Valence Transition in Yb Hydrides*

©

by

R.

Oldenbourg Verlag,

-0044-3336/89

Valence

Transition

in Yb

_Hydrides*

Stefan

Biichler,

Rene Monnier and Leo

_Degiorgi

Laboratorium fur

Festkorperphysik,

ETH Zurich,CH-8093Zurich,Switzerland Louis

_Schlapbach

Universite de

Fribourg,

Perolles,CH-1700

Fribourg,

Switzerland

Ybexhibitsanunstable4f

configuration

in certain

_compounds.

Ithas been shown

experimentally

and

_{theoretically}

that 3d core-level

_spectra

ofsome rareearthions exhibit satellite features which canbe

_{quantitatively}

relatedtothe4f_occupancyin the

_ground

state. We

_present

3d

_{photoelectron}

_spectra of various

_Yb-hydrides,

measured with Si Kcc radiation at 1740 eV

_photon

energy. The results indicate divalent Yb in

YbHi_.8<x<2and mixed

_valency

in the

_trihydrides

(YbH2<x<2.6)-Introduction

At moderate _pressure and _temperature Yb forms a solid solution

phase

with

hydrogen,

anonmetallic

dihydride phase

(YbHi g<x<2.o)

andametallic

trihydride

phase

(YbH.j02.5n).

The

_dihydride

is orthorhombic and

_nonmagnetic,

whereas the

_trihydride

has

been

_reported

tobe of feestructureand

magnetic

/l/.

Apparently

theamountof

hydrogen

uptake

is relatedtovalence

_changes,

which havenotbeen studied

previously.

Photoelectron _spectroscopy has been shown to be a

powerful

tool for

studying

valence transitionsinrareearthmaterials

/2,3,4,5,6,7/

including hydrides

/8,9,10,11/.

Photoemission_spectraof valence band andcorelevels ofsome rareearth ions

_(La,

Ce,

Sm, Yb,

Lu)

show features whichcan be understood ina

many-electron

_picture

only.

* _{Presentedatthe International}

Symposium

onMetal

—

Hydrogen

Systems,

Fundamen-tals and

_{Applications,}

Stuttgart,

FRG,

September

4—9, 1988.

Model calculations

11,5,61

based on the

_degenerate

Anderson

_impurity

hamiltonian describe

_{spectroscopic}

data

_quite

well. These models involve initial and final states of

mixed 4f occupancy andas

important

interaction a

hybridisation

termbetween 4fand valencestates. Inthecaseof Yb the

groundstate lg>

is the

superposition

oftwo states

with13 and 14 4f

electrons,

respectively:

The firstterm

_corresponds

to

divalent,

the secondtodivalent Yb.

_Similarly,

the final statelf> of the 3d

_{photoemission}

_{process is written}as :

For the sake of

_brevity

we usethe obvious abbreviations

4f13

and

_4f14,

_{respectively,}

for thetwocontributionstothe final and the

_ground

state.

Alocalized4felectronscreensa3d holemuchbetter thanacontinuumstate.Inthe

photoemission

process the

outgoing

electron is much _stronger

_coupled

to a

_poorly

screened ion thantoonewithafull 4f orbital. Therefore the kinetic_energyof the former is lower andin the 3d

_{photoemission}

_experiment

the

4f13

finalstate_appearstobemore

strongly

bound than the

4f14.

In thecaseof the Yb

_hydrides

the difference in

_binding

energyis almost 10eV,

making

it

_possible

to_separatethetwo_componentsof the final statein the_spectra. A

_comparison

of

_experimental

intensities of the

4f13

and

4f14

final

states with model calculations allowstheextraction of the_parametersof the modified Anderson

hamiltonian,

in

_particular

the4f

_binding

_energyand the

_{hybridisation strength.}

With theseparametersone cancalculate the

ground

state

_lg>

and thus_{the 4f occupancy.}

The relative intensities of the different 4P final states contributions in the

photoemission

spectraare adirectmeasureof the 4f occupancyinthe

_ground

state as

Gunnarsson and Schonhammer/3/have shown in thecaseof

Ce,

and Monnieretal.161in

thecaseof Yb

/12/.

InYb the

missing

ofafinalstatecontribution

_implies

the

_missing

of the

_{corresponding ground}

state. Therefore 3d

_{photoemission}

is a _strong tool for

determining

the Yb

_valency.

Experimental

The XPS _spectra were taken in a VG Escalab 5 _spectrometer ata base pressure 1 •

10"lu

mbar. Si Ka radiation_wasused and the overall resolution

was 1.6 eV FWHM

lg>

=

ocg

I4fl33dl°val3>

+

(3g

Ufl43dl°val2>

(1)

of the Au

_4fj/2

_peak

at 84.0 eV.

_Kcc3

₄ satellites were

numerically

corrected in the

spectra. The nonmetallic

YbH.2.0

showeda shift in

binding

_{energy due} to

_charging

effects. The

_energies

werecalibrated witha

_gold

wireonthe surface of the

hydride.

Yb metal

_(99.9%,

Research

_Chemicals)

wascleanedinthe

preparation

chamber of the _spectrometer

_{by repetitive}

Ar+

bombardement,

filing

and

_outgassing

at 400°Cat

p=

10"9

mbar.

Hydrogenation

_was

performed

in _a

specially designed high

_pressure

cell inside the _{spectrometer.} The

_hydriding

conditions are summarized in Table 1.

Hydrogen

wasfirst cleaned

by

a

_LN2

cooled_trapanda_getterstation downtoabout

lppm

contamination level. Table 1:

Hydride

YbHi.g

YbH2.Q

YbH2.2

YbH2,6

YbD2,6

pressure

(bar)

50-->1 50 50 50 50 temperature

(°C)

400

4C0.12°C/h>375

400.1.0°C/h>350

400

2A°9/.i1>

250

4002l°_9/_h>

250 exposure time

(hours)

10 24+4

Hydridingconditions_ofseveral _hydrides. In_YbHjjjthepressurewas50 barsduring30 minutes. The hydrideswere_preparedina_highpressurecell inside the_{spectrometer,the deuteride in}a_{separate reaction}

cell.

The

_samples

wereof

_{cylindrical shape}

with about 1 cmof diameter. The

_hydride

samples

were_{of dark grey colour and}

_brittle,

but didnot

_disintegrate

into

_powder.

To

reduce_oxygendiffusiontothe surface the

_samples

werecooled with

_{liquid nitrogen}

to=

100K in the

_{analyzing position.}

The

_hydrides

werecleaned with adiamond file in situ

immediately

before

_taking

the_spectra. Thecontaminationasdetermined from

Ols, Cls,

and

_{Yb4p peaks}

measured with

_MgKa

radiationwasbelow 2%.

Structure,

_composition

and

_homogeneity

of the

_samples

werechecked after

photoemission

measurements

_by

X-ray diffraction and volumetric determination of the

hydrogen

liberated upon dissolution of partsof the

samples

in dilute acid.

The deuteridewas

prepared

intwo_steps.Afterafirst deuteration of the Yb

_metal,

it was

pulverized

inan_argon

atmosphere

and deuterated

again.

The

_powder

wasthen filled

in a _{copper coated vanadium vessel. Neutron} measurements were

performed

at the multidetector

_powder

diffractometer DMCatS APHIR. The

_wavelength

was 1.708

A.

Results

Fig.

1 shows the Yb

_3ds/2

XPS _spectra of Yb

metal,

several Yb

_hydrides

and Yb+ 100L

O2 (1

L= 1 s at

10~6 Torr).

The

hydride

_spectra arecharacterized

by

two

main features. The first is at about 1522 eV

_binding

energy and

corresponds

to a

Ufl43d9val2>

finalstate

(4f14).

It isa

single

line and is broadened

only by

lifetime and instrumental effects. The second is centered around 1530 eV and

_corresponds

_mainly

toa

I4f133d9val3>

finalstate

(4f13).

The3d and 4f holes

_produce

a

_multiplet

_structure,which

significantly

broadens this contribution. This

_multiplet

ismost

_clearly

seen asthe_strong

peak

in the Yb+100L

_O2

_spectrum, which is included in the

_figure

as areference .

Yb2C>3

is knowntobe divalent

_having

4f13

_{configuration.}

The small

4f14

finalstate

_signal

is due to

_remaining

divalent

Yb,

resulting

from poor oxidation kinetics at room

temperatures.

Themost

_interesting

andclearest result is the increase of the

_weight

ofthe

4f13

_peak

fromaboutzeroin the

_dihydrides

toabout 50 %in

YbH2.6-

The intermediate

_compound

YbH2.2,

which isatwo

_phase

mixture of

_YbH.2

and

_YbH.2.5,

shows somescatterof the

4f13

finalstaterelative

_intensity

arounda meanvalue of 30%.

The_specdaof thetwo

_dihydrides,

_YbHi.g

and

_YbH.2.0,

aredominated

by

the

4f14

finalstate.Inthe

_region

of the

4f13

finalstatethere isa_verybroad

peak

of low

intensity.

Thecenterof this feature is at

_{significantly}

_{higher binding}

_energythan that ofthe

4f13

final state,

suggesting

a_{very low}

4f13

finalstate

intensity.

This broad

peak

may be caused

by

banddansition losses of the

4f14

finalstate.The

_{backscattering}

elecdon energyloss specda

(EELS)

ofthe

_dihydrides

show broad losses

_starting

4.5eV below the elastic

line,

withamaximumat 14eV.

The divalent Yb metal exhibitsa

_pronounced

4f14

finalstate

_peak.

Between 1525 and 1535 eV elecdon losses from the

4f14

finalstatedominate the_spectrum.Abulkanda

surface

_plasmon

5.4eV and 9.4

eV,

respectively,

below the

4f14 line,

overlap

witha

broad

_peak

caused

_by

band dansition losses.

_Again

these losses have beenseeninrecent

EELS

_experiments

/13/.

The

_experimental

results

_clearly

showavalence dansition from divalent

_YbH.2

with

4f14

configuration

tomixed valent

_YDH2.6

with

_4f13/4f14

_{configuration.}

Our calculations of the 3d core level

_spectrum

of

_YbH.2.6

based on the Gunnarsson-Schonhammer formulation of the

_degenerate

Anderson

_impurity

_{model agrees}

_{qualitatively}

and

quantitatively

very well witha50 %

4f13

/ 4f14

mixed valent

configuration

_{/l 1/.}

Magnetic properties, specially

the absence of

_{magnetic ordering}

atlow_temperature

XPS SiKrx

I

I

1540 1530 1520 1510

M-

_{BINDING ENERGY}

_(eV)

fig.1: Yb_-W5/2corelevel spectra_ofYbmetal. Yb_hydridesand Yb+100L_O2.A cleartransition_from

divalent _Ybll2

_(4f14)

tomixed valenl_Ybll2.6

{4fl3l4f14)

occurs.Si

KC134

satellitestructures_{have been}

numericallysubtracted.4findicatesa

4fx3d9val3

_finalstale_{configuration.}Indie Yb spectrumabulk and

Inorderto

_clarify

the

_question

of

_homogeneous

or

inhomogeneous

mixed valencea

structurerefinement ofneutrondiffraction_patternsof

_YDD2.6

wasstarted. Some

_peaks

in

addition to the _pattern of the fee structure _suggest a

_larger

unit cell due to small

displacements

of the Yb ions anda

possible

smearing

outofsomeoctahedralDsites/14/. It isa

pleasure

tothank J. Schefer and P. Fischer from theneutrondiffraction group at Paul Scherrer Institut for the

preliminary

structure

results,

T. Greber and

H.C.

_Siegmann

for fruitful

discussions,

the Swiss National

_Energy

Research Foundation

_(NEFF)

and the National ScienceFoundation

_(NF)

for financial_support.

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