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PHASE TRANSITION AND MAGNETISM IN Eu3S4
O. Massenet, J. Coey, F. Holtzberg
To cite this version:
JOURNAL DE PHYSIQUE Colloque C4, supple'ment au no 10, Tome 37, Octobre 1976, page C4-297
PHASE TRANSITION AND MAGNETISM IN
E u ~ S ~
0. MASSENET and J. M. D. COEY Groupe des Transitions de Phases, C. N. R. S.
BP 166, 38042 Grenoble Cedex, France F. HOLTZBERG
Centre de Recherche sur les Tr&s Basses TempCratures, C . N. R. S. BP 166, 38042 Grenoble Cedex, France
RBsum6. - Des mesures magnktiques, d'effet Mossbauer et de chaleur specifique ont Bte faites sur Eu&. L'augmentation. d'entropie observke en chauffant au passage de la transition ?i 160 K est de 3,3 joules/K/mole. Elle est associee B un desordre partiel de charges sur les ions europium qui s'etablit au-dessus de 160 K. Ce compose s'ordonne ferromagn6tiquement en dessous de 3,l K et la susceptibilite magnetique au-dessus de cette temperature est la somme d'une contribution de type Van Vleck due aux ions Eu+++ et d'une contribution type Curie-Weiss due aux ions Euff.
Abstract. - Specific heat, magnetic and Mossbauer effect measurements are reported for Eu&. The increase of entropy on heating through the transition at 160 K is 3.3 joules/K/mole ; it is associated with partial charge disordering of Eu++ and Eu+++ ions. The compound orders ferro- magnetically at 3.1 K and the magnetic susceptibility above this temperature is explained as the sum of a Van Vleck contribution due to Eu+++ and a Curie-Weiss part due to Eu++.
Eu3S4 has been reported as showing a cristallogra- phic transition at 160 K [I]. Above this temperature, it has a Th3P4 cubic unit cell with equivalent sites for all the Eu atoms. Below the transition, the symmetry of the unit cell is believed to become tetragonal and the Eu sites should split into two groups of inequivalent sites in the ratio 1 : 2, the first group populated by E u f + and the second group by E u + + + according to the formula (Eu++) [Eu"'], S4 [2]. Davis
et al. [I] have shown that the resistivity jumps by a factor of about 50 at the transition, with semiconduc- tor-like behaviour on either site of it. Berkooz et al. [3] have interpreted the change of shape of the Mossbauer spectrum between 83 K and room temperature in terms of thermally activated electron hopping among the Eu sites, with a characteristic frequency for the valence
fluctuation in the range 1 0 ~ - 1 0 ~ ~ S-I. However, their resistivity data show no evidence of the phase transition near 160 K. Low temperature Mossbauer work by Gorlich et al. [4] indicates that Eu,S, orders magneti- cally at 3.8 K.
In the present paper, we present the results of a study of the magnetic properties of Eu3S4 at low temperature and some quantitative measurements of the heat capa- city in the region of the transition.
Eu3S4 powder samples were prepared by mixing EuS powder from a ground-up crystal with a quantity of sulphur in slight excess of the stoichiometric propor- tions. The mixture was sealed in an evacuated quartz tube and fired at 600 OC for 3 days. This gave a black powder which showed only the lines due to the cubic
Th3P4 unit cell with a, = 8.507
A,
in a Zeeman- Bohling X-ray diffraction photograph. No lines of EuS were visible, but a more sensitive test for the absence of EuS is given by the magnetic measurements describ- ed below.1. Structure and lattice parameter. -There are two series of sulfide compounds with the Th3S, struc- ture, R3S4 and R2S3 which are stable for all rare earths from La to Gd with the exception of Eu. The latter series can be represented by the formula R,/, ,/, S,, where the vacancies are randomly distributed. When R is a trivalent rare earth, there is essentially no diffe- rence in lattice constant for the two series (Fig. 1) [5,6]. In the Th,P4 structure, I 3d, the rare earth atoms are in fixed positions 12 a, determined by the space group, whereas the sulphur atoms are in 16 c positions deter- mined by a single site parameter u. For u = 1/12, the sulphur atom is equidistant from its six nearest rare earth neighbours. Although u deviates from 1/12 in the real structure [7,8], we have assumed the ideal position in estimating the Eu3S4 lattice parameter. Figure 1 shows a smooth variation of lattice parameter with atomic number, except for these compounds Eu3S4 and Sm3S4 where the large increase in lattice constant can be attributed to the presence of divalent rare earth ions. This is very similar to what is observed in the RS series with NaCl structure. The R-S bond distance, the sum of the rare earth and sulphur ionic radii is 0.346 1 a, for u = 1/12 in the Th3P4 structure [7]. The rare earth ionic radii are determined from the NaCl
C4-298 0 . MASSENET, J. M. D. COEY AND F. HOLTZBERG
FIG. 1. -Lattice constant parameter a0 of R3S4 and R2S3
compounds structure. R = rare earth in the series from La to Gd.
x = RsS4 compounds. = RzS3 compounds.
type compounds for which the sulphur radius is taken to be 1.84
A.
Using the rare earth radii and a sulphur ionic radius of 1.88A
we can obtain the observed values of a, in the Th,P4 type series. The increase in sulphur radius between the two types of structure may be attributed to the increase in coordination number of the rare earth ion from six to eight. If we attribute an average valence of 813 to the europium in Eu3S4 and take the ionic radii of Eu' + and Eu++' to be 1.14and 1.03
A,
we calculate for the Th3P4 structure a lattice parameter a. = 8.51 which is in good agreementwith our measured value. We emphasise that the choice of sulphur radius of 1.88
A
consistent with the lattice parameters of the R2S3 series is not necessarily appro- priate for the fluctuating valence compounds Eu3S4 and Sm3S,. The agreement between the calculated and observed lattice parameter for Eu,S4 in perhaps fortuitous.2. Specific heat. - Specific heat measurements have been made on these powders between 120 and
400 K using a Perkin-Elmer DSC2 differential scanning
calorimeter. Measurements were made on samples of
10 mg or 35 mg at a heating rate of 10 Klmn. The
apparatus was calibrated using , a standard sample of benzoic acid. A large peak of specific heat was measur-
ed around 160 K (Fig. 1). The width of about 7 K was intrinsic to the sample and not an instrumental effect since the same form was found for the peak on heating at 1.25 K/mn. The form of the peak is similar on cooling. From the area of the peak, one calculates an increase of enthalpy at the transition which amounts to :
AH = 530 joules/mole
.
The difference of entropy is A S = AHIT,
A S = 3.3 joules/mole
.
Above the transition with complete disorder, there would be 3 possible ways to locate one Eu+' and two Eu"++ ions on three equivalent sites while below
the transition, there remains only one configuration possible. From this simple consideration, one can deduce that complete charge disordering at the transi- tion should induce an increase of entropy
A S =
R
Log 3 = 9.2 joules/K/mole.
The experimental value of 3.3 joules/K/mole suggests that there remains a large amount of short range order in the distribution of europium valencies above the transition.
3. Magnetic properties.
-
Magnetic susceptibility measurements were performed between 300 K and liquid helium temperature. These showed no anomaly at the charge ordering transition around 160 K. Fur- thermore, there was no anomaly at the EuS Curie temperature (T, = 16 K), which indicates that there isless than 0.1
%
of EuS present as an impurity phase. The experimental curve of 1 / ~ versus T is represented on figure 2. It seems to consist of two linear segmentsT/K
100 150 200 2 50 300 350 t
FIG. 2. - Specific heat C, versus temperature T. Insert : detail of the transition on an expended temperature scale.
with different slopes above and below 100 K. The high and low temperature parts follow a Curie-Weiss law
with the following parameters : above 100 K
B1
=-
29 Cl = 12.2 below 100 K 0 2 = + 2c,=
9.30, and C, values are in good agreement with those obtained by Davis et al. [I] but these authors did not
extend their measurements below 80 K.
Low temperature magnetization were performed down to 1.9 K.
A
ferromagnetic ordering temperatureT,
= 3.1 K was deduced from low field scans, usingPHASE TRANSITION AND MAGNETISM IN E u ~ S ~ C4-299
A I . compared to the characteristic Mossbauer frequency
-30,, 10' s-' and there is a single Mossbauer line in the
!
On average position. Below the charge ordering
3
~
transition there are two distinct lines for E u f f andEu++ + with an intensity ratio of 1 : 2. Below the Curie
. 2 0
FIG. 3. - Magneticsusceptibility versus temperature. experi- mental point x the same after substractini the Van Vleck
1 T - 2
susceptibility of Eu+++. Solid line : curve -
x
= - 8.5.
Dashed 1 T - 2line : theoretical susceptibility of Eu++ -
x
= -- 7 . 8 8 'In the high field limit, the magnetization extrapolates
o
2 L 6 8 10to 66 emulgr. If one assumes that only the Eu+* spins H/ kOe
order (J = 7/2), while the E U + + + (J = 0) sublattice FIG. 4. - Magnetization versus H and versus 1 / H at 1.9 K. bears no moment, then the expected value of satura-
tion magnetization would be M , = 67 emulg, which is in very good agreement with the experimental value. According to this model, the molar susceptibility
xtOt
of Eu3S, above T, should be the sum of the suscepti-bility of one mole of E u + + plus 2 moles of Euf + + Xtot = xc.,".(Eu+
'1
+
2 x ~ . ~ . ( E ~ + + +).
We calculated at different temperatures xV.,.(Euf ++) using the Van Vleck formula and assuming that only the first excited state of Eu+ + + located at A = 500 K
above the ground state contribute to the susceptibility in the range 3-300 K. We then substracted this calculat- ed value from the experimental susceptibility
x,,,
and the obtained value should then give the Curie-Weiss susceptibility of the E u + + + sublattice xC.,.(Euf +). The corresponding data are plotted as crosses on figure 2. One notices that they now fall almost exactly on a straight line with C = 8.5 and 6 = 2 K. The expected value of the Curie constant for 1 mole of E u + + is C = 7.88, i. e. somewhat less than what is actually obtained.We have also made Mossbauer effect measurements on I5'Eu between 1.6 and 300 K. Our results are very similar to those reported in [3] and [4]. At room tem- perature, the charge fluctuation frequency is rapid
point, magnetic hyperfine structure appears for the Eu+
"
resonance whereas the Eu+'
*
linewidth merely increases from 3.4 to 7.2 mmls as temperature goes from 4.2 K to 1.6 K. Extrapolated to T = 0 we found Hint = 300 +_ 10 kG for Eu++, in agreement with [4]and Hint = 50 f 10 kG for Euf + ', which might be explained by dipolar and supertransferred hyperfine effects.
4. Conclusion. - From Mossbauer results, one finds that Eu3S, is a fluctuating valence compound with a single site occupied by the two valence states of Eu between room temperature and 160 K. Unlike the
true mixed valence compounds such as SmB6 or SmS un- der pressure for which the susceptibility is temperature independent at all temperatures, here we find, above the ferromagnetic ordering temperature, a susceptibi- lity which is interpretable as a simple sum of Van Vleck type (Eu'" +) and Curie-Weiss type (Euf + ) contri- butions. In this case, the spin lattice relaxation is faster than the valence fluctuation and therefore, we can observe the normal susceptibility of the two valence states. This is an example of the inhomogeneous mixed
valence state discussed by Varma [9].
References
[I] DAVIS, H. H., BRANSKY, I., TALLAN, N . M., J. Less Comm. [6] FLAHAUT, J., GUITTARD, M., PATRE, M., PARODO, M. P.,
Metal 22 (1970) 193-199. GOLABI, S. M. and DOMARGE, L., Acta Crystallogr. 19 [2] CARTER, F. L., J. Solid State Chem. 5 (1972) 300. (1964) 14.
131 BERKOOZ, O.3 M.9 SHTRIKMAN, S., State [7] HOLTZBERG, F. and METHFESSEL, S., J. Allpl. Phys. 37 (1966)
Commun. 6 (1968) 185-188.
. .--
141 GORLICH, E., HRYNKIEWICZ, H. U., KMIEC, R., LATKA, K., 1455.
TOMALA, K., Phys. Stat. Sol. 64 (1974) K 147-151. 181 Cox, W. L., STEINFINK, H. and BRADLEY, W. F., J. lnorg.
651 Progress in the Sciences and Technology of Rare Earth, Chem. 5 (1966) 318.
