Chemistry Reference
In-Depth Information
(A)
(B)
FIGURE 7.5
Ferryl. (Fe
‾
O). termination. of. the. α-Fe
2
O
3
. (0001). surface. with. molecular.
orbital. (MO). plots. superimposed. over. the. atomic. positions.. The. MO. in. pane. (A). displays.
bonding.with.a.dz2-type.orbital.from.Fe.while.that.in.(B).exhibits.bonding.through.a.dxz-
type.orbital..Such.orbital.symmetries.are.not.available.for.bonding.with.oxygen.in.the.case.of.
the.α-Al
2
O
3
.(0001).surface,.which.may.explain.the.observed.differences.in.surface.termina-
tions.between.Al
2
O
3
.and.metal.oxides.with.the.same.atomic.structure.Cr
2
O
3
.and.Fe
2
O
3
.
microscopy.and.anticipated.based.on.infrared.relection.absorption.spectroscopy.
84
.
This.is.one.of.the.many.instances.where.quantum.chemical.calculations.were.able.
to.explain.and.inform.experimental.observation..Likewise,.such.calculations.offer.
insight.into.the.dissimilar.behavior.between.Al
2
O
3
,.Cr
2
O
3
,.and.Fe
2
O
3
.surface.ter-
minations..The.energetically.available.
d
.states.with.Cr.and.Fe.metals.stabilize.the.
metal.double.bond.to.oxygen.and.permit.that.to.be.a.favored.surface.termination.
with.suficient.oxygen.pressure.(see.Figure.7.5)..However,.in.the.case.of.alumina,.
the. lack. of. energetically. accessible.
d
. states. available. for. bonding. results. in. the.
theoretical. Al
‾
O. termination. being. much. less. favorable,. and. thus. not. observed.
experimentally.
The.experimentally.observed.α-Fe
2
O
3
.(11
_
02).surface.termination.also.appears.
to. vary. as. a. function. of. surface. preparation,. though. perhaps. to. a. lesser. extent.
than. for. α-Fe
2
O
3
. (0001).. While. the. stoichiometric. termination. is. favored. under.
UHV.conditions,.Lo.et.al..showed.that.an.oxygen.overlayer.is.present.under.high.
O
2
. partial. pressures. approaching. atmospheric. pressure.
85
. In. the. stoichiometric.
termination,. there. is. a. large. expansion. in. the.
z
. direction. between. the. layer-1. O.
anions. and. the. layer-2. Fe. cations,. which. is. driven. by. the. ivefold-coordinated.
layer-2. Fe. cations. recessing. into. the. bulk.. In. addition,. these. two. atomic. layers.
also. exhibit. small,. but. not. insigniicant,. in-plane. (
xy
). motion.. Both. the. in-plane.
rotation. and. the. out-of-plane. relaxation. result. in. the. formation. of. shorter. Fe-O.
bonds.between.the.under..coordinated.atomic.layers.1.and.2,.compared.to.the.bulk.
α-Fe
2
O
3
.structure..While.these.atomic.motions.are.observed.using.both.classical.
energy.minimizations.and.DFT.calculations,.the.motion.of.the.layer-1.O.anions.
is.more.accurately.modeled.using.a.quantum.mechanical.approach..For.instance,.
classical.molecular.dynamics.simulations.indicate.that.the.layer-1.O.anions.recess.
signiicantly. into. the. bulk. α-Fe
2
O
3
,. presumably. so. that. the. large. negative. point.
charges. localized. on. the. O. anions. may. be. screened. by. the. Fe. cations.. In. fact,.
density. functional. calculations. show. that. the. layer-1. O. anions. relax. less. signii-
cantly.than.predicted.by.classical.molecular.dynamics.simulations..In.the.quan-
tum.mechanical.treatment,.the.electrons.are.delocalized.over.the.entire.solid,.such.
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