Computational Investigations of Metal Oxide Surfaces - Metallic Systems: A Quantum Chemist's Perspective - page 202

techniques,.and.new.materials.will.be.required.for.continued.advancement.through.
the.twenty-irst.century.
Many.of.our.most.signiicant.dilemmas.concerning.materials.may.include.metal.
oxides. in. their. eventual. solution.. Computer. hardware. development. predicaments.
have.been.highly.publicized,.with.new.materials.and.production.techniques.seeming.
necessary.to.continue.the.advancement.trajectory..Energy.produced.through.fossil.
fuel.combustion.has.many.drawbacks,. including.the.lack.of.sustainable.resources,.
emissions,.and.health.and.safety,.which.motivate.interest.in.developing.alternative.
and.renewable.sources..Catalysts.are.needed.in.such.varied.applications.to.facilitate.
novel.pathways.for.chemical.reactions,.to.break.down.environmental.pollutants,.and.
to.dramatically.permit.more.energy-eficient.chemical.production..The.materials.sci-
ence.of.thin.ilms,.interfaces,.and.small.particles.must.be.advanced..Metal.oxides.are.
likely.to.igure.prominently.in.each.of.these.issues.
The. physical. understanding. required. for. these. advances. will. be. made. possible.
only. by. experiment. and. theory. via. simulations. working. hand. in. hand. to. provide.
new.insight.into.these.complex.systems..The.qualitative.information.gleaned.from.
electronic.and.atomic.structure.calculations.can.provide.a.general.understanding.of.
materials.behavior.and.atomic-level.explanations.for.certain.trends.observed.experi-
mentally,.in.some.cases.allowing.predictions.for.materials.improvements.based.on.
theory..A.quantitative.analysis.of.calculations.permits.a.more.rigorous.comparison.
with. experimental. indings. and. furnishes. a. basis. for. predicting. values. that. may.
be dificult.to.measure.via.experimental.techniques..It.is.vital.to.have.computational.
justiication,.such.as.density.functional.calculations,.to.explain.behavior.in.complex.
systems,.such.as.heterogeneous.catalysts. 1 .A.recent.review.by.Norskov.et.al. 2 .gives.
some.lavor.of.the.progress.that.has.been.made.in.the.computational.understanding.of.
catalysis..This.review.and.the.references.herein.describe.how.recent.advances.allow.
strong. comparisons. with. experiment. and. even. aid. in. design. through. a. predictive.
understanding.gleaned.from.calculations..The.ability.to.control.structure.and.isolate.
interactions.in.calculations.permits.predictions.of.material.improvements.that.gener-
ally.would.not.be.possible.to.determine.simply.through.experimental.measurements.
7.1.1 m etal o xide m odelS in c alculationS
Frequently,.metal.oxides.have.a.variety.of.possible.crystalline.structures.as.well.as.
aperiodic. structures. that. satisfy. a. given. stoichiometric. ratio.. Naturally,. the. exter-
nal.conditions,.usually.measured.in.temperature.and.pressure,.determine.the.ther-
modynamically. preferred. structure. under. the. given. thermodynamic. conditions,.
while. growth. conditions. may. introduce. kinetic. limitations. to. achieving. this. most.
favorable. structure.. In. practical. applications,. a. particular. phase. may. be. essential.
to. achieve. and. stabilize. over. a. wider. range. of. physical. conditions. than. the. phase.
diagram.would.permit..For.instance,.bulk.ZrO 2 .exhibits.several.phases.(including.
cubic,.tetragonal,.and.monoclinic).with.distinct.material.properties.and.electronic.
structure..In.thermal.barrier.applications,.the.tetragonal.to.monoclinic.phase.tran-
sition.must.be.suppressed.due.to.the.unfavorable.physical.properties.of.the.mono-
clinic. phase. in. matching. to. a. metal. alloy. substrate. 3 . Accordingly,. yttria,. Y 2 O 3 ,. is.
added. in.<10%. quantities. to. inhibit. the. transition. to. the. monoclinic. phase,. which.