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Occurrence of superconductivity in lanthanum sesquicarbide
Journal of the Less-Common Metals Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Occurrence
of superconductivity
Lanthanum cubic
sesquicarbide
in lanthanum
(La&)
which
121
sesquicarbide*
crystallized
in the body-centered over the entire
Dgc (PuzCs-type) structure was found to be superconducting
homogeneity
range with the transition temperature
varying
from a low of 5.9”K
to
a high of II.o”K. A similar behaviour has been observed for the isostructural yttrium sesquicarbidel. This is believed to be the first reported occurrence of superconductivity in a structure of this type. To determine
the effect
of composition
on the superconducting
transition
temperature of the lanthanum sesquicarbide, a series of samples were prepared in which the mole ratio of carbon to lanthanum metal was varied from 0.5 to 1.58. The samples were prepared by arc-melting weighed amounts of lanthanum metal (99 + yO purity) and spectrographic-quality graphite. Spectrochemical analysis of the lanthanum metal indicated
that the weight percent of the major impurities
present was:
Y 0.001, Yb 0.001, Ca 0.02, Mg ~0.01, Fe 0.01, Al, 0.01, Si 0.05 wt.:,& To insure homogeneity each of the preparations was melted a total of four times with the carbide button being turned over between each melting. The :uperconducting transition temperature susceptibility
measurements.
of the arc-melted material was determined by magnetic Measurements
were also made on samples annealed at
for various time periods. X-ray diffraction patterns were taken of all samples, and the lattice parameter of the sesquicarbide phase determined where possible. 1350°C
In their study of the lanthanum-carbon system SPEDDING, GSCHNEIDNER AND lattice parameters of four compositions in the homogeneity range
DAANE~ reported
of the lanthanum sesquicarbide. Using their data, the following linear equation relating composition to lattice parameter was calculated by the method of least squares : C/M = 15.45 ao - 134.7 where C/M is the carbon to metal mole ratio, and a0 is the lattice parameter of the sesquicarbide phase in A. All four sets of their data fall on this straight line within the error limits of the lattice parameters reported. For the present investigation,
the com-
position of the sesquicarbide phase present in the various samples was calculated using this equation, and the parameter determined by X-ray diffraction. A tabulation of the samples studied and the results obtained
are given in Table I. The superconducting
transition was fairly sharp for the samples containing the compositions at the limits of the homogeneity range (samples I, 2,3,4, IO and II) with the entire transition occurring over a temperature span of about 0.3”K. The samples containing the intermediate compositions had very broad transitions occurring over temperature spans of one to three degrees. The transition temperatures reported are the values obtained by extrapolation of the susceptibility curve and correspond to the “onset” of superconductivity. Annealing the samples caused the stoichiometry of the sesquicarbide phase (as shown by the lattice parameter) to shift toward the higher carbon content but had little effect in improving the sharpness of the superconducting transition. Two additional experiments were conducted to determine the effect of the lanthanum metal phase on * Work done under the auspices of the U. S. Atomic Energy Commission. J.
Less-CommonMetals,
17 (1969)
121-123
SHORT ~OM~~~~ICATIONS
122 TABLE Sample No.
I Nominal corn~as~~i~~~
I
0.5
2
0.75
3 4 5 6 ; 9 10
1.00 1.18 I.18 1.35 I-35 1.38 1.38 1.58
II
1.58
...__..____...~.. Phases Heat present** tre&nent La, La, La, La,
La& La&3 La& La&
La&
LazC3 La&
La2C3 La&s, La& La.&, LaCz
arc-melt arc-melt arc-melt arc-melt goh/1350”C arc-melt 48 arc-melt hj135o”C 90/1350°C arc-melt 49 h/135o”C
*
__ Lathe ~a~~rne~er*~~[~}
Transitiolz tern~e~,atuY~ (“Eij
Calculated composition
8.8022 f
6.0
LaG.29
8.8036 8.8073 8.8069 8.8214 8.8095 8.8107 8.8115 8.8146 8.8162 8.8193
6.0
LaG.31
14 26 4 3 31 5 2 4 f 3 i j & 5
+ zt f * zt f+
Carbon-to-Ianthanunl mole ratio of starting materials. ** Phases detected by X-ray diffraction. *** Parameter of sesquicarbide phase. 7 Large variation in lattice parameter and two transition temperatures phases.
5.9 5.9 5.9x9.7 11 9.1 8.8 IO 9.6 9.9
LaG .37 LaG.37
t LaG.41 LaG LaG.44 .43 LG.49 LaG,si
LaC1.56 --
indicate presence of two sesquicarbide
the transition temperature of samples 1-4. Pure lanthanum metal was saturated with carbon by arc melting the lanthanum in the presence of I mole- o/O C. The resulting sample was found to be superconducting with a sharp transition at +3”K_ X-ray diffraction showed that the addition of the small amount of carbon had stabilized the hightemperature (f.c.c.) phase of the metal. In a second experiment a small amount of hafnium metal was added to a portion of sample 6 (arc-melted LaCr.35) and the mixture heated to the melting point of the sesquicarbide. X-ray diffraction study of the resulting material showed the presence of hafnium carbide and a decrease in the lattice parameter of the sesquicarbide phase from 8.8095 A to 8.8082 A. No lanthanum metal phase could be detected. The superconducting transition of this material was also sharp at 59°K. These results suggest that the lantl~anum metal phase and the lowcarbon lanthanum sesquicarbide phase are both superconducting, and the sharp transition observed is due to the similarity in the transition temperature of the two materials. The occurrence of the highest observed transition temperature at the intermediate composition of LaCr.41 rather than at the highest stoichiometry of the homogeneity range is surprising. Further experiments are, therefore, plannedin which minor changes in composition over the range of LaCr.43 to LaCr.3, will be studied to determine if transition temperatures even higher than II.OOK are possible. Previous work by various investigators on binary metal carbides with superconducting transition temperatures above I"K have involved materials crystallizing in the f.c.c. (NaCI-type) structure. This includes work on niobium and tantalum carbide@, molybdenum carbided, tungsten carbides, and technetium carbide6. The hexagonal molybdenum carbide7 is an exception. Although superconductivity has been reported in the hexagonal Nb& and TaC.$, previous work by two of the present authors on carefully prepared, singlephase dimetal carbides has failed to show such superconductivity. The work indicated rather that the observed superconductivity was probably due to the presence of a metal phase at the grain boundaries. The close agreement between the reported transition temperatures and the transition temperatures of the pure metal supports this view. J. Less-Common
Metals, 17 (1969) 121-123
123
SHORT COMMUNICATIONS Los Alamos University
Scientific
A. L. GIORGI E. G. SZKLARZ
Laboratory,
of California,
Los Alanzos,
N. M. 87544
M.C. KRUPKA N. H. KRIKORIAN
(U.S.A.)
I M. C. KRUPKA, A. L. GIORGI, N. H. KRIKORIAN AND E. G. SZKLARZ, J,
Less-CommonMetals, 17
(‘969) 91.
SPEDDING, K. GSCHNEIDNER. JR. AND A. H. DAANE, inK. GSCHNEIDNER (ed.), RaveEarth AZloys, Van Nostrand, Princeton, N.J., 1961, p. 141. A. L. GIORGI, E. G. SZKLARZ, E. K. STORMS, ALLEN L. BOWMAN AND B. T. MATTHIAS, Phys. Rev., 125 (1~62) 837. J. JOHNSTON, Univ. of Cahf., Lawrence Radiation Lab., Rept. UCRL-II~~O, 1964. R. H. WILLENS AND E. BUEHLER, A#. Phys. Letters, 7 (1965) 25. .4. L. GIORGI AND E. G. SZKLARZ, J. Less-Common Metals, II (1966) 455. B. T. MATTHIAS AND J. K. HULM, Phys. Rev., 87 (1952) 799. J. 1~. HULM AND G. HARDY, Third I?ztern. Con-f. on Low Temperature Physics and Chemistry, Houston, Texas, Dec. 1953 (MIT Library).
2 F. H. 3 4 5
6 7 8
Received
August 29th, 1968 J. Less-Common
Metals, 17 (1969) IZ.-123
Occurrence
of superconductivity
Lanthanum cubic
sesquicarbide
in lanthanum
(La&)
which
121
sesquicarbide*
crystallized
in the body-centered over the entire
Dgc (PuzCs-type) structure was found to be superconducting
homogeneity
range with the transition temperature
varying
from a low of 5.9”K
to
a high of II.o”K. A similar behaviour has been observed for the isostructural yttrium sesquicarbidel. This is believed to be the first reported occurrence of superconductivity in a structure of this type. To determine
the effect
of composition
on the superconducting
transition
temperature of the lanthanum sesquicarbide, a series of samples were prepared in which the mole ratio of carbon to lanthanum metal was varied from 0.5 to 1.58. The samples were prepared by arc-melting weighed amounts of lanthanum metal (99 + yO purity) and spectrographic-quality graphite. Spectrochemical analysis of the lanthanum metal indicated
that the weight percent of the major impurities
present was:
Y 0.001, Yb 0.001, Ca 0.02, Mg ~0.01, Fe 0.01, Al, 0.01, Si 0.05 wt.:,& To insure homogeneity each of the preparations was melted a total of four times with the carbide button being turned over between each melting. The :uperconducting transition temperature susceptibility
measurements.
of the arc-melted material was determined by magnetic Measurements
were also made on samples annealed at
for various time periods. X-ray diffraction patterns were taken of all samples, and the lattice parameter of the sesquicarbide phase determined where possible. 1350°C
In their study of the lanthanum-carbon system SPEDDING, GSCHNEIDNER AND lattice parameters of four compositions in the homogeneity range
DAANE~ reported
of the lanthanum sesquicarbide. Using their data, the following linear equation relating composition to lattice parameter was calculated by the method of least squares : C/M = 15.45 ao - 134.7 where C/M is the carbon to metal mole ratio, and a0 is the lattice parameter of the sesquicarbide phase in A. All four sets of their data fall on this straight line within the error limits of the lattice parameters reported. For the present investigation,
the com-
position of the sesquicarbide phase present in the various samples was calculated using this equation, and the parameter determined by X-ray diffraction. A tabulation of the samples studied and the results obtained
are given in Table I. The superconducting
transition was fairly sharp for the samples containing the compositions at the limits of the homogeneity range (samples I, 2,3,4, IO and II) with the entire transition occurring over a temperature span of about 0.3”K. The samples containing the intermediate compositions had very broad transitions occurring over temperature spans of one to three degrees. The transition temperatures reported are the values obtained by extrapolation of the susceptibility curve and correspond to the “onset” of superconductivity. Annealing the samples caused the stoichiometry of the sesquicarbide phase (as shown by the lattice parameter) to shift toward the higher carbon content but had little effect in improving the sharpness of the superconducting transition. Two additional experiments were conducted to determine the effect of the lanthanum metal phase on * Work done under the auspices of the U. S. Atomic Energy Commission. J.
Less-CommonMetals,
17 (1969)
121-123
SHORT ~OM~~~~ICATIONS
122 TABLE Sample No.
I Nominal corn~as~~i~~~
I
0.5
2
0.75
3 4 5 6 ; 9 10
1.00 1.18 I.18 1.35 I-35 1.38 1.38 1.58
II
1.58
...__..____...~.. Phases Heat present** tre&nent La, La, La, La,
La& La&3 La& La&
La&
LazC3 La&
La2C3 La&s, La& La.&, LaCz
arc-melt arc-melt arc-melt arc-melt goh/1350”C arc-melt 48 arc-melt hj135o”C 90/1350°C arc-melt 49 h/135o”C
*
__ Lathe ~a~~rne~er*~~[~}
Transitiolz tern~e~,atuY~ (“Eij
Calculated composition
8.8022 f
6.0
LaG.29
8.8036 8.8073 8.8069 8.8214 8.8095 8.8107 8.8115 8.8146 8.8162 8.8193
6.0
LaG.31
14 26 4 3 31 5 2 4 f 3 i j & 5
+ zt f * zt f+
Carbon-to-Ianthanunl mole ratio of starting materials. ** Phases detected by X-ray diffraction. *** Parameter of sesquicarbide phase. 7 Large variation in lattice parameter and two transition temperatures phases.
5.9 5.9 5.9x9.7 11 9.1 8.8 IO 9.6 9.9
LaG .37 LaG.37
t LaG.41 LaG LaG.44 .43 LG.49 LaG,si
LaC1.56 --
indicate presence of two sesquicarbide
the transition temperature of samples 1-4. Pure lanthanum metal was saturated with carbon by arc melting the lanthanum in the presence of I mole- o/O C. The resulting sample was found to be superconducting with a sharp transition at +3”K_ X-ray diffraction showed that the addition of the small amount of carbon had stabilized the hightemperature (f.c.c.) phase of the metal. In a second experiment a small amount of hafnium metal was added to a portion of sample 6 (arc-melted LaCr.35) and the mixture heated to the melting point of the sesquicarbide. X-ray diffraction study of the resulting material showed the presence of hafnium carbide and a decrease in the lattice parameter of the sesquicarbide phase from 8.8095 A to 8.8082 A. No lanthanum metal phase could be detected. The superconducting transition of this material was also sharp at 59°K. These results suggest that the lantl~anum metal phase and the lowcarbon lanthanum sesquicarbide phase are both superconducting, and the sharp transition observed is due to the similarity in the transition temperature of the two materials. The occurrence of the highest observed transition temperature at the intermediate composition of LaCr.41 rather than at the highest stoichiometry of the homogeneity range is surprising. Further experiments are, therefore, plannedin which minor changes in composition over the range of LaCr.43 to LaCr.3, will be studied to determine if transition temperatures even higher than II.OOK are possible. Previous work by various investigators on binary metal carbides with superconducting transition temperatures above I"K have involved materials crystallizing in the f.c.c. (NaCI-type) structure. This includes work on niobium and tantalum carbide@, molybdenum carbided, tungsten carbides, and technetium carbide6. The hexagonal molybdenum carbide7 is an exception. Although superconductivity has been reported in the hexagonal Nb& and TaC.$, previous work by two of the present authors on carefully prepared, singlephase dimetal carbides has failed to show such superconductivity. The work indicated rather that the observed superconductivity was probably due to the presence of a metal phase at the grain boundaries. The close agreement between the reported transition temperatures and the transition temperatures of the pure metal supports this view. J. Less-Common
Metals, 17 (1969) 121-123
123
SHORT COMMUNICATIONS Los Alamos University
Scientific
A. L. GIORGI E. G. SZKLARZ
Laboratory,
of California,
Los Alanzos,
N. M. 87544
M.C. KRUPKA N. H. KRIKORIAN
(U.S.A.)
I M. C. KRUPKA, A. L. GIORGI, N. H. KRIKORIAN AND E. G. SZKLARZ, J,
Less-CommonMetals, 17
(‘969) 91.
SPEDDING, K. GSCHNEIDNER. JR. AND A. H. DAANE, inK. GSCHNEIDNER (ed.), RaveEarth AZloys, Van Nostrand, Princeton, N.J., 1961, p. 141. A. L. GIORGI, E. G. SZKLARZ, E. K. STORMS, ALLEN L. BOWMAN AND B. T. MATTHIAS, Phys. Rev., 125 (1~62) 837. J. JOHNSTON, Univ. of Cahf., Lawrence Radiation Lab., Rept. UCRL-II~~O, 1964. R. H. WILLENS AND E. BUEHLER, A#. Phys. Letters, 7 (1965) 25. .4. L. GIORGI AND E. G. SZKLARZ, J. Less-Common Metals, II (1966) 455. B. T. MATTHIAS AND J. K. HULM, Phys. Rev., 87 (1952) 799. J. 1~. HULM AND G. HARDY, Third I?ztern. Con-f. on Low Temperature Physics and Chemistry, Houston, Texas, Dec. 1953 (MIT Library).
2 F. H. 3 4 5
6 7 8
Received
August 29th, 1968 J. Less-Common
Metals, 17 (1969) IZ.-123