Photon cascade emission in Pr³⁺ doped fluorides with CaF₂ structure: Application of a model for its prediction

Photon

cascade

emission

in

Pr

3+

_doped

_fluorides

_with

_CaF

2

structure:

Application

of

a

model

for

its

prediction

Benjamin

Herden

_,

_Amador

_{García-Fuente}

_,

_Harry

_{Ramanantoanina}

_,

_Thomas

_Jüstel

_,

Claude

Daul

_,

_Werner

_Urland

a_{DepartmentofChemistry,UniversityofFribourg,CheminduMusée9,1700Fribourg,Switzerland}

b_{DepartmentofChemicalEngineering,MünsterUniversityofAppliedSciences,Stegerwaldstrasse39,48565Steinfurt,Germany}

Inthiswork,wepredictandmeasuretheopticalbehaviourofPr3+_{indifferentbinaryandternaryfluorides.}

WeuseavalidatedmodelbasedonLigandFieldTheoryandDensityFunctionalTheorytocalculatethe

multipletenergylevelsarisingfromtheground[Xe]4f2_{andexcited[Xe]4f}1_5d1_{electronconfigurations}

ofPr3+_{initschemicalenvironment.Moreover,theluminescencespectraoftheconsideredmaterials}

wererecorded.Inoverallthetheoreticaldeterminationcorroboratestotheexperimentalfindings.The

phenomenonofthephotoncascadeemissionisparticularlystressed,beingimportantforthedesignof

modernphosphorswithquantumefficiencieslargerthan100%.

1. Introduction

Divalent and trivalentlanthanideions arewidely appliedin phosphors,lasergainmediaandscintillatorcrystalsorceramics. Especially trivalent praseodymium(Pr3+_{) shows a largevariety}

ofdifferentapplicationsasactivatorion, viz.UVphosphors[1], lasermaterials[2],up-[3]anddown-conversion[4,5]phosphors. Thedown-conversionphosphorsalsocalledquantumcutter sys-temsorphotoncascadeemissionmaterials(PCE),exhibitatwoor morephotonluminescenceprocess,whereatleasttwolowenergy photonsareemittedperonehighenergyphotonabsorbed[6,7]. Therefore,quantumefficiencieslargerthan100%areachievable. PCEmaterialscanbeusefulinplasmadisplaypanelsorHg-free dischargelamps,inwhichthephosphorsareexcitedbyvacuum UVradiationfromtheXeexcimerdischarge[8].

TheobservationofPCEinPr3+_{phosphorswassimultaneously}

describedbySommerdijketal.[4]andbyPiperetal.[5]in1974 forYF3.TherePiperetal.reportedquantumefficienciesofabout

140%, which have also been foundfor ␤-NaLaF4 [9].Aside the

usualfluoridehosts,Pr3+_{dopedoxidichosts,viz.SrAl}

12O19[10],

LaMgB5O10[11],andLaB3O6[12]weredemonstratedtoshowPCE,

whose phenomenon canonlybe observedifthe lowestenergy level oftheexcited[Xe]4f1_5d1 _{electronconfiguration ofPr}3+ _is

locatedabovethe1_S

0energyleveloftheground[Xe]4f2electron

∗ Correspondingauthor.

E-mailaddress:[email protected](W.Urland).

configurations[5].Itisofcrucialimportancetodelimitatethe mul-tipletenergylevelsarisingfromtheground[Xe]4f2_{andtheexcited}

[Xe]4f1_5d1_{electronconfigurationsofPr}3+_{initschemical}

environ-ment.Itturnsoutthatanoverlapbetweenbothmultipletenergy levelsiscontrolledbyabalancebetweentwointeractionssuchas thenephelauxeticeffectandtheligandfieldsplitting[13].

Sincefluorideligandsexhibiting asmallnephelauxeticeffect [13],thepositionofthelowestenergylevelofthe[Xe]4f1_5d1

con-figurationdependsmainlyontheligandfieldsplittingofthe5d orbitals.Thisligandfieldsplittingismainlydependentonthe coor-dinationsphereofthePr3+_ion.

We considerhereinfour differenthostmatrices,(CaF2, SrF2,

BaF2,and ␣-NaYF4)allcrystallizingin thecubic CaF2 structure

type(spacegroupFm−3m).ThePr3+_{isdopedintheunique}

eight-foldcoordinatedcationsitehavingOhsymmetry[14].Thecubic

␣-phaseofNaYF4isthehigh-temperaturepolymorph,unlikethe

lowtemperatureonewhichpossessesthehexagonal(spacegroup P−6)␤-phase[15].In␣-NaYF4thecationsiteisrandomlyoccupied

by½Na+_and½Y3+_[16].

The luminescenceof thesePr3+ _{activatedfluorideshasbeen}

experimentallydescribedbyseveralworkinggroups[17–19].The alkalineearthfluorides(CaF2:Pr3+,SrF2:Pr3+,andBaF2:Pr3+)show

inallthecasesbroadbandemissionduetothefactthatthelowest energylevelofthe[Xe]4f1_5d1_{configurationislocatedbelowthe}1_S

0

energylevel[17].Thustheligandfieldsplittingofthe5dorbitalsis expectedtobelarge.Ontheotherhand,the␣-NaYF4:Pr3+shows

lineemissionproperforPCEmaterials[18,19].Heretheligandfield interactionofthe5dorbitals iscertainlyweaker.Thesefindings

Published in &KHPLFDO3K\VLFV/HWWHUV±

which should be cited to refer to this work.

cannotbesolelyexplainedbythedifferentsizesofthereplaced cations,Ca2+_:126pm,Sr2+_:140pm,Ba2+_:156pmandY3+_:116pm

incontrasttoPr3+_{:126.6pm(allgivenforcoordinationnumber}

(CN)8[20]),sincethechemicalenvironmentandthesymmetryis inallcasesidentical.Rodnyietal.[21]reportedabouttheenergy shiftofthelowestlevelofthe[Xe]4f1_5d1_{configurationasfunction}

ofthePr3+_{concentrationinBaF}

2 crystals.Forhigher

concentra-tions(3.0%)PCEwasobservedduetotheformationofPr3+_-based

clusters.

Hereweapplyourvalidatedmodel[13]toexplainandpredict theabovementionedPCEphenomenon.Thismodelisalso essen-tialfortailoringphosphorstogeneratewarm-whitelight[22].The structureofthePr3+_{dopedsystemsisoptimizedusingperiodical}

calculationsbasedonDensityFunctionalTheory(DFT)bymeans oftheVASPcode[23,24].ThePBEfunctionalisusedto parame-terizetheexchange-correlationpotential[25].Thestructureofthe dopedsystemisobtainedtakingintoaccountasuper-cellmodel composedbyonePr3+_{ionin32unit-cellscorrespondingtoadopant}

concentrationof3%.ItisimportanttomentionthatthePr3+_ionsare

handledasisolatedions.Aselectivecutofthisoptimizedstructure isperformedtakingintoaccountthePr3+_{centreanditssurrounding}

ligands,subjected toa molecularcalculationusingthe Amster-damDensityFunctional (adf2013.01)programpackage [26,27]. WeusethecomputationalmodelnamedLFDFT[13,28,29],whose outputenablesthecomputationsoftheSlater–Condonparameters, spin–orbitcouplingconstantsandligandfieldpotential. Prepara-tionandopticalcharacterizationofCaF2:Pr3+,Na+;SrF2:Pr3+,Na+;

BaF2:Pr3+,Na+;and␣-NaYF4:Pr3+areperformed(seeSection4).The

dopinglevelwasisallcasesadjustedto1.0%toavoidclusteringof Pr3+_{inthepowdersamples.Thepreparedphosphorswereof}

sin-glephaseandluminescencespectrawererecordedfromtheVUV tothenearinfraredregion.

2. Resultsanddiscussion

ThecalculationsforthePr3+_{dopedalkalineearthfluoridesand}

␣-NaYF4 yieldtheparameterslistedinTable1.Inthetabletwo

columnsaredevotedtothe␣-NaYF4systemrelatingthetwo

spe-cialextremesituationswhichareillustratedbytakingthesecond coordinationspherecontainingexclusivelyNa+_(hereafterO

h(Na))

Table1

ElectrostaticandligandfieldparametersforallPr3+_{dopedhostswithO} hsite

sym-metry(incm−1_).

Host CaF2 SrF2 BaF2 ␣-NaYF4 ␣-NaYF4

Symmetry Oh Oh Oh Oh(Na) Oh(Y)

F2(ff)a 322.8 322.9 322.9 322.9 322.8 F4(ff)a 41.7 41.7 41.7 41.7 41.7 F6(ff)a 4.4 4.4 4.4 4.4 4.4 G1(fd)a 358.9 356.9 351.2 350.2 365.3 F2(fd)a 226.7 225.3 222.7 221.4 229.4 G3(fd)a 30.7 30.5 30.1 29.9 31.2 F4(fd)a 17.4 17.3 17.1 17.0 17.6 G5(fd)a 4.8 4.7 4.7 4.7 4.9 ␨4fb 710 710 710 710 710 ␨5db 945 945 945 945 945 e␴(f)c 599 538 491 450 673 e␲(f)c 257 232 213 164 279 e␴(d)c 14114 13009 12154 11652 16210 e␲(d)c 4705 4336 4051 3886 5404 AOM(fd)d 15183 17918 20030 21162 9851 a_F

k(ff), Fk(fd) and Gk(fd) refer to the interelectronic effect known as

Slater–Condonparameterswidelydescribedintextbooks[30,31].

b_{Spin–orbitcouplingconstantsforthe4fand5dorbitalswereobtainedusingthe}

relativisticZORAapproach[13].

c _{TheligandfieldinteractionisbetterrepresentedwithrespecttotheAngular}

OverlapModel(AOM)parameters[32].InthepresentcasefourAOMparameters areavailableduetotheproblemoftwo-open-shell4fand5delectrons.

d _{Wereferto[32]anddefine}

AOM(fd)accordingly.

Figure1.CalculatedmultipletenergylevelsofPr3+_dopedCaF

2,SrF2,andBaF2

correspondingtothe[Xe]4f2_{(red)andthe[Xe]4f}1_5d1_{configurations(blue).The}

structuresrepresentthecentralPr3+_{ion(grey)surroundedbyeightF}−_(green)and

thesecondcoordinationspherecontainingthecorrespondingalkalineearthcations. (Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderis referredtothewebversionofthisarticle.)

orY3+_(hereafterO

h(Y))ions.Inbothsituations,Ohsymmetryofthe

coordinationofthePr3+_isachieved.

ThetheoreticalresultsaregraphicallyrepresentedinFigure1, wherethemultipletenergylevelsoriginatingfromthe[Xe]4f2_and

the[Xe]4f1_5d1 _{configurationsaregiveninredand blue,}

respec-tively.Anoverlapbetweenbothmultipletenergylevelsisnoticed. ThusPCEisnotobservedasitisconfirmedbyourrelated exper-imental work (see Figure 2) which is also in linewith earlier observationsbyLawsonetal.[17].

Duetotheweakligandfieldinteractionofthe4forbitalsofPr3+

(seeparametersinTable1),inFigure1themultipletenergy lev-elsarisingfromthe[Xe]4f2_{configurationsseemtobeidenticalin}

anysystems.Ontheotherhand,theinfluenceofthechemical sur-roundingonthe5dorbitalsisstrongerwhichresultsinadifferent splittingofthe[Xe]4f1_5d1_{configurations.Inthepresentsituation}

the5dorbitalsofPr3+_splitovere

gandt2girreducible

representa-tionsoftheOhpointgroup.Therefore,themultipletenergylevels

fromthe[Xe]4f1_5d1_{configurationsaresplitintwodifferentbands}

fullyrecognizedinFigure1butalsointheexperimental character-izationbelow(seeFigure2).

TheionicradiiofCa2+_(126pm)andPr3+_{(126.6pm)forCN8are}

almostidentical,whereastheionicradiiofSr2+_(140pm)andBa2+

(156pm)aremuchlarger.Thismeans,thatthesizeofthecation whichisreplacedbyPr3+_{isincreasing(seeTable2)intheseries}

ofCaF2,SrF2andBaF2leadingtoadecreaseofthesplittingforthe

5dorbitalsofPr3+_{.Thetotalsplittingofthecalculatedmultiplet}

energylevelsbetweenegandt2gisspecifiedwith20910cm−1for

CaF2:Pr3+,19272cm−1forSrF2:Pr3+,and18006cm−1forBaF2:Pr3+.

TheluminescencespectraofthePr3+_{dopedalkalineearth}

fluo-ridesaremeasured(seeSection4)andgiveninFigure2.Sincewe focusonthelowestenergylevelofthe[Xe]4f1_5d1 _{configuration,}

excitationspectraweremonitoredintheVUVpartofthespectral range.Theexcitationspectraindicatethattherearetwoseparated bandswhichcanbecomparedtotheegandt2glevelsknownfor

Ohsymmetry.Thesplittingbetweenthesetwobandsisdecreasing

fromabout18230cm−1forCaF2:Pr3+,Na+(1.0%)and14580cm−1

forSrF2:Pr3+,Na+(1.0%)to13260cm−1forBaF2:Pr3+,Na+(1.0%).The

values were derived taking the energy difference between the twomaximaoftheexcitationbands,whereatitis importantto announcethat themeasurementsweredoneatroom tempera-ture(298K).Ourtheoreticalresultsoverestimatethembyabout 20%,althoughthecaseofCaF2:Pr3+areperfectlyinlinewiththe

Figure2.RoomtemperatureluminescencespectraofCaF2:Pr3+,Na+(1.0%),SrF2:Pr3+,Na+(1.0%)andBaF2:Pr3+,Na+(1.0%).VUVexcitationspectraareplottedontheleftand

emissionspectraontheright.Thedashedgreylinerepresentstheenergypositionofthe1_S

0energylevel.

observation of Oskam et al. [35]. They reported a ligand field splittingofthe5dorbitalsofabout21000cm−1derivedfrom exper-imentalwork(measuredat6K).Thediscrepancyoriginatesfrom thetemperaturedependenceofthet2g levels,meanwhiletheeg

levelsremainatthesameenergy.Thiswasalready observedby Loh [36] in former investigations of theUV absorption of Pr3+

dopedalkalineearthfluorides.Sincetheeglevelsremainatthe

sameenergy,changesintheemissionspectracannotbeobserved. Thehighestenergylevel1_S

0oftheground[Xe]4f2configuration

isexpectedtobelocatedcloseto47000cm−1asitisdetermined theoretically(seeFigure1)andwhichwasalreadydescribedin lit-erature[4,35].Therefore,thelowestenergylevelofthe[Xe]4f1_5d1

configurationisinallcasesoverlappingwiththehighestenergy level1_S

0.PCEforthesephosphorsisnotobservedwhereby the

emissionspectrainFigure2mainlyshowbroadbandemissionfrom thelowestenergylevelofthe[Xe]4f1_5d1_{configurationintheUV}

partofthespectralrange.IncaseofSrF2:Pr3+,Na+anintermediate

bandbetweentheegandthet2gonesappearswhichisduetoasmall

amountofresidualPrF3[37].However,thisleadstohigher

intensi-tiesofintraconfigurational[Xe]4f2_−[Xe]4f2_{transitionswhichcan}

onlybeobservedinthecaseofSrF2:Pr3+,Na+.

Incomparisontothebinaryalkalineearthfluoridestheternary ␣-NaYF4isinvestigated.The␣-polymorphexhibitsthecubicCaF2

structuretypewithNa+_andY3+_{randomlydistributedonthecation}

sites.Here,theaveragebondlengthiscomparabletotheoneofCaF2

(seeTable2).

TheresultsofthecalculationsaregiveninFigure3,following thesamerepresentationsasinFigure1.Thetwoabovementioned extremesituationsoftheOhsymmetryarecalculatedseparately.

Anoverlapbetweenbothmultipletenergylevelsisnoticedagain. Thus PCEisnotexpectedin bothsituations whichisin contra-dictiontoourexperimentalresults(seeFigure4)andalsotothe experimentalworkpublishedbySommerdijketal.[18].

Themultipletenergylevelsarisingfromthe[Xe]4f2

configu-rationsinFigure3seemtobeagainnearlyidenticalforthetwo differentOhsituations.Theinfluenceofthechemicalsurrounding

onthe5dorbitalsis muchstrongerwhich resultsina different splittingforthe[Xe]4f1_5d1_{configurations.Na}+_{hasalargerionic}

radiusthanY3+_{,leadingtosmallersplittingofthe5dorbitalsof}

Pr3+_{.Therefore,thelowestenergylevelofthe[Xe]4f}1_5d1

config-urationislocatedathigherenergybutstillbelowthe1_S 0energy

level.Thetotalsplittingofthecalculatedmultipletenergylevels betweenegandt2gisspecifiedwith17255cm−1forOh(Na)and

24015cm−1fortheOh(Y)symmetry.

Theluminescencespectraof ␣-NaYF4:Pr3+(1.0%)aregivenin

Figure4.Theexcitationspectrumshowsonebroadbandranging from48000to72000cm−1withtwonextneighbouredmaxima. Thehighestenergylevel1_S

0oftheground[Xe]4f2configurationis

Table2

Cation–anionbondlengthindifferenthosts(inpm).Literaturevaluesgivethebondlengthoftheundopedhostswhereasthecalculatedonesdisplaythevaluesofthe theoreticalPr–Fbond.Incaseof␣-NaYF4thedistancefromtheliteratureshowstheaveragevalue.

Host CaF2 SrF2 BaF2 ␣-NaYF4 ␣-NaYF4

Symmetry Oh Oh Oh Oh(Na) Oh(Y)

Literature(undoped) 236.6[33] 251.1[33] 268.5[33] 235.7[34] 235.7[34]

Calculated(doped) 237.2 240.9 244.0 245.7 231.8

Figure3.CalculatedmultipletenergylevelsofPr3+_{doped␣-NaYF}

4,corresponding

tothe[Xe]4f2_{(red)andthe[Xe]4f}1_5d1_{configurations(blue).Thetwogiven}

sym-metriesdifferinthesecondcoordinationspherecontainingjustNa+_(O

h(Na))orY3+

(Oh(Y)).ThestructuresrepresentthecentralPr3+ion(grey)surroundedbyeightF−

(green)andthesecondcoordinationspherecontainingNa+_(yellow)orY3+_(blue).

(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderis referredtothewebversionofthisarticle.)

stilllocatedatabout47000cm−1(seeFigure1and3).Therefore,the lowestenergylevelofthe[Xe]4f1_5d1_{configurationislocatedabove}

thehighestenergylevel1_S

0whichmeansthatPCEforthisphosphor

ispresent.Theemissionspectrumof␣-NaYF4:Pr3+isdominated

bysharp intraconfigurational[Xe]4f2_−[Xe]4f2 _{transitions which}

isingoodagreementwithpreviousresultsofSommerdijketal.

[18].TheemissionlinesintheUVpartofthespectralrange origi-natefromthe1_S

0energylevelwiththemaximumat24600cm−1

due to the 1_S

0–1I6 transition and representing the first

pho-ton.The emissionlinesin thevisiblepartof thespectral range originate from the 3_P

0 energy level and representing the

sec-ond photon. In theUV range at around 43500cm−1 there is a smallamountofbroademissionwhichcannotbeassignedtoany [Xe]4f2_–[Xe]4f2_{transitionknownforPr}3+_{andwhichwasalready}

describedelsewhere[19].Thiscouldleadbacktoasmallamountof interconfigurational[Xe]4f1_5d1_–[Xe]4f2_{emissionoriginatingfrom}

thelowestenergylevelofthe[Xe]4f1_5d1_{configuration.Incontrary}

to LaPO4:Pr3+ where the combination of sharp

intraconfigura-tional[Xe]4f2_–[Xe]4f2_{transitions andbroadinterconfigurational}

[Xe]4f1_5d1_–[Xe]4f2 _{transitions isonlyobservedatlow}

tempera-tures[38]herethisisalreadyoccursatroomtemperature. Toexplainthedifferencesbetweentheabovegiven theoreti-calresults(seeFigure3)andtheexperimentalmeasurements(see Figure4),newmodelswereperformedassumingadistortionof theOhsymmetry[39].Thisdistortionoriginatesfromthesecond

coordinationsphereofPr3+_{whichconsistsof12sitesrandomly}

occupiedbyNa+_(132pm)andY3+_{(116pm).Unlikeasmentioned}

in[40]thesecondcoordinationsphereisaffectingtheanionswhich arethenresponsibleforthechangeoftheligandfield.Sincethe ionicradiiaswellastheionicchargeofthesetwoionsareso dif-ferentdistortionofthesymmetryisexpected.Takingintoaccount thesecondcoordinationsphere,212_{=4096differentstructural}

con-formationsarepossible,fromwhichonlytwoextremesituations exhibitOhsymmetry.Duetothelargenumberofstructural

con-formations,hereweanalyzeonlythesituationwherethesecond coordinationspherecontainsexactly6Na+_and6Y3+_ionsbeside

thetwoextremesituationsdiscussedbefore(formoreinformation seeSection4).ThemodelparametersoftheOhcases(seeTable1)

aremodifiedforthedistortedconformationsusingtheAOMforthe angulardistortionandDFTfortheradialpart.Thusthedescentin symmetrygoesfromD3dtoonlyC1.Theresultsofthesecalculations

aregiveninFigure5.

Figure4.Roomtemperatureluminescencespectraof␣-NaYF4:Pr3+(1.0%).VUVexcitationspectrumisgivenontheleftandemissionspectrumontheright.Thedashedgrey

linerepresentstheenergypositionofthe1_S

0energylevel.

Figure5.Calculatedmultipletenergylevelsofthe[Xe]4f1_5d1_{configurationsofthe}

Pr3+_{doped␣-NaYF}

4indifferentsymmetries.Thesolidredlinecharacterizesthe

energyofthe1_S

0energyleveloftheground[Xe]4f2configuration.Thestructures

representthecentralPr3+_{ion(grey)surroundedbyeightF}−_{(green)andthesecond}

coordinationspherecontainingNa+_(yellow)andY3+_{(blue).(Forinterpretationof}

thereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversion ofthisarticle.)

Themultipletenergylevelsarisingfromthe[Xe]4f2

configura-tionsisnotpresentedinFigure5,sincetheinfluenceofthechemical surroundingisverylow.Theinfluenceofthechemicalsurrounding onthe5dorbitalsforthegivensymmetriesismuchstrongerwhich resultsforeachcaseinadifferentsplitting.Therefore,weachieve symmetrieswherethelowestenergylevelofthe[Xe]4f1_5d1

con-figuration islocatedbelow thehighestenergy level1_S

0 and do

notshowPCElikeD_3d.However,forsymmetriesviz.C_2h,C2v,Cs,

andC1thelowestenergylevelofthe[Xe]4f15d1configurationis

locatedabovethehighestenergylevel1_S

0whichleadstoPCE(see

Figure5).Noticethat lowersymmetrystructureswouldalsobe morecommonamongthe4096differentstructuralconfigurations thanhigherones.ThisexplainsthePCEprocessin␣-NaYF4asan

effectofloweringthesymmetryduetotheeffectofthesecond coordinationsphere,aswellasthesmallamountof interconfigu-rational[Xe]4f1_5d1_–[Xe]4f2_{emission,whichwouldcomefromPr}3+

ionsoccupyinghighersymmetrysites.

3. Conclusions

Insummary,thealkalineearthfluorides,(CaF2,SrF2andBaF2)do

notshowPCEsincethePr3+_{islocatedatacrystallographicposition}

withOhsymmetry.Thesplittingofthe5dorbitalsinthis

symme-tryislarger,insofarasthelowestenergylevelofthe[Xe]4f1_5d1

configuration is always locatedbelow the highest energy level

1_S

0 of theground [Xe]4f2 configuration. Our calculations fit to

theexperimental data,which showmainly interconfigurational [Xe]4f1_5d1_−[Xe]4f2_{emission.␣-NaYF}

4offersaswellOh

symme-tryforthecationposition.Thecalculationofsuchasystemwhere thePr3+ _{is locatedat a crystallographic positionwithO}

h

sym-metrywould leadtoamaterialwithoutshowingPCE. Thisisin contradictionwiththeexperimentaldatawhichshowmainly intra-configurational [Xe]4f2_−[Xe]4f2 _{emission lines.By lowering the}

symmetry starting froma highD3d tothe lowC1 this

extraor-dinary finding can be explained. The lower symmetries which describeadistortionofthecrystallographicpositioncanbe eas-ilyexplainedbythesecondcoordinationsphere.Moreover,with thesedifferentsymmetriesitispossibletoexplaintheemission

spectrawhichshowsinadditionasmallpartofinterconfigurational [Xe]4f1_5d1_−[Xe]4f2_{transitionsintheUVpartofthespectralrange.}

4. Experimental

4.1. Computationaldetails

VASPcalculationswereperformedtodeterminethe surround-ingstructureofPr3+_{inaperiodiclattice.PBEfunctionalwasused}

fortheexchange-correlationpotential.TheProjectorAugmented Wavemethod(PAW)wasusedtoreproducetheinteractionofthe valenceelectronswiththenucleiandcoreelectrons.Aplane-wave basissetwithacutoffenergyof400eVwasusedtoreproducethe electronicstates.Theatomicpositionswererelaxeduntilallforces weresmallerthan0.001eV/ ˚A.Fromtherelaxedstructure, molec-ularcalculationswereperformedwithADF2013.01toobtainthe electrostaticandligandfieldparameters.ATZ2P+basissetwasused forPr,andaTZPbasissetwasusedforF.Pointchargesareincluded atthedistanceofthePr3+_{secondcoordinationspheretoreproduce}

theeffectofthelatticepotential.ThedistortedstructuresofNaYF4

areobtainedfrommolecularcalculationsbyfixingthepositionsof theNa+_andY3+_{ionsinthesecondsphere,allowingtorelaxthePr}3+

andF−ions.

4.2. Experimentaldetails

Allphosphors were preparedby precipitationfrom aqueous solutionattheDepartmentofChemicalEngineering,Münster Uni-versity ofApplied Sciences. Theconcentrationof Pr3+_{in theso}

preparedphosphorswasadjustedto1%.

Incaseofthealkalineearthfluoridesthealkalineearthnitrates, viz. Ca(NO3)2·4H2O (ProAnalysi, Merck), Sr(NO3)2, (ProAnalysi,

Merck),andBa(NO3)2,(ProAnalysi,Merck),weredissolvedinwater

togetherwithstoichiometricamounts ofPr(NO3)3·6H2O(99.9%,

AlfaAesar)andsubsequentdropwiseadditionofdissolvedNH4HF2

(95%,AlfaAesar)untilprecipitationoccurred.Afterfiltrationand drying1%NaFwasaddedforchargecompensation.Themixtures werethenfilledintoglassy-carboncruciblesandsinteredfor6hat 900◦Cinanitrogenstream.Thenitrogen,whichwasusedtoavoid thereactionofthefluorideswithoxygen,wasfurtherpurifiedby dryingwithmolecularsieveandKOH.

Forthe␣-NaYF4astoichiometricmixtureofYCl3·6H2O(99.9%,

TreibacherIndustrieAG)andPrCl3·6H2O(99.9%,Sigma–Aldrich)

wasdissolvedinwaterandcombinedwithasolutionofatentimes molarexcess ofNaF(ProAnalysi,Merck).Precipitationoccurred immediatelyandafterfiltrationanddryingthemixturewasfilled intoa glassy-carboncrucibleand sinteredfor6hat600◦_{C ina}

nitrogenstream.Thistemperaturewaschosentoavoidthe trans-formationofthecubic␣-phaseofNaYF4tothehexagonal␤-phase.

4.3. Characterization

Allluminescencemeasurementswereperformedatthe Depart-ment of Chemical Engineering, Münster University of Applied Sciences.

Forallluminescencemeasurementsthepowdersampleswere pressedintoaPTFE-sampleholder,wherebythesurfacewas struc-turedbyusingawhitefleeceafterwards.Allinvestigatedpowder samples were excited at 160nm and the luminescence spec-tra recorded with a VUV spectrometer (Edinburgh Instruments FS920).ThespectrometerwasequippedwithaVUV monochroma-torVM504fromActonResearchCorporation(ARC)andadeuterium lampasanexcitationsource.Thesamplechamberwasflushedwith driedNitrogeninordertopreventabsorptionofVUVradiationby waterandoxygen.Excitationandemissionspectrawererecorded intherangesof120–250nmand200–800nm,respectively.The

emissionspectrawerecorrectedbyusingacorrectionfileobtained from a tungstenincandescent lamp certifiedby NPL. The rela-tiveVUVexcitationintensitiesofthesampleswerecorrectedby dividingthemeasuredexcitationspectraofthesampleswiththe excitationspectrumofsodiumsalicylate(o-C6H4OHCOONa)under

thesameexcitationconditions.Themeasurementswereperformed atambienttemperatureandpressurewithastepsizeof0.5nm.

Acknowledgements

ThisworkissupportedbytheSwissNationalScience Founda-tion(SNF),theSwissStateSecretariatforInnovationandResearch, theEuropeanCooperationinScienceandTechnology(COST)Action EUFEN (CM1006) and the Bundesministerium für Bildung und Forschung(BMBF)project13N9353.Dr.FanicaCimpoesufromthe UniversityofBucharestandDr.JörgNordmannformUniversityof Osnabrückarewarmlyacknowledgedforthefruitfuldiscussionand interestonthistopic.

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