- Email: [email protected]
Structural and magnetic properties of the ternary rare earth (RE) compound series REInAu2 (RE La to Lu, Y)
Journal of the Less-Common
STRUCTURAL RARE EARTH
Metals, 120 (1986)
101 - 112
101
AND MAGNETIC PROPERTIES OF THE TERNARY (RE) COMPOUND SERIES REInAu, (RE z La TO
Lu, Y) M. 3. EESNUS,
J. P. KAPPLER,
~~~~~~r~~~~Strasbourg R. LAHIOUEL,
M. F. RAVET and A. MEYER
1, LX.S.E.S.,
instifut
de F~~~~~~~~67084 Stru~b~~rg jFrance)
J. PIERRE and E. SIAUD
.Laboratoire Louis N&et, Centre National de ta Recherche Grenoble (France)
Scientifique,
I66
X, 38042
G. NIEVA and J. SERENI Centro Atomico, (Received
Bariloche (Argentina)
November
2,1985)
Results of structural, magnetic and heat capacity studies are reported for the REInAq series (except europium). A crystallographic transformation from cubic to tetragonal structure accurs for the compounds with lanthanum, cerium, praseodymium and neodymium. With the exception of LaInAuZ, the phase transition has no observable effect on the magnetic susceptibility. Compounds formed with gaduliniurn, terbium, dysprosium and holmium order antiferromagnetically at low temperature. In this series YbInAu, is confirmed to be in an unstable valence state with the maximum W + 1 = 8 ground state degeneracy.
Ternary rare earth (RE) in~~~tallics with the Weusfer-type structure L2, have been recently discovered. They have the general formula REMM2’ where M is an s-p type element and M’ a transition metal. Some CsCl type compounds with M = Al, Ga, In and M’ = Au were first described by Matthias et al. [l]. Galera et al. [Z] investigated the structure and magnetic properties of the REInAg, series and showed that these compounds crystallize with a Heusler phase fCsCl-type derivative) structure. Recently, fshikawa et al. [3] and Johnson and Shelton [4] reported on the properties of compounds with M’ = Pd. We report in this paper on the structural and magnetic properties of the REInAu, series (lanthanum to lutetium) and the results of heat capacity studies concerning the cerium and ytterbium compounds as well as the nonmagnetic references LaInAu, and YInAu,. Among this series YbTnAu:, is recognized as an intermediate valent (IV) compound. @ Elsevier Sequoia/Printed
in The Netherlands
102
Experimental details are summarized in Section 2. In the first part of Section 3 we report on the cubic-to-tetragonal structure transformation which is shown to occur for the light RE compounds (lanthanum to neodymium). The second part of Section 3 deals with the magnetic and calorimetric properties with a deeper insight into the cerium and ytterbium compounds.
2. Experimental details The polycrystalline samples used for this study were prepared by induction melting the requisite amounts of the constituent metals in a cold copper crucible under a purified argon atmosphere. The purity grades of the starting materials were 99.9% or 99.99% for the RE, 99.999% for indium and 99.999% for gold. The structure and single-phase states of all alloys studied were checked by X-ray diffraction powder patterns using Cu Ko radiation. Supplementary X-ray and neutron diffraction investigations as a function of temperature concerned the cerium, praseodymium, neodymium, thulium and yttrium compounds. M~etization measurements were taken with vibrating sample magnetometers operating in the field ranges 0 - 2 T and between 1.5 and 300 K, and by an induction method at 1.5 and 4.2 K in fields up to either 7 or 15 T (Service National des Champs Intenses, Grenoble). A conventional four-probe a.c. technique with lock-in detection was used to measure the resistivity. Heat capacity measurements were performed in the temperature range 1.5 - 20 K (0.4 - 35 K for CeInAu,) by standard heat pulse techniques.
3. Results and discussion 3.1. Crystallographic study and phase transformation All compounds from cerium to lutetium crystallize at room temperature with the cubic CsCl type (B2) structure; some weak superstructure lines, characteristic of a Heusler-type structure were also observed [6]. These lines are, depending on the sample, the hkl odd, 311,331, 511 and 711 lines. The whole Bragg reflections can be indexed on the basis of a cubic cell twice that of the CsCl cell. Thus by analogy with the REInAg, series we think that this kind of Heusler-type ordering may exist for the whole series. The lattice parameters obtained are given in Table 1. Except for LaInAu,, which is found to have a tetragonal structure at room temperature (Table 1) the room temperature lattice constants agree with the data of Marazza et ai. [ 71. Compounds with cerium, praseodymium and neodymium undergo a structural transition on lowering the temperature, mainly characterized by large hysteretic jumps in the resistivity at the transformation temperatures 7’M [6]. Some typical thermal variations in the resistivity are shown in Fig. 1.
103 TABLE 1 Crystallographic transition temperature TM and lattice parameters of the cubic and tetragonal phases at various temperatures for the REInAuz series RE
Ta (K)
a (8)
c (A)
v”3
La
RT
7.048
7.318
7.135
Ce
RT 240 210 40
7.090 7.079 7.007 6.980
7.246 7.268
7.084 7.073
1.034 1.041
RT 100 30
7.046 7.022 6.991
7.111 7.137
7.050 7.038
1.013 1.021
RT 80
7.055 6.993
7.184
7.055
1.027
RT 20
7.030 6.902
7.060
6.953
1.023
Sm
RT
6.973
Gd
RT
6.940
Tb
RT
6.913
DY
RT
6.885
Ho
RT
6.870
Er
RT
6.864
Tm
RT 12
6.856 6.838
Yb
RT
6.860
Lu
RT
6.822
Y
RT 80
6.893 6.883
Ce0.75Y0.25
Pr Nd
(A)
c/ah
TM(K)
1.038
355 k 5 230 f 5
110+10
150flO 70 f 2oc
aRT, room temperature. bRatio c/a of tetragonal distortion. CStrongly dependent on state and heat treatment of the sample.
The resistivity jumps decrease strongly with decreasing atomic volume, from about 60% in LaInAu, to about 6% in NdInAu*; at the same time an increased hysteresis is observed. TM values, as derived from the hysteresis loops, show a rapid decrease when going from lanthanum to neodymium and also as YInAuz retains the cubic structure at low temperature, in the yttriumsubstituted cerium series (Fig. 2). X-ray analysis as a function of temperature reveals a tetragonally deformed structure below TM. Further, low temperature neutron diffraction patterns taken on CeInAu, reveal additional superstructure lines such as the 001 line. TM values and the lattice parameters for some temperatures are given in Table 1. Within the limit of experimental error, there is no observable volume change between the two phases (Fig. 3);
4ooY
0
100
200
TbGd
Sm
Nd_Pr Ce
La
300 TtKf
Fig, 1. Temperature dependence of the resistivity for CeInAua, (inset, resistive behaviour of LaInAus in the vicinity of TM).
PrInAuz and LaInAua
Fig. 2. Transformation temperatures TM in the REInAus series as a function of the ionic radii R3+. For the yttrium-substituted cerium-series a fictitious radius was determined by linear interpolation between cerium and yttrium. (Vegard’s law is obeyed throughout the series for the lattice parameters.)
t
I
a,
Fig. 3. Splitting of the lattice constant owing to the tetragonal distortion of the unit ceil in CeInAuz (circles} and Cec.7sYo.251nAuz (squares). Tetragonal phase: open symbols lattice parameter a; full symbol lattice parameter c; cubic phase, half-shaded symbols; crosses, V1’3. (The broken lines are guides to the eye.)
105
values of the c/a ratio range between 1.038 and 1.023. The splitting of the lattice constant due to the tetragonal distortion of the cubic cell is shown in Fig. 3 for CeInAu, and for one yttrium-substituted alloy. For CeInAu, one notes a rather sharp increase in the c/a ratio near !I’,, whereas this behaviour (characteristic of a first-order transition) is smeared for Ce0.75Y,,251nAu2, i.e. as TM decreases. Except for ytterbium, the lattice constants (or V”3) show the usual linear correlation with the ionic R3’ ratio. These transformations appear to be analogous to the martensitic transformations reported to occur in numerous CsCl-type compounds like, for example, CeAg [ 81, LaAg, ~ xInx [ 91 or RECd [lo]. In the REInAuz series, this structural instability seems to be strongly correlated to the size of the rare earth atom as the transition temperature TM shows an almost regular increase with increasing ionic radii (Fig. 2) as does also the c/a ratio (Table 1). Unlike the resistivity, where the observed anomalies reach the high value of about 60%, the phase transformation has no observable effect on the Curie-Weiss susceptibilities (Fig, 4). It must, however, be mentioned that, in the case of LaInAu, which displays the small temperature independent magnetism of a 4f0 state, a rapid increase is observed at the transformation, the susceptibility varying from 6.6 X lo-’ e.m.u. mol-’ to 9.7 X lo-’ e.m.u. mol--‘.
0
100
4. Magnetic
and heat capacity
-.l 203
3cll TCKI Fig. 4. Reciprocal susceptibility as a function of temperature for PrInAuz and CeInAuz. The structural transition temperatures TM are indicated by the arrows; straight lines, hightemperature Curie-Weiss dependences.
studies
4.1. Magnetic charac teds tics The REInAu, series is characterized by rather small ordering tempera= 11.5 K). The gadolinium, terbium, dysprosium and tures (e.g. TNGdInAu,
106 TABLE 2 Order temperatures TN, magnetization parameter ep for the REInAuz series
M
RE
M, effective magnetic moments j&f and Weiss
BJ
Ce
la
1.05b
Peff (FBI
(PB)
2.14
2.57 2.54 2.54 2.51 3.55 3.70
Ce0.5Y0.5 Ce0.25Y0.75 Ce0.1 Y0.9
Pr Nd Sm Gd Tb DY HO
11.5 a.2 6.0 2.6
Er Tm Yb
1.03b 1.78= 0.15b 6.8b 7.4b 7.Sc 7.Sc 6.7b 5.65b 0.17b
3.20 3.27 0.71 7.0 9.0 10.0 10.0 9.0 7.0 4.0
4 WI
g{J(J+1)}i'2
-15
-13 -7 -12 -5 -a
d
d
8.0 9.7 10.8 10.8 9.63 7.06 e
-14 -7 -5 -5 -4 -35 e
2.54
3.58 3.62 0.85 7.94 9.72 10.65 10.61 9.58 7.56 4.25
aTemperature of the specific heat maximum. b15 T.
c7.5T.
dNo Curie-Weiss behaviour. eIV behaviour. holmium compounds order ~tife~om~etic~ly at low temperature (Table ‘2); no magnetic order is detected for the other compounds down to 1.5 K. A specific heat investigation of CeInAu, shows an anomaly at about 1 K (AC = 4 J mol-’ K-‘) which may reveal magnetic order. The magnetic properties are summarized in Table 2 and illustrated in Figs. 4 - 7 through some representative x-‘(T) variations and low temperature high-field magnetization data. At high temperature the susceptibility exhibits Curie-Weiss behaviour with 8 values ranging from -5 to -35 K, for all samples excluding the non-magnetic RE, yttrium, samarium and ytterbium. The effective paramagnetic moments peff obtained (Table 2) are close to those of the free tripositive ions. In the case of SmInAuz satisfactory agreement is achieved for the temperature dependence of the susceptibility, by taking into account the contribution of the J = 7/2 excited level and an exchange field equivalent to a Curie-Weiss parameter of -10 K, as deduced from low temperature data (up to 10 K) which yields C = 0.081, i.e. close to the theoretical value for Sm3+ and thus permits us to ignore the small crystal field effects. Crystal fields affect the low temperature susceptibility and magnetization, especially for the light rare earths (cerium, praseodymium, neodymium) compounds. As discussed above, these latter compounds undergo a structural transition from cubic to tetragonal symmetry. In this case at least five independent parameters are involved in the CEF hamiltonian which hampers
107
_I 0
20
10 TCKI
Fig. 5. Inverse susceptibilities only guides to the eye.)
for
TbInAu 2, HoInAuz
and DyInAuz.
(The
full lines are
Fig. 6. Low temperature (1.7 K) magnetization us. field for several compounds REInAq series. The gJ values for the different RE are indicated on the right-hand
of the axis.
determination of the CEF effects; thus due to the additional uncertainties introduced by the increased number of parameters no quantitative analysis of these effects was carried out. Typical high field magnetization curves taken at 1.7 K are displayed in Fig. 6. Excepting GdInAuz which saturates near 7 pg in the highest field available, the low temperature moments of all compounds are largely reduced in comparison with the gJpB values (Table 2). 4.2. The cases of CeInAu, and YbInAu, For CeInAu* the lack of lattice constant anomaly as well as X-ray absorption spectroscopy (XAS) measurements suggest an almost integral valence. The low temperature specific heat shows a broad anomaly at about 1 K (Fig. 8) with AC * 4 J mol-’ K-‘, which spreads out to about 3 K and shows magnetic ordering. The form of the anomaly gives an indication of
108
r IO.
. ;
#.
.
. l
+o +o
l
+O+O
.
l
o
+o+o*o
@dJ
Otomo+
o*
m
9
.
.
.
0,5_
0’
.
0’ .
0+
Ce 1.7K 0 Ce 4.2K + Cea~kx 1
0’ . o+ . .+O
l
o+
OO
50
1aJ
1 i0 HCkCd
Fig. 7. Magnetization
us. field for CeInAuz and one yttrium-substituted
:
v
sample, Ceo.zs-
:
:
.....‘#
0 f\ ,.,,~ ,,,_.... .... 0
10
23 TCKI
Fig. 8. Heat capacity data for CeInAu2 in the 0.4 - 20 K range, C us. T; Inset, magnetic contribution to the specific heat AC/T and normalized entropy SIR In 2 us. T.
precursor or .short-range order effects at T > 1 K, likely to be due to the possibility of lattice disorder. The magnetic dilution of cerium was investigated in the substitution series Ce, _,Y,InAu, through high-field magnetization (Fig. 7) and susceptibility measurements. In the concentration range investigated 0 < x < 0.9 and within the magnitude of experimental precision no differences could be evidenced in the magnetic behaviour. The effective moments peff deduced from the high-temperature susceptibility data decrease from 2.57 to 2.51 (CeoerYo.,InAuz) while the 8, values scatter between -15 and -7 K (Table 2). The heat capacity study concerns CeInAuz, Ceo.iYo..$nAuz and the two reference compounds LaInAuz and YInAu, (Fig. 9 and Table 3). The
109
Ceo,YJnAul /’ /- -_x’
, /’
,./
YInAy.
-.l
-.
1
20
0
T2 CK’)
Fig. 9. Heat capacity data, show least-squares fits. TABLE
4%
C/T us. T2 for YInAu*
and Cee.iYo.aInAu2.
The broken
lines
3
Electronic specific heat coefficient, y; Debye temperature, 0~ and room-temperature magnetic susceptibility, x293 K for the REInAu? series (for details of 0~ see text) RE
Y (mJ mol-’
La Ce
1.9 130 f 11.8 40 + 1.9
Ce0.1Y0.9
Yb Y aLow
temperature
BD
KP2 )
+ 0.3 2 + 0.2 0.7 + 0.3
susceptibility,
(K) 140 125 118 105 139
x293
K
(lop5
cm3 molP1)
6.6
485a 8.0
x0,
reference compounds behave normally and the behaviour observed can be reproduced by C = yT + /3T3 in the temperature range up to about 3.5 K. Least squares fits yield the value of 1.9 mJ mol-’ Ke2 for the linear term of the specific heat. The Debye temperatures, 139 and 140 K near 0 K for the yttrium and lanthanum samples, go through a minimum of about 120 K near 9 K. For CeInA+, owing to the large magnetic anomaly (Fig. 8) only a restricted linear C(T)/T variation is available. Analysis of the data yields in the high temperature region a tentative y value of 130 mJ mol-’ Ke2, where y denotes the term of the specific heat linear in temperature between 5.5 and 11 K. A further analysis with the assumption that the lattice term may be in a first approximation taken equal to that of LaInAu,, i.e. subtracting the LaInAu2 data from the measured values, leads to a near constant C(T)/T value of about 150 + 10 mJ K--2 (Ce atom))‘, up to the limit of the
110
measurements, which may be identified, keeping in mind the above approximation, with the electronic contribution y. In the yttrium-substituted sample, for which the magnetic anomaly is shifted towards fairly low temperatures, only a small upturn is observed. For this alloy, the electronic contribution, deduced from data analysis in the 3 - 7.5 K temperature range, is 11.8 mJ mol-’ K-2, i.e. roughly proportional to the cerium concentration. The entropy variation associated with the anomaly observed in CeInAu,, defined as j-AC/T dT with AC = C,,,, - (YT + /3T3) reaches 0.8 R In 2 at T - 4.5 K (Fig. 8, inset); thus the cerium atoms in CeInAu, are in a doublet crystal field ground state, as required by the low temperature tetragonal symmetry, the 2J + 1 = 6, cerium levels being split into 3 doublets as a result of crystal field interaction [ll]. The y values in the cerium system, largely enhanced with respect to the values of the non-magnetic reference, compare with those typically observed in IV and Kondo systems or more generally in cerium-based compounds where they can be accounted for by f-d hybridization effects. Lm X-ray absorption spectra of CeInAu2 show the characteristic line of integral valent RE atoms, which seems to exclude the possibility for the enhanced y values to be related to an unstable IV state. As already mentioned the lattice constants, except that of ytterbium, show a linear correlation with the ionic R3+ radii which suggests that ytterbium is in an unstable valence state. Lattice parameter studies yield a valence of 2.83, whereas the value deduced from L,,, edge measurements [12] is 2.68. Susceptibility measurements show that YbInAu, presents behaviour typical of IV materials; namely at high temperature a Curie-Weiss like temperature variation, and at decreasing T a broad maximum, followed by the low temperature demagnetization. After subtraction of an impurity term, assumed to be Curie-like, a finite almost temperature independent susceptibility may be defined. The analysis of our low and high field magnetization data yields the values of x = 4.85 X 10d3 cm3 mol-’ and T,,, - 80 K for the low-temperature magnetic susceptibility and the temperature of the susceptibility maximum in agreement with previous determinations [ 51. Typical values of about 1% Yb3+ ions (part being in the Yb203 phase as discussed below) are deduced, both from the Curie-like impurity contribution and from the low temperature saturating component of the M(H) curves. The experimental heat capacity results are given in Fig. 10. The behaviour observed can be reproduced by C = yT + /3T3 in the temperature range 2.7 - 8 K, with y = 40 mJ mol-’ KP2 and /3 = 6.6 mJ mol-’ KP4. In addition the C(T) dependence is characterized by the presence of a small peak located at 2.2 K, typical of a second-order phase transition. The same anomaly, caused by the presence of the impurity phase Yb203 has already been reported to occur in ytterbium compounds and especially in YbCuAl [ 13,141. The estimated entropy associated with our anomaly is about 0.04 J mol-’ K-‘, which if attributed to the second-order phase transition of the magnetic ytterbium atoms (in the Yb203 phase) would correspond to about 2% - 3% of the ytterbium atoms in this sample. The Debye temperature deduced from our data with the usual assumption that the T3 coefficient
111
.
100 T2 (K*) Fig. 10. C/T against T* for YbInAu2.
Broken
line shows
least squares
fit.
arises only from the phonon contribution is 105 K, i.e. an anomalous low value compared with that of the non-magnetic isomorphous references, even after correction for mass difference or taking into account the thermal variation of ~9~ of the non-magnetic partners. From Fermi-liquid approaches, one expects for IV compounds an electronic T3 term in the low temperature specific heat. An estimation of this contribution to the 0 term, within the framework of the model of Newns and Hewson [ 151, gives the value of +2.3 X 10h3 J mall’ Kh4. This contribution is not negligible when compared with the experimental /3value of 6.6 X 10-3, which thus indicates that a large part of the T3 term arises from electronic effects. This gives the tentative value of 122 K, in line with the values observed in this temperature range, for the Debye temperature of intermediate valent YbInAuz. Finally, the R ratio, expressed in the usual manner, as for example in the picture of Fermi liquids is R =
(n2kB2/peff2)x
with p,rf2 = gJ2pB2J(J + 1). For YbInAu, our values of the T = 0 susceptibility x0 and the electronic specific heat coefficient y yield R = 1.28 close to the theoretical limit value of R = 1+
&
= 1.14
for ytterbium compounds; YbInAu, thus displays, as usually observed in cerium-based mixed valence compounds [16] the maximum 2J + 1 ground state degeneracy. 5. Conclusions Compounds of the ReInAu, series formed with the light RE, from lanthanum to neodymium, undergo a structural transformation from cubic
112
to tetragonal symmetry. The size of the RE atom plays an important role, the transformation temperatures displaying an almost regular decrease with decreasing RE ionic radii or by dilution of the RE in the constricted YInAuz matrix. All compounds have normal valence state behaviour, except YbInAq which is confirmed to be mixed valent with the maximum 2J + 1 = 8 ground state degeneracy, and CeInAu, which despite undergoing a magnetic transition in a 2s + 1 = 2 ground state, presents in the paramagnetic region a strongly enhanced temperature independent electronic specific heat.
References 1 B. T. Matthias, E. Corenzwit, J. M. Vandenberg, H. Harz, M. B. Maple and R. N. Shelton, J. Less-Common Met., 46 (1976) 339. 2 R. M. Galera, J. Pierre, E. Siaud and A. P. Murani, J. Less-Common Met., 97 (1984) 151. 3 M. Ishikawa, J. L. Jorda and A. Junod, in W. Buckel and W. Weber (eds.), Superconductivity in d- and f-band metals, Karlsruhe, 1982, p. 141. 4 M. J. Johnson and R. N. Shelton, Solid State Commun., 52 (1984) 839. 5 H. Oesterreicher and F. T. Parker, Phys. Rev. B, 16 (1977) 5009. 6 M. J. Besnus, J. P. Kappler, A. Meyer, J. Sereni, E. Siaud, R. Lahiouel and J. Pierre, Physica B, 130 (1985) 240. 7 R. Marazza, R. Ferro and D. Rossi, 2. Metallk., 66 (1975) 111. 8 D. Schmitt, P. Morin and J. Pierre, J. Magn. Magn. Mater., 8 (1978) 249. 9 H. Balster, H. Ihrig, A. Kockel and S. Methfessel, 2. Phys. B, 21 (1975) 241. 10 R. Aleonard and P. Morin, J. Magn. Magn. Mater., 42 (1984) 151. 11 U. Walter, J. Phys. Chem. Solids, 45 (1984) 401. 12 D. Muller, S. Hussain, E. Cattaneo, H. Schneider, W. Schlabitz and D. Wohlleben, in P. Wachter and H. Boppart (eds.), Valence Instabilities, North-Holland, 1982, p. 463 13 R. Pott, R. Schefzyk, D. Wohlleben and A. Junod, 2. Phys. B, 44 (1981) 17. 14 J. C. P. Kiaasse, F. R. de Boer and P. F. de Chatel, Physica B, 106 (1981) 178. 15 D. M. Newns and A. C. Hewson, J. Phys. F, 10 (1980) 2429. 16 M. J. Besnus, J. P. Kappler and A. Meyer, Physica B, 130 (1985) 127.
STRUCTURAL RARE EARTH
Metals, 120 (1986)
101 - 112
101
AND MAGNETIC PROPERTIES OF THE TERNARY (RE) COMPOUND SERIES REInAu, (RE z La TO
Lu, Y) M. 3. EESNUS,
J. P. KAPPLER,
~~~~~~r~~~~Strasbourg R. LAHIOUEL,
M. F. RAVET and A. MEYER
1, LX.S.E.S.,
instifut
de F~~~~~~~~67084 Stru~b~~rg jFrance)
J. PIERRE and E. SIAUD
.Laboratoire Louis N&et, Centre National de ta Recherche Grenoble (France)
Scientifique,
I66
X, 38042
G. NIEVA and J. SERENI Centro Atomico, (Received
Bariloche (Argentina)
November
2,1985)
Results of structural, magnetic and heat capacity studies are reported for the REInAq series (except europium). A crystallographic transformation from cubic to tetragonal structure accurs for the compounds with lanthanum, cerium, praseodymium and neodymium. With the exception of LaInAuZ, the phase transition has no observable effect on the magnetic susceptibility. Compounds formed with gaduliniurn, terbium, dysprosium and holmium order antiferromagnetically at low temperature. In this series YbInAu, is confirmed to be in an unstable valence state with the maximum W + 1 = 8 ground state degeneracy.
Ternary rare earth (RE) in~~~tallics with the Weusfer-type structure L2, have been recently discovered. They have the general formula REMM2’ where M is an s-p type element and M’ a transition metal. Some CsCl type compounds with M = Al, Ga, In and M’ = Au were first described by Matthias et al. [l]. Galera et al. [Z] investigated the structure and magnetic properties of the REInAg, series and showed that these compounds crystallize with a Heusler phase fCsCl-type derivative) structure. Recently, fshikawa et al. [3] and Johnson and Shelton [4] reported on the properties of compounds with M’ = Pd. We report in this paper on the structural and magnetic properties of the REInAu, series (lanthanum to lutetium) and the results of heat capacity studies concerning the cerium and ytterbium compounds as well as the nonmagnetic references LaInAu, and YInAu,. Among this series YbTnAu:, is recognized as an intermediate valent (IV) compound. @ Elsevier Sequoia/Printed
in The Netherlands
102
Experimental details are summarized in Section 2. In the first part of Section 3 we report on the cubic-to-tetragonal structure transformation which is shown to occur for the light RE compounds (lanthanum to neodymium). The second part of Section 3 deals with the magnetic and calorimetric properties with a deeper insight into the cerium and ytterbium compounds.
2. Experimental details The polycrystalline samples used for this study were prepared by induction melting the requisite amounts of the constituent metals in a cold copper crucible under a purified argon atmosphere. The purity grades of the starting materials were 99.9% or 99.99% for the RE, 99.999% for indium and 99.999% for gold. The structure and single-phase states of all alloys studied were checked by X-ray diffraction powder patterns using Cu Ko radiation. Supplementary X-ray and neutron diffraction investigations as a function of temperature concerned the cerium, praseodymium, neodymium, thulium and yttrium compounds. M~etization measurements were taken with vibrating sample magnetometers operating in the field ranges 0 - 2 T and between 1.5 and 300 K, and by an induction method at 1.5 and 4.2 K in fields up to either 7 or 15 T (Service National des Champs Intenses, Grenoble). A conventional four-probe a.c. technique with lock-in detection was used to measure the resistivity. Heat capacity measurements were performed in the temperature range 1.5 - 20 K (0.4 - 35 K for CeInAu,) by standard heat pulse techniques.
3. Results and discussion 3.1. Crystallographic study and phase transformation All compounds from cerium to lutetium crystallize at room temperature with the cubic CsCl type (B2) structure; some weak superstructure lines, characteristic of a Heusler-type structure were also observed [6]. These lines are, depending on the sample, the hkl odd, 311,331, 511 and 711 lines. The whole Bragg reflections can be indexed on the basis of a cubic cell twice that of the CsCl cell. Thus by analogy with the REInAg, series we think that this kind of Heusler-type ordering may exist for the whole series. The lattice parameters obtained are given in Table 1. Except for LaInAu,, which is found to have a tetragonal structure at room temperature (Table 1) the room temperature lattice constants agree with the data of Marazza et ai. [ 71. Compounds with cerium, praseodymium and neodymium undergo a structural transition on lowering the temperature, mainly characterized by large hysteretic jumps in the resistivity at the transformation temperatures 7’M [6]. Some typical thermal variations in the resistivity are shown in Fig. 1.
103 TABLE 1 Crystallographic transition temperature TM and lattice parameters of the cubic and tetragonal phases at various temperatures for the REInAuz series RE
Ta (K)
a (8)
c (A)
v”3
La
RT
7.048
7.318
7.135
Ce
RT 240 210 40
7.090 7.079 7.007 6.980
7.246 7.268
7.084 7.073
1.034 1.041
RT 100 30
7.046 7.022 6.991
7.111 7.137
7.050 7.038
1.013 1.021
RT 80
7.055 6.993
7.184
7.055
1.027
RT 20
7.030 6.902
7.060
6.953
1.023
Sm
RT
6.973
Gd
RT
6.940
Tb
RT
6.913
DY
RT
6.885
Ho
RT
6.870
Er
RT
6.864
Tm
RT 12
6.856 6.838
Yb
RT
6.860
Lu
RT
6.822
Y
RT 80
6.893 6.883
Ce0.75Y0.25
Pr Nd
(A)
c/ah
TM(K)
1.038
355 k 5 230 f 5
110+10
150flO 70 f 2oc
aRT, room temperature. bRatio c/a of tetragonal distortion. CStrongly dependent on state and heat treatment of the sample.
The resistivity jumps decrease strongly with decreasing atomic volume, from about 60% in LaInAu, to about 6% in NdInAu*; at the same time an increased hysteresis is observed. TM values, as derived from the hysteresis loops, show a rapid decrease when going from lanthanum to neodymium and also as YInAuz retains the cubic structure at low temperature, in the yttriumsubstituted cerium series (Fig. 2). X-ray analysis as a function of temperature reveals a tetragonally deformed structure below TM. Further, low temperature neutron diffraction patterns taken on CeInAu, reveal additional superstructure lines such as the 001 line. TM values and the lattice parameters for some temperatures are given in Table 1. Within the limit of experimental error, there is no observable volume change between the two phases (Fig. 3);
4ooY
0
100
200
TbGd
Sm
Nd_Pr Ce
La
300 TtKf
Fig, 1. Temperature dependence of the resistivity for CeInAua, (inset, resistive behaviour of LaInAus in the vicinity of TM).
PrInAuz and LaInAua
Fig. 2. Transformation temperatures TM in the REInAus series as a function of the ionic radii R3+. For the yttrium-substituted cerium-series a fictitious radius was determined by linear interpolation between cerium and yttrium. (Vegard’s law is obeyed throughout the series for the lattice parameters.)
t
I
a,
Fig. 3. Splitting of the lattice constant owing to the tetragonal distortion of the unit ceil in CeInAuz (circles} and Cec.7sYo.251nAuz (squares). Tetragonal phase: open symbols lattice parameter a; full symbol lattice parameter c; cubic phase, half-shaded symbols; crosses, V1’3. (The broken lines are guides to the eye.)
105
values of the c/a ratio range between 1.038 and 1.023. The splitting of the lattice constant due to the tetragonal distortion of the cubic cell is shown in Fig. 3 for CeInAu, and for one yttrium-substituted alloy. For CeInAu, one notes a rather sharp increase in the c/a ratio near !I’,, whereas this behaviour (characteristic of a first-order transition) is smeared for Ce0.75Y,,251nAu2, i.e. as TM decreases. Except for ytterbium, the lattice constants (or V”3) show the usual linear correlation with the ionic R3’ ratio. These transformations appear to be analogous to the martensitic transformations reported to occur in numerous CsCl-type compounds like, for example, CeAg [ 81, LaAg, ~ xInx [ 91 or RECd [lo]. In the REInAuz series, this structural instability seems to be strongly correlated to the size of the rare earth atom as the transition temperature TM shows an almost regular increase with increasing ionic radii (Fig. 2) as does also the c/a ratio (Table 1). Unlike the resistivity, where the observed anomalies reach the high value of about 60%, the phase transformation has no observable effect on the Curie-Weiss susceptibilities (Fig, 4). It must, however, be mentioned that, in the case of LaInAu, which displays the small temperature independent magnetism of a 4f0 state, a rapid increase is observed at the transformation, the susceptibility varying from 6.6 X lo-’ e.m.u. mol-’ to 9.7 X lo-’ e.m.u. mol--‘.
0
100
4. Magnetic
and heat capacity
-.l 203
3cll TCKI Fig. 4. Reciprocal susceptibility as a function of temperature for PrInAuz and CeInAuz. The structural transition temperatures TM are indicated by the arrows; straight lines, hightemperature Curie-Weiss dependences.
studies
4.1. Magnetic charac teds tics The REInAu, series is characterized by rather small ordering tempera= 11.5 K). The gadolinium, terbium, dysprosium and tures (e.g. TNGdInAu,
106 TABLE 2 Order temperatures TN, magnetization parameter ep for the REInAuz series
M
RE
M, effective magnetic moments j&f and Weiss
BJ
Ce
la
1.05b
Peff (FBI
(PB)
2.14
2.57 2.54 2.54 2.51 3.55 3.70
Ce0.5Y0.5 Ce0.25Y0.75 Ce0.1 Y0.9
Pr Nd Sm Gd Tb DY HO
11.5 a.2 6.0 2.6
Er Tm Yb
1.03b 1.78= 0.15b 6.8b 7.4b 7.Sc 7.Sc 6.7b 5.65b 0.17b
3.20 3.27 0.71 7.0 9.0 10.0 10.0 9.0 7.0 4.0
4 WI
g{J(J+1)}i'2
-15
-13 -7 -12 -5 -a
d
d
8.0 9.7 10.8 10.8 9.63 7.06 e
-14 -7 -5 -5 -4 -35 e
2.54
3.58 3.62 0.85 7.94 9.72 10.65 10.61 9.58 7.56 4.25
aTemperature of the specific heat maximum. b15 T.
c7.5T.
dNo Curie-Weiss behaviour. eIV behaviour. holmium compounds order ~tife~om~etic~ly at low temperature (Table ‘2); no magnetic order is detected for the other compounds down to 1.5 K. A specific heat investigation of CeInAu, shows an anomaly at about 1 K (AC = 4 J mol-’ K-‘) which may reveal magnetic order. The magnetic properties are summarized in Table 2 and illustrated in Figs. 4 - 7 through some representative x-‘(T) variations and low temperature high-field magnetization data. At high temperature the susceptibility exhibits Curie-Weiss behaviour with 8 values ranging from -5 to -35 K, for all samples excluding the non-magnetic RE, yttrium, samarium and ytterbium. The effective paramagnetic moments peff obtained (Table 2) are close to those of the free tripositive ions. In the case of SmInAuz satisfactory agreement is achieved for the temperature dependence of the susceptibility, by taking into account the contribution of the J = 7/2 excited level and an exchange field equivalent to a Curie-Weiss parameter of -10 K, as deduced from low temperature data (up to 10 K) which yields C = 0.081, i.e. close to the theoretical value for Sm3+ and thus permits us to ignore the small crystal field effects. Crystal fields affect the low temperature susceptibility and magnetization, especially for the light rare earths (cerium, praseodymium, neodymium) compounds. As discussed above, these latter compounds undergo a structural transition from cubic to tetragonal symmetry. In this case at least five independent parameters are involved in the CEF hamiltonian which hampers
107
_I 0
20
10 TCKI
Fig. 5. Inverse susceptibilities only guides to the eye.)
for
TbInAu 2, HoInAuz
and DyInAuz.
(The
full lines are
Fig. 6. Low temperature (1.7 K) magnetization us. field for several compounds REInAq series. The gJ values for the different RE are indicated on the right-hand
of the axis.
determination of the CEF effects; thus due to the additional uncertainties introduced by the increased number of parameters no quantitative analysis of these effects was carried out. Typical high field magnetization curves taken at 1.7 K are displayed in Fig. 6. Excepting GdInAuz which saturates near 7 pg in the highest field available, the low temperature moments of all compounds are largely reduced in comparison with the gJpB values (Table 2). 4.2. The cases of CeInAu, and YbInAu, For CeInAu* the lack of lattice constant anomaly as well as X-ray absorption spectroscopy (XAS) measurements suggest an almost integral valence. The low temperature specific heat shows a broad anomaly at about 1 K (Fig. 8) with AC * 4 J mol-’ K-‘, which spreads out to about 3 K and shows magnetic ordering. The form of the anomaly gives an indication of
108
r IO.
. ;
#.
.
. l
+o +o
l
+O+O
.
l
o
+o+o*o
@dJ
Otomo+
o*
m
9
.
.
.
0,5_
0’
.
0’ .
0+
Ce 1.7K 0 Ce 4.2K + Cea~kx 1
0’ . o+ . .+O
l
o+
OO
50
1aJ
1 i0 HCkCd
Fig. 7. Magnetization
us. field for CeInAuz and one yttrium-substituted
:
v
sample, Ceo.zs-
:
:
.....‘#
0 f\ ,.,,~ ,,,_.... .... 0
10
23 TCKI
Fig. 8. Heat capacity data for CeInAu2 in the 0.4 - 20 K range, C us. T; Inset, magnetic contribution to the specific heat AC/T and normalized entropy SIR In 2 us. T.
precursor or .short-range order effects at T > 1 K, likely to be due to the possibility of lattice disorder. The magnetic dilution of cerium was investigated in the substitution series Ce, _,Y,InAu, through high-field magnetization (Fig. 7) and susceptibility measurements. In the concentration range investigated 0 < x < 0.9 and within the magnitude of experimental precision no differences could be evidenced in the magnetic behaviour. The effective moments peff deduced from the high-temperature susceptibility data decrease from 2.57 to 2.51 (CeoerYo.,InAuz) while the 8, values scatter between -15 and -7 K (Table 2). The heat capacity study concerns CeInAuz, Ceo.iYo..$nAuz and the two reference compounds LaInAuz and YInAu, (Fig. 9 and Table 3). The
109
Ceo,YJnAul /’ /- -_x’
, /’
,./
YInAy.
-.l
-.
1
20
0
T2 CK’)
Fig. 9. Heat capacity data, show least-squares fits. TABLE
4%
C/T us. T2 for YInAu*
and Cee.iYo.aInAu2.
The broken
lines
3
Electronic specific heat coefficient, y; Debye temperature, 0~ and room-temperature magnetic susceptibility, x293 K for the REInAu? series (for details of 0~ see text) RE
Y (mJ mol-’
La Ce
1.9 130 f 11.8 40 + 1.9
Ce0.1Y0.9
Yb Y aLow
temperature
BD
KP2 )
+ 0.3 2 + 0.2 0.7 + 0.3
susceptibility,
(K) 140 125 118 105 139
x293
K
(lop5
cm3 molP1)
6.6
485a 8.0
x0,
reference compounds behave normally and the behaviour observed can be reproduced by C = yT + /3T3 in the temperature range up to about 3.5 K. Least squares fits yield the value of 1.9 mJ mol-’ Ke2 for the linear term of the specific heat. The Debye temperatures, 139 and 140 K near 0 K for the yttrium and lanthanum samples, go through a minimum of about 120 K near 9 K. For CeInA+, owing to the large magnetic anomaly (Fig. 8) only a restricted linear C(T)/T variation is available. Analysis of the data yields in the high temperature region a tentative y value of 130 mJ mol-’ Ke2, where y denotes the term of the specific heat linear in temperature between 5.5 and 11 K. A further analysis with the assumption that the lattice term may be in a first approximation taken equal to that of LaInAu,, i.e. subtracting the LaInAu2 data from the measured values, leads to a near constant C(T)/T value of about 150 + 10 mJ K--2 (Ce atom))‘, up to the limit of the
110
measurements, which may be identified, keeping in mind the above approximation, with the electronic contribution y. In the yttrium-substituted sample, for which the magnetic anomaly is shifted towards fairly low temperatures, only a small upturn is observed. For this alloy, the electronic contribution, deduced from data analysis in the 3 - 7.5 K temperature range, is 11.8 mJ mol-’ K-2, i.e. roughly proportional to the cerium concentration. The entropy variation associated with the anomaly observed in CeInAu,, defined as j-AC/T dT with AC = C,,,, - (YT + /3T3) reaches 0.8 R In 2 at T - 4.5 K (Fig. 8, inset); thus the cerium atoms in CeInAu, are in a doublet crystal field ground state, as required by the low temperature tetragonal symmetry, the 2J + 1 = 6, cerium levels being split into 3 doublets as a result of crystal field interaction [ll]. The y values in the cerium system, largely enhanced with respect to the values of the non-magnetic reference, compare with those typically observed in IV and Kondo systems or more generally in cerium-based compounds where they can be accounted for by f-d hybridization effects. Lm X-ray absorption spectra of CeInAu2 show the characteristic line of integral valent RE atoms, which seems to exclude the possibility for the enhanced y values to be related to an unstable IV state. As already mentioned the lattice constants, except that of ytterbium, show a linear correlation with the ionic R3+ radii which suggests that ytterbium is in an unstable valence state. Lattice parameter studies yield a valence of 2.83, whereas the value deduced from L,,, edge measurements [12] is 2.68. Susceptibility measurements show that YbInAu, presents behaviour typical of IV materials; namely at high temperature a Curie-Weiss like temperature variation, and at decreasing T a broad maximum, followed by the low temperature demagnetization. After subtraction of an impurity term, assumed to be Curie-like, a finite almost temperature independent susceptibility may be defined. The analysis of our low and high field magnetization data yields the values of x = 4.85 X 10d3 cm3 mol-’ and T,,, - 80 K for the low-temperature magnetic susceptibility and the temperature of the susceptibility maximum in agreement with previous determinations [ 51. Typical values of about 1% Yb3+ ions (part being in the Yb203 phase as discussed below) are deduced, both from the Curie-like impurity contribution and from the low temperature saturating component of the M(H) curves. The experimental heat capacity results are given in Fig. 10. The behaviour observed can be reproduced by C = yT + /3T3 in the temperature range 2.7 - 8 K, with y = 40 mJ mol-’ KP2 and /3 = 6.6 mJ mol-’ KP4. In addition the C(T) dependence is characterized by the presence of a small peak located at 2.2 K, typical of a second-order phase transition. The same anomaly, caused by the presence of the impurity phase Yb203 has already been reported to occur in ytterbium compounds and especially in YbCuAl [ 13,141. The estimated entropy associated with our anomaly is about 0.04 J mol-’ K-‘, which if attributed to the second-order phase transition of the magnetic ytterbium atoms (in the Yb203 phase) would correspond to about 2% - 3% of the ytterbium atoms in this sample. The Debye temperature deduced from our data with the usual assumption that the T3 coefficient
111
.
100 T2 (K*) Fig. 10. C/T against T* for YbInAu2.
Broken
line shows
least squares
fit.
arises only from the phonon contribution is 105 K, i.e. an anomalous low value compared with that of the non-magnetic isomorphous references, even after correction for mass difference or taking into account the thermal variation of ~9~ of the non-magnetic partners. From Fermi-liquid approaches, one expects for IV compounds an electronic T3 term in the low temperature specific heat. An estimation of this contribution to the 0 term, within the framework of the model of Newns and Hewson [ 151, gives the value of +2.3 X 10h3 J mall’ Kh4. This contribution is not negligible when compared with the experimental /3value of 6.6 X 10-3, which thus indicates that a large part of the T3 term arises from electronic effects. This gives the tentative value of 122 K, in line with the values observed in this temperature range, for the Debye temperature of intermediate valent YbInAuz. Finally, the R ratio, expressed in the usual manner, as for example in the picture of Fermi liquids is R =
(n2kB2/peff2)x
with p,rf2 = gJ2pB2J(J + 1). For YbInAu, our values of the T = 0 susceptibility x0 and the electronic specific heat coefficient y yield R = 1.28 close to the theoretical limit value of R = 1+
&
= 1.14
for ytterbium compounds; YbInAu, thus displays, as usually observed in cerium-based mixed valence compounds [16] the maximum 2J + 1 ground state degeneracy. 5. Conclusions Compounds of the ReInAu, series formed with the light RE, from lanthanum to neodymium, undergo a structural transformation from cubic
112
to tetragonal symmetry. The size of the RE atom plays an important role, the transformation temperatures displaying an almost regular decrease with decreasing RE ionic radii or by dilution of the RE in the constricted YInAuz matrix. All compounds have normal valence state behaviour, except YbInAq which is confirmed to be mixed valent with the maximum 2J + 1 = 8 ground state degeneracy, and CeInAu, which despite undergoing a magnetic transition in a 2s + 1 = 2 ground state, presents in the paramagnetic region a strongly enhanced temperature independent electronic specific heat.
References 1 B. T. Matthias, E. Corenzwit, J. M. Vandenberg, H. Harz, M. B. Maple and R. N. Shelton, J. Less-Common Met., 46 (1976) 339. 2 R. M. Galera, J. Pierre, E. Siaud and A. P. Murani, J. Less-Common Met., 97 (1984) 151. 3 M. Ishikawa, J. L. Jorda and A. Junod, in W. Buckel and W. Weber (eds.), Superconductivity in d- and f-band metals, Karlsruhe, 1982, p. 141. 4 M. J. Johnson and R. N. Shelton, Solid State Commun., 52 (1984) 839. 5 H. Oesterreicher and F. T. Parker, Phys. Rev. B, 16 (1977) 5009. 6 M. J. Besnus, J. P. Kappler, A. Meyer, J. Sereni, E. Siaud, R. Lahiouel and J. Pierre, Physica B, 130 (1985) 240. 7 R. Marazza, R. Ferro and D. Rossi, 2. Metallk., 66 (1975) 111. 8 D. Schmitt, P. Morin and J. Pierre, J. Magn. Magn. Mater., 8 (1978) 249. 9 H. Balster, H. Ihrig, A. Kockel and S. Methfessel, 2. Phys. B, 21 (1975) 241. 10 R. Aleonard and P. Morin, J. Magn. Magn. Mater., 42 (1984) 151. 11 U. Walter, J. Phys. Chem. Solids, 45 (1984) 401. 12 D. Muller, S. Hussain, E. Cattaneo, H. Schneider, W. Schlabitz and D. Wohlleben, in P. Wachter and H. Boppart (eds.), Valence Instabilities, North-Holland, 1982, p. 463 13 R. Pott, R. Schefzyk, D. Wohlleben and A. Junod, 2. Phys. B, 44 (1981) 17. 14 J. C. P. Kiaasse, F. R. de Boer and P. F. de Chatel, Physica B, 106 (1981) 178. 15 D. M. Newns and A. C. Hewson, J. Phys. F, 10 (1980) 2429. 16 M. J. Besnus, J. P. Kappler and A. Meyer, Physica B, 130 (1985) 127.