High pressure thorium-rare-earth carbide superconductors

Journal of the Less-Common Metals, (; Elsevier Sequoia S.A., Lausanne

HIGH PRESSURE THORIUM-RARE-EARTH SUPERCONDUCTORS*

MILTON

C. KRUPKA,

ANGELO

217

30 (1973) 217-223 Printed in The Netherlands

L. GIORGI

Los Alamos Scientific Laboratory. University

and

CARBIDE

EUGENE

G. SZKLARZ

of Calijornia. Los Alamos. N.M. 87544 (U.S.A.)

(Received June 26, 1972)

SUMMARY

The high pressure preparation of superconducting thorium sesquicarbide permitted its use as a matrix whereby the effects of the addition of incremental quantities of rare-earth elements on the transition temperature could be studied. Thorium-rare-earth carbide superconducting solid solutions of general formula (Th. R.E.)& (R.E. = Ho, Er, Lu as well as SC) have been synthesized under conditions of high pressure and high temperature. Transition temperatures of 4.1 to 11.7 K were observed and are dependent upon the rare-earth element and the thorium/rare-earth atomic ratio. The occurrence of superconductivity in these qystems is limited by the thermodynamic stability of the superconducting phase and the conversion from the superconducting state to one of paramagnetism. Light rare-earth elements such as Ce, Pr, and Nd also form superconducting solid solutions but with depressed transition temperatures. The intermediate rareearth elements Gd, Tb and Dy form magnetic solid solutions at the concentration levels investigated. The (Th, R.E.),C, solid solutions crystallize in the body-centered cubic Pu2C, (D5,) structure.

INTRODUCTION

The preparation of a new class of high pressure-high temperature superconductors of general formula (Th, R.E.),C, (R.E. =Y, La) was reported previously’.2. Later work succeeded in preparing single phase Th2Cs3 which despite an apparent valence electron per atom ratio of 4.0 was found to be superconducting4. These materials crystallize in the body-centered cubic (b.c.c.) Pu,C, (D5,) structure. It was of interest, therefore, to synthesize ternary solid solutions containing rare earths other than yttrium or lanthanum. EXPERIMENTAL

Ternary alloys with variable Th/R.E. atomic ratios but constant carbon con* Work done under the auspices

of the US. Atomic

Energy

Commission.

218

M. C. KRUPKA,

A. L. GIORGI,

E. S. SZKLARZ

tent were prepared by standard arc-melt techniques (except for Tm and Yb) after which they were subjected to high pressure-high temperature treatment to produce the b.c.c. sesquicarbide phase. Purities of the rare-earth elements, crystal bar thorium metal and spectrographic grade carbon have been mentioned previously’g”*6. The various (Th, R.E.),C, compositions were synthesized in the pressure range 3@-40 kb., the temperature range 1350’-14OO’C and times of 510 min. The ‘belt” die system’ was used in this investigation. Calibration, high pressure cell assembly details and operational sequence have been previously described6. Temperature measurements were made by Pt/Pt-Rh(l3%) thermocouples designed into the cell assembly. No correction was made for the e.m.f.-pressure effect. X-ray diffraction powder patterns were obtained from arc-cast buttons and quenched high pressure samples. The sesquicarbide phases were indexed to b.c.c. symmetry, Pu$, (D5,) structure. Lattice parameters were calculated using theNelsonRiley extrapolation8 and the least squares treatment of Hess’. RESULTS AND DISCUSSION

The addition of the rare-earth elements to the Th,C, matrix produced effects ranging from depression to enhan~ment of the superconducting transition temperature as well as conversion within a given system from a superconducting state to a magnetic one. Table I provides a summary of the data obtained for the respective systems. The addition of the light rare-earth elements Ce, Pr and Nd at the Th/R.E. atomic ratio of 9/l resulted in depressions of the transition temperature of the order AT, = 0.1-1.0 K from a base value of 4. I K. It was di~cult to determine the degree of solubility of these rare-earth elements in the sesquicarbide matrix since the expected changes of lattice parameter for the solid solution Th,C,-(R.E.),C, (R.E.= Ce, Pr, Nd) would be small”. To be certain that solution did occur at least in one case, an additional preparation for the element Pr, (Th,,sPr,,,),C, was made so that an extra increment of lattice size change would be measured. Major interest would lie in elements which would enhance transition temperatures. In view of the depressions observed, no further work was done with the light rare-earth elements. The elements Pm, Sm, and Eu were not investigated. The addition of the rare-earth elements Gd, Tb and Dy at the Th/R.E. atomic ratio of 9/l resulted in solid solutions exhibiting paramagnetism. No superconducting transition temperature was observed. The results obtained with the element Ho described below, however, suggest that some smaller quantity of Dy, c 9/l Th/Dy atomic ratio, may produce a small transition temperature enhancement above 4.1 K normally associated with Th,C,. The addition of the element Ho produced the first significant enhancement of the transition temperature of Th,C,. Of interest is the observation that the ternary solution becomes magnetic upon increase of the Ho content beyond the Th/Ho atomic ratio of 7/3. The relationships between transition temperature us. composition and lattice parameter us. composition (Vegard plot) are shown in Figs. 1 and 2. Although not all compositions were. synthesized, the data suggest that a complete solid solution series could be prepared. Holmium sesquicarbide. HO&~, can be made

HIGH TABLE

PRESSURE

Th-RARE-EARTH

CARBIDE

I

SUPERCONDUCTING (Th, R.E.),C, SOLID

TRANSITION SOLUTIONS

System composition

TEMPERATURES

Superconducting transition temperature

R.E. element

Nominal Th/R.E. atomic ratio _

CK)

Ce Pr

9/l 911 812 911

<4 <4 <4 <4 magnetic magnetic magnetic magnetic 5.4 5.5 5.2 magnetic 6.8 8.2 8.2 8.1 7.0 4.6 magnetic magnetic magnetic magnetic 10.3 10.9 11.7 11.6 6.1 6.8 1.2 7.1

Nd Gd Tb DY Ho

Er

LU

SC

219

SUPERCONDUCTORS

9/l 9/l 90 812 9/l 812 713 614 9.510.5 9/l 812 713 614 515 416 317 218 119 9,il 812 713 614 911 812 713 614 515 416 317

6.5 6.0 5.4

Fig. 1. Superconducting

transition

temperature

Fig. 2. Lattice parameter

w. composition

us. composition

for (Th,Ho),C,.

AND

LATTICE

PARAMETERS

Lattice parameter a&) (b.c.c. sesquicarbide phase)

8.5445 +4 8.5563 + 4 8.5602 + 5 8.5555 k4 8.5439 k4 8.5415 +4 8.5482 f 7 8.5296 + 7 8.5448 k 5 8.526 L 1 8.499 +2 8.471 +2 8.5434*4 8.5400 + 8 8.523 k2 8.486 k2 8.455 k2 8.412 +3 8.361 k2 8.297 i2 8.237 +4 8.194 +3 8.5278 + 7 8.495 f 1 8.461 k 1 8.426 + 1 8.5362 i9 8.531 F 1 8.5297 & 3 8.529 k 1 8.543 *2 8.550 i5 Not determined

for (Th. Ho)&,

OF

220

M. C. KRUPKA,

A..L. GIORGI,

E. S. SZKLARZ

directly from arc-melt or annealing techniques lo . Therefore the synthesis of this series would require a gradation of the pressure parameter from ambient to the 30-40 kb. level. A high pressure-high temperature ternary single phase solid solution was prepared across the entire composition range in the case of the element Er. This is shown in Fig. 3. The high pressure-high temperature preparation of the b.c.c. crystal structure of Er,C, has been reported previously l1 . As observed with the element Ho, conversion from a superconducting to a magnetic state occurs at an intermediate composition level, in this case between the Th/Er atomic ratios of 5/5 and 4/6 (see Fig. 4). A complete Th-Lu-C solid solution series could not be made under the prevailing preparatory conditions, the major limitation being one of the thermodynamic stability of the sesquicarbide phase at the composition M&Z,. The Vegard plot is shown in Fig. 5. The Lu-C system is compiex and has not been studied completely. In the composition region of interest, a stable two-phase region exists at ambient pressure with phases of approximate composition LuC!~.~~~’ and LuC,. Phases existing at high pressure are unknown. Body-centered cubic Lu,C, has never been prepared. The restricted solubility of Lu in the sesquicarbide matrix is unfortunate since the enhancement of the transition temperature is the greatest of all the rareearth elements studied in this investigation (see Fig. 5). Only the elements Y and La previously reported produce a comparable effect’ - 3. The highly significant transition temperature enhancements observed with Y and Lai-3_suggested that the remaining Group III member, SC,might have a similar 5

-4

8.60

/

/

/

/

G9.0

/

i

/

/

/

,

!

/

/

1

BODY-CENTEREDCUBIC

SOLID

SOLUTION

COMWSITIONS -PARAMAGNETISM EXHlSlT

-i

‘h&

S/1

812

713 614 515 * 3/7 ThlEr ATOMIC RATIO

--

218

Fig. 3. Lattice parameter vs. composition for (Th, Er),C,. Fig. 4. Transition temperature us. composition for (Th, Er),C,.

ThzCa 90

812

,/a

614

ThlLu

Fig. 5. Lattice parameter US.composition for (Th,

W&.

Fig. 6. Transition temperature vs. composition for (Th, Lu),C3.

5/s

ATOMIC

46

317

RATIO

218

119 L”,Ca

HIGH

PRESSURE

ThdWRE-EARTH

CARBIDE

WPERCONDUCTORS

221

effect. Indeed this is the case, although the absolute magnitude observed is limited. The data shown in Figs. 7 and 8 require additional interpretation. As was noted for the element Lu, the major limitation is again the thermodynamic stability of the b.c.c. sesquicarbide phase. The apparent limit of solubility of SC in the b.c.c. sesquicarbide matrix (under the preparatory conditions used) occurs in the vicinity of the Th/Sc atomic ratio of 7/3 for which a maximum transition temperature is observed. At this approximateratioastableSc-CphaseofapproximatecompositionScC,,,,isobserved which increases in quantity as the Th/Sc ratio decreases. As the amount of this phase increases, less and less of the Th,C, matrix, together with a steadily decreasing amount of dissolved SC, is formed (note the lattice parameter reversal in the Vegard plot of Fig. 7). Disappearance ofthe superconducting phase occurs in the vicinity ofthe Th/Sc atomic ratio 4/6 to 3/7. Thus the transition temperatures observed for relatively low Th/Sc atomic ratios in reality belong to those solutions with high Th/Sc atomic ratios which are, however, present in progressively smaller quantity until total disappearance occurs.

Fig. ?. Lattice

parameter

Fig. 8. Transitlon

cs. composition

temperature

for (Th, Sc),C,.

us. composition

for (Th, Sc),C,

The elements Tm and Yb could not be incorporated into the sesquicarbide matrix. A number of methods were used all of which proved unsuccessful. The arcmelting technique could not be used because of excessive volatilization of the rare earth additives. Pre-reaction in a tantalum bomb (1300°C and 12 h) produced a low symmetry crystal modification of (R.E.),C,. However, solid state solution with Th,C, (in proper molar ratio) under high pressure and high temperature failed to occur. Possibly the time factor was too short. Finally, powder mixtures of the rare-earth element, Th, and C were reacted in situ under high pressure-high temperature conditions, but only relatively pure b.c.c. Th,C, formed to give 40-4.1 K transition temperatures. In one experiment with Tm, a broad transition temperature range was observed with an extrapolated T, of 5.2 K. Lattice parameter evidence, however. suggested minimal solubility. Although the difficulty encountered with the element Yb might be expected, the inability to incorporate the element Tm into the lattice is disappointing since the data from all systems containing Ho, Er and Lu implies that a solid solution so obtained with Tm should also be superconducting, (see Fig. 9). Further work on preparatory techniques is suggested. The experimental curves shown in Fig. 9 represent high thorium content compositions only. The curves of the Y and La additives continue to higher superdonducting temperatures not shown1-3. The Ho and Er curves were terminated because of conversion to the magnetic state.

M. C. KRUPKA,

8.60

8.50

8.56

8.54

LATTICE

8.52

8.50

8.48

PARAMETER,

Fig. 9. Rare earth carbide

8%

8.44

8?)2

A. L. GIORGI,

E. S. SZKLARZ

8.40

a0 -i

solid solution

superconduc

Data for Y and La were obtained

from refs. 1-3.

The mechanism of enhancing the superconducting transition temperature by the use of rare-earth elements is not understood. In all cases, however, solubility in the sesquicarbide lattice and phase stability are required so that the appropriate electronic interactions may occur. The maximum transition temperature observed for the elements Ho, Er, Lu and SC was seen to occur in the region of the Th/R.E. atomic ratio of g/2-7/3. This is equivalent to a valence electron per atom ratio of -3.9. The maximum transition temperatures observed for the additive elements Y and La occur at the valence electron per atom ratio of N 3.7, i.e., at higher rare-earth element concentrations. The significance of this small change in ratio is not readily apparent, The instability of the ternary phases containing Lu and SC would certainly place restrictions on reaching higher transition temperatures. It would be of interest to determine those experimental conditions under which stable solutions containing greater quantities of Lu and SC exist. In the case of Ho and Er, the results suggest that the magnetic properties may be the dominant factor. Cooper et al. have demonstrated that a new peak of superconducting transition temperatures exists in the general valence electron per atom ratios of 3.7-3.9 for a number of crystal structures of which the Pu,C, (D5,) structure is one13. This crystal structure represents a new class from which high superconducting transition temperatures, > 15 K, have been observed. Only the P-W and NaCl B-l crystal structures have previously been associated with high transition temperatures. Perhaps of greater importance is the observation that metallic elements other than the usual niobium and vanadium are involved. The conversion from a superconducting state to one of paramagnetism within a given system is of great interest. Further studies would require the preparation of samples with smaller compositional differences in the transition range. Low temperature magnetic susceptibility measurements on samples in the composition range where these materials just become magnetic should show whether the appearance of the magnetic state completely suppresses superconductivity or simply lowers the transition temperature with the magnetic and superconducting properties coexisting. The preparation of a complete ternary solid solution series demonstrating a regular variation of lattice parameter provides additional evidence for the validity of the Vegard-type relationship for phases quenched from high pressure, at least to 40 kb.

HIGH

PRESSURE

Th-RARE-EARTH

CARBIDE

SUPERCONDUCTORS

223

ACKNOWLEDGEMENTS

The authors wish to thank Mr. C. Radosevich for his capable assistance in the preparation of stock materials and Mrs. G. P. Boicourt for reading of the many X-ray diffraction films. REFERENCES M. C. Krupka,A. L. Giorgi. N. H. Krikorian and E. G. Szklarz. J. Less-Common Metals, 19 (1969) 113. A. L. Giorgi. E. G. Szklarz, N. H. Krikorian and M. C. Krupka, J. Less-Common Metals, 22 (1970) 131. M. C. Krupka, J. Less-Common Metals, 20 (1970) 135. M. C. Krupka and M. G. Bowman, Colloque In?. SW Les PropriMs Physiques Des Solides Sous Pression. Grenoble, Frunce, Sept. 8-10. 1969: Proc. Pub). 1970. 5 M. C. Krupka. N. H. Krikorian and T. C. Wallace, Proc. 7th Rare Earth Rex Conf:. October 28-30. 1968. Caronudo. Cub/: 6 M. C. Krupka. A. L. Giorgi, N. H. Krikorian and E. G. Szklarz, J. Less-Common Metuls. 17 (1969) YI. 7 H. T. Hall, Ret). Sci. Instr., 31 (1960) 125. 8 J. B. Nelson and D. P. Riley, Proc. Phys. Sot. (London), 57 (1945) 160. 9 J. B. Hess, Acta Cryst., 4 (1951) 125. 10 F. H. Spedding, K. Gschneidner, Jr. and A. H. Daane, J. Am. Chem. Sot.. 80 (1958) 4499. 11 M. C. Krupka and N. H. Krikorian, Proc. 8th Rare Earth Res. Conf, April 19-22, 1970. Reno. Nevada. 12 M. C. Krupka, unpublished information. 13 A. S. Cooper, E. Corenzwit, L. D. Longinotti. B. T. Matthias and W. H. Zachariasen, Proc. Nut. Acud. Sci.. 67 (1970) 313. 1 2 3 4