Di ec de e mina ion o he Fe3+/2+ cha ge ansi ion le el in BaTiO3and iso alen ly
subs i u ed Ba0.82Ca0.18Ti0.92Z 0.08O3by X- ay pho oelec on spec oscopy
Sa i a Chaoudha y,1Anna M. Paulik,2Niklas Be elmann,1Ka ha ina N.S. Lohaus,1Lisanne
Gossel,1Melissa A. La sson,1Heba allah Ali,3Raoul Blume,4Ju ij Ko uza,2and And eas Klein1
1Technical Uni e si y o Da ms ad , Ins i u e o Ma e ials Science,
Elec onic S uc u e o Ma e ials, 64287 Da ms ad , Ge many
2G az Uni e si y o Technology, Ins i u e o Chemis y and Technology o Ma e ials, 8010 G az, Aus ia
3Fo schungszen um J¨ulich GmbH, Ins i u e o Ene gy Technologies,
Fundamen al Elec ochemis y, 52425 J¨ulich, Ge many
4Max Planck Ins i u e o Chemical Ene gy Con e sion,
Depa men o He e ogeneous Reac ions, 45470 M¨ulheim an de Ruh , Ge many
(Da ed: No embe 7, 2025)
Cha ge ansi ion le els o dopan s in oxides and o he semiconduc o s a e key ac o s a ec ing
a wide ange o ma e ial p ope ies. Despi e hei impo ance, only e y ew cha ge ansi ion le -
els a e known quan i a i ely. This wo k aims o alida e he di ec expe imen al de e mina ion o
cha ge ansi ion le els o dopan s in oxides by means o X- ay pho oelec on spec oscopy (XPS).
The app oach is used o de i e he ene gy le el associa ed o he Fe3+/2+ ansi ion, which is de-
e mined as 2.45 ±0.05 eV and 2.65 ±0.05 eV abo e he alence band maximum o BaTiO3and
Ba0.82Ca0.18Ti0.92Z 0.08O3, espec i ely. The o me ag ees wi h he mog a ime ic and elec ic
measu emen s. The esul s consolida e ha XPS is a e sa ile and eliable echnique o expe imen-
ally de e mine cha ge ansi ion le els, which can be used o e eal sys ema ic dependencies on
concen a ion, empe a u e, and hos ma e ial. I is u he demons a ed ha high- empe a u e
nea -ambien p essu e XPS pe o med a a synch o on is ideally sui ed o he de e mina ion o
cha ge ansi ion le els.
INTRODUCTION
Chemical doping, i.e. he e o alen chemical subs i u-
ion, is one o he mos widely es ablished app oaches o
he modi ica ion o he unc ional p ope ies o mode n
elec onic ma e ials. Depending on he ma e ial sys em,
dopan ype, and p ocessing condi ions, he ma e ial’s
eac ion o he addi ion o he e o alen ions can include
ionic compensa ion, elec onic compensa ion, dopan seg-
ega ion, pola on o ma ion, o he change o dopan ’s
oxida ion s a e [1]. Cha ge- ansi ion le els (CTLs) a e
he ene gy le els a which he dopan changes i s cha ge
s a e when he Fe mi le el mo es ac oss i . The posi-
ion o he Fe mi le el, de e mined by o e all cha ge neu-
ali y, is he e o e also a ec ing he cha ge s a e o he
dopan . As a consequence, he cha ge ansi ion ene gies
a e di ec ly a ec ing de ec concen a ions [2] and hence
p ope ies o a ma e ial.
BaTiO3is one o he mos ex ensi ely used ma e ials
o dielec ic ene gy s o age [3, 4] and lead- ee piezoelec-
ic applica ions [5]. Widely s udied de ec s in BaTiO3
a e Fe and Mn, which can be p esen in a ious oxida-
ion s a es om +2 o +5 [2, 6–8]. I on and manganese
a e ypically accep o - ype de ec s, which a e used o
supp ess elec ical conduc i i y o BaTiO3dielec ics [9]
and o e oelec ic ha dening [10]. Compa ed o single-
alence accep o s such as Mg o Al, mul i alen Mn has
been shown o be mo e e icien in supp essing esis ance
deg ada ion o BaTiO3[11], which ex ends he ope a ion
empe a u e o mul ilaye ce amic capaci o s [9, 12, 13].
Excep o ew examples, including Fe and Mn, expe -
imen al da a abou cha ge ansi ion le els o dopan s
in BaTiO3a e sca ce [1]. The a ailable da a a e de-
i ed om de ec chemical expe imen s, such as he mo-
g a ime y [14, 15] o empe a u e and oxygen pa ial
dependen conduc i i y measu emen s [8, 16]. In o ma-
ion abou de ec cha ge s a es o ene gy le els o de-
ec s can also be ex ac ed om o he echniques, such as
elec on pa amagne ic esonance, pho o- and ca hodolu-
minesce, M¨ossbaue spec oscopy, o deep le el ansien
spec oscopy [6–8, 17–20]. Howe e , none o hese allow
o a di ec de e mina ion o he cha ge ansi ion le els,
i.e, in o ma ion om o he expe imen s o speci ic dedi-
ca ed p epa a ion is equi ed o ei he iden i y he de ec
species o assign he obse ed ansi ion le el. The p es-
ence o mo e han jus one dopan o de ec may u he
complica e o e en disable he analysis.
I has been demons a ed ecen ly o 0.2 mol%
anadium-doped BaTiO3 ha X- ay pho oelec on spec-
oscopy (XPS) can di ec ly and selec i ely iden i y
CTLs [21]. In ha pa icula case, compa ison wi h
li e a u e could only be pe o med wi h heo e ical de-
ec calcula ions. To consolida e he de e mina ion o
CTLs by means o XPS, his s udy epo s on he ex-
pe imen al de e mina ion o he Fe3+/2+ cha ge ansi-
ion le el in BaTiO3, which is well es ablished o lie a
an ene gy o 2.4 eV abo e he alence band maximum
[2, 14, 19]. Howe e , de e mining he cha ge ansi-
ion le el o Fe is mo e challenging han ha o V, as
he mos in ense Fe 2pemission line is sp ead o e a
2
much wide ene gy ange han ha o he V 2pemis-
sion [22]. Doping concen a ions o 1 % o less will
hus no be de ec able wi hin a easonable measu emen
ime. We ha e selec ed Fe a he han Mn in he p esen
case, as Mn-doped BaTiO3exhibi s a much s onge
endency o c ys allize in he hexagonal phase han Fe-
doped BaTiO3[23]. We u he apply he app oach o
Fe-doped Ba0.82Ca0.18Ti0.92Z 0.08O3(BCZT), which is
widely s udied as a lead- ee piezoelec ic, due o i s su-
pe io pe o mance as compa ed o BaTiO3[24–28].
EXPERIMENTAL
The 5 mol% Fe-doped Ba0.82Ca0.18Ti0.92Z 0.08O3sam-
ple was syn hesized ia con en ional solid-s a e p ocess-
ing using BaCO3, CaCO3, TiO2, Z O2, and Fe2O3as
s a ing ma e ials. Ba0.82Ca0.18Ti0.92Z 0.08O3, subs i u-
ionally doped wi h 5 mol% o Fe on he pe o ski e B-
si e, was sin e ed a 1500 ◦C. De ails on he solid-s a e
p ocessing pa ame e s can likewise be ound in he Sup-
plemen a y In o ma ion. No hexagonal pe o ski e phase
was obse ed in he sin e ed Ba0.82Ca0.18Ti0.92Z 0.08O3
sample, which is consis en wi h p e ious li e a u e e-
po s on Ba0.85Ca0.15Ti0.9Z 0.1O3[29]. De ails o sam-
ple p ocessing o BaTiO3a e men ioned in he Supple-
men a y In o ma ion. A inal sin e ing empe a u e o
1300 ◦C was selec ed o BaTiO3, which e ec i ely sup-
p essed he o ma ion o he hexagonal phase (see Fig.
S1 in he Supplemen a y In o ma ion).
As he sin e ed samples a e elec ically highly insula -
ing and no sui able o XPS analysis, he samples a e e-
duced a 1100 ◦C in a A /5 %H2mix u e o 12 h in a ube
u nace. The samples a e quickly cooled o oom em-
pe a u e by a an. In o de o emo e undesi ed su ace
phases, which can eme ge a e he educ ion s ep [30],
he samples a e g ound wi h SiC sand pape and hen
polished using Phoenix 4000 semiau oma ic machine us-
ing polishing clo hs wi h 6, 3, 1 and 0.25 µm diamond
pas e. The same p ocedu e o educ ion and su ace
p epa a ion has been applied in a p e ious expe imen
[21].
XPS analysis and di e en su ace ea men s, which
a e equi ed o change he Fe mi le el and he oxida ion
s a e o Fe a he su ace, a e pe o med in he Da m-
s ad In eg a ed Sys em o Ma e ials Resea ch (DAISY-
MAT) [31], which combines a Physical Elec onics PHI
5700 (Physical Elec onics, Chanhassan, MN) wi h se -
e al acuum chambe s o su ace p ocessing and hin
ilm deposi ion. Oxygen plasma ea men s a e pe -
o med using a ec a MK II plasma sou ce ope a ed in
a omic mode. In he DAISY-MAT sys em, XPS analysis
o he Ba0.82Ca0.18Ti0.92Z 0.08O3and BaTiO3samples is
conduc ed a oom empe a u e u ilizing monoch oma ic
Al Kα adia ion wi h a pho on ene gy o 1486.6 eV a a
akeo angle o 45 ◦. Due o hei low in ensi y, Fe 2p
FIG. 1. XPS su ey spec a o Fe-doped BaTiO3and Fe-
doped Ba0.82Ca0.18Ti0.92Z 0.08O3a e wo consecu i e oxy-
gen plasma ea men s. Ca bon con amina ion (C 1s) a e e -
ec i ely emo ed by hese ea men s.
spec a a e eco ded wi h a pass ene gy o 23.5 eV, while
all o he co e le els and he alence band spec a a e
eco ded wi h a pass ene gy o 11.75 eV o be e ene gy
esolu ion. Binding ene gies a e calib a ed on a daily
basis wi h he Fe mi edge emission o a spu e -cleaned
sil e oil. The Gaussian b oadening o he Fe mi edge is
0.4 eV. Fe-doped BaTiO3samples a e also analyzed using
nea -ambien p essu e XPS a he ISISS bending dipole
beamline and ends a ion o he BESSY II synch o on in
Be lin, Ge many [32]. The Fe spec a a e measu ed using
a pho on exci a ion ene gy o 970 eV and a pass ene gy
o 20 eV. All o he de ailed spec a a e eco ded using a
pho on exci a ion ene gy o 720 eV wi h a pass ene gy o
10 eV. The a ia ion o he pho on ene gies is accoun ed
o by eco ding he O 1sspec a wi h i s and second
o de di ac ion o he beamline monoch oma o .
RESULTS AND DISCUSSION
Figu e 1 displays he XPS su ey spec a o 2 mol%
Fe-doped BaTiO3and 5 mol% Fe-doped BCZT a e e-
duc ion, polishing and oxygen plasma ea men . All ex-
pec ed co e le el lines (Ba, Ca, Z , Ti, Fe, and O) a e
obse ed in he spec a, and he C 1semissions ha e al-
mos disappea ed. The Fe 2psignal is no iceable o he
highe doped BCZT, bu ba ely o he BaTiO3sample.
Excep o samples measu ed di ec ly a e inse ion
in o he acuum sys em, he Ba 3d, Ba 4d, and Ti 2p
co e le els o BaTiO3and BCZT exhibi no p onounced
changes in line shapes bu some a ia ion o binding en-
3
e gies. The same is he case o he Ca 2pand Z 3demis-
sions o BCZT. The espec i e spec a a e displayed in
he supplemen a y in o ma ion. Fe 2pco e le el and a-
lence band spec a o he BaTiO3and he BCZT sample,
eco ded a e inse ion in o he acuum chambe (”pol-
ished” s a e) and a e wo consecu i e oxygen plasma
ea men s, a e p esen ed in Fig. 2.
The changes in he oxida ion s a e o Fe a e clea ly
isible in he Fe 2pspec a, while he shi o he Fe mi
le el can be disce ned om he a ia ion o he alence
band onse . The Fe3+ oxida ion s a e is iden i ied by he
peak maximum a a binding ene gy o ≈711 eV and by
he sa elli e peak wi h a b oad maximum a ≈719.5 eV.
The sepa a ion be ween he maximum and he sa elli e
is cha ac e is ic o F e3+ [33, 34]. In con as , he F e2+
s a e is cha ac e ized by a b oade pa ially spli maxi-
mum a ≈710 eV and by a sa elli e emission a ≈716 eV.
The posi ions o he cha ac e is ic ea u es a e indica ed
in he le panel o Fig. 2.
Acco ding o he cha ac e is ic ea u es o wo oxida-
ion s a es o Fe, one can assign he spec a as ollows:
The Fe in he as polished ( educed) samples is p esen
in he Fe2+ oxida ion s a e, while a pu e Fe3+ s a e is
p esen a e he second oxygen plasma ea men o
BaTiO3. The i s oxygen plasma ea men esul s in an
oxida ion o he su ace, e iden om shi o he peaks
o lowe binding ene gies (see Fig. S4 in he Supplemen-
a y In o ma ion). Ne e heless, he Fe-doped BaTiO3
samples exhibi s s ill some Fe2+, e iden no only om
he small addi ional shoulde a low binding ene gies o
he main Fe 2ppeak, bu also om he p esence o bo h
sa elli es. The beha io o he BCZT sample is analo-
gous. The Fe2+ oxida ion s a e o he educed samples is
es o ed a e a subsequen annealing o he samples in
acuum a 300 ◦C. Howe e , he Fe 2pin ensi y is also
inc eased a e his annealing. This migh be due o some
seg ega ion o Fe o he su ace.
The shi o he Fe mi le el is e iden om he shi o
he low binding ene gy onse o he alence bands. Bo h
samples exhibi ≈0.4 eV highe alence band onse s in
he polished and acuum-annealed s a e as compa ed o
he oxygen plasma ea ed samples. The shi s o he a-
lence band maxima induced by he oxygen plasma ea -
men is in pa allel o ha obse ed in he co e le el spec-
a. The co esponding shi s a e displayed in Fig. 3. The
cha ge ansi ion le els a e indica ed by he dashed lines.
Thei posi ion is ≈50 meV highe han he Fe mi le el
a e he i s oxygen plasma ea men , o which a mix-
u e o a small amoun o Fe2+ and a dominan Fe3+
oxida ion s a e is obse ed. The addi ion o 50 meV is
mo i a ed by he he mal b oadening o he Fe mi dis-
ibu ion unc ion a oom empe a u e. Acco dingly,
he Fe3+/2+ cha ge ansi ion le els a e de e mined as
2.45 ±0.05 eV and 2.65 ±0.05 eV abo e he alence band
maxima o BaTiO3and BCZT, espec i ely.
The de e mina ion o he cha ge ansi ion le el ac-
co ding o he p ocedu e ou lined abo e is s aigh o -
wa d o implemen and no special expe imen al equip-
men is equi ed. A s anda d XPS spec ome e wi h a
monoch oma ic X- ay sou ce and p ope binding ene gy
calib a ion [36, 37] is su icien . Sample p ocessing can,
bu does no need o be pe o med in-si u. Impo an
equi emen s o sample p ocessing a e i) ha su icien
elec ical conduc i i y is achie ed o a oid sample cha g-
ing, ii) he su ace composi ion is main ained, iii) he
ea men s lead o shi s o he Fe mi le el a leas a
he su ace, and i ) ha he su ace con amina ion by
adso ba es is kep low o su icien signal in ensi y. The
cons ain o he elec ical conduc i i y o he sample is
li ed i he measu emen s a e pe o med a ele a ed em-
pe a u es. Fo example, empe a u es o 500 ◦C o highe
a e expec ed o be su icien o ully oxidized BaTiO3
samples o a oid cha ging du ing XPS measu emen [38].
Such measu emen s ha e o be pe o med in con olled
a mosphe e, howe e , as exchange o oxygen wi h he en-
i onmen a such empe a u es esul s in changes o he
Fe mi le el. Nea -ambien p essu e XPS is ideally sui ed
o his pu pose [32, 39, 40].
Fe 2pNAP-XP spec a eco ded om wo 2 mol% Fe-
doped BaTiO3pelle s a 350 −400 ◦C in di e en gas a -
mosphe es a e displayed in he uppe pa o Fig. 4. One
sample has been educed in A /H2and polished using he
same pa ame e s as hose used o he BaTiO3sample e-
po ed abo e. The second sample is p ocessed iden ically,
bu no educed be o e he measu emen . This sample is
e e ed o as oxidized. Fo bo h samples, changes o he
oxida ion s a e o Fe om +2 o +3 a e e iden . A clea
con ibu ion o Fe2+ is obse ed o he oxidized sam-
ple in educing a mosphe e, while he oxida ion s a e o
he educed sample changes o Fe3+ in oxidizing a mo-
sphe e. In p inciple, he binding ene gies and oxida ion
s a es should no depend on sample p e- ea men . The
obse ed di e ences indica e ha he samples a e no in
equilib ium wi h he en i onmen . This is no su p ising
as bulk equilib a ion o such samples akes many hou s
e en a 600 ◦C. The absence o equilib ium o he bulk
does no a ec he ex ac ion o he cha ge ansi ion
le els, as he Fe mi le el and oxida ion s a es a e only
p obed wi hin he measu emen dep h o a ew nanome-
e s.
The espec i e Fe mi le el posi ions ex ac ed om he
alence band and O 1sco e le el spec a a e p esen ed in
he lowe panel o Fig. 4. O e all, he oxidized sample
exhibi s lowe binding ene gies co esponding o a lowe
Fe mi le el. The di e ence is o be expec ed o oxidized
and educed samples. By combining he Fe mi le els and
he Fe 2pspec a, he Fe3+/2+ cha ge ansi ion le el is
de i ed as ≈2.6 eV o he oxidized and as 2.42±0.05 eV
o he educed sample, espec i ely. While he la e
alue is in e y good ag eemen wi h ha de e mined by
he s anda d oom empe a u e XPS measu emen o he
educed sample and wi h expe imen al da a epo ed in
4
FIG. 2. Backg ound-sub ac ed Fe 2pco e le el (le ) and alence band spec a ( igh ) o Fe-doped BaTiO3and
Ba0.82Ca0.18Ti0.92Z 0.08O3a e educ ion and polishing, and a e wo consecu i e O2plasma ea men s. Fea u es in he
Fe 2pspec a associa ed wi h Fe2+ and Fe3+ a e indica ed. The Fe 2pspec a o he plasma ea ed samples a e shi ed by
0.4 eV o highe binding ene gies o emo e he shi o he Fe mi le el, which is e iden om he shi o he alence band
maxima.
li e a u e [2, 14, 19], he appa en di e ence o he ox-
idized sample is likely an expe imen al a i ac induced
by esidual cha ging o he sample. In he expe imen al
se up used, he empe a u e in educing (H2) a mosphe e
was limi ed o 400 ◦C. A his empe a u e, he esis i -
i y o he sample may no be low enough o comple ely
a oid cha ging. The educed sample does no su e om
low conduc i i y. The lowe empe a u e is bene icial in
he case o he educed sample, as comple e oxida ion o
he bulk is no possible wi hin he measu emen ime,
lea ing mos o he sample in a educed (conduc ing)
s a e.
The Fe3+/2+ cha ge ansi ion le els in BaTiO3and
BCZT di e by abou 0.2 eV (see Fig. 3). This di e -
ence may pa ially be ela ed o he sample composi ion,
as a signi ican di e ence o he Fe3+/2+ cha ge ansi-
ion le el in BaTiO3and S TiO3has also been epo ed
[19]. Howe e , he CTLs in Fig. 3 a e aken ela i e o
he alence bands o he wo ma e ials, which may also
ha e a di e en ene gy [31]. The Z -s a es in BCZT a e
expec ed o mos ly a ec he conduc ion band posi ion
[41–43], while he pa ial subs i u ion o Ba by Ca is
likely o lowe he alence band maximum [43]. This is
in ag eemen wi h he alence band maximum o BaTiO3
being abou 0.2 eV abo e ha o S TiO3[43, 44].
In o de o examine he ela i e ene gies o he alence
band maxima (i.e. he band alignmen ) o BaTiO3and
BCZT, he in e ace o ma ion wi h RuO2is s udied by
s epwise deposi ion o RuO2o Sn-doped In2O3(ITO)
on o di e en undoped and doped BCZT subs a es. The
band alignmen can hen be e alua ed using he me hod
desc ibed by K au and cowo ke s [45]. Un o una ely,
he binding ene gy shi s o he di e en BCZT co e le -
els induced by RuO2deposi ion a e di e en and also
a y om sample o sample (see Fig. S7 in he Supple-
men a y In o ma ion). The lowes Fe mi le el posi ion o
1.8±0.1 eV abo e he alence band maximum is obse ed
o RuO2deposi ion on o a educed 5 % Fe-doped BCZT
sample. This alue is 0.1 eV highe han ha de e mined
p e iously a he BaTiO3/RuO2in e ace (1.7 eV) [30],
sugges ing he alence band o BCZT o be 0.1eV lowe
han ha o BaTiO3. This di e ence co esponds well
wi h he di e ence in ene gy gap be ween BaTiO3and
20 % Ca-subs i u ed BaTiO3o 0.1 eV [46] and also co -
esponds well wi h a lowe alence band maximum o
CaTiO3compa ed o BaTiO3[43]. An in e ace expe -
imen , in which ITO was deposi ed a 300 ◦C on o an
undoped educed BCZT sample, e eals a Fe mi le el
o 2.8±0.1 eV abo e he alence band maximum. The
di e ence o he Fe mi le el a he BCZT/RuO2in e -
5
FIG. 3. Fe mi le el posi ion ela i e o he alence band
o Fe-doped BaTiO3(le ) and Ba0.82Ca0.18Ti0.92Z 0.08O3
( igh ) eco ded a e educing and polishing, wo consecu-
i e oxygen plasma ea men s, and acuum annealing o he
samples. Excep o he acuum annealed BaTiO3sample,
he shi s o he co e le els a e in pa allel o hose o he
alence band maximum, which is e iden a e sub ac ing
he ene gy di e ences indica ed in he g aphs. The numbe s
used o BaTiO3a e iden ical o hose used in p e ious wo k
[30, 35], which a e de e mined om a la ge se o di e en
samples. The dashed ho izon al lines indica e he Fe3+/2+
cha ge- ansi ion le el, ep esen ing he ene gy a which Fe
changes i s alence s a e.
ace o 1.0 eV is only sligh ly less han ha obse ed a
o he oxide ma e ials [31]. This indica es he absence o
in e ace Fe mi le el pinning, which can s ongly mod-
i y he band alignmen [31]. The Fe mi le el posi ion
a in e aces be ween high- empe a u e deposi ed ITO
ilms and single c ys alline BaTiO3is also de e mined as
2.8 eV abo e he alence band maximum (see Fig. S8
in he Supplemen a y In o ma ion), in good ag eemen
wi h he esul s o BCZT. Using hese esul s and con-
side ing he Fe3+/2+ cha ge ansi ion le els de e mined
abo e, he ene gy le el diag ams displayed in Fig. 5 we e
de i ed.
The ag eemen o he Fe3+/2+ cha ge ansi ion le el
in BaTiO3de e mined by XPS wi h hose de i ed om
he mog a ime ic and elec ical measu emen s [2, 14,
19] con i ms he eliabili y o he app oach. A main ad-
an age o he de e mina ion o he CTL by XPS com-
pa ed o o he echniques is ha i is a di ec me hod,
e ealing he oxida ion s a e o he measu ed species and
he Fe mi le el om he same se o spec a. The en-
e gy band alignmen o di e en ma e ials can also be
de e mined by XPS [45, 47, 48], enabling o s udy he
FIG. 4. ( op) Fe 2pspec a eco ded on an as-sin e ed (oxi-
dized) and a educed 2 mol% Fe-doped BaTiO3bulk ce amic.
Spec a a e backg ound sub ac ed, no malized o he peak
heigh o be e compa ison and shi ed on he ene gy axis
o emo e he e ec o he Fe mi le el a ia ion. Labels a
he le o he spec a indica e he p essu e in mba and
gas a mosphe e. The measu emen empe a u es a e indi-
ca ed a he igh . (bo om) The Fe mi le el posi ion ela i e
o he alence band maximum unde di e en measu emen
condi ions o as-sin e ed (oxidized) and educed 2 mol% Fe-
doped BaTiO3samples is de e mined om O1s co e-le el and
alence band spec a. Ho izon al dashed lines indica e he
cha ge ansi ion le els ex ac ed om he Fe 2pspec a.
ans e abili y o he CTLs om one ma e ial o ano he .
This ans e abili y o CTLs would inally enable o p e-
dic he e ec o speci ic dopan s wi hou p io knowl-
edge and open he pa h o Fe mi le el enginee ing [1].
The alignmen o de ec ene gy le els has al eady been
demons a ed o ansi ion me al impu i ies in semicon-
duc o s [49–52], o hyd ogen in semiconduc o s [53], and
o oxygen acancies in a ious oxides [54]. Ne e heless,
he ex en o which cha ge ansi ion le els o ansi ion
me al species in oxides can be ans e ed emains o be
elabo a ed. In his con ex , i is also impo an o ana-
lyze how CTLs depend on concen a ion and on empe -
a u e. A compa ison o he Fe3+/2+ and Co3+/2+ an-
si ions in (La,S )Fe3−δand (La,S )Co3−δwi h hose in
6
FIG. 5. Ene gy le el diag ams o he Fe3+/2+ cha ge ansi-
ion le els o BaTiO3and Ba0.82Ca0.18Ti0.92Z 0.08O3.
Bi(Fe,Co)O3indica es ha he ene gy le els migh no
depend signi ican ly on he concen a ion o he species
[55, 56], which would make he ans e abili y a he gen-
e al. The empe a u e dependence o he de ec ene gy
le els is ele an o unde s anding high empe a u e de-
ice ope a ion and sample p ocessing. I is well-known
ha he band gap o ma e ials dec eases wi h empe a-
u e [16, 57, 58]. Howe e , knowledge abou how much
he CTLs change wi h empe a u e, which is also e-
qui ed o quan i a i e de ec modelling, is e y limi ed.
A ailable da a, o example o Fe-doped S TiO3[16], a e
ex ac ed om empe a u e and p(O2) dependen con-
duc i i y measu emen s. Such measu emen s a e a ec ed
by all de ec s p esen in he ma e ial and hus depend on
he de ec model used o desc ibe he da a. Di ec mea-
su emen s, such as hose o e ed by he XPS app oach
desc ibed in his con ibu ion, can become a majo ool
o be e in o med de ec models. Nea -ambien p es-
su e XPS is ideally sui ed o his pu pose.
The di e ence in CTL in BaTiO3and
Ba0.82Ca0.18Ti0.92Z 0.08O3o 0.2 eV is a leas pa -
ially ela ed o a lowe alence band maximum o he
la e , which can be a ibu ed o he pa ial subs i u-
ion o Ba by Ca. Taking his in o accoun , he Fe3+/2+
ansi ion in BaTiO3 emains abou 0.1 eV lowe in
ene gy compa ed o ha in Ba0.82Ca0.18Ti0.92Z 0.08O3
on an absolu e ene gy scale. While his is wi hin
he expe imen al unce ain y, i may also pa ially be
ela ed o he highe Fe concen a ion in he s udied
Ba0.82Ca0.18Ti0.92Z 0.08O3sample. A a de ec concen-
a ion o 5 %, de ec in e ac ions canno be igno ed.
This migh a ec he CTL. Fu u e wo k has o add ess
he concen a ion and hos dependence o he de ec
ene gy le els explici ly.
Finally, he in o ma ion on he exac posi ion o he
CTL is o di ec ele ance o he use o hese ma e i-
als in dielec ic and piezoelec ic applica ions. This is o
pa icula impo ance since he dopan s used in indus-
ial composi ions a e o en ansi ion me al ions wi h
one o mo e CTLs ha lie inside he band gap, such as Fe.
Fo example, he co- i ing o elec oce amics wi h base-
me al elec odes equi es he use o speci ic a mosphe es
[9], which may shi he Fe mi le el owa ds a CTL and
consequen ly esul in a change o he dopan ’s oxida ion
s a e. This could change he conduc i i y beha io o he
ma e ial. Mo eo e , in he case o piezoce amics, he oxi-
da ion s a e o accep o dopan s o en de ines he amoun
o cha ge-compensa ing oxygen acancies and he ype o
de ec complexes [59]. Fe has been shown o inco po a e
in o he B-si e o he pe o ski e la ice o BaTiO3-based
ma e ials. The e o e, one oxygen acancy is o med o
e e y wo Fe3+ ions, while one oxygen acancy is c e-
a ed o compensa e one Fe2+ ion. Di alen ions we e
hus shown o ha e s onge in luence on he educ ion
o he Cu ie empe a u e, which depends on he concen-
a ion o bo h he accep o -dopan -ions and ··
O[60], and
could also o m di e en ypes o de ec complexes han
i alen ions. In addi ion, hese de ec s ha e an elec o-
s a ic and elas ic in e ac ion wi h e oelec ic domain
walls [61] and hus ha e a di ec in luence on ma e ial’s
unc ional p ope ies, such as piezoelec ic coe icien s,
pola iza ion, and aging/ a igue beha io .
SUMMARY AND CONCLUSIONS
The ene gy le el o he Fe3+/2+ cha ge ansi ion in
BaTiO3has been quan i ied using XPS and nea ambi-
en p essu e XPS wi h in-si u manipula ion o he Fe mi
le el a he su ace. The ex ac ed CTL o EF−EVB =
2.45±0.05 eV is he same as hose ex ac ed om he mo-
g a ime ic and elec ical measu emen s [2, 14, 19], con-
i ming he eliabili y o he XPS app oach. The capaci y
o XPS o di ec ly de e mine CTLs o e s he unique op-
po uni y o s udy he ans e abili y o de ec ene gy
le els and hei dependence on concen a ion and em-
pe a u e. I is expec ed ha gene al doping ecipes o
elec oce amic oxides can be es ablished om sys ema ic
s udies o hese dependencies.
The compa ison o he Fe3+/2+ CTL in 5 % Fe-doped
BCZT and in 2 % Fe-doped BaTiO3 e eals a di e ence o
only 0.1 eV on an absolu e ene gy scale. The la e has
been ob ained om ene gy band alignmen . The close
ag eemen sugges s ha de ec ene gy le els can be ans-
e ed om BaTiO3 o Ba0.82Ca0.18Ti0.92Z 0.08O3. The
de ailed knowledge o de ec p ope ies o BaTiO3should
hus cons i u e a di ec s a ing poin o se up quan i-
a i e de ec models o BCZT o be e unde s and he
ma e ial and enhance he piezoelec ic p ope ies.
7
SUPPLEMENTARY MATERIAL
The Supplemen a y Ma e ial con ains addi ional in o -
ma ion on sample p ocessing, addi ional pho oelec on
spec a and da a on band alignmen om in e ace s ud-
ies.
ACKNOWLEDGEMENT
The p esen ed wo k has been ca ied ou in he
amewo k o he collabo a i e esea ch cen e FLAIR
(Fe mi le el enginee ing applied o oxide elec oce am-
ics), which is suppo ed by he Ge man Resea ch Foun-
da ion (DFG), p ojec -ID 463184206 – SFB 1548, and by
he Aus ian Fonds zu F¨o de ung de wissenscha lichen
Fo schung (FWF, Aus ian Science Fund), p ojec
[G an -DOI: 10.55776/I6450]. Measu emen s ca ied ou
a he ISISS s a ion a he BESSY II ope a ed by he
Helmhol z-Zen um Be lin ¨u Ma e ialien und Ene gie
we e suppo ed wi hin p oposal-ID 251-13190-ST. Fo
he pu pose o open access, he au ho has applied a
CC BY public copy igh licence o any Au ho Accep ed
Manusc ip e sion a ising om his submission.
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