THERMAL EXPANSlON OF V3Si

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THERMAL EXPANSlON OF V3Si

M. Milewits, S. Williamson

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-408

THERMAL EXPANSION OF V

S i *

M. Milewits and S.J.Williamson

Department of Physios New York University, New York, N.Y. 10003 U.S.A.

Résumé.- Des mesures dilatométriques effectuées entre 4,2 K et 293 K sur cinq échantillons de V3Si

monocristallins n'ont pas permis de confirmer le coefficient de dilatation négatif extraordinaire observé juste en dessus de Tc dans des échantillons polycristallins par Smith et al. Dans les deux

échantillons présentant la transformation martensitique l'anomalie dilatométrique associée à la transformation est observée au moins jusque 60 K.

Abstract.- Measurements of the thermal expansion coefficient of five single crystals of V,Si in the temperature range 4.2 - 293 K fail to reveal the .unusual negative values just above T reported by Smith et. al. for polycrystalline samples. In two of the samples exhibiting a lattice transforma-tion at low temperature associated anomalies are found to extend up to at least 60 K.

1.INTRODUCTION.- The high temperature superconduc-tors with the A-]5 crystal structure such as V3S1 and Nb,Sn exhibit anomalous elastic properties abo-ve the superconducting transition temperature Tc. Various explanations have been proposed to account for this behavior, including models based on the electronic band structure, presence of lattice de-fects, and anharmonic components of the interatomic potential /l/. Thermal expansion provides a direct indication of anharmonicity. Recently Smith et al. /2,3/ on the basis of thermal expansion measurements on nearly stoichiometric samples of V3Si have repor-ted the observation just above T = 16.5 K of a

J c

contraction on warming over a span of several tens of degrees Kelvin. They interpret the corresponding negative thermal expansion coefficient a=L-1(dL/dT), where L is the sample lenght, as arising from anhar-monic properties of the cubic lattice. As the stu-dies of Smith e£ al^.were performed on polycrystalli-ne samples wethought it worthwile to establish whe-ther good quality single crystals dsplay the same behavior. We report here our results of strain mea-surements for five single crystals of V3Si, two of which display the lattice transformation from the cubic A-15 phase at high temperatures to a tetrago-nal phase below 21 K, and three of which remain cu-bic at all temperatures.

2.EXPERIMENTAL RESULTS.- A capacitance dilamometer /4/ was used to measure changes in sample lenght along the JjOO] direction for three non-transforming single crystals having T = 16.1 K and a result re-is

gistance ratio RRR - R(295 K)/R(17 K)=8. Measure-Supported by the National Science Foundation

(DMR 76- 02043)

ments of the elastic constants of these samples were reported by Larson and Ruoff /5/. Two trans-forming crystals were also studied, one having Tc=16.8 K and the other Tc=16.6 K with a nominal

value of RRR= 60. Malta and Bucher have previously measured the specific heat and magnetic susceptibi-lity of the latter sample /€/.

0 | 1 1 , f , . V3Si - 5 - . ' *o , • x . »'

< "

Fig.l : Thermal expansion for non-transforming (RRR=8) and transforming (RRR=60) single crystals of V3Si.

Figure 1. illustrates representative results for the three non-transforming crystals of V-Si. Figu-re 2 shows the corFigu-responding thermal expansion coefficient a, which has a relatively small value up to 50 K but increases at higher temperatures. Quite different behavior is found for the two

transforming samples: on warming through T w 21 K there is a marked increase in strain which can be identified as the expansion of the lattice parame-ter along the a-axis of the tetragonal phase when

transforming into the cubic phase. This temperatu-

re of 21 K is customarily denoted as

the

transfor-

mation temperature. However it is clear from Figu-

re 2 that the transformation is broad, with the

high temperature limit being in excess of 60 K whe-

reahas a local minimum. The second transforming

sample shows virtually identical behavior above

21 K; but the change in strain between Tc and 21 K

is only 45% of the first sample's.

3.DISCUSSION.- These experimental results yield no

evidence for negative values of

a

above Tc, unlike

the behavior of the polycrystalline samples repor-

ted bt Smith

5

&/2,3/

This demonstrated absence

of a negative

a

in non-transforming single crystals

removes the objection raised by Smith et al.to

electronic models for

explaining the thermal expan-

sion, as proposed for example by Barsch and Rogowski

171.

I

i

.

' . . . . . . A

*

.

_.

Fig. 2

Thermal expansion coefficient

for the

non-transforming (RRRk8) and transforming (RRR=60)

samples.

In transforming samples the transformation

itself dominatesabelow

60 K. At least some fea-

tures of the broad transformation are not unique

to our samples Fawcett /8/ found in a different

transforming sample that the total increase in a-

axis strain from 4.2 K to 30 K was 8.8 x

which is identical with our value for the sample

having RRR=60. Also his value

05

a

~6 x

~o-~K-'

is close to our value of

1

0

K-1. Similar agre-

ement is found for our second transforming

sample above 21 K; below that temperature we do not

see the full tetragonal development, perhaps becau-

se the sample does not form a'single domain. The

agreement for the three samples above 21 K suggests

that this broad temperature span within which a is

anomalously large may be an intrinsic property of

transforming samples.

This evidence for tetragonality at temperatu-

res extending up to at least 60 K may account for

the anomalously strong second-harmonic conversion

in ultrasonic wave propagation observed by Testardi

191 in a transforming sample at 40

K and 77 K.This

effect, forbidden for A-15 symmetry, was interpre-

ted as supporting defect models for explaining the

elastic properties of these materiales/lO,ll/.

Strain measurements yield bulk properties and

cannot reveal the internal nature of the sample.

Hence neutron diffraction studies are underway to

determine the microscopic nature of VBSi at Ts60K.

We thank R.E.Larson and E.Bucher for providing

samples and acknowledge informative discussions

with G.Barsch, J.Birman, S.Foner, T.K.Lee and

Y-Shapira.

References

/I/ Testardi,L.R. in Physical Acoustic, Mason,W.P.

and Thurston,R.N. Eds.(Academic Press.New York,

1972) vol.10, p.193

/2/ Smith, T.F., Finlayson,T.R. and Shelton,R.N.,

.I.Less-Conmon Metals

9

(1975) 21.

/3/ Smith,T.F., Fin1ay~on~T.F.

and Traft,A.,

Commun.on Phys.L (1976) 167.

/4/ White, G.K., Cryogenics

1

(1961) 151.

/5/ Larson,R.E. and Ruoff,A.L., J.Appl.Phys.44

(1973) 1021.

161 Maita,J.P. and Bucher,E., Phys.Rev.Lett.29

(1972) 931.

/7/ Barsch,G.R. and Rogowski,D.A., Mater.Res.Bul1.

8 (1973) 1459.

-

/8/ Fawcett,E., Phys.Rev.Lett.

2

(1971) 829.

/9/

Testardi,L.R., Phys.Rev.Lett.l (1973) 37.

/10/Varma,C.M., Phillips,J.C. and Chui,S.T.,

Phys.Rev.Lett.2 (1974) 1223.