A15Nb3Si produced by high-pressure annealing of amorphous sputter deposits

Solid State Communications, Printed in Great Britain.

AlS-NbsSi

Vol. 42, No. 5, pp. 381-384,1982.

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0038- 1098/82/ 17038 l-04$03.00/0 Pergamon Press Ltd. OF AMORPHOUS SPUTTER DEPOSITS

H. Iwasaki?, W.K. Wang*, N. Toyota, T. Fukase and H. Fujimori The Research Institute

for Iron, Steel and Other Metals, Tohoku University,

Sendai 980, Japan

and Y. Akahama and S. Endo High Pressure Research Laboratory, Faculty of Engineering Toyonaka 560, Japan

Science, Osaka University,

(Received 4 December 1981 by J. Kanamori)

A high-pressure annealing technique has been used to convert an amorphous Nl-23.7 at.% Si alloy, prepared by sputtering, into the Al5 phase alloy. X-ray diffraction examination of the alloy samples quenched to ambient conditions has shown that they are in an almost single-phased state with a lattice parameter of 0.5120 nm and a high degree of atomic ordering. An onset of superconductivity has been detected at 8.9 K and a temperature derivative of upper critical field is 14 kOe/K (MA/4n mK).

formation of a single-phased Al5 structure giving sharp diffraction lines. In this method of synthesis, transformation from the amorphous into the Al5 structure proceeds over the period of several days and atomic order can well develop. However, a steep rise in the melting temperature of the alloy with increasing Si content placed a practical upper limit on the composition for the amorphous alloy and the synthesized Al 5 phase could contain silicon only by 19 at.%. To was measured to be 3.4 K. In the present paper, we report the results of highpressure synthesis experiments using amorphous sputter deposits of the Nb-Si alloy as starting materials. It is possible to get the Al5 phase richer in Si content and improve TC. We suggest here a high-pressure annealing of amorphous alloys as a new,-powerful method of synthesizing crystalline phases.

1. INTRODUCTION CONTINUED and extended efforts have been made to synthesize a superconducting AlS-NbsSi with an expectation that it may have a high transition temperature TC. Although many researchers claimed their success in the synthesis [l] , the materials obtained invariably contained a contaminating phase (or phases) in addition to the Al5 phase and it was not certain that high superconducting transition temperature, around 18 K, measured on these material was really that characteristic of the Al 5-NbsSi. If the composition of starting materials deviated appreciably from stoichiometry, however, the Al5 structure could be produced in a single-phased state. But its Tc was low [2,3] . Recently, Haase and Meyer [4] expressed doubt about high T, reported previously for AlS-NbsSi and suggested that the measured superconducting transition was that of another compound Nb,Si having nearly the same lattice parameter. However, identification of this new compound was not convincing. Hence there still remains a pr ;,ern of investigating superconducting properties of AlS-NbsSi using samples which contain the structure in a single-phased state and have the composition near the stoichiometry. The present authors [5] showed that high-pressure annealing of amorphous Nl-Si alloy, prepared by a liquid-quenching technique, favorably lead to the

2. SAMPLE PREPARATION AND HIGH PRESSURE TECHNIQUE Original binary alloy samples were prepared by arcmelting appropriately weighted amounts of niobium and silicon under argon gas atmosphere. The melted alloy was in the form of button 35 mm in diameter and 6 mm in thickness. Sputtering was performed in a d.c. trianode plasma system [6] using the alloy button as target. The substrate was made of copper, which was cooled by flowing water during operation. The system was filed with highpurity argon gas to a pressure of 5 Pa. Target voltage was kept at 1000 V and total current 50 mA. The sputtered material uniformly covered the substrate to a

* Qn leave from Institute of Physics, Chinese Academy of Sciences, Beijing, China. t To whom all the correspondence should be addressed. 381

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Table 1. Analysis of the X-ray diffraction pattern recorded from the Al 5-NbaSi phase formed by annealing at 10 GPaand 1023 K. a = 0.5120 mn. Iobs is the observed intensity corrected for Lorentz-polarization and absorption effects and normalized at the 2 10 reflection. I,* = m * / F(hkl) I*, where an isotropic temperature factor with B = 0.0056 nm* is assumed hkl

Fig. 1. X-ray diffraction pattern of NI-23.7 at .% Si alloy annealed at 10 GPa and 1023 K for 173 k set (48 hr) and quenched to ambient conditions, showing the formation of AlS-NbaSi. Monochromated Cu& radiation. Arrow indicates diffraction line from the coexisting Ti,P-type NbsSi. thickness of 300 pm. It was removed from the substrate by mechanical cutting. X-ray diffraction examination showed that the material was in an amorphous state. Chemical analysis showed the composition to be Nb23.7 at.% Si. High-pressure experiments were carried out using a 6-8 type multi-anvil apparatus [7]. Several pieces of small fragments of the amorphous alloy, embedded in boron nitride powder, were put in the center of semisintered MgO octahedron, 4 mm in an edge length, which served as pressure transmitting medium. Graphite tube immediately surrounding the alloy sample in the octahedron served as a heater. Pressure was first raised to 10 GPa and a.c. current was conducted through the heater. After being kept at the pressure and increased temperature for periods longer than a day, the sample was quenched to ambient conditions.

3. STRUCTURAL

CHARACTERIZATION

Figure 1 shows an X-ray diffraction pattern, taken with monochromated Cuba! radiation, of the Nl-23.7 at .% Si alloy annealed at 10 GPa and 1023 K for 173 k set (48 hr) and quenched. It can clearly be seen that crystallization has taken place and the alloy gives many sharp diffraction lines. All of them can be indexed in terms of an Al5 structure with the lattice parameter of

110 200 210 211 220 310 222 320 321 400 411,330 420 421 332 422 510,431 520,432 521 440

0.3617 0.2558 0.2290 0.2089 0.1808 0.1618 0.1477 0.1420 0.1368 0.1280 0.1206 0.1145 0.1117 0.1093 0.105 0.1005 0.09509 0.09352 0.09057

0.3620 0.2560 0.2290 0.2090 0.1810 0.1619 0.1478 0.1420 0.1368 0.1280 0.1207 0.1145 0.1117 0.1092 0.1045 0.1004 0.09508 0.09348 0.0905 1

24 41 345 145 15 20 114 227 201 132 18 87 293 62 < 10 33 362 102 144

19.9 46.4 345 .o 157.0 12.5 22.1 125.8 213.1 200.9 134.2 22.4 76.8 298.6 71.0 11.8 32.7 331.1 104.8 144.3

a = 0.5120 + 0.0004 nm. There are, however, a few, weak extra lines, which are identified as those from a coexisting Ti,P-type Nb,Si, the normal pressure phase. An estimate based on a formula given in [8] shows that the volume fraction of the Al5 phase is more than 97%. If annealing temperature was too low, no indication of crystallization was obtained, while if it was too high, the amorphous alloy decomposed into multi-phases. In order to determine the degree of atomic order in the Al5 lattice, integrated intensity of the diffraction lines was measured on an X-ray diffractometer. During intensity measurement, a tip of the alloy sample, mounted on a conventional goniometer-head, was rotated so as to eliminate preferred orientation effect. The intensity was corrected for Lorentz-polarization and absorption effects. The Al5 structure belongs to a space group Pm3n and there are two kinds of atomic site, 6c and 2a. An occupancy model has been sought which gives the best fit of the calculated intensity to the observed one. The results are such that the 6c site is occupied by 0.97 Nb + 0.03 Si atoms and the 2a site by 0.14 Nb + 0.86 Si atoms, while the highest ordering scheme attainable in the present slightly non-stoichiometric alloy is such that the 6c site is exclusively occupied by Nb atoms and the 2~ site by 0.052 Nb + 0.948 Si atoms. The degree of

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Al&NbaSi

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A15-Nb,SI (23.7X51)

4

6

6

IO

T(K)

Fig. 2. AC susceptibility of the A15 phase, formed in Nb-23.7 at.% Si alloy, plotted as a function of temperature. x’ and x” represent respectively the real and imaginary parts of the susceptibility. atomic order S’ defined by van Reuth and Waterstrat [9] is determined to be 0.87. Table 1 lists the calculated interplanar spacing dd and intensity I,_, of reflections compared respectively with the observed ones, dabs and lobs * From the occupancy probabilities of the 6c site, an average length of Nb atom chains running in the (100) direction is calculated to be 32 times half a lattice parameter. 4. SUPERCONDUCTING

PROPERTIES

AC susceptibility of the A15 phase sample was measured by a Hartshorn-bridge technique under constant magnetic field as a function of temperature. Figure 2 shows the results at zero field. Real part of the susceptibility, x’, begins to decrease at 8.9 K, indicating an onset of superconducting transition. The transition width, defined here as a full width of x” (imaginary part) peak, is 2 K. Susceptibility measurement of the mixed-to-normal transition was performed up to 60 kOe (MA/4?rm), which yielded a plot shown in Fig. 3. The slope of the straight line gives a temperature derivative of upper critical field dHo,/dT of 14 kOe/K (MA/4nmK). 5. DISCUSSION Since an amorphous state is only metastable, it is easily transformed into crystalline state upon heating. In the case of binary alloys, this transformation is a decomposition into multi-phased state. Beneficial effect of application of high pressure to the amorphous alloy is

4

6 T(K)

6

10

Fig. 3. Upper critical field of the Al5 phase, formed in Nb-23.7 at.% Si alloy, as a function of temperature. firstly to suppress the decomposition reaction and lead to the formation of a crystalline phase having the same composition as that of the amorphous phase. Secondly, high pressure favors the formation of a crystalline phase having a higher packing density, thus resulting in the formation, in the case of Nb-Si alloys, of the denser AlS-type Nb3Si in preference to the Ti3P-type Nb3Si. To the best of the author’s knowledge, the high-pressure annealing of amorphous alloys is the first method by which near-stoiciometric AlS-NbaSi is produced in an almost single-phased state. Presence of a small amount of the Ti,P-type phase in the high-pressure annealed Nb-23.7 at.% Si alloy sample is due to an increased stability of this phase relative to the Al5 phase in the alloy of composition closer to the stoichiometry. The experimental observations reported in the papers [ 10, 1 l] that it becomes progresively difficult to get the Al5 phase with increasing Si content lend support to this interpretation. Recent progress in the techniques of materials science has made it possible to prepare many kinds of alloys in an amorphous state and a high-pressure annealing of these alloys can be used to produce crystalline phases which are not obtainable by usual methods of synthesis. Although the increase in Si content of the Al5 phase achieved by using amorphous sputter deposits as starting materials has improved Tc, it is still lower than the highest Tc reported previously as that of “A15Nb$i”. Attempt is here made to plot the Tc value of 8.9 K measured in the present study and that of 3.4 K in our previous study [5] in the relation between the lattice parameter and Tc proposed by Haase and Meyer [4] for the real AlS-Nb,Si and a consistent result is

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obtained. If the relation is linearly extrapolated to a = 0.508 nm, an expected lattice parameter value of the stoichiometric AlS-NbaSi, Tc will be N 15 K, considerably lower than the predicted value on the basis of comparison of Tc’s of various Al5 superconductors. The results of the present study are thus in favor of the suggestion of Haase and Meyer. It is, however, to be noted that Nb$i very recently recovered from shockwave compression has given Tc of 18 K [ 121. Besides, the transition is accompanied by a clear specific-heat anomaly [13]. The measured lattice parameter 0.5091 nm is close to the ideal value, but, unfortunately, X-ray diffraction pattern from the recovered material is contaminated by a number of diffraction lines from coexisting phases. It was suggested that Tc of Nb-based Al5 phases is greatly affected by the integrity of the Nb atom chains [ 141. Sweedler and Cox [ 151 showed experimentally for AlS-NbsAl a remarkable dependence of Tc on the degree of integrity, thereby the degree being varied by neutron irradiation. Kawamura and Tachikawa [ 161 measured intensity of the X-ray diffraction lines from th Al5 phase formed in liquid-quenched Nb-22 at .% Si alloy and obtained S’ = 0.64, i.e. the Nb atom chain had 11-12 times half the lattice parameter. Tc of their sample was 6 K. On the other hand, our Al5 sample has the chain three times as long as it, but Tc is higher only by 3 K. This fact suggests that Tc of AlS-Nb,Si is not affected by the integrity of the chain in the way as Tc of AlS-NbsAl is. More investigations which can relate Tc of AlS-NbaSi to the structural parameters are required in order to shed light in the nature of superconductivity of that compound.

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Assistance of Mr S. Hashimoto in X-ray intensity measurement is greatly acknowledged. The present work has been supported in part by Grant-in-Aid for Scientific Research from Ministry of Education, Science and Culture and also by the fund from the Mitsubishi Foundation. REFERENCES 1.

2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Acknowledgement - The authors wish to express their appreciation to Profs Y. Muto and T. Masumoto of Tohoku University for their interest in the present work.

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