Change in valence of Ce ions induced by hydrogen absorption : CeRu2

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Submitted on 1 Jan 1979

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Change in valence of Ce ions induced by hydrogen

absorption : CeRu2

G.X. Tessema, J. Peyrard, A. Nemoz, J.P. Senateur, A. Rouault, R. Fruchart

To cite this version:

G.X. Tessema, J. Peyrard, A. Nemoz, J.P. Senateur, A. Rouault, et al.. Change in valence of Ce ions

induced by hydrogen absorption : CeRu2. Journal de Physique Lettres, Edp sciences, 1979, 40 (5),

pp.105-107. �10.1051/jphyslet:01979004005010500�. �jpa-00231577�

L-105

Change

in valence of

Ce

ions induced

_by

_{hydrogen absorption : CeRu2}

_(*)

G. X.

Tessema,

J.

_Peyrard,

A. Nemoz

Centre de Recherches sur les Très Basses _{Températures,} C.N.R.S., B.P. 166X, 38042 Grenoble Cedex, France

J. P.

_Senateur,

A. Rouault and R. Fruchart

Equipe de Recherches du C.N.R.S. _(**),

Section du Génie Physique de l’ENSEGP, INPG, B.P. 46, 38042 Saint-Martin-d’Hères, France

(Re~u le 23 aofit 1978, revise le 3 janvier 1979, accepte le 17 janvier 1979)

Résumé. 2014 On a _préparé les _hydrures ternaires des _{composés supraconducteurs} de structure _{cubique phase} de

Laves, CeRu2 et _{LaRu2. On constate,} avec la conservation de la structure

_cubique,

une forte dilatation du réseau. Pour les deux _hydrures, des mesures de

_{susceptibilité}

révèlent la

_disparition

de la

_{supraconductivité}

et, dans le cas

de

_CeRu2,

_{l’apparition} d’une transition _magnétique à T ~ 1,4 K. Le

_CeRu2Hx

a _{pu être} désorbé. _Après

désorption

de _CeRu2, la _{supraconductivité} est rétablie. Les mesures de l’aimantation du _CeRu2Hx et la valeur de sa constante

de _{réseau, comparée} à celle du _LaRu2Hx, semblent

_indiquer

_qu’avec

_{l’hydrogénation,}

le cérium a _changé de

valence pour devenir trivalent. Abstract. 2014

Ternary hydrides of

_{superconducting}

cubic _{Laves-phases CeRu2} and LaRu2 have been _prepared. The cubic structure is _preserved and a considerable lattice _expansion is observed. _Magnetic

_{susceptibility}

mea-surements show the

_{disappearance}

of the

_{superconducting}

transition for both

_hydrides,

and in the case of _CeRu2, the appearance of a

_{susceptibility}

_peak at T ~ 1.4 K. For _{CeRu2 hydride,} the _hydrogen could be removed and

superconductivity reestablished. _{Magnetization} measurements of

_CeRu2

_hydride and _comparison of its lattice parameter with that of

_LaRu2

_hydride indicate that, with _{hydrogenation,} the valence of Ce has changed from 4 to 3.

LE JOURNAL DL PHYSIQUE - LETTRES _TOME

40, 1 er MARS 1979,

Classification

Physics Abstracts

74. 70L - 75 . 30K - 75 . 50C

Rare-earth intermetallic

_compounds

are well-known

to absorb

_{large quantities}

of

_hydrogen.

Such

_hydrides

are of _great interest for

_{technological}

reasons

(for

example

energy

storage).

From a more fundamental

point

of

view,

controlled introduction of

_hydrogen

into these

_compounds

is a

_good

tool for modification of the structure of the material and hence, some

physical properties

like

_{superconductivity}

and magne-tism.

We have studied the cubic

_{Laves-phase compounds}

CeRu2

and

_LaRu2.

From a

comparison

of its lattice

parameters with those of the trivalent

_RE-Ru2

it

appears that cerium is tetravalent in

CeRu2

[1].

Both

Ce and La have no localized 4f electrons in these

compounds.

Furthermore,

these two

_compounds

are

superconducting

and are

_reported

to absorb

_large

quantities

of

_{hydrogen [2].}

The

_samples

were

_prepared

_by

induction

melting.

The

_CeRu2

was sealed in a tantalum container and

annealed at 1 100 ~C for 10

_days,

and at 1 300 OC I

for few hours. The

_LaRu2

was annealed at 1 150 °C

for 3 hours in the induction furnace.

Different values of the

_{superconducting}

transition

temperature

(7~)

have been

_published

for these

compounds.

The

_7~

varies between 4.9 K and 6.2 K

for

_CeRu2

and between 3.08 K and 4.4 K for.

LaRu2 [3-4].

We have obtained a

_Tc

of 5.8 K for

CeRu2

and 3.6 K for

_LaRu2.

A careful

_{investigation}

in the case of

_CeRu2

has revealed that this lower value

is due to contamination

_by

tantalum. We are uncertain as to the reason for

_LaRu2.

, For

hydrogenation

small

pieces

of the

sample

were

_placed

in a stainless steel reactor connected to a vacuum line. After each

_{hydrogenation}

the structure

of the

_compounds

was checked

_{by X-ray}

_analysis.

Attempts

to

_hydrogenate

from 200 to 700 ~C with

pressures above 1 atm., have all led to the dissociation of the

_starting

materials. Lines associated with Ru

could be identified in the

_X-ray

_patterns of both

compounds.

However,

hydrogenation

at room

tempe-rature has been conducted

_{successfully.}

The

_sample

is

L-106 JOURNAL DE PHYSIQUE - LETTRES

activated

_by

_going

several times

_through

_pressure

(10 bars)-vacuum

cycles.

Then

_hydrogen

_pressure

(10 bars)

is introduced and the

_sample

is left for four

days.

A second method which consists in

_using

_higher

pressure

(110 bars)

and

_letting

the

_sample

absorb for

one

_day

has led to the same results

_concerning

the data

presented

in this _{paper. The}

_samples

are broken into

tiny

particles

by

hydrogenation.

X-ray

diffraction measurements have been carried

out at room _temperature, in a vacuum chamber. For

both

_{hydrogenated compounds,}

the

_X-ray

_diagrams

show that the cubic

_Laves-phase

structure is

_preserved

upon the formation of the ternary

hydrides

CeRu2Hx

and

_LaRu2Hx,

where x ~

_4,

as found

_by

_preliminary

volumetric measurements. A lattice

expansion

and a

significant

line

_broadening

is observed.

As can be seen from table I the relative increase of

the lattice

_parameter

_Aa/a

of

_CeRu2Hx

is

_larger

than

that of the

_hydride

_LaRu2Hx.

It is

_important

to

notice that the lattice _parameters for the two

_hydrides

are much nearer the same value than those of the

starting

materials. The _present

_experiments

do not

allow us to determine the location of the

_hydrogens.

Table I.

ao : lattice _parameter of the _starting material.

a : lattice _parameter of the _hydride.

t1.a/a : relative _expansion of the lattice.

Desorption experiments

under vacuum have been

attempted

for both

_hydrides.

The

_sample

is introduced into a _quartz tube and heated at a fixed _temperature for more than 3 hours. In the case of

CeRu2 Hx

no

change

in the

_X-ray

_diagrams

is observed for

tempe-ratures below 150 °C.

_Desorption

starts at 150 °C with the

_X-ray

_{diagram showing}

lines both of the initial

_compound

and the

_hydride.

When

_increasing

the

_desorption

_temperature we observe that the rela-tive

_intensity

of the two series of lines

_changes

in favour of

_CeRu2.

After

_desorption

at 200 °C

_only

the lines of

_CeRu2

were seen.

During

this

evolution,

the values of the lattice

_parameters

of both

_phases,

respectively

ao and a, do not

_change.

The width of the lines of

_CeRu2

are as

_sharp

as those of the

_starting

material.

For

_LaRu2Hx,

after a

desorption experiment

at

150 °C,

the

X-ray

patterns

give

evidence of Ru as well as the

_hydride.

Thus,

desorption

of

_LaRu2Hx

was not successful.

The

_temperature

_dependence

of the AC

₍₂₀

_Hz)

susceptibility

is measured

_by

a sensitive mutual inductance

_bridge.

_{Hydrogenation}

leads to a sudden

disappearance

of the

_{superconducting}

transition as

Fig. 1. -

Temperature dependence of the AC susceptibility. a : Hydride of _CeRu2, obtained by hydrogenation, at 10 bars during

4 _{days. b :} Desorbed hydride of _CeRu2 at 200 °C. c : Desorbed

hydride of CeRu2 at 200 ~C, followed by thermal treatment at

700 ~C. ’

shown in

_figure

1

(in

figure

la the small

superconduct-ing

transition at 7" ~ 5.8 K is

_probably

a contribution of

_{non-hydrogenated grains)}

and to a _clear

parama-gnetic

susceptibility.

Furthermore,

a

_peak

of

suscep-tibility

is observed at T ~ 1.4 K

_indicating

the onset

of

_{magnetic ordering.}

_Desorption

at 200 °C reduces the

_paramagnetic

_part of

_{susceptibility,}

and no

_peak

is observed down to 1.3 K

_(Fig.

_lb).

To reestablish the

superconducting

transition,

a thermal treatment at

700 °C is needed.

The

_7c

defined at the

_mid-point

of the transition is

the same as that of the

_starting

material,

i.e.

7~5.8

K

(Fig.

Ic)

but the transition is broader. However at

4.5 K almost

_{15 %}

of the

_compound

is still normal and the transition is

_completed

_only

at 2.5 K. The

reason for this is not known. For the

_hydride

of

LaRu2,

the

_{disappearance}

of the

_{superconducting}

transition is not associated with _any

_special

feature

in the

_{susceptibility,}

within the

_temperature

range

explored.

In order to

_investigate

the

_paramagnetic

suscepti-bility

of the

_CeRu2Hx,

_{magnetization}

measurements in a field _up to 80 kOe have been conducted in the

temperature

range 1.4-4.2 K.

Figure

2 shows the

magnetization

versus the

applied

field at 1.47 K.

In contrast to the Pauli like

_{paramagnetism}

of

CeRu2 [4],

the

_{hydrogenated compound}

shows a Brillouin like

_{magnetization,}

which must be due to

trivalent cerium ions.

By assuming

that all cerium ions

L-107 VALENCE CHANGE BY HYDROGEN ABSORPTION

Fig. 2. -

Magnetization measurements of the hydride of CeRu2 at 1.47 K. Insert shows the _{reciprocal susceptibility} versus

tem-perature.

the

_magnetic

moment _per cerium atom from the

saturation

_{magnetization.}

The calculated

value,

0.65 ± _{0.03 J.1B’} is lower than the moment of

_{2.14 ~}

for free Ce3+ ion. This is

_possibly

due to the fact that the

_ground

state of trivalent cerium in the

_hydride

is the

_{r 7 as}

found in cubic

_{CeAl2 [5]}

and

_{Ce.,Lal -.,Th}

_[6].

In the

_high

field

_region,

we observe a Van Vleck

type

susceptibility

which may be due to _{the presence}

of

_crystalline

field effects.

From the curve

_1/~

versus T

plotted

in the insert

of

_figure

2 we have deduced a Curie constant which is

_slightly

lower than that of a

_{r 7 ground}

state. This curve seems also to confirm the appearance of magne-tic

_ordering

at about 1 K. The nature of this

_ordering

is _yet to be

_{investigated.}

In

conclusion,

the

_{disappearance}

of

superconducti-vity,

the appearance of the

_paramagnetic

suscepti-bility,

with a saturation

_{magnetization}

of 0.65 f1B

per Ce

ion,

and the very close values of the lattice

parameters of

_CeRu2Hx

and

_LaRu2Hx

lead one to

conclude that

_hydrogen

_absorption

has induced an almost

_perfect

_change

of valence of cerium from tetravalent into trivalent ion. Other

_experiments

are

in progress to test the

_validity

of this

_{interpretation.}

Acknowledgments.

- The authors wish to thank J. Palleau and B. Tissier for the

_experimental

assistance

and 0.

Bethoux,

G. Chouteau and B. Cornut for numerous

_helpful

discussions.

References

[1] GSCHNEIDNER, K. A., Rare-Earth Alloys (D. Van Nostrand

Company, Inc.) 1961, p. 380.

CORNUT, B., Thèse Université de Grenoble (1976).

[2] SHALTIEL, D., JACOB, J. and DAVIDOV, D., J. Less-Common Metals 53 (1977) 117.

[3] HILLENBRAND, B. and _{WILHELM, M., Phys.} Lett. 33A (1970)

61.

LAWSON, A. C., ENGEL, U. and BABERSCHKE, K., Phys. Lett. 49A

(1974) 373.

[4] ENGEL, U., Thesis Berlin (1974).

[5] BARBARA, B., BOUGHERLE, J. _{X., BUEVOZ,} J. L., ROSSIGNOL,

M. F. and _{SCHWEIZER, J.,} Solid State Commun. 24 ₍₁₉₇₇₎ 481.

[6] PEÑA, O., TOURNIER, R., BENOIT, A. and FLOUQUET, J., Solid State Commun. 21 ₍₁₉₇₇₎ 971, 974.