Martensitic transformations in V3Ga and Nb3Al

Mat. Res. Bull. Vol. 9, pp. 277-Z8Z, in the United States.

1974.

Pergamon

Press, Inc.

Printed

MARTENSITIC TRANSFORMATIONS IN V3Ga AND Nb3AI

R. Viswanathan Department of Applied Physics and Information Science University of California, San Diego La Jolla, California 92037

(Received D e c e m b e r

Z7, 1973; C o m m u n i c a t e d

by B. T. Matthias)

ABSTRACT It is shown by low-temperature heat capacity measurements using ac technique that martensitic transformation in small samples of V3Ga and Nb3AI can be prevented by covering them nearly completely with copper plating.

Introduction The occurrence of lattice transformation has not so far been clearly established for V3Ga and Nb3AI, which belong to the same class of high-transition-temperature superconductors with ~-W structure as Nb3Sn. By recent low-temperature heat capacity measurements using the ac technique it was shown indirectly (I) that martensitic transformations do occur in both V3Ga and Nb3AI.

Since it is possible to inhibit the phase transformation

of Nb3Sn by copper plating (2), it will be interesting to study similar effects in these two compounds, if indeed they undergo martensitic transformation.

Reported here are results of such an investigation, which

indicate that V3Ga and Nb3AI behave in a way quite similar to Nb3Sn, so far as lattice instability is concerned.

Experiments The V3Ga sample was prepared from high-purity elements by arc melting followed by a 1300°C anneal for two hours and a 700°C anneal for three days. *

This work was supported by U.S. Atomic Energy Commission contract AEC-AT- (04-3)-34. Z77

Z78

TRANSFORMATIONS

IN V 3 G a

The Nb3AI sample was prepared b y a r c for 50 hours.

AND

Nb3AI

Vol. 9, No.

3

casting, and followed by a 725°C anneal

These samples are the same as V3Ga(1) and Nb3AI(III) reported

in an earlier publication (i), with respective inductive transition onsets at 15.2°K and 18.6°K.

The low-temperature heat capacity was measured by

ac calorimetry technique with a laser beam as a heat source (3).

Results and Discussions The low-temperature heat capacity data for V3Ga and Nb3AI are given in Figs. i and 2 as conventional C/T-vs.-T 2 plots.

Table i lists all relevant

parameters for comparison, including literature data (4,5).

The electronic

heat capacity (y) and Debye temperature (8D) values in these measurements were obtained by simple straight-line extrapolation from above the superconducting transition temperature, only.

and hence are suitable for comparison

The onset of the inductive transition is shown by a dotted arrow in

each figure. When the samples are copper-plated on only one side to attach the thermocouples,

they show large heat capacity anomalies at temperatures 2 to 3 °

below the inductive T

values. Only small amounts of untransformed phases c seem to persist in these samples at low temperatures. When the samples are completely covered by further copper plating, however, the bulk anomaly shifts to higher temperatures coinciding with the inductive T c.

We interpret

this to mean, by analogy with the results for Nb3Sn (2), that copper plating inhibits the martensitic transformation in both V3Ga and Nb3AI, and thus stabilizes the cubic phase. The above result is especially interesting for Nb3AI, in which martensitic transformation has not so far been directly observed, and which is quite difficult to prepare as a stoichiometric single-phase compound. If the lower temperature anomaly at 15.6°K were really interpreted as the result of a variation in composition -- as might be indicated by the lattice parameter -- then it would be difficult to explain the shift of the bulk T

C

for a completely plated sample to 18.6°K: a figure corresponding to the T

of a stoichiometric compound.

Based on these and earlier data (2,6,7)

C

one might further speculate that lattice instabilities and their inhibition by copper cladding are quite common among high-T One may easilyinterpret

C

B-W superconductors.

the increased values of critical field (H c)

and critical current (Jc) in copper-covered V3Ga (8) and Nb3Sn (9) to be in part the consequence of stabilization of the higher-T c cubic phase.

Vol. 9, No. 3

TRANSFORMATIONS

300

IN V3Ga A N D Nb3AI

|

Z79

!

I

AO C~

200

0

E E

:}

i

•

•

I00

(,111) 1 1 on one side only copper 91ated on all sides

!

,

13.4o152

,3~a °

!. •O

T2 in °K2 i

0

I

100

I

200

I

300

400

FIG. 1 Heat capacity data for V3Ga before and after copper plating on all sides I

!

I

o o

i o ~

300

oo Oo

ol

200

118.6°

k

£ Aoo

E

15.6

~o

Nb3AI ('91)

100

(, on one side

£

copper plated Ao

/

/o on all sides

•

,~e'" 0

!

T 2 in °K2 I lO0

I 200

I 300

I 400

FIG. 2 Heat capacity data for Nb3AI before and after copper plating on all sides,

Reference 5

Reference 4

Nb3AI , #91

Reference 4

V3Ga , #Iii

Sample

t

I Cu-coated, all sides

one s i d e o n l y

Cu-coated,

I Cu-eoated, all sides

t

one s i d e o n l y

Cu-coated,

5.181

5. 185

4.822

4. 819

Lattice Parameter

NONE

290 30

42

42

NONE

45

0

-?-

160

NONE

70

123

i0

at high temp.

5

125

at low temp.

AC/T (mJ/gm-mole.°K 2)

280

222

235

302

295

302

8D (°K)

32

30

30

97

96

96

y (mJ/gm-mole.°K 2)

TABLE 1 Relevant Parameters of Copper-Coated V3Ga and Nb3AI from Heat Capacity Data

Oo

Z O

~O

O

Z

ffl

0 Z

0

> Z cn

O0 0

Vol. 9, No.

3

TRANSFORMATIONS

IN V 3 G a

AND

Nb3AI

Conclusion By low-temperature heat capacity measurements using ac technique, it is shown here that martensitic transformation does occur in both V3Ga and Nb3AI , and that it can be prevented in small samples by completely covering them with copper plating.

Acknowledgement The author would like to thank Mr. D. C. Johnston for interesting discussions,

and Mr. C. T. Wu, Dr. H. L. Luo and Dr. G. W. Webb for

supplying the samples.

References i.

R. Viswanathan, (in press).

C. T. Wu, H. L. Luo and G. W. Webb, Solid State Comm.

2.

R. Viswanathan and D. C. Johnston, Mater. Res. Bull. 8, 589 (1973).

3.

R. Viswanathan and H. L. Luo, Proc. of Symp. on Nuclear and Solid-State Phys., Chandigarh, India, December 1972, 15C, 361.

4.

A. Junod, J. L. Staudenmann, J. Muller and P. Spitzili, J. Low-Temp. Phys. 5, 25 (1971).

5.

R. H. Willens, T. H. Geballe, A. S. Gossard, J. P. Malta, A. Menth, G. W. Hull, Jr. and R. R. Soden, Solid State Comm. ~, 837 (1969).

6.

R. Viswanathan and H. L. Luo, Solid State Comm. 9, 1733 (1971).

7.

R. Viswanathan, H. L. Luo and L. J. Vieland, Proc. 13th Internat. Conf. Low-Temp. Phys. LT-13, Boulder, Colorado, 1972 (in press).

8.

K. Tachikawa and Y. Tamaka, Jap. J. Appl. Phys. 6, 782 (1967).

9.

D. F. Fairbanks, U.S. Patent No. 3,352,008 (November 14, 1967).

ZSI