Structure of Liquid Metal Surfaces: A First Principles Perspective - Metallic Systems: A Quantum Chemist's Perspective - page 146

that,. being. a. prototypical. free. electron. metal,. sodium. as. a. system. will. provide.
clear.insights.into.the.effects.of.metallic.bonding.on.the.surface.structure,.without.
complications.like.partial.covalent.character.present.in.the.bonding.of.some.met-
als.such.as.Ga..Having.access.to.a.system.in.which.the.bonding.is.purely.metallic.
(or.close.to.it).would.be.especially.important.when.examining.the.existence.and.
importance. of. Friedel. oscillations.. Furthermore,. experimental. x-ray. relectivity.
measurements. have. proven. more. dificult. and. less. clear-cut. in. illuminating. sur-
face.layering.for.the.low.surface.tension.(e.g.,.alkali).metals.than.in.the.high.sur-
face.tension.metals.measured.earliest,.so.performing.simulations.on.alkali.metals.
would. be. expected. to. provide. valuable. insights. to. complement. the. experimental.
efforts.being.made.
These.simulations.considered.the.free.surfaces.of.liquid.sodium.at.two.tempera-
tures,. 400.and. 500.K,. which.correspond. to.conditions. just. above. the.melting. tem-
perature.of.sodium.( T M .=.373.K),.and.somewhat.above.it..As.the.existence.and.details.
of. in-plane. order. (see. Section. 5.4). was. an. important. item. of. consideration. in. the.
study,. the. simulations. were. performed. with. two. cross-sectional. geometries. in. the.
plane.of.the.surface,.the.idea.being.that.having.simulation.cells.with.different.cross-
sectional.shapes.would.help.to.assess.the.inluence.of.inite-size.effects.on.the.exis-
tence.(if.any).of.order.parallel.to.the.surface..The.cell.shapes.simulated.are.indicated.
in.Figure.5.4,.and.the.dimensions.of.the.cells.in.Table.5.1..Slabs.of.liquid.atoms.as.
models.of.free.surfaces.were.obtained.by.irstly.melting.bulk.systems.(i.e.,.with.no.
vacuum.regions).at.high.temperatures.( T .=.800.K.for.2.ps,.followed.by. T .=.600.K.for.
2.ps)..The.“(001)”.cell.contained.160.atoms,.and.the.starting.bulk.simulation.cell.con-
tained.10.atomic.layers.along.the. z -direction.(that.would.later.become.the.direction.

(001)

18.24 Å

18.24 Å

y y

18.24 Å 34.03 Å

x z

(111)

19.34 Å 16.75 Å

19.34 Å 34.93 Å

FIGURE 5.4 Unit. cell. geometries. used. in. simulating. the. free. liquid. surfaces. of. sodium.
[70-74]..The.upper.part.shows.the.shape.of.the.“(001)”.cells;.the.lower.part.shows.the.shape.
of.the.“(111)”.cells..The.dimensions.shown.are.for.temperature. T .=.500.K.