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Superconductivity and phase compositions in annealed amorphous Nb-Si alloys under high pressure
~
Solid State Communications, Vol.58,No.5, pp.285-288, 1986. Printed in Great Britain.
0038-1098/86 $3.00 + .00 Pergamon P r e s s L t d .
SUPERCONDUCTIVITYAND PHASE COMPOSITIONS IN ANNEALED AMORPHOUS Nb-St ALLOYS UNDER HIGH PRESSURE W. K. Wang, Y. J. Wang, S. A. He
I n s t i t u t e of Physics, Chinese Academy of Science Betjing, People's Republic of China J. H. Huang Massachusetts I n s t i t u t e of Technology, Cambridge, HA 02]39, USA J. D. Boyer, C. Y. Huang Los Alamus National Laboratory, Los Alames, NM 87545 USA H. Iwasakt I n s t i t u t e f o r Iron, Steel and Other Metals, Tohoku U n i v e r s i t y , Sendat, 3apan ( R e c e i v e d F e b . l ~ 1 9 8 6 by W.Y.Kuan)
We report the results of amorphous Nb81St19 and Nb77Si23 a l l o y s annealed at high pressure but at a pressure lower than that used to synthesize single-phase A]5 Nb3St. The lower pressure gives rise to the decomposition of the a l l o y s i n t o a multi-phase state consisting of AI5 Nb3SI, F-NbsSt3, Ll2-type Nb3Si and Nb. The l a t t i c e constants are smaller and Tc higher than those obtained f o r the single-phase samples. We predict the Tc of stotchlometric AI5 Nb3St to be 25 K.
A15 phase, i t was thought that t t was possible to obtain the l a t t e r phase at higher pressure. The f t r s t attempt to synthesize A15 Nb3St in this manner was made by L~gar and Hall [19]. They employed mtxed elemental powders subjected to s t a t i c pressures up to 7 GPa and temperatures up to 2000°C. At low temperatures, they obtained a tetragonal phase, which became hexagonal as the temperature appraoched 2000°C. They thus obtained Nb5St3, but no A15 structure phase was found. Wa]terstrat et al [20] also attempted synthesis at ]0 GPa with sputtered metastable bcc Nb3St and at 6 GPa with Ti3P-type Nb3Sl. They also f a i l e d to produce any A]5 structure. Wang e t a ] [21] have also i n v e s t i gated the high pressure e f f e c t s , and have found that the competing Nb5SJ3 has to be suppressed in order to obtain AI5 Nb3St. Encourged by the successful synthesis of A]5 Nb3Sn from elemental powders with shock waves from the detonation of explosives which could y i e l d short-duration hgth-pressure pulses of 20-25 GPa, Pan et al [22] subjected to a sample of the stable Tt3P-type Nb3Si phase to explosive compression with a peak pressure estimated to greater than lO0 GPa. They claimed to have obtained about 5% of the sample In the A15 phase with Tc = 18.5 - 19.0 K and a = 5.08 A. Later, Dew-Hughes and Linse [23] repeated the same experiment and obtained the same value of Tc with a = 5.12 A. S i m i l a r l y , Oltnger and Newkirk [24] also obtained approximately the
A f t e r I t s f i r s t discovery In 1953 [ 1 ] , superc o n d u c t i v i t y in A38-type compounds with the A15 structure has been investigated i n t e n s i v e l y . Based on the known A15 superconductors, many authors have found the empirical c o r r e l a t i o n s between the superconducting t r a n s i t i o n temperature, Tc, and some physical parameters [ 2 - 6 ] . From the e x t r a p o l a t i o n of the c o r r e l a t i o n between Tc and the mass of the B element, Dew-Hughes and R l v l t n [2] predicted a Tc of 38 K f o r Nb3Si. But, from that between Tc and the atomic volume Dew-Hughes [3] obtained a predict i o n of only 25 K. By e x t r a p o l a t i o n from the l a t t i c e constants and Tc of V3Sn, V3St, and Nb3Sb, Geller [4] predicted a value of Tc = 30 K f o r Nb3Si with a l a t t i c e constant of a = 5.06 ± 0.02 X . This value of Tc was also predicted by Noolandi and Testardi [6] by means of a correlation between the reduction in Tc and the increase in the l a t t i c e parameter. Encouraged by these extrapolations, many authors have attempted to synthesize this compound by conventional methods such as electron-beam coevaporat|on [?,8] dual target sputtering [9-12], chemical vapor deposition [13-15],
l i q u i d quenching [ ] 6 , 1 7 ] and pulse annealing of amorphous a l l o y s [18]. Aside from Ref. l l , a l l these methods gave a Tc even lower than that of pure Nb (9.2 K). In view of the fact that the s p e c i f i c volume of the tetragonal Tt3P-type Nb3St phase is about 3% larger than that of the extrapolated 285
286
ANNEALED AMORPHOUS Nb-Si ALLOYS UNDER HIGH PRESSURE
same Tc but with a = 5.091 A. However,the x-ray determination of the structure was d i f f i cult in a l l these experiments. I t has been speculated that this lower value of Tc than the extrapolated value might be attributable to the result of non-stoichlmetrlc compositions which in turn cause the l a t t i c e constant a to be larger than Geller's extrapolated value [4]. I t was recognized that the amorphous state is unstable and that the Al5 structure could be produced by annealing amorphous Nb-Sl a l l o y s at htgh pressure.J25,26] For the samples with the compositions of Nb81Sll9 and Nb77Si23, these authors obtained single-phase samples with the A15 s t r u c t u r e . The l a t t i c e constants are 5.155 and 5.120 A and Tc are 3.4 and 8.9 K, respectively. These Tc'S are, however, even lower than that of pure Nb. I t is clear t h a t , in order to get higher Tc, compositions closer to the stoicheometry (Nb?5Si25) are needed. Nevertheless, in t h i s case, the pressure required f o r annealing is i n h i b i t e d l y high. At pressures much higher than those used to produce s i n g l e phase A15 Nb8lSil9 and Nb77St23, a supersaturated s o l i d s o l u t i o n , such as fcc Nb(St), r e s u l t s . In both these cases, the crystallization process from an amorphous state does not involve any long-distance atomic migrations and phase decompositions. At s l i g h t l y lower pressure, the atomic diffusion can take place completely, giving rise to the decomposition of an amorphous alloy into a multl-phase state. T h i s rather complete atomic diffusion at proper pressure might be helpful in attaining the stoichiometric Al5 phase with a smaller l a t t i c e constant and hence a higher Tc. The present work is motivated by this observation, and hence we investigate the crystallization and Tc of the samples at somewhat lower annealed pressure. In order to compare with the previous results [25], the same compositions, Nb-lg and -23 at % Si, have been chosen to be studied in this work. The Ig% amorphous samples were prepared by splat melting and the 23% ones by the sputtering method. The Bridgman anvils and belt-type high pressure apparatus was used. The annealing procedure similar to those used in the previous work [25,26] is employed in this work. After the high pressure-temperature annealing procedure, the specimens were carefully peeled off from the BN pressure medium for characterization. The typical size of a sample is ~0.02 mmfor the lg% specimens and ~0.5 mm for the 23% ones. The x-ray powder d i f f r a c t i o n method with a 114.6 cm diameter camera and CuK~ radiation was employed to determine crystal structures. Tc were measured by the ac magnetic susceptib i l i t y technique. Figure l(a) displays the x-ray diffraction pattern of a Ig% sample (No. l ) annealed at 4 6Pa and 800°C for 35 minutes. In spite of overlapping, the d i f f r a c t i o n llne positions and intensities can be analyzed and interpreted in terms of a multi-phase state consisting of Al5 Nb3Si, r-Nb§Si3 and Nb. The l a t t i c e constant for the Al5 phase is 5.10 A, and the superconducting transition takes place at 13.1K with a width of ~ 3 K. T h i s high value of Tc is believed to be due to the Al5 phase.
Vol. 58, No. 5
(a)
Fig. l
X-ray diffraction pattern of amorphous Nb-19 at % Si alloy annnealed at (a) 4 GPa and 800°C for 35 min, (b) 4 GPa and 750°C for 1440 min.
For the sample of another 19% sample (No. 2) annealed at 4 GPa and 750°C for 1440 minutes, the lines as shown in Fig. l(b) can be best fit to a multi-phase state, AI5 Nb3Si + Tl3P-type Nb3Si + Nb + an amorphous alloy. The lattice constant of the Al5 phase is 5 . 1 1 A and T c is II.9 K. Again, this high value of T c is attributed to the Al5 phase, because the Ti3P phase is not superconducting down to 1.3 K. Sample No. 3 was the result of annealing a 19% amorphous alloy at 4 GPa and 8 5 0 % for 480 minutes. The x-ray result of this sample demonstrated the coexistence of a Nb solid solution with Ll2-type Nb3Si and an amorphous Nb-Si alloy. T c of this sample is 7.52 K, which is believed to arise from the Nb solid solution because Ll 2 type Nb3Si is not superconducting above 0.5 K. As summed up in Table I, all these three samples were prepared at 4 GPa. Nontheless, we have shown that the annealing temperature is rather c r i t i c a l to get the high Tc; a difference of 50°C in annealing temperature makes a considerable difference in Tc. Masumoto et al [27] have shown that an amorphous Nb-Si alloy annealed at ambient pressure decomposes into Nb and Ti3P-type Nb3Si through a metastable multi-phase state. The results of our low pressure annealing samples presented above are consistent with this observation of Masumoto et a l . Nonetheless,in our case, the multi-phase state is more complicated, becuase i t consists of four phases: Al5 Nb3Si, F-Nb5Si3, Ti3P-type Nb3Si and Ll2-type Nb3Si. In fact, the r-Nb5Si 3 and Ll2-type Nb3Si phases have been found in crystalline Nb-Si alloys annealed at high pressure [28]. Sample No. 4 is a specimen obtained by annealing an amorphous Nb77Si23 sample at ambient pressure and 650°C for 43,000 minutes. Only the Ti3P phase results, and i t is not a superconductor. When the a l l o y is instead annealed at 5 GPa and 750°C for 840 minutes, i t decomposes i n t o Ti3P type Nb3Si in coexistence with an amorphous phase. T~ of 2.27 K was obtained and i t could be a t t r i b u t a b l e to the amorphous phase. From t h i s observation, i t is clear that in order to acquire the A;5 phase at
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287
ANNEALED AMORPHOUS Nb-Si ALLOYS UNDER HIGH PRESSURE
Table I .
High pressure annealing and Tc.
No.
Alloy
Annealing Condttlon
Phase Composition
I
Nb81Silg
4 GPa, 800°C 35 min.
A15 ( a = 5.10A) +F-Nb~St 3 + Nb
13.1
Nb81Silg
4 GPa, 750°C 1440 min.
A15 (a = 5 . l l A ) + Tl3P-type Nb3Si ÷ Nb
11 .g
3
Nb81Stlg
4 GPa, 850°C 480 min.
L12-type Nb3St + Nb
4
Nb77Si23
0.01HPa, 650°C 43000 min
5
Nb77St23
7.52
Ti3P
5 GPa, 750°C 840 min
t h i s composition, somewhat higher pressure ts needed and vie expect Tc to surpass 15 K. In Fig. 2, we present the r e l a t i o n s h i p between Tc and the l a t t i c e constant a. ~e have also included the results f o r the s i n g l e phase 19 and 23~ samples [25,26]. As depicted, a smooth curve can be drawn through the data points f o r a l l the samples prepared from amorphous a ] l o y s . The Tc of the samples obtained by explosive compression [24] f a l l s on t h i s curve. From t h i s curve, we obtain a Tc of -25 K at a = 5.08 A for stotchiometrtc A15 Nb3St, which is in agreement with the extrapolated value of Tc acquired by DewHughes [ 3 ] . The t r a n s i t i o n of an amorphous a ] ] o y under high pressure depends on i t s composition, pressure, annealing temperature and time. For a metal-metalloid binary system near the eutectic composition, the t r a n s i t i o n process can be displayed in a pressure-temperature t r a n s t i o n diagram. As shown in Fig. 3, the diagram is
Tc(K)
<1.3
2.27
TJ3P • Amorphous
divided i n t o three regtons depend|ng on pressure. In the low pressure regton (A), the decomposition takes place, and thus an amorphous a l l o y normal]y decomposes i n t o an elemental metal (or d | l u t e s o l i d s o l u t i o n ) and a metastable l n t e r m e t a l l t c compound. The high pressure reg|ons (B and C) are the non-decompostt|on regtons. In Region B, via the atomic rearrangmont, an amorphous a l ] o y annealed under high pressure transforms t t s e l f i n t o a s|ngle-phase l n t e r m e t a l l t c compound [25,26]. In constrast, the u l t r a high pressure tn Region C suppresses the ordertng process of the atomtc arrangement in an amorphous a l ] o y , forming a disordered supersaturated s o l i d s o l u t i o n such as the fcc supersaturated s o l i d s o l u t i o n obtained for the amorphous Nb-SI a l l o y [26]. In the present work, our amorphous a l l o y s decompose into Nb and metastable A]5 phase, and hence the t r a n s i t i o n s Invo]ved belong to Region A. In t h i s case, the A15 phase thus obtained
MATRIX METAL + STABLE COMPOUND
25
/
20
.AT ,X.ETA ÷/ META TA..
/
pn-
15
k-
z
lo
p-
-w
5
AMORPHOUS
0
5.07
5.08
5.09
5.10
5.11
5.12
5.13
i 5.14
5.15
5.16
A
Ftg. 3. Relationship between Tc and l a t t i c e parameter a.
I C PRESSURE
LATTICEPARAMETER(A) Fig. 2.
! B
Temperature-pressure-transit|on dtagram of amorphous metal-metalloid btnary system near the eutecttc composit|on.
288
ANNEALED AMORPHOUS Nb-Si ALLOYS UNDER HIGH PRESSURE
has i t s composition closer to stoicheometric Nb3St than the slngle-phase A15 compound formed at higher pressure (Region B) s t a r t i n g from the amorphous sample with the same composit i o n . This is why our samples have smaller l a t t i c e constants and higher Tc. More work in Region A has to be made in order to get higher Tc. Ultimately, i t is s t i l l desirable, however, to have an u l t r a high-pressure annealing
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apparatus f o r the purpose of synthesizing s i n g l e phase yet stotcheometric A15 Nb3Si s t a r t i n g from stoicheometrtc amorphous Nb3Si so that the question about high Tc of A15 Nb3Si can be s e t t l e d . Acknowledgements - Two of the authors (JDB and CYH) were supported by the U.S. Department of Energy.
References 1. G. F. Hardy and 3. K. Hulm, Phys. Rev. B7, 884 (1953). 2. D. Dew-Hughes and V. G. RtvlJn, Nature 250, 723 (1974). 3. 0. Dew-Hughes, Cryogenics 15, 435 (19757. 4. S. Geller, Appl. Phys. ! , 321 (19757. 5. F. E. Wang, 3. Phys. Chem. Solids 35, 273 (1974); Sol. State Commun. 23, 803 (1977). 6. 3. Noolandl and L. R. Testardl, Phys. Rev. B 15, 5462 (1977). 7. R. O. Felderman, R. H. Hammond and T. H. Geballe, App%. Phys. Lett. 35, 818 (1979); R. D. Feldmen and R. H. Hammond, 3. Appl. Phys. 52 1427 (1981). 8 Y. Tarutanl and U. Kawabe, 3. Low Temp. Phys. 41, 553 (1980). 9 G. R. Johnson and D. H. Douglas, 3. Low Temp. Phys. 14, 575 (1974). 10. T. Ogushi, K. Nisht, H. Nagat, T. Numata and K. Qbara, 3. Low Temp. Phys. 41 13 (1980). 11. E. Amberger and U. Siefken, 3. Less - Common Met. 75, 273 (1980). 12. H. Kawamura and K. Tachikawa, Phys. Lett. 55A, 65 (1975). 13. S. Patdasst and 3. Spitz, 3. Less - Common Met. 61, 213 (1978). 14. G. R. Stewart, L. R. NewkJrk and R. A. Valencia, Phys. Rev. B20, 3647 (1979). 15. E. L. Haase and O. Meyer, IEEE Trans. Mag. Mag.-17, 541 (1981). 16. R. M. Waterstrat, F. Haenssler and 3. M~ller, 3. Appl. Phys. 50, 4763 (1979). 17. K. Togano, H. Kumakura and K. Tachikawa, Phys. Lett. 76A, 83 (19BO).
18. T. Shlbata, J. F. Gibbons, J. Kwa, R. D. Feldmen and T. H. Geballe, J. Appl. Phys. 52, 1537 (1981). 19. J. M. LGgar and H. T. H a l l , J. Less Common Met. 32, 181 (1973). 20. R. M. gaterstrat, F. Haenssler, J. M~ller, S. D. Dahlgren, and 3.0. W i l l i s , 3. Appl. Phys. 49, I143 (1978). 21. W. K. Wang, C. H. Zi, H. Chin and Y. C. Chang, Wuli 9, 309 (1980). 22. V. M. Pan, V. P. Aiekseevskli, A. G. Popov, Yu. I. B e l e t s k i i , L. M. Yupko, and V. V. Varosh, 3.E.T.P. Letters 21, 228 (1975). 23. D. Dew-Hughes and V. D. Linse, 3. Appl. Phys. 50, 3500 (1979). 24. B. Ollnger and L. R. Newklrk, Sol. Stat. Commun. 37, 613 (1981). 25. C. Suryanarayana, W. K. Wang, H. Iwasakl and T. Masumoto, Sol. Stat. Commun. 34, 861 (1980); H. lwasakl, W. K. Wang, N. Toyota, T. Fukase, H. Fujimeri, S. Endo and Y. Akahama, Sol. Stat. Commun. 42, 381 (1982); W. K. Wang, H. lwasakl, C. Suryanarayana, T. Masumoto, N. Toyota, T. Fukase and F. Koglku, 3. Mater. Sci. 17, 1523 (IgB3). 26. W. K. Wang, Tohoku University thesis, 1981, unpublished. 27. T. Masumoto, A. InDue, S. Sakai, H. Kimura and A. Hoshi, Trans. 31M 21, I15 (1980). 28. Y. X. Zhau, X. P. Xu, H. Chen and W. K. Wang, Wull 8, 489 (1979); W. K. Wang, X. P. Xu, H. Chen and Y. X. Zhau, Wuli 9 309 (IgBO).
Solid State Communications, Vol.58,No.5, pp.285-288, 1986. Printed in Great Britain.
0038-1098/86 $3.00 + .00 Pergamon P r e s s L t d .
SUPERCONDUCTIVITYAND PHASE COMPOSITIONS IN ANNEALED AMORPHOUS Nb-St ALLOYS UNDER HIGH PRESSURE W. K. Wang, Y. J. Wang, S. A. He
I n s t i t u t e of Physics, Chinese Academy of Science Betjing, People's Republic of China J. H. Huang Massachusetts I n s t i t u t e of Technology, Cambridge, HA 02]39, USA J. D. Boyer, C. Y. Huang Los Alamus National Laboratory, Los Alames, NM 87545 USA H. Iwasakt I n s t i t u t e f o r Iron, Steel and Other Metals, Tohoku U n i v e r s i t y , Sendat, 3apan ( R e c e i v e d F e b . l ~ 1 9 8 6 by W.Y.Kuan)
We report the results of amorphous Nb81St19 and Nb77Si23 a l l o y s annealed at high pressure but at a pressure lower than that used to synthesize single-phase A]5 Nb3St. The lower pressure gives rise to the decomposition of the a l l o y s i n t o a multi-phase state consisting of AI5 Nb3SI, F-NbsSt3, Ll2-type Nb3Si and Nb. The l a t t i c e constants are smaller and Tc higher than those obtained f o r the single-phase samples. We predict the Tc of stotchlometric AI5 Nb3St to be 25 K.
A15 phase, i t was thought that t t was possible to obtain the l a t t e r phase at higher pressure. The f t r s t attempt to synthesize A15 Nb3St in this manner was made by L~gar and Hall [19]. They employed mtxed elemental powders subjected to s t a t i c pressures up to 7 GPa and temperatures up to 2000°C. At low temperatures, they obtained a tetragonal phase, which became hexagonal as the temperature appraoched 2000°C. They thus obtained Nb5St3, but no A15 structure phase was found. Wa]terstrat et al [20] also attempted synthesis at ]0 GPa with sputtered metastable bcc Nb3St and at 6 GPa with Ti3P-type Nb3Sl. They also f a i l e d to produce any A]5 structure. Wang e t a ] [21] have also i n v e s t i gated the high pressure e f f e c t s , and have found that the competing Nb5SJ3 has to be suppressed in order to obtain AI5 Nb3St. Encourged by the successful synthesis of A]5 Nb3Sn from elemental powders with shock waves from the detonation of explosives which could y i e l d short-duration hgth-pressure pulses of 20-25 GPa, Pan et al [22] subjected to a sample of the stable Tt3P-type Nb3Si phase to explosive compression with a peak pressure estimated to greater than lO0 GPa. They claimed to have obtained about 5% of the sample In the A15 phase with Tc = 18.5 - 19.0 K and a = 5.08 A. Later, Dew-Hughes and Linse [23] repeated the same experiment and obtained the same value of Tc with a = 5.12 A. S i m i l a r l y , Oltnger and Newkirk [24] also obtained approximately the
A f t e r I t s f i r s t discovery In 1953 [ 1 ] , superc o n d u c t i v i t y in A38-type compounds with the A15 structure has been investigated i n t e n s i v e l y . Based on the known A15 superconductors, many authors have found the empirical c o r r e l a t i o n s between the superconducting t r a n s i t i o n temperature, Tc, and some physical parameters [ 2 - 6 ] . From the e x t r a p o l a t i o n of the c o r r e l a t i o n between Tc and the mass of the B element, Dew-Hughes and R l v l t n [2] predicted a Tc of 38 K f o r Nb3Si. But, from that between Tc and the atomic volume Dew-Hughes [3] obtained a predict i o n of only 25 K. By e x t r a p o l a t i o n from the l a t t i c e constants and Tc of V3Sn, V3St, and Nb3Sb, Geller [4] predicted a value of Tc = 30 K f o r Nb3Si with a l a t t i c e constant of a = 5.06 ± 0.02 X . This value of Tc was also predicted by Noolandi and Testardi [6] by means of a correlation between the reduction in Tc and the increase in the l a t t i c e parameter. Encouraged by these extrapolations, many authors have attempted to synthesize this compound by conventional methods such as electron-beam coevaporat|on [?,8] dual target sputtering [9-12], chemical vapor deposition [13-15],
l i q u i d quenching [ ] 6 , 1 7 ] and pulse annealing of amorphous a l l o y s [18]. Aside from Ref. l l , a l l these methods gave a Tc even lower than that of pure Nb (9.2 K). In view of the fact that the s p e c i f i c volume of the tetragonal Tt3P-type Nb3St phase is about 3% larger than that of the extrapolated 285
286
ANNEALED AMORPHOUS Nb-Si ALLOYS UNDER HIGH PRESSURE
same Tc but with a = 5.091 A. However,the x-ray determination of the structure was d i f f i cult in a l l these experiments. I t has been speculated that this lower value of Tc than the extrapolated value might be attributable to the result of non-stoichlmetrlc compositions which in turn cause the l a t t i c e constant a to be larger than Geller's extrapolated value [4]. I t was recognized that the amorphous state is unstable and that the Al5 structure could be produced by annealing amorphous Nb-Sl a l l o y s at htgh pressure.J25,26] For the samples with the compositions of Nb81Sll9 and Nb77Si23, these authors obtained single-phase samples with the A15 s t r u c t u r e . The l a t t i c e constants are 5.155 and 5.120 A and Tc are 3.4 and 8.9 K, respectively. These Tc'S are, however, even lower than that of pure Nb. I t is clear t h a t , in order to get higher Tc, compositions closer to the stoicheometry (Nb?5Si25) are needed. Nevertheless, in t h i s case, the pressure required f o r annealing is i n h i b i t e d l y high. At pressures much higher than those used to produce s i n g l e phase A15 Nb8lSil9 and Nb77St23, a supersaturated s o l i d s o l u t i o n , such as fcc Nb(St), r e s u l t s . In both these cases, the crystallization process from an amorphous state does not involve any long-distance atomic migrations and phase decompositions. At s l i g h t l y lower pressure, the atomic diffusion can take place completely, giving rise to the decomposition of an amorphous alloy into a multl-phase state. T h i s rather complete atomic diffusion at proper pressure might be helpful in attaining the stoichiometric Al5 phase with a smaller l a t t i c e constant and hence a higher Tc. The present work is motivated by this observation, and hence we investigate the crystallization and Tc of the samples at somewhat lower annealed pressure. In order to compare with the previous results [25], the same compositions, Nb-lg and -23 at % Si, have been chosen to be studied in this work. The Ig% amorphous samples were prepared by splat melting and the 23% ones by the sputtering method. The Bridgman anvils and belt-type high pressure apparatus was used. The annealing procedure similar to those used in the previous work [25,26] is employed in this work. After the high pressure-temperature annealing procedure, the specimens were carefully peeled off from the BN pressure medium for characterization. The typical size of a sample is ~0.02 mmfor the lg% specimens and ~0.5 mm for the 23% ones. The x-ray powder d i f f r a c t i o n method with a 114.6 cm diameter camera and CuK~ radiation was employed to determine crystal structures. Tc were measured by the ac magnetic susceptib i l i t y technique. Figure l(a) displays the x-ray diffraction pattern of a Ig% sample (No. l ) annealed at 4 6Pa and 800°C for 35 minutes. In spite of overlapping, the d i f f r a c t i o n llne positions and intensities can be analyzed and interpreted in terms of a multi-phase state consisting of Al5 Nb3Si, r-Nb§Si3 and Nb. The l a t t i c e constant for the Al5 phase is 5.10 A, and the superconducting transition takes place at 13.1K with a width of ~ 3 K. T h i s high value of Tc is believed to be due to the Al5 phase.
Vol. 58, No. 5
(a)
Fig. l
X-ray diffraction pattern of amorphous Nb-19 at % Si alloy annnealed at (a) 4 GPa and 800°C for 35 min, (b) 4 GPa and 750°C for 1440 min.
For the sample of another 19% sample (No. 2) annealed at 4 GPa and 750°C for 1440 minutes, the lines as shown in Fig. l(b) can be best fit to a multi-phase state, AI5 Nb3Si + Tl3P-type Nb3Si + Nb + an amorphous alloy. The lattice constant of the Al5 phase is 5 . 1 1 A and T c is II.9 K. Again, this high value of T c is attributed to the Al5 phase, because the Ti3P phase is not superconducting down to 1.3 K. Sample No. 3 was the result of annealing a 19% amorphous alloy at 4 GPa and 8 5 0 % for 480 minutes. The x-ray result of this sample demonstrated the coexistence of a Nb solid solution with Ll2-type Nb3Si and an amorphous Nb-Si alloy. T c of this sample is 7.52 K, which is believed to arise from the Nb solid solution because Ll 2 type Nb3Si is not superconducting above 0.5 K. As summed up in Table I, all these three samples were prepared at 4 GPa. Nontheless, we have shown that the annealing temperature is rather c r i t i c a l to get the high Tc; a difference of 50°C in annealing temperature makes a considerable difference in Tc. Masumoto et al [27] have shown that an amorphous Nb-Si alloy annealed at ambient pressure decomposes into Nb and Ti3P-type Nb3Si through a metastable multi-phase state. The results of our low pressure annealing samples presented above are consistent with this observation of Masumoto et a l . Nonetheless,in our case, the multi-phase state is more complicated, becuase i t consists of four phases: Al5 Nb3Si, F-Nb5Si3, Ti3P-type Nb3Si and Ll2-type Nb3Si. In fact, the r-Nb5Si 3 and Ll2-type Nb3Si phases have been found in crystalline Nb-Si alloys annealed at high pressure [28]. Sample No. 4 is a specimen obtained by annealing an amorphous Nb77Si23 sample at ambient pressure and 650°C for 43,000 minutes. Only the Ti3P phase results, and i t is not a superconductor. When the a l l o y is instead annealed at 5 GPa and 750°C for 840 minutes, i t decomposes i n t o Ti3P type Nb3Si in coexistence with an amorphous phase. T~ of 2.27 K was obtained and i t could be a t t r i b u t a b l e to the amorphous phase. From t h i s observation, i t is clear that in order to acquire the A;5 phase at
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287
ANNEALED AMORPHOUS Nb-Si ALLOYS UNDER HIGH PRESSURE
Table I .
High pressure annealing and Tc.
No.
Alloy
Annealing Condttlon
Phase Composition
I
Nb81Silg
4 GPa, 800°C 35 min.
A15 ( a = 5.10A) +F-Nb~St 3 + Nb
13.1
Nb81Silg
4 GPa, 750°C 1440 min.
A15 (a = 5 . l l A ) + Tl3P-type Nb3Si ÷ Nb
11 .g
3
Nb81Stlg
4 GPa, 850°C 480 min.
L12-type Nb3St + Nb
4
Nb77Si23
0.01HPa, 650°C 43000 min
5
Nb77St23
7.52
Ti3P
5 GPa, 750°C 840 min
t h i s composition, somewhat higher pressure ts needed and vie expect Tc to surpass 15 K. In Fig. 2, we present the r e l a t i o n s h i p between Tc and the l a t t i c e constant a. ~e have also included the results f o r the s i n g l e phase 19 and 23~ samples [25,26]. As depicted, a smooth curve can be drawn through the data points f o r a l l the samples prepared from amorphous a ] l o y s . The Tc of the samples obtained by explosive compression [24] f a l l s on t h i s curve. From t h i s curve, we obtain a Tc of -25 K at a = 5.08 A for stotchiometrtc A15 Nb3St, which is in agreement with the extrapolated value of Tc acquired by DewHughes [ 3 ] . The t r a n s i t i o n of an amorphous a ] ] o y under high pressure depends on i t s composition, pressure, annealing temperature and time. For a metal-metalloid binary system near the eutectic composition, the t r a n s i t i o n process can be displayed in a pressure-temperature t r a n s t i o n diagram. As shown in Fig. 3, the diagram is
Tc(K)
<1.3
2.27
TJ3P • Amorphous
divided i n t o three regtons depend|ng on pressure. In the low pressure regton (A), the decomposition takes place, and thus an amorphous a l l o y normal]y decomposes i n t o an elemental metal (or d | l u t e s o l i d s o l u t i o n ) and a metastable l n t e r m e t a l l t c compound. The high pressure reg|ons (B and C) are the non-decompostt|on regtons. In Region B, via the atomic rearrangmont, an amorphous a l ] o y annealed under high pressure transforms t t s e l f i n t o a s|ngle-phase l n t e r m e t a l l t c compound [25,26]. In constrast, the u l t r a high pressure tn Region C suppresses the ordertng process of the atomtc arrangement in an amorphous a l ] o y , forming a disordered supersaturated s o l i d s o l u t i o n such as the fcc supersaturated s o l i d s o l u t i o n obtained for the amorphous Nb-SI a l l o y [26]. In the present work, our amorphous a l l o y s decompose into Nb and metastable A]5 phase, and hence the t r a n s i t i o n s Invo]ved belong to Region A. In t h i s case, the A15 phase thus obtained
MATRIX METAL + STABLE COMPOUND
25
/
20
.AT ,X.ETA ÷/ META TA..
/
pn-
15
k-
z
lo
p-
-w
5
AMORPHOUS
0
5.07
5.08
5.09
5.10
5.11
5.12
5.13
i 5.14
5.15
5.16
A
Ftg. 3. Relationship between Tc and l a t t i c e parameter a.
I C PRESSURE
LATTICEPARAMETER(A) Fig. 2.
! B
Temperature-pressure-transit|on dtagram of amorphous metal-metalloid btnary system near the eutecttc composit|on.
288
ANNEALED AMORPHOUS Nb-Si ALLOYS UNDER HIGH PRESSURE
has i t s composition closer to stoicheometric Nb3St than the slngle-phase A15 compound formed at higher pressure (Region B) s t a r t i n g from the amorphous sample with the same composit i o n . This is why our samples have smaller l a t t i c e constants and higher Tc. More work in Region A has to be made in order to get higher Tc. Ultimately, i t is s t i l l desirable, however, to have an u l t r a high-pressure annealing
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apparatus f o r the purpose of synthesizing s i n g l e phase yet stotcheometric A15 Nb3Si s t a r t i n g from stoicheometrtc amorphous Nb3Si so that the question about high Tc of A15 Nb3Si can be s e t t l e d . Acknowledgements - Two of the authors (JDB and CYH) were supported by the U.S. Department of Energy.
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18. T. Shlbata, J. F. Gibbons, J. Kwa, R. D. Feldmen and T. H. Geballe, J. Appl. Phys. 52, 1537 (1981). 19. J. M. LGgar and H. T. H a l l , J. Less Common Met. 32, 181 (1973). 20. R. M. gaterstrat, F. Haenssler, J. M~ller, S. D. Dahlgren, and 3.0. W i l l i s , 3. Appl. Phys. 49, I143 (1978). 21. W. K. Wang, C. H. Zi, H. Chin and Y. C. Chang, Wuli 9, 309 (1980). 22. V. M. Pan, V. P. Aiekseevskli, A. G. Popov, Yu. I. B e l e t s k i i , L. M. Yupko, and V. V. Varosh, 3.E.T.P. Letters 21, 228 (1975). 23. D. Dew-Hughes and V. D. Linse, 3. Appl. Phys. 50, 3500 (1979). 24. B. Ollnger and L. R. Newklrk, Sol. Stat. Commun. 37, 613 (1981). 25. C. Suryanarayana, W. K. Wang, H. Iwasakl and T. Masumoto, Sol. Stat. Commun. 34, 861 (1980); H. lwasakl, W. K. Wang, N. Toyota, T. Fukase, H. Fujimeri, S. Endo and Y. Akahama, Sol. Stat. Commun. 42, 381 (1982); W. K. Wang, H. lwasakl, C. Suryanarayana, T. Masumoto, N. Toyota, T. Fukase and F. Koglku, 3. Mater. Sci. 17, 1523 (IgB3). 26. W. K. Wang, Tohoku University thesis, 1981, unpublished. 27. T. Masumoto, A. InDue, S. Sakai, H. Kimura and A. Hoshi, Trans. 31M 21, I15 (1980). 28. Y. X. Zhau, X. P. Xu, H. Chen and W. K. Wang, Wull 8, 489 (1979); W. K. Wang, X. P. Xu, H. Chen and Y. X. Zhau, Wuli 9 309 (IgBO).