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NEEL AND CURIE TEMPERATURES FOR SIX OF THE CRYSTAL STRUCTURES OF CERIUM AND
PRASEODYMIUM
G. Meaden, N. Sze, G. Krithivas, M. Zuckermann
To cite this version:
G. Meaden, N. Sze, G. Krithivas, M. Zuckermann. NEEL AND CURIE TEMPERATURES FOR SIX OF THE CRYSTAL STRUCTURES OF CERIUM AND PRASEODYMIUM. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-375-C1-377. �10.1051/jphyscol:19711129�. �jpa-00213946�
JOURNAL DE PHYSIQUE Colloque C I, supplkment au no 2-3, Tome 32, Fivrier-Mars 1971, page C 1 - 375
NEEL AND CURIE TEMPERATURES FOR SIX OF THE CRYSTAL STRUCTURES OF CERIUM AND PRASEODYMIUM
G. T. MEADEN, N. H. SZE, G. KRITHIVAS
Department of Physics, Dalhousie University, Halifax, Nova Scotia and M. J. ZUCKERMANN
Eaton Laboratory, McGill University, Montreal, Quebec
Rbum6. - Par des mkthodes diverses on a r6ussi a obtenir le Ce et le Pr en structures crystallines differentes B 4 OK. Des mesures electriques nous ont permis de dCcouvrir leurs transitions magnetiques. Dans le Ce-a (cfc'), Ce-8 (hexa- gonale), Pr-a (hexagonale) et Pr-y (ccc) on trouve en dessous de 24, 14, 22 et 18 OK des effets rksistifs qui sont caracte- ristiques de I'antiferromagn6tisme. Dans le Pr-/3 (cfc) et peut-&tre le Ce-y (cfc), les anomalies resistives suggkrent plutBt le ferromagnetisme a 9 et a 8 OK. On discute I'importance de ces resultats pour le Ce-a et le Ce-y.
Abstract. - By various means we have succeeded in obtaining Ce and Pr in different crystal structures at 4 OK.
Electrical measurements have permitted detection of the magnetic transitions. Resistance effects characteristic of anti- ferromagnetism are apparently displayed by a-Ce (fcc'), 8-Ce (dhcp), a-Pr (dhcp) and y-Pr (bcc). below 23, 14, 22 and 18 OK respectively. For fiPr (fcc) and possibly y-Ce (fcc), the resistance anomalies are more like ferromagnetic ones with transitions at 9 and 8 OK. The importance of these preliminary results for a- and y-Ce is indicated.
I. Introduction. - At 300 OK annealed Ce is fcc (y-phase) and trivalent with one 4 f electron/atom [I].
On cooling below 260 OK y-Ce becomes unstable and begins transforming to dhcp P-Ce (also trivalent), while below 100 OK P- and y-Ce partially transform t o a-Ce (collapsed fcc). Neutron diffraction work by Wilkinson et al. [2] has shown that P-Ce is antiferro- magnetic (AFM) below N 13 OK, but little is known about ordered magnetism in a- and y-Ce. It has however been deduced that at high temperatures and pressures a-Ce has no disordered moments to any appreciable extent [3] whereas y-Ce does (-J 2.5 pB) [4]. a-Pr (dhcp) is AFM below - 25 OK, according to neutron diffraction studies [5]. A fcc form (P-Pr) which has recently been identified by Bucher et al. [6]
is ferromagnetic (FM) below 8.7 OK. Nothing is known of the y-phase (bcc).
11. Results on Praseodymium. - Firstly, the high- temperature phase diagram had t o be clarified in order to delineate the equilibrium crystallographic transition temperatures. By differential thermal ana- lysis we found the 8 2 y transformation at l 128 +
1 OK, but could not detect the a 2 P change. A possible lower limit is 975 OK because quenching produced the a-phase and subsequently the 22 OK magnetic transi- tion. Although the Pr was polycrystalline, the resistive transition had the familiar hump-back form of nume- rous AFM monocrystals. Quenching from 1 11 5 OK retained only P-Pr because a single magnetic transition appeared at 9 OK with a form suggestive of F M (cf the susceptibility work of Bucher et al. [ 6 ] ) . Quenching from the y-field (at 1 150 OK) was only partially success- ful because 3 magnetic transitions appeared (Fig. l), those at 90 and 22 OK corresponding to /I- and a-Pr.
The new transition suggests AFM and implies a Nee1 point for y-Pr at 18 OK.
111. Results on Cerium. - Annealed Ce when slow- cooled produces a t 4 OK an a-/? phase mixture with
/ RESISTIVITY OF PRASEODYMIUM OUENCHED FROM 1153'K
FIG. 1. - Resistivity at low temperatures of praseodymium quenched from the y-phase (bcc) at 1 153 O K . This curve shows the occurrence of magnetic transitions due to the a-, 0- and
y-phases.
very little y (0-5 %) [2] [7]. Figure 2, curve 1, shows a nearly linear resistivity with an anomaly below 14.5 OK due t o AFM P-Ce. The next curve shows the effect of quenching from 3000 t o 4 OK at a very high rate (< 10 seconds). This partially inhibits 8-formation and increases the a-fraction a t 4 OK. The main new feature is the anomaly below 23 OK, which resembles AFM Mn, Dy, etc. Lastly, curve 3 was obtained after lightly cold-working a quenched sample by scratching its surface at 4 OK. This process converts almost the entire sample to a-Ce, according to McHargue and Yakel [7]. The result strongly suggests that a-Ce is AFM below -- 23 OK. Note that the curves are norma- lized to unity a t 4 OK ; in absolute terms the resistivity of P-Ce is higher than that of a-Ce [81.
The next experiments were designed to increase the fraction of y-Ce retained at 4 OK. Unannealed Ce was used. Firstly, the control curve (Fig. 3, no. l), obtained
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711129
C 1 - 3 7 6 G. T. MEADEN, N. H. SZE, G. KRITHIVAS
FIG. 2. - Relative resistance of annealed cerium at low tem- peratures. Curve 1 and Inset : warming curve for slow-cooled Ce showing the Nee1 point of P-Ce at 14.5 OK. Curve 2 : for Ce quenched from 300° to 4 OK.Curve 3 : for quenched Ce cold- worked at 4 O K . All curves are normalized to unity at 4.2 OK.
The ordinates refer to curve 3. Curve 2 is displaced by 0.1 and curve 3 by 0.2 with respect to curve 3.
FIG. 3. - Relative resistance if unannealed cerium at low temperatures. Curve 1.: warming curve for slow-cooled Ce.
Curve 2 : for the same sample when clamped. Curve 3 : for the same sample when no longer clamped. All curves are norma- lized to unity at 4.2 OK. The ordinates refer to curve 2. Curves 1
and 3 are both displaced by 0.2 relative to curve 3.
after slow cooling, revealed a magnetic transition at 15OK (P-Ce) and another transition of unknown origin a t 35 OK (possibly related to the appearance, on warming, of an intermediate a-y form [9]). Next, the specimen was clamped rigidly in a special frame and cooled to 4 OK. It was hoped that the constraint would impede to some extent the fcc y + a transition which, if free, occurs with a 16 % volume contraction ; the y-Ce lattice might conceivably submit to a kind of negative pressure effect. As a result, two extra transi- tions appeared, one near 21 OK and one at 8 OK (curve 2). Next, a third run was done without the clamping constraint (curve 3), and the 8 OK anomaly was found to be no longer detectable. Other runs have confirmed the qualitative nature of these results.
Previously, a heat capacity anomaly at 7 OK has been noted [lo]. The form of the 8 OK resistance anomaly suggests FM, but AFM without appreciable magnetic superzone scattering (as in Eu [Ill) is not excluded.
The crystal phase is not yet known but fcc y-Ce is a possibility because of the nature of the clamping experiment. The fcc forms of Pr and Nd are already known to be FM below 8.70 and 29 OK respectively.
IV. Discussion. - Apart from the work of Elliott et al. [8] on P-Ce these are the first observations of the effects of magnetic transitions on the transport properties of Ce and Pr. Emphasis has been on deter- mining the techniques for obtaining the various phases at low temperatures. For P-Ce and a-Pr the work confirms the NCel points, and for P-Pr the Curie point, provided by other means. It also appears that a-Ce and y-Pr could be AFM, and y-Ce either FM or AFM at 8 OK. However, we stress that the work is preliminary as the inferences drawn here are based mainly on resistance and thermopower measurements and the planned low-temperature X-ray and magneti- zation experiments have yet to be done.
Although magnetic measurements have never been made on pure a-Ce, it has appeared likely that at high pressures and high temperatures a-Ce may be without (disordered) localized 4f-electrons while possessing either spin-compensated or band-like 4 f- states [3] [12] 1131. This may be reconciled with an AFM low-temperature, low-pressure ground state by noting that, because the a-y first-order phase boundary in Ce is akin to the Mott metal-insulator transition (as in V203) [14] [15], our low-temperature ordered phases may constitute an entirely new phase (g-Ce, say) which is the equivalent of the AFM, low-temperature, low-pressure, insulating region of V203. On heating, the c-phase would have a first-order transition to either the disordered, magnetic y-phase or the non- magnetic a-phase, depending on the temperature and pressure. All the Ce phases are metallic, as opposed to V203, because conduction electrons are always present. The delocalizing process would be one where the localized f-electron (in y- or g-Ce) moves into an existing conduction band or spin-compensated state (in a-Ce). These possibilities will be examined in our further studies.
We thank D. J. W. Geldart for discussions and the National Research Council of Canada for a grant in aid of this research.
NEEL AND CURIE TEMPERATURES FOR SIX OF THE CRYSTAL STRUCTURES OF CERIUM C 1 - 377
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