Emission Red Shift and Unusual Band Narrowing of Mn2+ in

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J. Phys. Chem. A 2010, 114, 6927–6934

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Emission Red Shift and Unusual Band Narrowing of Mn2+ in NaCaPO4 Phosphor Liang Shi,† Yanlin Huang,‡ and Hyo Jin Seo*,† Department of Physics, Pukyong National UniVersity, Busan 608-737, Republic of Korea, and College of Chemistry, Chemical Engineering and Materials Science, Soochow UniVersity, Suzhou 215123, China ReceiVed: February 26, 2010

Concentration dependence of Mn2+ luminescence in NaCaPO4/Mn2+ is investigated by structural analyses and optical and laser excitation spectroscopies in the temperature range 19-300 K. NaCaPO4/Mn2+ forms solid solution over the Mn2+ concentration range 1.0-22 mol %. We observe the red shift and unusual band narrowing of Mn2+ emission by increasing Mn2+ concentration in NaCaPO4. The lifetime of Mn2+ emission lengthens unexpectedly for higher Mn2+ concentration. The results are discussed in relation with crystal structure, photon reabsorption, exchange interaction, and energy transfer and energy migration in NaCaPO4/ Mn2+. I. Introduction An emission shift occurs by controlling surrounding environments of optically active ions doped in host materials such as semiconductors,1,2 organic materials,3 and dielectrics.4-8 The control of emission shift leads to tune a color point of phosphor.4,7 Recently, considerable attention has been paid to the study of color point tuning of phosphors because of its potential application in improved white light-emitting diodes (LEDs).4,5 The available white light LEDs in market are produced by combination of blue light from the LED and the yellow emission from the most widely applied YAG/Ce phosphor. However, the available white LED gives a relatively cool white light; therefore, it requires a phosphor with red-shifted emission to obtain pc-LEDs (phosphor converted LEDs) emitting warmer white light by the blue LED chip excitation. The modified surroundings influence the crystal field strength acting on activators, the covalency between dopant ion and ligands, and the interaction with lattice vibration leading to emission shift. The emission shift has been achieved by changing concentration of activator ions (e.g., (Sr,Ca,Ba)Si2O2N2/Eu2+,4 Sr2SiO4/Eu2+,6 NaCaPO4/Eu2+, Mn2+,9-11 Ca3(SiO4)Cl2/Eu2+/ Mn2+,12 and LaAl(Si6-zAlz)(N10-zOz) (ref 5)) and by partial substitution of host cations in the lattice by foreign cations with either larger or smaller ionic radii (e.g., (Sr,Ba,Ca)SiO4/Eu2+,6 (Sr,Ba,Ca)3SiO5/Eu2+,14 (Y,Gd,La)3Al5O12/Ce3+,13 Y3(Al,Ga)5O12/ Ce3+,6 (Ca,Sr)MgSi2O7/Eu2+).15 In these phosphors, it has been suggested that the emission red shift is due to the energy transfer and energy migration,4 structural changes of host lattice and changes in crystal field strength,4-6,13,14 photon reabsorption,16 and exchange interaction.17-21 However, a different mechanism was applied to the emission red shift in individual phosphors depending on the type of host lattice and dopant ion. It is hard to understand clearly the mechanisms of the emission shift because numerous factors are involved in this phenomenon accompanying changes in spectral features (band broadening or narrowing), relaxation dynamics, and luminescence efficiencies. Usually the band broadens homogeneously and inhomogeneously with temperature and lattice disorder, respectively. The changes in bandwidth by doping with foreign * Corresponding author. E-mail: [email protected]. † Pukyong National University. ‡ Soochow University.

cations or dopant ions are often observed in a variety of phosphors. However, no discussion of changes in bandwidth has been given in detail in the above-mentioned reports, which were only focused on emission shift for color tuning. Moreover, the band narrowing by increasing dopant concentration has rarely been reported. The Mn2+ ions doped in a host lattice show a broad band emission due to the 4T1 f 6A1 transition within the 3d shell in which the electrons are strongly coupled to lattice vibration and affected by crystal field strength and site symmetry. The different crystal field strength on Mn2+ can tune the emission color point, which varies from green (strong crystal field) to orange/red (weak crystal field). Because of the spin and parity selection rules, Mn2+ luminescence in a host lattice has a relative long lifetime (on the order of 10 ms). Therefore, the Mn2+ dopant ion can serve as a sensitive probe of chemistry and structure of its host and interaction between Mn2+ and surrounding oxygen ions. Recently, MIMIIPO4-type (MI and MII are monovalent alkali metal ions and diavalent alkaline earth ions, respectively) phosphors doped with rare earth and transition metal ions have been reported in KSrPO4,21-22 NaCaPO4,9 KBaPO4,23 LiBaPO4,24 LiSrPO4,25 and MBaPO4 (M ) Na, K).26 In these types of phosphors, a large amount of dopant ions can be introduced because two kinds of cations (alkali and alkaline earth cations) have ionic radii large enough for substitution. Eu2+-, Tb3+-, and Mn2+-doped NaCaPO4 have been reported for the application of the white LEDs in which the NaCaPO4 phosphors exhibit high concentration quenching of the luminescence, high thermal stability, and emission red shift by increasing concentration.9-11 In this article, we investigate luminescence properties of NaCaPO4/Mn2+ phosphors and observe energy transfer, emission red shift, and band narrowing with increasing Mn2+ concentration. Two different types of substitution sites are identified for Mn2+ in NaCaPO4 lattice. The unexpected emission-band narrowing and lengthening of the measured lifetimes with increasing Mn2+ concentration are reported. The mechanisms are discussed in relation with the photon reabsorption process together with energy transfer and energy migration, structural properties of host lattice, and exchange interaction in the Mn2+ pair in NaCaPO4.

10.1021/jp101772z  2010 American Chemical Society Published on Web 06/14/2010

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Shi et al.

Figure 2. Excitation and emission spectra of NaCaPO4/Mn2+ (1.0%) at room temperature. Whereas the monitoring wavelength for the excitation spectrum is 620 nm, the emission spectrum was reordered under the excitation at 420 nm.

Figure 1. Projection of half a unit cell on the (010) plane of the NaCaPO4 lattice.

II. Experimental Section NaCaPO4/Mn2+ phosphors were synthesized by solid-state reaction. The Mn2+ concentration is varied from 1.0 to 22 mol % relative to Ca2+. The starting materials were a stoichiometric mixture of reagent grade Na2CO3, CaCO3, NH4H2PO4, and MnCO3. The mixtures were heated to 350 °C and then kept at this temperature for 10 h. The obtained powders were thoroughly mixed and then heated to 750 °C and kept at this temperature for 8 h. After that, the samples were mixed and heated to 900 °C for 10 h in crucibles along with the reducing agent (active carbon). X-ray diffraction (XRD) data were collected on a Rigaku D/Max diffractometer operating at 40 kV, 30 mÅ with Bragg-Brentano geometry using Cu KR radiation (λ ) 1.5405 Å). The excitation spectrum was recorded on a Perkin-Elmer LS-50B luminescence spectrometer with Monk-Gillieson-type monochromator and a 20 kW xenon discharge lamp. The timeintegrated emission at the temperature range 19-300 K was obtained by 355 nm pulsed Nd/YAG (yttrium aluminum garnet) laser (Spectron Laser Systems SL802G). The samples were placed at cold finger in a He gas recycled cryostat. The decays were recorded by the 500 MHz digital oscilloscope (LeCroy 9350A). III. Results 1. Crystal Structures and Substitution Sites for Mn2+. Phosphates of MIMIIPO4 (MΙ ) Na+, K+; MII ) Ca2+, Sr2+, Ba2+) formula have a structure related to the β-K2SO4 type. The structure of NaCaPO4 is described as a superstructure of β-K2SO4.27,28 As shown in Figure 1, the Na+ ions form zigzag edge-sharing chains with alternating [PO4] tetrahedra along the a axis, and the Ca2+ ions are located in chains with slight

alternative displacements over the a axis. There exist three different Na sites (Na(I), Na(II), and Na(III)) and three different Ca sites (Ca(I), Ca(II), and Ca(III)) in the NaCaPO4 lattice, as shown in Figure 1. Each kind of substitution site has a slightly different average distance between the cation and ligands. The average distances of Na(I)-O, Na(II)-O, and Na(III)-O are 2.74, 2.73, and 2.74 Å, which are larger than those of Ca(I)-O (2.45 Å), Ca(II)-O (2.48 Å), and Ca(III)-O (2.50 Å), respectively. It is known that dopant ions such as Mn2+ and Eu2+ can substitute for both Na(I, II, and III) and Ca(I, II, and III) sites, which are common in this kind of phosphate.29,30 In NaCaPO4, the Na+ ions are found in a ten-fold coordination considering the distances 200 K. The emission band A is observed only at low temperature and low Mn2+ concentration. For Mn2+ concentration of 22% in NaCaPO4 (Figure 3b), the emission band B shows a large red shift and narrowed bandwidth in comparison with 1.0% Mn2+-doped NaCaPO4 (Figure 3a). The emission band A for higher Mn2+ concentration remarkably weakens even at 19 K, as indicated by arrow in Figure 3b. The luminescence is enhanced by increasing Mn2+ concentration until a maximum intensity reaches 16% Mn2+; then, it decreases at the concentration >16% because of concentration quenching. The normalized emission spectra of NaCaPO4/Mn2+ for various Mn2+ concentration (1.0-22%) at 19 and 300 K are shown in Figure 4. Red shift and band

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Figure 5. (a) Bandwidth and (b) peak position of Mn2+ emission B as a function of Mn2+ concentration in NaCaPO4 at various temperatures (19-300 K).

narrowing of emission B are significant with increasing Mn2+ concentration in NaCaPO4. The intensity of emission A is reduced to a considerable degree with increasing Mn2+ concentration at low temperature (19 K) (Figure 4a), and the band A cannot be recognized at 300 K for all Mn2+ concentration of 1.0-22% (Figure 4b). The bandwidth and peak position are estimated as a function of Mn2+ concentration from Figures 3 and 4, and the results are displayed in Figure 5. A large red shift of ∼1450 cm-1 at the temperature range 19-300 K is found with increasing Mn2+ concentration from 1.0 to 22% (Figure 5b), but the temperature-dependent blue shift from 19 and 300 K is small: about 15 cm-1 for 1.0% Mn2+ and 5 cm-1 for 22% Mn2+. (See Figures 3 and 5b.) The band narrowing of emission B is surprising with increasing Mn2+ concentration in the temperature range under investigation. A band broadening is usually observed with increasing dopant concentration in the emission spectra of Mn2+-, Eu2+-, or Ce3+-doped host materials because the increase in dopant concentration gives rise to lattice disorder. The substitutional disorder on the Ca site (and/or the Na site) can induce an inhomogeneous broadening of the Mn2+ emission band. On the contrary, the emission band B narrows by increasing Mn2+ concentration from 1.0 to 22%. The band narrowing is larger for lower temperatures (e.g., 1500 cm-1 at 19 K and 900 cm-1 at 300 K). The mechanisms of band narrowing by increasing Mn2+ concentration in NaCaPO4/Mn2+ are discussed in Section IV.

Shi et al.

Figure 6. Decay curves of Mn2+ emission B of (a) NaCaPO4/ Mn2+(1.0%) and (b) NaCaPO4/Mn2+(22%) at various temperatures (19-300 K).

Figure 7. Decay curves of Mn2+ emission A of NaCaPO4/Mn2+(1.0%) (solid lines) and NaCaPO4/Mn2+(22%) (open squares) at different temperatures (19-300 K).

4. Luminescence Decays of the Mn2+ Ions in NaCaPO4. Luminescence decays of emissions A and B of NaCaPO4/Mn2+ (Figures 6 and 7) and their average lifetime values (Figures 8 and 9) were obtained at various concentrations (1.0-22%) and temperatures (19-300 K). Figure 6 shows decay curves of emission B of Mn2+ (0.01 and 22%) in NaCaPO4 at the temperature range 19-300 K. The decays consist of two

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Figure 8. Average decay times of emissions A (open symbols) and B (closed symbols) of NaCaPO4/Mn2+(1.0-22%) as a function of temperature. The solid line in part a is the result of fit into eq 1. The dotted line in part b is only guide to the eye.

components, that is, fast and slow components. The fast component is attributed to the rapid energy transfer from Mn2+(Ca) ion to closely spaced neighbor Mn2+(Ca) ion in the initial stage after the laser pulse, but at high Mn2+ concentration, the energy diffusion among the assembled Mn2+(Ca) ions suppresses the fast component (e.g., 22% Mn2+ in Figure 6b). No other fast component was detected in a few and tens of nanosecond time regions after the laser pulse. The slow components of emission B are slightly deviated from single exponential for all the Mn2+ concentrations. No significant changes in decay behaviors are observed in different temperature range 19-300 K as observed in Figure 6a,b. The average lifetime values of emission B as a function of temperature for different Mn2+ concentration in NaCaPO4/Mn2+ (1.0-22%) were estimated from the decay curves (e.g., Figure 6a,b), and the results are displayed in Figure 8b. The obtained lifetime values of emission B are ranged between 8.0 and 11 ms (Figures 8b and 9b), which are typical for the parity and spin-forbidden 4 T1 f 6A1 transition of Mn2+. Figure 7 shows the decay curves of emission A at various temperatures. We notice that the temporal behavior of emission A is extremely different from that of emission B. The decays of emission A are nonexponential and rapidly shorten with increasing temperature. The nonexponential decay is attributable to a variety of surrounding environments of Mn2+(A) because of charge compensators formed near Mn2+(A). The temperaturedependent lifetime values of emission A behave similarly to the temperature-dependent emission intensity, as seen in Figure 3. At temperatures >170 K, no changes in decay curves are observed between different Mn2+ concentrations (Figure 7c,d), but as the temperature decreases from 170 to 19 K, the slope of the curves belonging to lower concentrations is getting steeper in the time region 0 to 1.2 ms (Figure 7a,b). At low Mn2+ concentration, similarly to the early time region of emission B,

Figure 9. Average decay times of emissions A (open symbols) and B (closed symbols) of NaCaPO4/Mn2+(1.0-22%) as a function of Mn2+ concentration at various temperatures (19-300 K). (a) Solid lines and (b) dotted line are guides for the eye.

the fast energy transfer is dominant from the Mn2+(Na) ions to the nearest quenching centers in the initial stage, but at high Mn2+ concentration, the energy diffusion among Mn2+(Na) ions occurs, loosing the fast decays. (See Figure 7a,b.) The average lifetime values of emission A are plotted as a function of temperature (Figure 8a) and as a function of concentration (Figure 9a). When the Mn2+ ions substitute for the Na sites, the charge compensation should occur in the lattice, providing defect centers such as oxygen interstitials, Na vacancies, and F centers, which play a role of luminescence quenching centers. It may be considered to be the energy transfer from Mn2+(Na) to Mn2+(Ca) partially influencing the quenching behavior. The thermal quenching process of emission A is described by the form -1 -∆E/kT τ-1 exp(A) ) τ0 (A) + Ke

(1)

where τ0(A) is the intrinsic lifetime of the emission A, K is the nonradiative rate factor (i.e., nonradiative rate in condition of ∆E ) 0), and ∆E is the activation energy of nonradiative recombination. The best fit, shown by solid line in Figure 8a, describes the main feature of the temperature dependence. The

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resulting best-fit parameters are obtained to be τ0(A) ) 1.0 ms, K ) 1.5 × 104 s-1, and ∆E ) 410 cm-1. The average lifetimes of emission B shorten from approximately 11 to 8.0 ms in the Mn2+ concentration range 1.0-8.0%; then, the lifetimes increase from 8.0 to 9.5 ms with increasing Mn2+ concentration up to 22% (Figure 9b). In general, the lifetime shortens by increasing dopant concentration because the energy diffusion among dopant ions makes the decay rate (τ-1) fast; that is, τ-1 ) τint-1 + Wdiff, where τint-1 is intrinsic decay rate and Wdiff is the energy diffusion rate.34 Moreover, at high Mn2+ concentration, the Mn2+ ions can form exchange-coupled pairs, and the lifetime can shorten by the exchange interaction.17 The mechanism of lifetimes as a function of Mn2+ concentration in NaCaPO4/Mn2+ is complicated because several factors are involved in the emission decay process. We suggest that energy transfer and energy migration, photon reabsorption, and exchange interaction are complexly responsible for the mechanisms of emission decays as well as emission shift and band narrowing in NaCaPO4/Mn2+, which are discussed in the next Section. IV. Discussion 1. Structure and Energy Transfer in NaCaPO4/Mn2+. It is expected that in NaCaPO4/Mn2+ the excitation energy migrates within the Mn2+ ions at different crystallographic sites of Ca(I), Ca(II), and Ca(III); then, at last the emission takes place in the Mn2+ ions at a site with the lowest energy among the three Ca(I, II, and III) sites. This type of energy migration can result in the emission red shift with increasing Mn2+ concentration. Furthermore, it can be responsible for band narrowing of emission B. However, it has been reported that no emission shift occurs in Eu2+-doped NaCaPO4 by increasing Eu2+ concentration.9,11 From the structural analysis, the crystallographic sites of Ca(I), Ca(II), and Ca(III) have a similar surrounding, and the Mn2+ ions at these sites are too close to be clearly distinguished.17 This indicates that the energy transfer and energy diffusion are not main causes of the emission red shift in NaCaPO4/Mn2+, even though these effects cannot be ignored. The XRD results of NaCaPO4/Mn2+ show the shrinkage of the volume of the unit cell due to the doping by smaller Mn2+ ions. The emission red shift of Mn2+ could be related to the stronger crystal field strength acting on Mn2+ because of the shortened Mn-O distances. A similar phenomenon has been reported in Ce3+-doped LaAl(Si6-zAlz)(N10-zOz) by Takahashi et al.5 They observed shrinkage of the lattice constant by Ce3+ doping and suggested the possibility of color point tuning for LaAl(Si6-zAlz)(N10-zOz)/Ce3+ for white light LEDs. However, it is not clear whether the strong crystal field strength affects band narrowing of emission B in NaCaPO4/Mn2+. Poort et al.15,35,36 reported a long-wavelength emission of Eu2+ in M2XO4 (M ) Ba, Sr; X ) Si, Al) and MIMIIPO4 (MI ) K; MII ) Sr, Ba), in which the cation sites form chains in the lattices. The Eu2+ ions in the chains experience positive charges because of cation neighbors in the chain direction in addition to the negative charges of the nearest anion neighbors. Therefore, the d orbital of Eu2+ at a Ba or Sr site on a raw is preferentially oriented, resulting in delocalization of excited state with a large Stokes shift of the emission band. A similar situation is assumed for NaCaPO4/Mn2+ in which the Ca sites form zigzag chains and the a axis increases in the chain direction with increasing Mn2+ concentration. It has been reported that the length of the c axis increases along the chain direction with increasing Sr2+ concentration relative to Ca2+ in (Ca,Sr)2MgSi2O7/Eu2+.15,35,36

Shi et al. Therefore, a blue shift of Eu2+ emission occurs because the orientation of a d orbital will energetically become less favorable in this sequence. However, in contradiction to the blue shift, the red shift occurs preferably in NaCaPO4 as the length of the a axis increases along the chain direction with increasing Mn2+ concentration.10 We assume that the effect of delocalization of Mn2+ excited states toward chain direction is smaller than that of Eu2+ or negligible for Mn2+, and the Mn2+(Na) ions with charge compensators make the chain effect incompetent in NaCaPO4/Mn2+. 2. Exchange Interaction in NaCaPO4/Mn2+. When Mn2+ ions form pairs in the Mn2+-doped host lattice, their luminescence properties can be different from those of isolated Mn2+ ions because the interaction between Mn2+ ions in pairs occurs by the exchange mechanism. The distances between the two nearest neighbors of the Ca2+ (or Na+) sites for Mn2+ in NaCaPO4 are short enough (3.42 to 3.78 Å) to form the Mn2+ ions into exchange-coupled pairs. Splittings of the ground state (6A1, S ) 5/2) and the first excited state (4T1, S ) 3/2) in exchange-coupled pairs (two Mn2+ ions, Mn(I) and Mn(II)) are governed by the linear combination of two spin states SI and SII, that is, (SI + SII), ..., (SI - SII).17,20,21 This results in the spin state S ) 5, 4, 3, 2, 1, and 0 for the ground state (6A1) and S ) 4, 3, 2, and 1 for the first excited state (4T1). The Mn2+ exchange pair, therefore, reduces the energy difference between the ground and first excited states, leading to emission red shift. The change in the spin selection rule from spin-forbidden to spin-allowed gives rise to shortening of lifetime. The effect of Mn2+ concentration on luminescence properties associated with exchange-coupled Mn2+ pairs was discussed in Zn2SiO4,17 CaS,18 LaMgB5O10,37,38 MgAl2O4, and ZnGa2O4.17 For example, the emission red shift was reported to be ∼320 cm-1 in MgAl2O4/Mn2+ (0.2-10 mol %) and 250 cm-1 in Zn2SiO4/Mn2+ (0.1-5.0 mol %) accompanying the lifetime shortening from 7.2 to 4.5 ms and from 15 to 1.8 ms, respectively. The emission red shift in NaCaPO4/Mn2+ by increasing Mn2+ concentration is partially explained by the exchange interaction mechanism. However, the lifetimes become longer with increasing Mn2+ concentration from 8.0 to 22%, which cannot be explained by the exchange interaction in Mn2+ pairs. Moreover, no emission band narrowing is caused by the exchange mechanism. In the case of Mn2+, the broad emission bands of isolated Mn2+ ions and Mn2+ pairs of ions have a large overlap and cannot be spectrally resolved.17 3. Photon Reabsorption, Lifetime, and Bandwidth of Mn2+ Emission in NaCaPO4. The possible causes of band narrowing can be conceived of optical interaction between Mn2+ ions and the interaction of Mn2+ with lattice vibrations (electron-phonon interaction) in heavily Mn2+-doped NaCaPO4, but the heavily doping of the lattice by Mn2+ ions seems unlikely to suppress the electron-phonon interaction in NaCaPO4/Mn2+ because phonon energies of stretching and banding modes between P and O ions are not noticeably changed by the replacement of Ca2+ ions (or Na+ ions) with the Mn2+ ions in the NaCaPO4 lattice.39 The optical interaction between Mn2+ ions with a large spectral overlap between emission and groundstate absorption causes photon reabsorption (or radiation self-trapping).39-43 The higher energy part of the emission is reabsorbed resonantly by the low-energy part of the absorption, resulting in emission red shift, band narrowing, and lifetime lengthening. The higher the ion concentration, the higher the reabsorption can repopulate the emitting levels with increasing extrinsic lifetime. The reduced emitted photon flux at short wavelengths results in emission red shift and band narrowing

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because the photon flux at longer wavelength side is unaffected by the reabsorption process.42 We suggest that the band narrowing and emission red shift in NaCaPO4/Mn2+ with increasing Mn2+ concentration can be caused by the reabsorption process. The photon reabsorption has been intensively investigated in laser crystals because the high reabsorption probability is responsible for energy loss and makes it complicated to determine the laser parameters such as intrinsic and extrinsic lifetimes, extrinsic fractional thermal loading, and the intrinsic or extrinsic quantum yield.42-47 In crystals, the photon reabsorption depends not only on the ion concentration but also on the optical path length through the sample. According to Auzel et al.,39,40 in the powdered sample with grain diameter of a few tens of micrometers, the photon reabsorption takes place, although the effect is three times smaller in powdered sample than in crystal. The measured luminescence decay rate is given by the sum of radiative (WR), nonradiative (WNR), and exchange-induced (Wexchange) decay rates as -1 τ-1 exp(B) ) τR (B) + WNR + Wexchange

(2)

where τR-1(B) corresponds to the radiative decay rate WR of emission B. By the reabsorption of the emitting photons with repeating the entire process, the radiative decay rate with optical reabsorption of emission B is given by39-41

[

τR-1(B) ) τ-1 0 (B) 1 +

( )]

9 N 2π N0

2

(3)

where τ0-1(B)is the intrinsic decay rate of a single isolated ion, N is the ion doping concentration, and No is in relation with critical transfer distance (R0) proportional to the spectral overlap ensuring energy conservation during the transfer as

N0 )

-1

( 34 πR ) 3 0

(4)

The second term of eq 3 lengthens the intrinsic lifetime τ0(B) with increasing Mn2+-concentration. In NaCaPO4/Mn2+, the lifetime lengthening by photon reabsorption in eq 3 competes with lifetime shortening by nonradiative relaxation and exchange interaction (second and third terms, respectively, in eq 2). It is assumed that the effect of exchange interaction and energy migration on the lifetime values in the Mn2+ concentration region 1.0-8.0% exceeds that of the photon reabsorption in NaCaPO4/Mn2+. Upon increasing concentration from 8.0 to 22%, the photon reabsorption greatly influences the lifetime, as opposed to the other factors that shorten the lifetime with increasing Mn2+ concentration. V. Conclusions We observe two emission bands of Mn2+ at shorter (A) and longer (B) wavelength sides in NaCaPO4/Mn2+. By the structural analyses, we attribute that the emission A is originated from the Mn2+ ions at Na sites, whereas the emission B comes from the Mn2+ ions substituted for Ca sites. The emission A is quenched not only by increasing temperature but by increasing Mn2+ concentration in NaCaPO4/Mn2+. The defect centers formed by charge compensation for the Mn2+ ions in the Na

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