Core-level photo-emission and -absorption spectra in Rare Earth hydroxide series

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Submitted on 1 Jan 1990

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Core-level photo-emission and -absorption spectra in

Rare Earth hydroxide series

J.C. Parlebas, E. Beaurepaire, T. Ikeda, A. Kotani

To cite this version:

Core-level

_{photo-emission}

and

_-absorption

_spectra

in Rare

Earth

_hydroxide

series

J. C. Parlebas

_(1),

E.

_Beaurepaire

_(1),

T. Ikeda

₍₂₎

and A. Kotani

₍₂₎

(1)

IPCMS

_(*)

CNRS, Université Louis Pasteur, 4 rue Blaise Pascal, 67070

_Strasbourg,

France

(2)

Department

of

_{Physics, Faculty}

of Science, Tohoku

_University,

Sendai 980,

Japan

(Requ

le 24 octobre 1989,

accepte

le 19 decembre

₁₉₈₉₎

Résumé. 2014 Nous calculons les

spectres de

_{photoémission}

3d des

_composés

_R(OH)3

avec

R = La, Ce, Pr, Nd et Sm et ce, en utilisant le modèle d’Anderson à une

_impureté.

_{Ensuite par}

l’analyse

des résultats

_{expérimentaux correspondants,}

nous pouvons estimer la valeur des

paramètres

fondamentaux de notre _{modèle pour la série}

_envisagée

des

_hydroxydes.

De même,

nous

_{interprétons}

les _spectres

_{d’absorption 2p}

de

La(OH)3,

Ce(OH)3

et

_Sm(OH)3

_qui

ont été observés recemment : pour ce faire, en

_plus

des

_paramètres

_{précédemment}

introduits, nous avons

besoin de deux nouveaux

_{paramètres (Ufd}

et

_{Udc) qui}

sont

_spécifiques

aux processus

d’absorption.

A

_partir

de notre étude

_{systématique}

de la série des

_hydroxydes

de lanthanides

_(du

moins de La à

Sm)

nous avons déduit que le _spectre de

_{photoémission}

obtenu

_{expérimentalement}

dans

Ce(OH)3

n’est

_probablement

pas celui d’un échantillon

_authentique

2014 à

cause de la contamina-tion de surface. Au contraire, nous _pouvons

_prédire

la forme

théorique

d’un _spectre de

photoémission

3d

_intrinsèque

à

_Ce(OH)3

et en

_plus

nous

_{présentons quelques}

résultats

expérimentaux préliminaires (mais nouveaux)

sur ce

_composé.

Abstract. 2014 We calculate the

3d-photoemission

spectra of

_R(OH)3

_{compounds (with}

R = La,

Ce, Pr, Nd and

_{Sm) by using}

the

_{single impurity}

Anderson model. Then

_{by analysing}

the

corresponding experimental

data we are able to estimate the

_key

_parameter values of the

hydroxides

considered within our model calculation. We also

_analyse

the

_{2p-photoabsorption}

spectra of

_{La(OH)3, Ce(OH)3}

and

_Sm(OH)3

which have been observed

_{recently :}

in addition to the _parameters

_already

introduced we need two new _parameters

(Ufd

and

Udc)

which are

_specific

to the

_{photoabsorption}

_{process. From} our

_{systematic study}

of the lanthanide

_hydroxide

series

_(at

least from La to

_Sm)

we deduce that the available

_{experimental 3d-photoemission}

_spectrum of

Ce(OR)3

is

_probably

not consistent with a

_{genuine sample}

of

Ce(OH)3 2014

due to surface

contamination. Nevertheless we are able to

_predict

the

_{corresponding}

intrinsic

_{3d-photoemission}

spectrum and in addition we _present some new

_{preliminary experimental}

measurements on that

compound.

Classification

Physics

Abstracts 78.70D - 79.60E

(*)

UI~IR 46.

1. Introduction.

In _{this paper} we are interested in the core-level

_spectroscopy

of Rare Earth

_{hydroxides :}

_(i)

a

first class of these

_compounds

would

correspond

to

_formally

« tetravalent »

hydroxides

i.e.

R (OH )4

where R is a Rare Earth element.

_{Unfortunately}

these

tetrahydroxides

do not seem

to form well defined

_{crystals :}

for

_example

a

_sample

of nominal

_composition

_{Ce {OH )4}

has been obtained in an

_amorphous

_{state ;} its 3d-XPS

_{(photoemission)}

_spectrum

_{(see [1]}

and Refs.

_therein)

is _{then very similar} to that of

_{Ce02 crystals,}

i.e,

_essentially

a three

_peak

structure

_{(see [2, 3]}

and Refs.

_{therein) ;}

also,

the essential feature of the

_2p-XAS

_(absorption)

spectrum

for the considered

_{Ce (OH )4}

sample

is the occurrence of a

_two-peak

structure like in

Ceo2 compounds

[1, 3-4] ; (ii)

another class of more common Rare Earth

_hydroxides

is

_given

by

« trivalent »

crystals,

i. e.

R (OH )3.

In the

_present

_paper we

_only

focus on this second class

of

_compounds

which are

_interesting

for

_example

in connection with their

_possible

role in

catalysis ;

in

_particular,

we

_develop

a

_systematic

theoretical

_analysis

of the

_experimental

data

on 3d-XPS and

_2p-XAS

spectra

in

_R(OH)3

with R =

La, Ce, Pr,

Nd and Sm.

As

_regards

the 3d-XPS

_spectra

for the series of

_{R (OH )3,}

the most extensive measurements

have been carried out _{several years ago}

_by

Tatsumi et al.

_[5],

_{essentially showing}

-

except

for

Sm (OH )3 --

a

_two-peak

structure : for

_example

_(i)

the

_spectrum

of

_{La {OH )3}

resembles that obtained for

_{La2o3 [6]}

whereas

_(ii)

the

_spectrum

of

_{Ce (OH )3}

also exhibits two

_peaks,

as in

Ce2o3

[3, 7]

but the energy

spacings

between the

_peak

_positions

are

_quite

different from one

result

_(oxide)

to the other

_(hydroxide).

We will discuss the data on

_{Ce (OH )3}

later in this paper. More

recently Beaurepaire

et al.

[1]

measured the

2p-XAS

of

R (OH )3

trihydroxides

with R =

La,

Ce and Sm :

they

found _one

peak

_at the

absorption

threshold due to the

_dipole

allowed transition

_{2p -}

_5d,

i.e. the usual « white line » in the

_{L3 edge}

_spectrum.

The theoretical

_analysis

of 3d-XPS within the framework of the filled band

_{single impurity}

Anderson model has been first

_proposed

for

_Ce02,

_Ce203

and

_La203

_[2,

3,

6]

and then carried

out for

_R02

and

_R203

_[8]

where R

_belongs

to _{the whole Rare Earth series. The purpose of the}

present paper is to

_apply

a similar theoretical

analysis

to 3d-XPS

_spectra

in

_{R (OH )3}

compounds

and to extend it to

_2p-XAS

_spectra

as

_well,

in the same

_trihydroxide

series. In

section 2 we

_present

our model

_study

and the

_{corresponding}

formalism for a

_systematic

calculation of 3d-XPS and

_2p-XAS

_spectra

in

_{R (OH )3.}

The results of the calculations with R =

La, Ce, Pr,

Nd and Sm _as well _as the

analysis

of the

experimental

data are

_given

in

section

_3,

_pointing

out

_(as

_already

_mentioned)

a coherence

_problem,

as for the data on the 3d-XPS

_spectrum

in

_{Ce (OH )3.}

In section 4 we discuss this

_questionable

case, propose a

theoretical

_prediction

of the 3d-XPS

_spectrum

and show some new

_preliminary

measurements

on it.

2. Formalism.

The Hamiltonian

_{Ha describing}

the initial state of the

_system

is

_given

_by

_{[2-3] :}

(2.1)

where

_E,(k)

is _{the energy} of the valence band with the index

_{k ( =1,}

... ,

N ) specifying

the

energy

levels, £

f is the 4f

level,

Uff

is the Coulomb interaction between 4f electrons and

V is the

_{hybridization}

between 4f and valence states. The

_operator

_ai,

_(i

=

k,

f)

represents

created and the 4f level is

pulled

down

_by

the core hole

_potential.

_Therefore,

the Hamiltonian is

_given

by :

where -

_Uf~

is the core hole

potential acting

on the 4f electron. The

_spectrum

of 3d-XPS is

represented

by :

where

_~g)

is the

_ground

state of

_Ho

with energy

_Eg,

and

_{the { ! f ) }’s}

are the

_eigenstates

of H with

_energies

_{~Ef} ;}

_EB

is the

_binding

energy and r denotes the

spectral

broadening

corresponding

to the lifetime of the core

_hole,

as well as the

experimental

resolution. As far as

2p-XAS

is

_concerned,

a core electron is

_{photo-excited}

to the Rare Earth 5d

_band,

so that the final state is described

_by

the Hamiltonian

_[3,

4,

6]

where the

_{photo-excited}

5d electron is assumed to

_couple

with the 4f electron

_through

an

interaction

_Ufd

and with the core hole

_through

the

_{potential -}

_Ud,.

The

_{corresponding}

absorption

spectrum

is

_expressed

as :

w

_being

the energy of the

_incoming

_photon.

The Hamiltonians

_Ho,

H and

_HXAS

are

_{numerically diagonalized}

in the

_subspace

of

4 fn,

4 fn + 1

v and

4 fn + 2 v2 configurations

(v

_being

a hole in the valence band and

n =

0, 1, 2,

...

respectively

for

La,

Ce,

Pr...)

for a finite

system

where

êv (k )

and

£a (k )

are

_expressed

as :

Here W and

_Wd

_respectively

_represent

the widths of the valence and conduction bands. The

ground

states to

_diagonalize

the Hamiltonian for the initial and final states of 3d-XPS are

given

by

fn ) , ~

~‘ + 1, v (k ) )

and

_{I fn + z, v (kl, k2 ) )}

_[8]

where

_{f )}

is the

_ground

state of the 4 fn

_{configuration}

written as :

with

_f°~

_denoting

the state in which the valence band is filled and the 4f level is

empty,

and the indices _{v 1... V n} are chosen

_arbitrarily

from the

_A~

indices. Then the

_{state If)}

is

_coupled

through

V with the 4 fn +

1 v

configuration given by :

Finally

the state

_{I fn + 1,}

_{v_ (k )~}

is further

_{coupled through}

V with the

following

states in the

4 fn + 2

y2 configurations :

The matrix elements of

_Ho

and H are

_{given by :}

where

H

=

Ho and iFf

= Ef for the XPS initial state and

H

_{= H and iFf}

=

ef -

L~c

for the final

state.

As

_regards

the final state of

_2p-XAS,

we add a 5d electron to that of

XPS,

so that the

ground

states are :

These results are for

example

a

_{straightforward}

extension of those for La

_compounds

(n

=

0) given

in

[6]

to

arbitrary

values of n.

3. Numerical results and

_analysis

of

_experimental

data.

The

_experimental

data

_[5]

of 3d-XPS

_spectra

in

_{R (OH )3}

_compounds

for R =

La,

... Sm are

shown in

_figure

1. The

_analysis

of these data is made in the same manner as the

_analysis

in

Fig.

1. -

Experimental

data of

_3dsl2-XPS

_spectra in

_{R(OH)3 compounds}

.for R =

La(a) ; Ce(b) ;

Pr(c) ; Nd(d)

and

_Sm(e)

_(after

Tatsumi et al.

_[5]).

Deconvolution of the satellite structure is

_{given by}

the dashed lines.

reference

_[8]

and the results are shown from

_figure

2a to

_figure

2e. The

_parameter

values estimated from this

analysis

are listed in table I and the

changes

of

for various Rare Earth elements are

_plotted

in

_figures

3 and 4. It is found that all the

Fig.

2. - Calculated 3d-XPS

spectra

(full line)

in

_R(OH)3

_compounds

for R =

La(a) ; Ce(b) ; Pr(c) ;

Nd(d)

and

_Sm(e).

The

_intensity

is

_expressed

in

_arbitrary

units. The

_experimental

data of

_figure

1 are

reported by

dashed lines. Also the _necessary

_background

_{(to adjust theory}

with

experiment)

is indicated

at the bottom of each _{spectrum :} it is calculated

_{by using equation}

_(3.2)

of reference

_[8],

except for

Sm(OH)3

where it is taken as a

_straight

line. However the detailed

_description

of how to take the

background

does not seem to be so

_important.

_

Earth

_element).

The value of

_Veft

for Ce is

_{anomalously large.}

We shall come back to this

point.

Table I. - List

of

the

_parameter

values estimated

_from

the

_{analysis of}

the 3d-XPS spectra in

R(OH)3

compounds

with R =

La, Ce, Pr,

Nd and

Sm ;

in the last line the deduced calculated

values

of

n f

(the

number

of f

electrons on the R

_ion)

are

_reported.

Fig. 3.

Fig. 4.

Fig.

3. -

Charge

transfer

_energies

in the initial

₍₄₎

and final

_(.:1f)

states

_along

the Rare Earth series for

R(OH)3

compounds

with R = La,

... Sm. See

equations {3.1)

and

(3.2)

for the definitions and table I

for the

_explicit

values of the _parameters.

Fig.

4. - Effective

hybridization V,ff (Eq. (3.3)

and Tab.

I) along

the Rare Earth series for

R(OH)3

Fig.

5. -

Experimental

data

_(dots)

of

_2p-XAS

in

_{R(OH)3 compounds}

for R = La, Ce and _Sm,

observed

_{by [1]}

together

with a

_{phenomenological}

fit

_{(full line).}

In our

_analysis,

we assume that the core hole

_{potential Uf,}

is the same whenever

_arising

from a

3d

_(like

in

_3d-XPS)

or a

_2p

(like

in

2p-XAS)

core hole. The width of the 5d conduction band is

fixed at

_Wd

= 6 eV and the values of

Ufd

and

_Udc

which have been introduced in Hamiltonian

(2.4)

are then varied as

_adjustable

_parameters.

The value of T is also

_{adjusted appropriately.}

The result for

_{La (OH )3}

is shown in

_figure

6a and b. In

_figure

6b the calculated

~’xAS(~ )

is shown for

_{(Ufa, Ua~) _ (0}

_eV,

0

_{eV), (2}

eV,

3

_eV)

and

₍₄

_eV,

5

_eV).

It is found that

_{FxAS (w )}

_{depends strongly}

on

_Ufa

and

_Ua~.

The result for

( Ufa, Udc) = (4

_eV,

5

_eV)

is in very

good

agreement

with the

_experimental

data as shown in

_figure

6a,

where the

_background

Fig.

6. - Calculated

2p-XAS

spectra for

_{La(OH)3 compounds}

_(with

r = 2

eV)

and

expressed

in

arbitrary

units,

(a)

for

_{( Ufa, Ua~ ) _}

₍₄

eV, 5

_{eV )}

and

_using

the

_background

shown at the bottom of the

figure -

the

_{corresponding}

_experimental

data has been drawn with a dotted line ;

(b)

for

is taken into account

_by

_{convoluting Fxps}

_{(EB )}

with a

_step

function. For the treatment of the

background

contribution,

see

_[9].

The results for

_Ce(OH)3

and

_Sm(OH)3

are shown in

_figure

7 and

_figure

_8,

_{respectively.}

The most

_striking

fact found from this

analysis

is

_that,

in order to

_reproduce

the

_experimental

2p-XAS

of

_{Ce (OH )3,}

we must assume

_{anomalously large}

values of

_Ufd

and

_Udc,

as seen from

figure

7a and b.

_According

to the

analysis

of

_2p-XAS

so far made for

_LaF3,

_La203,

Ce02,

CeF4,

etc., the values of

(Ufd, Udc)

are

_always

around

_{(4 eV,}

_{5 eV) [3,}

_{4, 6,}

_9].

Therefore,

the results of our

_analysis

for

La (OH )3

and

Sm (OH )3

are

_reasonable,

but that for

Ce (OH )3

is

_strange.

If we use

(Ufd, Udc) = (4

eV ,

5

_{eV ),}

for

_{Ce (OH )3,}

we have a satellite

on the

_higher

_energy side of the

_2p-XAS

main

_peak,

in

_disagreement

with

_experiment.

The

anomalously large

values of

_Ufa

and

_{Udc correspond}

to the

_{anomalously large}

value of

V~.

This is

_{indispensable}

to

_reproduce

the 3d-XPS of

_{Ce (OH )3}

_by

Tatsumi et al.

_[5].

With the above results in

mind,

we would like to

_point

out a

_possibility

that the

Fig.

7. -

Calculated

_2p-XAS

_spectra for

_{Ce(OH)3 compounds (with}

r = 2

eV)

and

expressed

in

arbitrary

units,

(a)

for

_{( Ufa, Ua~ ) _ (8}

_eV, 9

_eV)

and

_using

the

background

shown at the bottom of the

figure -

the

_{corresponding experimental}

data has been drawn with a dotted line ;

(b)

for

(Ufd~ Udc) = (0

eV, 0

_{eV) (1) ; (4}

_eV, 5

_{eV) (2) ; (6}

eV, 7

_{eV) (3)}

and

₍₈

eV, 9

_{eV) (4).}

Fig.

8. - Calculated

2p-XAS

spectra for

Sm(OH)3 compounds (with

1’ = 2

eV)

and

expressed

in

arbitrary

units,

(a)

for

_{( Ufd, Ud~ ) _ (3}

eV, 4

_eV)

and

_using

the

_background

shown at the bottom of the

figure -

the

_{corresponding experimental}

data has been drawn with a dotted line ;

(b)

for

experimental

data of 3d-XPS of

_{Ce (OH )3 [5]}

might

not be intrinsic. For

_example,

the data

might

be much influenced

_by

some

_impurity

_phase,

different from

Ce (OH )3,

on the surface of the

_sample.

_Now,

we assume that the value of

_Veff

of

genuine

Ce (OH )3

is obtained

by

an

interpolation

between the values for

_{La (OH )3}

and

_{Pr (OH )3}

_(so

that

_Veff

= 1.5

_eV).

Then we

calculate 3d-XPS with this value of

_Veff

and obtain the result shown in

_figure

9.

_Furthermore,

when we

_analyse

the

2p-XAS

of

Ce(OH)3

with

Veff

=1.5

eV,

as shown in

_figure

10,

we can

obtain a

_good

_agreement

with the

_experimental

data with reasonable values of

_Ufa

and

Udc

(~=5eV, ~=6eV).

Fig.

9. -

Recalculated 3d-XPS in

_{Ce(OH)3 compounds}

by using

an

_interpolated

value,

Veff

=1.5 _eV,

between the values of

_La(OH)3

and

_Pr(OH)3

compounds.

Other _parameters are d = 12.0 eV,

df

= - 0.5 eV,

Uff

= 10.0 eV, W = 3.0 eV and r = 1.5 eV. The

intensity

is

expressed

in

arbitrary

units.

Fig.

10. - Recalculated

2p-XAS

in

_Ce(OH)3

_{compounds by using}

an

_interpolated

value,

Veff

= 1.5 eV,

between the values of

_La(OH)3

and

_Pr(OH)3

_compounds

and

_{by introducing}

_{(Ufa, Udc) =}

(5

eV, 6

_{eV ).}

Other _parameters are

_kept

as in

_figure

9. The

_intensity

is

_expressed

in

_arbitrary

units. The

background

is shown at the bottom of the

_figure

and the

_experimental

data is

_{reported by}

a dotted line.

4.

_Concluding

remarks.

From the

_previously

considered theoretical

_analysis

of

_experimental

data we are able to fit the 3d-XPS

_spectra

of the

_{R (OH )3}

series with R

_ranging

from La to Sm elements and with the

key

parameter

values

_{varying smoothly along}

the

_{series, except}

for the case of

_{hybridization}

in Ce

hydroxides. Similarly

the

_2p-XAS

_spectra

of the series were

interpreted by introducing

two

additional necessary

parameters

( Uf~

and

_{Ud~) .}

In the

_particular

case of

_{Ce (OH )3}

_compounds

we would like to infer that the intrinsic 3d-XPS

_spectrum

should be as shown

_by

the

recalculated curve of

_figure

9. In order to corroborate this

conclusion,

let us

_finally

_present

some tentative

_preliminary

results on the

_experimental

3d-XPS

_spectra

of

_{Ce (OH )3}

compounds.

For

_simplicity

we

_only

show

(like

in

_{Fig. 1),}

the

_{3d5~2 spin-orbit}

contribution to

the _spectra

_(Fig.

_11).

The

_spectrum

shown in

_figure

11a

_corresponds

to a

_rough

surface of

Ce(OH)3

(which

most

_probably

includes a CO

_adsorption

_layer)

whereas the

_spectrum

of

Fig.

11. -

Experimental

tentative data of 3d-XPS _spectra in

_Ce(OH)3

_compounds.

For

_{simplicity only}

the

_{3ds/2 spin-orbit}

contribution is shown,

(a) using

a

_sample

with a

_rough

surface ;

(b) using

the same

sample

after a surface

_sputtering

treatment.

Ce02 compounds :

in

_{particular, figure}

llb is

_{quite comparable}

- in relative intensities and

energy

spacing

of the

_peaks

- with

figure

Ib,

wrongly

attributed to

_{Ce (OH )3.}

On the

contrary the energy

spacing

of the

_{peak positions}

in

_figure

11a is smaller

_(of

the order of

magnitude

of about 3

_eV)

and therefore in

_agreement

with our theoretical

prediction

of

figure

9. As a conclusion it would be _very

_interesting

and

_important

to reexamine more

precisely

the 3d-XPS

_spectra

of clean

_{Ce (OH )3}

samples

and confirm whether the

_present

conclusion is

_actually

correct.

Acknowledgments.

The authors would like to thank Dr. F. Le Normand for

_providing

the

_sample

of

Ce (OH )3

used to obtain the

_preliminary

results of

_figure

11. Part of the

_present

_paper was

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