Structures of High-Density and Low-Density Water

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1、VOLUME84,NUMBER13PHYSICALREVIEWLETTERS27MARCH2000StructuresofHigh-DensityandLow-DensityWaterAlanK.Soper*ISISFacility,RutherfordAppletonLaboratory,Chilton,Didcot,OxonOX110QX,UnitedKingdomMariaAntoniettaRicciDipartimentodiFisica“E.Amaldi,”Universita’diRomaTre,IstitutoNazionalepe

2、rlaFisicadellaMateria,Unita’diRomaTre,viadellaVascaNavale84,00146-Roma,Italy(Received12November1999)Thethreesite-sitepartialstructurefactorsforwaterhavebeenmeasuredasafunctionofpressure,usingneutrondiffraction,atatemperatureof268K.Itisfoundthatthemeasuredstructurefunc-tionsimp

3、lyacontinuoustransformationwithincreasingpressurefromalow-densityformofwater(r
0.0295moleculesÅ3),withanopen,hydrogen-bondedtetrahedralstructure,toahigh-densityLformofwater(r
0.0402moleculesÅ3),withnontetrahedralO-O-Oanglesandacollapsedsec-Hondcoordinationshell,whichimpliesb

4、rokenhydrogenbondsbetweenthefirstandsecondcoordina-tionshells.PACSnumbers:61.20.Qg,61.12.–q,61.25.–fThreedifferentconjectureshavebeenproposedoverthemationinliquidwater,andtouncoverthelocalmolecu-lastdecadetoaccountforthephasebehaviorofmetastablelararrangementsthatoccurintheset

5、woformsofwater.wateratlowtemperature.ThesearetheretracingspinodalNeutrondiffractionexperimentsoncompressedwater,[1,2],thesecondcriticalpoint[3–7],andthesingularityusingisotopesubstitutiononthewaterhydrogens,werefreescenarios[8,9].Anotherrecentmodelshowsthatbothperformedinatemp

6、eratureregimewheretheanomalousthesecondcriticalpointandsingularityfreescenarioscanpropertiesofwateraremostvisible[17,18],namelybereproducedbyachangeinthestrengthofthehydrogenclosetotheiceI/iceIIItriplepoint(T
251K,bondinginteractioninthemodel[10].ThefirstconjectureP
209MPa),si

7、ncethisisthelowesttemperatureatimpliesanomalousincreasesoftheresponsefunctionsinwhichwaterisastableliquid.Thetemperaturechosentheproximityofthespinodalcurve,thesecondassumesthewasslightlyabovethis,268K,sothatthestructuraltrendexistenceofafirstordertransitionbetweentwodifferent

8、asafunctionpressure(26,209,and400MPa,correspond-formsofliquid