V

vanderWaalsforces691●VVVacancies—areonesortofpointdefectsinsolidswhicharevacant(empty)crystallographicsitesinacrys-tallattice.Thevacancyformationmaybeassociatedwithnumerousfactors,includingthermodynamicpropertiesofacrystalatagiventemperatureandtotalpressure(thermallyactivateddisorder),chemicalequilibrationwithenvironment,intercalationofimpuritiesordopantionsinthelattice(doping-induceddisorder),interfacialprocesses,externalfactorssuchasradiationandelec-tromagneticfields,etc.Thevacancydiffusionmech-anismisobservedwhenionsinthelatticemayjumpbetweenvacantsites.Inthiscase,theiondiffusioncoefficientisproportionaltothevacancyconcentration(seealsoNernst–Einsteinequation).Asthisprocesscanbevisualizedasvacancymigration,thevacanciesmaybeconsideredasionicchargecarriers,althoughtherealchargecarriersareions.Importantsolidelec-trolyteswiththevacancydiffusionmechanismaresta-bilizedzirconia,dopedceriumdioxide,BIMEVOX,andnumerousperovskites.Vacantsitesatthecrystalsurfaceandinthenear-surfacelayersaffectstronglytheexchangecurrentdensityandcatalyticproperties.Seealsoionconductors.Refs.:[i]WestAR()Solidstatechemistryanditsapplications.Wiley,Chichester;[ii]RickertH()Electrochemistryofsolids.Anintroduc-tion.Springer,BerlinVKVacuumspectroscopysurfaceanalyticalmethodsValenceband—Theenergybandofasolidisanen-ergyintervalwithaquasicontinuumdistributionofal-lowedenergylevelsforelectrons.Insolid-statephysics,fromthetheoreticalpointofview,low-lyingelectroncorelevelsarewelldescribedbytight-bindingwavefunctionsandareinertformanypurposes.Energeti-callyhigherlyingbands,whichareeitherfilled,partiallyfilledorempty,areinthiscontextreferredtoasvalencebands.Thesevalencebandsdeterminetheelectronicbe-haviorofsolidsinavarietyofcircumstances.Whenre-latedtosemiconductors,however,thetermvalencebanddenotesthehighestfullyoccupiedenergybandatT=.Ifthetemperatureisnotzerothereisanonvan-ishingprobabilitythatsomeelectronswillbethermallyexcitedfromthevalencebandacrosstheenergygapintotheconductionband,leadingtounoccupiedelectronstates,calledholes,inthevalenceband,andhole-typeconduction.Seealsosemiconductor.Refs.:[i]BlakemoreJS()Semiconductorstatistics.Dover,NewYork;[ii]AshcroftW,MerminND()Solidstatephysics.SaundersCollege,PhiladelphiaIHValvemetals—Metalsthatformacompact,electronicinsulatingpassivelayerwhenanodizedinaqueouselec-trolyte,exhibitingasymmetricconductivity:blockinganodicreactions,exceptatveryhighvoltages.Valvemet-alsincludealuminum,titanium,tantalum,zirconium,hafnium,andniobium.Someothermetals,suchastin,mayexhibitvalve-metalpropertiesunderspecificcon-ditions.Inelectrochemicalcells,valvemetalsfunctiongen-erallyasacathode,butnotgenerallyasananodeduetotheinsulatingoxidelayerthatformsonthemetalsurfaceunderanodicconditions.Thisoxideishighlyre-sistanttothepassageofcurrent,andmayserveforcorro-sionprotection.Inmostcasestheoxidepassivatingfilmsareeitheramorphous,orcomposedofverysmallcrys-tallinedomains(withhighdefectconcentration)withinanamorphousmatrix.Theanodicoxidelayerscanbegrownuptonmthick.Valvemetalsareusedinelectroniccomponents,suchastantalumcapacitors,microwavefield-effecttransis-tors,gatematerials,etc.Refs.:[i]HasselAW,DiesingbD()ThinSolidFilms:;[ii]StrehblowHH()Passivityofmetals.In:AlkireRCandKolbDM(eds)Advancesinelectrochemicalscienceandengineering,vol..Wiley-VCH,Weinheim,pp–;[iii]KernW,SchuegrafKK()De-positiontechnologiesandapplications:introductionandoverview.In:SeshanK(ed)Handbookofthinfilmdepositiontechniques:principles,methods,equipmentandapplications.WilliamAndrew,Noyes,pYGvanderWaalsforces—areattractive(orrepulsive)short-rangeforcesbetweenmolecularentities(orbe-tweengroupswithinthesamemolecularentity)otherthanthoseduetobondformationortotheelectrostaticinteractionofionsorofionicgroupswithoneanotherorwithneutralmolecules.Theyinclude:dipole–dipoleinteractions(orientationinteraction,Casimirforce),dipole-induceddipoleinteraction(inductioninterac-tion,Debyeforce),andinstantaneouslyinduceddipole-induceddipoleforces(dispersioninteraction,Lon-donforces).AllthreekindsofvanderWaalsforceshavethesamepotentialdependenceonintermoleculardis-tancer,i.e.,theyareproportionaltor−(usuallythedis-tancedependenceofthepotentialenergyisgivenwhich692van’tHoff,JacobusHenricus●Visr−).Generally,theLondonforcesstronglydominate.Typicalenergyis.–kJmol−[i,ii].Refs.:[i]HunterRJ()FoundationsofColloidalScience,ndedn.ppOxfordUniversityPress,Oxford;[ii]AtkinsPW()PhysicalChemistry.OxfordUniversityPress,Oxford,p;[iii]Is-raelachviliJ()Intermolecularandsurfaceforces,AcademicPress,LondonWKvan’tHoff,JacobusHenricusHoff,JacobusHenricusvan’tVariance—Aparameterusedtoindicatethepreci-sionofameasurementwhichisequaltothesquareofthestandarddeviation[i].Ref.:[i]SkoogD,WestD,HollerF,CrouchSR()Fundamentalsofanalyticalchemistry,thedn.Brooks/Cole,BelmontFGVarleycell—ThiswasavariantoftheDaniellcell.Seealsozinc,Zn+/Znelectrodes,Zn+/Zn(Hg)elec-trodes,Zinc-airbatteries(cell),andLeclanchécell.Ref.:[i]Hauc