RESISTANCE ANOMALIES IN PRASEODYMIUM UNDER PRESSURE

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RESISTANCE ANOMALIES IN PRASEODYMIUM

UNDER PRESSURE

M. Heinrichs, J. Wittig

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplPment au no 8 , Tome 39, aolit 1978, page _C6-1072

R E S I S T A N C E ANOMALIES I N PRASEODYMIUM UNDER PRESSURE

M. Heinrichs and J. Wittig

I n s t i t u t

far

Festkiirperforschung, KFA JiiLich, 0-51 7 0 Jiilich, Germany

Rdsum6.- La caractdristique R-T du Pr prdsente une interessante anomalie au-dessus de 40 kbar. Quand la tempdrature diminue, la rdsistance passe par un minimum suivi d'un maximum. La rdsistivitd r6si- duelle est maximum vers 60 kbar. Une correspondance dtroite entre les diagrammes de phase P-T de Pr et du La devient ainsi apparente.

Abstract.- The R-T characteristic of £cc Pr shows an interesting anomaly at pressures above 40 kbar. With increasing temperature the resistance passes a maximum followed by a minimum. The residual resistivity reveals a maximum at approximately 60 kbar. A close correspondence between the P-T phase diagrams of Pr and La thus becomes apparent.

A point of inflection had been reported in the R-T characteristic for lanthanum under pressure. Futhermore, a maximum occurs in the residual resistivity at253 kbar/l/. Pr has the same outer elec- trons as La besides the 4f2 configuration. One should hence expect very similar metallic proper- ties apart from Pr's magnetism. We have searched for the corresponding phenomena in Pr and have observed them indeed at slightly higher pressures than for La.

Figure 1 shows R-T curves for £cc-pr2 at indicated pressures. A hump is seen in this family of curves.

pressure and is just below room temperature at 75 kbar. Irreversibilities between cooling and war- ming are very small and of the order of the width of the lines. At 130 kbar the R-T characteristic is still highly anomalous (figure 1) : the temperature coefficient is vanishingly small down to = 150 K. Below = 50 K, the resistance drops steeply.

Figure 2 shows the residual resistance as a function of pressure for one particular pressure cell.

PRESSURE l kbar 300

C

Fig. 2 : Residual resistance vs. pressure. The pressure changed at room temperature in the indicated sequence.

I I

-

Starting at data point 1 , the pressure has been stepwise increased at room temperature.A maximum occurs between 60 and 65 kbar. On the pressure re- leasing cycle (open circles, 6..

.

IS), the maximum ~ i g . 1 : R vs. T for £cc-Pr at indicated pressures. is less pronounced. We attribute this to some other

perturbing influence of unknown arigi'n which leads It shifts to higher temperatures with increasing to a decrease of R q S 2

K

after a constant pressure

E

W'

75 k bar

3 S

g

m-

100, 100 200 300 Temperaturel K

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run between 300'K-and 4.2 K (see e.g. the pairs of data points 8,9 or 16,17). The existence of a maximum in the residual resistivity has however been clearly established by changing the pressure at liquid He temperature. In figure 3, the pressure was increased from 53 kbar to 82 kbar (1...7).

I I I

60 70 80

PRESSURE1 kbar

Fig. 3 : Residual resistance vs. pressure. The pressure was changed at liquid He temperature.

The resistance increases presumably due to cold working with a tendency towards saturation. During the release of pressure (open circles, 7. ..15) RqeZK passes a maximum near 60 kbar. This maximum was E-

producibly observed upon again increasing the pressure (figure 3, triangles, 15

...

22). The sample was never warmed above 10K in these series of experi- ments.

We have thus observed qualitatively the same anomalies in Pr at slightly higher pressures than for La / l / . Hence a very close correspondence between the P-T phase diagrams of both metals becomes apparent. Figure 4 shows a tentative low temperature phase diagram. The location of the dhcp-fcc phase boundary is as yet uncertain for Pr. One of the interesting features is a possible phase boundary at higher pressure (bold lines) associated with the points-of-inflectinn anomaly. The slope of the pha-

se boundary is nearly the same for Pr and La. It had been suggested that La may undergo a subtle fcc+fccl transformation upon crossing the points- of-inflection anomaly. An isostructural transformation is a lattice instability and will be probably accompanied by phonon-softening. This may have a profound influence on the resistivity and thus cause the points-of-inflection. However it should be noted that the existing X-ray data /3/ are not

precise enough to exclude the occurence of a small lattice distortion from the cubic symmetry.

300

] L a = d h c p \ fcc

,

I

~r I.::

\

d h c ~

':

fcc

/

,j'f

0 20 L0 ₆₀ ₈₀

PRESSURE / kbar

Fig. 4 : Tentative P-T phase diagram for La and Pr.

The maximum of the residual resistivity at 53 and =60 kbar in La !l/ and Pr respectively is attributed to another subtle phase transformation at low temperature (figure 4). ~lztzel and Frits- che /4/ have recently suggested that an electronic Lifshitz-transition /5/ may be the origin of this phenomenon. In summary we think the present data point toward an extremely similar electronic band structure for Pr and La.

References

/l/ Balster,H. and Wittig,J., J. Low Temp. Phys. 21 (1975) 377

-

/2/ Piermarini,G.J. and Weir,C.E., Science

144

(1964) 69

/ 3 / Syassen,K. and Holzapfel,W.B., Solid State Commun.

16

(1975) 533

141 Glstzel,~. and Fritsche,L., Phys. Stat. Sol. (b)

2

(1977) 85