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The magnetic structure of EuAl2Si2
Journal of the Less-Common
Metals,
146 (1989)
327 - 335
327
THE MAGNETIC STRUCTURE OF EuAl& P. SCHOBINGER-PAPAMANTELLOS Znstitut fiir Kristallographie
und Petrographie,
ETH, CH-8092
Ziirich (Switzerland)
F. HULLIGER Laboratorium
fiir Festkiirperphysik,
ETH, CH-8093 Ziirich (Switzerland)
(Received August 26,1988)
summary EuA12Si2 crystallizes in the trigonal CaAl$i,-type structure, W5, space group P&l (No. 164), with a = 4.178(l) A, c = 7.249(3) A and c/a = 1.735 at 295 K. Europium is located in la, aluminium and silicon in 2d with zA1 = 0.629(3) and Zsi = 0.268(3) at 60 K. Below TN = 35 K EuA1,Siz orders antiferromagnetically with k = [0, 0, l/2] and the europium spins oriented parallel within the (O,O,l) planes. The isostructural EuAl,Ge* (a = 4.219(l) A, c = 7.317(4) A and c/a = 1.734) orders at TN = 27 K.
1. Introduction The most popular structure type for ternaries with a composition ABzXz, the tetragonal ThCr#i, type, occurs mainly with transition elements on the tetrahedral B sites. If no d electrons are engaged in the bonding, i.e. if the element B is one of Group IIB, IIIB, or Li, Be, Mg and Mn2+(d5), then the trigonal Mg,Sb2 or CaA12Si2 type of structure [l] is adopted in most cases. The ThCr2Si2 type is seen mainly in metallic phases whereas the CaA12Si2-type representatives are usually normal valence compounds [2 - 51. The CaA12Si2structure can be derived from a hexagonal close-packing of the anions (silicon or X) in which half the octahedral holes and half the tetrahedral holes are occupied in an ordered manner by calcium or the element A and by aluminium or the element B respectively (with the ideal values zx = l/4 and zs = 5/8, if we describe the unit cell in the usual way with the element A at the origin). If we omit half of these tetrahedral atoms we arrive at the CuScS2 type, and if we omit all the tetrahedral atoms we end up with the Cd12-type structure. In the known representatives [l] the c/a value varies between 1.55 and 1.74, compared with the ideal value 1.633 for an anion sphere packing. A report on a metallic CaAl,Si,-type representative with excess valence electrons, GdA12Si2 [6], roused our interest in this family of compounds. 0022-5088/89/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
328
The gadolinium compound was found to be antiferromagnetic below TN = 21 K. We then wondered whether the corresponding Eu*+ compound, that we supposedly had synthesized for the first time, could be a ferromagnetic semiconductor, somewhat in analogy to the rocksalt compounds GdP, GdAs, . . . us. EuS, or in direct analogy to EuZn2P2 or EuZn2As2 [7] which are isoelectronic with Eti12Si2. Obviously parallel to our preliminary investigation of EuA12Si2, the ternary alloy system Eu-Al-Si was studied by Zarechnyuk and Yanson [8]. In their report the authors gave a reference to their own work on EuA12Si2from the year 1972. Finally we became aware of Russian papers on further metallic CaA12Si2-type representatives: YA12Si2 [9], LnA12Si2 (Ln = Pr, Tb . . . Lu) [lo], LnA12Ge2 (Ln = La . . . Nd, Sm . . . Tm, Lu, Y) [ll]. Since in the case of GdA12Si2 [6] the metallic character of these Ln3+A12Si2 phases was verified by resistivity measurements and the crystal structure was refined on a single crystal our first spontaneous conjecture was thus refuted, namely that these compounds could have the composition Ln2,3Cli&12Si2 (the chemical analysis yielded GdA12.i5Si1.s5), which would have brought them back to the normal valence compounds. The fact that EuA12Si2was thus known for years and the ascertainment of antiferromagnetism instead of the expected ferromagnetism dampened our enthusiasm for this compound and it was mainly the high neutron absorption of europium that still acted as a challenge. 2. Experimental Polycrystalline EuA12Si2 was synthesized by arc melting EuSi2 + 2Al in an atmosphere of purified argon. The purity of the starting elements was 99.9% for europium and 99.99% for aluminium and silicon. The resulting metallic-grey material was fairly stable in air. The X-ray diffraction pattern revealed a small percentage of unreacted EuSi2. The room-temperature lattice constants were derived from a Guinier pattern taken with copper Koli radiation and silicon added as internal standard (assuming a = 5.43047 A at 295 K). The magnetic measurements were carried out on a 10 mg specimen in a moving-sample magnetometer in the temperature range 1.5 - 300 K and in magnetic fields up to 10 T. Neutrondiffraction measurements were performed at 60 K in the paramagnetic state and at 9 K in the magnetically ordered state, using a wavelength of 1.184 A. The diffraction data were collected with the doubleaxis multicounter system at the Saphir reactor at Wurenlingen. In view of the high absorption cross-section (uEu = 1600 barns at X, = 1.08 A [12]) we used a tubular sample of thickness 0.5 mm and outer diameter 5 mm pressed into a vanadium holder. The step increment of the diffraction angle 28 was 0.10”. The count rate of the two strongest nuclear reflections (hkl = 101 and 110) was 4 counts min-l. The measured neutron data were corrected for absorption by a computer program written by SchZirli [13], based
329
on a procedure described by Paalman and Pings [14]. The final data were evaluated by a Rietveld profile analysis [15]. The neutron scattering lengths used in the refinement were taken from Koester and Yelon [16] or Sears [17] and the magnetic form factor of EuZi from Freeman and Desclaux VSI. The temperature dependence of the intensities of some magnetic reflections was determined on a double-axis spectrometer with neutrons of wavelength 1.05 A. In this case, for reasons of time, we measured only the peak intensities. 3. Results and discussion The magnetic meas~ements are .of a standard type and are not shown. Well above the ordering temperature, from 45 to 300 K, EuAl,Si, displays a perfect Curie-Weiss behaviour. The effective moment pp = 7.2 & derived from the slope of the l/x(T) straight line turned out to be somewhat smaller than the expected free-ion value for Eu2+, 7.94pa. The positive value of the paramagnetic Curie temperature 8, = + 30 K points to a predominance of f~omagnetic interactions. The peak in the low-field (0.1 T) magnetic susceptibility defines the Neel temperature as TN = 35.5 K. In fields higher than about 0.5 T the peak vanishes and only a knee in the x(T) curve remains. The magnetization curve taken at 1.5 K shows a discontinuity between 0.3 and 0.7 T. The initial slope is 0.13~~ T-l, increasing to 0.17,~ T-i above 0. 7T. This points to a spin-flip transition in our polycrystalline material. At 1.5 K, magnetic saturation is reached at Bsat = 3.3 T. Instead of the expected 7~~ (Eu atom))i the saturation moment was only of the order of 6~~ (Eu atom))‘. We measured the same low saturation moment on isostructural EuAl,Ge, (a = 4.219(l) A, c = 7.317(4) A and c/u = 1.734) although the germanide sample was less phase-pure than the silicide. EuA12Ge2 revealed a magnetic behaviour very similar to that of EuA12Si2. Its Neel temperature is T, = 27 K. The spin-flip transition appears to take place below 0.5 T and saturation at 1.5 K is reached at around 4 T. The magnetic properties of the europium compounds resemble those of GdA12Si2. From our measurements on a polycrystal~ne GdA12Si2 sample we deduce TN = 22 K, Bsat (1.5 K) = 5.3 T, and an indication for a spin-flip transition was found between 0.7 and 1.6 T. The magnetic measurements point to a rather simple magnetic structure in these hexagonal compounds and this conjecture was verified for EuA12Si2. The neutrond~fraction patterns for the paramagnetic state and for the magnetically ordered state are juxtaposed in Fig. 1. The parameters resulting from the refinements are summarized in Table 1. As a consequence of the high absorption the R factors are quite large. On a qualitative basis we may say that the refined 60 K nuclear data reproduce the room-temperature
Fig. 1. Neutron diffraction patterns of EuAI$Siz in the paramagnetic state at 60 K and in the magnetically ordered state at 9 K. The full curves represent the calculated profiles. The difference diagram Z,,b - Zdc is given below each curve. In the 9 K pattern only the magnetic peaks are indexed. AI1 possible reflections (except the suppressed reflections (001) and (111)) are indicated on the top frame, the nuclear reflections above and nuclear + magnetic reflections below.
TABLE 1 Structural and magnetic parameters of EuAi&z refined from the neutron diffraction data (p 1 c). The orientation of the magnetic moment of the europium atom within the (001) planes remains undetermined
0 (A) c (A)
sAl ZSi PEP (PB)
En R,
R WP
295 K
60 K
9K
4.178(l) 7.249(3)
4.168(3) 7.223(6) O-629(3) 0.268(3)
4.162(2) 14.416(8) 0.315 0.134 5.8(l) 0.12 0.057 0.22
0.16 0.30
At 295 K and 60 K the space group is Z&n1 (No. 164), with europium in la: O,O,O: aluminium and silicon in 2d: *(l/3, 2/3, z). In the doubled cell at 9 K (neglecting the low-symmetry distortions which are below the limit of our resolution) europium is located in la and lb (0,0,1/2). Aluminium and silicon occupy two sets of 2d with Z’ z/2 and z” = l/2 - z’. The value z’ was not refined; instead we assumed z’ = z/2.
331 TABLE 2 Interatomic distances (A) in EuAl$& at 9 K up to 4.1 A. The Si-Si distances are added to give an idea about the deformation of the close packing Eu-6 Si 3.08(2) ---_--6 Al
3.60(2)
Al-3Si 1 Si 3 Al 3 Eu
2.52(2) 2.60(3) 3.04(2) 3.60(2)
Si-3 Al 2.52(2) 1 Al 2.60(3) 3 Eu 3.08(2) ----_-3 Si 4.12(3) 6 Si 4.162(2) 3 Si 4.55(3)
X-ray data. The information resulting from the 9 K data is more conclusive due to the higher intensity of the magnetic reflections and the extinction rules. The nuclear distances at 9 K, relevant for chemical bonding, are given in Table 2. In the 9 K diffraction pattern (Fig. 1) the appearance of the magnetic peaks indicates a cell enlargement in the c direction co~esponding to the wave vector k = [0, 0, $1. From the observed non-zero intensities of the (O,O, l/2) reflections we deduce that the magnetic moments of the europium atoms are confined to the (0, 0, 1) plane. The refinement of the magnetic intensities led to a collinear antiferromagnetic structure made up by an alternating +/-- stacking of ferromagnetic layers perpendicular to the trigonal axis. Calculated and observed intensities are listed in Table 3 and the magnetic cell is illustrated in Fig. 2. The Eu2+ 4f moments are oriented perpendicular to the c-axis. The value of the resulting moment, 5.8(l) pa Eu atom-‘, is considerably lower than the expected value of - 6.8c(a at T ;?;:T,/4. It confirms the magnetization measurements and thus the possibility of the presence of some Eu3+ may be faced. As the magnetic moments lie in the hexagonal plane their direction cannot be derived from powder data [19]. Therefore, we cannot decide whether the symmetry of the magnetic structure is monoclinic or triclinic. The former case would correspond to an orientation of the magnetic moments either along or perpendicular to the G-axis with the magnetic space groups C,2/m (Sh$) and &2/e (Shah) respectively, while for an arbitrary moment direction in the (001) plane the space group is triclinic P,i (Shz) (201. The temperature dependence of the intensities of the magnetic lines (0, 0, 3/2) and (1,0, l/2) is illustrated in Fig. 3. The resulting Neel temperature TN = 35 f 1 K is in good agreement with the result of the susceptibility measurements. Knowing the magnetic structure we can estimate the exchange interactions. The magnetic environment of each europium atom is as follows: Eu(?)-6
Eu(?) at a = 4.162 A (J,)
332
TABLE 3 The low-angle part of the observed and calculated integrated neutron-diffraction tensities of EuAl$$ at 9 K corresponding to a collinear antiferromagnetic structure
in-
hkJ
Znue
Zmsg
Ztot
Zobs
ESD
0 0 l/2 0 0 312 100 002 1 0 l/2 101 1 0 312 0 0 512 102 003 1 0 512 110 1 1 l/2 0 0 II2 103 1 1 312 200 112 004 2 0 l/2 1 0 712 201 2 0 312 1 1 512 202 104 0 0 912 113
-
12150 1199
12150 1199 27 36 1912 1388 1446 342 1435 353 832 1728 436 128 719 396 8 62 49 262 428 477 232 296 671 164 54 I.060
12570 1156 40 54 2020 1539 1374 325 1557 376 876 1772 418 121 544 470 3 22 18 240 394 500 246 312
159 101 31 40 96 100 84 20 89 95 102 89 50 15 99 98 4 34 25 58 46 96 50 64 78 18 12 98
27 36
-
1388 -
-
1435 353 832 1728 -
-
-
-
-
-
-
8 62 49
-
671 164
1060
1446 342
-
719
477
1912
-
-
436 128 396
262 428 232 296
-
54
569
137 27 1038
The indexing refers to the chemical cell. The intensities correspond to total counts. ESD = estimated standard deviation of z&s The weak reflections (001) and (111) coincided with impurity reflections and are therefore omitted.
Eu(f )-6 Eu( t) at 31’2a = 7.209 A (J2) 2 Eu( 4) at c/2 = 7.208 li (J3) 8, from l/x(T) Jp = Jt,
+
corrected for demagnetization is +30 K. From
Jt, = 6J1 + 6J2 + 25, =
Jaf = Jw -J+l=&J1+&J2-&J3=
3k0, 2S(S + 1) 3kTN
2S(S + 1)
we deduce J,/k = -0.13 K and (J1 + J,)/k K we end up with Jl/k = +0.6 K.
= +0.52 K. Assuming J,/k
= -0.1
333
Fig. (0,
2. Temperature 0, 312) and
dependence
of the peak intensities of the magnetic
reflections
(1,0, 112). For clarity error bars are given for the (0, 0, 3f2) reflections
only. The full curves are based on the molecular-field model with S = 7/2, adapted at the lowest temperature, T = 10 K.
Our meas~ements provide only a rough value for the saturation Bsat and a crude estimate for the spin-flip field Bnip [Zl f B exch = (B?,, + B:J4 Nip
field
a 1.65 T
= (2BexchBanis -BaiZ)1’2
z t2Bexck&nti)1’2
which yields Banis = 0.1 T; whereas Bexch = 2l~t~l{~)/g~~ and hence iJt&k = with Jtr/k = 2-J,lk = EBBB exchfk W = 0.28 K, which is to be compared 3(8, - TN)/4S(S + 1) * 0.26 K from the low-field data. The agreement is satisfactory if we take into account the accuracy of the polycrystal data. The exchange constant J, in EuA12Si2 is more than twice as large as in insulating EuS [22] which has similar newest-ne~hbo~ Eu-Eu distances. In EuAlSi, that we expect to crystallize in the ordered AlB2 version, the BaAlSi type [ 11, the NQel temperature should be very close to that of EuA12Si2 since the hexagonal europium layers which contribute the predominant magnetic coupling are nearly identical, only the interlayer coupling is slightly increased.
334 25
20 -ii ‘E a 0 “0 =
2. .t$
15
f -c IO
5
0
IO
20
40
30
T(K)
Fig. 3. Upper part: A (110) section of the magnetic structure of EuAlzSip. The arrows represent the europium moment component in the (110) plane. Lower part: Projection of the nuclear structure onto the basal plane. Largest circles, silicon; smallest circles, aluminium.
In the isoelectronic compounds EuZnzX2 (X 2 P, As and Sb) [7] the ei, values are also positive and decrease with a larger unit cell (43,ZO and 6 K respectively). These p&tides thus may show the same magnetic order as EuAl,Sis. If we omit the aluminium atoms in EuA12Gez the c axis shrinks considerably, and the a axis also slightly contracts by 0.12 A in the CeCdz-type structure of the resulting EuGes (a = 4.102 A, c = 4.995 A [l]). Obviously Ji is also pr~ominant here, the Neel temperate is 42 K [23]. It may, however, be dangerous to extrapolate too far, as is demonstrated by the example of EuC,. In this metallic first-stage graphite intercalation compound Eu-Eu = 4.31 (Ji) and 5.47 a (the latter distance, however, is not along the c axis as in our examples since in the h.c.p. europium substructure the hexagonal layers are shifted relative to each other). In Eu&, the 4f spins of the europium atoms also lie in the basal plane, but in contrast to EuAl,Sis their arrangement is not collinear but triangular antiferromagnetic [24,25], which leads to a larger a axis (a’ = 30s). Unfortunately the quality of our sample was not sufficient for unequivocally answering the question about its possible semiconductor properties. We notice that in the potentially non-metallic silicides and germanides the axial ratio c/a is considerably higher than the ideal value for anion closepacking whereas in the metallic silicides and germanides c/a is (slightly) below this value. Strangely, the larger radius of divalent europium and ytterbium does not manifest itself in the a value anywhere in the lanthanide
335
series (although they contract on going from LaAlzXz to LuA12X2), although the c axis is considerably affected.
Acknowledgments The authors aye highly indebted to Dr. Peter Fischer and the staff of the Laboratorium fiir Neutronensteuung LNS, ETH ZiCch, for experimentaI help. Financial support by the Swiss National Science Foundation is also gratefully acknowledged.
References 1 P. VilIars and L. IX Calve& Pearson’s Handbook of ~~sial~o~~~~~~ fzota for ZntermetaZZicPhases, Vols. 1 - 3, American Society for Metals, Metals Park, OH, 1985. 2 P. Kliifers and A. Mewis, 2. Naturfomch., 32b (1977) 753. 3 P. Khifers, A. Mewis and H. U. Schuster, 2. Kristallogr., 149 (1979) 211. 4 Chong Zheng and R. Hoffmann, J. Solid State Chem., 72 (1988) 58. 5 R. Ramirex, R. Nesper and H. G. von Scbnering, Z. Nuturfo~~h.~ 42a (1987) 670. 6 R. Neaper, H. G. von Schnering and J. Curda, Z. Naturf#~~h.~ 37b (1982) 1514. 7 G. Zwiener, H. Neumann and H. U. Schuster, 2. Naturfomch., 36b (1981) 1195. 8 0. S. Zarechnyuk and T, I. Yanson, Dokl, Akad. Nauk Ukrain. RSR, Ser. 23, 4 (1982) 31. 9 A. A, Murav’era, 0. S. Zarechnyuk and E. I. Gladyshevskii, Znorg. Mater., 7 (1971) 34. 10 V. V. NemoshkaIenko, V. Ya. Nagornii, B. P. Mamko, P, K. Nikolyuk, P. V. GeI’, R. V. Lutaiv and M, Il. Koterhn, L&rain. F&. Zh., 26 (1981) 1831. 11 0. S. Zarechnyuk, A. 0. Muraviova and E. I. Gladyshevskii, Dopou. Akud. Nauk Ukrain. RSR, Ser. A, 32 (1970) 753. 12 G. G, Bacon, Neutron Diffraction, Clarendon, Oxford (1975), p. 73. 13 M. Schiirli, private communication. 14 H. II, Paaiman and C. G. Pings, J. Appl Phys.. 33 (1962) 2635. 15 H. M. RietveIdd, RCN Rept. 104, Petten, The Netherlands (1976f. 16 L. Koester and W, B. Yefon, Summary of Low-Ene~ Neutron Slathering Lengths and Cross Sections, Brookhaven National Laboratory, Upton, NY, 1984. 17 V. F. Sears, Thermal-Neutron Scattering Lengths and Cross Sections for CondensedMatter Research, AECL-8490, Chalk River Nuclear Laboratories Chalk River, Ontario, 1984. 18 A. J. Freeman and J. P. Desclaux, J. Magn, Magn. Mater., 12 (1979) 11. 19 G. Shirane, Acta Crystallogr., Sect A, 31 (19593 282. 20 V, A. Koptsik, Shub~ikou~e Gruppy - Sp~vochnik po Simmetrii i F~i~hesk~rn Svotstvam Kristaflicheskikh Struktur (Shubnikov Groups - A Handbook on Symmetry and Physical properties of Crystal Structures), Moscow University Press, Moscow, 1966, p. 100. 21 A. Herpin, Thiorie du Magnttisme. Presses Universitaires de France, Paris, 1968, p. 491. 22 P. Waehter, in Ii. A. Gschneidner, Jr. and L. Eyring (eds), handbook on the Physics and ~hern~~ of Rare Karths~ North-HoIIand, Amsterdam, 1979, p. 507, 23 M. Loewenhaupt, 2. Physik, 267 (1974) 219. 24 H. Suematsu, K. Ohmatsu and R. Yoshizaki, SoZid State Commun., 38 (1981) 1103. 25 T. Sakakibara and M. Date, J. Phys. Sot. Jpn., 53 (1984) 3599.
Metals,
146 (1989)
327 - 335
327
THE MAGNETIC STRUCTURE OF EuAl& P. SCHOBINGER-PAPAMANTELLOS Znstitut fiir Kristallographie
und Petrographie,
ETH, CH-8092
Ziirich (Switzerland)
F. HULLIGER Laboratorium
fiir Festkiirperphysik,
ETH, CH-8093 Ziirich (Switzerland)
(Received August 26,1988)
summary EuA12Si2 crystallizes in the trigonal CaAl$i,-type structure, W5, space group P&l (No. 164), with a = 4.178(l) A, c = 7.249(3) A and c/a = 1.735 at 295 K. Europium is located in la, aluminium and silicon in 2d with zA1 = 0.629(3) and Zsi = 0.268(3) at 60 K. Below TN = 35 K EuA1,Siz orders antiferromagnetically with k = [0, 0, l/2] and the europium spins oriented parallel within the (O,O,l) planes. The isostructural EuAl,Ge* (a = 4.219(l) A, c = 7.317(4) A and c/a = 1.734) orders at TN = 27 K.
1. Introduction The most popular structure type for ternaries with a composition ABzXz, the tetragonal ThCr#i, type, occurs mainly with transition elements on the tetrahedral B sites. If no d electrons are engaged in the bonding, i.e. if the element B is one of Group IIB, IIIB, or Li, Be, Mg and Mn2+(d5), then the trigonal Mg,Sb2 or CaA12Si2 type of structure [l] is adopted in most cases. The ThCr2Si2 type is seen mainly in metallic phases whereas the CaA12Si2-type representatives are usually normal valence compounds [2 - 51. The CaA12Si2structure can be derived from a hexagonal close-packing of the anions (silicon or X) in which half the octahedral holes and half the tetrahedral holes are occupied in an ordered manner by calcium or the element A and by aluminium or the element B respectively (with the ideal values zx = l/4 and zs = 5/8, if we describe the unit cell in the usual way with the element A at the origin). If we omit half of these tetrahedral atoms we arrive at the CuScS2 type, and if we omit all the tetrahedral atoms we end up with the Cd12-type structure. In the known representatives [l] the c/a value varies between 1.55 and 1.74, compared with the ideal value 1.633 for an anion sphere packing. A report on a metallic CaAl,Si,-type representative with excess valence electrons, GdA12Si2 [6], roused our interest in this family of compounds. 0022-5088/89/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
328
The gadolinium compound was found to be antiferromagnetic below TN = 21 K. We then wondered whether the corresponding Eu*+ compound, that we supposedly had synthesized for the first time, could be a ferromagnetic semiconductor, somewhat in analogy to the rocksalt compounds GdP, GdAs, . . . us. EuS, or in direct analogy to EuZn2P2 or EuZn2As2 [7] which are isoelectronic with Eti12Si2. Obviously parallel to our preliminary investigation of EuA12Si2, the ternary alloy system Eu-Al-Si was studied by Zarechnyuk and Yanson [8]. In their report the authors gave a reference to their own work on EuA12Si2from the year 1972. Finally we became aware of Russian papers on further metallic CaA12Si2-type representatives: YA12Si2 [9], LnA12Si2 (Ln = Pr, Tb . . . Lu) [lo], LnA12Ge2 (Ln = La . . . Nd, Sm . . . Tm, Lu, Y) [ll]. Since in the case of GdA12Si2 [6] the metallic character of these Ln3+A12Si2 phases was verified by resistivity measurements and the crystal structure was refined on a single crystal our first spontaneous conjecture was thus refuted, namely that these compounds could have the composition Ln2,3Cli&12Si2 (the chemical analysis yielded GdA12.i5Si1.s5), which would have brought them back to the normal valence compounds. The fact that EuA12Si2was thus known for years and the ascertainment of antiferromagnetism instead of the expected ferromagnetism dampened our enthusiasm for this compound and it was mainly the high neutron absorption of europium that still acted as a challenge. 2. Experimental Polycrystalline EuA12Si2 was synthesized by arc melting EuSi2 + 2Al in an atmosphere of purified argon. The purity of the starting elements was 99.9% for europium and 99.99% for aluminium and silicon. The resulting metallic-grey material was fairly stable in air. The X-ray diffraction pattern revealed a small percentage of unreacted EuSi2. The room-temperature lattice constants were derived from a Guinier pattern taken with copper Koli radiation and silicon added as internal standard (assuming a = 5.43047 A at 295 K). The magnetic measurements were carried out on a 10 mg specimen in a moving-sample magnetometer in the temperature range 1.5 - 300 K and in magnetic fields up to 10 T. Neutrondiffraction measurements were performed at 60 K in the paramagnetic state and at 9 K in the magnetically ordered state, using a wavelength of 1.184 A. The diffraction data were collected with the doubleaxis multicounter system at the Saphir reactor at Wurenlingen. In view of the high absorption cross-section (uEu = 1600 barns at X, = 1.08 A [12]) we used a tubular sample of thickness 0.5 mm and outer diameter 5 mm pressed into a vanadium holder. The step increment of the diffraction angle 28 was 0.10”. The count rate of the two strongest nuclear reflections (hkl = 101 and 110) was 4 counts min-l. The measured neutron data were corrected for absorption by a computer program written by SchZirli [13], based
329
on a procedure described by Paalman and Pings [14]. The final data were evaluated by a Rietveld profile analysis [15]. The neutron scattering lengths used in the refinement were taken from Koester and Yelon [16] or Sears [17] and the magnetic form factor of EuZi from Freeman and Desclaux VSI. The temperature dependence of the intensities of some magnetic reflections was determined on a double-axis spectrometer with neutrons of wavelength 1.05 A. In this case, for reasons of time, we measured only the peak intensities. 3. Results and discussion The magnetic meas~ements are .of a standard type and are not shown. Well above the ordering temperature, from 45 to 300 K, EuAl,Si, displays a perfect Curie-Weiss behaviour. The effective moment pp = 7.2 & derived from the slope of the l/x(T) straight line turned out to be somewhat smaller than the expected free-ion value for Eu2+, 7.94pa. The positive value of the paramagnetic Curie temperature 8, = + 30 K points to a predominance of f~omagnetic interactions. The peak in the low-field (0.1 T) magnetic susceptibility defines the Neel temperature as TN = 35.5 K. In fields higher than about 0.5 T the peak vanishes and only a knee in the x(T) curve remains. The magnetization curve taken at 1.5 K shows a discontinuity between 0.3 and 0.7 T. The initial slope is 0.13~~ T-l, increasing to 0.17,~ T-i above 0. 7T. This points to a spin-flip transition in our polycrystalline material. At 1.5 K, magnetic saturation is reached at Bsat = 3.3 T. Instead of the expected 7~~ (Eu atom))i the saturation moment was only of the order of 6~~ (Eu atom))‘. We measured the same low saturation moment on isostructural EuAl,Ge, (a = 4.219(l) A, c = 7.317(4) A and c/u = 1.734) although the germanide sample was less phase-pure than the silicide. EuA12Ge2 revealed a magnetic behaviour very similar to that of EuA12Si2. Its Neel temperature is T, = 27 K. The spin-flip transition appears to take place below 0.5 T and saturation at 1.5 K is reached at around 4 T. The magnetic properties of the europium compounds resemble those of GdA12Si2. From our measurements on a polycrystal~ne GdA12Si2 sample we deduce TN = 22 K, Bsat (1.5 K) = 5.3 T, and an indication for a spin-flip transition was found between 0.7 and 1.6 T. The magnetic measurements point to a rather simple magnetic structure in these hexagonal compounds and this conjecture was verified for EuA12Si2. The neutrond~fraction patterns for the paramagnetic state and for the magnetically ordered state are juxtaposed in Fig. 1. The parameters resulting from the refinements are summarized in Table 1. As a consequence of the high absorption the R factors are quite large. On a qualitative basis we may say that the refined 60 K nuclear data reproduce the room-temperature
Fig. 1. Neutron diffraction patterns of EuAI$Siz in the paramagnetic state at 60 K and in the magnetically ordered state at 9 K. The full curves represent the calculated profiles. The difference diagram Z,,b - Zdc is given below each curve. In the 9 K pattern only the magnetic peaks are indexed. AI1 possible reflections (except the suppressed reflections (001) and (111)) are indicated on the top frame, the nuclear reflections above and nuclear + magnetic reflections below.
TABLE 1 Structural and magnetic parameters of EuAi&z refined from the neutron diffraction data (p 1 c). The orientation of the magnetic moment of the europium atom within the (001) planes remains undetermined
0 (A) c (A)
sAl ZSi PEP (PB)
En R,
R WP
295 K
60 K
9K
4.178(l) 7.249(3)
4.168(3) 7.223(6) O-629(3) 0.268(3)
4.162(2) 14.416(8) 0.315 0.134 5.8(l) 0.12 0.057 0.22
0.16 0.30
At 295 K and 60 K the space group is Z&n1 (No. 164), with europium in la: O,O,O: aluminium and silicon in 2d: *(l/3, 2/3, z). In the doubled cell at 9 K (neglecting the low-symmetry distortions which are below the limit of our resolution) europium is located in la and lb (0,0,1/2). Aluminium and silicon occupy two sets of 2d with Z’ z/2 and z” = l/2 - z’. The value z’ was not refined; instead we assumed z’ = z/2.
331 TABLE 2 Interatomic distances (A) in EuAl$& at 9 K up to 4.1 A. The Si-Si distances are added to give an idea about the deformation of the close packing Eu-6 Si 3.08(2) ---_--6 Al
3.60(2)
Al-3Si 1 Si 3 Al 3 Eu
2.52(2) 2.60(3) 3.04(2) 3.60(2)
Si-3 Al 2.52(2) 1 Al 2.60(3) 3 Eu 3.08(2) ----_-3 Si 4.12(3) 6 Si 4.162(2) 3 Si 4.55(3)
X-ray data. The information resulting from the 9 K data is more conclusive due to the higher intensity of the magnetic reflections and the extinction rules. The nuclear distances at 9 K, relevant for chemical bonding, are given in Table 2. In the 9 K diffraction pattern (Fig. 1) the appearance of the magnetic peaks indicates a cell enlargement in the c direction co~esponding to the wave vector k = [0, 0, $1. From the observed non-zero intensities of the (O,O, l/2) reflections we deduce that the magnetic moments of the europium atoms are confined to the (0, 0, 1) plane. The refinement of the magnetic intensities led to a collinear antiferromagnetic structure made up by an alternating +/-- stacking of ferromagnetic layers perpendicular to the trigonal axis. Calculated and observed intensities are listed in Table 3 and the magnetic cell is illustrated in Fig. 2. The Eu2+ 4f moments are oriented perpendicular to the c-axis. The value of the resulting moment, 5.8(l) pa Eu atom-‘, is considerably lower than the expected value of - 6.8c(a at T ;?;:T,/4. It confirms the magnetization measurements and thus the possibility of the presence of some Eu3+ may be faced. As the magnetic moments lie in the hexagonal plane their direction cannot be derived from powder data [19]. Therefore, we cannot decide whether the symmetry of the magnetic structure is monoclinic or triclinic. The former case would correspond to an orientation of the magnetic moments either along or perpendicular to the G-axis with the magnetic space groups C,2/m (Sh$) and &2/e (Shah) respectively, while for an arbitrary moment direction in the (001) plane the space group is triclinic P,i (Shz) (201. The temperature dependence of the intensities of the magnetic lines (0, 0, 3/2) and (1,0, l/2) is illustrated in Fig. 3. The resulting Neel temperature TN = 35 f 1 K is in good agreement with the result of the susceptibility measurements. Knowing the magnetic structure we can estimate the exchange interactions. The magnetic environment of each europium atom is as follows: Eu(?)-6
Eu(?) at a = 4.162 A (J,)
332
TABLE 3 The low-angle part of the observed and calculated integrated neutron-diffraction tensities of EuAl$$ at 9 K corresponding to a collinear antiferromagnetic structure
in-
hkJ
Znue
Zmsg
Ztot
Zobs
ESD
0 0 l/2 0 0 312 100 002 1 0 l/2 101 1 0 312 0 0 512 102 003 1 0 512 110 1 1 l/2 0 0 II2 103 1 1 312 200 112 004 2 0 l/2 1 0 712 201 2 0 312 1 1 512 202 104 0 0 912 113
-
12150 1199
12150 1199 27 36 1912 1388 1446 342 1435 353 832 1728 436 128 719 396 8 62 49 262 428 477 232 296 671 164 54 I.060
12570 1156 40 54 2020 1539 1374 325 1557 376 876 1772 418 121 544 470 3 22 18 240 394 500 246 312
159 101 31 40 96 100 84 20 89 95 102 89 50 15 99 98 4 34 25 58 46 96 50 64 78 18 12 98
27 36
-
1388 -
-
1435 353 832 1728 -
-
-
-
-
-
-
8 62 49
-
671 164
1060
1446 342
-
719
477
1912
-
-
436 128 396
262 428 232 296
-
54
569
137 27 1038
The indexing refers to the chemical cell. The intensities correspond to total counts. ESD = estimated standard deviation of z&s The weak reflections (001) and (111) coincided with impurity reflections and are therefore omitted.
Eu(f )-6 Eu( t) at 31’2a = 7.209 A (J2) 2 Eu( 4) at c/2 = 7.208 li (J3) 8, from l/x(T) Jp = Jt,
+
corrected for demagnetization is +30 K. From
Jt, = 6J1 + 6J2 + 25, =
Jaf = Jw -J+l=&J1+&J2-&J3=
3k0, 2S(S + 1) 3kTN
2S(S + 1)
we deduce J,/k = -0.13 K and (J1 + J,)/k K we end up with Jl/k = +0.6 K.
= +0.52 K. Assuming J,/k
= -0.1
333
Fig. (0,
2. Temperature 0, 312) and
dependence
of the peak intensities of the magnetic
reflections
(1,0, 112). For clarity error bars are given for the (0, 0, 3f2) reflections
only. The full curves are based on the molecular-field model with S = 7/2, adapted at the lowest temperature, T = 10 K.
Our meas~ements provide only a rough value for the saturation Bsat and a crude estimate for the spin-flip field Bnip [Zl f B exch = (B?,, + B:J4 Nip
field
a 1.65 T
= (2BexchBanis -BaiZ)1’2
z t2Bexck&nti)1’2
which yields Banis = 0.1 T; whereas Bexch = 2l~t~l{~)/g~~ and hence iJt&k = with Jtr/k = 2-J,lk = EBBB exchfk W = 0.28 K, which is to be compared 3(8, - TN)/4S(S + 1) * 0.26 K from the low-field data. The agreement is satisfactory if we take into account the accuracy of the polycrystal data. The exchange constant J, in EuA12Si2 is more than twice as large as in insulating EuS [22] which has similar newest-ne~hbo~ Eu-Eu distances. In EuAlSi, that we expect to crystallize in the ordered AlB2 version, the BaAlSi type [ 11, the NQel temperature should be very close to that of EuA12Si2 since the hexagonal europium layers which contribute the predominant magnetic coupling are nearly identical, only the interlayer coupling is slightly increased.
334 25
20 -ii ‘E a 0 “0 =
2. .t$
15
f -c IO
5
0
IO
20
40
30
T(K)
Fig. 3. Upper part: A (110) section of the magnetic structure of EuAlzSip. The arrows represent the europium moment component in the (110) plane. Lower part: Projection of the nuclear structure onto the basal plane. Largest circles, silicon; smallest circles, aluminium.
In the isoelectronic compounds EuZnzX2 (X 2 P, As and Sb) [7] the ei, values are also positive and decrease with a larger unit cell (43,ZO and 6 K respectively). These p&tides thus may show the same magnetic order as EuAl,Sis. If we omit the aluminium atoms in EuA12Gez the c axis shrinks considerably, and the a axis also slightly contracts by 0.12 A in the CeCdz-type structure of the resulting EuGes (a = 4.102 A, c = 4.995 A [l]). Obviously Ji is also pr~ominant here, the Neel temperate is 42 K [23]. It may, however, be dangerous to extrapolate too far, as is demonstrated by the example of EuC,. In this metallic first-stage graphite intercalation compound Eu-Eu = 4.31 (Ji) and 5.47 a (the latter distance, however, is not along the c axis as in our examples since in the h.c.p. europium substructure the hexagonal layers are shifted relative to each other). In Eu&, the 4f spins of the europium atoms also lie in the basal plane, but in contrast to EuAl,Sis their arrangement is not collinear but triangular antiferromagnetic [24,25], which leads to a larger a axis (a’ = 30s). Unfortunately the quality of our sample was not sufficient for unequivocally answering the question about its possible semiconductor properties. We notice that in the potentially non-metallic silicides and germanides the axial ratio c/a is considerably higher than the ideal value for anion closepacking whereas in the metallic silicides and germanides c/a is (slightly) below this value. Strangely, the larger radius of divalent europium and ytterbium does not manifest itself in the a value anywhere in the lanthanide
335
series (although they contract on going from LaAlzXz to LuA12X2), although the c axis is considerably affected.
Acknowledgments The authors aye highly indebted to Dr. Peter Fischer and the staff of the Laboratorium fiir Neutronensteuung LNS, ETH ZiCch, for experimentaI help. Financial support by the Swiss National Science Foundation is also gratefully acknowledged.
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