (3) 0.3391 (4) 0.3381 (3) 0.3363 (4) 0.3385 (3) 0.3358 (3)
0.2326 (3) 0.2353 (5) 0.2345 (3) 0.2329 (4) 0.2351 (3) 0.2359 (4)
0.993 (6) 0.949 (8) 0.941 (4) 0.959 (6) 0.921 (5) 0.936 (6)
0.653 (5) 0.654 (7) 0.637 (4) 0.625 (5) 0.646 (5) 0.674 (5)
0.927 (5) 0.935 (9) 0.968 (5) 0.961 (6) 0.938 (6) 0.997 (7)
3.9 4.0 2.6 3.5 4.6 5.1
8.9 11.3 5.9 7.4 7.5 10.5
7.3 11.5 6.1 8.6 7.4 10.7
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42 Table 2 Atomic percentage in REFe6 Sn4 Ge2 compounds measured by microprobe analysis RE
%RE
%Fe
%Sn
%Ge
Y Gd Tb Dy Ho Er
7.19 (0.93) 7.47 (0.97) 7.64 (0.99) 7.50 (0.98) 7.62 (0.99) 7.82 (1.02)
46.64 (6.03) 46.49 (6.02) 46.44 (6.03) 46.15 (5.99) 46.36 (6.02) 46.51 (6.06)
30.54 (3.95) 30.43 (3.95) 30.18 (3.92) 30.76 (3.99) 30.88 (4.01) 31.04 (4.04)
15.63 (2.02) 15.60 (2.03) 15.74 (2.05) 15.58 (2.02) 15.14 (1.97) 14.63 (1.90)
The number of atoms assuming a REx (Fe1−y−z Sny Gez )12 formula is given in parentheses.
39
temperature of the iron sublattice. It is also worth noting that the ordering point of the REFe6 Sn4 Ge2 compounds is more close to the N´eel temperature of the germanides than to that of the stannides, thus suggesting a non-linear dependence on the Ge concentration. The thermomagnetic curves are characterized by a strong increase of the magnetisation at low temperature without displaying an evident ordering point. However, the first derivate dM/dT displays a more or less pronounced anomaly which might be attributed to the ordering of the RE sublattice. The transition temperatures decrease from 42.7 K
Fig. 3. Thermomagnetic curves of the REFe6 Sn4 Ge2 compounds (RE = Y, Gd–Er) under an applied field of 0.2 T (Inset shows a zoom of the low temperature part of the curves).
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42
40
Fig. 4. Magnetisation loops of the REFe6 Sn4 Ge2 compounds (RE = Y, Gd–Er) recorded at 4.2 and 60 K.
Table 3 Magnetic properties of the REFe6 Sn4 Ge2 compounds (reported transition temperatures of ternary germanides and stannides [12] are given for comparison)
Y Gd Tb Dy Ho Er
TN (K)
TN Sn (K)
TN Ge (K)
Tt (K)
Tt Sn (K)
Tt Ge (K)
Mmax (µB /fu)
µRE (theor) (µB /fu)
Hc (T)
518 518 518 516 517 515
558 554 553 559 559 560
486 489 490 489 484 484
– 42.7 25.4 15.4 12.2 7.3
– 45 19 14 8 4
– 29.3 7.8 7.5 8.0 3.1
0.09 3.8 3.9 4.2 5.3 3.7
0 7 9 10 10 9
– 0.35 0.28 0.21 0.15 –
Mmax : maximum magnetisation under H = 1.5 T; Hc : critical field of the metamagnetic transition.
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42
for the gadolinium compound to 7.3 K for the erbium compound. The magnetisation versus field curve of YFe6 Sn4 Ge2 indicates a very small susceptibility of the iron sublattice (χ ≈ 3 × 10−5 emu/g). This suggests a strong antiferromagnetic behaviour of this sublattice and should be related to the weak N´eel point of these compounds. The isotherm curves of the paramagnetic RE compounds recorded at 4.2 K display, except for ErFe6 Sn4 Ge2 , a metamagnetic transition characterized by a hysteretic behaviour and by a rather weak critical field decreasing on going from the gadolinium compound (Hc = 0.35 T) to the holmium compound (Hc = 0.15 T). The maximum magnetisation values, ranging between 3.8 µB /fu for the Gd compound to 5.4 µB /fu for the Ho compound, remain smaller than the theoretical free RE ion value. According to the Fe magnetic moment measured in YFe6 Ge6 (µFe = 1.88 µB ) and YFe6 Sn6 (µFe = 2.14 µB ) [20,21], the measured magnetisation value does not also fairly well account for a ferrimagnetic arrangement of the RE and iron sublattices.
4. Discussion Substituting a part of the tin atoms by germanium atoms in REFe6 Sn6 compounds yields deep modifications of the crystal chemistry of these phases. The simple hexagonal HfFe6 Ge6 -type structure is stabilised for REFe6 Sn4 Ge2 although the corresponding REFe6 Sn6 and REFe6 Ge6 compounds display more complicated orthorhombic structures which have been described as intergrowths of HfFe6 Ge6 and ScFe6 Ga6 blocks [1]. The origin of this effect is probably connected to a geometrical factor and to the different sizes of the Ge and Sn atoms. This assumption is corroborated by several facts: • The occurrence of the intergrowth structures in the REFe6 Sn6 and REFe6 Ge6 compounds is related to the size of the RE element since the compounds of the smallest ones (Sc, Lu, Tm) are isotypic with HfFe6 Ge6 . This evolution suggests that some RE contacts become too tight in the HfFe6 Ge6 structure when the size of RE element increases. • The occurrence of the intergrowth structures in the REMn6 Sn6 compounds is limited to the largest RE elements only (Pr, Nd, Sm) [22] and most of these compounds crystallize in the HfFe6 Ge6 structure. The relative behaviour of iron and manganese compounds should be related to the different sizes of these elements: the manganese compounds, characterized by larger unit cell, display less tight RE–Sn contacts. • Finally, the single crystal refinement of the TbFe6 Sn6 type DyFe6 Sn6 compound indicated atomic displacements yielding the increase of several Dy–Sn distances within the HfFe6 Ge6 blocks [5].
41
All these features suggest that the stabilisation of the intergrowth structures is governed by an optimisation of some RE–Sn(Ge) contacts within the pseudo-hexagonal (RE,Sn(Ge)) planes. On the other hand, the study of HfFe6 Ge6 -type REMn6 Sn6−x Gax , ErFe6 Sn6−x Gax and REMn6 Sn6−x Gex compounds indicated that the substituting atom Ga or Ge is mainly located at the Sn 2(c) site [15,16,23]. This site preference has been related to chemical bonding consideration: the occupation of this site and the corresponding atomic shifts in the HfFe6 Ge6 structure enable a tightening of the heteroatomic RE–Mn(Fe) contacts. The substitution of Sn by the smaller Ga or Ge atoms yields a decrease of the unit cell volume but the distance between the atom lying in the 2(c) position and the RE atom remains larger than the distance in the hypothetical corresponding HfFe6 Ge6 -type germanide or gallide as suggested by the comparison of the Lu–Ge distance ˚ in the HfFe6 Ge6 -type LuFe6 Ge6 compound (d = 2.942 A) ˚ in ErFe6 Sn4 Ge2 . [2] and the Er–Ge distance (d = 3.049 A) Therefore, the RE–X2(c) distance in the REFe6 Sn4 Ge2 compounds becomes much less critical than in the corresponding REFe6 X6 compounds (X = Ge, Sn). As it is suggested that this contact is responsible for the stabilisation of the intergrowth structures along the REFe6 X6 series, it seems understandable that the HfFe6 Ge6 structure may be stabilised in the substituted REFe6 Sn4 Ge2 compounds. The stabilisation of the HfFe6 Ge6 structure for the REFe6 Sn4 Ge2 compounds lead to a strongly different tridimensional arrangement of the rare earth atoms with respect to their arrangements in the intergrowth structures observed for the ternary REFe6 Ge6 and REFe6 Sn6 compounds. Particularly, the RE atoms build infinite hexagonal (0 0 1) planes stacked along the c axis in the HfFe6 Ge6 -type while limited RE slabs building complicated stackings are characteristic of the intergrowth structures. Therefore, it is suggested that these various arrangements may lead to different coupling scheme. However, according to the present magnetisation measurements, the ordering temperatures of the REFe6 Sn4 Ge2 compounds remain close to those measured in the corresponding ternary stannides thus suggesting that the strength of the magnetic interactions does not drastically change in the substituted stannides. Most of the neutron diffraction studies undertaken on the REFe6 X6 compounds (X = Ge, Sn) show that the magnetic arrangement of the RE sublattice was characterized by two components ferromagnetically and antiferromagnetically coupled. This observation should be related to the present results of the magnetisation versus field study. It has been observed that the curves displayed a metamagnetic behaviour with a weak critical field giving rise to a maximum magnetisation value significantly lower than the theoretical free ion value. This might indicate that the RE sublattice in the REFe6 Sn4 Ge2 compounds is characterized, like in the ternary compounds, by two magnetic components: a weakly antiferromagnetic one giving rise to the metamagnetic behaviour and an other one more strongly antifer-
42
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42
romagnetic accounting for the reduced maximum magnetisation. This suggests that the main features of the RE ordering in ternary compounds are retained in the substituted stannides.
5. Conclusions A better understanding of the magnetic properties of the HfFe6 Ge6 -type REFe6 Sn4 Ge2 compounds needs now further investigations with either neutron or M¨ossbauer experiments. The relatively simple crystal structure of these compounds and the fair ordering of the RE atoms should make possible an accurate determination of the magnetic structures. It will also be interesting to examine the correlations between the three-dimensional arrangement of the RE sublattice and its magnetic behaviour.
References [1] B. Chafik El Idrissi, G. Venturini, B. Malaman, Mater. Res. Bull. 26 (12) (1991) 1331. [2] G. Venturini, R. Welter, B. Malaman, J. Alloys Compd. 185 (1992) 99. [3] Y.B. Wang, D. Wiarda, D.H. Ryan, J.M. Cadogan, IEEE Trans. Magn. 30 (6) (1994) 495. [4] X.L. Rao, M.D. Coey, J. Appl. Phys. 81 (8) (1997) 5227.
[5] O. Oleksyn, H. Bohm, Z. Kristall. 213 (5) (1998) 270. [6] O. Oleksyn, H.U. Nissen, R. Wessicken, Philos. Mag. Lett. 77 (5) (1998) 275. [7] O. Zaharko, P. Schobinger-Papamantellos, C. Ritter, J. RodriguezCarvajal, K.H.J. Buschow, J. Magn. Magn. Mater. 187 (1998) 293. [8] P. Schobinger-Papamantellos, O. Oleksyn, J. Rodriguez-Carvajal, G. Andr´e, E. Bruck, K.H.J. Buschow, J. Magn. Magn. Mater. 182 (1–2) (1998) 96. [9] O. Oleksyn, P. Schobinger-Papamantellos, J. Rodriguez-Carvajal, E. Br¨uck, K.H.J. Buschow, J. Alloys Compd. 257 (1997) 36. [10] O. Zaharko, P. Schobinger-Papamantellos, J. Rodriguez-Carvajal, K.H.J. Buschow, J. Alloys Compd. 288 (1–2) (1999) 50. [11] D.H. Ryan, J.M. Cadogan, J. Appl. Phys. 79 (8) (1996) 6004. [12] J.M. Cadogan, D.H. Ryan, J. Alloys Compd. 326 (2001) 166. [13] J.M. Cadogan, D.H. Ryan, I.P. Swainson, J. Phys. Condens. Matter 12 (2000) 8963–8971. [14] J.M. Cadogan, D.H. Ryan, O. Moze, Suharyana, M. Hofmann, J. Phys. Condens. Matter 15 (10) (2003) 1757–1771. [15] H. Ihou-Mouko, G. Venturini, J. Alloys Compd. 396 (1–2) (2005) 59–63. [16] G. Venturini, J. Alloys Compd. 398 (1–2) (2005) 42–47. [17] M. Franc¸ois, G. Venturini, B. Malaman, B. Roques, J. Less Common Met. 160 (1990) 197–213. [18] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [19] T. Mazet, B. Malaman, J. Magn. Magn. Mater. 219 (2000) 33. [20] J.M. Cadogan, D.H. Ryan, I.P. Swainson, O. Moze, J. Phys. Condens. Matter 10 (24) (1998) 5383. [21] J.M. Cadogan, Suharyana, D.H. Ryan, O. Moze, W. Kockelmann, J. Appl. Phys. 87 (9) (2000) 6046. [22] F. Weitzer, A. Leithejasper, K. Hiebl, P. Rogl, Q.N. Qi, J.M.D. Coey, J. Appl. Phys. 73 (13) (1993) 8447. [23] C. Lefevre, G. Venturini, B. Malaman, J. Alloys Compd. 354 (1–2) (2003) 47–53.
Crystallographic and magnetic properties of HfFe6Ge6-type REFe6Sn4Ge2 compounds (RE = Y, Gd–Er) G. Venturini Laboratoire de Chimie du Solide Min´eral, Universit´e Henri Poincar´e-Nancy I, associ´e au CNRS (UMR 7555), B.P. 239, 54506 Vandoeuvre les Nancy Cedex, France Received 18 March 2005; accepted 31 March 2005 Available online 29 April 2005
Abstract The REFe6 Sn4 Ge2 (RE = Y, Gd–Er) compounds have been synthesized and studied by powder X-ray diffraction and magnetisation measurements. These compounds crystallize in the hexagonal HfFe6 Ge6 structure although the parent ternary compounds REFe6 X6 (X = Ge, Sn) display more complicated orthorhombic crystal structure. This evolution is discussed and interpreted on the basis of the relaxation of some RE–X contacts in the quaternary compounds. The iron sublattice order antiferromagnetically above room temperature (554 ≤ TN ≤ 560 K) while the paramagnetic RE compounds display a second transition at low temperature (7.3 ≤ Tt ≤ 42.7 K). The magnetisation versus field curves display a metamagnetic behaviour at 4.2 K. The corresponding value of the magnetisation suggests a non-collinear ordering of the RE sublattice. © 2005 Elsevier B.V. All rights reserved. Keywords: Rare earth iron tin germanium; Crystal-chemistry; Antiferromagnetism
1. Introduction The crystallographic and magnetic properties of the REFe6 Ge6 and REFe6 Sn6 compounds have been extensively studied [1–5]. Most of the paramagnetic rare earth element compounds crystallize in rather complicated structures resulting from different locations of the RE element in the host CoSn-type sub-structure. Several studies have evidenced some problems regarding the crystallographic ordering of the RE sites and have shown partial disorder depending on the thermal history of the samples [6–10]. The magnetic studies suggested an independent magnetic ordering of the RE and Fe sublattices and, in most of the cases, a non-collinear arrangement of the RE moments with a simultaneous presence of ferro-, antiferro- and/or helimagnetic components [8–14]. These problems are complicated by the presence of several different crystallographic RE sites and by the atomic disorder on these sites.
E-mail address: [email protected]. 0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.03.058
Recent studies of ErFe6 Sn6−x Gax and REMn6 Sn4 Ge2 (RE = Nd, Sm) substituted compounds have shown that the partial substitution of Sn by the smaller atoms Ge or Ga enabled the stabilisation of the more simple hexagonal HfFe6 Ge6 structure [15,16]. This one, being characterized by only one crystallographic transition metal and rare earth site, should allow a more simple examination of the order of the two magnetic sublattices and of their interplay. Attempts to synthesize the REFe6 Sn4 Ge2 compounds under the HfFe6 Ge6 form succeeded for RE = Y, Gd–Er. The crystallographic properties and the bulk magnetisation measurements of these phases are presented. 2. Experimental The REFe6 Sn4 Ge2 compounds have been synthesized from the elements melted in an induction furnace. The resulting ingots are annealed at 1123 K for 1 month. After this first stage, it is found that the samples still contain large amounts of a CeNiSi2 -type REFex Sn2 phase [17]. The ingot is therefore finely ground, compacted again into pellets and annealed for
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42
38
one additional month. The samples are checked by powder X-ray (Guinier camera Co K␣1 ). It is observed that the main remaining impurities are elemental tin and RE2 O3 oxide. The powder diffraction data have been collected on a Philips Xpert Pro powder diffractometer (Cu K␣) and the crystal structure has been refined using the FULLPROF software [18]. The magnetisation measurements have been performed on a MANICS magneto-susceptometer in the temperature range 4.2–700 K and in fields up to 1.5 T. The N´eel point of the iron sublattice which is not clearly observed by magnetisation measurements has been checked using a Mettler DSC30 differential scanning calorimeter.
3. Results 3.1. Crystallographic properties The analysis of the powder diffraction data of the REFe6 Sn4 Ge2 compounds clearly evidences the typical (h k 3) lines of the hexagonal HfFe6 Ge6 structure [P6/mmm; Hf in 1(a) (0, 0, 0); Fe in 6(i) (1/2, 0, z); Ge1 in 2(d) (1/3, 2/3, 1/2); Ge2 in 2(c) (1/3, 2/3, 0); Ge3 in 2(e) (0, 0, z)] (Figs. 1 and 2). A summary of the crystallographic properties is given in Table 1. Different refined occupation factors were found for the three tin sites with a particular low value for the Sn2 site thus indicating the preferential location of the germanium atoms on the 2(c) site. Correlatively, the zFe coordinate significantly deviates from the idealized zFe ≈ 0.25 value usually measured in ternary HfFe6 Ge6 compounds. This feature indicates that the iron atoms move towards the 2(c) site occupied by the smaller germanium atom in fair accordance with the calculated inter˚ dFe–Ge(2c) = 2.555 A ˚ in atomic distances (dFe–Sn(2d) = 2.763 A; GdFe6 Sn4 Ge2 ). The results of microprobe analysis confirm the initial stoichiometry of the samples (Table 2). There is not any significant under-stoichiometry in the RE element as observed in several HfFe6 Ge6 -type REFe6 Sn6 compounds [19]. 3.2. Magnetic properties The thermomagnetic and isotherm curves are displayed in Figs. 3 and 4 and a summary of the magnetic properties is gathered in Table 3. A very weak N´eel point may be observed at 518 K in the thermomagnetic curve of YFe6 Sn4 Ge2
Fig. 1. Representation of the HfFe6 Ge6 -type ErFe6 Sn4 Ge2 compound.
Fig. 2. Observed and calculated X-ray pattern of ErFe6 Sn4 Ge2 (upper tik: HfFe6 Ge6 -type lines, medium tik: Sn lines, lower tik: Er2 O3 lines).
but this transition is less detectable in the curves of the other RE compounds due to the large paramagnetic contribution of the RE element. However, this transition displays a small endothermic peak upon heating in differential thermal analysis and the N´eel temperature ranging between 553 and 560 K, has been measured by this way. As previously reported for the corresponding ternary germanides and stannides [12], there is no strong influence of the RE element on the N´eel
Table 1 Refined cell parameters, atomic coordinates and occupation factors of REFe6 Sn≈4 Ge≈2 compounds (RE = Y, Gd, Tb, Dy, Ho, Er) RE
˚ a (A)
˚ c (A)
zSn
zFe
mSn1
mSn2
mSn3
χ2
RBragg
Rf
Y Gd Tb Dy Ho Er
5.2936 (1) 5.3041 (1) 5.2876 (1) 5.2879 (1) 5.2875 (1) 5.2806 (1)
8.6649 (2) 8.6910 (2) 8.6510 (2) 8.6579 (2) 8.6623 (1) 8.6554 (2)
0.3385 (3) 0.3391 (4) 0.3381 (3) 0.3363 (4) 0.3385 (3) 0.3358 (3)
0.2326 (3) 0.2353 (5) 0.2345 (3) 0.2329 (4) 0.2351 (3) 0.2359 (4)
0.993 (6) 0.949 (8) 0.941 (4) 0.959 (6) 0.921 (5) 0.936 (6)
0.653 (5) 0.654 (7) 0.637 (4) 0.625 (5) 0.646 (5) 0.674 (5)
0.927 (5) 0.935 (9) 0.968 (5) 0.961 (6) 0.938 (6) 0.997 (7)
3.9 4.0 2.6 3.5 4.6 5.1
8.9 11.3 5.9 7.4 7.5 10.5
7.3 11.5 6.1 8.6 7.4 10.7
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42 Table 2 Atomic percentage in REFe6 Sn4 Ge2 compounds measured by microprobe analysis RE
%RE
%Fe
%Sn
%Ge
Y Gd Tb Dy Ho Er
7.19 (0.93) 7.47 (0.97) 7.64 (0.99) 7.50 (0.98) 7.62 (0.99) 7.82 (1.02)
46.64 (6.03) 46.49 (6.02) 46.44 (6.03) 46.15 (5.99) 46.36 (6.02) 46.51 (6.06)
30.54 (3.95) 30.43 (3.95) 30.18 (3.92) 30.76 (3.99) 30.88 (4.01) 31.04 (4.04)
15.63 (2.02) 15.60 (2.03) 15.74 (2.05) 15.58 (2.02) 15.14 (1.97) 14.63 (1.90)
The number of atoms assuming a REx (Fe1−y−z Sny Gez )12 formula is given in parentheses.
39
temperature of the iron sublattice. It is also worth noting that the ordering point of the REFe6 Sn4 Ge2 compounds is more close to the N´eel temperature of the germanides than to that of the stannides, thus suggesting a non-linear dependence on the Ge concentration. The thermomagnetic curves are characterized by a strong increase of the magnetisation at low temperature without displaying an evident ordering point. However, the first derivate dM/dT displays a more or less pronounced anomaly which might be attributed to the ordering of the RE sublattice. The transition temperatures decrease from 42.7 K
Fig. 3. Thermomagnetic curves of the REFe6 Sn4 Ge2 compounds (RE = Y, Gd–Er) under an applied field of 0.2 T (Inset shows a zoom of the low temperature part of the curves).
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42
40
Fig. 4. Magnetisation loops of the REFe6 Sn4 Ge2 compounds (RE = Y, Gd–Er) recorded at 4.2 and 60 K.
Table 3 Magnetic properties of the REFe6 Sn4 Ge2 compounds (reported transition temperatures of ternary germanides and stannides [12] are given for comparison)
Y Gd Tb Dy Ho Er
TN (K)
TN Sn (K)
TN Ge (K)
Tt (K)
Tt Sn (K)
Tt Ge (K)
Mmax (µB /fu)
µRE (theor) (µB /fu)
Hc (T)
518 518 518 516 517 515
558 554 553 559 559 560
486 489 490 489 484 484
– 42.7 25.4 15.4 12.2 7.3
– 45 19 14 8 4
– 29.3 7.8 7.5 8.0 3.1
0.09 3.8 3.9 4.2 5.3 3.7
0 7 9 10 10 9
– 0.35 0.28 0.21 0.15 –
Mmax : maximum magnetisation under H = 1.5 T; Hc : critical field of the metamagnetic transition.
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42
for the gadolinium compound to 7.3 K for the erbium compound. The magnetisation versus field curve of YFe6 Sn4 Ge2 indicates a very small susceptibility of the iron sublattice (χ ≈ 3 × 10−5 emu/g). This suggests a strong antiferromagnetic behaviour of this sublattice and should be related to the weak N´eel point of these compounds. The isotherm curves of the paramagnetic RE compounds recorded at 4.2 K display, except for ErFe6 Sn4 Ge2 , a metamagnetic transition characterized by a hysteretic behaviour and by a rather weak critical field decreasing on going from the gadolinium compound (Hc = 0.35 T) to the holmium compound (Hc = 0.15 T). The maximum magnetisation values, ranging between 3.8 µB /fu for the Gd compound to 5.4 µB /fu for the Ho compound, remain smaller than the theoretical free RE ion value. According to the Fe magnetic moment measured in YFe6 Ge6 (µFe = 1.88 µB ) and YFe6 Sn6 (µFe = 2.14 µB ) [20,21], the measured magnetisation value does not also fairly well account for a ferrimagnetic arrangement of the RE and iron sublattices.
4. Discussion Substituting a part of the tin atoms by germanium atoms in REFe6 Sn6 compounds yields deep modifications of the crystal chemistry of these phases. The simple hexagonal HfFe6 Ge6 -type structure is stabilised for REFe6 Sn4 Ge2 although the corresponding REFe6 Sn6 and REFe6 Ge6 compounds display more complicated orthorhombic structures which have been described as intergrowths of HfFe6 Ge6 and ScFe6 Ga6 blocks [1]. The origin of this effect is probably connected to a geometrical factor and to the different sizes of the Ge and Sn atoms. This assumption is corroborated by several facts: • The occurrence of the intergrowth structures in the REFe6 Sn6 and REFe6 Ge6 compounds is related to the size of the RE element since the compounds of the smallest ones (Sc, Lu, Tm) are isotypic with HfFe6 Ge6 . This evolution suggests that some RE contacts become too tight in the HfFe6 Ge6 structure when the size of RE element increases. • The occurrence of the intergrowth structures in the REMn6 Sn6 compounds is limited to the largest RE elements only (Pr, Nd, Sm) [22] and most of these compounds crystallize in the HfFe6 Ge6 structure. The relative behaviour of iron and manganese compounds should be related to the different sizes of these elements: the manganese compounds, characterized by larger unit cell, display less tight RE–Sn contacts. • Finally, the single crystal refinement of the TbFe6 Sn6 type DyFe6 Sn6 compound indicated atomic displacements yielding the increase of several Dy–Sn distances within the HfFe6 Ge6 blocks [5].
41
All these features suggest that the stabilisation of the intergrowth structures is governed by an optimisation of some RE–Sn(Ge) contacts within the pseudo-hexagonal (RE,Sn(Ge)) planes. On the other hand, the study of HfFe6 Ge6 -type REMn6 Sn6−x Gax , ErFe6 Sn6−x Gax and REMn6 Sn6−x Gex compounds indicated that the substituting atom Ga or Ge is mainly located at the Sn 2(c) site [15,16,23]. This site preference has been related to chemical bonding consideration: the occupation of this site and the corresponding atomic shifts in the HfFe6 Ge6 structure enable a tightening of the heteroatomic RE–Mn(Fe) contacts. The substitution of Sn by the smaller Ga or Ge atoms yields a decrease of the unit cell volume but the distance between the atom lying in the 2(c) position and the RE atom remains larger than the distance in the hypothetical corresponding HfFe6 Ge6 -type germanide or gallide as suggested by the comparison of the Lu–Ge distance ˚ in the HfFe6 Ge6 -type LuFe6 Ge6 compound (d = 2.942 A) ˚ in ErFe6 Sn4 Ge2 . [2] and the Er–Ge distance (d = 3.049 A) Therefore, the RE–X2(c) distance in the REFe6 Sn4 Ge2 compounds becomes much less critical than in the corresponding REFe6 X6 compounds (X = Ge, Sn). As it is suggested that this contact is responsible for the stabilisation of the intergrowth structures along the REFe6 X6 series, it seems understandable that the HfFe6 Ge6 structure may be stabilised in the substituted REFe6 Sn4 Ge2 compounds. The stabilisation of the HfFe6 Ge6 structure for the REFe6 Sn4 Ge2 compounds lead to a strongly different tridimensional arrangement of the rare earth atoms with respect to their arrangements in the intergrowth structures observed for the ternary REFe6 Ge6 and REFe6 Sn6 compounds. Particularly, the RE atoms build infinite hexagonal (0 0 1) planes stacked along the c axis in the HfFe6 Ge6 -type while limited RE slabs building complicated stackings are characteristic of the intergrowth structures. Therefore, it is suggested that these various arrangements may lead to different coupling scheme. However, according to the present magnetisation measurements, the ordering temperatures of the REFe6 Sn4 Ge2 compounds remain close to those measured in the corresponding ternary stannides thus suggesting that the strength of the magnetic interactions does not drastically change in the substituted stannides. Most of the neutron diffraction studies undertaken on the REFe6 X6 compounds (X = Ge, Sn) show that the magnetic arrangement of the RE sublattice was characterized by two components ferromagnetically and antiferromagnetically coupled. This observation should be related to the present results of the magnetisation versus field study. It has been observed that the curves displayed a metamagnetic behaviour with a weak critical field giving rise to a maximum magnetisation value significantly lower than the theoretical free ion value. This might indicate that the RE sublattice in the REFe6 Sn4 Ge2 compounds is characterized, like in the ternary compounds, by two magnetic components: a weakly antiferromagnetic one giving rise to the metamagnetic behaviour and an other one more strongly antifer-
42
G. Venturini / Journal of Alloys and Compounds 400 (2005) 37–42
romagnetic accounting for the reduced maximum magnetisation. This suggests that the main features of the RE ordering in ternary compounds are retained in the substituted stannides.
5. Conclusions A better understanding of the magnetic properties of the HfFe6 Ge6 -type REFe6 Sn4 Ge2 compounds needs now further investigations with either neutron or M¨ossbauer experiments. The relatively simple crystal structure of these compounds and the fair ordering of the RE atoms should make possible an accurate determination of the magnetic structures. It will also be interesting to examine the correlations between the three-dimensional arrangement of the RE sublattice and its magnetic behaviour.
References [1] B. Chafik El Idrissi, G. Venturini, B. Malaman, Mater. Res. Bull. 26 (12) (1991) 1331. [2] G. Venturini, R. Welter, B. Malaman, J. Alloys Compd. 185 (1992) 99. [3] Y.B. Wang, D. Wiarda, D.H. Ryan, J.M. Cadogan, IEEE Trans. Magn. 30 (6) (1994) 495. [4] X.L. Rao, M.D. Coey, J. Appl. Phys. 81 (8) (1997) 5227.
[5] O. Oleksyn, H. Bohm, Z. Kristall. 213 (5) (1998) 270. [6] O. Oleksyn, H.U. Nissen, R. Wessicken, Philos. Mag. Lett. 77 (5) (1998) 275. [7] O. Zaharko, P. Schobinger-Papamantellos, C. Ritter, J. RodriguezCarvajal, K.H.J. Buschow, J. Magn. Magn. Mater. 187 (1998) 293. [8] P. Schobinger-Papamantellos, O. Oleksyn, J. Rodriguez-Carvajal, G. Andr´e, E. Bruck, K.H.J. Buschow, J. Magn. Magn. Mater. 182 (1–2) (1998) 96. [9] O. Oleksyn, P. Schobinger-Papamantellos, J. Rodriguez-Carvajal, E. Br¨uck, K.H.J. Buschow, J. Alloys Compd. 257 (1997) 36. [10] O. Zaharko, P. Schobinger-Papamantellos, J. Rodriguez-Carvajal, K.H.J. Buschow, J. Alloys Compd. 288 (1–2) (1999) 50. [11] D.H. Ryan, J.M. Cadogan, J. Appl. Phys. 79 (8) (1996) 6004. [12] J.M. Cadogan, D.H. Ryan, J. Alloys Compd. 326 (2001) 166. [13] J.M. Cadogan, D.H. Ryan, I.P. Swainson, J. Phys. Condens. Matter 12 (2000) 8963–8971. [14] J.M. Cadogan, D.H. Ryan, O. Moze, Suharyana, M. Hofmann, J. Phys. Condens. Matter 15 (10) (2003) 1757–1771. [15] H. Ihou-Mouko, G. Venturini, J. Alloys Compd. 396 (1–2) (2005) 59–63. [16] G. Venturini, J. Alloys Compd. 398 (1–2) (2005) 42–47. [17] M. Franc¸ois, G. Venturini, B. Malaman, B. Roques, J. Less Common Met. 160 (1990) 197–213. [18] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [19] T. Mazet, B. Malaman, J. Magn. Magn. Mater. 219 (2000) 33. [20] J.M. Cadogan, D.H. Ryan, I.P. Swainson, O. Moze, J. Phys. Condens. Matter 10 (24) (1998) 5383. [21] J.M. Cadogan, Suharyana, D.H. Ryan, O. Moze, W. Kockelmann, J. Appl. Phys. 87 (9) (2000) 6046. [22] F. Weitzer, A. Leithejasper, K. Hiebl, P. Rogl, Q.N. Qi, J.M.D. Coey, J. Appl. Phys. 73 (13) (1993) 8447. [23] C. Lefevre, G. Venturini, B. Malaman, J. Alloys Compd. 354 (1–2) (2003) 47–53.