the presence of a passive 
 surface film is key to the exceptional 
 corrosion resistance of stainless steel alloys .
consequently , 
 much effort has targeted the characterization and enhancement of this 
 protective layer , which is often composed , at least partially , of 
 chromia .
such work includes fundamental studies of 
 single crystal surfaces of -cr2o3 to 
 gain atomic scale insight into pertinent properties , e.g. , refs ( 310 ) .
to date , however , most of these measurements have been conducted 
 in ultra high vacuum ( uhv ) , limiting their relevance with regard to 
 mechanistic understanding of corrosion performance in engineering 
 environments . targeting this omission ,
the current study is concerned 
 with determining the surface structure of -cr2o3(0001 ) in the presence of h2o vapor through acquisition 
 of surface x - ray diffraction ( sxrd ) data ; water is an essential ingredient 
 for many corrosion phenomena . the structure of -cr2o3(0001 ) as a 
 function of both h2o partial pressure and temperature
has 
 previously been explored by costa et al . through ab initio modeling . as a starting point for these 
 calculations ,
a clean surface terminated by a single layer of 3-fold 
 coordinated chromium atoms was assumed , as depicted in figure 1a ; this surface termination 
 is labeled cr 
o3cr on the basis 
 of its first three atomic layers ( the subscript indicates the average 
 number of atoms in each 1  1 unit cell ) .
near room temperature , 
 it was concluded that two other terminations become energetically 
 favorable in the presence of h2o . at
lower h2o partial pressures , dissociative adsorption was proposed to be the 
 most likely scenario with each surface cr becoming decorated with 
 two hydroxyls ( oh ) , i.e. ( oh)2cr  o3 ; a ( oh)cr  o3 termination 
 was found to be energetically unfavorable . increasing the h2o partial pressure resulted in the attachment of an intact h2o molecule to each dihyroxylated cr to form a new surface 
 termination , i.e. ( h2o(oh)2)cr  o3. ( a ) schematic illustration of the clean -cr2o3(0001)(1  1 ) surface employed by costa 
 et al . in their ab initio calculations of the interaction 
 of h2o with this substrate . to the left ( right ) 
 is a side ( plan ) view
( b ) and ( c ) similar models of stable oh / h2o decorated 
 terminations predicted by costa et al . at
hydrogen bonding is indicated by means of dashed 
 lines ; the smallest spheres are hydrogen atoms .
the 1  1 surface 
 unit cell is indicated in the plan view in part a. parts b and c of figure 1 illustrate the ( oh)2cr  o3 and ( h2o(oh)2)cr  o3 adsorbate phases predicted in ref ( 6 ) , including the location 
 of the acidic hydrogen resulting from dissociative adsorption of h2o .
this moiety is bound to the topmost substrate oxygen atoms , 
 forming a second distinct oh species .
hydrogen bonds formed between 
 surface adsorbates ( oh and h2o ) are also indicated in figure 1 , parts b and c. 
 these theoretical structures are consistent with experimental characterization 
 performed under uhv conditions of thin films of -cr2o3(0001 ) exposed to h2o , in that at room temperature , 
 dissociative adsorption is evident .
moreover , the 
 existence of two distinct oh species , as well as interadsorbate hydrogen 
 bonding , is apparent from vibrational data .
here , the validity of the theoretical study of costa et al . is explored experimentally .
analysis of sxrd 
 data acquired at a h2o partial pressure of 30 mbar 
 indicates that the -cr2o3(0001 ) surface 
 is decorated by oh , but not exactly as predicted .
this work builds 
 on a previous sxrd study examining the impact of o2 on 
 the surface structure of -cr2o3(0001 ) .
experimental 
 work was carried out at the diamond light source ( dls ) 
 synchrotron facility , employing the surface village s off - line 
 uhv chamber for sample preparation , and beamline i07 for sxrd measurements . in situ cleaning of the single crystal -cr2o3(0001 ) sample ( supplied by pi - kem ltd . )
involved repeated 
 cycles of ar bombardment and annealing in uhv to approximately 
 1200 k. low energy electron diffraction ( leed ) , and auger electron 
 spectroscopy ( aes ) facilities were employed for sample characterization ; 
 leed and aes data can be found in supporting information .
it should be noted that following acquisition of leed and aes data , 
 the sample underwent a further cycle of ar bombardment 
 and annealing to minimize the possibility of surface damage due to 
 electron beam impingement .
following completion of surface preparation , 
 the sample was exposed 
 to   1000 l ( langmuir ) of h2o vapor . 

prior to dosing , the h2o had been degassed through repeated 
 freeze  pump - thaw cycles .
the sample was then transferred under 
 vacuum to i07 s diffractometer in eh1 , using 
 a custom - built vacuum - suitcase and uhv baby chamber 
 combination .
the latter ( base pressure 1  10 mbar ) incorporates a dome shaped x - ray transparent beryllium window 
 suitable for undertaking sxrd measurements .
once located on the beamline 
 the sample was exposed to a further 1000 l of h2o vapor ; henceforth this surface will be referred to as cr2o3h2ouhv .
the purpose of 
 dosing h2o prior to commencing diffraction measurements 
 was to mitigate the risk of any surface contamination during the sample 
 transfer process .
sxrd data were collected at an incidence angle 
 of 1 with 
 the substrate at room temperature , using a photon energy of hv = 17.7 
 kev and a 2d pilatus photon detector .
initially , a systematic series 
 of x - ray reflections was acquired from cr2o3h2ouhv .
more specifically , for a given ( h , k)-integer , data were measured as a function of l to facilitate generation of so - called crystal truncation 
 rods ( ctrs ) ; fractional - order rods ( fors ) were also surveyed .
h , k , and l are the reciprocal lattice 
 vectors , and are defined with reference to the real space ( 1  
 1 ) unit cell of the -cr2o3(0001 ) surface , 
 described by lattice vectors ( a1 , a2 , a3 ) which are parallel to the , ,   directions , 
 respectively . the magnitudes of these lattice vectors are a1 = a2 = a = 4.957  , and a3 = c = 13.592  , where a and c are the bulk lattice constants .
subsequent to compiling surface diffraction data from cr2o3h2ouhv in uhv , the h2o partial pressure was increased in a stepwise fashion by 
 appropriate backfilling of the baby chamber with 
 h2o .
we note that above 1  10 mbar a static volume of h2o was employed rather than 
 obtaining an equilibrium pressure through balancing the rates of h2o inflow and pumping , i.e. the baby chamber 
 was no longer continuously pumped . for each h2o partial 
 pressure , the intensity of the ( 1 , 0 , 2.9 ) reflection was monitored 
 to identify changes in the -cr2o3(0001 ) 
 surface structure .
selection of this reflection was based upon its 
 sensitivity to such variation as a function of o2 partial 
 pressure . on the basis of these measurements 
 ( see below ) , a further systematic series of x - ray reflections was 
 acquired from -cr2o3(0001 ) at a h2o partial pressure of 30 mbar ; henceforth this surface 
 will be referred to as cr2o3h2o30 mbar .
it should be noted that as 30 mbar of h2o is equivalent to 100% relative humidity with the substrate 
 at room temperature , one would expect the surface to be submerged 
 beneath multiple monolayers of h2o in this environment .
to facilitate fully quantitative structure 
 determination , the raw 
 diffraction data acquired at uhv and p(h2o )  30 
 mbar were integrated and corrected to 
 enable plots of structure factor versus perpendicular momentum transfer 
 for each ctr to be compiled .
this procedure resulted in a total of 
 1054 ( 1142 ) nonequivalent reflections from six ctrs for cr2o3h2ouhv ( cr2o3h2o30 mbar ) .
concerning 
 fors , no evidence for any surface unit cell other than 1  1 
 was found .
for surface structure determination , we adopted the 
 usual approach 
 of generating simulated sxrd data for a series of potential model 
 structures , and iteratively refining structural ( and nonstructural ) 
 parameters to find the overall best fit between experiment and theory . 

reduced  was used to evaluate the goodness 
 of the fit ; this is defined as follows : n is the number of measured structure factors , p the number of parameters optimized during fitting , and fiexp(hkl ) and fith(hkl ) are the experimental and theoretically calculated 
 structure factors , respectively .
 behaves such that a value of 1 indicates that experiment 
 and theory are essentially coincident , with agreement decreasing with 
 increasing . values of  significantly 
 less than 1 suggest that the magnitudes of experimental uncertainties 
 have been overestimated .
the quoted precision of each fitted parameter 
 is determined by systematically varying the parameter about its optimal 
 value , and for each step optimizing all other parameters , until  has increased by 1/(n 
figure 2 displays 
 the intensity of the ( 1 , 0 , 2.9 ) reflection as a function of increasing 
 h2o partial pressure ; please note , as described above , 
 the sample had already been dosed with 2000 l of h2o prior to acquisition of these data . upon exposure of the sample 

to 30 mbar of h2o vapor , there is an increase of 
 20% in the signal .
this increase is fully reversible ( i.e. , 
 there is an 20% decrease in intensity upon reducing the pressure 
 down to 8  10 mbar ) , which indicates 
 that the change occurring at 30 mbar of h2o is not maintained 
 at lower vapor pressures .
the inset in figure 2 compares rocking scans acquired in uhv ( 1 
  10 mbar ) and 30 mbar , demonstrating 
 the significance of the variation in reflection intensity . furthermore , 
 this comparison shows that there is no appreciable variation in the 
 width of the reflection , indicating that terrace size is not significantly 
 influenced by the presence of h2o .
these data suggest that 
 the presence of 30 mbar of h2o vapor leads to a 
 modification of the surface structure of -cr2o3(0001 ) . this supposition will be confirmed below , through 
 analysis of the ctr data sets acquired from cr2o3h2ouhv and cr2o3h2o30 mbar .
plot of the intensity 
 of the ( 1 , 0 , 2.9 ) reflection as a function 
 of h2o partial pressure ; the -cr2o3(0001 ) sample had been dosed with 2000 l of h2o prior to acquisition of these data . dashed line is a guide 
 for the eye .
inset displays ( 1 , 0 , 2.9 ) rocking scans acquired at 
 uhv ( thin line ) and 30 mbar of h2o ( bold line ) .
initially , attention was focused 
 upon the diffraction data acquired 
 from cr2o3h2ouhv . to begin the search for a structural solution , the clean surface 
 structure ( cr2o3cleanuhv ) 
 determined in recent quantitative leed ( leed - iv ) and sxrd studies
this surface exhibits a topmost partially occupied 
 double layer of cr atoms ( cr0.31cr0.61o2.4 from leed - iv , and cr0.22cr0.31o3 from sxrd ) .
given that 
 the current measurements were undertaken following exposure to 2000 
 l of h2o , terminations with surface cr atoms bonded to 
 one or more oh / h2o species were tested .
it should be noted 
 that h atoms were not explicitly included during generation of simulated 
 of sxrd data , due to their negligible x - ray scattering , i.e. only 
 an oxygen atom was added for each oh / h2o .
refinement of 
 these oh / h2o decorated structures , including atomic coordinates , 
 site occupation , and a surface roughness parameter (  ) , resulted 
 in  values of 1.7 , 2.1 , and 1.8 for cr atoms bound 
 to one , two , or three oh / h2o species , respectively . for 
 completeness
optimization of this structure resulted 
 in a  of 1.2 , indicating that the sxrd data provide 
 no substantive evidence for adsorbed
figure 3 displays 
 a comparison of the experimental ctrs acquired from cr2o3h2ouhv with the best - fit 
 theoretical simulations . to achieve this fit 41 parameters
were optimized , 
 i.e. 35 atomic coordinates , a scale factor , a surface roughness parameter , 
 and fractional occupancy factors for cr(1 ) , cr(2 ) , o(1 ) , and cr(3 ) 
 ( these atoms are identified in figure 4 ) ; debye  waller factors for all atoms were maintained 
 at bulk values , i.e. , 0.5  . as may be expected for 
 a  value of 1.2
, there is a good level of agreement 
 between theory and experiment . in a number of regions away from bragg 
 peaks , however , the uncertainty in the experimental structure factor 
 is relatively large and can encompass zero .
given this situation , 
 which may lead one to question the reliability of the optimum structure , 
 a further structure refinement was undertaken excluding all data points 
 where the error in the structure factor includes zero .
employing this 
 more limited data set did not result in any significant changes in 
 the structural solution , and so was not considered further . comparison 
 of experimental ctr data ( solid markers with error bars ) , 
 acquired from -cr2o3(0001 ) in uhv subsequent 
 to exposure to 2000 l of h2o ( cr2o3h2ouhv ) , and theoretical best - fit 
 simulations ( solid red lines ) .
schematic models of the -cr2o3(0001 ) 
 surface structure determined from sxrd data acquired in uhv , following 
 exposure to 2000 l of h2o ( cr2o3h2ouhv ) . at the bottom ( top ) 

the surface geometry 
 emerging from analysis of the data acquired 
 from cr2o3h2ouhv is illustrated in figure 4 .
corresponding atomic coordinates are listed in table 1 ; all nearest neighbor 
 cr  o interatomic distances are physically reasonable .
a topmost 
 partially occupied cr double layer is maintained in the optimized 
 structure , although the fractional occupancies of both cr sites are 
 less than those determined previously for cr2o3-cleanuhv ; layer occupancies determined 
 from analysis of sxrd data from cr2o3h2ouhv ( the present study ) and cr2o3-cleanuhv are indicated 
 in figure 4 .
atomic 
 layer spacings perpendicular to the -cr2o3(0001 ) surface derived from both the present results and the earlier 
 measurements are listed in table 2 . as with the fractional occupancies , 

as 
 mentioned in ref ( 7 ) , a plausible explanation for these discrepancies in surface structure 
 is that they arise from small variations in sample preparation methods , 
 e.g. anneal temperature .
furthermore , it should be remembered that 
 the latest sxrd data were recorded after exposure to 2000 l of h2o , which may have induced surface modification , even though 
 no clear evidence of surface bound oh / h2o was found during 
 analysis of the diffraction data .
for example , a proportion of topmost 
 oxygen atoms may in reality be oh s due to reaction with the 
 acidic hydrogen resulting from dissociative adsorption of h2o .
the optimum value of the surface roughness parameter (  
 = 0.42 ) , along with lower fractional occupancies of surface layers , 
 may also reflect surface modification induced by h2o exposure ; 
  = 0.2 was obtained during fitting of the sxrd data acquired 
 from cr2o3cleanuhv in ref . greater surface roughness following h2o exposure
is apparently consistent with stm images acquired from 
 a thin film of -cr2o3(0001 ) , which suggest that h2o induces geometric 
 disordering within terraces .
fractional occupancy 
 is indicated 
 by a non - integer subscript in the  atom  column ; the 
 overall occupancy of oxygen atoms in the layer containing o(1 ) is 
 1.11  0.12 , as there are three symmetry equivalent oxygen atoms 
 per ( 1  1 ) unit cell .
atomic coordinates for the bulk - terminated 
 cr  cr  o3-structure are also listed .
figure 4 provides a key to 
 the identity of the atoms , and the axes x , y , and z. an asterisk (  ) indicates 
 that the parameter has been held constant during optimization .
structure determination 
 commenced with refinement of 
 the coordinates of the optimized cr2o3h2ouhv structure .
a best - fit  of 
 3.2 was obtained , suggesting that the presence of 30 mbar 
 h2o results in surface modification beyond mere relaxation . 

consequently , terminations of the optimum cr2o3h2ouhv structure , where surface cr 
 atoms are bound to one or more oh / h2o species , were tested . 

refinement of these oh / h2o decorated structures , resulted 
 in  values of 1.1 , 2.1 , and 2.3 for cr atoms bound 
 to one , two , or three oh / h2o species , respectively ,
i.e. , 
 a structure where each surface cr is bound to a single oh / h2o species is favored .
more specifically , it is concluded that oh / h2o is adsorbed atop cr , at a distance of 2.09  ; off atop 
 adsorption was also tested , but found to increase . a comparison of the experimental ctrs with the best - fit theoretical 
 simulations is shown in figure 5 .
comparison of experimental ctr data ( solid markers with error bars ) , 
 acquired from -cr2o3(0001 ) at p(h2o )  30 mbar ( cr2o3h2o30 mbar ) , and theoretical best - fit simulations 
 ( solid red lines ) .
also included are theoretically simulated data 
 ( broken blue line ) for optimum cr2o3h2ouhv geometry .
figure 6 depicts 
 the surface structural model employed to obtain the best - fit displayed 
 in figure 5 , in which 
 the oxygen atoms of adsorbed oh / h2o species are labeled 
 with 1 , 2 , 3 , and 4. as illustrated , 
 the best fit was obtained with oh / h2o ( o(1 )  

o(4 ) ) located atop cr(1 ) and cr(2 ) , as well as above any cr(3 ) 
 and cr(4 ) atoms available for bonding due to fractional occupation 
 of the topmost oxygen layer ( o(1 ) ) .
it should be borne in mind that 
 the presence of o(3 ) and o(4 ) does not result in unphysical 
 interatomic distances , as the fractional occupancy of these atoms 
 is governed by fractional occupancy of o(1 ) .
optimum atomic coordinates 
 are listed in table 3 . here , again , all nearest neighbor cr  o interatomic distances 
 are physically reasonable . during fitting 35 atomic coordinates were 
 varied .
in addition , as above , a scale factor , a surface roughness 
 parameter , and fractional occupancy factors for cr(1 ) , cr(2 ) , o(1 ) , 
 and cr(3 ) were also optimized .
waller factors for all 
 atoms were again maintained at bulk values , i.e. 0.5  .
the optimum surface roughness parameter ,  = 0.39 , is very 
 similar to that obtained for cr2o3h2ouhv , indicating that immersion in p(h2o )  30 mbar does not induce further surface 
 roughening .
furthermore , it should be noted that o(1 ) and 
 o(2 ) were constrained to have the same fractional occupancies 
 as cr(1 ) and cr(2 ) , respectively .
similarly , the fractional occupations 
 of o(3 ) and o(4 ) were fixed to be equal to the fraction 
 of available cr(3 ) and cr(4 ) atoms , respectively .
oh / h2o bond lengths ( i.e. , cr(1)o(1 ) , cr(2)o(2 ) , 
 cr(3)o(3 ) , and cr(4)o(4 ) ) were constrained 
 to have the same value during optimization .
ball and stick model 
 ( side view ) of the surface termination of 
 -cr2o3(0001 ) employed for fitting the 
 sxrd data acquired at p(h2o )  
 30 mbar ( cr2o3h2o30 mbar ) .
larger ( smaller ) spheres are oxygen ( chromium ) atoms ; the smallest 
 spheres are hydrogen atoms , which are employed to indicate location 
 of adsorbed oh / h2o .
the oxygen atoms of adsorbed oh / h2o species are labeled with 1 , 2 , 3 , 
 and 4. numerical labeling of atoms is employed for identification 
 purposes .
fractional 
 occupancy is indicated 
 by a non - integer subscript in the  atom  column ; the 
 overall occupancy of oxygen atoms in the layer containing o(1 ) is 
 1.2  0.09 , as there are three symmetry equivalent oxygen atoms 
 per ( 1  1 ) unit cell .
atomic coordinates for the bulk - terminated 
 cr  cr  o3-structure are also listed .
an asterisk (  ) indicates that the 
 parameter has been held constant during optimization .
figure 7 summarizes 
 the change in surface termination of -cr2o3(0001 ) determined through analysis of the sxrd data acquired from 
 cr2o3h2ouhv and 
 cr2o3h2o30 mbar . in the presence of 30 mbar of h2o , each under - coordinated 
 surface
the lack of oh / h2o on cr2o3h2ouhv is unexpected given that previous 
 investigations of h2o adsorption on -cr2o3(0001 ) have revealed the presence of adsorbed oh at 
 room temperature even in uhv ; h2o exposures 
 in these earlier studies were significantly lower than those employed 
 in the present work .
one possible explanation for this discrepancy 
 is that the impinging x - ray beam could induce adsorbate desorption . 

such a process would still occur at p(h2o )  30 
 mbar , but the oh / h2o overlayer would be dynamically maintained 
 due to the continuous flux of surface impinging h2o molecules . 
 alternatively
, it may be that under uhv conditions the surface coverage 
 of oh / h2o species is simply significantly lower than at 
 30 mbar ( e.g. , oh / h2o may only be located at specific defect 
 sites ) , and so the diffraction data are not sensitive to their presence . 

this possibility seems to contradict the previous studies , which suggest that adsorption is not restricted to minority sites 
 at room temperature in uhv .
however , it should be noted that these 
 studies were undertaken on thin films of -cr2o3(0001 ) , rather than a suitably oriented single crystal .
it 
 could very well be that this difference in substrate leads to variation 
 in h2o adsorption , i.e. thin films of -cr2o3(0001 ) may display significant concentrations 
 of sites active for dissociative adsorption of h2o that 
 are not present to any significant extent on the single crystal substrate . 
 finally , it is concluded in ref ( 4 ) that surface - bound oh is expected to slowly desorb from 
 -cr2o3(0001 ) at around room temperature . 

such loss of oh , if it occurs , would likely result in sxrd data being 
 acquired in uhv from a surface with a coverage of oh well below saturation . 
 however , this desorption process is probably not the origin of the 
 observed lack of adsorbed oh / h2o on cr2o3h2ouhv .
if it were , one might 
 expect an oh / h2o overlayer close to saturation coverage 
 to be maintained on the surface at h2o partial pressures 
 well below 30 mbar , which is not reflected by the plot in figure 2 , i.e. , would expect 
 increase in diffracted signal to occur at lower p(h2o ) .
cartoon 
 of the variation in surface termination of -cr2o3(0001 ) with water partial pressure , as determined 
 through analysis of the sxrd data acquired from cr2o3h2ouhv and cr2o3h2o30 mbar . concerning
the structure determined at p(h2o ) 
  
 30 mbar , it can be seen that it is inconsistent with the predictions 
 emerging from the calculations of costa et al .
they conclude that a ( oh)cr  o3 
 termination is energetically unfavorable , contradicting our experimental 
 structure determination .
it may be argued that this discrepancy is 
 again due to the influence of the x - ray beam .
however , in our opinion , 
 that the x - ray beam would selectively desorb oh / h2o species 
 from each surface cr leaving one remaining is doubtful .
one more plausible 
 reason for this difference , as stated in the experimental 
 methods , is that the sxrd data have been acquired from a surface 
 submerged beneath multiple monolayers of h2o , due to the 
 relative humidity being 100% .
in contrast , costa et al . do not explicitly include multiple layers 
 of water in their modeling , which may 
 be the origin of the variation in interfacial structure .
an alternative 
 explanation for the divergence between the ab initio predictions and experiment is that the initial clean substrate termination 
 used for the calculations is significantly different to that determined 
 by diffraction
. it could be that the more disordered surface found 
 in the experiment hinders the formation of an extended network of 
 oh / h2o hydrogen bonding , which emerges from the first - principles 
 modeling , resulting in cr binding to multiple oh / h2o species 
 becoming energetically unfavorable .
another possible reason for the 
 difference is that there is a significant energy barrier to the attachment 
 of additional oh / h2o to each cr , and so can not be realized 
 with the substrate at room temperature .
further ab initio modeling , which more closely mimics both the experimentally determined 
 -cr2o3(0001 ) termination and the measurement 
 environment , is required to test these hypotheses .
for example , ab initio molecular dynamics could be employed to better 
 simulate the submerged substrate .
regarding the precise nature of the adsorbed species at 30 mbar 
 of h2o , only the cr(oh / h2o ) interatomic 
 distances ( i.e. , cr(1)o(1 ) , cr(2)o(2 ) , 
 cr(3)o(3 ) , and cr(4)o(4 ) ) provides 
 any direct insight . as indicated above , a value of 2.09  , with 
 an error bar ranging from 0.01  for cr(3)o(3 ) 
 and cr(4)o(4 ) to 0.07  for cr(1)o(1 ) , 
 has been obtained for this parameter ; the spread in the magnitude 
 of the errors is a result of the variation in fractional occupancies 
 ( lower occupancy leads to a larger error . ) .
oh moiety should be 
 1.97  , increasing to 2.1  for adsorbed h2o . on this basis
, one might propose that at 30 mbar 
 of h2o , cr2o3(0001 ) is decorated 
 with adsorbed molecular h2o rather than oh . however , this 
 deduction must be regarded with caution due to the approach employed 
 for fitting the diffraction data , i.e. to limit the number of parameters 
 optimized , the possibility for each substrate atom to adopt more than 
 one location was not incorporated , even where there were variations 
 in local environment . for example , regarding the cr2o3h2o30 mbar data , this approach 
 results in employing only two atoms ( cr(3 ) and cr(4 ) ) to describe 
 the first subsurface double layer of cr atoms , even though some are 
 bound to o(1 ) and others to o(3 ) or o(4 ) atoms .
given 
 that this change in local coordination is likely to lead to somewhat 
 different atomic coordinates , the cr-(oh / h2o ) inter atomic 
 distance ( 2.09  ) is less well - defined than indicated above . 

consequently , we conclude that it is not possible to uniquely identify 
 the adsorbate ( oh or h2o ) present on cr2o3h2o30 mbar from the current sxrd 
 data set .
however , the weight of other evidence suggests that dissociative adsorption is 
 more likely , i.e. the adsorbate is oh . comparing the present 
 results to those obtained for the interaction 
 of h2o with other ( 0001 ) surfaces of corundum - type metal 
 oxides , of particular interest are near ambient pressure photoemission 
 data from -fe2o3(0001 ) . in that study , it was concluded that as the partial pressure 
 of h2o increases , the surface becomes increasingly decorated 
 with surface oh , attaining a maximum coverage of 1 monolayer 
 at 10 mbar .
adsorbed h2o is 
 also observed , suggested to be located above the oh layer , i.e. a 
 three layer fe2o3(0001)/oh / h2o interface 
 is formed .
one might also expect similar layering on -cr2o3(0001 ) at 30 mbar of h2o , but only 
 a single oh / h2o layer is evident from analysis of the sxrd 
 data .
however , it should be remembered that sxrd is sensitive to adsorbed 
 layers displaying order both parallel and perpendicular to the surface 
 plane of the substrate .
hence , the analysis presented here should 
 not be interpreted as indicating that only a single layer of oh / h2o is present on cr2o3(0001 ) at 30 mbar 
 h2o , but that only this layer is sufficiently ordered to 
 be apparent in sxrd .
again , 30 mbar of h2o is equivalent 
 to 100% relative humidity with the substrate at room temperature , 
 and so the surface is expected to be submerged beneath multiple monolayers 
 of h2o .
another result 
 worthy of mention is the local adsorption geometry 
 of oh obtained from photoelectron diffraction ( phd ) measurements in 
 uhv performed following exposure of v2o3(0001 ) 
 to h2o .
in contrast to the 
 current study , no evidence for oh atop surface v atoms was found . 

instead , only the surface oxygen atoms ( equivalent to o(1 ) in figures 4 and 6 ) are hydroxylated through attachment of h , presumably derived 
 from h2o dissociation ; the location of the dissociated 
 oh fragment is not identified .
this difference may be due to the initial 
 v2o3(0001 ) surface being terminated by vanadyl 
 groups ( v = o ) , rather than under - coordinated v atoms . of further interest
is a brief consideration 
 of the previous sxrd 
 study probing the surface structure -cr2o3(0001 ) as a function of oxygen partial pressure .
it is pleasing to note that away from uhv ( p(h2o )  30 mbar ( current study ) , and p(o2 ) = 1  
 10 mbar ) the optimum 
 surface structures are not identical , i.e. data have not simply been 
 acquired from similarly contaminated surfaces due to extrinsic components 
 of the ambient environment .
for both h2o and o2 , adsorption occurs atop under - coordinated surface cr atoms , but 
 the cr 
o bond distance is significantly shorter in the presence 
 of o2 ( 1.57  0.03  ) .
o 
 distance is consistent with the formation of surface chromyl ( cr = o ) 
 groups .
furthermore , unlike h2o / oh decorated -cr2o3(0001 ) , the chromyl 
 terminated surface remains intact following reduction of the o2 partial pressure .
to summarize , sxrd data 
 have been acquired from -cr2o3(0001 ) 
 as a function of h2o partial 
 pressure . in uhv , after exposure to 2000 l of h2o , the surface is terminated by a partially occupied double layer 
 of chromium atoms ; the lack of adsorbed oh / h2o is concluded 
 to be most likely a result of either adsorption only at defects , or 
 x - ray induced desorption .
this surface geometry is largely consistent 
 with those determined in recent leed - iv and sxrd studies of clean -cr2o3(0001 ) in uhv , although there are differences 
 in the values of atomic coordinates and fractional layer occupancies . 
 at  30 mbar of h2o
this result is not consistent 
 with the ab initio calculations of costa et al .
, which predict that surface termination evolves 
 as a function of h2o partial pressure at around room temperature 
 as cr  o3cr  ( oh)2cr  o3  ( h2o(oh)2)cr  o3. one possible explanation 
 for this discrepancy between theory and experiment is that the calculations 
 do not explicitly take into account multiple layers of interfacial 
 h2o , which is the expected sxrd measurement environment , 
 as the relative humidity is 100% .
 surface x - ray diffraction has been 
 employed to elucidate the surface 
 structure of -cr2o3(0001 ) as a function 
 of water partial pressure at room temperature . 
 in ultra high vacuum , 
 following exposure to 2000 langmuir of h2o , the 
 surface is found to be terminated by a partially occupied double layer 
 of chromium atoms . 
 no evidence of adsorbed oh / h2o is found , 
 which is likely due to either adsorption at minority sites , or x - ray 
 induced desorption . at a water partial pressure of 30 mbar , 
 a single oh / h2o species is found to be bound atop each 
 surface cr atom . 
 this adsorption geometry does not agree with that 
 predicted by ab initio calculations , which may be 
 a result of some differences between the experimental conditions and 
 those modeled . 