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Submitted on 1 Jan 1979
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Pressure-induced valence instability in Sm4Bi3
A. Jayaraman, E. Bucher
To cite this version:
A. Jayaraman, E. Bucher. Pressure-induced valence instability in Sm4Bi3. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-350-C5-351. �10.1051/jphyscol:19795122�. �jpa-00218907�
JOURNAL DE PHYSIQUE Collogue C5, supplement au n° 5, Tome 40, Mai 1979, page C5-350
Pressure-induced valence instability in Sm4Bi3
A. Jayaraman
Bell Laboratories, Murray Hill, NJ 07974, U.S.A.
and E. Bucher
University of Konstanz, Department of Physics, D-7750 Konstanz, F.R.G.
Résumé. — Sm4Bi3 a une structure cubique (type Th3P4) avec 3 ions Sm2 + et un ion Sm3 +. A 26 kbar, ce composé présente une transition isostructurale de premier ordre. La résistance décroît brutalement d'un facteur 3 ainsi que le volume (décroissance de 10 %). Les ions Sm2+ passent à l'état 3+ et le matériau présente une transition de l'état semi-métallique à l'état métallique.
Abstract. — Sm4Bi3 has the anti-Th3P4 (cubic) structure with three Sm2+ ions and one Sm3+ ion. The compound undergoes a first-order, isostructural transition at about 26 kbar. The resistance abruptly decreases by a factor of three and the volume by about 10 %. The Sm2+ ions undergo a change towards the 3+ state in the transition and the material goes from semimetallic to a metallic state.
1. Introduction. — Many rare-earth germanides, antimonides and bismuthides of the formula R4B3 where B is Ge, Sb or Bi are known to crystallize in the anti-Th3P4 structure (cubic) [1-3], with the R occupy- ing the phosphorus sites (this has been referred to as the Gd4Bi3 structure-type in reference [2]). It is believed that in Sm4Bi3 (a = 9.815 A) three of the samarium ions are divalent [2, 4]. The possibility that the divalent samarium ions in Sm4Bi3 may undergo a change in their valence state towards the 3+ state at moderately high pressures prompted us to investigate this compound.
2. Experimental results. — The results of resis- tance measurements under hydrostatic pressure are shown in figure 1 for two different temperatures.
At 298 K an abrupt decrease in the resistance of the sample occurs at about 26 kbar. When the sample is pressurized at higher temperatures, the magnitude of the abrupt decrease in resistance decreases and is
preceded by a more gradual change; as the tempe- rature is increased the latter behavior dominates over the abrupt decrease. This suggests that the phase boundary may be terminating at a critical point.
The data obtained from the high pressure X-ray studies are plotted in figure 2. The striking lattice parameter decreases (from 9.7 A to 9.4 A + 0.05), without any change in the structure (isostructural
Fig. 1. — Resistance discontinuity associated with the pressure- induced transition in Sm4Bi3 at 298 K (left) and 500 K (right).
Fig. 2. — Lattice parameter versus pressure for Sm4Bi3 at 298 K.
There is an abrupt lattice parameter decrease from 9.7 A to 9.4 A in the isostructural transition.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19795122
PRESSURE-INDUCED VALENCE INSTABILITY IN Sm,Bi, C5-351
transition), strongly suggests that a change in valence toward a higher valence state is involved. A conti- nuous compression curve would be valuable, but we are unable to carry out this experiment for lack of adequate sample volume at this time. However the abrupt resistance drop, combined with the abnor- mal lattice parameter change at about the same pressure, confirm that the transition is a strongly first-order one at ambient temperature.
Pressure-temperature studies, carried out using the abrupt resistance change, as well as differential thermal analysis (DTA) carried out under hydro- static pressure indicated that the transition boundary is very steep [dT/dP very large] and that it may have a positive slope. In order to confirm that the slope is positive, we determined the sign of the latent heat change and found that heat is evolved in going from the low pressure phase (semimetallic) to the high pressure phase (metallic) ; this definitely proves that the semimetallic phase is the higher entropy phase.
Based on average ionic radii, Gambino [2]
postulated the formula Sm+:Sm+2Bi3 lJ 2 2 for Sm4Bi3, YblZYb+3Bi3 for Yb4Bi3 and ~ u ; ~ B i , for Eu4Bi3.
Since the anticipated valence transition occurs in Sm4Bi3, we expect such a transition also in the case of Eu4Bi, and Yb4Bi3, but these may require much higher pressures than Sm,Bi,.
The magnetic susceptibility of Sm4Bi3 [4] appears to closely resemble SmB2Bi3 [4] (a fictitious compound). However the measured susceptibilities are even lower (by about 30 %) in the range of tempe- rature 1-300 K, with a weak Curie-like anomaly superposed at the lower temperature end. The absence of magnetic ordering and the smaller susceptibility values suggest a mixed valence state (ICF state), for Sm4Bi3 at ambient pressure. Our lattice parameter data for the pressure-induced transformation suggests that the Sm ions may be in the integral valence state 3 ' .
In the latter event, the high pressure phase should exhibit magnetic ordering.
References
[l] HOHNKE, D. and PARWE, E., Acra Cryst. 21 (1966) 435.
[2] GAMBINO, R. J., J. Less-Comm. Metals 12 (1967) 344.
[3] YOSHIHARA, K., TAYLOR, J. B.. CALVERT, L. D. and DES- PAULT, J. G., J. Less-Comm. Metals 41 (1975) 329-37.
[4] BUCHER, E., COOPER, A. S., JACCARD, D. and SIERRO, J., Valence Instabilities and Related Narrow-Band Phenomena, ed.
R. D. Parks (Plenum) 1977, p. 529.
