ViewArticleOnlineR)CheckforupdatesView JournalJournalofMaterialsChemistryCAcceptedManuscriptThisarticlecanbecitedbeforepaqenumbershavebeenissued,todothispleaseuse:FTang,ZC.SuH.Ye,S.J.Xu,G.Wang.Y.Cao,W.GaoandX.Pan,J.Mater.Chem.C,2017,DOl:10.1039/C7TC04695BThisisanAcceptedManuscript,whichhasbeenthroughtheRoyalSocietyofChemistrypeerreviewprocessandhasbeenacceptedforpublication.JournalofMaterialsChemistryCAcceptedManuscriptsarepublishedonlineshortlyafteracceptance,beforetechnicalediting.formattingandproofreadingUsingthisfreeservice,authors canmaketheirresultsavailabletothe community.in citableform,beforewepublish theeditedarticle.Wewillreplacethis AcceptedManuscriptwiththeeditedandformattedAdvanceArticleas soonas itisavailableYoucanfindmoreinformationaboutAcceptedManuscriptsintheauthorguidelinesPleasenotethattechnical editingmayintroduceminorchangestothetextand/orgraphics,which.mayaltercontent.Thejournal'sstandardTerms&Conditionsandtheethicalguidelines,outlinedinourauthorandreviewerresourcecentrestill apply.Inno175eventshalltheRoyalSocietyofChemistrybeheldresponsibleforanyerrors oromissions in this AcceptedManuscriptoranyconsequencesarisingfromtheuseofanyinformationitcontainsROYALSOCIETYrsc.li/materials-cOFCHEMISTRY
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the author guidelines. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the ethical guidelines, outlined in our author and reviewer resource centre, still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. Accepted Manuscript rsc.li/materials-c Journal of Materials Chemistry C Materials for optical, magnetic and electronic devices www.rsc.org/MaterialsC ISSN 2050-7526 PAPER Nguyên T. K. Thanh, Xiaodi Su et al. Fine-tuning of gold nanorod dimensions and plasmonic properties using the Hofmeister eff ects ~ Volume 4 Number 1 7 January 2016 Pages 1–224 Journal of Materials Chemistry C Materials for optical, magnetic and electronic devices View Article Online View Journal This article can be cited before page numbers have been issued, to do this please use: F. Tang, Z.C. Su, H. Ye, S. J. Xu, G. Wang, Y. Cao, W. Gao and X. Pan, J. Mater. Chem. C, 2017, DOI: 10.1039/C7TC04695B
Page1of 8Journal of Materials Chemistry CCHEMISTTC04695BJournal NameARTICLEBoostingupphonon-induced luminescenceinredfluoridephosphors via composition variation driven structuralReceived 00th January 20xx,transformationsAccepted 00th January 20xxFei Tang,"Zhicheng Su,"Honggang Ye,"Shijie Xu,aWang Guo,Yongge Cao,Wenpei GaoandDOI:10.1039/×0xx00000xXiaoqing Pan dpawww.rsc.org/In this study, a series of (KNa-)2siFs:Mn red phosphors with systematic composition variations of alkali metals were兰synthesized via a low-temperature full-solution approach. Driven by the composition variation, a sequence of continuousstructural phase transformations, ie., from trigonal to mixed, then to orthorhombic, and eventually to cubic phase, is8evidently observed in this series of red phosphors. More excitingly, phonon-induced luminescence is promoted as theOmost efficient anddominant light emission mechanism in cubic phosphorof K,SiFs:Mn*tat room temperature.As a result,Uthe overall emission intensity of cubic KzsiFs:Mn*t is increased by more than 5 folds with respect to that of trigonalNazSiF:Mn*+,High-resolution x-ray diffraction, electron paramagnetic resonance and micro-Raman scattering experimentsAconsistently reveal a decisive relationship between fluorescence property and crystallinestructure.OKNaSiFa:Mn4+ is intermediate of those of homo-dialkalineIntroductionK2SiF6:Mn4+andNazSiF6:Mn4+redphosphors.7OtherEadvantages of transition-metal Mn4+-activated phosphorstIn recent years, the demand for high-efficiency red phosphorsinclude abundant resources of involved elements and mildShas rapidlyraised up notonlybecause of the critical roleofproduction conditions.Furthermore, thermal stability andsuch phosphors in the improvement of color rendering indexwtemperature robustness of Mn4+-activated fluoride phosphors(CRl)forphosphor-convertedwhitelight-emittingdiodes(pc-are the last but more technologically critical properties forWLEDs),butalsoduetothe significancethat the additionoftheir practical applications in pc-WLEDs.18-20theredcomponentcanefficientlyenlargethecolorgamutofback lighting in the LED-based display devices.1-8 Therefore,Ontheotherhand,pioneertheoretical studieshavefirmlyhuntingforsuchhigh-efficiencyredphosphorshasemergedasshownthattheforbiddenelectronictransitions inafreeatoman interesting but challenging subject in materials science andSmaybecome allowedand efficient when theatomissolid state lighting. Very recently, manganese ion (Mn4+)-incorporated into a crystal due to the mixing of impurityactivated fluorides and oxides have attracted an increasingelectronicstateswithhostphononsvialocalelectron-phononattention due to their high-efficiency yield and narrow-bandV coupling.21-26 Such transitions, usually termed as phonon-red light emissions.9-17 For example, Adachi et al. reported the0assisted or phonon-induced optical transitions or simplysynthesisof thehetero-dialkalinehexafluorosilicate redvibronic transitions,27 have been theoretically predicted tophosphorKNasiFg:Mn4+byetchingSiwafersinaaexhibitoutstandingtemperaturebehaviorssuchasHF/KMnOa/NaMnO4-HzOmixedsolution.7Theyemployedthetemperature induced great increase in emission probabilityX-ray diffraction analysis to reveal that the synthesizeddue to the deep participation of thermal phonons.26 Thesephosphorhasanorthorhombicstructurewiththespacegroupfoutstanding theoretical studies provide a solid base andOD26 - pnma .Moreover,they further investigated theguideline for developing high-efficiency and temperature-luminescenceproperties of thephosphor,and found that therobust impurity-activated phosphorsforpc-WLEDs.Inturn,theadevelopmentofsuchphosphorsalsooffersagreatchangefor=testing these theoretical predictions and pushing the furtherDepartment of Physics, and Shenzhen Institute of Research and Innovation (HKUISIRI), The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. Chinaprogress in relevant theoretical studies.Therefore, conducting5FujanInstituteofResearchontheStructureofMatter,ChineseAcademyfa systematicinvestigation ofMn4+-activatedfluorideSciences,Fuzhou350002, P.R.China0phosphorsof(K,Na1-x)2SiFwithavaryingalkalimetal.Department of Physics, Renmin University of China, Beijing100872, P.R.China.Department of Chemical Engineering andMaterials Science,Universityofcomposition is of both technical and scientific significance.California-rvine, Irvine,CA,USACorresponding author: Email:sjxu@hku.hkElectronicSupplementary Information (ESl)available:[Rietveld structuralrefinement on powder XRD patternsJ.See DOI: 10.1039/x0xx00000xThis journal is@TheRoyal Society of Chemistry2OxxJ.Name.,2013,00,1-3|1Please do not adjust margins
Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins Please do not adjust margins a.Department of Physics, and Shenzhen Institute of Research and Innovation (HKUSIRI), The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China. b.Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China. c.Department of Physics, Renmin University of China, Beijing 100872, P. R. China. d.Department of Chemical Engineering and Materials Science, University of California–Irvine, Irvine, CA, USA. * Corresponding author: Email: sjxu@hku.hk Electronic Supplementary Information (ESI) available: [Rietveld structural refinement on powder XRD patterns]. See DOI: 10.1039/x0xx00000x Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ Boosting up phonon-induced luminescence in red fluoride phosphors via composition variation driven structural transformations Fei Tang, a Zhicheng Su, a Honggang Ye, a Shijie Xu, a,* Wang Guo, b Yongge Cao, c Wenpei Gao d and Xiaoqing Pan d In this study, a series of (KxNa1-x)2SiF6:Mn4+ red phosphors with systematic composition variations of alkali metals were synthesized via a low-temperature full-solution approach. Driven by the composition variation, a sequence of continuous structural phase transformations, i.e., from trigonal to mixed, then to orthorhombic, and eventually to cubic phase, is evidently observed in this series of red phosphors. More excitingly, phonon-induced luminescence is promoted as the most efficient and dominant light emission mechanism in cubic phosphor of K2SiF6:Mn4+ at room temperature. As a result, the overall emission intensity of cubic K2SiF6:Mn4+ is increased by more than 5 folds with respect to that of trigonal Na2SiF6:Mn4+ . High-resolution x-ray diffraction, electron paramagnetic resonance and micro-Raman scattering experiments consistently reveal a decisive relationship between fluorescence property and crystalline structure. Introduction In recent years, the demand for high-efficiency red phosphors has rapidly raised up not only because of the critical role of such phosphors in the improvement of color rendering index (CRI) for phosphor-converted white light-emitting diodes (pcWLEDs), but also due to the significance that the addition of the red component can efficiently enlarge the color gamut of back lighting in the LED-based display devices.1-8 Therefore, hunting for such high-efficiency red phosphors has emerged as an interesting but challenging subject in materials science and solid state lighting. Very recently, manganese ion (Mn4+)- activated fluorides and oxides have attracted an increasing attention due to their high-efficiency yield and narrow-band red light emissions.9-17 For example, Adachi et al. reported the synthesis of the hetero-dialkaline hexafluorosilicate red phosphor KNaSiF6:Mn4+ by etching Si wafers in a HF/KMnO4/NaMnO4·H2O mixed solution.7 They employed the X-ray diffraction analysis to reveal that the synthesized phosphor has an orthorhombic structure with the space group D2h 16 − pnma . Moreover, they further investigated the luminescence properties of the phosphor, and found that the KNaSiF6:Mn4+ is intermediate of those of homo-dialkaline K2SiF6:Mn4+ and Na2SiF6:Mn4+ red phosphors.7 Other advantages of transition-metal Mn4+-activated phosphors include abundant resources of involved elements and mild production conditions. Furthermore, thermal stability and temperature robustness of Mn4+-activated fluoride phosphors are the last but more technologically critical properties for their practical applications in pc-WLEDs.18-20 On the other hand, pioneer theoretical studies have firmly shown that the forbidden electronic transitions in a free atom may become allowed and efficient when the atom is incorporated into a crystal due to the mixing of impurity electronic states with host phonons via local electron-phonon coupling.21-26 Such transitions, usually termed as phononassisted or phonon-induced optical transitions or simply vibronic transitions,27 have been theoretically predicted to exhibit outstanding temperature behaviors such as temperature induced great increase in emission probability due to the deep participation of thermal phonons.26 These outstanding theoretical studies provide a solid base and guideline for developing high-efficiency and temperaturerobust impurity-activated phosphors for pc-WLEDs. In turn, the development of such phosphors also offers a great change for testing these theoretical predictions and pushing the further progress in relevant theoretical studies. Therefore, conducting a systematic investigation of Mn4+-activated fluoride phosphors of (KxNa1-x)2SiF6 with a varying alkali metal composition is of both technical and scientific significance. Page 1 of 8 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
Page 2of 8Journal ofMaterials ChemistryCARTICLEJournal NameIn this article, we report a comprehensive study ofto form a golden-yellow solution. Then, both KHEa.and.NaHF2microstructural phasetransformationandresultantpromotionpowderswereadded into the aboveSolutfoi3heTFeeof phonon-induced luminescence in (KxNa1-x)2SiF:Mn4+ redcomposition of K and Na in compounds can be preciselyphosphorswithsystematicrelativecompositionvariationsofKcontrolled by changing the amount of KHF2 and NaHF2and Na metals for the first time. By using high-resolution x-raypowders.HzSiF6 solution was subsequently dropwised into thetdiffraction(XRD),high-resolutiontransmissionelectronobtainedsolutionandthesolutioncolorchangedfromgolden-P(TEM),andmicro-Ramanyellow to gold.Followed the filtration of thegolden solution,microscopelightscatteringEtechniques, we firmly show clear evidence for the structuralthe final redphosphorswithdifferent alkali metalCphase transformation from trigonal to orthorhombic andcompositions were prepared through washing and drying thesnurfurther to cubic phase in the phosphors for varying the relativeprecipitants for several times.composition of K with respect to Na from o to 1. MoreCharacterization techniques. XRD patterns of the powderexcitingly,thephonon-induced luminescence is boostedup sosignificantly that the best internal luminescence quantumsamples were measured on an X-ray diffractometer (Type D8?efficiency (QE) of 97.6% (external QE=73.0%) is obtained inAdvance EcO, Bruker,UK).Rietveld refinement work on the2K2SiF6:Mn4+ phosphor with cubic crystalline structure at roommeasured XRD data was carried out using the GSAS package.WeTEMtemperature.AalsoemployelectronparamagneticBoth low-and high-resolutionmeasurementswerearesonance (EPR)technique to examine the hyperfine states ofperformedonaJEOLJEM-2100TEMinstrumentoperatedate3d3 electrons of Mn ions within host local lattice structures.200okV.EPR spectra of thepowder samples were recorded on北Thetime-resolvedPLspectraof thisseriesof phosphorsalsoa BRUKER-BIOSPIN EPRinstrumentwithspectral resolutionofPoffersome consistent evidence for the boosting up of phonon-kHz.Photoluminescence excitation (PLE) spectra were1einduced light emission due to the lattice phase transition.measuredon ahome-assembled setup witha Xenon lampODifferent from the study on a particular fluoride phosphor,e.g.(Muller,Germany)as the excitation sourcezs by monitoring theUK2SiFg:Mn4+ or Na2SiFg:Mn4+, this work clearly shows us for thestrongest emission peak for each phosphor. High-resolution PLAfirst time the evolution progress of crystalline structure andspectra were measured at room-temperature on a home-related properties with the composition variation of K+ and Natassembled high-resolution PL setup29 using the 477 nm laserOions in red fluoride phosphorsbeam of an Ar+-Kr+ mixed gas laser as the excitation lightsource. Room-temperature Raman scattering spectrum ofKunsExperimentaleach phosphor was registered on a confocal micro-Ramansystem (WITech-Alpha) using the 514.5 nm line of an Ar+ ioneaeChemical and materials. The raw powders of potassiumgas laser as the excitation light source. Room-temperaturepermanganate (KMnOa), sodium hydrogen difluoride (NaHF2)time-resolved PL (TRPL) decay traces were recorded on aand potassiumhydrogen fluoride(KHF2)werepurchasedfromnanosecondtime-resolvedspectrometer(FLS920dSinopharm Chemical Reagent Co., China. Hydrofluoric acidspectrofluorimeter, Edinburgh Instruments Ltd).(HF), hydrogen peroxide solution (HzOz) and fluorosilicic acidResults anddiscussionsolution (H2SiFs) were purchased from Aladdin Reagent Co.(Shanghai). All these chemicals were directly used without anyfurther treatments.Mn4+-activated fluoride phosphors.SynthesisofAsdescribed previously elsewhere,13 the studied Mn4+-activatedfluoride phosphors were synthesized at low temperature of -16 oC by employing a two-step chemical co-precipitationmethod. Firstly, we prepared K2MnF6 powder as the Mn4+source material, and the specific preparation process is asfollows:KHF2powder was firstly dissolved into HF solutionunder vigorous stirring operation to form a uniform solution.Then,acertainamountofblackKMnO4powderwaspouredinto the above solution with further stirring operation for onehour to produce Modena solution.Followed by the slowly-dropping H2O2 solution, a brown-yellow solution will begenerated with some precipitant.After filtrating the obtainedsolution,a certain amount of brown precipitant can be gained.By drying and washing the participant for three times, weobtainedKzMnFs powder.The secondstepis toprepare(K.Na1-x)2SiF6:Mn4+ phosphors with x=1, 0.75, 0.5, 0.25, 0. Theprocedure is described as follows:The as-preparationsynthesizedKzMnF6powderwasfirstlymixedwithHFsolution21J.Name.,2012,00,1-3This journal is The Royal Society of Chemistry 20xxPlease donotadjust margin
ARTICLE Journal Name 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins In this article, we report a comprehensive study of microstructural phase transformation and resultant promotion of phonon-induced luminescence in (KxNa1-x)2SiF6:Mn4+ red phosphors with systematic relative composition variations of K and Na metals for the first time. By using high-resolution x-ray diffraction (XRD), high-resolution transmission electron microscope (TEM), and micro-Raman light scattering techniques, we firmly show clear evidence for the structural phase transformation from trigonal to orthorhombic and further to cubic phase in the phosphors for varying the relative composition of K with respect to Na from 0 to 1. More excitingly, the phonon-induced luminescence is boosted up so significantly that the best internal luminescence quantum efficiency (QE) of 97.6% (external QE=73.0%) is obtained in K2SiF6:Mn4+ phosphor with cubic crystalline structure at room temperature. We also employ electron paramagnetic resonance (EPR) technique to examine the hyperfine states of 3d3 electrons of Mn ions within host local lattice structures. The time-resolved PL spectra of this series of phosphors also offer some consistent evidence for the boosting up of phononinduced light emission due to the lattice phase transition. Different from the study on a particular fluoride phosphor, e.g. K2SiF6:Mn4+ or Na2SiF6:Mn4+, this work clearly shows us for the first time the evolution progress of crystalline structure and related properties with the composition variation of K + and Na+ ions in red fluoride phosphors Experimental Chemical and materials. The raw powders of potassium permanganate (KMnO4), sodium hydrogen difluoride (NaHF2) and potassium hydrogen fluoride (KHF2) were purchased from Sinopharm Chemical Reagent Co., China. Hydrofluoric acid (HF), hydrogen peroxide solution (H2O2) and fluorosilicic acid solution (H2SiF6) were purchased from Aladdin Reagent Co. (Shanghai). All these chemicals were directly used without any further treatments. Synthesis of Mn4+-activated fluoride phosphors. As described previously elsewhere,13 the studied Mn4+ -activated fluoride phosphors were synthesized at low temperature of - 16 oC by employing a two-step chemical co-precipitation method. Firstly, we prepared K2MnF6 powder as the Mn4+ source material, and the specific preparation process is as follows: KHF2 powder was firstly dissolved into HF solution under vigorous stirring operation to form a uniform solution. Then, a certain amount of black KMnO4 powder was poured into the above solution with further stirring operation for one hour to produce Modena solution. Followed by the slowlydropping H2O2 solution, a brown-yellow solution will be generated with some precipitant. After filtrating the obtained solution, a certain amount of brown precipitant can be gained. By drying and washing the participant for three times, we obtained K2MnF6 powder. The second step is to prepare (KxNa1-x)2SiF6:Mn4+ phosphors with x=1, 0.75, 0.5, 0.25, 0. The preparation procedure is described as follows: The assynthesized K2MnF6 powder was firstly mixed with HF solution to form a golden-yellow solution. Then, both KHF2 and NaHF2 powders were added into the above solution. The relative composition of K and Na in compounds can be precisely controlled by changing the amount of KHF2 and NaHF2 powders. H2SiF6 solution was subsequently dropwised into the obtained solution and the solution color changed from goldenyellow to gold. Followed the filtration of the golden solution, the final red phosphors with different alkali metal compositions were prepared through washing and drying the precipitants for several times. Characterization techniques. XRD patterns of the powder samples were measured on an X-ray diffractometer (Type D8 Advance ECO, Bruker, UK). Rietveld refinement work on the measured XRD data was carried out using the GSAS package. Both low- and high-resolution TEM measurements were performed on a JEOL JEM-2100 TEM instrument operated at 200 kV. EPR spectra of the powder samples were recorded on a BRUKER-BIOSPIN EPR instrument with spectral resolution of 1 kHz. Photoluminescence excitation (PLE) spectra were measured on a home-assembled setup with a Xenon lamp (Müller, Germany) as the excitation source28 by monitoring the strongest emission peak for each phosphor. High-resolution PL spectra were measured at room-temperature on a homeassembled high-resolution PL setup29 using the 477 nm laser beam of an Ar+ -Kr+ mixed gas laser as the excitation light source. Room-temperature Raman scattering spectrum of each phosphor was registered on a confocal micro-Raman system (WITech-Alpha) using the 514.5 nm line of an Ar+ ion gas laser as the excitation light source. Room-temperature time-resolved PL (TRPL) decay traces were recorded on a nanosecond time-resolved spectrometer (FLS920 spectrofluorimeter, Edinburgh Instruments Ltd). Results and discussion Journal of Materials Chemistry C Page 2 of 8 Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
Page3of 8JournalofMaterialsChemistryCJournalNameARTICLE(a)(b)photographs of the synthesized powder phospboreletramwhichwe seethatthe samplecolorgraaualyzAdngesforX-0faint orange-yellow of NazSiFs:Mn4+ phosphor to bright goldenyellow of K2SiF6:Mn4+phosphor.This color change may implyLOsignificant change in both structure and PL properties, as1P321shown and evidenced later.x=0.25(a)NaSiFe:Mn"(b)PDF872-1115trigonalx=0.50(KaNaau)SiFMn()smixedPnma(KaNaSiFaMn)BE国PDFX81-0009童orthorhombic服工牌身x=0.75(KaNaus):SiFe:Mn*cubicKiSiFa:MnPDF81-2264cubic电20 (degrees)20 (degrees)x=1Fm-3mFigure 2. (a) XRD diffraction patterns of (KxNai-x)2SiF6:Mn4*redphosphors with x=1,0.75, 0.50, 0.25, and0 from bottom to top.(K.Na1-x)SiF6:Mn4+OKONaOSi OFOMn(b) Replotted XRD patterns at low diffraction angles.Structural5phase transformation can be clearly seen as the relativeFigure 1. (a)Three types of crystalline structures of K2SiF6:Mn4+,content of K and Na is systematically varied.KNaSiF6:Mn4+ and NazSiF6:Mn4+ phosphors with space groupsof Fm-3m,Pnma andP321.(b)Photographs of theas-synthesizedpowderphosphors(KNa1-x)zSiF:Mn4+(frombottomtotop:x=1,0.75,0.50,0.25,and0)Inordertoelucidatethestructuralphasetransformationinthe studied series of (K,Na1-x)2SiFg:Mn4+ red phosphors, fineXRD measurements were carried out, and the results areShown in Figure 1(a) are the three types of crystallineillustrated in Figure 2(a).The XRD patterns at low diffractionstructures of K2SiF6:Mn4+ (cubic), KNaSiFg:Mn4+ (orthorhombic)angles are enlarged in Figure 2(b),so thatthe solid evidenceAo uo persiedandNazSiFg:Mn4+phosphors(trigonal).Theirfull-solutionfor phasetransformation can be clearly seen.From thesynthesis process may be concisely described by the followingexperimental xRDpatterns,three majorlattice phaseSreactionequationsstructurescan be determined:cubic phase characterized byaxK2MnF6+2(1-x)KHF2+(1-x)H2SiF6HE_)spacegroup of Fm3mforK2siF6:Mn4+, trigonal phase witha?(1) space group of P321 for NazSiFg:Mn4+,and orthorhombicK2Si(lr)Mn.F6 + 4(1 x)HFphase with a space groupof Pnma for (Ko.5Nao.s)2SiFg:Mn4+or0simply KNaSiFg:Mn4+, For (Ko.25Nao.75)2SiFg:Mn4+ phosphor, itfor K2SiF6:Mn4+phosphor:possess a mixed phase of trigonal and orthorhombic structures.xK2MnF6+2NaHF2 +(1-x)H2SiF6—H)(2)Asshown inFigure2(a),theexperimental XRDpatternswereNa2Sil)Mn.F6+ 2xKF +2(2 - x)HFfinely simulated with standard base data (e.g.JCPDF 81-2264,JCPDF81-0009andJCPDF72-1115).Toexaminetheinfluencefor Na2SiFg:Mn4+ phosphor; andof alkali metal composition variation on the crystallineOxK2MnF6+(1-x)KHF2 +NaHF2 +(1-x)H2SiF6structures of the phosphors more accurately,we did fine(3) H>KNaSi()Mn.F6+ xKF+(4-3x)HFtheoretical simulations on the measured XRD result of eachsample with Rietveld method, as depicted in Figure S1-S5 inunfor KNaSiFa:Mn4+ phosphor.the supporting document. Obtained lattice parameters for theInaboveequations,xrepresentstheconcentrationofMn4+ion.three major types of crystalline structures are tabularized inNote that inK2iF6:Mn4+phosphor,K+ion originates frombothTable 1.K,MnFand KHFz,whereasNa element inthecombinatorialphosphors is mainly supplied by the NaHF2 compound. Moredetaileddescriptiononthelow-temperaturesynthesiscanbereferred to one previous publication.13 Figure 1(b) presentsThis journal is @TheRoyal Society of Chemistry2OxxJ.Name.,2013,00,1-33Please donotadjustmargins
Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins Figure 1. (a) Three types of crystalline structures of K2SiF6:Mn4+ , KNaSiF6:Mn4+ and Na2SiF6:Mn4+ phosphors with space groups of Fm-3m, Pnma and P321. (b) Photographs of the assynthesized powder phosphors (KxNa1-x)2SiF6:Mn4+ (from bottom to top: x=1, 0.75, 0.50, 0.25, and 0). Shown in Figure 1(a) are the three types of crystalline structures of K2SiF6:Mn4+ (cubic), KNaSiF6:Mn4+ (orthorhombic) and Na2SiF6:Mn4+ phosphors (trigonal). Their full-solution synthesis process may be concisely described by the following reaction equations: HF 2 6 2 2 6 2 (1- ) 6 K MnF 2(1- )KHF (1- )H SiF K Si Mn F 4(1 )HF x x x x x x , (1) for K2SiF6:Mn4+ phosphor； HF 2 6 2 2 6 2 (1- ) 6 K MnF 2NaHF (1- )H SiF Na Si Mn F 2 KF 2(2 )HF x x x x x x , (2) for Na2SiF6:Mn4+ phosphor; and 2 6 2 2 2 6 HF (1- ) 6 K MnF (1- )KHF NaHF (1- )H SiF KNaSi Mn F KF (4-3 )HF x x x x x x x , (3) for KNaSiF6:Mn4+ phosphor. In above equations, x represents the concentration of Mn4+ ion. Note that in K2SiF6:Mn4+ phosphor, K + ion originates from both K2MnF6 and KHF2, whereas Na element in the combinatorial phosphors is mainly supplied by the NaHF2 compound. More detailed description on the low-temperature synthesis can be referred to one previous publication.13 Figure 1(b) presents photographs of the synthesized powder phosphors, from which we see that the sample color gradually changes from faint orange-yellow of Na2SiF6:Mn4+ phosphor to bright golden yellow of K2SiF6:Mn4+ phosphor. This color change may imply significant change in both structure and PL properties, as shown and evidenced later. Figure 2. (a) XRD diffraction patterns of (KxNa1-x)2SiF6:Mn4+red phosphors with x=1, 0.75, 0.50, 0.25, and 0 from bottom to top. (b) Replotted XRD patterns at low diffraction angles. Structural phase transformation can be clearly seen as the relative content of K and Na is systematically varied. In order to elucidate the structural phase transformation in the studied series of (KxNa1-x)2SiF6:Mn4+ red phosphors, fine XRD measurements were carried out, and the results are illustrated in Figure 2(a). The XRD patterns at low diffraction angles are enlarged in Figure 2(b), so that the solid evidence for phase transformation can be clearly seen. From the experimental XRD patterns, three major lattice phase structures can be determined: cubic phase characterized by a space group of Fm3̅m for K2SiF6:Mn4+ , trigonal phase with a space group of P321 for Na2SiF6:Mn4+, and orthorhombic phase with a space group of Pnma for (K0.5Na0.5)2SiF6:Mn4+ or simply KNaSiF6:Mn4+. For (K0.25Na0.75)2SiF6:Mn4+ phosphor, it possess a mixed phase of trigonal and orthorhombic structures. As shown in Figure 2(a), the experimental XRD patterns were finely simulated with standard base data (e.g. JCPDF 81-2264, JCPDF 81-0009 and JCPDF 72-1115). To examine the influence of alkali metal composition variation on the crystalline structures of the phosphors more accurately, we did fine theoretical simulations on the measured XRD result of each sample with Rietveld method, as depicted in Figure S1-S5 in the supporting document. Obtained lattice parameters for the three major types of crystalline structures are tabularized in Table 1. Page 3 of 8 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
JournalofMaterialsChemistryCPage4of8ARTICLEJournalNameTable 1.Rietveldrefinedlatticeparameters for thebecauseof both strongewAuantumopticalpropertiesphosphorswiththreetypesofcrystallineconfinement effect and remarkable redGctin0/ong9rgesynthesizedelectron-phonon interaction.30 In particularly,it may favor thestructures.phonon-induced luminescence and thus may result in highluminescence efficiency at roomtemperature and even higher.PhospCrystalLatticeLatticeLatticeUnit CellhorsVolumeStructureParametParameterParameterb (A)c (A)(A3)era (A)8.1258288.1258288.125828536.541K2SiFe:CubicMn4+士土土±0.0180.0000900.0000900.000090KNasiF9.3304595.5093029.804473Orthorho503.9926:Mn**土土mbic土±0.051200nm200.nm500nm0.0003590.0002110.000342o(e)NazSiFtrigonal8.8671808.8671805.047439343.6956:Mn4+士±土±0.0110.0001160.0001160.0000892nm200nmInadditiontotheXRDdataandanalysis,weemployedhigh-Figure 3. (a), (b), and (c) show TEM images of the synthesizedresolution TEM+selected area electron diffraction (SAED)tocubic K2SiFg:Mn4+, orthorhombic KNaSiFg:Mn4+,and trigonaldo a direct microstructural characterization on the samples.NazSiFg:Mn4+,respectively. The inset figures in (a), (b), and (c)Figure 3(a), (b)and (c) show typical TEM images of cubicaopeooashowtheir corresponding SAED patterns.(d)High-resolutionKNaSiFg:Mn4+,K,SiF6:Mn4+orthorhombicandtrigonalTEM image of the K-richcombinatorial phosphor.The insetNa2SiFg:Mn4+ phosphors,respectively.Their respective SAEDillustratesafastFouriertransformationpatternofthisHR-TEMimage.(e)TEM image of theNa-rich phosphorand its SAEDpatterns are depicted as the inset figures in upper rightcorners of respective TEM images. Figure 3(d) depicts a high-patterns (the inset figure).resolutionTEMimageoftheK-richphosphor(K75%+Na25%)Its fast Fourier transformation patterns are shown in the insetZfigure.Two different crystallineplanes canbe identified withNaSiFs:Mntalmostequal interplanarspacingof~0.307nm,asmarkedintrigonalthe image. Nevertheless, the measured angle between thecrystalline planes differsfrom the theoretical value,suggesting(ne) is d(KasNans)SiFe:Mnthe appearance of serious lattice distortion in the K-richmixedphosphor. Such lattice distortion shall be induced by thepartial substitution of K by Na. As for the Na-rich phosphor (K(Ka.soNanso)SiFe:Mn*25%+Na75%),itsTEMandcorrespondingSAEDpatternsareAshown in Figure 3(e). Compared with the SAED patterns oforthorhombicother phosphors, the SAED patterns of the Na-rich phosphor(KasNa)SiFe:Mn"display more complicated features.Such results are expectedbecausetheNa-richphosphorpossessesamixedlatticephasecubicof trigonal and orthorhombic structures,justifiedfrom the XRDKasiFs:Mnt0data.Thecharacteristic SAEDpatternsconsistingof rings anddiscrete bright spots in Figure 3(a), (b), (c)and (e) indicate thatcubicAallthesynthesizedphosphorsseemofnano-scaled590600610620630640650660polycrystalline.However,considering the sharp xRD patternsWavelength (nm)and theSEM images(not shownhere,but referredto thepreviousonespublishedelsewhere13),suchaspecialmaterialFigure 4. PL spectra of (KxNa1-x)2SiF6:Mn4+ (with x=1, 0.75, 0.50,structure composed of distorted nanocrystals +amorphous0.25,and0)phosphorsmeasuredatroomtemperature.Thesurrounding matrix is believed to be most likely caused by thedashed downward arrows in the figure indicate the zero-destructiondueto the bombardment of high-energy electronphonon lines (ZPL).Dbeam on thefluoride phosphors in TEMmeasurement.If suchstructure is produced under the bombardment of high-energyelectron or laserbeam, and it may exhibit some extraordinary4/J.Name.,2012,00,1-3This journal is@TheRoyal Societyof Chemistry20xxPleasedonotadjust margins
ARTICLE Journal Name 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Table 1 ． Rietveld refined lattice parameters for the synthesized phosphors with three types of crystalline structures. Phosp hors Crystal Structure Lattice Paramet er a (Å) Lattice Parameter b (Å) Lattice Parameter c (Å) Unit Cell Volume (Å3 ) K2SiF6: Mn4+ Cubic 8.125828 ± 0.000090 8.125828 ± 0.000090 8.125828 ± 0.000090 536.541 ±0.018 KNaSiF 6:Mn4+ Orthorho mbic 9.330459 ± 0.000359 5.509302 ± 0.000211 9.804473 ± 0.000342 503.992 ±0.051 Na2SiF 6:Mn4+ trigonal 8.867180 ± 0.000116 8.867180 ± 0.000116 5.047439 ± 0.000089 343.695 ±0.011 In addition to the XRD data and analysis, we employed highresolution TEM + selected area electron diffraction (SAED) to do a direct microstructural characterization on the samples. Figure 3(a), (b) and (c) show typical TEM images of cubic K2SiF6:Mn4+, orthorhombic KNaSiF6:Mn4+, and trigonal Na2SiF6:Mn4+ phosphors, respectively. Their respective SAED patterns are depicted as the inset figures in upper right corners of respective TEM images. Figure 3(d) depicts a highresolution TEM image of the K-rich phosphor (K 75% + Na 25%). Its fast Fourier transformation patterns are shown in the inset figure. Two different crystalline planes can be identified with almost equal interplanar spacing of ~0.307 nm, as marked in the image. Nevertheless, the measured angle between the crystalline planes differs from the theoretical value, suggesting the appearance of serious lattice distortion in the K-rich phosphor. Such lattice distortion shall be induced by the partial substitution of K by Na. As for the Na-rich phosphor (K 25% + Na 75%), its TEM and corresponding SAED patterns are shown in Figure 3(e). Compared with the SAED patterns of other phosphors, the SAED patterns of the Na-rich phosphor display more complicated features. Such results are expected because the Na-rich phosphor possesses a mixed lattice phase of trigonal and orthorhombic structures, justified from the XRD data. The characteristic SAED patterns consisting of rings and discrete bright spots in Figure 3(a), (b), (c) and (e) indicate that all the synthesized phosphors seem of nano-scaled polycrystalline. However, considering the sharp XRD patterns and the SEM images (not shown here, but referred to the previous ones published elsewhere13 ), such a special material structure composed of distorted nanocrystals + amorphous surrounding matrix is believed to be most likely caused by the destruction due to the bombardment of high-energy electron beam on the fluoride phosphors in TEM measurement. If such structure is produced under the bombardment of high-energy electron or laser beam, and it may exhibit some extraordinary optical properties because of both strong quantum confinement effect and remarkable reduction of long-range electron-phonon interaction. 30 In particularly, it may favor the phonon-induced luminescence and thus may result in high luminescence efficiency at room temperature and even higher. Figure 3. (a), (b), and (c) show TEM images of the synthesized cubic K2SiF6:Mn4+, orthorhombic KNaSiF6:Mn4+, and trigonal Na2SiF6:Mn4+, respectively. The inset figures in (a), (b), and (c) show their corresponding SAED patterns. (d) High-resolution TEM image of the K-rich combinatorial phosphor. The inset illustrates a fast Fourier transformation pattern of this HR-TEM image. (e) TEM image of the Na-rich phosphor and its SAED patterns (the inset figure). Figure 4. PL spectra of (KxNa1-x)2SiF6:Mn4+ (with x=1, 0.75, 0.50, 0.25, and 0) phosphors measured at room temperature. The dashed downward arrows in the figure indicate the zerophonon lines (ZPL). Journal of Materials Chemistry C Page 4 of 8 Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B