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Short Notes phys. stat. sol. (a) - Kz3 (1983) 77, Subject classification: 14.2; 1.3; 21.6 Institute of Solid State Physics, Academy of Sciences of the USSR, Chernogolovka1) Superconductivity of Pd- Au-H Solid Solutions BY V.E. ANTONOV, T.E. ANTONOVA, I.T. BELASH, E.G. PONYATOVSKII, and V.I. RASHUPKIN

Kz3

It was revealed in /1/ that hydrogen implantation into alloys of palladium with noble metals (copper, silver, and gold) can convert them to superconductors whose superconducting transition temperature, Tk, reaches much higher values than that of Pd-H solutions. The discovery of this effect has stimulated to a considerable extent the extensive studies of the last few years on the superconducting properties of hydrogen solutions in the palladium alloys. A great number of works has been conducted in order to analyse what lies at the bottom of the effect (see review /2/). Further experiments have shown, however, that at least in the case of the Pd-Cu-H and Pd-Ag-H solutions with f. c. c. metal sublattice the existence of superconductivity with anomalously high Tk values is not an equilibrium property of massive, homogeneous samples /3/. The technique for hydrogen com-

pression to high pressures enabled us to saturate the Pd60Cu40 and PdsoAgZo alloys (having nearly optimum compositions for achieving the highest Tk values on hydrogen implantation /l/) with hydrogen up to concentrations also approximating the optimum ones according to /I/. It turned out that at H-tometal atomic ratio n

5

1 the Tk(n) dependence for the PdsOAgzo-H solutions is

close to that for the Pd-H solutions, and in the Pd60Cu40-H solutions superconductivity is absent at nz0.6 and T

22

K.

The present note is devoted to the investigation of hydrogen solutions in the Pd-Au alloys containing 3, 9, 22.5, 50, and 75 at% Au. The ingots were melted from Pd (99.99%) and Au (99.999%) in an induction furnace in vacuum. After a 10 h homogenization in vacuum at 1000 C and water-quenching these ingots were rolled into strips 0.15 mm thick, then subjected to stress-relief annealing in vacuum at 1000 OC for 5 min and again quenched in water. The
1 ) 142432 Chernogolovka, Moscow District, USSR.
0


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physica status solidi (a) 77

specimens in the form of discs of 5 mm diameter were cut out of these strips. Hydrogenation of Pd-Au samples was conducted by exposure under hydrogen pressures up to 8 GPa. Hydrogen compression was performed by the method suggested in /4/ (a detailed description of the method is given in /5/). The pressure and the temperature were measured accurate to 20.3 GPa and +I0 C, respectively. After the hydrogenation was completed, the high-pressure chamber was rapidly cooled down to -120 C, then the pressure was
0 0

lowered and the samples were taken out of the chamber and placed for storage into liquid nitrogen to prevent hydrogen losses. At atmospheric pressure a pronounced release of hydrogen 'from specimens began at T %

-

50 OC. The

hydrogen concentration was measured to an accuracy of 5%, the technique is described in details in /6/. The Tk values were determined by the induction method at T 2 2 K. An X-ray study was carried out by a phototechnique at T = -190 OC using a DRON-2.0 diffractometer with FeK, radiation. Under normal conditions Pd and Au form continuous disordered solid solutions with the f. c. c. lattice /'7/. According to /8/, the gold alloying of palladium results in a rapid decrease in the critical temperature of the separation of the Pd-H solutions into the two isomorphous phases, poor and rich in hydrogen, and at T

25 OC hydrogen should form continuous interstitial solutions (based on the f. C.C. metal sublattice) with the Pd-Au alloys containing 2 17 at%,Au. Our recent experiments, however, have shown that at high pressure some new phase transitions followed by a diffusional redistribution of atoms of the metallic matrix may take place in the Pd-Me-H systems (dissolution into the poor and rich in palladium phases in the Pd-Ni-H /9/ and Pd-Pt-H /1 O/ systems; ordering in the Pd60Cu40-H system /6/), the temperature, T*, above which these transitions occur with a detectable rate, being approximately equal for all the systems in hand running as high as

2

rl

and

r2,

*

(probably, orthorhombic) structure at P

250 OC. Besides, gold was found to form a hydridewith rather complicated 2 2.8 GPa and T 5 300 OC /11/.
H2 To examine whether analogous phenomena occur in the Pd-Au-H system,

we have studied two series of samples. The samples of the first series were exposed at P exposed initia y at 350

X-ray study has shown the samples of both series to be single-phase and to have near values of the parameters, a, of their f.c.c. metal sublattices, see

3

= 6.5 GPa and
OC

T = 200 OC < T* for 24 h. The second ones were

for 24 h and then at 200 OC for another 24 h. The


Short Notes

K25
Fig. 1. Parameters a of the f. c. c. metal lattice at -1 90 OC and atmospheric pressure for the initial Pd-Au alloys ( + ) and for the Pd-Au-H samples = 6.5 GPa: ( 0 ) 200 OC for 24 exposed at P (
(0, 0 )
OC for 24 h; the same samples after partial hydrogen release at normal conditions

) 350 OC for 24 h and then 200

HZ

395

20 Lo

Au(of%)-

ijo

*O

loo Fig. 1. The hydrogen concentrations of these

samples proved to be also the same within the experimental error (the n-values are listed in Fig. 1).

As is seen from Fig. 1, the samples of both series exhibited close values of

a and n as well after partial losses of hydrogen through several days exposure at normal conditions (a noticeable release of hydrogen from the samples ceased in several hours; the diffraction pattern showed relatively narrow lines that evidenced a homogeneous hydrogen distribution over the samples volume, though its concentration of the samples was not thermodynamically equilibrium for normal conditions, compare with /8/). These results allowed one to conclude that no irreversible phase transitions (analogous to those observed in some other Pd-Me-H system /6, 9, 10/ and Au-H system /ll/) took place at P under study.
=

H2

6.5 GPa and T 5 350 OC in the Pd-Au-H system

One more fact corroborating this conclusion is worth presenting. It has been shown in /12/ that at n

5 0.7 the dependences AVO(") of the increment

of the unit-cell volume are very close for all the investigated hydrogen solutions
in the f. c. c. disordered palladium-based alloys. The dependence AVO(") obtained in /12/ is plotted in Fig. 2 by a broken line. To extend the dependence

m"Q

9

I':;.i

6

-4
0,

2
0

A

02 0.4

06

08

10

n-

Fig. 2. Increase in the volume of the unit cells, AVO, versus hydrogen concentration, n, of the Pd-Au alloys containingA 0, 0 3;V 9, 22.5, 0 50 at% Au. Half-blackened symbols stand for the same samples but after partial release of hydrcgen under normal conditions. The broken line - see the text


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physica status solidi (a) 77

Tk, as a function of hydrogen concentration, n,

Fig. 3. Superconducting transition temperature,

of the Pd-Au alloys with A 0, 0 3, V 9, 0 22.5 at% Au. Symbols with arrows stand for the samples possessing no superconductivity at T 2 2 K. The broken line shows the Tk(n) dependence for the Pd-H solutions /2/
'05 06

07

08

09 n-

10

for n

2

0.6 we used the data of /13/ for the Pd-H

"a, solutions. As is seen

from Fig. 2, the values AVO(") for the samples of both series synthesized in the present work agree well with this dependence. Thus, one can ascertain that only ordinary r-solutions on the base of the starting disordered Pd-Au alloys

are formed in the Pd-Au-H system at high pressure and T 5 350 OC.
The samples of hydrogen solutions in alloys containing 2 22.5 at%Au obtained in the present work were revealed to possess no superconductivity at

2 K. To get a more comprehensive information on the T (n) dependences k for the Pd Au -H and Pd Au -H solutions we have prepared additional 97 3 91 9 5 8 GPa and 200 5 T 350 OC for 24 h. The width samples by exposure to P
T

of the steps observed on the temperature dependences of the signal of disbalance of an ac bridge in the ranges of sample transition to the superconducting state did not exceed 0.15 K which was indicative of a homogeneous hydrogen distribution over the samples volume. The Tk values were estimated from the step midpoint locations. The results of measurements are depicted in Fig. 3. As is seen from the figure, the Tk(n) dependences for the Pd and Pd /3/, Au -H 97 3

Hz

Au H solutions are close to that for the Pd-H solutions. 91 9Thus, the gold alloying of palladium, as well as copper and silver alloying
does not result in a distinct increase in Tk of the Pd-H solutions in the

case of the sample hydrogenation under conditions close to thermodynamical equilibrium ones. As to the anomalously high Tk values observed in /l/, their appearance seems to be attributable to the specific properties of the metastable state of the thin (a1500

8)

hydrogen-bearing layer obtained during hydrogen

implantation at a low temperature. References /1/ B. STRITZKEFt, Z. Phys.

268,

261 (1974).

/2/ B. STRITZKER and H. WUHL, in: Topics in Applied Physics, Vol. 29, Ed. G. ALEFELD and J. VOLKL, Springer-Verlag, 1978 (p. 243).


Short Notes /3/ V.E. ANTONOV, I.T. BELASH, E.G. PONYATOVSKII, and V.I. 31, RASHUPKIN, Zh. eksper. teor. Fiz., Pisma - 451 (1980). /4/ I.T. BELASH and E.G. PONYATOVSKII, Patent No. 741105 (USSR).

K27

/5/ E.G. PONYATOVSKII, V.E. ANTONOV, and I.T. BELASH, Proc. 8th AIRAPT/19th EHPRG Conf., 17 to 22 August 1981, Vol. 1, Uppsala (Sweden) (p. 80). /6/ V.F. DEGTYAREVA, V.E. ANTONOV, I.T. BELASH, and E.G. PONYA66, TOVSKII, phys. stat. sol. (a) - 77 (1981). /7/ M. HANSEN and K. ANDERKO, Constitution of Binary Alloys, McGrawHill Publ. Co., Inc., New York/Toronto/London 1958. /8/ M. MAELAND and T.B. FLANAGAN, J. phys. Chem. - 3575 (1965). 69, /9/V.E. ANTONOV, T.E. ANTONOVA, I.T. BELASH, E.G. PONYATOVSKII, V.I. RASHUPKIN, and V.G. THIESSEN, phys. stat. sol. (a)- 71 77, (1983).

/lo/ V.E. ANTONOV, T.E. ANTONOVA, I.T. BELASH, E.G. PONYATOVSKII, and V.I. RASHUPKIN, phys. stat. sol.(a) 17, No. 2 (1983). -

/ll/V.E.

ANTONOV, T.E. ANTONOVA, I.T. BELASH, A.E. GORODETSKII,

and E.G. PONYATOVSKII, Dokl. Akad. Nauk SSSRE, 376 (1982). /12/ B. BARANOWSKI, S, MAYCHRZAK, and T.B. FLANAGAN, J. Phys. F 1, 258 (19n). /13/ J.E. SCHIRBER and B. MOROSIN, Phys. Rev. B - 117 (1975). 12, (Received February 10, 1983)