Chemistry Reference
In-Depth Information
that. it. is. not. necessary. for. the. surface. ions. to. move. as. signiicantly. to. achieve.
effective.charge.screening.
In. the. oxygen-rich. termination,. the. layer-1. O. anions. still. recess. signiicantly.
into. the. bulk. as. a. result. of. electrostatic. repulsion. from. the. layer-
i
. O. atoms,. but.
the. layer-2. Fe. cations. do. not. relax. as. much. into. the. bulk. as. they. do. in. the. stoi-
chiometric.termination..Since.the.layer-2.Fe.cations.are.now.sixfold.coordinated,.
compared.to.being.ivefold.coordinated.in.the.stoichiometric.termination,.there.is.
less.driving.force.for.a.signiicant.relaxation.of.the.layer-2.Fe.cations.into.the.bulk..
Instead,.it.is.the.layer-
i
.O.anions.that.are.highly.unsaturated;.as.a.result,.the layer-2.
Fe. cations. assume. an. increased. formal. oxidation. state. of. (IV). in. order. to. keep.
the.slab.charge.neutral..While.this.surface.termination.has.many.structural.simi-
larities. with. ferryl-terminated. α-Fe
2
O
3
. (0001),. it. differs. in. the. orientation. of. the.
Fe
‾
O.bond.to.the.surface.since.the.topmost.Fe.cations.are.coordinated.differently.
in.α-Fe
2
O
3
.(11
_
02).and.α-Fe
2
O
3
.(0001)..Both.of.these.atomic.layer.relaxations.are.
depicted.in.Figure.7.6.
Hematite.is.commonly.precipitated.out.of.standing.water.or.mineral.hot.springs,.
or.formed.as.a.result.of.weathering.processes.in.soils..Thus,.the.structure.and.reac-
tivity.of.the.iron.(hydr)oxide-water.interface.are.of.prime.importance.in.controlling.
C
s
Layer
d
(Å) %∆
1
} 0.472 +37
2
3
5
} 0.585 -26
} 0.848 +7
} 0.430 +25
1.352 -4
7
8
10
} 0.368 +7
} 0.804 +2
} 0.784 -1
} 0.343 -1
(A)
C
s
Layer
d
(Å) %∆
i
2
4
5
7
8
10
} 0.101 -71
} 0.795 +1
} 0.706 -11
} 0.435 +26
1.324 -7
} 0.390 +13
} 0.811 +3
} 0.762 -4
} 0.332 -4
(B)
FIGURE 7.6
Stoichiometric. (A). and. oxygen-rich. (B). terminations. of. the. α-Fe
2
O
3
. (1
‾
02).
surface.with.atomic.layer.relaxations.in.the.
c
s
.direction.noted.
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