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Superconductivity in A15 Nb3(Ge, Ga) and Nb(Ge, B) compounds
Physica 10 7B (1981) 267-268 North-Holland Publishing Company
EC 4
SUPERCONDUCTIVITY IN AI5 Nb3(Ge,Ga) AND Nb-(Ge,B) COMPOUNDS J. D. Thompson, M. P. Maley, L. R. Newkirk F. A. Valencia and K. C. Lim Los Alamos N a t i o n a l Laboratory Los Alamos, New Mexico 87545
We report results of a preliminary investigation of the superconducting properties, Tc and Hc2(0) , of stolchiometric pseudobinaries Nb3(GexGal_ x) and pseudobinaries Nb-(Ge,B) prepared by chemical vapor deposition. We find that the addition of third elements Ga and B to AI5 Nb3Ge depresses T c relative to that of the binary but that the incorporation of B holds promise for enhanced values of Hc2(O).
I.
INTRODUCTION
Nb3Ge exhibits the highest upper critical field Hc2(O) at T = O K of any binary AI5 compound[l] and, as such, becomes a candidate conductor for the production of high field magnets[2]. Therefore, it is interesting to determine if Hc2 can be enhanced further by the addition of third elements. We have chosen to study Ca and B additions each for distinctly different reasons. Gallium, because of its larger atomic radius compared to Ge, is expected to expand the AI5 lattice and thus increase the stability of Nb3Ce. On the other hand, B substitution for Ce in the AI5 lattice is expected to decrease the lattice parameter a o and possibly increase the superconducting transition temperature Tc, provided the limit of lattice stability is not exceeded. In both cases, the introduction of a "defect" into the A3B lattice is expected to enhance the residual resistivity Po; however, for Ga additions the expected improvement in lattice stability may counter this effect.
found t o be stoichiometric. Such an analysis was n o t possible for the B-containing compounds because of the soft x-rays emitted by B and because n o prediction exists for a o of Nb3B. Therefore, results for these samples will be reported as a function of known B content in the deposition gases. The Tc o f a l l s a m p l e s was m e a s u r e d i n d u c tively. F o r f i e l d s up t o 9T, Hc2(T) was m e a s u r e d resistively o n selected samples using a 4-termlnal ac technique. 3.
RESULTS
We show i n F i g . 1 t h e i n d u c t i v e o n s e t Tc and a o a s f u n c t i o n s o f Ca f r a c t i o n f o r t h e compounds Nb3(GexGal_x). Inductive transit i o n w i d t h s were s h a r p , t y p i c a l l y 1 K or l e s s b e t w e e n l 0 and 90X o f t h e t o t a l s i g n a l .
Recent studies on Nb3Sn ~nd,V3Si[3] have noted a universal dependence of Nc2 = ~Hc2/~T near T c and T c on p o. For initially increasing Po, Tc decreases and Hc2 increases. For non-Pauli-paramagnetically limited materials in the dirty limit,[4] Hc2(O) = 0.693 Hc2 T c.
I
I
I
I
I
I
I
25
(I)
Therefore, Hc2(O) may be enhanced, provided Hc2 increases more rapidly than Tc decreases as 0 o is made larger. 2.
I
I
i
0~ 515 o
15
g
_o o
EXPERIMENTAL
Nb3Ge and its pseudobinaries were prepared by chemical vapor deposition (CVD)[5]. All Nb3(GexCal_ x) compounds were examined by x-ray fluoresence to determine the Ca:Ge ratio. This ratio, combined with measured ao'S and Vegard's law, was used to determine if the compound was stoichlometric. Results are reported on only those Ga-containing samples 0378-4363/81/0000-0000/$02.50
© North-Holland Publishing Company
5.14le- ~i 0 OJ Nb3Ge
Fig.
1.
fraction
1 02
I
1
I
1
I
0.4 0.5 0.6 0.7 GO FRACTION (GoGOGe)
03
I O,S
1 09
I0 1.0 Nb3G°
Tc and a o a s
f u n c t i o n s o f Ca in stoichiometric Nb3(CexCal_x). 267
268
22.0 ,
~
•
\-
o
~..}.~
~,,,,,
TABLE I
0,, TD=885 oC ~)a, TD =945°C
5.150
S.mmary of High Field Properties O
_>_--
Deposition Temp.
21,O
O
~L~
o
20.0 ~J bJ )~w 5.1401
19.0 ~ !
d 5A35~)
I
BORON F R A C T I O N ( ~ ) I N
]
I
I
7
~18.O
~-
GAS PHASE (Or °/o)
---LDi°c)__
lc_i~)+ ~ z . ~ L
Nb3(Ge0.92Gao.08)
880 885
Nb3(c~£.gsCoo.]~)
885
Nb{Ga Nb-(Ge,Bo.[6)* Nb-(Ge.B0.94)* ND-(Ce,BI.94)* Nb-(~e,Bo.03)*
---
885 885 885 945
20.2 19.0 18.6 21.8
Nb3Ge
21.1 20.5
~2(o~ (32
2,25
32.~ 3t.3
20.4
2.22
30.8
20,3
2.43 2.07 2.55 2.5~ 1.88
34.1 2g.l 34.2 34.1 27.9
2.29
+ Inductive onset ++ From Ref. 9 * Samples denoted by at.% B fraction in deposition gases.5
Fig. 2. T c and a o as functions of at. % B in the deposition gases.
From Fig. I we observe that initial additions of Ga produce a very shallow decrease in T c that is accompanied by a monotonic increase in a o. The dashed curve represents a smooth extrapolation of Tc's for Ge-rich pseudobinaries to the T c of Nb3Ga. The minimum in T c at x = 0.5 might be expected from an increased 0 o because at this ratio the "defect" concentration is a maximum. Values for T c and a o as functions of B content in the deposition gases are shown in Fig. 2. The monotonically sharp decrease in T c and corresponding increase in a o were not expected. Because no analysis was possible to determine either the Nb:(Ge + B) ratio or the Ge:B ratio in these samples, we can only speculate as to the origin of these observations. Possibly, because of their small radius, the B atoms are positioning themselves interstitially in the AI5 lattice instead of substltutlonally for the Ge atoms. This effect would expand the cell size. Furthermore, the monotonic decrease in T c suggests that the compounds may b~ shifting away from stoichiometry as B is added. From available information, we cannot assess the relative importance of either mechanism. Finally, we note that the inductive transition widths are relatively broad, 2 - 2.5 K, indicative of inhomogeneity, arising possibly from disorder, strain or compositional variations. Measured values of ~ 2 and values of Hc2(O) calculated from Eq. (I) determined on selected samples are presented in Table I. Hc2(O) for Nb3(GexGal_ x) follows the behavior of T c vs x, while Hc2 remains essentially constant. Because in the dirty limit, Hc2 ~ OoY, where y is the electronic heat capacity coefficient, these results imply that OoY remains nearly constant with Ga fraction at large x and that Hc2(O) is governed primarily by T c. Contrary to these results,
the addition of a small amount of B depresses Hc2 by %10%. Such a large decrease in Hc2 suggests that y is reduced, possibly by deviations from stoichiometry[6], because third element additions and off-stoichiometry drive up 0o[7]. With further additions of B, Hc2 and Hc2(O) increase, implying that Do must increase substantially to compensate for the effect of decreasing y and T c. We note that the value of Hc2 = 2.59 T/K for Nb-(Ge,BI.94) is higher than any Hc2 we have found for binary Nb3Ge prepared by CVD[8]. These preliminary results encourage us to believe that B additions to Nb3Ge may result in superior values of Hc2(O).
REFERENCES [I] Foner, S., McNiff, E. J., Gavaler, J. R, and Janocho, M. A., Phys. Lett. 47A (1974) 485-486. [2] Thompson, J. D., Maley, M. P., Newkirk, L. R., Valencia, F. A., Bartlett, R. J., and Carlson, R. V., Solid State Commun. 28 (1978) 729-732. [3] Orlando, T. P., McNiff, E. J., Foner, S., and Beasley, M. R., , Phys. Rev. 19 (1979) 4545-4561. [4] Helfand, E. and Werthamer, N. R., Phys. Rev. 114 (1966) 288-294. [51Maley, M. P., Newkirk, L. R., Thompson, J. D. and Valencia, F. A., IEEE Trans. Mag. MAC-17 (1981) 533-540. [6] Stewart, G. R., Newkirk, L. R. and Valencia, F. A., Phys. Rev. 20 (1979) 3647-3652. [7] Lutz, H., Weismann, H., Kammerer, O. F. and Strongin, Myron, Phys. Rev. Left. 36 (1976) 1576-1579. [8] Thompson, J. D., Maley, M. P., Newkirk, L. R. and Valencia, F. A., IEEE Trans. Mag. MAG-15 (1979) 516-519. [9] Foner, S., McNiff, E. J., Webb, G. W., Vieland, L. J., Miller, R. E. and Wickfund, A., Phys. Left. 38A (1972) 323-324.
EC 4
SUPERCONDUCTIVITY IN AI5 Nb3(Ge,Ga) AND Nb-(Ge,B) COMPOUNDS J. D. Thompson, M. P. Maley, L. R. Newkirk F. A. Valencia and K. C. Lim Los Alamos N a t i o n a l Laboratory Los Alamos, New Mexico 87545
We report results of a preliminary investigation of the superconducting properties, Tc and Hc2(0) , of stolchiometric pseudobinaries Nb3(GexGal_ x) and pseudobinaries Nb-(Ge,B) prepared by chemical vapor deposition. We find that the addition of third elements Ga and B to AI5 Nb3Ge depresses T c relative to that of the binary but that the incorporation of B holds promise for enhanced values of Hc2(O).
I.
INTRODUCTION
Nb3Ge exhibits the highest upper critical field Hc2(O) at T = O K of any binary AI5 compound[l] and, as such, becomes a candidate conductor for the production of high field magnets[2]. Therefore, it is interesting to determine if Hc2 can be enhanced further by the addition of third elements. We have chosen to study Ca and B additions each for distinctly different reasons. Gallium, because of its larger atomic radius compared to Ge, is expected to expand the AI5 lattice and thus increase the stability of Nb3Ce. On the other hand, B substitution for Ce in the AI5 lattice is expected to decrease the lattice parameter a o and possibly increase the superconducting transition temperature Tc, provided the limit of lattice stability is not exceeded. In both cases, the introduction of a "defect" into the A3B lattice is expected to enhance the residual resistivity Po; however, for Ga additions the expected improvement in lattice stability may counter this effect.
found t o be stoichiometric. Such an analysis was n o t possible for the B-containing compounds because of the soft x-rays emitted by B and because n o prediction exists for a o of Nb3B. Therefore, results for these samples will be reported as a function of known B content in the deposition gases. The Tc o f a l l s a m p l e s was m e a s u r e d i n d u c tively. F o r f i e l d s up t o 9T, Hc2(T) was m e a s u r e d resistively o n selected samples using a 4-termlnal ac technique. 3.
RESULTS
We show i n F i g . 1 t h e i n d u c t i v e o n s e t Tc and a o a s f u n c t i o n s o f Ca f r a c t i o n f o r t h e compounds Nb3(GexGal_x). Inductive transit i o n w i d t h s were s h a r p , t y p i c a l l y 1 K or l e s s b e t w e e n l 0 and 90X o f t h e t o t a l s i g n a l .
Recent studies on Nb3Sn ~nd,V3Si[3] have noted a universal dependence of Nc2 = ~Hc2/~T near T c and T c on p o. For initially increasing Po, Tc decreases and Hc2 increases. For non-Pauli-paramagnetically limited materials in the dirty limit,[4] Hc2(O) = 0.693 Hc2 T c.
I
I
I
I
I
I
I
25
(I)
Therefore, Hc2(O) may be enhanced, provided Hc2 increases more rapidly than Tc decreases as 0 o is made larger. 2.
I
I
i
0~ 515 o
15
g
_o o
EXPERIMENTAL
Nb3Ge and its pseudobinaries were prepared by chemical vapor deposition (CVD)[5]. All Nb3(GexCal_ x) compounds were examined by x-ray fluoresence to determine the Ca:Ge ratio. This ratio, combined with measured ao'S and Vegard's law, was used to determine if the compound was stoichlometric. Results are reported on only those Ga-containing samples 0378-4363/81/0000-0000/$02.50
© North-Holland Publishing Company
5.14le- ~i 0 OJ Nb3Ge
Fig.
1.
fraction
1 02
I
1
I
1
I
0.4 0.5 0.6 0.7 GO FRACTION (GoGOGe)
03
I O,S
1 09
I0 1.0 Nb3G°
Tc and a o a s
f u n c t i o n s o f Ca in stoichiometric Nb3(CexCal_x). 267
268
22.0 ,
~
•
\-
o
~..}.~
~,,,,,
TABLE I
0,, TD=885 oC ~)a, TD =945°C
5.150
S.mmary of High Field Properties O
_>_--
Deposition Temp.
21,O
O
~L~
o
20.0 ~J bJ )~w 5.1401
19.0 ~ !
d 5A35~)
I
BORON F R A C T I O N ( ~ ) I N
]
I
I
7
~18.O
~-
GAS PHASE (Or °/o)
---LDi°c)__
lc_i~)+ ~ z . ~ L
Nb3(Ge0.92Gao.08)
880 885
Nb3(c~£.gsCoo.]~)
885
Nb{Ga Nb-(Ge,Bo.[6)* Nb-(Ge.B0.94)* ND-(Ce,BI.94)* Nb-(~e,Bo.03)*
---
885 885 885 945
20.2 19.0 18.6 21.8
Nb3Ge
21.1 20.5
~2(o~ (32
2,25
32.~ 3t.3
20.4
2.22
30.8
20,3
2.43 2.07 2.55 2.5~ 1.88
34.1 2g.l 34.2 34.1 27.9
2.29
+ Inductive onset ++ From Ref. 9 * Samples denoted by at.% B fraction in deposition gases.5
Fig. 2. T c and a o as functions of at. % B in the deposition gases.
From Fig. I we observe that initial additions of Ga produce a very shallow decrease in T c that is accompanied by a monotonic increase in a o. The dashed curve represents a smooth extrapolation of Tc's for Ge-rich pseudobinaries to the T c of Nb3Ga. The minimum in T c at x = 0.5 might be expected from an increased 0 o because at this ratio the "defect" concentration is a maximum. Values for T c and a o as functions of B content in the deposition gases are shown in Fig. 2. The monotonically sharp decrease in T c and corresponding increase in a o were not expected. Because no analysis was possible to determine either the Nb:(Ge + B) ratio or the Ge:B ratio in these samples, we can only speculate as to the origin of these observations. Possibly, because of their small radius, the B atoms are positioning themselves interstitially in the AI5 lattice instead of substltutlonally for the Ge atoms. This effect would expand the cell size. Furthermore, the monotonic decrease in T c suggests that the compounds may b~ shifting away from stoichiometry as B is added. From available information, we cannot assess the relative importance of either mechanism. Finally, we note that the inductive transition widths are relatively broad, 2 - 2.5 K, indicative of inhomogeneity, arising possibly from disorder, strain or compositional variations. Measured values of ~ 2 and values of Hc2(O) calculated from Eq. (I) determined on selected samples are presented in Table I. Hc2(O) for Nb3(GexGal_ x) follows the behavior of T c vs x, while Hc2 remains essentially constant. Because in the dirty limit, Hc2 ~ OoY, where y is the electronic heat capacity coefficient, these results imply that OoY remains nearly constant with Ga fraction at large x and that Hc2(O) is governed primarily by T c. Contrary to these results,
the addition of a small amount of B depresses Hc2 by %10%. Such a large decrease in Hc2 suggests that y is reduced, possibly by deviations from stoichiometry[6], because third element additions and off-stoichiometry drive up 0o[7]. With further additions of B, Hc2 and Hc2(O) increase, implying that Do must increase substantially to compensate for the effect of decreasing y and T c. We note that the value of Hc2 = 2.59 T/K for Nb-(Ge,BI.94) is higher than any Hc2 we have found for binary Nb3Ge prepared by CVD[8]. These preliminary results encourage us to believe that B additions to Nb3Ge may result in superior values of Hc2(O).
REFERENCES [I] Foner, S., McNiff, E. J., Gavaler, J. R, and Janocho, M. A., Phys. Lett. 47A (1974) 485-486. [2] Thompson, J. D., Maley, M. P., Newkirk, L. R., Valencia, F. A., Bartlett, R. J., and Carlson, R. V., Solid State Commun. 28 (1978) 729-732. [3] Orlando, T. P., McNiff, E. J., Foner, S., and Beasley, M. R., , Phys. Rev. 19 (1979) 4545-4561. [4] Helfand, E. and Werthamer, N. R., Phys. Rev. 114 (1966) 288-294. [51Maley, M. P., Newkirk, L. R., Thompson, J. D. and Valencia, F. A., IEEE Trans. Mag. MAC-17 (1981) 533-540. [6] Stewart, G. R., Newkirk, L. R. and Valencia, F. A., Phys. Rev. 20 (1979) 3647-3652. [7] Lutz, H., Weismann, H., Kammerer, O. F. and Strongin, Myron, Phys. Rev. Left. 36 (1976) 1576-1579. [8] Thompson, J. D., Maley, M. P., Newkirk, L. R. and Valencia, F. A., IEEE Trans. Mag. MAG-15 (1979) 516-519. [9] Foner, S., McNiff, E. J., Webb, G. W., Vieland, L. J., Miller, R. E. and Wickfund, A., Phys. Left. 38A (1972) 323-324.