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Luminescent properties and energy transfer of thermal stable Ca10Na (PO4)7:Ce3+, Mn2+ red phosphor under UV excitation
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 9037–9043 www.elsevier.com/locate/ceramint
Luminescent properties and energy transfer of thermal stable Ca10Na (PO4)7: Ce3 þ , Mn2 þ red phosphor under UV excitation Xinguo Zhanga,b,n, Chunyan Zhoua, Jiahui Songa, Liya Zhoua, Menglian Gongb a School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
b
Received 2 December 2013; received in revised form 11 January 2014; accepted 25 January 2014 Available online 31 January 2014
Abstract A new red phosphor Ca10Na(PO4)7:Ce3 þ , Mn2 þ was synthesized by a solid-state reaction method. Its luminescent properties were investigated by excitation/emission spectra and decay curves. Ca10Na(PO4)7:Ce3 þ exhibits strong violet emission under UV excitation. For Ce3 þ , Mn2 þ co-doped Ca10Na(PO4)7, the Mn2 þ red emission is enhanced dramatically with the optimum Ce3 þ co-doping. The energy transfer from Ce3 þ to Mn2 þ was proposed to be of resonance-type via an electric dipole–dipole mechanism, and the energy transfer efficiency was also calculated by decay data. The relationship between occupied sites and emission peak shift of Mn2 þ ion in the Ca10Na(PO4)7 lattice was identified and discussed. Ca10Na(PO4)7:Ce3 þ , Mn2 þ exhibits strong sensitized Mn2 þ red emission and excellent thermal stability, which can serve as a good candidate for UV-excited red emitting phosphor. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Ca10Na(PO4)7:Ce3 þ ; Color-tunable; Energy transfer; Luminescence; Mn2 þ
1. Introduction Phosphate is an excellent host for luminescent materials due to its low sintering temperature, high luminous efficiency and good stability [1,2]. Moreover, rare-earth or transition metal activated phosphate shows good luminescent properties, which make it a suitable host for luminescent materials [3,4]. β-Ca3 (PO4)2-type compounds (space group R3c) have been extensively studied as a good candidate for bioceramics [5,6]. Different compounds have been obtained by iso- and heterovalent substitutions of Ca2 þ with M þ (monovalent cation Li, Na, K), Me2 þ (bivalent cation Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd, Sr, Pb), R3 þ (rare earth ions RE, Y, Bi, Ga, In Sb, Sc, Cr) in β-Ca3(PO4)2 structure [7]. The above-mentioned new compounds derived from β-Ca3(PO4)2 structure offer various
n Corresponding author at: School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China. Tel./fax: þ86 771 3233718. E-mail address: [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2014.01.116 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
potential hosts for the investigation of new luminescent materials [8,9]. It is suggested that manipulating coordination of Mn2 þ ion by mixing different anions is one way to enlarge opportunities for novel phosphor with promising properties for further application [10]. Owing to the weak excitation bands of Mn2 þ , sensitizers with high transition efficiency such as Eu2 þ or Ce3 þ with the 4f–5d allowed transition are normally used to enhance the intensity of Mn2 þ emission by efficient Eu2 þ /Ce3 þ -Mn2 þ energy transfer [11,12]. Efficient Mn2 þ sensitized emission has been reported in many β-Ca3(PO4)2type compounds, such as Ca9Al(PO4)7:Ce3 þ , Mn2 þ [13]., Ca8MgY(PO4)7:Eu2 þ , Mn2 þ [14], and Ca9MgLi(PO4)7:Eu2 þ , Mn2 þ [15]. Ca10Na(PO4)7 (CNP) was firstly reported by Morozov et al. in 1997, which is isostructural to β-Ca3(PO4)2 [16]. Currently, the luminescent properties of red-emitting Ca10Na(PO4)7:Eu3 þ and yellow-emitting Ca10Na(PO4)7:Eu2 þ have been reported by Dou et al. [17] and Yu et al. [18]. However, to the best of our knowledge, little effort has been made on the synthesis and luminescent properties of Ce3 þ and
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Mn2 þ co-doped Ca10Na(PO4)7 (CNP). In this paper, Ca10Na (PO4)7:Ce3 þ , Mn2 þ was found to be a red-emitting phosphor. The energy transfer mechanism between Ce3 þ and Mn2 þ has been systematically investigated. 2. Experimental Ca10(1 x)Na(PO4)7:xCe3 þ (CNP:Ce3 þ ) and Ca10(1 0.05 y) Na(PO4)7:0.05Ce3 þ , yMn2 þ (CNP:Ce3 þ , Mn2 þ ) phosphors were prepared by a solid-state reaction; x and y are the doped ratios of Ce3 þ and Mn2 þ . Stoichiometric amounts of raw materials CaCO3 (A.R.), Na2CO3 (A.R.), NH4H2PO4 (A.R.), CeO2 (99.99%) and MnCO3 (A.R.) were thoroughly mixed by grinding. They were pre-sintered at 600 1C for 3 h in air, and re-sintered in a reducing atmosphere (N2:H2 ¼ 90:10) at 1050 1C for 3 h. X-ray powder diffraction (XRD) patterns of the products were recorded on a Rigaku D/max-IIIA diffractometer with Cu Kα radiation (λ ¼ 1.5403 Å). Photoluminescent excitation (PLE) and emission (PL) spectra, fluorescence lifetime, as well as temperature-dependent PL spectra of the phosphors were recorded on an EDINBURGH FLS920 Combined Fluorescence Lifetime and Steady State Spectrometer and a 450 W xenon lamp was used as the excitation source.
Fig. 1. Representative XRD patterns of CNP:Ce3 þ , Mn2 þ samples and JCPDS no. 45-0339.
3. Results and discussions 3.1. Structure and powder characteristics of CNP Double calcium and sodium phosphates Ca10Na(PO4)7 are crystallized in the trigonal system, and are isostructural to β-Ca3(PO4)2. The variations of the unit-cell parameters and the volumes (a, c, and V) as functions of the radius of the alkali metal cation for these compounds are similar to the variations of these parameters for the compounds with the composition Ca9MgM(PO4)7 (M ¼ Li, Na, K) and Ca9R(PO4)7 (R ¼ Al, La–Lu) [16]. There are five cation sites in Ca10Na(PO4)7:M1 (seven-coordinated), M2 (six-coordinated), M3 (seven-coordinated), and M5 (six-coordinated) site are occupied by Ca2 þ , and M4 (three-coordinated) site is occupied by Na þ . Due to the similarity of ionic radius and charge, both Ce3 þ (r¼ 1.01 Å, C.N.¼ 6; r¼ 1.07 Å, C.N.¼ 7) and Mn2 þ (r¼ 0.83 Å, C.N. ¼ 6; r¼ 0.90 Å, C.N. ¼ 7) are supposed to occupy Ca2 þ (r¼ 1.00 Å, C.N. ¼ 6; r ¼ 1.06 Å, C.N. ¼ 7) sites, rather than Na þ site. Ca10Na(PO4)7 is isostructural to β-Ca3(PO4)2; then the tendency of rare-earth elements’ occupation for Ca10Na (PO4)7 should be similar to those for phosphates isotypic with β-Ca3(PO4)2. As reported by Lazoryak et al. [19], rare-earth elements can statistically occupy the M1, M2 and M5 positions of the β-Ca3(PO4)2 structure together with Ca2 þ . In Ca9R (PO4)7 structure, rare-earth elements prefer to occupy M1–M3 positions, while small cations (Al3 þ , Fe3 þ ) like to occupy M5 position, and the cationic positions M4 and M6 are completely empty [20]. Therefore it is assumed that Ce3 þ will mainly occupy the M1 and M2 sites. Since the effective ionic radius for Ce3 þ and Ca2 þ is similar, Ce3 þ may also occupy the M3 site in the host. Mn2 þ prefers to occupy the M5 site in the
Fig. 2. PL (λem ¼ 300 nm) and PLE (λex ¼ 370 nm) spectra of CNP:xCe3 þ (x¼ 0.01 0.09) phosphors.
Ca10Na(PO4)7 host, while the possibility of Mn2 þ occupying M1 and M2 sites cannot be excluded. The composition and phase purity of the as-prepared powder samples were first examined by XRD. Fig. 1 shows the representative XRD patterns of CNP:Ce3 þ , Mn2 þ samples annealed at 1050 1C for 3 h. It is obvious that the diffraction peaks of all these samples can be exactly assigned to pure hexagonal phase of Ca10Na(PO4)7 (JCPDS no. 45-0339), indicating that the obtained samples are single phase and the co-doped Ce3 þ and Mn2 þ ions do not cause any significant changes in the host structure. 3.2. Photoluminescence excitation and emission spectra The ground configuration and the excited configuration of Ce3 þ are 4f1 and 5d1, respectively. Since the 4f–5d transition is parity allowed, the emission transition is a fully allowed one. Usually Ce3 þ ion shows broad excitation and emission bands. Fig. 2 demonstrates the PL and PLE spectra of CNP:Ce3 þ . The PLE spectrum shows a broad absorption band from
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200 nm to 350 nm, with two peaks located at 265 and 300 nm. Excited by 300 nm UV, the sample exhibits an obvious asymmetric violet emission band around 370 nm, which indicates the existence of different luminescent peaks. The emission spectra of CNP:xCe3 þ (x ¼ 0.01–0.09) phosphors excited by 300 nm are also shown in Fig. 2. It can be observed that all the emission spectra exhibit a similar profile with various relative intensities. The emission intensities enhance with the increase of Ce3 þ concentration and reach the maximum at x¼ 0.05. However, the photoluminescence intensities decrease with further increase of Ce3 þ concentration due to concentration quenching. It is accepted that concentration quenching is mainly caused by energy transfer among Ce3 þ ions, the probability of which increases as the concentration of Ce3 þ increases. If we consider energy transfer in the given crystal structure, the critical distance (Rc) is defined as the distance for which the probability of nonradiative energy transfer equals the probability of radiative emission of Ce3 þ , as pointed out by Blasse [21]. So the following equation can be used to calculate the Rc value. Rc ¼ 2
3V 4πxc N
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Fig. 3. PL (λem ¼ 300 nm) and PLE spectra (λex ¼370 nm) of CNP:0.05Ce3 þ and CNP:0.05Mn2 þ (a) and those of CNP:0.05Ce3 þ , 0.07Mn2 þ (b). (Inset) Photograph of CNP:0.05Ce3 þ and CNP:0.05Ce3 þ , 0.07Mn2 þ sample under UV lamp (λem ¼254 nm, 6 W).
1=3
where xc is the critical concentration, N is the number of cations in the unit cell and V is the volume of the unit cell. By taking the experimental and analytic values of V, N and xc (3521.3 Å3, 60, and 0.05, respectively), the critical transfer distance is calculated to be about 13.08 Å. As a comparison, the distances between the nearest two sites among M1, M2 and M3 sites are 3.655 Å (M1–M2), 3.964 Å (M1–M3) and 6.276 Å (M2–M3). Based on these results, the optimal Ce3 þ content for Mn2 þ co-doped is found to be 0.05. Since the transitions of Mn2 þ are d-d spin and parity forbidden according to the spin selection rule, the emission and excitation intensities are very weak. The PLE spectrum of CNP:0.07Mn2 þ sample (Fig. 3(a) consists of several peaks centered at 350, 365, 405, 421 and 500 nm, corresponding to the well-known transitions of Mn2 þ from ground level 6 A1 (6S) to 4E (4D), 4T2 (4D), [4A1 (4G), 4E (4G)], 4T2 (4G) and 4T1 (4G) levels. The PL spectrum is a red emission centered at 650 nm, which is assigned to the 4T1–6A1 forbidden transition of Mn2 þ . The Ce3 þ emission has a significant spectral overlap on the excitation band of Mn2 þ , which is in favor of the resonance type energy transfer from Ce3 þ to Mn2 þ [22]. As seen in Fig. 3(b), the PLE spectrum of CNP:0.05Ce3 þ , 0.07Mn2 þ is similar to that for the single Ce3 þ -doped sample. Thus, compared with the PL spectrum of the single Ce3 þ -doped sample, besides the violet–blue emission band with decreased intensity from Ce3 þ , a broad and strong red band appears, which is due to Mn2 þ sensitized emission. The result indicates that the efficient energy transfer from Ce3 þ to Mn2 þ happens in the Ce3 þ –Mn2 þ co-doped sample. This phenomenon is easy to understand since the Ce3 þ emission covers some Mn2 þ excitation band and the excitation spectrum of CNP:Ce3 þ , Mn2 þ monitoring the Mn2 þ -derived
Fig. 4. PL spectra of CNP:0.05Ce3 þ , yMn2 þ (y¼0.01–0.13) phosphors (λex ¼300 nm).
emission has a similar profile with the excitation bands of CNP:Ce3 þ . Besides, Ce3 þ is larger than the host cation Ca2 þ , and Mn2 þ is smaller than Ca2 þ , i.e. rCe3 þ (114 pm) 4rCa2 þ (112 pm) 4 rMn2 þ (96 pm); Ce3 þ /Mn2 þ clusters probably form in the crystal lattice due to the ion-size compensation effect [23]. In the clusters, Mn2 þ is located near Ce3 þ ; this also benefits effective energy transfer. 3.3. Concentration quenching and energy transfer The PL spectra of CNP:0.05Ce3 þ , yMn2 þ (y ¼ 0.01–0.13) under excitation of 300 nm are displayed in Fig. 4. The intensity of Ce3 þ UV emission decreases with increase of doped Mn2 þ concentration, while the PL intensity of Mn2 þ emission increases until the Mn2 þ concentration-quenching happens at above y ¼ 0.07. The observed variations in the emission intensities of Ce3 þ and Mn2 þ strongly confirm the continuous energy transfer from Ce3 þ to Mn2 þ .
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Fig. 5. Experimental emission spectrum (solid line, range: 500–800 nm), fitted curve (red dashed line), and deconvoluted Gaussian components (green dashed lines) of CNP:0.05Ce3 þ , yMn2 þ (y¼ 0.01, 0.03, 0.05, and 0.07. λex ¼ 300 nm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The profiles of Mn2 þ emission spectra (range: 500–800 nm) of CNP:0.05Ce3 þ , yMn2 þ (y¼ 0.01, 0.03, 0.05, 0.07) exhibit different shapes under 300 nm excitation, as shown in Fig. 5. By Gaussian fitting, three broad emission peaks at 640, 680, and 745 nm can be observed. The Mn2 þ broadband emission originates from 4T1–6A1 transition. The energy separations between the 4T1(4G) state and the 6A1(6S) ground state are very sensitive to the crystal-field strength and decrease with increase in crystal field strength [24]. Since the luminescence wavelength due to Mn2 þ is sensitive to the magnitude of the crystal field, several emission bands are observed when different types of Mn2 þ sites exist in a host crystal. The crystal field strength is affected by many factors such as coordination number, site symmetry and size of the coordinating polyhedron. Based on the data from a large number of literatures, Dorenbos [25–28] concluded that the larger the size of the coordinating polyhedron, the smaller the crystal field splitting of 5d levels and shorter the emission wavelength. Similar
conclusion could be applied for Mn2 þ 3d levels, which indicates that larger coordinating polyhedron results in lower crystal field strength and a blue-shift of Mn2 þ emission wavelength [29]. According to P.H. Hollway and P.D. Rack, the crystal field strength of d-orbital splitting could be expressed as [30] Dq ¼
ze2 r 4 6R5
where Dq is a measurement of the crystal-field strength, z is the charge or valence of the anion, e is the charge of an electron, r is the radius of the d wave function, and R is the distance between the central ion and its ligands. When Mn2 þ occupies a certain position in a host, the crystal field strength that affects the ion decreases as the space containing Mn2 þ becomes larger, as expected from the above-mentioned equation. For decrease in the field, the transition energy between 4 T1(4G) and 6A1(6S) is predicted to increase, and the emission wavelength of Mn2 þ will shift to shorter wavelengths [31].
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Thus, the size of coordinating polyhedron could be qualitatively measured by the average cation–anion distance. According to the crystal structure of Ca10Na(PO4)7, M1 site is coordinated with seven O atoms with the average Ca–O distance 2.4285 Å, and the average Ca–O distance of M2 (C.N. ¼ 6) and M5 (C.N. ¼ 6) is 2.3893 Å and 2.2575 Å, respectively. As a consequence, the crystal field strength is larger for Mn2 þ at the M5 site than at the M1 and M2 sites. Thus, the emission band at lower energy ( 745 nm) is assigned to the Mn2 þ located at the M5 site, and the other emission bands at higher energy ( 680 nm and 640 nm) originate from the Mn2 þ located at the M2 and M1 sites, respectively. The ratio of these three peaks varies with increasing Mn2 þ contents. This phenomenon is due to the fact that the radius of Mn2 þ (0.83–0.96 Å) is more close to that of M5 (C.N. ¼ 6, and r (Ca2 þ ) ¼ 1.00 Å), so it is reasonable that Mn2 þ ions tend to occupy M5 sites first, and the corresponding emission peak prevails at low doping content (x¼ 0.01). With increasing Mn2 þ contents, the percentage of Mn2 þ , which occupied M1 and M2 sites, will rise, and the intensities of corresponding peaks will increase. This results in the change of spectral shape and the blue-shift of emission wavelength. The PL decay curves of Ce3 þ in CNP:0.05Ce3 þ , yMn2 þ phosphors were measured with excitation at 300 nm and monitored at 370 nm, and are shown in Fig. 6. For CNP:0.05Ce3 þ , the decay curves can be fitted successfully based on the double-exponential equation I ¼ A1 expð t=τ1 Þ þ A2 expð t=τ2 Þ where I represents the luminescence intensity; A1 and A2 are constants; t is time; τ1 and τ2 are the decay times for the exponential components, respectively. For CNP:0.05Ce3 þ , the lifetime values (τ1, τ2) were determined to be 11.92 ns and 30.18 ns. The results reveal that there are two lattice sites occupied by Ce3 þ ions in Ca10Na(PO4)7 host. Thus, the average decay time (τ) can be determined using the following
Fig. 6. Ce3 þ decay time of CNP:0.05Ce3 þ , yMn2 þ under 300 nm excitation.
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equation: τ¼
A1 τ21 þ A2 τ22 A1 τ1 þ A2 τ2
As seen in Fig. 6, Ce3 þ emission decay curves of CNP:0.05Ce3 þ , xMn2 þ sample become more and more nonexponential with increasing Mn2 þ content. The average decay time τ reduces monotonically from 27.06 to 1.86 ns as x increases from 0 to 0.13 which is good evidence for Ce3 þ Mn2 þ energy transfer. The energy transfer efficiency (ηT) from a sensitizer to an activator ion can be calculated by the following relation, which has been discussed by Paulose et al. [32] τs ηT ¼ 1 τs0 where τs and τs0 are the decay lifetimes of the sensitizer (Ce3 þ ) ion with and without activator (Mn2 þ ) ion present, respectively. The ηT from Ce3 þ to Mn2 þ in CNP host are calculated as a function of the Mn2 þ concentration and presented in the inset of Fig. 4. The ηT was found to increase with increasing Mn2 þ content, and reached 93% at y ¼ 0.13. Generally, there are two main aspects responsible for the resonant energy-transfer mechanism: one is an exchange interaction and the other is multipolar interaction. It is known that if energy transfer results from the exchange interaction, the critical distance between the sensitizer and activator should be shorter than 5 Å [33]. The critical distance Rc for energy transfer from the Ce3 þ to Mn2 þ ions can be calculated using the concentration quenching method 3V 1=3 Rc ¼ 2 4πxc N where N is the number of available sites for the dopant in the unit cell, xc is the total concentration of Ce3 þ and Mn2 þ , and V is the volume of the unit cell. For the Ca10Na(PO4)7 host, N ¼ 60 and V¼ 3521.3 Å3. The critical concentration xc, at which the luminescence intensity of Ce3 þ is half of that with the absence of Mn2 þ , is about 0.05þ 0.05 ¼ 0.10. Therefore, the critical distance (Rc) of energy transfer was calculated to be about 10.38 Å. This value is much higher than 4 Å, indicating little possibility of energy transfer via the exchange interaction mechanism. Thus, the energy transfer between the Ce3 þ and Mn2 þ ions mainly takes place via electric multipolar interactions. On the basis of Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the following relation can be obtained [34,35]: τS0 p Ca=3 τS where C is the concentration of Mn2 þ , and a= 6, 8, and 10 for dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. Plots of τs0/τs and Ca/3 based on the above equation are shown in Fig. 7. Linear behavior was observed only when a¼ 6, implying that energy transfer from Ce3 þ to Mn2 þ occurred via the dipole–dipole mechanism. The CIE chromaticity diagram and coordinates for the CNP:0.05Ce3 þ , yMn2 þ phosphors with different doping
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Fig. 7. Dependence of τs0/τs of Ce3 þ on CMn 6/3, CMn 8/3, and CMn 10/3.
contents of Mn2 þ are calculated according to the relevant PL spectrum under excitation of 300 nm, which are exhibited in Fig. 8. The color tone of the phosphors can be tuned from violet through purplish red and finally to red with the increase of Mn2 þ content from 0 to 0.13 due to efficient Ce3 þ -Mn2 þ energy transfer with the increase of Mn2 þ concentration. And the corresponding CIE coordinates are calculated to change from (0.161, 0.019) to (0.682, 0.302). The CIE coordinate of optimal red phosphor CNP:0.05Ce3 þ , 0.07Mn2 þ is (0.679, 0.298), which is close to that of NSTC (National Television System Committee) standard red (0.67, 0.33). 3.4. Temperature-dependent PL properties In general, the thermal quenching behavior of phosphors is important because it has considerable influence on the light output and color rendering index [36,37]. Fig. 9 shows the temperature-dependent emission spectra of CNP:0.05Ce3 þ , 0.07Mn2 þ phosphors from room temperature to 190 1C. The relative peak intensities of Ce3 þ decreased generally with temperature, and it reached 75% of the initial value at 190 1C. The thermal stability of Mn2 þ emission is higher than that of Ce3 þ emission. The Mn2 þ emission intensity almost keeps constant with rising temperature, which indicates that CNP:0.05Ce3 þ , 0.07Mn2 þ has an excellent thermal stability, and makes it a possible red phosphor for further application. As seen in Fig. 9, the Mn2 þ emission wavelength blueshifts slightly with increasing temperature. Peak positions at 30 1C and 190 1C are 656 and 640 nm, respectively. It is considered that thermal active phonon-assisted excitation from lower energy sublevel to higher energy sublevel in excited states occurs [38,39]. Elevated temperature causes electrons at lower energy level to jump to higher energy levels by phonon assistance whereas the nonradiative transitions from excited states to ground states are prevented. At higher temperature,
Fig. 8. CIE chromaticity diagram and coordinates for the CNP:0.05Ce3 þ , yMn2 þ phosphors. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
more electrons are populated at higher-energy excited sublevel, but the nonradiative transitions through the crossing point between the excited state and the ground state in configurational coordinate diagram are decreased. The height of higherenergy emission peak is increased, and that of lower-energy emission peak is decreased. As a result, the blue-shift behavior is observed with increasing temperature. The shift of CIE coordinates with increasing temperature has been calculated and demonstrated in Table 1, and it can be observed that the CIE shift is negligible in a wide range of temperature. 4. Conclusion Luminescent properties and energy transfer process of Ca10Na(PO4)7:Ce3 þ , Mn2 þ phosphor are studied. Co-doping
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References
Fig. 9. Temperature-dependent PL spectra of CNP:0.05Ce3 þ , 0.07Mn2 þ under 300 nm excitation.
Table 1 The shift of CIE coordinates with increasing temperature of CNP: 0.05Ce3 þ , 0.07Mn2 þ phosphor. Temperature (1C)
CIE x
CIE y
30 50 70 90 110 130 150 170 190
0.683 0.681 0.679 0.676 0.673 0.670 0.668 0.665 0.662
0.303 0.306 0.309 0.313 0.316 0.320 0.323 0.327 0.331
Ce3 þ /Mn2 þ is an effective approach to enhance the red emission of Mn2 þ . The suitable spectrum overlapping of Ce3 þ emission to Mn2 þ excitation as well as the formation of Ce3 þ /Mn2 þ clusters may be the causes of enhanced Mn2 þ luminescence. The different shape of Mn2 þ emission with increasing Mn2 þ content is due to the Mn2 þ occupation of different sites. The energy transfer from Ce3 þ to Mn2 þ was proposed to be of resonance-type via an electric dipole–dipole mechanism. The selected sample Ca10Na(PO4)7:0.05Ce3 þ , 0.07Mn2 þ shows strong red emission and a good thermal stability with increasing temperature up to 200 1C. All the results have demonstrated that Ca10Na(PO4)7:Ce3 þ , Mn2 þ is potentially useful as a UV excited red-emitting phosphor. Acknowledgments This work was supported by the Scientific Research Foundation of Guangxi University (Grant no. XBZ120573) and National Natural Science Foundation of China (No. 61264003). The authors thank Prof. Menglian Gong and Prof. Jianxin Shi (Sun Yat-sen University, Guangzhou, China) for their help in PL/PLE measurement.
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 9037–9043 www.elsevier.com/locate/ceramint
Luminescent properties and energy transfer of thermal stable Ca10Na (PO4)7: Ce3 þ , Mn2 þ red phosphor under UV excitation Xinguo Zhanga,b,n, Chunyan Zhoua, Jiahui Songa, Liya Zhoua, Menglian Gongb a School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
b
Received 2 December 2013; received in revised form 11 January 2014; accepted 25 January 2014 Available online 31 January 2014
Abstract A new red phosphor Ca10Na(PO4)7:Ce3 þ , Mn2 þ was synthesized by a solid-state reaction method. Its luminescent properties were investigated by excitation/emission spectra and decay curves. Ca10Na(PO4)7:Ce3 þ exhibits strong violet emission under UV excitation. For Ce3 þ , Mn2 þ co-doped Ca10Na(PO4)7, the Mn2 þ red emission is enhanced dramatically with the optimum Ce3 þ co-doping. The energy transfer from Ce3 þ to Mn2 þ was proposed to be of resonance-type via an electric dipole–dipole mechanism, and the energy transfer efficiency was also calculated by decay data. The relationship between occupied sites and emission peak shift of Mn2 þ ion in the Ca10Na(PO4)7 lattice was identified and discussed. Ca10Na(PO4)7:Ce3 þ , Mn2 þ exhibits strong sensitized Mn2 þ red emission and excellent thermal stability, which can serve as a good candidate for UV-excited red emitting phosphor. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Ca10Na(PO4)7:Ce3 þ ; Color-tunable; Energy transfer; Luminescence; Mn2 þ
1. Introduction Phosphate is an excellent host for luminescent materials due to its low sintering temperature, high luminous efficiency and good stability [1,2]. Moreover, rare-earth or transition metal activated phosphate shows good luminescent properties, which make it a suitable host for luminescent materials [3,4]. β-Ca3 (PO4)2-type compounds (space group R3c) have been extensively studied as a good candidate for bioceramics [5,6]. Different compounds have been obtained by iso- and heterovalent substitutions of Ca2 þ with M þ (monovalent cation Li, Na, K), Me2 þ (bivalent cation Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd, Sr, Pb), R3 þ (rare earth ions RE, Y, Bi, Ga, In Sb, Sc, Cr) in β-Ca3(PO4)2 structure [7]. The above-mentioned new compounds derived from β-Ca3(PO4)2 structure offer various
n Corresponding author at: School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China. Tel./fax: þ86 771 3233718. E-mail address: [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2014.01.116 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
potential hosts for the investigation of new luminescent materials [8,9]. It is suggested that manipulating coordination of Mn2 þ ion by mixing different anions is one way to enlarge opportunities for novel phosphor with promising properties for further application [10]. Owing to the weak excitation bands of Mn2 þ , sensitizers with high transition efficiency such as Eu2 þ or Ce3 þ with the 4f–5d allowed transition are normally used to enhance the intensity of Mn2 þ emission by efficient Eu2 þ /Ce3 þ -Mn2 þ energy transfer [11,12]. Efficient Mn2 þ sensitized emission has been reported in many β-Ca3(PO4)2type compounds, such as Ca9Al(PO4)7:Ce3 þ , Mn2 þ [13]., Ca8MgY(PO4)7:Eu2 þ , Mn2 þ [14], and Ca9MgLi(PO4)7:Eu2 þ , Mn2 þ [15]. Ca10Na(PO4)7 (CNP) was firstly reported by Morozov et al. in 1997, which is isostructural to β-Ca3(PO4)2 [16]. Currently, the luminescent properties of red-emitting Ca10Na(PO4)7:Eu3 þ and yellow-emitting Ca10Na(PO4)7:Eu2 þ have been reported by Dou et al. [17] and Yu et al. [18]. However, to the best of our knowledge, little effort has been made on the synthesis and luminescent properties of Ce3 þ and
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Mn2 þ co-doped Ca10Na(PO4)7 (CNP). In this paper, Ca10Na (PO4)7:Ce3 þ , Mn2 þ was found to be a red-emitting phosphor. The energy transfer mechanism between Ce3 þ and Mn2 þ has been systematically investigated. 2. Experimental Ca10(1 x)Na(PO4)7:xCe3 þ (CNP:Ce3 þ ) and Ca10(1 0.05 y) Na(PO4)7:0.05Ce3 þ , yMn2 þ (CNP:Ce3 þ , Mn2 þ ) phosphors were prepared by a solid-state reaction; x and y are the doped ratios of Ce3 þ and Mn2 þ . Stoichiometric amounts of raw materials CaCO3 (A.R.), Na2CO3 (A.R.), NH4H2PO4 (A.R.), CeO2 (99.99%) and MnCO3 (A.R.) were thoroughly mixed by grinding. They were pre-sintered at 600 1C for 3 h in air, and re-sintered in a reducing atmosphere (N2:H2 ¼ 90:10) at 1050 1C for 3 h. X-ray powder diffraction (XRD) patterns of the products were recorded on a Rigaku D/max-IIIA diffractometer with Cu Kα radiation (λ ¼ 1.5403 Å). Photoluminescent excitation (PLE) and emission (PL) spectra, fluorescence lifetime, as well as temperature-dependent PL spectra of the phosphors were recorded on an EDINBURGH FLS920 Combined Fluorescence Lifetime and Steady State Spectrometer and a 450 W xenon lamp was used as the excitation source.
Fig. 1. Representative XRD patterns of CNP:Ce3 þ , Mn2 þ samples and JCPDS no. 45-0339.
3. Results and discussions 3.1. Structure and powder characteristics of CNP Double calcium and sodium phosphates Ca10Na(PO4)7 are crystallized in the trigonal system, and are isostructural to β-Ca3(PO4)2. The variations of the unit-cell parameters and the volumes (a, c, and V) as functions of the radius of the alkali metal cation for these compounds are similar to the variations of these parameters for the compounds with the composition Ca9MgM(PO4)7 (M ¼ Li, Na, K) and Ca9R(PO4)7 (R ¼ Al, La–Lu) [16]. There are five cation sites in Ca10Na(PO4)7:M1 (seven-coordinated), M2 (six-coordinated), M3 (seven-coordinated), and M5 (six-coordinated) site are occupied by Ca2 þ , and M4 (three-coordinated) site is occupied by Na þ . Due to the similarity of ionic radius and charge, both Ce3 þ (r¼ 1.01 Å, C.N.¼ 6; r¼ 1.07 Å, C.N.¼ 7) and Mn2 þ (r¼ 0.83 Å, C.N. ¼ 6; r¼ 0.90 Å, C.N. ¼ 7) are supposed to occupy Ca2 þ (r¼ 1.00 Å, C.N. ¼ 6; r ¼ 1.06 Å, C.N. ¼ 7) sites, rather than Na þ site. Ca10Na(PO4)7 is isostructural to β-Ca3(PO4)2; then the tendency of rare-earth elements’ occupation for Ca10Na (PO4)7 should be similar to those for phosphates isotypic with β-Ca3(PO4)2. As reported by Lazoryak et al. [19], rare-earth elements can statistically occupy the M1, M2 and M5 positions of the β-Ca3(PO4)2 structure together with Ca2 þ . In Ca9R (PO4)7 structure, rare-earth elements prefer to occupy M1–M3 positions, while small cations (Al3 þ , Fe3 þ ) like to occupy M5 position, and the cationic positions M4 and M6 are completely empty [20]. Therefore it is assumed that Ce3 þ will mainly occupy the M1 and M2 sites. Since the effective ionic radius for Ce3 þ and Ca2 þ is similar, Ce3 þ may also occupy the M3 site in the host. Mn2 þ prefers to occupy the M5 site in the
Fig. 2. PL (λem ¼ 300 nm) and PLE (λex ¼ 370 nm) spectra of CNP:xCe3 þ (x¼ 0.01 0.09) phosphors.
Ca10Na(PO4)7 host, while the possibility of Mn2 þ occupying M1 and M2 sites cannot be excluded. The composition and phase purity of the as-prepared powder samples were first examined by XRD. Fig. 1 shows the representative XRD patterns of CNP:Ce3 þ , Mn2 þ samples annealed at 1050 1C for 3 h. It is obvious that the diffraction peaks of all these samples can be exactly assigned to pure hexagonal phase of Ca10Na(PO4)7 (JCPDS no. 45-0339), indicating that the obtained samples are single phase and the co-doped Ce3 þ and Mn2 þ ions do not cause any significant changes in the host structure. 3.2. Photoluminescence excitation and emission spectra The ground configuration and the excited configuration of Ce3 þ are 4f1 and 5d1, respectively. Since the 4f–5d transition is parity allowed, the emission transition is a fully allowed one. Usually Ce3 þ ion shows broad excitation and emission bands. Fig. 2 demonstrates the PL and PLE spectra of CNP:Ce3 þ . The PLE spectrum shows a broad absorption band from
X. Zhang et al. / Ceramics International 40 (2014) 9037–9043
200 nm to 350 nm, with two peaks located at 265 and 300 nm. Excited by 300 nm UV, the sample exhibits an obvious asymmetric violet emission band around 370 nm, which indicates the existence of different luminescent peaks. The emission spectra of CNP:xCe3 þ (x ¼ 0.01–0.09) phosphors excited by 300 nm are also shown in Fig. 2. It can be observed that all the emission spectra exhibit a similar profile with various relative intensities. The emission intensities enhance with the increase of Ce3 þ concentration and reach the maximum at x¼ 0.05. However, the photoluminescence intensities decrease with further increase of Ce3 þ concentration due to concentration quenching. It is accepted that concentration quenching is mainly caused by energy transfer among Ce3 þ ions, the probability of which increases as the concentration of Ce3 þ increases. If we consider energy transfer in the given crystal structure, the critical distance (Rc) is defined as the distance for which the probability of nonradiative energy transfer equals the probability of radiative emission of Ce3 þ , as pointed out by Blasse [21]. So the following equation can be used to calculate the Rc value. Rc ¼ 2
3V 4πxc N
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Fig. 3. PL (λem ¼ 300 nm) and PLE spectra (λex ¼370 nm) of CNP:0.05Ce3 þ and CNP:0.05Mn2 þ (a) and those of CNP:0.05Ce3 þ , 0.07Mn2 þ (b). (Inset) Photograph of CNP:0.05Ce3 þ and CNP:0.05Ce3 þ , 0.07Mn2 þ sample under UV lamp (λem ¼254 nm, 6 W).
1=3
where xc is the critical concentration, N is the number of cations in the unit cell and V is the volume of the unit cell. By taking the experimental and analytic values of V, N and xc (3521.3 Å3, 60, and 0.05, respectively), the critical transfer distance is calculated to be about 13.08 Å. As a comparison, the distances between the nearest two sites among M1, M2 and M3 sites are 3.655 Å (M1–M2), 3.964 Å (M1–M3) and 6.276 Å (M2–M3). Based on these results, the optimal Ce3 þ content for Mn2 þ co-doped is found to be 0.05. Since the transitions of Mn2 þ are d-d spin and parity forbidden according to the spin selection rule, the emission and excitation intensities are very weak. The PLE spectrum of CNP:0.07Mn2 þ sample (Fig. 3(a) consists of several peaks centered at 350, 365, 405, 421 and 500 nm, corresponding to the well-known transitions of Mn2 þ from ground level 6 A1 (6S) to 4E (4D), 4T2 (4D), [4A1 (4G), 4E (4G)], 4T2 (4G) and 4T1 (4G) levels. The PL spectrum is a red emission centered at 650 nm, which is assigned to the 4T1–6A1 forbidden transition of Mn2 þ . The Ce3 þ emission has a significant spectral overlap on the excitation band of Mn2 þ , which is in favor of the resonance type energy transfer from Ce3 þ to Mn2 þ [22]. As seen in Fig. 3(b), the PLE spectrum of CNP:0.05Ce3 þ , 0.07Mn2 þ is similar to that for the single Ce3 þ -doped sample. Thus, compared with the PL spectrum of the single Ce3 þ -doped sample, besides the violet–blue emission band with decreased intensity from Ce3 þ , a broad and strong red band appears, which is due to Mn2 þ sensitized emission. The result indicates that the efficient energy transfer from Ce3 þ to Mn2 þ happens in the Ce3 þ –Mn2 þ co-doped sample. This phenomenon is easy to understand since the Ce3 þ emission covers some Mn2 þ excitation band and the excitation spectrum of CNP:Ce3 þ , Mn2 þ monitoring the Mn2 þ -derived
Fig. 4. PL spectra of CNP:0.05Ce3 þ , yMn2 þ (y¼0.01–0.13) phosphors (λex ¼300 nm).
emission has a similar profile with the excitation bands of CNP:Ce3 þ . Besides, Ce3 þ is larger than the host cation Ca2 þ , and Mn2 þ is smaller than Ca2 þ , i.e. rCe3 þ (114 pm) 4rCa2 þ (112 pm) 4 rMn2 þ (96 pm); Ce3 þ /Mn2 þ clusters probably form in the crystal lattice due to the ion-size compensation effect [23]. In the clusters, Mn2 þ is located near Ce3 þ ; this also benefits effective energy transfer. 3.3. Concentration quenching and energy transfer The PL spectra of CNP:0.05Ce3 þ , yMn2 þ (y ¼ 0.01–0.13) under excitation of 300 nm are displayed in Fig. 4. The intensity of Ce3 þ UV emission decreases with increase of doped Mn2 þ concentration, while the PL intensity of Mn2 þ emission increases until the Mn2 þ concentration-quenching happens at above y ¼ 0.07. The observed variations in the emission intensities of Ce3 þ and Mn2 þ strongly confirm the continuous energy transfer from Ce3 þ to Mn2 þ .
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Fig. 5. Experimental emission spectrum (solid line, range: 500–800 nm), fitted curve (red dashed line), and deconvoluted Gaussian components (green dashed lines) of CNP:0.05Ce3 þ , yMn2 þ (y¼ 0.01, 0.03, 0.05, and 0.07. λex ¼ 300 nm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The profiles of Mn2 þ emission spectra (range: 500–800 nm) of CNP:0.05Ce3 þ , yMn2 þ (y¼ 0.01, 0.03, 0.05, 0.07) exhibit different shapes under 300 nm excitation, as shown in Fig. 5. By Gaussian fitting, three broad emission peaks at 640, 680, and 745 nm can be observed. The Mn2 þ broadband emission originates from 4T1–6A1 transition. The energy separations between the 4T1(4G) state and the 6A1(6S) ground state are very sensitive to the crystal-field strength and decrease with increase in crystal field strength [24]. Since the luminescence wavelength due to Mn2 þ is sensitive to the magnitude of the crystal field, several emission bands are observed when different types of Mn2 þ sites exist in a host crystal. The crystal field strength is affected by many factors such as coordination number, site symmetry and size of the coordinating polyhedron. Based on the data from a large number of literatures, Dorenbos [25–28] concluded that the larger the size of the coordinating polyhedron, the smaller the crystal field splitting of 5d levels and shorter the emission wavelength. Similar
conclusion could be applied for Mn2 þ 3d levels, which indicates that larger coordinating polyhedron results in lower crystal field strength and a blue-shift of Mn2 þ emission wavelength [29]. According to P.H. Hollway and P.D. Rack, the crystal field strength of d-orbital splitting could be expressed as [30] Dq ¼
ze2 r 4 6R5
where Dq is a measurement of the crystal-field strength, z is the charge or valence of the anion, e is the charge of an electron, r is the radius of the d wave function, and R is the distance between the central ion and its ligands. When Mn2 þ occupies a certain position in a host, the crystal field strength that affects the ion decreases as the space containing Mn2 þ becomes larger, as expected from the above-mentioned equation. For decrease in the field, the transition energy between 4 T1(4G) and 6A1(6S) is predicted to increase, and the emission wavelength of Mn2 þ will shift to shorter wavelengths [31].
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Thus, the size of coordinating polyhedron could be qualitatively measured by the average cation–anion distance. According to the crystal structure of Ca10Na(PO4)7, M1 site is coordinated with seven O atoms with the average Ca–O distance 2.4285 Å, and the average Ca–O distance of M2 (C.N. ¼ 6) and M5 (C.N. ¼ 6) is 2.3893 Å and 2.2575 Å, respectively. As a consequence, the crystal field strength is larger for Mn2 þ at the M5 site than at the M1 and M2 sites. Thus, the emission band at lower energy ( 745 nm) is assigned to the Mn2 þ located at the M5 site, and the other emission bands at higher energy ( 680 nm and 640 nm) originate from the Mn2 þ located at the M2 and M1 sites, respectively. The ratio of these three peaks varies with increasing Mn2 þ contents. This phenomenon is due to the fact that the radius of Mn2 þ (0.83–0.96 Å) is more close to that of M5 (C.N. ¼ 6, and r (Ca2 þ ) ¼ 1.00 Å), so it is reasonable that Mn2 þ ions tend to occupy M5 sites first, and the corresponding emission peak prevails at low doping content (x¼ 0.01). With increasing Mn2 þ contents, the percentage of Mn2 þ , which occupied M1 and M2 sites, will rise, and the intensities of corresponding peaks will increase. This results in the change of spectral shape and the blue-shift of emission wavelength. The PL decay curves of Ce3 þ in CNP:0.05Ce3 þ , yMn2 þ phosphors were measured with excitation at 300 nm and monitored at 370 nm, and are shown in Fig. 6. For CNP:0.05Ce3 þ , the decay curves can be fitted successfully based on the double-exponential equation I ¼ A1 expð t=τ1 Þ þ A2 expð t=τ2 Þ where I represents the luminescence intensity; A1 and A2 are constants; t is time; τ1 and τ2 are the decay times for the exponential components, respectively. For CNP:0.05Ce3 þ , the lifetime values (τ1, τ2) were determined to be 11.92 ns and 30.18 ns. The results reveal that there are two lattice sites occupied by Ce3 þ ions in Ca10Na(PO4)7 host. Thus, the average decay time (τ) can be determined using the following
Fig. 6. Ce3 þ decay time of CNP:0.05Ce3 þ , yMn2 þ under 300 nm excitation.
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equation: τ¼
A1 τ21 þ A2 τ22 A1 τ1 þ A2 τ2
As seen in Fig. 6, Ce3 þ emission decay curves of CNP:0.05Ce3 þ , xMn2 þ sample become more and more nonexponential with increasing Mn2 þ content. The average decay time τ reduces monotonically from 27.06 to 1.86 ns as x increases from 0 to 0.13 which is good evidence for Ce3 þ Mn2 þ energy transfer. The energy transfer efficiency (ηT) from a sensitizer to an activator ion can be calculated by the following relation, which has been discussed by Paulose et al. [32] τs ηT ¼ 1 τs0 where τs and τs0 are the decay lifetimes of the sensitizer (Ce3 þ ) ion with and without activator (Mn2 þ ) ion present, respectively. The ηT from Ce3 þ to Mn2 þ in CNP host are calculated as a function of the Mn2 þ concentration and presented in the inset of Fig. 4. The ηT was found to increase with increasing Mn2 þ content, and reached 93% at y ¼ 0.13. Generally, there are two main aspects responsible for the resonant energy-transfer mechanism: one is an exchange interaction and the other is multipolar interaction. It is known that if energy transfer results from the exchange interaction, the critical distance between the sensitizer and activator should be shorter than 5 Å [33]. The critical distance Rc for energy transfer from the Ce3 þ to Mn2 þ ions can be calculated using the concentration quenching method 3V 1=3 Rc ¼ 2 4πxc N where N is the number of available sites for the dopant in the unit cell, xc is the total concentration of Ce3 þ and Mn2 þ , and V is the volume of the unit cell. For the Ca10Na(PO4)7 host, N ¼ 60 and V¼ 3521.3 Å3. The critical concentration xc, at which the luminescence intensity of Ce3 þ is half of that with the absence of Mn2 þ , is about 0.05þ 0.05 ¼ 0.10. Therefore, the critical distance (Rc) of energy transfer was calculated to be about 10.38 Å. This value is much higher than 4 Å, indicating little possibility of energy transfer via the exchange interaction mechanism. Thus, the energy transfer between the Ce3 þ and Mn2 þ ions mainly takes place via electric multipolar interactions. On the basis of Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the following relation can be obtained [34,35]: τS0 p Ca=3 τS where C is the concentration of Mn2 þ , and a= 6, 8, and 10 for dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. Plots of τs0/τs and Ca/3 based on the above equation are shown in Fig. 7. Linear behavior was observed only when a¼ 6, implying that energy transfer from Ce3 þ to Mn2 þ occurred via the dipole–dipole mechanism. The CIE chromaticity diagram and coordinates for the CNP:0.05Ce3 þ , yMn2 þ phosphors with different doping
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Fig. 7. Dependence of τs0/τs of Ce3 þ on CMn 6/3, CMn 8/3, and CMn 10/3.
contents of Mn2 þ are calculated according to the relevant PL spectrum under excitation of 300 nm, which are exhibited in Fig. 8. The color tone of the phosphors can be tuned from violet through purplish red and finally to red with the increase of Mn2 þ content from 0 to 0.13 due to efficient Ce3 þ -Mn2 þ energy transfer with the increase of Mn2 þ concentration. And the corresponding CIE coordinates are calculated to change from (0.161, 0.019) to (0.682, 0.302). The CIE coordinate of optimal red phosphor CNP:0.05Ce3 þ , 0.07Mn2 þ is (0.679, 0.298), which is close to that of NSTC (National Television System Committee) standard red (0.67, 0.33). 3.4. Temperature-dependent PL properties In general, the thermal quenching behavior of phosphors is important because it has considerable influence on the light output and color rendering index [36,37]. Fig. 9 shows the temperature-dependent emission spectra of CNP:0.05Ce3 þ , 0.07Mn2 þ phosphors from room temperature to 190 1C. The relative peak intensities of Ce3 þ decreased generally with temperature, and it reached 75% of the initial value at 190 1C. The thermal stability of Mn2 þ emission is higher than that of Ce3 þ emission. The Mn2 þ emission intensity almost keeps constant with rising temperature, which indicates that CNP:0.05Ce3 þ , 0.07Mn2 þ has an excellent thermal stability, and makes it a possible red phosphor for further application. As seen in Fig. 9, the Mn2 þ emission wavelength blueshifts slightly with increasing temperature. Peak positions at 30 1C and 190 1C are 656 and 640 nm, respectively. It is considered that thermal active phonon-assisted excitation from lower energy sublevel to higher energy sublevel in excited states occurs [38,39]. Elevated temperature causes electrons at lower energy level to jump to higher energy levels by phonon assistance whereas the nonradiative transitions from excited states to ground states are prevented. At higher temperature,
Fig. 8. CIE chromaticity diagram and coordinates for the CNP:0.05Ce3 þ , yMn2 þ phosphors. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
more electrons are populated at higher-energy excited sublevel, but the nonradiative transitions through the crossing point between the excited state and the ground state in configurational coordinate diagram are decreased. The height of higherenergy emission peak is increased, and that of lower-energy emission peak is decreased. As a result, the blue-shift behavior is observed with increasing temperature. The shift of CIE coordinates with increasing temperature has been calculated and demonstrated in Table 1, and it can be observed that the CIE shift is negligible in a wide range of temperature. 4. Conclusion Luminescent properties and energy transfer process of Ca10Na(PO4)7:Ce3 þ , Mn2 þ phosphor are studied. Co-doping
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References
Fig. 9. Temperature-dependent PL spectra of CNP:0.05Ce3 þ , 0.07Mn2 þ under 300 nm excitation.
Table 1 The shift of CIE coordinates with increasing temperature of CNP: 0.05Ce3 þ , 0.07Mn2 þ phosphor. Temperature (1C)
CIE x
CIE y
30 50 70 90 110 130 150 170 190
0.683 0.681 0.679 0.676 0.673 0.670 0.668 0.665 0.662
0.303 0.306 0.309 0.313 0.316 0.320 0.323 0.327 0.331
Ce3 þ /Mn2 þ is an effective approach to enhance the red emission of Mn2 þ . The suitable spectrum overlapping of Ce3 þ emission to Mn2 þ excitation as well as the formation of Ce3 þ /Mn2 þ clusters may be the causes of enhanced Mn2 þ luminescence. The different shape of Mn2 þ emission with increasing Mn2 þ content is due to the Mn2 þ occupation of different sites. The energy transfer from Ce3 þ to Mn2 þ was proposed to be of resonance-type via an electric dipole–dipole mechanism. The selected sample Ca10Na(PO4)7:0.05Ce3 þ , 0.07Mn2 þ shows strong red emission and a good thermal stability with increasing temperature up to 200 1C. All the results have demonstrated that Ca10Na(PO4)7:Ce3 þ , Mn2 þ is potentially useful as a UV excited red-emitting phosphor. Acknowledgments This work was supported by the Scientific Research Foundation of Guangxi University (Grant no. XBZ120573) and National Natural Science Foundation of China (No. 61264003). The authors thank Prof. Menglian Gong and Prof. Jianxin Shi (Sun Yat-sen University, Guangzhou, China) for their help in PL/PLE measurement.
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