Progress in the T-X Phase Diagram of the Solid Solution CeH2 — CeH3

© by

Oldenbourg Verlag,

-0044-3336/89

Progress

in the T-X Phase

_Diagram

of the Solid

Solution

_CeH2

-

CeH3

Part II: Neutron

Diffraction

_{Investigations. Trihydrides*}

K.Conder,

J.Schefer1,

E.Kaldis,

Cai

Ru-Xiu2

.

Laboratorium für

_{Festkörperphysik}

_,

8093

_Zürich,

Switzerland 1_{Paul Scherrer}

Institute,

PSI

_Ost,

CH-5232

_Villigen,

Switzerland 2 _{On leave}

ofabsence from

_University

of

_Wuhan,

China.

In this _paper we

_present

first neutron diffraction

_{investigations}

on solid solutions

CeDg—CeDg

prepared

in our

laboratory.

Several new

_{superstructures}

have been

found and the

_highlight

of this

_{investigations}

is the existence of a non—cubic

trihydrides

_(CeD^,

_CeH^)

which was

proved

both

by X—ray

and neutron

diffractions.

Introduction

Aim of this

_{investigation}

was to _support the

phase

diagrams

studies of the

system

_CeDg—CeD^

presented

in the Part I of this work

_using

neutron diffraction

so that information both from the deuterium and the metal sublattice structures

could be

_gathered.

The results have shown the existence of several new

superstructures

for someof which_structuremodelsare

_proposed.

For

_La,

Ce and Pr both

_dihydrides

and

_trihydrides

are

thought

to

crystallize

in

the same fee

CaFg

structure.

_However,

this structural characterization of the

trihydrides

was made

long

time _ago in

samples

with an

appreciable

deviation

(>5%)

of the

_stochiometry

and not

_always

the

_highest

_purity.

In this paper we

present

for the first timesomeevidence that the

CeH^

(the

same is truefor

Lall^)

deviates from the cubic

symmetry,

if it is _pure and

_{stoichiometric,}

similar to the heavier

_{RE—Hydrides}

whichare

hexagonal.

Experimental

Some details about the

_synthesis

of the deutendes and

_hydrides

have been

_reported

before

_[1,2]

and in the Part I of this work.The deuterium for the

_synthesis

₍

AGA Zürich

-purity

99.8%:

_N2<60

ppm,

~2 <40

ppm,

O2<20

ppm and

H^O.2%

)

was

additionaly purified by passing through

a

palladium

diffusion unit at

340°C.

Both

_trihydrides

and trideuterides were

synthesized

at

500°C.

At this _temperature and under

_H~2

_pressure of 6 bar the

_composition

_CeD2

_^

_(or

_CeH2

_^)

is reached.

Cooling

(5°C/h)

under constant pressure of 6 bar to room

_temperature

leads to

higher hydrogen

contents.

Two methodswereused to

_investigate

the

phase

relationships:

—

X—ray powder

diffraction

_{investigations}

at room

_temperature

using

a

Debye—Scherrer

camera

(

for

trihydrides

and

trideuterides).

—

and for theseven deuterated

samples

available up to now, neutron diffraction in

the range 10<T<350 K. A double axis multicounterdiffractometer

_(DMC)

wasused

at the 10 MW SAPHIR reactorof

PSI/Würenlingen,

witha

focusing

Ge^

j

single

crystal

monochromator

_{(A=0.1708 nm).}

Results and Discussion

First Neutron Diffraction

_{Investigations}

These were

performed

for the

compositions

CeD2

23(293

K),

CeD2 gg(293

and 10

K),

_CeD2

₆₄₍₁₅₃

and7

_K)

and

_CeD2

₇₅₍₂₉₃

and 213

_K).

Further

_{investigations}

in

the trideuteride

_region

are

_reported

in thenext section.

_Figure

1 showsanoverview of the_spectraofsomeof these

_{compositions.}

CeD2.23

This

_composition

shows at room

_temperature

a

_tetragonal

structure in

_agreement

with

_Fig.6

ofPart I

_[7].

This structureis similar to that found

by

Scheferet al

_[3]

for a

sample

CeD2

_2Q at T=4.2 K. Rietveld refinement

_using

the

_tetragonal

space

group 14

gives

rather

satisfactory

error factors

Rj=0.059

and

Rwp=0.114

and

lattice constants a=b=0.5547 nm and c=l. 1121 nm.

Figure

2 shows the observed

intensity

as well as the

negligible

difference between observed and calculated

CeDx

Fm3m

2-Theta [·]

Fig.

1

_Change

ofthe neutron

intensity

(logaritmic)

as afunction of the 20

angle

for

various stoichiometries and

temperatures.

The calculated reflections for the

tetragonal

body

centered structureand the cubic Fm3m space group are shown in

intensities. An open

question, however,

remains the

_application

of the structure

model

₍₁₄₎

whose

_{fully occupied}

lattice

_corresponds

to

_CeDg

7y for the

composition

of

only

_CeD2

_23· Further structure refinements should

_give

the

answer. vv y w w _yywvwvwvwwv^pvw wwvw wv vywwwv obs cale difi _LL 2-Thela

Fig.2

Neutron diffraction

_spectra

of

_CeD2

₂₃ at room

_temperature

(T=293

K).

Tetragonal

space group 14 a=b=0.5547 nm and c=_{1.1121 nm;}

agreement

factors:

integrated

RT=0.059,

weighted profile

_R^^O.114.

At room

temperature

thestructureis cubic

(Fm3m),

in

_agreement

with our

X—ray

data

_(see

Part I

_[7]).

At 10 a

_{superstructure}

is present.

Indexing

is on the way now.

At 150 a

superstructure

appears, i.e. at the

temperature

where the

_X-ray

data

[7]

cleary

show

only

the cubic structure. The conclusion is that the cerium

sublattice of the

superstructure

must have cubic face centred

_symmetry

and that the

superstructure

appears duetoshifts at thedeuterium atoms. At 15 thesame

(220) (200) (311) (331) (412) (511) (531) (222) (400) ₍₄₂₀₎

jL

(333) (440) _l_1_1— tT 1500

i

«

1

^ 1000 5 500

a

₃ 10 20 30 >t0 50 60 70 80 90 100 110 120 1 30 10 20 30 "»0 50 60 70 80 90 100 110 120 130 2 -Theta (°)

Fig.3

a)

Neutron diffraction

_spectra

of the

_CeD2

75 at room

temperature.

The

cubic_symmetry Fm3m is

_cleary

shown

_by

the

_indexing.

b)

At

_higher

_resolution,

no

_impurity

lines_{appear in} the

_background.

with a=b=0.8719 nm and c=0.5514 nm.

However,

a

larger

unit cell

_might

be

necessary in order tohavea deuterium_occupancyin

_agreement

with the measured

stoichiometry.

CeD275

Figure

3a shows the pure Fm3m cubic structure at room

_temperature,

resulting

from the statistical distribution of deuterium on _{tetrahedral and octahedral sites}

displaced

along

the <111> directions from the ideal

_positions.

The

_spectra

show

clearly

that no contamination is

_present

in the

_samples

_{(Fig. 3).}

_{At T=213} a

structure with a clear

tetragonal splitting

is

existing

(Fig.4).

This structure

_is,

however,

different from the

_tetragonal

one atx=2.23.

_Indexing

is on the way now.

It _seems,

_therefore,

that the cerium sublattice iscubic_upto 2.05

_{(in x),}

tetragonal

up to

2.6,

cubic_up to

2.65,

tetragonal

up to2.85and then

again

cubic.

In

_general,

the neutron diffraction data are in

good

_agreement

with the

_X-ray

data of

_Fig.6

_presented

in Part I of this work

_[7].

i

£] loooo •

1

g 5000 fi a e o

a

Fig.4

Neutron diffraction

_spectra

of the

_CeDg

_phase

at T=216 K.

_Clearly

_(in

agreement

with

_Fig.6

of Part I

_[7])

the structure is

_tetragonal,

as the

splitting

shows

_{(arrow pairs).}

Noncubic Structure of

_{CeHg_<c}

Most

unexpected

was the

discovery

_{by X—rays}

[4]

that the

_trihydrides

and trideuterides of cerium

_{(CeH^ g^—CeH^ gg)}

are not cubic. We _mean, of course

samples

whose

_hydrogen

content has been

_exactly

_(±0.005)

determined

by

the

volumetric method

_[5]

and which have been

_synthesized

and handled under

conditions of very

_high

purity.

_Figure

5 shows the neutron diffraction

spectra

of

CeD2

g7o±o 005 at ^ an(^

^

^' ^ne

near*v

cubic

reflections

are shown in

brackets,

easily recognizable

duetotheir much

_higher

intensities

_{(approx. lOOx).}

In addition 11 feeble reflections areshown

(broken arrows)

in the

_angle

_{range up} to

120

.

Even more clear

however,

is the

_change

ofstructure shown in the

_X-ray

i

-1-1-1-1-20 40 60 80 100 120 2-Theta _[·]

Fig.5

Neutron diffraction

spectra

of a trideuteride

sample

with

composition

CeD^

97g±QQQ51 both at T=350K and T=15K. The

indexing

of the

quasi-cubic

reflections

_{(solid arrows)}

has been done for Fm3m

_{(a=0.549 nm)}

but the

_agreement

between observed and calculated ones is_very bad

_(Fig.6).

In addition 11 less

_strong

reflectionsare

existing

(see

broken

arrows).

No

appreciable changes

appearbetween

thetwo

temperatures.

For the calculation of the cubic reflections the Fm3m

symmetry

was

assumed,

using

a lattice constant a=0.55250±0.00004 nm

extrapolated,

for x=2.97 from

slightly

H—poorer

deutendes.

Clearly both,

20 values and calculated intensities show

_appreciable

differences from the observed ones,

indicating

the existence of

Calculated cubic Fm3m Observed

_{(D—Scherrer]}

h k 20

_intensity

2

intensity

1 1 1 200 22 0 3 1 1 2 2 2 40 0 3 1 2 0 2 2 1 1 3 3 4 0 3 1 4 2 0 0 20 3 3 2 2 4 4 5 1 1 1 27.95 32.38 46.45 55.08 57.76 67.79 74.85 77.14 86.16 92.85 92.85 104.12 111.14 113.55 113.55 123.71 132.19 135.28 150.00 169.32 169.32 1000 513 405 451 130 64 184 170 137 117 39 55 215 109 27 123 139 147 74 616 616 26.65 28.17 28.91 32.70 46.71 47.07 54.49 55.61 58.08 68.31 73.79 75.51 77.41 86.50

strong

medium medium week

strong

medium

strong

medium week week medium medium medium medium not

sharp

Table 1

Calculated Fm3m for a=0.5525 nm and observed

X-ray

(Debye—Scherrer)

reflections ofthe non cubic trideuteride

_phase

CeD2

g7U±n 005' shows

clearly

the

_appreciable

deviations of the observed reflections from the calculated cubic

ones. Inadditiontothese

_nearly

cubic

_{reflexions, however,}

severalother

_strong

and mediumreflectionsexist.

In

_principle,

four

possible

cases may be considered for the

_{understanding}

of the

new diffraction

pattern

of

CeHg^:

a)

Contamination

the Pd—Purifier

_adjusted

at

_higher

_temperatures for deuterium diffusion. Several

samples

synthetized

under these conditions gave

reproducible

spectra. A further direct

_proof

of the absenceofcontaminationsis

_given

in

_Fig.3.

The

_CeÜ2

_sample

which shows

_exclusively

_only

the cubic reflections has been

_synthesized

under

exactly

thesame

purity

conditions asthe

CeÜ2

Qy

sample.

b)

Coexistence of Two Phases.

Under this

_assumption

and

_according

to the diffraction _patterns, one of the two

phases

should be cubic and one noncubic. It is

easily

shown that this is not the

case.

Figure

6 shows the

_attempt

to fit the

strong,

nearly

cubic reflections to the spacegroup Fm3m. For T=350 the error _factors are

_R,=0.11

and R

=0.32,

and for T=13 :

Rj=0.097

and

Rwp=0.26,

in bothcases

unacceptably

high.

Also an

_inspection

of Table 1 shows thatnocubic

phase

can be fitted tothedata.

c)

One

_Slightly

Noncubic Phase.

Our

_original

ideawas

_that,

similartothe heavier

_{RE—hydrides,}

a

hexagonal

cerium

trihydride

might

exist.

_Indexing

of both neutron and

_X—ray

spectra

for a

hexagonal

lattice gave

high

R-factors. Calculations with ABCAB and several other

sequencesof closed

packed

structures_gavetoomanyreflections. The

indexing

work

is continued.

d)

Two Noncubic Phases.

The

_only

indication for this case comes from NMR—measurements

[6].

Measurement ofthe Proton Relaxation Time

_Tj

at 40MHz forone ofour

_samples

in the

_temperature

range 416<T<900 gave two

_signals.

_Judging

from the

_high

background

of the

_X—ray

films

_(in

contrast to

H—poorer

samples)

we

thought

originally,

that a _second

amorphous phase

is

existing

in these

_samples.

_However,

theneutrondiffractiondid notshowan

amorphous

_component.

Concluding,

it can be said that the up to now

existing

evidence indicates a

superstructure

for the

_trihydrides

of cerium and lanthanum

_[4].

How far this

superstructure

could

_produce

the

_unexpected

_signals

of the

_proton

resonance is not

Obs cale din 80 2-Theta -2. w

Fig.6

Difference neutron

spectra

_0oos~Icaic)

at T=15K for the trideuteride

CeDg

97o±o 005' ^edifference

particulary

inthe first reflection is very

strong.

References

1. Kaldis

_E.,

Boroch E. and Tellefsen

_M.,

J.Less Comm.Metals

129,

57

_(1987).

2. Boroch E. and Kaldis

_E.,

_Inorganica

Chimica Acta

_140,

89

_(1987).

3. Schefer

J.,

Fischer

_P.,

_Hälg

_W.,

Osterwalder

J.,

Schlapbach

L. and

_Jorgensen

J.D.,

J.Phys.C:Solid

State

_Phys.

_17,

1575

_(1984).

4. Conder . and Kaldis

_E.,

tobe

_published.

5. Conder . and Kaldis

_E.,

J.Less

_Comm.Metals,

_{in press.} 6. Barnes

_R.,

Ames

Laboratory,

Iowa, private

communication.