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Low temperature heat capacity studies on intermetallic and semimetallic rare earth compounds
K. Gschneidner, Jr, T. Takeshita, B. Beaudry, O. Mcmasters, S. Taher, J. Ho, G. King, J. Gruber
To cite this version:
K. Gschneidner, Jr, T. Takeshita, B. Beaudry, O. Mcmasters, et al.. Low temperature heat capacity studies on intermetallic and semimetallic rare earth compounds. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-114-C5-115. �10.1051/jphyscol:1979539�. �jpa-00218956�
JOURNAL DE PHYSIQUE Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-114
Low temperature heat capacity studies on intermetallic and semimetallic rare earth compounds
K. A. Gschneidner, Jr. (*), T. Takeshita (*), B. J. Beaudry (*), O. D. McMasters (*), S. M. A. Taher (*• **), J. C. Ho (**), G. B. King (**) and J. B. Gruber (*• ***)
(*) Ames Laboratory-DOE, Iowa State University, Ames, IA 50011, U.S.A.
(**) Dept. of Physics, Wichita State University, Wichita, K.S 67208, U.S.A.
(***) Dept. of Physics, North Dakota State University, Fargo, ND 58102, U.S.A.
Résumé. — Nous présentons les résultats de mesures de la chaleur spécifique à basses températures (1-20 K) de quelques phases de type RM5 (qui absorbent bien de l'hydrogène) et de quelques systèmes de type La3S4-La2S3. Nous présentons aussi des résultats sur le supraconducteur LaSt 33-LaS140.
Abstract. — The results of low temperature (1-20 K) heat capacity measurements on some RM5 phases, which are good hydrogen absorbers, and on some La3S4-La2S3 compounds are reported and discussed. Also included are some measurements on the superconducting LaSs 33-LaSj 4 0 phases.
1. Introduction. — Low temperature (1-20 K) heat capacity measurements of normal metals, alloys and intermetallic compounds yield information about the integrated density of states at the Fermi level and the vibrational characteristics of the solid. The former is proportional to the electronic specific heat constant, y, and the latter is the Debye temperature at absolute zero, 6D.
2. Hydride forming intermetallic compounds. — The heat capacity of the several R(T5_XM;|.) compounds, where R = a rare earth or Th, T = a transition metal and M = a non-transition metal, has been measured from 1 to 20 K. The results for several Th(Ni5_;cAl;c) series of alloys, where 0 < x < 3, are shown in figure 1 along with amount of hydrogen absorbed [1]. The difference in the hydrogen absorp- tion characteristics between ThNi5 and the isostructu- ral LaNis phase is quite surprising, especially in view of their nearly identical y and 8D values (compare y = 5.72 [2] and y = 6.08 [3], and 0D = 341 [2] and 0D = 351 [3] for LaNi5 with those shown in figure 1 for ThNi5). Although there is one more electron per formula unit in ThNi5 than in LaNi5 and this might account for the difference in their hydroge- nation behaviors, the small difference in y values suggests they have similar band structures. In addition the magnitude of the y values suggests that both phases have unpaired d electrons in bands at the Fermi surface — this, however, does not necessarily mean that the Ni d-bands are unfilled, since both La and Th could give rise to partially filled d-bands.
The decrease in y when Al is substituted for Ni might be correlated with the increase in the ability of these materials to absorb hydrogen. This corre-
' • 560
/
T h N i5-xA lx / - 5 4 0
o y I 10 • * e0 I • 520
• No. H atoms/RM^
9 • / - 5 0 0
E 8 • / - 480
" 7 • / - 4 6 0
1 o / 2
*" 6 k / - - 440 c£
° \ I
<p \ '
I 5 - \ / - 420
0 \ /
e A » '
1 4 - \ / - ^ . I "400
A
£ 3 • / \ - 380 I - / "~\
/' \
0 « 1 1 \ 1 0 1 2 3 4 5
x
Fig. 1. — Electronic specific heat constant, y, Debye temperature, 9D, and number of H atoms absorbed per R M5 formula unit in the ThCNij.^L,) system.
lation, however, is not too likely in view of what has been discussed concerning LaNis and ThNi5. The magnitude of y suggests that there is mostly ji-like and little if any d-like character in the bands at the Fermi surface of the Al containing alloys.
The increase in dD with Al additions is not too sur- prising, since Al, which substitutes for the Ni atoms
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979539
LOW TEMPERATURE HEAT CAPACITY STUDIES ON INTERMETALLIC AND SEMIMETALLIC C5-115
in the 3(g) positions, is about 15
%
larger than Ni in atomic diameters. The 40%
increase inOD
when x increases from 2 to 3 and as the 3(g) positions are filled indicates this alloy composition has a much more rigid lattice and thus is unable to absorb H,.3. The Lass,-La$, solid solution. - The low temperature (1-20 K) heat capacity results for a series of La-S alloys in the La3S4-La$, solid solution region are shown in figure 2. The alloys were prepared by direct combination of La and S at low temperatures, followed by melting in a W crucible. For alloys with a S : R ratio less than 1.40 the alloys become super- conducting at 8.2 K. As is evident in figure 2, Tc is independent of concentration for the three composi- tions measured, but the jump in the heat capacity at T , decreases with increasing S : R ratio, indicating that only portions of the samples are superconductive.
One possible explanation is that the so-called R3S4- R,S3 solid solution region H consists of two phases, one near the R3S4 stoichiometry and the other near the R,S, stoichiometry. Optical metallography and microprobe analyses do not support this phase separation, but we can not rule out a narrow two phase region
<
1 at.%
S. Another possible expla- nation is that La,S,, which has been reported to undergo a cubic to tetragonal distortion at 90 K [4], only partially transforms as the S : R ratio increases, and when this ratio reaches-
1.40 no transformation occurs. This could easily account for the reduced areas under the curves below Tc as the S : R ratio2 4 6 8 10 12
T ( K )
Fig. 2. - Low temperature heat capacity (CIT) vs. temperature for a series of LaS, alloys.
increases, assuming the untransformed material does not become superconducting.
Acknowledgment. - This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Material Sciences and Chemical Sciences Division.
Reference
[I] TAKESHITA, T. and WALLACE, W. E., J. Less-Common Metals 55 (1977) 61.
[2] NASU, S., NEUMANN, H. H., MARZOUK, N., CRAIG, R. S. and WALLACE, W. E., J. Phys. Chem. Solids 32 (1971) 2779.
[3] THOME, D. K. and GSCHNEIDNER, K. A., Jr, Unpublished results (1976).
141 BUCHER, E., ANDRES, K., DI SALVO, F. J., MAITA, J. P., GOS- SARD, A. C., COOPER, A. S. and HULL, G. W., Jr, Phys.
Rev. B 11 (197.5) 500.
