solid oxide fuel cells 
 ( sofcs ) are very 
 promising as a source for sustainable and renewable power generation . 
 despite the efforts of researchers worldwide ,
a more detailed , atomic 
 scale understanding of the elementary , interface controlled processes 
 of sofcs is necessary to tailor their performance and lifetime .
sofcs 
 basic principle of operation is as follows : oxygen is reduced to o on the cathode side , and then oxygen ions are transported 
 through the electrolyte to the anode side where they react with hydrogen 
 from pure hydrogen or hydrocarbons to form water and release electrons , 
 which travel back to the cathode side through the external circuit 
 giving electrical power .
conventional sofcs are operated at 
 elevated temperatures ( 10001250 k ) which results in higher 
 costs of the cell and specific material requirements ( durability , 
 electrode / electrolyte interface stability ) .
bottlenecks for the sofc operation are the cathodic oxygen exchange 
 reaction on the surface of the electrode or the oxygen transport through 
 the electrode / electrolyte interface .
lowering 
 operational temperatures while keeping a high ion transport rate is 
 one of the key tasks in today s sofcs development .
regarding the electrode material , perovskites 
 are suitable candidates for intermediate temperature range ( 8001000 
 k ) sofcs .
the mixed ionic - electronic conductor la1xsrxcoo3 ( lsc ) is a promising cathode material , due 
 to its high oxygen conductivity and fast oxygen surface kinetics . in order to independently investigate the cathode side of the fuel 
 cell and to be able to deduce factors and parameters influencing sofc 
 performance ,
conventional 
 model systems usually consist of a perovskite thin film as an electrode 
 deposited on yttria - stabilized zirconia ( ysz ) single crystals serving 
 as electrolyte .
a further microstructuring of the perovskite thin 
 films has the advantage of a negligible effect of the counter electrode 
 and thus no need for a reference electrode .
sofc model electrodes are traditionally studied by electrochemical 
 impedance spectroscopy ( eis ) and time - of - flight secondary ion mass 
 spectrometry ( tof - sims ) .
eis delivers fingerprint type information 
 on the underlying transport processes under operando conditions , whereas 
 tof - sims is a destructive technique employed ex situ after the experiment 
 to establish laterally resolved oxygen diffusion profiles via isotope 
 labeling .
in addition , state - of - the - art 
 ambient pressure photoemission spectroscopy can reveal the chemical 
 binding state of surface atoms under fuel cell operation conditions .
despite recent progress made by the use 
 of model microelectrodes , there is still a lack of knowledge on the 
 atomic structure and chemical composition of buried electrode / electrolyte 
 interfaces of such systems . at these interfaces oxygen incorporation 
 or
release takes place under transport conditions , which may be accompanied 
 by a change of the interfacial cation composition , detrimental for 
 transport properties of the interface .
it is a challenge to obtain 
 an atomic - scale picture during the processes occurring at the interface , 
 because a nondestructive probe with high penetration power as well 
 as high interfacial sensitivity is needed , which is compatible with 
 the harsh operation conditions of atmospheric oxygen pressure and 
 elevated temperatures .
since the electrode is typically a few hundreds 
 of nanometers thick , any charged probe such as electrons or ions disqualify 
 for interface operando studies . here
we apply anomalous surface x - ray 
 diffraction ( sxrd ) using a microfocused x - ray beam to reveal the interfacial 
 electrode / electrolyte atomic structure and chemical composition as 
 a function of temperature , ambient pressure , and polarization under 
 operation conditions .
these challenging experiments allowed us to 
 resolve the complete 3d interfacial structure and composition with 
 subatomic resolution despite the presence of background scattering 
 and fluorescence from the 200 nm thick lsc electrode and the ysz substrate . 
 the structural information is contained in the so - called crystal truncation 
 rods ( ctrs ) , which are lines of diffracted intensity in reciprocal 
 space arising from the abrupt termination of the polished ysz single 
 crystal substrate .
such atomic - scale crystallographic information 
 on one particular interface in the sofc can serve as very important 
 input into further modeling of the oxygen transport mechanism . at 
 the moment , most of the information available under operando conditions 
 stems from impedance measurements , which are determined by all the 
 interfaces in the sofc . by including the details of one particular 
 interface the transport models can be further refined leading to a 
 better microscopic understanding . using synchrotron radiation 

the x - ray energy can be precisely tuned to the y and zr absorption 
 edges , thereby varying the scattering contrast between the two elements 
 and retrieving the layer - wise composition , disclosing information 
 on the segregation profiles .
in addition the synchrotron x - ray beam 
 can be effectively focused down to the size of the microelectrode , 
 thereby enabling to study the electrode / electrolyte interface and 
 also the bare ysz surface by lateral sample translation .
using highly 
 intense x - rays from an undulator source makes a fast data acquisition 
 possible , which is indispensible for such operando studies .
previous 
 anomalous sxrd experiments revealed that the bare and clean ysz(111 ) 
 surface in ultrahigh vacuum ( uhv ) exhibits an y enrichment under reducing 
 conditions and high temperature annealing . to mimic the transport 
 processes in a real fuel cell , we polarized the cathode microelectrode 
 with respect to a porous pt counter electrode below the single crystal 
 ysz(100 ) electrolyte under oxygen atmosphere at elevated temperatures , 
 allowing controlled inward and outward oxygen ion transport .
the cathode 
 consists of a polycrystalline complex oxide electrode in contact with 
 a single crystal electrolyte , a situation which is representative 
 for a real sofc .
our results disclose an operation condition dependent 
 variation in the interfacial structure , y composition and cation vacancy 
 concentration as compared to bulk ysz .
such nonstochiometry is expected 
 to have strong impact upon the interfacial oxygen ion conductivity . 
 for comparison ,
a free part of the ysz(100 ) surface was investigated , 
 which exhibits compositions and relaxations that do not depend on 
 the oxygen pressure and external voltage .
two samples were subjects of our 
 study : the first was a cathode model system consisting of a la0.6sr0.4co3 ( lsc ) microelectrode 
 on an yttria - stabilized zirconia ( ysz ) single crystal ( 9.5 mol % of 
 y2o3 ) in ( 100 ) orientation and miscut of < 0.1. 
 the second was a pristine ( 100 ) ysz single crystal with identical 
 specifications from the same manufacturer .
measurements on the clean 
 ysz(100 ) substrate are compared with those taken from the bare part 
 beside the microelectrode from the first sample .
a 200 nm thin lsc 
 film was deposited on the ysz(100 ) substrate by pulsed laser deposition 
 ( pld ) at 900 k and 0.04 mbar o2 pressure followed by wet 
 chemical etching with 0.1 mol / l hcl in deionized water to prepare 
 a squared 400  400 m microelectrode .
the 
 well - defined geometry of the electrode facilitated the interpretation 
 of impedance spectroscopy measurements performed before and after 
 the experiment demonstrating that electrode survived the x - ray experiment 
 without significant beam damage .
the sample with lsc microelectrode 
 on top was studied at the european synchrotron radiation facility 
 ( esrf ) , beamline id03 , by means of anomalous 
 sxrd at the y and zr k - edges under controlled oxygen environment and 
 temperature , as well as applied bias voltage , in a dedicated mobile 
 vacuum chamber for combined solid state electrochemistry 
the experiment was performed in z axis 
 diffraction mode with fixed incidence angle , allowing to keep the 
 beam footprint on the sample surface constant . by convention
, a unit cell is chosen whereby the reciprocal 
 ( continuous ) l direction is parallel to the surface 
 normal and therfore along the ctr directions .
the in - plane lattice 
 parameters span the surface and determine the ( integer ) diffraction 
 indices ( h , k ) of each ctr .
diffraction 
 data are collected by obtaining integrated intensities at several 
 ( h , k , l ) positions 
 in reciprocal space and extracting structure factors f(hkl ) , which can be compared with calculated ones 
 stemming from model structures . the chamber is equipped with a turbo 
 molecular pump , which is supported by an external pre - pump to allow 
 ultrahigh vacuum conditions , with a ceramic heater ( sample temperature 
 up to 1100 k ) , leak valves for controlled gas dosing , and 
 two pressure gauges for vacuum and elevated pressure ranges , a piezo 
 translation stage to position a contact tip , an optical microscope 
 to locate the electrodes , and an x - ray transparent beryllium window . 

platinum paste was brushed on the back side of the ysz single crystal 
 and served as counter - electrode for the electrical measurements .
anomalous sxrd data sets were collected 
 by measuring crystal truncation rods ( ctrs ) at the k - edge energies 
 of y and zr , respectively ( 17.038 and 17.998 kev ) , using a 2d detector 
 in stationary mode .
the graphite analyzer 
 in front of the detector allowed an efficient fluorescence background 
 suppression . when the beam energy is tuned to one of the absorption 
 edges , the real part of the atomic form factor of the corresponding 
 atoms
( in addition , the imaginary part of the complex atomic scattering 
 factor increases due to absorption processes . ) exploiting this anomalous 
 diffraction effect allows us to distinguish between y and zr , which 
 have almost identical atomic form factors at x - ray energies away from 
 their absorption edges . by including data measured at several x - ray 
 energies in the structure refinement , it is possible to determine 
 the individual compositions with a higher accuracy .
the ysz crystal with lsc microelectrode on top and the brushed 
 pt as a counter - electrode at the bottom are placed on a heating stage . 

the tip is attached to a piezo - electric translation stage , which is 
 positioned on the upper part of the chamber .
tip and counter - electrode 
 can be connected to the impedance analyzer and a power source via 
 feedthroughs of the vacuum chamber .
a ge fluorescence detector was 
 placed at 90 to the x - ray beam in the horizontal plane in order 
 to minimize elastic scattering .
an analyzer ( graphite ( 0001 ) crystal ) 
 was put in signal path to suppress the fluorescence background .
( a ) 
 fluorescence spectrum from a sample area without the electrode ; ( b ) 
 fluorescence spectrum from a sample area with the electrode ; ( c ) an 
 image of ctr signal together with a region of interest ( roi)yellow 
 box . in order to illuminate only the 
 area underneath one 
 electrode ,
baez 
 ( kb ) optics resulting in a beam size of 5.5 m ( vertical ) and 
 5.7 m ( horizontal ) .
such a small beam cross section ensures 
 that its footprint at the incident angle of 0.9 was slightly 
 smaller than the electrode size ( 400  400 m ) .
the beam stayed on the same spot of the electrode throughout the 
 measurements involving sample rotation which was verified by tracking 
 the sr fluorescence signal from the lsc electrode with an energy - dispersive 
 ge detector placed perpendicular to the direction of the primary beam ; 
 see figure 1 .
the electrode 
 was first characterized under reducing conditions , and then we used 
 a step - by - step approach to reach operando conditions , while recording 
 crystal truncation rod data at each step to follow the buried interface 
 evolution .
an extensive set of ctrs was taken at five different conditions 
 ( altogether , 1216 structure factors for both energies ) : reducing condition 
 ( p = 1.0  10 mbar , 300 
 and 775 k ) , oxidizing conditions ( p(o2 ) = 18 mbar , 775 k ) and operational conditions ( p(o2 ) = 18 mbar , 775 k , 500 mv and + 250 mv ) .
the 
 measurements under reducing conditions and cathodic bias were thus 
 close to the thermodynamic stability limit of lsc , i.e. , ( surface ) 
 phase decomposition . during cathodic 
 and anodic polarization the electrical current through the electrode 
 was monitored confirming that the model fuel cell was operating .
the 
 bare single crystal ysz(100 ) reference sample was studied at the max 
 planck beamline at the ngstrm - quelle karlsruhe ( anka ) by sxrd at 10 kev photon energy under uhv conditions 
 after annealing at 700 k in oxygen atmosphere ( p(o2 ) = 1.0  10 mbar ) for 120 min .
a special version of rod was used , 
 featuring the possibility to refine anomalous ctr data sets taken 
 at different energies at the same time .
an overview 
 of the sxrd experiments from the lsc / ysz(100 ) interface is given in figure 2 , which represents 
 the ctr data ( open circles ) measured at three different conditions 
 ( p = 1.0  10 mbar , 775 
 k ; p(o2 ) = 18 mbar , 775 k ; p(o2 ) = 18 mbar , 500 mv ) and the fits ( solid lines ) .
( additional data obtained for other conditions and for the free ysz(100 ) 
 surface in between the microelectrodes is shown in the supporting information .
also shown are the results 
 obtained for the clean ysz(100 ) surface in uhv . )
significant changes 
 in the ctr signals for the different conditions are discernible , related 
 to structural changes at the interface as uncovered by the fit to 
 the data and discussed in the following .
we used the idealized structure 
 model of caf2 type , fm3m space group , where y and zr are placed statistically on 
 bulk positions according to the nominal formula y0.174zr0.826o0.193 and a cubic lattice constant of a = 5.145  deduced from the experiment . note that 
 the lsc films do not contribute to the ctr signal because they exhibit 
 a polycrystalline structure .
ysz , with 
 its complex defect structure , can also 
 be described by an elaborate so - called zr - shift model , but , as discussed previously in ref ( 8) , a simple zirconia fluorite 
 model is sufficient to describe x - ray data taken at momentum transfers 
 covered in our experiment .
ctr data from the electrode / electrolyte interface 
 and fits for three different conditions : 300 k , p = 1.0  10 mbar ( blue line ) ; 775 k , p(o2 ) = 18 mbar ( green line ) , 775 k , p(o2 ) = 18 mbar , u = 500 
 mv ( red line ) .
one data set consists of eight ctrs  four taken 
 at zr k - edge ( a ) , and the other four taken at y k - edge ( b ) .
all eight 
 ctrs taken under each particular condition are fitted simultaneously . 
 on the ( 1 , 1 )
rod the signal was cut by one of the supporting 
 steel rods of the be window . as a starting model for the
electrode - covered as well as 
 the free ysz(100 ) surface an oxygen termination was chosen . the ysz 
 crystal structure is polar along its 100 direction 
 due to the alternating stacking of oxygen and metal ion layers ( see figure 3a ) . at the lsc
/ ysz interface charge compensation to 
 overcome the diverging dipole moment can take place via bonding to 
 atoms from the lsc layer . during our operando studies
the free part 
 of the sample surface is likely to be partially hydroxylated , which 
 can also stabilize the surface .
when in vacuum , such intrinsically 
 unstable polar surfaces are known to commonly show reconstructions , 
 in order to decrease their otherwise very high surface energy . based 
 on the results from a low energy ion scattering study the surface oxygen atoms
are reported to occupy nonbulk 
 positions , situated at metal bridge sites above metal ions from the 
 second metal layer below the surface .
density functional theory calculations support the experimentally obtained ( 1  
 1 ) surface reconstruction but only together with y segregation .
however , 
 our uhv data taken after annealing up to 750 k do not allow us to 
 make a clear distinction between a bulk oxygen - terminated or the proposed 
 ( 1  1 ) model . in any case all our 
 fit results do indicate a surface morphology composed of nanoscaled 
 islands of uniform well - defined 0.25 nm height , almost identical to 
 what was found for the ysz(111 ) surface .
( a ) side view 
 of the first three atomic layers ( i , ii , 
 iii ) and the lsc film placed on the oxygen ( red spheres ) terminated 
 ysz surface . yttrium atoms ( green spheres ) are randomly placed on 
 zirconium ( blue spheres ) sites within the lattice .
( b  f ) : chemically 
 resolved occupancy profiles of the first three atomic bilayers of 
 ysz(100 ) below the lsc electrode for the conditions indicated , where 
 the third layer represents bulk values as a reference and the first 
 and second layer occupancies were fitted .
( g ) reference profile for a bare ysz(100 ) surface 
 area far away from the electrode .
atomic 
 site occupancies (  ) , displacements (  ) , and debye 
waller 
 factors ( b ) are those three structural parameters 
 refined within our model .
three parameters (  ,  , b ) were varied in the first atomic layer , only two (  , 
 b ) are used for the second atomic layer , from the third layer on all 
 the parameters are fixed to the bulk ysz values of the ideal fluorite 
 model taken from ref ( 14 ) ( see figure 3 and table 1 ) .
the refinement 
 also included physically acceptable limits for each parameter used 
 in the fit .
since the scattering contribution of oxygen is significantly 
 weaker as compared to zirconium and yttrium , the oxygen occupancy 
 parameters were fixed with respect to those of y and zr atoms according 
 to chemical formulas y2o3 and zro2 . by doing this
we restrain the oxygen occupancy parameter and assume 
 electroneutrality . to reduce the number of parameters
waller parameter and displacement 
 parameter was used for y and zr atoms within one atomic layer .
waller 
 factors with lower limits and starting values for all atom types were 
 equal to the corresponding bulk values . refining three atomic layers and
values in brackets 
 stand for the estimated standard deviations ( e.s.d . ) from the fitting 
 procedure .
the 
 atomic - layer - resolved chemical profiles at the lsc / ysz interface were 
 obtained , as summarized in figure 3 .
first , under oxygen poor conditions ( p = 10 mbar , 300 k ) we find that the first atomic 
 layer has a composition y0.5zr0.5 , while the 
 second layer is already close to the bulk chemical composition , indicating 
 a strong y segregation in the as prepared sample . to the best of our 
 knowledge
the strong y segregation to the top of ysz 
 layer presumably took place during pld deposition of the lsc layer 
 at 900 k and 0.04 mbar o2 pressure .
y segregation under 
 reducing conditions is in accordance with measurements and ab inito 
 thermodynamics calculations on free ysz surfaces , while the exact level of y is most likely also affected 
 by the lsc layer on top . in comparison
the free ysz surface several 
 millimeters away from the electrode shows a topmost layer consisting 
 of uniform islands , which is seen in the total coverage of about 70% . 

furthermore , these islands as well as the second layer beneath them 
 are depleted in y , and the trend is that under more oxidizing conditions 
 y starts to segregate to the near - surface region ( see supporting information ) .
although this trend 
 is observed , the total y concentration is very close to the error 
 bar and presumably the near - surface composition hardly changes .
the 
 island structure and coverage agree very well with the results from 
 the uhv prepared ysz(100 ) surface ( see supporting information ) .
the presence of islands , which can be interpreted 
 as clustering of atomic vacancies , may contribute to the polarity 
 compensation of the ysz(100 ) surface .
the high y occupancy in 
 the top ysz layer at the electrode / electrolyte interface remains present 
 at 775 k under reducing conditions ( p = 1.0  
 10 mbar ) , while around 10% cation vacancies ( likely 
 together with oxygen vacancies ) are formed .
when switching to oxidizing 
 conditions ( p(o2 ) = 18 mbar ) at 775 k 
 the cation occupancy strongly changes .
much less y is observed in 
 the first atomic layer , while an increase in zr concentration is present 
 and the cation vacancy concentration becomes more substantial .
this 
 clearly indicates that cations in ysz are mobile even at such comparatively 
 low temperatures
. a similar analysis carried out on the electrode 
 material would be desirable and in principle possible if single crystal 
 epitaxial lsc films were used .
measurements upon applied 
 voltage between the tip and the sample to mimic the oxygen reduction 
 reaction ( orr ) ( 500 mv ) and the reversed process ( + 250 mv ) 
 again indicate that the ysz / lsc interface is highly dynamic ( figure 3e , f ) . at 500 

mv voltage the y / zr composition at the interface is similar to the 
 conditions from before without voltage applied , whereas at + 250 mv 
 the interface becomes enriched in y. this might be a consequence of 
 the oxygen chemical potential changes in lsc upon positive voltage : 
 owing to the slow kinetics of the electrochemcial surface reaction 
 ( here release of oxygen ) the main chemical potential drop is at the 
 surface , particularly upon cathodic voltage but also for moderate 
 positive bias .
hence , positive bias voltage 
 leads to an increase of the chemical potential in lsc and thus significantly 
 lowers the oxygen vacancy concentration in lsc .
provided that oxygen 
 transport across the lcs / ysz interface is still close to equilibrium , 
 this concentration change in lsc has to be counterbalanced by an electrostatic 
 potential step ( nernst s equation ) with positive charges in 
 lsc and negative ones in ysz .
a possible mechanism for the substrate 
 could be played by y ions on zr sites .
in such a case an electroneutral 
 calculation of the oxygen vacancy concentration in ysz is no longer 
 possible .
we argue this is due to the fact that oxygen diffusion is 
 faster in lsc than in ysz , resulting in net reduction of the interface , 
 which leads to a y enrichment .
this might be related to a reduced 
 oxygen concentration at the lsc / ysz interface under positive potential , 
 due to the fact that the amount of oxygen vacancies in ysz is directly 
 related to the yttria concentration .
discussing the obtained cation 
 concentrations in a more crude way as interfacial stoichiometry changes 
 lead to the following considerations .
it is known that optimum doping 
 level for oxygen conduction in ysz is close to that found in the substrate .
this means that as soon as the interfacial region 
 deviates from this composition , it can be expected that the interface 
 forms an additional diffusion barrier .
according to our observation , 
 the ysz interfacial cation stoichiometry is almost identical to that 
 of the bulk for negative bias . indeed , impedance spectroscopy of the 
 related ( la , sr ) ( co
, fe)o system has shown that there is a clear trend 
 of the electrode / electrolyte interface resistance and capacitance 
 increasing when going from negative to positive bias voltage .
the results presented here suggest that part 
 of this trend can be explained by the near - surface stoichiometry changes 
 and show that the method presented here can be used to give very detailed 
 microscopic information on the origin of certain properties found 
 in sofcs .
one important conclusion concerning the long - term stabililty 
 of sofcs at the maximum investigate temperature of 775 k is that the 
 lsc / ysz(100 ) smooth interface morphology is found to stay unaltered 
 under oxygen transport conditions .
it has been reported that , at higher 
 temperatures above 970 k , new phases like srzro3 can form 
 which we did not observe in our experiments .
further evidence for the discussed y segregation behavior 
 comes from a close investigation of the interfacial relaxations .
the 
 first and second ysz substrate metal ion layer directly at the electrolyte / electrode 
 interface exhibits outward relaxations which may also partially be 
 explained by the chemical bonding of the outermost oxygen layer to 
 atoms from the lsc electrode ( see table 1 ) .
in contrast , the first metal ion layer 
 at the free surface far away from the electrode shows slight inward 
 relaxations for all conditions investigated , followed by the second 
 layer with small outward relaxations , in agreement with the uhv prepared 
 ysz(100 ) surface ( see also supporting information ) . to obtain more insight into the correlation of structural 
 and chemical changes at the interface
, it is instructive to compare 
 the y / zr ratio and the interfacial relaxations as a function of the 
 experimental conditions , as plotted in figure 4 .
interestingly , there is a clear trend visible : 
 for higher y / zr ratios , the outward relaxations are more pronounced . 

this is inline with the increased ionic radius of y of 
 1.02  as compared to 0.84  for zr , giving 
 additional evidence for the observed y segregation .
finally , the isotropic debye  waller parameters , 
 which were refined for  below the electrode 
data only , 
 exhibit enhanced values compared to the bulk likely because of some 
 local distortions at the interface . here
no systematic trend is discernible , 
 which may be related to the high error bars for the debye 
y / zr ratio ( left y axis ) and interfacial cation 
 out - of - plane relaxations ( right y axis ) for different 
 experimental conditions .
in our study we demonstrated that anomalous 
 sxrd can resolve the chemical composition of deeply buried interfaces 
 of sofc model systems under operational conditions with atomic resolution . 

the presented investigation of a polycrystalline lsc model microelectrode 
 on a ysz(100 ) substrate showed that the y cation occupancy at the 
 electrode / electrolyte interface strongly depends on the sample environment 
 and the applied potential .
the interface was found to be y rich after 
 lsc film deposition ; subsequent exposure to oxygen in the mbar pressure 
 regime at 775 k strongly reduced the y concentration at the interface , 
 which is traced back to the complex defect chemistry and thermodynamics 
 of lsc in contact with ysz . upon bias voltage application
further 
 changes are observed , indicating enhanced y concentration at the interface 
 for positive bias which is expected to be detrimental for the oxygen 
 ion transport through the interface .
the y segregation behavior is 
 supported by outward cation interfacial relaxation which correlates 
 with the amount of y at the interface .
the lsc / ysz(100 ) interface 
 is found to stay atomically smooth under oxygen transport conditions . 

reference measurements from a bare ysz(100 ) surface area indicate 
 stable surface composition and relaxations .
the observed interfacial 
 defects and variation from bulk stochiometry for different conditions 
 calls for further theoretical investigations to disentangle their 
 role for the rate limitation of the oxygen reduction reaction .
future 
 experiments on epitaxial electrode layers will allow the additional 
 investigation of the gas phase / electrode interface under oxygen incorporation 
 or removal conditions .
 we employed operando anomalous surface 
 x - ray diffraction to investigate the buried interface between the 
 cathode and the electrolyte of a model solid oxide fuel cell with 
 atomic resolution . 
 the cell was studied under different oxygen pressures 
 at elevated temperatures and polarizations by external potential control . 
 making use of anomalous x - ray diffraction effects at the y and zr 
 k - edges allowed us to resolve the interfacial structure and chemical 
 composition of a ( 100)-oriented , 9.5 mol % yttria - stabilized zirconia 
 ( ysz ) single crystal electrolyte below a la0.6sr0.4coo3 ( lsc ) electrode . 
 we observe yttrium 
 segregation toward the ysz / lsc electrolyte / electrode interface under 
 reducing conditions . under oxidizing conditions , 
 the interface becomes 
 y depleted . 
 the yttrium segregation is corroborated by an enhanced 
 outward relaxation of the ysz interfacial metal ion layer . at the 
 same time , an increase in point defect concentration in the electrolyte 
 at the interface was observed , as evidenced by reduced ysz crystallographic 
 site occupancies for the cations as well as the oxygen ions . 
 such 
 changes in composition are expected to strongly influence the oxygen 
 ion transport through this interface which plays an important role 
 for the performance of solid oxide fuel cells . 
 the structure of the 
 interface is compared to the bare ysz(100 ) surface structure near 
 the microelectrode under identical conditions and to the structure 
 of the ysz(100 ) surface prepared under ultrahigh vacuum conditions . 
Introduction
Experimental
Details and Data Analysis
Results and Discussion
Conclusions