Page5of16Journal ofMaterialsChemistryCView Article OnlingDOI:10.1039/C6TC02737GSP2300i). Finally, the signal was grasped by a thermoelectrically cooled CCDdetector (Andor).Room-temperature time-resolved photoluminescence (TRPL) spectra of thessamples wererecorded witha nanosecond time-resolved spectrometer(LP920 laserflash spectrometer,EdinburghInstruments Ltd).In the TRPLmeasurements,aQ-switched Nd:YAG laser(3th harmonic line at >=355 nm, 10 ns) was employed toilluminate the samples inside a 10 mm quartz cell.At a right angle the luminescencesignal was collected and guided into a monochromator before it was detected by a fastphotomultiplier tube, and recorded on a Tektronix model TDS 3012C digital signalanalyzer.Resultsand discussionAs mentioned earlier, four kinds of red phosphors were prepared in the present studyTheir synthesis may be described in the following reaction equations, respectively:for KTFM phosphor,xK,MnF,+(1-x)K, TiF。H>K,Tif,Mn,F.,(2)for KSFM phosphor,xK,MnF+2(1-x)KHE,+(1-x)H,SiFHF→K,Si-r)Mn,F,+4(1-x)HF,(3)forKGFM phosphor;xK,MnF,+2(1-x)KHF,+2(1-x)HF+(1-x)GeO, HF→K,Ge()Mn,F+2(1-x)H,O, (4)and for NaSFM phosphor,xK,MnF+2NaHF,+(1-x)H,SiF。HF→Na,Si(d-r)Mn,F+2xKF+(4-2x)HF,(5)in which x represents the concentration of Mn*+ ions.From the chemical equation (2)to (4), it can be seen that the B element compounds are different to each other, leadingtothreedifferentcrystallizationmechanisms.AsfortheK,TiF.Mn+alloy,it canbesynthesized by using K2TiF6 via a direct reaction with KMF in which cation exchangebetween Ti4+ and Mn++ takes place. This simpler reaction process can beaccomplished in a short time without using KHF2 powder as reagent or catalyzer'It isasimplebutefficientwaytoproduceKTFMphosphor.However,suchamethod doesnot efficiently work on the preparation of KSFM phosphor.Alternatively,H2SiFwaschosen as a B compound, and was assumed to spontaneouslyreact with Mn4+ and Kions before theformationof final product.In the caseofKGFM,GeO2powder waschosen as a B compound, and the relevant chemical reaction processes are morecomplicated compared with the former ones. It is anticipated that both the cationexchange and non-cation exchange reactions probably co-exist in the reaction process,andfinallyresultsinthegenerationofKGFMphosphors.ThepreparationofNaSFMis similar to that of KSFM except that KHF2 was replaced by NaHF2 as a reactant andcatalyzer, asshowninreaction equation(5)Shown in Fig. 2 are the XRD patterns of all the synthesized powders measured at5
5 SP2300i). Finally, the signal was grasped by a thermoelectrically cooled CCD detector (Andor). Room-temperature time-resolved photoluminescence (TRPL) spectra of the samples were recorded with a nanosecond time-resolved spectrometer (LP920 laser flash spectrometer, Edinburgh Instruments Ltd). In the TRPL measurements, a Q-switched Nd:YAG laser (3th harmonic line at λ=355 nm, 10 ns) was employed to illuminate the samples inside a 10 mm quartz cell. At a right angle the luminescence signal was collected and guided into a monochromator before it was detected by a fast photomultiplier tube, and recorded on a Tektronix model TDS 3012C digital signal analyzer. Results and discussion As mentioned earlier, four kinds of red phosphors were prepared in the present study. Their synthesis may be described in the following reaction equations, respectively: for KTFM phosphor, HF K MnF +(1- )K TiF K Ti Mn F 2 6 2 6 2 1- 6 x x x x → , (2) for KSFM phosphor; HF K MnF 2(1 ) KHF (1 ) H SiF K Si Mn F +4(1- )HF 2 6 2 2 6 2 (1- ) 6 x x x x x x + − + − → , (3) for KGFM phosphor; HF K MnF +2(1- )KHF +2(1- )HF+(1- )GeO K Ge Mn F +2(1- 2 6 2 2 2 (1- ) 6 2 x x x x x → x x )H O , (4) and for NaSFM phosphor; HF K MnF +2NaHF +(1- )H SiF Na Si Mn F +2 KF+(4-2 )HF 2 6 2 2 6 2 (1- ) 6 x x x x x x → , (5) in which x represents the concentration of Mn4+ ions. From the chemical equation (2) to (4), it can be seen that the B element compounds are different to each other, leading to three different crystallization mechanisms. As for the K2TiF6:Mn+4 alloy, it can be synthesized by using K2TiF6 via a direct reaction with KMF in which cation exchange between Ti4+ and Mn4+ takes place. This simpler reaction process can be accomplished in a short time without using KHF2 powder as reagent or catalyzer.1 It is a simple but efficient way to produce KTFM phosphor. However, such a method does not efficiently work on the preparation of KSFM phosphor. Alternatively, H2SiF6 was chosen as a B compound, and was assumed to spontaneously react with Mn4+ and K+ ions before the formation of final product. In the case of KGFM, GeO2 powder was chosen as a B compound, and the relevant chemical reaction processes are more complicated compared with the former ones. It is anticipated that both the cation exchange and non-cation exchange reactions probably co-exist in the reaction process, and finally results in the generation of KGFM phosphors. The preparation of NaSFM is similar to that of KSFM except that KHF2 was replaced by NaHF2 as a reactant and catalyzer, as shown in reaction equation (5). Shown in Fig. 2 are the XRD patterns of all the synthesized powders measured at Page 5 of 16 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
JournalofMaterialsChemistryCPage6of 16ViewArticle OnlinDOI:10.1039/C6TC02737G20 ranging from 10° to 80°.As seen in the figure, the obtained diffraction peaks canbe well indexed to the hexagonal structure of KMF (JCPDS 77-2133), the cubicstructureofKSFM(JCPDS75-0694)thehexagonalstructureofNaSFM(JCPDS33-1280)and thehexagonal structureof KTFM(JCPDS 08-0488),However, thediffraction patterns ofKGFM indicatea complexlattice structure probably consistingof two hexagonal phases with space groups of P3ml and P63mc respectively. It is0eworthnoticing that in a previous report, the double phases ofKGFMwas observedfora much higher synthesis temperature of above 473 k 21 We thus wish to say that acomplicatedcrystallization mechanismisoccurring inthe chemical synthesis ofKGFM phosphor.When GeO2 was added into the mixture solution of HE, KHF2 andKMF, two kinds of reactions probably simultaneously occur, and lead to thegeneration of H,GeFandK,GeF.In thiscase,taking into account the sameioniceeradii of Mn4+(0.054nm)and Ge4+(0.054nm)22,thegeneration ofKGFM could berealized through the two routes being similar as reaction equations (2) and (3)However,thedifferentreactionmechanismsmaydirectlyresultinvariationof crystalstructure of KGFM phosphor.(202)K2MnF6(203)(100)omo(104NB1910(111)(222)K2SiF6:Mn4(220)()(400)adaoin(311)(422)51)53142012(301)美Na2SiF6:Mn4+(11)20(30BOB(520)oof-(002)K2TiF6:Mn4101(2002)(100)20101K2GeF6:Mn4+(202-EOL红102030607080AO20 (50leunoFig.2XRDdifraction patternsof K,MnF6,K,SiFg:Mntt,NaSiF:MnK,TiF6:Mn4+andK2GeF6:Mn4+phosphors.In sharp contrast to the case of KGFM, the remaining three phosphors display highpurity in lattice structure (composition). For example, no KMF phase was identifiedfromtheirXRDpatterns.AninterestingcomparisoncomesfromcubicKSFMandhexagonal NaSFM.Thecrystal structureoftheformercompoundbelongsto thespace6
6 2θ ranging from 10o to 80o . As seen in the figure, the obtained diffraction peaks can be well indexed to the hexagonal structure of KMF (JCPDS 77-2133), the cubic structure of KSFM (JCPDS 75-0694), the hexagonal structure of NaSFM (JCPDS 33-1280) and the hexagonal structure of KTFM (JCPDS 08-0488). However, the diffraction patterns of KGFM indicate a complex lattice structure probably consisting of two hexagonal phases with space groups of P3m1 and P63mc respectively. It is worth noticing that in a previous report, the double phases of KGFM was observed for a much higher synthesis temperature of above 473 K.21 We thus wish to say that a complicated crystallization mechanism is occurring in the chemical synthesis of KGFM phosphor. When GeO2 was added into the mixture solution of HF, KHF2 and KMF, two kinds of reactions probably simultaneously occur, and lead to the generation of H2GeF6 and K2GeF6. In this case, taking into account the same ionic radii of Mn4+ (0.054nm) and Ge4+ (0.054nm)22, the generation of KGFM could be realized through the two routes being similar as reaction equations (2) and (3). However, the different reaction mechanisms may directly result in variation of crystal structure of KGFM phosphor. Fig. 2 XRD diffraction patterns of K2MnF6, K2SiF6:Mn4+, Na2SiF6:Mn4+ , K2TiF6:Mn4+ and K2GeF6:Mn4+ phosphors. In sharp contrast to the case of KGFM, the remaining three phosphors display high purity in lattice structure (composition). For example, no KMF phase was identified from their XRD patterns. An interesting comparison comes from cubic KSFM and hexagonal NaSFM. The crystal structure of the former compound belongs to the space Journal of Materials Chemistry C Page 6 of 16 Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G