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Structural chemistry and magnetic behaviour of the ternary silicides U2TSi3 (T = Mn, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Au)
ELSEVIER
Journal of Alloys and Compounds
233 (1996) 150-160
Structural chemistry and magnetic behaviour of the ternary silicides U,TSi, (T = Mn, Fe, Co, Ni, Ru, Rh, Pd, OS, Ir, Pt, Au) B. Chevalier*,
R. PBttgen’, B. Darriet, P. Gravereau,
Institut de Chimie de la Matitre Condenste
de Bordeaux,
ICMCB
(UPR-CNRS
J. Etourneau
9048), 33608 Pessac Ctdex, France
Received 12 June 1995
Abstract The crystallographic structures of many U,TSi, compounds with T a transition element were investigated by both X-ray and electron diffraction studies. These silicides crystallize in the ideal or derivative structure of the hexagonal AlB, type. A structural ordering between T and Si atoms exists in these various compounds; it is strongly influenced by the nature of the transition element. This structural ordering seems to be attributed to an electronic factor rather than to a steric factor; it is favoured with T elements having a small number of d electrons. Furthermore, these compounds exhibit interesting magnetic properties since for T = Fe, Ru and OS, they display a spin fluctuation behaviour at low temperatures whereas for T = Co, Ni, Rh, Pd, Ir, Pt and Au they order magnetically. Keywords:
Uranium; Ternary silicides; Crystal structure; Spin fluctuations; Ferromagnetism
1. Introduction
Recently many ternary silicides with general formula U,TSi, (T = Fe, Co, Ni, Cu, Ru, Rh, Pd, OS, Ir, Pt, Au) have been prepared [l-7]. The crystal structure of these compounds, with the exception of U,CuSi,, is derived from the hexagonal AlB, type..In most cases, the silicon and transition element atoms are, for these ternary silicides, randomly distributed into the trigonal prisms of a primitive hexagonal array of uranium atoms. For this distribution, a hexagonal unit cell similar to that of AlB,, is defined by (a,, a,,~,). However, our recent X-ray and electron diffraction studies on U,RuSi, and U,OsSi, show that silicon and ruthenium or osmium atoms are ordered in a two-dimensional (2D) network [8]. The latter silicides crystallize in a new derived structure of the hexagonal AlB, type having (2a,, 2u,, c,,) as parameters of the unit cell. * Corresponding author. ’Permanent address: Max Planck-Institiit fiir Festkoperforschung, Heisenbergstrasse
1, 70569 Stuttgart,
0925-8388/96/$15.00 0 1996 Elsevier SSDI 09258388(95)02006-3
Germany. Science
S.A. All rights
reserved
Some compounds U,TSi, with T = Fe, Co, Ni, Cu, Ru and Pt were investigated by magnetic measurements and were shown to have interesting physical properties. U,FeSi, and U,RuSi, exhibit a spin fluctuation behaviour at low temperatures [8,9] whereas U,CoSi,, U,NiSi, and U,CuSi, can be considered as “re-entrant spin glasses” systems [9], i.e. with the occurrence of a ferromagnetic state between the paramagnetic and low temperature spin glass states. On the contrary, some controversy exists concerning the magnetic properties of U,PtSi, which is classified as a spin glass [2] or a weak itinerant ferromagnetic system [3,4]. All these studies suggest that the physical properties of the U,TSi, compounds are strongly influenced by the nature of the transition element T which governs the strength of the Sf(U)-rid(T)) hybridization and consequently the nature of the uranium magnetic state. For a better understanding of this Sf(U)-rid(T)) hybridization, it is essential to study the magnetic properties of a great number of U,TSi, compounds. In this paper are reported both structural and magnetic properties of the U,TSi, silicides. The discussion of their magnetic behaviour takes into account both the distances d,_, and
B. Chevalier et al. I Journal of Alloys and Compounds
the degree of the crystallographic order between silicon and transition element atoms.
2. Experimental
the
details
Silicide
U,MnSi, U,FeSi, UzCoSi, U,NISI, U,RuSi, U,RhSi, U,PdSi, UzOsSi, U,IrSi, U,PtSi, U,AuSi,
analyses
of the ternary
Standard
UMn,Si, U,Fe,Si UCoSi UNi$, URu,Si, URh,Si, UPd,Si, U,Os,Ge, URu,Si, UIr,Si, UPt& UAu>Si,
silicides
151
Magnetization measurements were carried out between 4.2 and 300K using both a pendulum susceptometer and a superconducting quantum interference device magnetometer. The a.c. susceptibility measurements were performed using a Lake Shore Series 7000 AC susceptometer.
Starting materials for the preparation of the ternary silicides were platelets of uranium, powders or ingots of the transition elements and lumps of silicon, all with stated purities above 99.9%. The uranium platelets were cleaned with concentrated nitric acid to remove oxide impurities. They were not allowed to contact air prior to the reactions. U,TSi, silicides were prepared by melting of the elemental components in an induction levitation furnace under an atmosphere of magnesium-gettered argon. The buttons were flipped over and remelted several times to ensure homogeneity. After the melting process all samples were sealed in evacuated silica tubes and annealed at 800°C for two months. Microprobe examinations were carried out for several samples in a CAMEBAX scanning electron microscope. The analyses of the samples were based on the measurements of U Mel, , Mn Ke, , Fe Ka,, CoKa,, NiKa,, RuL(-w,, RhLa,, OsLa,, IrLol,, Pt Lo_,, Au Lol, and Si Ka, radiations, which were compared with those obtained by using a standard silicide. through their All samples were characterized Guinier powder patterns which were recorded with using 99.999% silicon (a = Cu Kor, radiation 543.07 pm) as an internal standard. The lattice constants were obtained by least-squares refinements of the Guinier powder data. For the investigation in the electron microscope (JEOL 2000 FX), parts of the samples were crushed in an agate mortar with methanol. Small crystal fragments were placed onma copper grid covered with an amorphous holey carbon film.
Table 1 Electron microprobe
233 (1996) l_TO-i60
3. Results and discussion 3.1. Electron microprobe
analysis
Microprobe examination reveals that the all obtained samples are homogeneous and contain no parasitic phase. The experimental atomic percentages deduced from the analysis are summarized in Table 1. It is worthwhile noting that the U,TSi, silicides exhibit a range of homogeneity on both silicon-poor and silicon-rich sides. However, in this work, the chemical composition of the sample studied is very close to the ideal stoichiometry U,TSi, U, 33.3%; T, 16.7%; Si, 50%. The experimental atomic ratio (T + Si): U exhibits only negligible deviations from the theoretical value. This observation tends to exclude the existence of vacancies in the arrangement of the silicon-transition element network. Such defects have been found previously for the binary silicides such as U,Si,, [lO,ll] and Er,Si+ [12]. 3.2. Transmission electron microscopy examination The first (TEM) examination of the U,TSi, series was performed recently on U,RuSi, which crystallizes in a new ordered structure of the hexagonal AlB, type [8]. The selected area electron diffraction patterns of U,RuSi, have revealed a superstructure which is explained by perfect order between ruthenium and silicon atoms occupying the boron site of the AlB, type.
U,TSi, Experiment
(at.%)
(T + Si):U
U
T
Si
33.6(4) 33.2(4) 33.4(6) 32.8(4) 32.9(4) 33.2(7) 33.0(4) 33.1(5)
16.5(2) 16.4(2) 16.2(6) 16.7(4) 15.5(S) 16.9(3) 16.0(5) 16.5(5)
49.9(4) 50.4(3) 50.4( 7) SOS(S) 51.6(4) 49.9( 6) X.0(6) 50.5(S)
1.98(4) 2.01(4) 1.99(8) 2.05(5) 2.04(S) 2.01(6) 2.03(6) 2.02(6)
33.9(5) 33.4(4) 33.2(4)
15.4(5) 15.9(4) 16.8(3)
50.6(5) 50.7(3) SO.O(4)
1.95(6) 1.99(4) 2.01(4)
152
B. Chevalier et al. I Journal of Alloys and Compounds
In order to understand the reasons which govern the occurrence of the ordered distribution between T and silicon atoms, we have extended the TEM investigation of the U,RhSi, and U,PdSi, compounds. Typical electron diffraction patterns projected along the [OOl] zone axis of the subcells of U,RuSi, (taken from Ref. [S], U,RhSi, and U,PdSi, are shown in Fig. 1. From this figure it can clearly be ruled out that the lattice parameters a and b of U,RuSi, are twice those of the corresponding primitive hexagonal AlB, cell. This result, which agrees with a perfect long-range order of Ru and Si atoms within the 2D Ru-Si arrangement, was confirmed by X-ray single-crystal structure refinement [8]. A projection of the crystal structure of U,RuSi, onto the (001) plane is given in Fig. 2. In the U,RuSi,-type structure (P6/mmm space group), Ru and Si atoms are not in the same plane perpendicular to the c axis since the former atoms are located at the 2d site (l/3,2/3,1 /2) whereas the silicon atoms are on the 120 split position (0.166, 2x, 0.4433) which is occupied at 50%. For U,RhSi,, the typical electron diffraction pattern obtained for the [OOl] zone axis is quite different of those observed for U,RuSi, (Fig. l(b)). Weak superstructure reflections appear along one direction of its reciprocal lattice which cannot be indexed in the primitive hexagonal AlB, cell. For the lattice of this silicide, contrary to what is observed in the U,RuSi,
(a)
233 (1996)
150-160
type, the [lOO] direction periodicity is twice that of the AlB, cell. Furthermore, additional diffuse lines are observed in a parallel direction to the [NO] axis. Taking into account all these observations, we suggest that U,RhSi, crystallizes with orthorhombic symmetry. All reflections of the superstructures can be indexed with the following lattice parameters: a = a,2/3, b = q, and c = c,,. The atomic arrangement in U,RhSi, may be described in the Pmmm space group with two positions for the uranium atoms (la (0,0,O) and If (l/2,1/2,0)), one position for silicon (2j (x = 0.33,0,1/2)) and one randomly occupied (21 (x = 0.90,1/2,1/2)) by 50% of rhodium and 50% of silicon atoms (Fig. 3(a)). A similar crystal structure was proposed recently for the non-stoichiometric binary silicides U,Si=, and Er,Si,, [11,12] in which the occurrence of the orthorhombic superstructure is due to a partial order of the silicon vacancies which occupy only one site of the two independent positions assigned to the silicon atoms. The electron diffraction patterns corresponding to U,PdSi, do not reveal a superstructure (Fig. l(c)). However, some very weak diffuse lines are observed along the [loo] axis in addition to fundamental spots resulting from the AlB,-type structure. This observation indicates a tendency towards an atomic ordering in the Pd-Si network suggesting that the crystal structure of U,PdSi, can be described in an ortho-
w
W
Fig. 1. Typical electron diffraction patterns along the [OOI] zone axis for (a) U,RuSi, better comparison all patterns are shown in the same enlargement.
(taken
from Ref. [8]), (b) U,RhSi,
and (c) U,PdSi,.
For
B. Chevalier et al. I Journal of Alloys and Compounds
Fig. 2. Crystal
structure
of U,RuSi,
projected
onto
233 (1996) 150-160
the (001) plane.
UzRhSig
U2PdSi3
(a)
(b)
b -
Fig. 3. Crystal structure of (a) U,RhSi, and (b) U,PdSi, projected onto the (001) plane: U at z, = 0; @, 50% Rh and 50% Si; 0,25% 75% -. E Si; 0, 25% - E Pd and 75% + E Si; 0, Si; all these atoms are located at z = l/2.
rhombic cell similar to that observed for U,RhSi, @hv3, Oh, ch) (Fig. 3(b)). However, in U,PdSi, the palladium and the silicon atoms are randomly distributed into the two sites 2j and 21 with a different atomic repartition. Certainly, the 2j site contains less palladium than the 21 site in comparison with that previously observed for U,RhSi,. At this point it is worthwhile noting that the additional superstructure reflections and diffuse lines for U,RhSi, and U,PdSi, could only be observed for the microcrystals investigated by TEM. However, a TEM study performed on U,RuSi,, U,RhSi, and
153
+ E Pd and
U,PdSi, shows that the crystallographic order between the transition element T and the silicon atoms is strongly influenced by the nature of T. It is favoured when the T element has fewer d electrons: it is perfect in the case of U,RuSi, (Ru, 4d7) and decreases gradually from U,RhSi, (Rh, 4d’) to U,PdSi3 (Pd, 4d”). This result suggests that the electronic factor could be responsible for the T-Si ordering in the U,TSi, silicides. This factor can be changed when the T element substitutes silicon atoms or by the occurrence of vacancies in the Si sublattice as for U,Si,, silicide [ll].
B. Chevalier et al. I Journal of Alloys and Compounds
154
3.3. Structural chemistry considerations
fact that the c,, parameter decreases in the sequence U,NiSi, -+ U,CoSi, -+ U,FeSi, + U,MnSi, cannot be explained by simple steric factor but rather by electronic considerations. Recently, we have found similar variations in the unit cell parameters vs. radius rT for a series of ternary stannides U,T,Sn with T = Fe, Co, Ni, Ru, Rh and Pd [14]. These compounds crystallize in the tetragonal ordered form of the U,Si,-type structure in which a U, trigonal prism contains the T atom. This type of structure is described as atomic planes perpendicular to the c axis with the sequence (T, Sn)-U-(T, Sn)-U very similar to that observed for the U,TSi, silicides: (T,Si)-U-(T, Si)-U. The presence of 2D (T, Sn) or (T, Si) sublattice perpendicular to the c axis in these compounds readily explains the dependence of the tetragonal or hexagonal parameter a or ah respectively. On the contrary, the U-T bond appears responsible for the variation of the c (tetragonal) or c,, (hexagonal) parameter in these structures. The low value of c,, corresponding to short U-T distances observed for U,TSi, with T = Fe, Mn, Ru, OS (Fig. 4) suggests a strong hybridization between the uranium 5f orbitals with those of the T atoms. It is interesting to compare the unit cell parameters of the U,TSi, compounds with those reported previously on the ternary silicides RE,TSi, (RE, rare earth). For instance, the structural properties of sever-
The crystallographic data concerning the U,TSi, ternary silicides studied in this work and these found in the literature are summarized in Table 2. For the sake of comparison, the unit cell parameters (a,,, c,,) of the hexagonal AlB,-type subcell are also given for all compounds. The X-ray powder pattern of U,MnSi,, reported here for the first time, displays weak reflections corresponding to the hexagonal U,RuSi,-type. In other words, the manganese and silicon atoms which are both surrounded by U, trigonal prisms are ordered in a 2D network. This result reinforces the explanation that the T-Si ordered sublattice in the U,TSi, compounds is observed when the T element has fewer d electrons (Mn, 3d5). The lattice parameters (a,, c,,) of all U,TSi, compounds are plotted in Fig. 4 as a function of the metallic radius (coordination number, 12) of the T element [13]. From this figure it can easily be seen that a,, increases linearly with rT. On the contrary, the c,, parameter decreases with increasing rT for the silicides containing a 3d transition element but increases for both 4d- and Sd-based compounds. We note that the c,, parameter for U,OsSi, is clearly smaller than that observed for U,RhSi, although ros = 135.3 pm > rRh = 134.5 pm (Fig. 4 and Table 2). In the same way, the
Table 2 Crystallographic Silicide
data
of the U,TSi,
Symmetry
ternary
Structure
233 (1996) 150-160
silicides Unit cell parameters
(pm)
AlB, subcell
Ref.
type 385.2( 1)
667.4(8) 804.5( 1)
407.4(2) 380.82(6)
400.4 401
386.4 384
4028(l)
0 H
U,RuSi,
384.6( 3) 804.5( 1)
U,FeSi, U,FeSi, U,FeSi,
H H H
AlB, AlBz
400.4 401
U,COSl,
H H
AlB, AlB,
U,IrSi, U,IrSi,
H H H H 0 H 0 H H H H H
U,PtSI,
H
U,PtSi, U,PtSi, U,AuSi,
H H H
H, hexagonal;
0, orthorhombic;
U,OSSl,
402.8( 1)
AIB,
H
USi, U,MnSi,
U,NiSi, U,NiSi, U,RuSi, U,RuSi, U,RhSi, U,RhSi, U,PdSi, U,PdSi, U,OSSl,
ah
b
USi,
U,CoSi,
c
a
U,RuSi, AlB, AlB, AlB, U,RuSi, AIB,
398.8 397.9 814.8(2) 407.5(2) 703.6( 1) 407.6(2) 706.7( 1) 408.5(2) 816.0(2) 406.7( 1)
398.8 397.9 814.8(2) 407.5(2) 406.2( 1) 407.6(2) 408.0( 1) 408.5(2) 816.0(2) 406.7(l)
388.3 394.9 385.5(2) 383.8( 1) 392.9(l) 388.3(2) 393.9(l) 393.5(2) 384.4( 1) 385.2( 1)
AlB,
407.2( 1)
407.2( 1)
389.5( 1)
AlB, AlB,
408.4 406.7( 2)
408.4 406.7(2)
397.3 396.4( 1)
AIB,
414.5(3)
414.5(3)
398.9(2)
a,, and c,, are the unit cell parameters
relative
to AlB,
Ch
5 Ia,
402.8( 1)
385.2( 1)
0.956
384.6(3) 402.25(5)
407.4( 2) 380.82(6)
1.059 0.947
400.4 401 400.3(l)
386.4 384 385.7( 1)
0.965 0.958 0.964
this work
398.8 398.7(2)
388.3 3883(l)
0.974 0.974
this work
397.9 397.9( 1) 407.4 407.5(2) 406.2( 1) 407.6(2) 408.0( 1) 408.5(2) 408.0( 2) 406.7(l) 406.5( 1) 407.2(l)
394.9 394.6( 1) 385.5(2) 383.8( 1) 392.9( 1) 388.3(2) 393.9( 1) 393.5(2) 384.4( 1) 385.2( 1) 391.4( 1) 389.5( 1)
0.992 0.992 0.946 0.942 0.967 0.953 0.965 0.963 0.942 0.947 0.963 0.957
408.4 406.7 407.3( 1) 414.5(3)
397.3 396.4( 1) 396.5( 1) 398.9(2)
0.973 0.975 0.973 0.962
type subcell.
1101 1111 this work ]91
VI [91 191 this work [8], this work
[71 this work ]71 this work ]71 [8], this work
[71 this work
[71 [31 171 this work [7], this work
B. Chevalier et al. I Journal of Alloys and Compounds
C
I_
II
1
1
III1
I
I
II
233 (1996)
150-160
I
I
155
I
I
I
U2TSi3
,
/
/
,
,,
.*
0 y60 s=
/0° ,0°’
t
,&
Au ,( .
O6
Pte pNi \ \ _ I &CO \
Rho
Co
.Pd’ ,
Pd
pounds should play a significant role in the magnetic properties of the U,TSi, silicides.
dOs
\
Ru
Fig. 5. Comparison of the unit cell parameters of the AlB, subcell of U,TSi, (ah (0) and c,, (O)), Gd,T,, ,Si, z (T = Fe, Co, Ni) and Gd,TSi, (T= Rh, Pd) (a,, (+) and ch (X)).
?? If
I 1 ‘c Fe RUO /’
G“*
fI
A’
Mn
1, I
125
I
rT (pm)
I
lr15
Fig. 4. Variation in the unit cell parameters as a function of the metallic radii r7 for U,TSi, silicides. For the sake of comparison, all the parameters are given for the AlB,-type subcell.
al compounds with RE = Gd have been investigated: Gd,T,,,Si,,2 (T = Fe, Co, Ni) adopts the hexagonal AlB, type [15] whereas Gd,TSi, (T = Rh, Pd) has a hexagonal structure of the Er,RhSi, type which derives from AlB, type [16,17]. Fig. 5 shows that the ah parameters of the Gd- or U-based silicides containing the same T element have close values. This result confirms that the ah parameter is essentially governed by the size of the T atom within the 2D (T, Si) network. On the contrary, the q, parameters of the Gd,TSi, compounds are greater than these observed for U,TSi, silicides, since the q, values are mainly influenced by the Gd-T or U-T bonds. This result is therefore in agreement with the metallic radius of gadolinium (rod = 18012pm) which is larger than that of uranium (rU = 156pm) [13]. To summarize, we can say that in the U,TSi, series with T = Mn, Fe, Co, Ni, the interatomic distance d,_, increases as rT decreases (see Table 3) suggesting a strong overlap between the 5f (U) and 3d (Mn or Fe) states. The U-T distances which influence the 5f(U)rid(T)) hybridization in the uranium intermetallic com-
3.4. Magnetic properties The magnetic data relative to the U,TSi, silicides obtained during this work as well as those available in the literature are reported in Table 3. The presence of small amounts of magnetic impurities in U,MnSi, (detected in grain boundaries by microprobe analysis) has not allowed us to determine the intrinsic magnetic properties of this silicide. 3.4.1. Silicides with T = Fe, Ru, OS
The reciprocal magnetic susceptibility of U,FeSi,, U,RuSi, and U,OsSi, follows a Curie-Weiss law X,’ = (T - 0,)/C, above 50 K, 60 K and 50 K respectively (Fig. 6). The effective magnetic moment of uranium (k”. = (LX,)“*) deduced from this behaviour (Table 3) is comparable with that determined for the binary silicide U,Si, (k,, = 3.23& per U atom) [ll]. Below 30 K, 60 K and 50 K respectively, the curves xi’ =f(T) for U,FeSi,, U,RuSi, and U,OsSi, exhibit both a positive deviation from the Curie-Weiss law and a tendency to take a constant value at low temperature down to 4.2 K. This behaviour can be ascribed to the existence of local spin fluctuations as a result of the occurrence of Kondo interactions. However, their paramagnetic behaviour leads to a large negative value for the Curie paramagnetic temperature 0p (Table 3). This agrees well with the Kondo effect coupling. It is worthwhile noting that the temperature /3, for U,OsSi, is smaller than that deter-
B. Chevalier et al.
156
Table 3 Average U-T
distances
Silicide
d,_, (pm)
U,MnSi,
300.3
U,FeSi,
301.3 301.0 301.2 301.1
U,CoSi,
data
of the U,TSi,
compounds
X0 ( X10m3 e.m.u. (U atom))‘)
BP(K)
2.77 3.35
0.732
-86 -132
Magnetic
temperatures
T,
T(x’)
(K)
2.03 2.01
1.15 1.19
0 2
10 10
8
0.769 0.92
11 32
25 26
22
30
0.784 1.232 1.057
6, -70 - 130 17 7 -186 3
1.045 1.993
19, -7 10 5
8 13.5 11.6
T(x”)
Magnetic
moments
~(2 T)
302.9 2.38 302.8 2.41
U,RuSi, U,RhSi, U,PdSi, U,OsSi, UJrSi,
303.6 305.9 307.0 304.0 305.6
U,PtSi,
308.3 2.1 307.6 2.15 311.5 1.65
(U atom))’
Ref.
I%
2.75 3.02 2.32 2.39 3.03 1.93
[91 this work
19.5 15.5 11
U2TSi3
+ ??
Fe Ru
+
00s
---talc.
100
300
T (K)
Fig. 6. Thermal dependence of the reciprocal of U,TSi, with T = Fe, Ru and OS.
magnetic
susceptibility
mined for U,FeSi, and U,RuSi,. The great extension of the 5d(Os) orbitals which favours the 5f(U)-5d(Os) hybridization enhances the conduction electron-5f
[91
0.28 0.16
0.05 0.06
this work
0.67 0.51
0.31 0.35
[91 this work
20.3 11.4
20.3 10.2
0.45 0.33
0.31 0.03
9.8
9.4
0.22
0.1
Cl81 [8], this work this work this work this work this work
10.8 11.6
10.0 10.8
0.31 0.34
0.02 0.11
t31 this work this work
0.6”
electron exchange interaction, tuations [19].
c
(h
this work
U,NiSi,
U,AuSi,
and magnetic
btt (j+ (U atom))‘)
I Journal of Alloys and Compounds 233 (1996) 150-160
leading to Kondo fluc-
3.4.2. Silicides with T = Co, Ni, Rh, Pd, Ir, Pt, Au Fig. 7 shows the thermal dependence of the reciprocal magnetic susceptibility of these ternary compounds. Contrary to previous results given in Refs. 193 and [3], no anomaly can be detected around 80-100 K in the xi’ =f(T) curve of both U,NiSi, and U,PtSi,. The observation of these anomalies is due to the presence of small amounts of impurity phases such as UNiSi, and UPtSi, which order ferromagnetically at respectively T, = 100 K and 85 K [2]. Above 30 K, the xi’ = f(T) curve for all silicides can be well described by a modified Curie-Weiss law x,,, = x0 + C, l(T - 19~) where x,, is the temperatureindependent contribution. The magnetic data derived from this analysis are summarized in Table 3. The Curie paramagnetic temperature 0, is positive, indicating the predominance of ferromagnetic interactions. The effective magnetic moment &rf,, ranging from 1.65& (U atom))’ (U,AuSi,) to 2.41& (U atom))’ (U,NiSi,), is smaller than those determined for the series U,TSi, with T = Fe, Ru, OS. The origin of these small values could be due to the character of the magnetic anisotropy in the paramagnetic domain which affects the calculation of bft, from the measurements performed on powder samples. However, we can exclude this effect since recent measurements performed on a single crystal of U,PtSi, [3] or U,NiSi, [X3] with an applied magnetic field parallel or perpendicular to the c axis reveal similar krr, values along the a and c directions. The values observed for hrr. would be rather due to an itinerant character of the 5f(U) electrons which leads to a decrease in the
B. Chevalier et al. I Journal of Alloys and Compounds
233 (1996)
150-160
1.57
Ni
+
i3
0 Pt ____ talc.
talc.
0 Fig
7. Thermal
100 dependence
T (K) of the reciprocal
+
?? W
300 magnetic
0 susceptibility
moment k,,, as for UCoGa (,q,,,. = 2.40& (U atom))’ [20]. The partial itinerancy of 5f(U) electrons in these ternary silicides may be governed by the strength of the Sf(U)-rid(T)) hybridization. In this view, it is note the series interesting to that, in U,RuSi,, --, U,RhSi, + U,PdSi,, the higher moment kfr, is obtained for U,RuSi, which exhibits a perfect structural arrangement between Ru and Si atoms (Table 3). It is well known that, in a series of compounds, the Sf(U)-rid(T)) hybridization depends on the geometric repartition of the T atoms. For these reasons, the involved hybridization may, for instance, be diffe::ent in U,RuSi, and U,PdSi,, which exhibit respectively a Ru-Si ordering and a Pd-Si disordering. Fig. 8 shows the thermal dependence of the magnetizaticn of U,CoSi, and U,IrSi, measured under an applied field (0.1 T) with decreasing temperature (FC curve). For U,RhSi,, the FC curve is compared with that obtained (ZFC curve) after cooling in zero field. The ZFC curve of U,RhSi, exhibits a maximum at about 14 K and then a strong decrease near 19.5 K with increasing temperature; the FC curve has at low temperatures a tendency to approach a constant value.
of U,TSi,
100
0
“0 3 \
with T = (a) Co, Rh, Ir and (b) Ni, Pd and Pt
I
I
UqTSi? L
--•
00
*o
.ee8be
P
300
-f (K)
I
r3
(b)
b
9
+
co
??
Rh
J
0 Ir
??
e
7
?? ??
0
c
zz
i
1
Hz 0.1 T
?? ? ?
-e
?? ?? ?? ??
e
C 10
T (K)
Fig. 8. Thermal dependence of the magnetization of U2CoSi,, UJrSi, (field cooling (FC) curve) and U,RhSi, (zero field cooling (ZFC) and FC curves). The applied magnetic field is 0.1 T.
B. Chevalier et al. I Journal of Alloys and Compounds
158
ples of U,TSi, are shown in Fig. 9. The behaviour of both U,NiSi, and U,RhSi,, characterized by a transition in the initial magnetization curve and a relatively large hysteresis loop, originates from narrow walls between ferromagnetic domains which are frozen by the strong anisotropy. Recently, the study of U,NiSi, single crystals by neutron diffraction shows that this compound orders ferromagnetically with uranium magnetic moment (0.6(1)h) perpendicular to the c axis of the AlB, subcell [18]. Our magnetization measurements giving 0.51c~, per U atom for H = 2 T is in agreement with this study. At 4.2 K, the magnetization of U,CoSi,, U,PdSi,, U,IrSi, and U,PtSi, as a function of the applied field exhibits a behaviour characteristic of a weak ferromagnet (Fig. 9). The hysteresis phenomenon is small as well as the magnetization which is especially strongly field dependent. At 4.2 K, the weak magnetization observed for H = 2 T (Table 3) could be an indication of the itinerant character of the 5f(U) electrons. It is worthwhile noting that the enhanced electronic specific heat coefficient y = 200 mJ Kp2 (mol U) _I, determined in the case of U,PtSi,, is in agreement with a reduced magnetic moment for uranium [3]. Fig. 10 shows the real x’ and imaginary x” components of the a.c. susceptibility for U,RhSi, and U,PdSi,. For the first silicide, the x’ and ,y”=f(T) curves exhibit a sharp peak at the same temperature, 20.3 K (Table 3). On the contrary, the peaks observed for U,PdSi, are broader and one order of magnitude smaller than that appearing for U,RhSi,. The x’ = f(T) curve exhibits a maximum at 11.4 K whereas the peak in the ,$‘=f(T) curve occurs at 10.2 K. Also x” is not zero above 11.4 K. These main features show that U,PdSi, behaves as a ferromagnetic-spin glass state [22]. The distribution between Pd and Si atoms which is almost statistical in U,PdSi, can allow random magnetic interactions in this compound. It is worthwhile noting also that U,TSi, with T = Ir, Pt and Au can be compared with U,PdSi, since the maxima in
This result characterizes a ferromagnet displaying noticeable domain-wall pinning effects. The Curie temperatures T,, given in Table 3 for each silicide, were estimated from the inflection point of the FC curve. The discrepancy shown for the T, value of U,PtSi, is due to their chemical composition, which is different. The U,T,_.,Si,+X silicides exhibit as do Th,Co, _XSi3+Xa composition homogeneity range [21] in which the hexagonal AlB, type is adopted for -0.40 =Zx G 0.10. The initial magnetization curves and the hysteresis loops measured at 4.2 K for the polycrystalline samI
I
I
III
U2TSi3
I
oooooo
od@@ O0
P
Pt
-0
T,4.2K C 0
2
20 H CT)
Fig. 9. Magnetic field dependence some U,TSi, compounds.
233 (1996) 150-160
at 4.2 K of the magnetization
of
UZRhSig 5 Oe 125
Hz
0
el 0
\” _
00
64 t
0
20
125
Hz
0
_
O 0 0 0
0
0.6
20
T (K) Fig. 10. Thermal
dependence
of the a.c. susceptibility
(,$ (0)
and x” (0))
for U,RhSi,
and U,PdSi,.
B. Chevalier et al. I Journal of Alloys and Compounds
their x’ = f(T) and x” = f(T) curves do not appear at the same temperature (Table 3). This behaviour has been confirmed for U,PtSi, where no anomaly is observed around T, = 8 K in specific heat measurements [3], indicating thus a spin glass state. The magnetic measurements performed on U,RhSi, and U,PdSi, reveal a different magnetic behaviour (Figs. 8 and 9). (i) At T, = 19.5 K the magnetization of U,RhSi, increases strongly; this transition leads to the occurrence of both the pronounced peaks in the thermal dependence of the a.c. susceptibility and a noticeable remanent magnetization (Table 3); these observations characterize a ferromagnet. (ii) On the contrary, the magnetic transition observed for U,PdSi, induces small changes (weak jump in the magnetiation at T,., low remanent magnetization at 4.2 K (Table 3)); this behaviour can be ascribed to a ferromagnetic-spin glass state as shown by the a.c. susceptibility measurements. The magnetic properties of these two ternary silicides can be understood on account of their structural properties. The more ordered arrangement of the Rh-Si sublattice in U,RhSi, corresponds to the occurrence of a long-range ferromagnetic ordering. On the contrary, the spin glass behaviour of U,PdSi, is due to the random distribution of the U-U interactions resulting from the very strong disordering between Pd and Si. However, neutron diffraction and specific heat measurements are be necessary in order to confirm the suggested magnetic state of U,RhSi, and U,PdSi,.
4. Conclusion Many transition elements T can replace silicon in USi, allowing the formation of the ternary silicides U,TSi,. These compounds crystallize in the hexagonal AlB, type or a derived structure. The structural arrangement of the T-Si sublattice is strongly dependent on the nature of the T element. For instance, the study of’U,RuSi,, U,RhSi, and U,PdSi, by electron diffraction reveals that these silicides adopt interesting superstructures of the AlB, type: (i) Ru and Si atoms are perfectly ordered in U,RuSi, which leads to the occurrence of a new hexagonal superstructure having the unit cell parameter a twice as great as that observed for the ideal AlB, type; (ii) the Rh-Si network in U,RhSi, exhibits a partial ordered arrangement corresponding to an orthorhombic superstructure; (iii) finally, in U,PdSi,, the distribution between Pd and Si is almost statistical. This study shows that the occurrence of a superstructure is increasingly favoured as the number of d electrons of the T element becomes smaller. At low temperatures, a non-magnetic ground state is observed for uranium in U,FeSi,, U,RuSi, and U,OsSi,. On the contrary, a ferromagnetic behaviour
233 (1996) 150-160
159
or a ferromagnet-spin glass state characterizes all the other ternary silicides. For the U,TSi, series as for the tetragonal UT,%, compounds, the magnetic state of uranium becomes more pronounced as the number of d electrons of the T element increases. Endstra et al. have shown that the magnetic properties of the UT,%, silicides are determined by the strength of the 5f(U)rid(T)) hybridization [19]. A decrease in the filling of the rid(T)) band induces an increase in the hybridization. The proposed “f-d hybridization model” based on the determination of the Sf(U)-rid(T)) overlap which explains well the physical properties of the UT& compounds could be used for the U,TSi, series. In order to understand better the structural properties (e.g. variation in the unit cell parameters) and the magnetic behaviour of some terms of the U,TSi, series, a band calculation structure using an augmented spherical-wave method is now in progress. This method provides information on the variation in Fermi energy with respect to bonding and antibonding states.
References Pinto, Acta Crystallogr., 21 (1966) 999. Geihel, C. Ksmmerer, E. Giiring, R. Moog, G. Sparn, R. Henseleit. G. Cordier, S. Horn and F. Steglich. J. Magn. Magn. Mater., 90-91 (1990) 435. [31 N. Sato, M. Kagawa, K. Tanaka, N. Takeda, T. Satoh, S. Sakatsume and T. Komatsubara, J. Phys. Sac. Jpn., 60 (1991) 757. [41 N. Sato, M. Kagawa, K. Tanaka, N. Takeda, T. Satoh and 7‘. Komatsubara, J. Magn. Magn. Mater., 108 (1992) 115. PI E. Hickey, Thesis no. 803, University of Bordeaux I, 1992. H. NoCl and R. PGttgen, 23Pmes Journees des [61 D. Kaczorowski, Actinides, Schwarzwald, 1993, abstract.
Ill M. de Lourdes
[21 C.
and D. Kaczorowski, J. Alloys Compd., 201 (1993) 157. B. Darriet. B. Chevalier, E. Hickey PI R. Piittgen, P. Gravereau, and J. Etourneau, 1. Mater. Chem., 4 (1994) 463. and H. No+ J. Phys.: Condens. Matter, 5 191 D. Kaczorowski (1993) 9185. [lOI A. Brown and J.J. Norreys, Nature (London), 183 (1959) 673. [Ill K. Remschnig, T. Le Bihan, H. Nosi and P. Rogl. .I. Solid State Chem., 97 (1992) 391. and J.A. [=I S. Auffret, J. Pierre, B. Lambert, J.L. Soubeyroux Chroboczek, Physica B, 162 (1990) 271. [I31 E. Taetum, K. Gschneider and J. Waber, cited in W.B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys. Wiley, New York, 1972, p. 1.51. and J. u41 F. Mirambet, B. Chevalier, L. Fourn&. P. Gravereau Etourneau, J. Alloys Compd., 203 (1994) 29. Met., 19 (1969) 173. [151 I. Mayer and M. Tassa, J. Less-Common Solid P61 B. Chevalier, P. Lejay, J. Etourneau and P. Hagenmuller, State Commun., 49 (1984) 753. J. Magn. [I71 P.A. Kotsanidis, J.K. Yakinthos and E. Gamari-Scale, Magn. Mater., 87 (1990) 199. M.F. Collins, C.V Stager, J.D. Garrett, J.E. WY A. Schrader, Greedam and Z. Tun, J. Magn. Magn. Mater.. 140-144 (1995) 1407.
I71 R. Piittgen
160
B. Chevalier et al.
[19] T. Endstra,
G.J. Nieuwenhuys
and J.A. Mydosh,
I Journal of Alloys and Compounds 233 (1996) 150-160 Phys. Rev. B,
48 (1993) 9595. [20] H. Nakotte, F.R. de Boer, L. Havela, P. Svoboda,V Sechovsky, Y. Kergadallan, J.C. Spirlet and J. Rebizant, J. Appl. Phys., 73 (1993) 6554.
[21] W.X. Zhong, W.L. Ng, B. Chevalier, J. Etourneau and P. Hagenmuller, Mater. Rex Bull., 20 (1985) 1229. [22] R.B. Goldfarh, K.V. Rao and H.S. Chen, Solid State Commun., 54 (1985) 799.
Journal of Alloys and Compounds
233 (1996) 150-160
Structural chemistry and magnetic behaviour of the ternary silicides U,TSi, (T = Mn, Fe, Co, Ni, Ru, Rh, Pd, OS, Ir, Pt, Au) B. Chevalier*,
R. PBttgen’, B. Darriet, P. Gravereau,
Institut de Chimie de la Matitre Condenste
de Bordeaux,
ICMCB
(UPR-CNRS
J. Etourneau
9048), 33608 Pessac Ctdex, France
Received 12 June 1995
Abstract The crystallographic structures of many U,TSi, compounds with T a transition element were investigated by both X-ray and electron diffraction studies. These silicides crystallize in the ideal or derivative structure of the hexagonal AlB, type. A structural ordering between T and Si atoms exists in these various compounds; it is strongly influenced by the nature of the transition element. This structural ordering seems to be attributed to an electronic factor rather than to a steric factor; it is favoured with T elements having a small number of d electrons. Furthermore, these compounds exhibit interesting magnetic properties since for T = Fe, Ru and OS, they display a spin fluctuation behaviour at low temperatures whereas for T = Co, Ni, Rh, Pd, Ir, Pt and Au they order magnetically. Keywords:
Uranium; Ternary silicides; Crystal structure; Spin fluctuations; Ferromagnetism
1. Introduction
Recently many ternary silicides with general formula U,TSi, (T = Fe, Co, Ni, Cu, Ru, Rh, Pd, OS, Ir, Pt, Au) have been prepared [l-7]. The crystal structure of these compounds, with the exception of U,CuSi,, is derived from the hexagonal AlB, type..In most cases, the silicon and transition element atoms are, for these ternary silicides, randomly distributed into the trigonal prisms of a primitive hexagonal array of uranium atoms. For this distribution, a hexagonal unit cell similar to that of AlB,, is defined by (a,, a,,~,). However, our recent X-ray and electron diffraction studies on U,RuSi, and U,OsSi, show that silicon and ruthenium or osmium atoms are ordered in a two-dimensional (2D) network [8]. The latter silicides crystallize in a new derived structure of the hexagonal AlB, type having (2a,, 2u,, c,,) as parameters of the unit cell. * Corresponding author. ’Permanent address: Max Planck-Institiit fiir Festkoperforschung, Heisenbergstrasse
1, 70569 Stuttgart,
0925-8388/96/$15.00 0 1996 Elsevier SSDI 09258388(95)02006-3
Germany. Science
S.A. All rights
reserved
Some compounds U,TSi, with T = Fe, Co, Ni, Cu, Ru and Pt were investigated by magnetic measurements and were shown to have interesting physical properties. U,FeSi, and U,RuSi, exhibit a spin fluctuation behaviour at low temperatures [8,9] whereas U,CoSi,, U,NiSi, and U,CuSi, can be considered as “re-entrant spin glasses” systems [9], i.e. with the occurrence of a ferromagnetic state between the paramagnetic and low temperature spin glass states. On the contrary, some controversy exists concerning the magnetic properties of U,PtSi, which is classified as a spin glass [2] or a weak itinerant ferromagnetic system [3,4]. All these studies suggest that the physical properties of the U,TSi, compounds are strongly influenced by the nature of the transition element T which governs the strength of the Sf(U)-rid(T)) hybridization and consequently the nature of the uranium magnetic state. For a better understanding of this Sf(U)-rid(T)) hybridization, it is essential to study the magnetic properties of a great number of U,TSi, compounds. In this paper are reported both structural and magnetic properties of the U,TSi, silicides. The discussion of their magnetic behaviour takes into account both the distances d,_, and
B. Chevalier et al. I Journal of Alloys and Compounds
the degree of the crystallographic order between silicon and transition element atoms.
2. Experimental
the
details
Silicide
U,MnSi, U,FeSi, UzCoSi, U,NISI, U,RuSi, U,RhSi, U,PdSi, UzOsSi, U,IrSi, U,PtSi, U,AuSi,
analyses
of the ternary
Standard
UMn,Si, U,Fe,Si UCoSi UNi$, URu,Si, URh,Si, UPd,Si, U,Os,Ge, URu,Si, UIr,Si, UPt& UAu>Si,
silicides
151
Magnetization measurements were carried out between 4.2 and 300K using both a pendulum susceptometer and a superconducting quantum interference device magnetometer. The a.c. susceptibility measurements were performed using a Lake Shore Series 7000 AC susceptometer.
Starting materials for the preparation of the ternary silicides were platelets of uranium, powders or ingots of the transition elements and lumps of silicon, all with stated purities above 99.9%. The uranium platelets were cleaned with concentrated nitric acid to remove oxide impurities. They were not allowed to contact air prior to the reactions. U,TSi, silicides were prepared by melting of the elemental components in an induction levitation furnace under an atmosphere of magnesium-gettered argon. The buttons were flipped over and remelted several times to ensure homogeneity. After the melting process all samples were sealed in evacuated silica tubes and annealed at 800°C for two months. Microprobe examinations were carried out for several samples in a CAMEBAX scanning electron microscope. The analyses of the samples were based on the measurements of U Mel, , Mn Ke, , Fe Ka,, CoKa,, NiKa,, RuL(-w,, RhLa,, OsLa,, IrLol,, Pt Lo_,, Au Lol, and Si Ka, radiations, which were compared with those obtained by using a standard silicide. through their All samples were characterized Guinier powder patterns which were recorded with using 99.999% silicon (a = Cu Kor, radiation 543.07 pm) as an internal standard. The lattice constants were obtained by least-squares refinements of the Guinier powder data. For the investigation in the electron microscope (JEOL 2000 FX), parts of the samples were crushed in an agate mortar with methanol. Small crystal fragments were placed onma copper grid covered with an amorphous holey carbon film.
Table 1 Electron microprobe
233 (1996) l_TO-i60
3. Results and discussion 3.1. Electron microprobe
analysis
Microprobe examination reveals that the all obtained samples are homogeneous and contain no parasitic phase. The experimental atomic percentages deduced from the analysis are summarized in Table 1. It is worthwhile noting that the U,TSi, silicides exhibit a range of homogeneity on both silicon-poor and silicon-rich sides. However, in this work, the chemical composition of the sample studied is very close to the ideal stoichiometry U,TSi, U, 33.3%; T, 16.7%; Si, 50%. The experimental atomic ratio (T + Si): U exhibits only negligible deviations from the theoretical value. This observation tends to exclude the existence of vacancies in the arrangement of the silicon-transition element network. Such defects have been found previously for the binary silicides such as U,Si,, [lO,ll] and Er,Si+ [12]. 3.2. Transmission electron microscopy examination The first (TEM) examination of the U,TSi, series was performed recently on U,RuSi, which crystallizes in a new ordered structure of the hexagonal AlB, type [8]. The selected area electron diffraction patterns of U,RuSi, have revealed a superstructure which is explained by perfect order between ruthenium and silicon atoms occupying the boron site of the AlB, type.
U,TSi, Experiment
(at.%)
(T + Si):U
U
T
Si
33.6(4) 33.2(4) 33.4(6) 32.8(4) 32.9(4) 33.2(7) 33.0(4) 33.1(5)
16.5(2) 16.4(2) 16.2(6) 16.7(4) 15.5(S) 16.9(3) 16.0(5) 16.5(5)
49.9(4) 50.4(3) 50.4( 7) SOS(S) 51.6(4) 49.9( 6) X.0(6) 50.5(S)
1.98(4) 2.01(4) 1.99(8) 2.05(5) 2.04(S) 2.01(6) 2.03(6) 2.02(6)
33.9(5) 33.4(4) 33.2(4)
15.4(5) 15.9(4) 16.8(3)
50.6(5) 50.7(3) SO.O(4)
1.95(6) 1.99(4) 2.01(4)
152
B. Chevalier et al. I Journal of Alloys and Compounds
In order to understand the reasons which govern the occurrence of the ordered distribution between T and silicon atoms, we have extended the TEM investigation of the U,RhSi, and U,PdSi, compounds. Typical electron diffraction patterns projected along the [OOl] zone axis of the subcells of U,RuSi, (taken from Ref. [S], U,RhSi, and U,PdSi, are shown in Fig. 1. From this figure it can clearly be ruled out that the lattice parameters a and b of U,RuSi, are twice those of the corresponding primitive hexagonal AlB, cell. This result, which agrees with a perfect long-range order of Ru and Si atoms within the 2D Ru-Si arrangement, was confirmed by X-ray single-crystal structure refinement [8]. A projection of the crystal structure of U,RuSi, onto the (001) plane is given in Fig. 2. In the U,RuSi,-type structure (P6/mmm space group), Ru and Si atoms are not in the same plane perpendicular to the c axis since the former atoms are located at the 2d site (l/3,2/3,1 /2) whereas the silicon atoms are on the 120 split position (0.166, 2x, 0.4433) which is occupied at 50%. For U,RhSi,, the typical electron diffraction pattern obtained for the [OOl] zone axis is quite different of those observed for U,RuSi, (Fig. l(b)). Weak superstructure reflections appear along one direction of its reciprocal lattice which cannot be indexed in the primitive hexagonal AlB, cell. For the lattice of this silicide, contrary to what is observed in the U,RuSi,
(a)
233 (1996)
150-160
type, the [lOO] direction periodicity is twice that of the AlB, cell. Furthermore, additional diffuse lines are observed in a parallel direction to the [NO] axis. Taking into account all these observations, we suggest that U,RhSi, crystallizes with orthorhombic symmetry. All reflections of the superstructures can be indexed with the following lattice parameters: a = a,2/3, b = q, and c = c,,. The atomic arrangement in U,RhSi, may be described in the Pmmm space group with two positions for the uranium atoms (la (0,0,O) and If (l/2,1/2,0)), one position for silicon (2j (x = 0.33,0,1/2)) and one randomly occupied (21 (x = 0.90,1/2,1/2)) by 50% of rhodium and 50% of silicon atoms (Fig. 3(a)). A similar crystal structure was proposed recently for the non-stoichiometric binary silicides U,Si=, and Er,Si,, [11,12] in which the occurrence of the orthorhombic superstructure is due to a partial order of the silicon vacancies which occupy only one site of the two independent positions assigned to the silicon atoms. The electron diffraction patterns corresponding to U,PdSi, do not reveal a superstructure (Fig. l(c)). However, some very weak diffuse lines are observed along the [loo] axis in addition to fundamental spots resulting from the AlB,-type structure. This observation indicates a tendency towards an atomic ordering in the Pd-Si network suggesting that the crystal structure of U,PdSi, can be described in an ortho-
w
W
Fig. 1. Typical electron diffraction patterns along the [OOI] zone axis for (a) U,RuSi, better comparison all patterns are shown in the same enlargement.
(taken
from Ref. [8]), (b) U,RhSi,
and (c) U,PdSi,.
For
B. Chevalier et al. I Journal of Alloys and Compounds
Fig. 2. Crystal
structure
of U,RuSi,
projected
onto
233 (1996) 150-160
the (001) plane.
UzRhSig
U2PdSi3
(a)
(b)
b -
Fig. 3. Crystal structure of (a) U,RhSi, and (b) U,PdSi, projected onto the (001) plane: U at z, = 0; @, 50% Rh and 50% Si; 0,25% 75% -. E Si; 0, 25% - E Pd and 75% + E Si; 0, Si; all these atoms are located at z = l/2.
rhombic cell similar to that observed for U,RhSi, @hv3, Oh, ch) (Fig. 3(b)). However, in U,PdSi, the palladium and the silicon atoms are randomly distributed into the two sites 2j and 21 with a different atomic repartition. Certainly, the 2j site contains less palladium than the 21 site in comparison with that previously observed for U,RhSi,. At this point it is worthwhile noting that the additional superstructure reflections and diffuse lines for U,RhSi, and U,PdSi, could only be observed for the microcrystals investigated by TEM. However, a TEM study performed on U,RuSi,, U,RhSi, and
153
+ E Pd and
U,PdSi, shows that the crystallographic order between the transition element T and the silicon atoms is strongly influenced by the nature of T. It is favoured when the T element has fewer d electrons: it is perfect in the case of U,RuSi, (Ru, 4d7) and decreases gradually from U,RhSi, (Rh, 4d’) to U,PdSi3 (Pd, 4d”). This result suggests that the electronic factor could be responsible for the T-Si ordering in the U,TSi, silicides. This factor can be changed when the T element substitutes silicon atoms or by the occurrence of vacancies in the Si sublattice as for U,Si,, silicide [ll].
B. Chevalier et al. I Journal of Alloys and Compounds
154
3.3. Structural chemistry considerations
fact that the c,, parameter decreases in the sequence U,NiSi, -+ U,CoSi, -+ U,FeSi, + U,MnSi, cannot be explained by simple steric factor but rather by electronic considerations. Recently, we have found similar variations in the unit cell parameters vs. radius rT for a series of ternary stannides U,T,Sn with T = Fe, Co, Ni, Ru, Rh and Pd [14]. These compounds crystallize in the tetragonal ordered form of the U,Si,-type structure in which a U, trigonal prism contains the T atom. This type of structure is described as atomic planes perpendicular to the c axis with the sequence (T, Sn)-U-(T, Sn)-U very similar to that observed for the U,TSi, silicides: (T,Si)-U-(T, Si)-U. The presence of 2D (T, Sn) or (T, Si) sublattice perpendicular to the c axis in these compounds readily explains the dependence of the tetragonal or hexagonal parameter a or ah respectively. On the contrary, the U-T bond appears responsible for the variation of the c (tetragonal) or c,, (hexagonal) parameter in these structures. The low value of c,, corresponding to short U-T distances observed for U,TSi, with T = Fe, Mn, Ru, OS (Fig. 4) suggests a strong hybridization between the uranium 5f orbitals with those of the T atoms. It is interesting to compare the unit cell parameters of the U,TSi, compounds with those reported previously on the ternary silicides RE,TSi, (RE, rare earth). For instance, the structural properties of sever-
The crystallographic data concerning the U,TSi, ternary silicides studied in this work and these found in the literature are summarized in Table 2. For the sake of comparison, the unit cell parameters (a,,, c,,) of the hexagonal AlB,-type subcell are also given for all compounds. The X-ray powder pattern of U,MnSi,, reported here for the first time, displays weak reflections corresponding to the hexagonal U,RuSi,-type. In other words, the manganese and silicon atoms which are both surrounded by U, trigonal prisms are ordered in a 2D network. This result reinforces the explanation that the T-Si ordered sublattice in the U,TSi, compounds is observed when the T element has fewer d electrons (Mn, 3d5). The lattice parameters (a,, c,,) of all U,TSi, compounds are plotted in Fig. 4 as a function of the metallic radius (coordination number, 12) of the T element [13]. From this figure it can easily be seen that a,, increases linearly with rT. On the contrary, the c,, parameter decreases with increasing rT for the silicides containing a 3d transition element but increases for both 4d- and Sd-based compounds. We note that the c,, parameter for U,OsSi, is clearly smaller than that observed for U,RhSi, although ros = 135.3 pm > rRh = 134.5 pm (Fig. 4 and Table 2). In the same way, the
Table 2 Crystallographic Silicide
data
of the U,TSi,
Symmetry
ternary
Structure
233 (1996) 150-160
silicides Unit cell parameters
(pm)
AlB, subcell
Ref.
type 385.2( 1)
667.4(8) 804.5( 1)
407.4(2) 380.82(6)
400.4 401
386.4 384
4028(l)
0 H
U,RuSi,
384.6( 3) 804.5( 1)
U,FeSi, U,FeSi, U,FeSi,
H H H
AlB, AlBz
400.4 401
U,COSl,
H H
AlB, AlB,
U,IrSi, U,IrSi,
H H H H 0 H 0 H H H H H
U,PtSI,
H
U,PtSi, U,PtSi, U,AuSi,
H H H
H, hexagonal;
0, orthorhombic;
U,OSSl,
402.8( 1)
AIB,
H
USi, U,MnSi,
U,NiSi, U,NiSi, U,RuSi, U,RuSi, U,RhSi, U,RhSi, U,PdSi, U,PdSi, U,OSSl,
ah
b
USi,
U,CoSi,
c
a
U,RuSi, AlB, AlB, AlB, U,RuSi, AIB,
398.8 397.9 814.8(2) 407.5(2) 703.6( 1) 407.6(2) 706.7( 1) 408.5(2) 816.0(2) 406.7( 1)
398.8 397.9 814.8(2) 407.5(2) 406.2( 1) 407.6(2) 408.0( 1) 408.5(2) 816.0(2) 406.7(l)
388.3 394.9 385.5(2) 383.8( 1) 392.9(l) 388.3(2) 393.9(l) 393.5(2) 384.4( 1) 385.2( 1)
AlB,
407.2( 1)
407.2( 1)
389.5( 1)
AlB, AlB,
408.4 406.7( 2)
408.4 406.7(2)
397.3 396.4( 1)
AIB,
414.5(3)
414.5(3)
398.9(2)
a,, and c,, are the unit cell parameters
relative
to AlB,
Ch
5 Ia,
402.8( 1)
385.2( 1)
0.956
384.6(3) 402.25(5)
407.4( 2) 380.82(6)
1.059 0.947
400.4 401 400.3(l)
386.4 384 385.7( 1)
0.965 0.958 0.964
this work
398.8 398.7(2)
388.3 3883(l)
0.974 0.974
this work
397.9 397.9( 1) 407.4 407.5(2) 406.2( 1) 407.6(2) 408.0( 1) 408.5(2) 408.0( 2) 406.7(l) 406.5( 1) 407.2(l)
394.9 394.6( 1) 385.5(2) 383.8( 1) 392.9( 1) 388.3(2) 393.9( 1) 393.5(2) 384.4( 1) 385.2( 1) 391.4( 1) 389.5( 1)
0.992 0.992 0.946 0.942 0.967 0.953 0.965 0.963 0.942 0.947 0.963 0.957
408.4 406.7 407.3( 1) 414.5(3)
397.3 396.4( 1) 396.5( 1) 398.9(2)
0.973 0.975 0.973 0.962
type subcell.
1101 1111 this work ]91
VI [91 191 this work [8], this work
[71 this work ]71 this work ]71 [8], this work
[71 this work
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B. Chevalier et al. I Journal of Alloys and Compounds
C
I_
II
1
1
III1
I
I
II
233 (1996)
150-160
I
I
155
I
I
I
U2TSi3
,
/
/
,
,,
.*
0 y60 s=
/0° ,0°’
t
,&
Au ,( .
O6
Pte pNi \ \ _ I &CO \
Rho
Co
.Pd’ ,
Pd
pounds should play a significant role in the magnetic properties of the U,TSi, silicides.
dOs
\
Ru
Fig. 5. Comparison of the unit cell parameters of the AlB, subcell of U,TSi, (ah (0) and c,, (O)), Gd,T,, ,Si, z (T = Fe, Co, Ni) and Gd,TSi, (T= Rh, Pd) (a,, (+) and ch (X)).
?? If
I 1 ‘c Fe RUO /’
G“*
fI
A’
Mn
1, I
125
I
rT (pm)
I
lr15
Fig. 4. Variation in the unit cell parameters as a function of the metallic radii r7 for U,TSi, silicides. For the sake of comparison, all the parameters are given for the AlB,-type subcell.
al compounds with RE = Gd have been investigated: Gd,T,,,Si,,2 (T = Fe, Co, Ni) adopts the hexagonal AlB, type [15] whereas Gd,TSi, (T = Rh, Pd) has a hexagonal structure of the Er,RhSi, type which derives from AlB, type [16,17]. Fig. 5 shows that the ah parameters of the Gd- or U-based silicides containing the same T element have close values. This result confirms that the ah parameter is essentially governed by the size of the T atom within the 2D (T, Si) network. On the contrary, the q, parameters of the Gd,TSi, compounds are greater than these observed for U,TSi, silicides, since the q, values are mainly influenced by the Gd-T or U-T bonds. This result is therefore in agreement with the metallic radius of gadolinium (rod = 18012pm) which is larger than that of uranium (rU = 156pm) [13]. To summarize, we can say that in the U,TSi, series with T = Mn, Fe, Co, Ni, the interatomic distance d,_, increases as rT decreases (see Table 3) suggesting a strong overlap between the 5f (U) and 3d (Mn or Fe) states. The U-T distances which influence the 5f(U)rid(T)) hybridization in the uranium intermetallic com-
3.4. Magnetic properties The magnetic data relative to the U,TSi, silicides obtained during this work as well as those available in the literature are reported in Table 3. The presence of small amounts of magnetic impurities in U,MnSi, (detected in grain boundaries by microprobe analysis) has not allowed us to determine the intrinsic magnetic properties of this silicide. 3.4.1. Silicides with T = Fe, Ru, OS
The reciprocal magnetic susceptibility of U,FeSi,, U,RuSi, and U,OsSi, follows a Curie-Weiss law X,’ = (T - 0,)/C, above 50 K, 60 K and 50 K respectively (Fig. 6). The effective magnetic moment of uranium (k”. = (LX,)“*) deduced from this behaviour (Table 3) is comparable with that determined for the binary silicide U,Si, (k,, = 3.23& per U atom) [ll]. Below 30 K, 60 K and 50 K respectively, the curves xi’ =f(T) for U,FeSi,, U,RuSi, and U,OsSi, exhibit both a positive deviation from the Curie-Weiss law and a tendency to take a constant value at low temperature down to 4.2 K. This behaviour can be ascribed to the existence of local spin fluctuations as a result of the occurrence of Kondo interactions. However, their paramagnetic behaviour leads to a large negative value for the Curie paramagnetic temperature 0p (Table 3). This agrees well with the Kondo effect coupling. It is worthwhile noting that the temperature /3, for U,OsSi, is smaller than that deter-
B. Chevalier et al.
156
Table 3 Average U-T
distances
Silicide
d,_, (pm)
U,MnSi,
300.3
U,FeSi,
301.3 301.0 301.2 301.1
U,CoSi,
data
of the U,TSi,
compounds
X0 ( X10m3 e.m.u. (U atom))‘)
BP(K)
2.77 3.35
0.732
-86 -132
Magnetic
temperatures
T,
T(x’)
(K)
2.03 2.01
1.15 1.19
0 2
10 10
8
0.769 0.92
11 32
25 26
22
30
0.784 1.232 1.057
6, -70 - 130 17 7 -186 3
1.045 1.993
19, -7 10 5
8 13.5 11.6
T(x”)
Magnetic
moments
~(2 T)
302.9 2.38 302.8 2.41
U,RuSi, U,RhSi, U,PdSi, U,OsSi, UJrSi,
303.6 305.9 307.0 304.0 305.6
U,PtSi,
308.3 2.1 307.6 2.15 311.5 1.65
(U atom))’
Ref.
I%
2.75 3.02 2.32 2.39 3.03 1.93
[91 this work
19.5 15.5 11
U2TSi3
+ ??
Fe Ru
+
00s
---talc.
100
300
T (K)
Fig. 6. Thermal dependence of the reciprocal of U,TSi, with T = Fe, Ru and OS.
magnetic
susceptibility
mined for U,FeSi, and U,RuSi,. The great extension of the 5d(Os) orbitals which favours the 5f(U)-5d(Os) hybridization enhances the conduction electron-5f
[91
0.28 0.16
0.05 0.06
this work
0.67 0.51
0.31 0.35
[91 this work
20.3 11.4
20.3 10.2
0.45 0.33
0.31 0.03
9.8
9.4
0.22
0.1
Cl81 [8], this work this work this work this work this work
10.8 11.6
10.0 10.8
0.31 0.34
0.02 0.11
t31 this work this work
0.6”
electron exchange interaction, tuations [19].
c
(h
this work
U,NiSi,
U,AuSi,
and magnetic
btt (j+ (U atom))‘)
I Journal of Alloys and Compounds 233 (1996) 150-160
leading to Kondo fluc-
3.4.2. Silicides with T = Co, Ni, Rh, Pd, Ir, Pt, Au Fig. 7 shows the thermal dependence of the reciprocal magnetic susceptibility of these ternary compounds. Contrary to previous results given in Refs. 193 and [3], no anomaly can be detected around 80-100 K in the xi’ =f(T) curve of both U,NiSi, and U,PtSi,. The observation of these anomalies is due to the presence of small amounts of impurity phases such as UNiSi, and UPtSi, which order ferromagnetically at respectively T, = 100 K and 85 K [2]. Above 30 K, the xi’ = f(T) curve for all silicides can be well described by a modified Curie-Weiss law x,,, = x0 + C, l(T - 19~) where x,, is the temperatureindependent contribution. The magnetic data derived from this analysis are summarized in Table 3. The Curie paramagnetic temperature 0, is positive, indicating the predominance of ferromagnetic interactions. The effective magnetic moment &rf,, ranging from 1.65& (U atom))’ (U,AuSi,) to 2.41& (U atom))’ (U,NiSi,), is smaller than those determined for the series U,TSi, with T = Fe, Ru, OS. The origin of these small values could be due to the character of the magnetic anisotropy in the paramagnetic domain which affects the calculation of bft, from the measurements performed on powder samples. However, we can exclude this effect since recent measurements performed on a single crystal of U,PtSi, [3] or U,NiSi, [X3] with an applied magnetic field parallel or perpendicular to the c axis reveal similar krr, values along the a and c directions. The values observed for hrr. would be rather due to an itinerant character of the 5f(U) electrons which leads to a decrease in the
B. Chevalier et al. I Journal of Alloys and Compounds
233 (1996)
150-160
1.57
Ni
+
i3
0 Pt ____ talc.
talc.
0 Fig
7. Thermal
100 dependence
T (K) of the reciprocal
+
?? W
300 magnetic
0 susceptibility
moment k,,, as for UCoGa (,q,,,. = 2.40& (U atom))’ [20]. The partial itinerancy of 5f(U) electrons in these ternary silicides may be governed by the strength of the Sf(U)-rid(T)) hybridization. In this view, it is note the series interesting to that, in U,RuSi,, --, U,RhSi, + U,PdSi,, the higher moment kfr, is obtained for U,RuSi, which exhibits a perfect structural arrangement between Ru and Si atoms (Table 3). It is well known that, in a series of compounds, the Sf(U)-rid(T)) hybridization depends on the geometric repartition of the T atoms. For these reasons, the involved hybridization may, for instance, be diffe::ent in U,RuSi, and U,PdSi,, which exhibit respectively a Ru-Si ordering and a Pd-Si disordering. Fig. 8 shows the thermal dependence of the magnetizaticn of U,CoSi, and U,IrSi, measured under an applied field (0.1 T) with decreasing temperature (FC curve). For U,RhSi,, the FC curve is compared with that obtained (ZFC curve) after cooling in zero field. The ZFC curve of U,RhSi, exhibits a maximum at about 14 K and then a strong decrease near 19.5 K with increasing temperature; the FC curve has at low temperatures a tendency to approach a constant value.
of U,TSi,
100
0
“0 3 \
with T = (a) Co, Rh, Ir and (b) Ni, Pd and Pt
I
I
UqTSi? L
--•
00
*o
.ee8be
P
300
-f (K)
I
r3
(b)
b
9
+
co
??
Rh
J
0 Ir
??
e
7
?? ??
0
c
zz
i
1
Hz 0.1 T
?? ? ?
-e
?? ?? ?? ??
e
C 10
T (K)
Fig. 8. Thermal dependence of the magnetization of U2CoSi,, UJrSi, (field cooling (FC) curve) and U,RhSi, (zero field cooling (ZFC) and FC curves). The applied magnetic field is 0.1 T.
B. Chevalier et al. I Journal of Alloys and Compounds
158
ples of U,TSi, are shown in Fig. 9. The behaviour of both U,NiSi, and U,RhSi,, characterized by a transition in the initial magnetization curve and a relatively large hysteresis loop, originates from narrow walls between ferromagnetic domains which are frozen by the strong anisotropy. Recently, the study of U,NiSi, single crystals by neutron diffraction shows that this compound orders ferromagnetically with uranium magnetic moment (0.6(1)h) perpendicular to the c axis of the AlB, subcell [18]. Our magnetization measurements giving 0.51c~, per U atom for H = 2 T is in agreement with this study. At 4.2 K, the magnetization of U,CoSi,, U,PdSi,, U,IrSi, and U,PtSi, as a function of the applied field exhibits a behaviour characteristic of a weak ferromagnet (Fig. 9). The hysteresis phenomenon is small as well as the magnetization which is especially strongly field dependent. At 4.2 K, the weak magnetization observed for H = 2 T (Table 3) could be an indication of the itinerant character of the 5f(U) electrons. It is worthwhile noting that the enhanced electronic specific heat coefficient y = 200 mJ Kp2 (mol U) _I, determined in the case of U,PtSi,, is in agreement with a reduced magnetic moment for uranium [3]. Fig. 10 shows the real x’ and imaginary x” components of the a.c. susceptibility for U,RhSi, and U,PdSi,. For the first silicide, the x’ and ,y”=f(T) curves exhibit a sharp peak at the same temperature, 20.3 K (Table 3). On the contrary, the peaks observed for U,PdSi, are broader and one order of magnitude smaller than that appearing for U,RhSi,. The x’ = f(T) curve exhibits a maximum at 11.4 K whereas the peak in the ,$‘=f(T) curve occurs at 10.2 K. Also x” is not zero above 11.4 K. These main features show that U,PdSi, behaves as a ferromagnetic-spin glass state [22]. The distribution between Pd and Si atoms which is almost statistical in U,PdSi, can allow random magnetic interactions in this compound. It is worthwhile noting also that U,TSi, with T = Ir, Pt and Au can be compared with U,PdSi, since the maxima in
This result characterizes a ferromagnet displaying noticeable domain-wall pinning effects. The Curie temperatures T,, given in Table 3 for each silicide, were estimated from the inflection point of the FC curve. The discrepancy shown for the T, value of U,PtSi, is due to their chemical composition, which is different. The U,T,_.,Si,+X silicides exhibit as do Th,Co, _XSi3+Xa composition homogeneity range [21] in which the hexagonal AlB, type is adopted for -0.40 =Zx G 0.10. The initial magnetization curves and the hysteresis loops measured at 4.2 K for the polycrystalline samI
I
I
III
U2TSi3
I
oooooo
od@@ O0
P
Pt
-0
T,4.2K C 0
2
20 H CT)
Fig. 9. Magnetic field dependence some U,TSi, compounds.
233 (1996) 150-160
at 4.2 K of the magnetization
of
UZRhSig 5 Oe 125
Hz
0
el 0
\” _
00
64 t
0
20
125
Hz
0
_
O 0 0 0
0
0.6
20
T (K) Fig. 10. Thermal
dependence
of the a.c. susceptibility
(,$ (0)
and x” (0))
for U,RhSi,
and U,PdSi,.
B. Chevalier et al. I Journal of Alloys and Compounds
their x’ = f(T) and x” = f(T) curves do not appear at the same temperature (Table 3). This behaviour has been confirmed for U,PtSi, where no anomaly is observed around T, = 8 K in specific heat measurements [3], indicating thus a spin glass state. The magnetic measurements performed on U,RhSi, and U,PdSi, reveal a different magnetic behaviour (Figs. 8 and 9). (i) At T, = 19.5 K the magnetization of U,RhSi, increases strongly; this transition leads to the occurrence of both the pronounced peaks in the thermal dependence of the a.c. susceptibility and a noticeable remanent magnetization (Table 3); these observations characterize a ferromagnet. (ii) On the contrary, the magnetic transition observed for U,PdSi, induces small changes (weak jump in the magnetiation at T,., low remanent magnetization at 4.2 K (Table 3)); this behaviour can be ascribed to a ferromagnetic-spin glass state as shown by the a.c. susceptibility measurements. The magnetic properties of these two ternary silicides can be understood on account of their structural properties. The more ordered arrangement of the Rh-Si sublattice in U,RhSi, corresponds to the occurrence of a long-range ferromagnetic ordering. On the contrary, the spin glass behaviour of U,PdSi, is due to the random distribution of the U-U interactions resulting from the very strong disordering between Pd and Si. However, neutron diffraction and specific heat measurements are be necessary in order to confirm the suggested magnetic state of U,RhSi, and U,PdSi,.
4. Conclusion Many transition elements T can replace silicon in USi, allowing the formation of the ternary silicides U,TSi,. These compounds crystallize in the hexagonal AlB, type or a derived structure. The structural arrangement of the T-Si sublattice is strongly dependent on the nature of the T element. For instance, the study of’U,RuSi,, U,RhSi, and U,PdSi, by electron diffraction reveals that these silicides adopt interesting superstructures of the AlB, type: (i) Ru and Si atoms are perfectly ordered in U,RuSi, which leads to the occurrence of a new hexagonal superstructure having the unit cell parameter a twice as great as that observed for the ideal AlB, type; (ii) the Rh-Si network in U,RhSi, exhibits a partial ordered arrangement corresponding to an orthorhombic superstructure; (iii) finally, in U,PdSi,, the distribution between Pd and Si is almost statistical. This study shows that the occurrence of a superstructure is increasingly favoured as the number of d electrons of the T element becomes smaller. At low temperatures, a non-magnetic ground state is observed for uranium in U,FeSi,, U,RuSi, and U,OsSi,. On the contrary, a ferromagnetic behaviour
233 (1996) 150-160
159
or a ferromagnet-spin glass state characterizes all the other ternary silicides. For the U,TSi, series as for the tetragonal UT,%, compounds, the magnetic state of uranium becomes more pronounced as the number of d electrons of the T element increases. Endstra et al. have shown that the magnetic properties of the UT,%, silicides are determined by the strength of the 5f(U)rid(T)) hybridization [19]. A decrease in the filling of the rid(T)) band induces an increase in the hybridization. The proposed “f-d hybridization model” based on the determination of the Sf(U)-rid(T)) overlap which explains well the physical properties of the UT& compounds could be used for the U,TSi, series. In order to understand better the structural properties (e.g. variation in the unit cell parameters) and the magnetic behaviour of some terms of the U,TSi, series, a band calculation structure using an augmented spherical-wave method is now in progress. This method provides information on the variation in Fermi energy with respect to bonding and antibonding states.
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[21] W.X. Zhong, W.L. Ng, B. Chevalier, J. Etourneau and P. Hagenmuller, Mater. Rex Bull., 20 (1985) 1229. [22] R.B. Goldfarh, K.V. Rao and H.S. Chen, Solid State Commun., 54 (1985) 799.