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Short Notes phys. stat. sol. (b) 142,K155 (1987) Subject classification: 74.10;Sl Institute of Sol'd State Physics Academy of Sciences of the USSR, Chernogolovka

K155

4

A Study on Superconductivities of High-pressure Phases Ru-H, Rh-H. and Ni-H Systems in the Re-H, BY V.E. ANTONOV, I.T. BELASH, O.V. ZHARIKOV. and A. V. PALNICHENKO A technique for compressing gaseous hydrogen to high pressures developed at the Institute of Solid State Physics of the Academy of Sciences of the USSR enabled the synthesis of solid Me-H solutions on the base of a number of VI to VIII group transition metals and their alloys with hydrogen concentrations exceeding those achieved earlier /I/. Many of the phases obtained are of interest as related to their superconducting properties. Under atmospheric pressure. all these phases are stable relative to decomposition into the metal and molecular hydrogen at liquid nitrogen temperature, but are unstable at room temperature. So, when the samples are placed into a conventional apparatus for measuring superconducting properties, an essential loss of hydrogen is observed. Earlier we studied superconductivities of the high-pressure Me-H phases in a specially designed installation with pumping4 out the He vapour at T 2 1.2 to 2 K. At present we have built an installation 3 with He vapour pumping-out, which permits loading the samples without their heating above the liquid nitrogen temperature and which enables their examination at temperatures down to 0.3 K. This note presents the results obtained on this installation on measuring the superconducting temperature in the Me-H phases on the base of Re, Ru, Rh, and Ni metals. 3. The samples were in the form of plates of about 3x3x0.2 mm in size. Hydrogena

tion on the samples was conducted by a 24 h exposure in an atmosphere of molecular hvdrogen at fixed magnitudes of temperature and pressure with subsequent "quenching" down to *-I50 to an accuracy of

%

by the method described in /l/. The pressure was determined

2 3 %,

the temperature was kept constant to within: 10K. The condi-

tions of hydrogenation provided saturation of the samples with hydrogen up to concen-

1) 142432 Chernogolovka, Moscow district. USSR.


K156

physica status solidi (b) 142
I

I

I

I

I

I

I

I

I

02

0.1

0

2

6

8

70

'
0. 7
Fig.2

F/g.I

0.2 n--t

Fig. 1. Hydrogen content, n, of the Re-H samples versus pressure, P of H2' their synthesis at T = 350 OC Fig. 2. Dependence of the superconducting temperature, Tc, on the hydrogen concentration, n, for the Re-H solid solutions

trations close to thermodynamic equilibrium ones at chosen values of T and P , and H2 7 the conditions of "quenching" ruled out possible losses of hvdrogen bv the samples when the pressure was decreased to the atmospheric one. Thevalues of
the superconducting temperature, Tc, in the samples obtained were estimated

from the step midpoints on the temperature dependence of their magnetic susceptibility, determined by the inductance method. An X-ray study of the samples was carried out by a phototechnique at T = 100 K using a DRON-2.0 diffractometer with CuKU radiation. The hydrogen content of the samples was determined from the amount of H2 liberated in their thermal decomposition in vacuum at temperatures up to 500 OC. the method was described in /2/; the error was dn

*

0.01 at n

0.1 and increased to d n

0.03 at n

*

1, where n is the

H-to-metal atomic ratio. Re-H

R300/R4.

*

A single crystal of rhenium metal with the electric resistance ratio
100 and Tc = 1.70 K was used as the starting material. The sam-

ples were hydrogenated at 350 OC and hydrogen pressures up to 9 GPa. It has


Short Notes

K157
7-

1
3

I
0-023

n-0
-

Fig. 3. Temperature dependences of the magnetic susceptibility, x , in the range of transition to the superconducting state for the initial Re samples (n = 0) and a Re-H solid solution with n = 0.23

0

I

-

been shown earlier /2/ that at T

W

170 to 250

OC

the hydrogen solubility in rhe-

nium increases monotonically with pressure reaching n 0.22 at PH = 9 GPa, 2 an increase in the hydrogen concentration of rhenium being accompanied by an approximately linear increase in the parameters of the initial h. c. p, metal lattice
=
,

The pressure dependence of the hydrogen solubility in rhenium metal at T =

350 OC obtained in the present work is plotted in Fig. 1. The concentration

of the Re-H solid solutions can be seen to increase monotonically with pressure

0.23, at P = 9 GPa. An X-ray examination H has shown that under atmospheric pressure an% T = 100 K the concentration dependences of the rhenium sublattice parameters in the Re-H samples obtained are close to those found in /2/. Incorporation of hydrogen into rhenium metal results in an approximately linear decrease in the Tc-values with the slope dTc/dn = -(5. Of 0.2)K/H atom (Fig. 2). the temperature interval of the superconducting transition remaining sufficiently narrow (Fig. 3), which points to a relatively homogeneous distribution of the hydrogen over the metal volume. After removal of the hydrogen from the Re-H samples by annealing in vacuum at 500 OC, the magnitude of the superconducting temperature returns tathe initial one, Tc
C

up to nearly the same values, n

=

1-70 K. The absence of

irreversible changes in the T value of rhenium after the hydrogenation-dehydrogenation cycle allows one to assume that the Tc values of the Re-H solutions under study are unambiguously related to their concentration. Ru-H The samples were cut from a single crystal obtained by electronbeam melting of ruthenium powder (99. 96 %) and had T = 0.495 K. Ruthenium C metal, as well as rhenium metal, has an h. c. p. crystal lattice, At 250 OC the solubility of hydrogen in ruthenium increases monotonically with pressure reaching n
%

0.03 at P H2

=

9 GPa /1, 3/.


K158

physica status solidi (b) 142

The Ru-H sample synthesized in the present work at T = 350 OC and PH = 2 = 9 GPa had the composition n = 0.035 0.01 and T = 0.455 K. The superconducting temperature of the sample annealed in vacuum at 500 OC increased to
C

its initial magnitude, 0.495 K. One may thus state that incorporation of hydrogen into 'the ruthenium metal leads to a decrease in its superconducting temperature with the initial slope dTc/dn of the order of -1 K/H atom. Rh-H Polycrystalline rhodium (99.98 %) was used. Hydrogenation of the
= 7 GPa. The hydride thus obtained sample was conducted at T = 350 OC and P H2 t had the composition n = 1. 02 0. 03 and f. c. c. metal sublattice with a a(0.4024 -

+ -

0.0002) nm under atmospheric pressure and T = 100 K, which is in agreement Rhodium metal is a superconductor with Tc = 3.25~10-~K /4/. Our measure-

with the data of /1, 3/. ments have shown the samples of the initial rhodium and its hydride to possess no superconductivity at T 2 0.3 K. The absence of superconductivity in the rhodium hydride is in line with the results of a quantum-mechanical calculation /5/. -3 Ni-H Polycrystalline nickel containing about 10 % Fe and less than

of Cr, Mn, Co, Cu, Mg. and A1 was used. A hydride with the composition n = = 1,06 0.03 was synthesized at T = 250 OC and P = 6.5 GPa. The hydride + H2 had an f. c. c, metal sublattice with a = (0.3737 - 0.0002) nm at atmospheric pressure and T = 100 K, which is in agreement with the data of /I/ for a nickel hydride with n = 1. 06. It has been shown earlier that under atmospheric pressure nickel hydrides with 0.7 5 n 5 1,18 are paramagnetic at T

2

4.2 K and that the NiH1. 18 hydride

sample examined in the 1.06 present work did not undergo a transition to a superconducting state at temperatures down to 0.3 K. The absence of superconductivity in the Ni hydride has been predicted theoretically/6/. The authors are grateful to N. S. Sidorov and N. A. Tulina for the high purity nickel and rhenium samples. References

is not superconducting at T 5 1.2 K /l/, The NiH

/1/ E. G. PONYATOVSKII, V. E. ANTONOV, and I. T. BELASH, in: Problems
in Solid State Physics, Ed, A.M. PROKHOROV and A. S. PROKHOROV, Izd. Mir, Moscow 1984 (p. 109)(in Russian).


Short Notes KII, and N.A. TULINA, Dokl. Akad. Nauk SSSR - 617 (1983). 269,

K159

/2/ V. E. ANTONOV, I. T. BELASH, V. YU. MALYSHEV, E. G. PONYATOVS/3/ V. E. ANTONOV, I. T. BELASH, V. W. MALYSHEV, and E. G. PONYATOVSKII, Platinum Metals Rev. - 158 (1984); Internat. J. Hydrogen 28, Energy - 193 (1986). 11,

/4/ CH. BUCHAL, F. POBELL, R.M. MUELLER, M. KUBOTA, and J.R.
OWERS-BRADLEY, Phys. Rev. Letters - 64 (1983). 50,
/5/ D. A.

PAPACONSTANTOPOULOS, E. N. ECONOMOU, B.M. KLEIN, and

L.L. BOYER, Phys. Rev. B - 17'7 (1979). 20.

/6/N.I.

91, KULIKOV, phys. stat. sol. (b) - 753 (1979).
(Received May 27, 1987)