Structure, luminescence property and abnormal energy transfer behavior of color-adjustable Ca₃Hf₂SiAl₂O₁₂:Ce³⁺,Mn²⁺ phosphors

Abstract

A series of color-adjustable phosphors Ca₃Hf₂SiAl₂O₁₂:Ce³⁺,Mn²⁺ were synthesized through a high temperature solid-state method. Ca₃Hf₂SiAl₂O₁₂ belongs to body-centered cubic crystal system and Ia3̄d (230) space-group. It was found that three different cation sites in the Ca₃Hf₂SiAl₂O₁₂ phase were occupied evenly by Ce³⁺ and Mn²⁺ ions. Different situations of Mn²⁺ occupying sites and energy transfer from Ce³⁺ to Mn²⁺ can appear with different Mn²⁺ content and the critical distance was calculated to be R_c1 = 10.8 Å, R_c2 = 10.1 Å and R_c3 = 12.6 Å after calculation of energy transfer from the Ce³⁺ to Mn²⁺ by using the concentration quenching method. Ca₃Hf₂SiAl₂O₁₂:Ce³⁺,Mn²⁺ phosphors exhibited a broad excitation band ranging from 300 to 450 nm and two broad asymmetric emission bands upon 400 nm excitation. The emission colors of Ca₃Hf₂SiAl₂O₁₂:Ce³⁺,Mn²⁺ could be tuned from blue-green (0.2303, 0.3265) to white (0.3350, 0.3388) by changing the ratio of Ce³⁺/Mn²⁺. The correlated color temperature can be adjusted from 12 763 K to 5379 K. It indicated that Ca₃Hf₂SiAl₂O₁₂:Ce³⁺,Mn²⁺ possesses potential applications in white-LEDs.

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1. Introduction

Nowadays, white light emitting diodes (W-LEDs) have attracted considerable attention and been used in many areas¹ for their various advantages such as small volume, high efficiency, long lifetime, energy saving and environment-friendly.^2,3 The initial method to get W-LED lamp is consisting three triple LED chips for red, green and blue.⁴ Though it is easy to fabricate, it has different thermal stability, different drive voltage, different degradation and expensive for different LED chips,^5–7 which restrains its application. Another feasible scheme for W-LED generation is using blue or ultraviolet chips combining with some phosphors which can be excited by them. For instance, YAG:Ce³⁺ yellow phosphor excited by a blue InGaN chip can generate white light by yellow emission from the phosphor and blue light from the chip. Though it has been commercialization and widely used, it cannot satisfy the optimum requirements. Different degradation for blue chip and phosphor leads to chromatic aberration. Lack of red light component causes high correlated color temperature (7765 K) and poor color rendering index which restricts its more broad applications. In order to obtain warm white light and high color rendering index, red phosphor is introduced to that system.^8–12 In addition to this method, another kind of W-LEDs can be fabricated by combing of the GaN-based blue chip with green and red phosphors or via coupling ultraviolet (UV) LED (350–420 nm) with blue, green and red phosphors.^13–15 As compared to YAG:Ce³⁺ yellow phosphor excited by a blue InGaN chip, this type of W-LEDs has low correlated color temperature and high color rendering index (R_a > 90), but low luminous efficiency due to the re-absorption between phosphors. Recently, more efforts to get satisfied warm white light have been focused on by using single-component phosphor which can emit white light doped with rare earth ions.^16–23 The single-component phosphor is more stable, more efficient and no phase separation, which can avoid the above mentioned problems. The single-composition white-light phosphor can be realized by co-doping a sensitizer and an activator such as Ce³⁺ and Mn²⁺ into the same host. It is accepted that the Ce³⁺ ion with the 4f configuration in solids shows efficient broad band luminescence due to the 4f–5d parity allowed electric dipole transition. Attributing to the extended radial wavefunctions of the 5d state, the Ce³⁺ ion usually has a large Stokes shift and its emission can be shifted from UV to the visible region of the electromagnetic spectrum, which is dependent on the host lattice. Moreover, the Ce³⁺ ion also acts as a good sensitizer, transferring a part of its energy to the activator ion. Being considered as a candidate of the activator, the transition-metal ion Mn²⁺ can give a broad emission band in the visible range, and the emission color of Mn²⁺ can vary from green to red, which is strongly affected by the crystal field of the host materials. Owing to the forbidden ⁴T₁–⁶A₁ transition of Mn²⁺ ion, the emission intensity of a Mn²⁺-ion singly doped phosphor is low under UV excitation. Based on the above-mentioned considerations, it is necessary to enhance the emission intensity of Mn²⁺-doped materials by introducing an efficient sensitizer Ce³⁺. More importantly, white light can be obtained by co-doping Ce³⁺ and Mn²⁺ ions under effective resonance-type energy transfer in a single phase host. Therefore, a single-phase white-light-emitting phosphor based on the mechanism of energy transfer has been realized by doping the sensitizers and activators in some host lattices such as Ca₃Sc₂Si₃O₁₂:Ce³⁺,Mn²⁺,²⁴ Ba₂Li₂Si₂O₇:Ce³⁺,Mn²⁺,²⁵ Ca₂Gd₈Si₆O₂₆:Ce³⁺,Mn²⁺,²⁶ Ca₃YGa₃B4O₁₅:Ce³⁺,Mn²⁺.²⁷

A new species of the garnet super-group Ca₃(Hf, Sn, Zr, Ti)₂SiAl₂O₁₂ was discovered in metasomatically altered carbonate-silicate xenoliths in ignimbrites of the upper Chegem Caldera, Northern Caucasus, Kabardino-Balkaria, Russia at 2013. It is said that Hf-garnet obtains higher in the conduction band, above 4.0 eV, which may obtain NUV excitation. We previously reported the discovery and crystal structure of Ca₃Hf₂SiAl₂O₁₂ (hereafter referred to as CHSA) and explored the potential of its Ce³⁺-activated derivative for use as a phosphor in LED. CHSA:Ce³⁺ emits a strong blue-green light under excitation from the UV to near-UV. Here, we reported Ce³⁺,Mn²⁺ co-doped CHSA and studied its energy transfer situation. According to changing different Mn²⁺ contents, we realized the adjustable CIE coordinates and color temperature.

2. Experimental section

CaCO₃ (99.9%), HfO₂ (99.99%), Al(OH)₃ (AR), H₂SiO₃ (AR), CeO₂ (99.99%) and MnCO₃ (99.9%) as the raw materials are stoichiometric and 5 wt% H₃BO₃ as the flux is also used in the synthesis process. Aluminum oxide crucibles (10 mm × 10 mm) and porcelain boat are utensils for sintering.

2.1 Synthesis

A series of Ca₃Hf₂SiAl₂O₁₂:Ce³⁺,Mn²⁺ samples are synthesized by two-step traditional high temperature solid-stated reaction. The relative amounts of materials are calculated and then are weighed by electronic balance accurate to 4 decimal places. All the raw materials and the flux are put into agate mortar with 8 mL ethanol added in it at the same time. Grind the mixture for 30 min until they are evenly blended and homogeneous. After the mixture dry, transfer the mixture into aluminum oxide crucibles (10 mm × 10 mm) and pre-fire at 800 °C for 2 h under air atmosphere, then, calcine them at 1480 °C for 5 h under flowing 95% N₂–5% H₂ atmosphere in the horizontal tube furnace. When they are cooled with 5 °C per min speed to room temperature, grind them to powders, yielding the resulting phosphor powder.

2.2 Characterization

The phase formation and crystal structure were analyzed by the X-ray powder diffraction (XRD) (D2 PHASER X-ray diffractometer, Germany) with graphite monochromator using Cu Kα radiation (λ = 1.54056 A), operating at 30 kV and 15 mA. The investigation range is 10° to 80° with scanning speed of 15° 2θ min⁻¹ for the series Ca₃Hf₂SiAl₂O₁₂:Mn²⁺ samples. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples are measured by a Fluorlog-3 spectrofluorometer equipped with 450 W xenon lamps (Horiba Jobin Yvon). The temperature-dependence luminescence properties are measured on the same spectrophotometer, which is combined with a self-made heating attachment and a computer-controlled electric furnace from room temperature (25 °C) to 250 °C with a heating rate of 100 °C min⁻¹ and a holding time of 5 min for each temperature point. The luminescence decay curves were obtained by FLS-920T fluorescence spectrophotometer as well. All the testes are carried out at room temperature.

3. Results and discussion

Ca₃Hf₂SiAl₂O₁₂ belongs to body-centered cubic crystal system and Ia3̄d (230) space-group which is shown in Fig. 1 observing from [100] crystal orientation. There are three sites for cations in this crystal structure and only one lattice site for Hf, Al/Si and Ca, respectively. In the present work, there is valency imposed double site-occupancy (R⁴⁺, R³⁺) at the Al/Si tetrahedron site,^28,29 which Al and Si atoms are in substitutional disorder, and the atomic position and anisotropic displacement parameters of Al and Si are constrained to be identical at Al/Si sites in the initial refinements. The yellow octahedrons (six coordination), [HfO₈]⁴⁻ is fully occupied by Hf, while the blue tetrahedrons site, [AlO₄]⁵⁻, [SiO₄]⁴⁻ is occupied by 2/3 Al and 1/3 Si. The green spheres located in the space between the polyhedrons represent the Ca site. The anion sites (oxygen sites) are located at the shared corners of the octahedra and tetrahedra. According to ions radius analysis, the radius of Ce³⁺ in eight coordinated is 1.283 Å which is close to Ca²⁺ 1.26 Å. It may provide a proof that Ce³⁺ will occupy Ca²⁺ site in this crystal structure. In the same time, the radius of Al³⁺/Si⁴⁺ in tetrahedron site is 0.53/0.40 Å and the radius of Hf⁴⁺ in octahedrons (six coordination) is 0.85 Å. Generally, the radius of Mn²⁺ is larger than Al³⁺/Si⁴⁺ and small but close to Hf⁴⁺, while smaller than Ca²⁺. It is considered that Mn²⁺ may occupy every cation site in this structure.

Fig. 1 The structure of Ca₃Hf₂SiAl₂O₁₂ and the coordination of Ca, Hf and Al/Si.

XRD patterns of the as-prepared CHSA:y%Mn²⁺ (1 ≤ y ≤ 14) phosphors were collected to verify the phase purity. As given in Fig. 2, it can be seen that all the diffraction peaks of the samples can be indexed to the corresponding calculated data basically, suggesting that CHSA:y%Mn²⁺ with different Mn²⁺ contents can be formed in the single-phased structure. From the right image, the peak shift of series XRD patterns, we can see that the peaks shift to smaller angle firstly with increasing Mn²⁺ content, and then shift to larger angle. It could provide a proof that Mn²⁺ can occupy not only one site. According to analysis above, in the low Mn²⁺ content, Mn²⁺ could occupy Al³⁺/Si⁴⁺ tetrahedrons site. The radius of Mn²⁺ in tetrahedrons is 0.80 Å, larger than Al³⁺/Si⁴⁺. According to2d sin θ = nλ, with the interplanar spacing (d) increasing, the sin θ will decrease and lead to the XRD peaks shift to smaller angle. With the increasing of Mn²⁺ content, Mn²⁺ will occupy Hf⁴⁺ or Ca²⁺ site. The radius of Mn²⁺ in six or eight coordination is smaller than Hf⁴⁺ or Ca²⁺. When Mn²⁺ substitutes Hf⁴⁺ or Ca²⁺, the interplanar spacing (d) will decrease and lead to XRD peaks shift to larger angle. The phenomenon of peaks shift could be as evidence for Mn²⁺ different occupying situation and it indicates that Mn²⁺ could substitute Al³⁺/Si⁴⁺ site. However, it cannot distinguish which site of Hf⁴⁺ and Ca²⁺ will be occupied by Mn²⁺. Another situation is that Mn²⁺ may substitute both Hf⁴⁺ and Ca²⁺. Because of the radius of Hf⁴⁺ and Ca²⁺ are both larger than Mn²⁺. Occupying one site of Hf⁴⁺ and Ca²⁺ or both sites of them will both lead to peak shift to larger angle.

Fig. 2 XRD patterns of series Ca₃Hf₂SiAl₂O₁₂:x%Mn²⁺ and calculated data (left image); peak shift of series XRD patterns (right image).

Fig. 3 presents the excitation and emission spectra of single doped and co-doped Ca₃Hf₂SiAl₂O₁₂ phosphors. As shown in Fig. 3a demonstrates the PLE and PL spectra of CHSA:1%Mn²⁺. The excitation spectrum consists of several bands centering at 362 nm, 410 nm, 455 nm and 480 nm, corresponding to the transitions of Mn²⁺ ion from ground level ⁶A₁(⁶S) to ⁴E(⁴D), ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)], and ⁴T₁(⁴G) levels, respectively. The broad emission band from 525 to 600 nm centered at 556 nm ascribed to the spin-forbidden ⁴T₁(⁴G)–⁶A₁(⁶S) transition of the Mn²⁺ ions.³⁰ The PLE and PL spectra of CHSA:1%Ce³⁺ sample are shown in Fig. 3b. The PLE spectrum monitored by 457 nm exhibits two distinct excitation bands at 325 and 400 nm, which could be ascribed to the electronic transitions from the ground state to the different crystal field splitting bands of excited 5d states of Ce³⁺. Upon 400 nm excitation, the PL spectrum consists of an asymmetric broad emission band from 450 to 550 nm with a maximum at 455 nm which is assigned to the 5d–4f transitions of Ce³⁺. It is well-known that the typical emission of Ce³⁺ should consist of a double band in view of the transitions of Ce³⁺ ions from 5d state to the ²F_5/2 and ²F_7/2 ground states, and the theoretical energy difference of this splitting between ²F_5/2 and ²F_7/2 level is about 2000 cm.³¹ As seen in Fig. 3a and b the significant spectral overlap occurs between the emission band of Ce³⁺ and the excitation band of Mn²⁺, which indicates the possible resonance type energy transfer from the Ce³⁺ to Mn²⁺ ions in CHSA matrix. The PLE and PL spectra of CHSA:1%Ce³⁺, 1%Mn²⁺ monitored at the emission of Mn²⁺ (556 nm) with 400 nm excitation are shown in Fig. 3c. Upon excitation at 400 nm, the PL spectrum of CHSA:1%Ce³⁺,1%Mn²⁺ phosphor appears not only as a blue-green band of Ce³⁺ ions also as a green band of the Mn²⁺ ions.

Fig. 3 PLE (left) and PL (right) spectra of Ca₃Hf₂SiAl₂O₁₂:Mn²⁺ (a); Ca₃Hf₂SiAl₂O₁₂:Ce³⁺ (b); Ca₃Hf₂SiAl₂O₁₂:Ce³⁺,Mn²⁺ (c).

From Fig. 4, we can see that CHSA:xCe³⁺ can emit cyan light (450–550 nm) under UV excitation and the emission spectra consist of two apparent asymmetric broad peaks before x = 0.03 which corresponds to the 5d–4f allowed transition of Ce³⁺, beyond x = 0.03, emission spectra turn to be a broad peak. Additionally, the inset (upper right) of Fig. 4 shows the Ce³⁺ content dependent emission intensity and peak position. It can be easily seen that the emission intensities have an obvious increasing trend with increasing Ce³⁺ concentration, and maximizes at x = 0.01, then the emission intensity decreases. The peak position has apparent red shift from 481 to 508 nm about 1105 cm⁻¹. It is well-known that the typical emission of Ce³⁺ should consist of a double band in view of the transitions of Ce³⁺ ions from 5d state to the ²F_5/2 and ²F_7/2 ground states. That's the reason why emission spectra consist of two apparent asymmetric broad peaks, and, with the increasing of Ce³⁺ content, the energy transfer of intra-Ce³⁺ becomes to be significant, this kind energy transfer that also leads to red shift and broader of emission and excitation spectra.³²

Fig. 4 PL properties under 400 nm excitation of series Ca₃Hf₂SiAl₂O₁₂:x%Ce³⁺ phosphors; inset: (upper right) dependence of emission intensity and peak position of Ca₃Hf₂SiAl₂O₁₂:x%Ce³⁺ phosphors on Ce³⁺ content.

The PL spectra of Ca₃Hf₂SiAl₂O₁₂:y%Mn²⁺ with different Mn²⁺ content are shown in Fig. 5. With 453 nm excitation, CHSA:Mn²⁺ can emit visible light which attributes ⁴T₁(⁴G)–⁶A₁(⁶S) transition of the Mn²⁺. Specially, in the low content, it is clearly that CHSA:Mn²⁺ can emit green light peaking at about 556 nm. With the increasing of Mn²⁺ content, another two peaks appear peaking at about 590 and 670 nm and emit orange and red light. According to above site occupying situations of Mn²⁺, the special emission spectra of CHSA:Mn²⁺ can be explained by different occupying site of Mn²⁺. In the low content of Mn²⁺, Mn²⁺ may substitute Al³⁺/Si⁴⁺ site. Generally speaking, the emission properties of Mn²⁺ are determined to the coordination number of the occupied site in the crystal structure mainly by Mn²⁺.^33,34 With the coordination number increasing, the emission of Mn²⁺ will shift to red range in the spectra. In this CHSA structure, Al³⁺/Si⁴⁺ are four coordinations. That is a relative small coordination and when Mn²⁺ occupies in this site, it will emit shortwave light. Therefore, in the low content of Mn²⁺ it will emit green light in this system. The occupying situation of Mn²⁺ in low content is certified by XRD peaks shift and emission spectra property. Furthermore, in high content of Mn²⁺ another two emission peaks appear. We conjecture that in high Mn²⁺ content, Mn²⁺ will occupy Hf⁴⁺ and Ca²⁺ site. Due to different coordination of Hf⁴⁺ and Ca²⁺, when Mn²⁺ occupy these two sites it will emit different light. Hf⁴⁺ has six coordinations and Ca²⁺ is eight as shown in Fig. 1. Therefore, when Mn²⁺ substitutes Hf⁴⁺ site it will emit orange light peaking at 590 nm while Mn²⁺ substitutes Ca²⁺ site it will emit orange light peaking at 670 nm, respectively. According to emission spectra properties, we conjecture that Mn²⁺ may substitute both Hf⁴⁺ and Ca²⁺. That can be as a supportive certification of Mn²⁺ occupying situation in high content.

Fig. 5 PL properties under 453 nm excitation of series Ca₃Hf₂SiAl₂O₁₂:y%Mn²⁺ phosphors.

The PL properties of Ce–Mn co-doped CHSA phosphors are shown in Fig. 6. It is clearly seen that the spectra are consist of two parts, one is at the range of 450–525 nm which can be attributed to Ce³⁺ emission; another part is at about 550–650 nm, which can be attributed to Mn²⁺ emission. Furthermore, as the general situation, with the increasing of Mn²⁺ content, the intensities of Ce³⁺ emission are decreasing gradually and the intensities of Mn²⁺ are increasing, respectively. More importantly, in the Ce–Mn co-doped situation, in the low Mn²⁺ content, specially, at 3%, the intensity of emission peaking at 556 nm reaches to highest. With the increasing of Mn²⁺ content, the emission peaking at 556 nm becomes weak and disappears in the spectra cause to concentration quenching and anther peak at 590 nm appear and then becomes weak. The emission peaks of Mn²⁺ just appear two peaking at about 556 and 590 nm which is different with the Mn²⁺ single-doped phosphor shown in Fig. 5. We surmise that the different energy transition efficiency between Ce³⁺ and Mn²⁺ may lead to this special situation. In general, the resonant-type energy transfer from a sensitizer to an activator in a phosphor may take place via the exchange interaction and electric multipolar interaction. In many cases, concentration quenching is due to energy transfer from one activator to another until an energy sink in the lattice is reached. The critical distance R_C for energy transfer from the Ce³⁺ to Mn²⁺ ions can be calculated using the concentration quenching method. According to Sato, the critical distance R_c can be described by³⁵

where N is the number of available sites for the dopant in the unit cell, x_c is the total concentration of the Ce³⁺ and Mn²⁺ ions, and V is the volume of the unit cell. According to previous study on the crystal structure of CHSA, the acquired values 1891.26 ų, respectively. N has different values cause to different occupying situation of Mn²⁺. There are three possible situations: (1) when Mn²⁺ occupies Al³⁺/Si⁴⁺, N₁ is 72; (2) when Mn²⁺ occupies Hf⁴⁺, N₂ is 71; (3) when Mn²⁺ occupies Ca²⁺, N₃ is 36. After calculation, the values of R_c in three situations are R_c1 = 10.8 Å, R_c2 = 10.1 Å and R_c3 = 12.6 Å. According to the results, we find that R_c1 ≈ R_c2 and they are smaller than R_c3 which demonstrates that when Mn²⁺ occupies Al³⁺/Si⁴⁺ and Hf⁴⁺ it has smaller critical distance. The smaller critical distance leads to more chances for Ce³⁺ to transfer energy to Mn²⁺. This result leads to the two emission peaks (556 and 590 nm) of Mn²⁺ in co-doped CHSA phosphor appear and the third peak at 670 nm obtains little energy from Ce³⁺ and too weak and does not appearing in co-doped emission spectra.

Fig. 6 PL spectra of Ca₃Hf₂SiAl₂O₁₂:1%Ce³⁺,y%Mn²⁺ phosphors on Mn²⁺ doping concentration (y) (λ_ex = 360 nm).

Temperature-dependent relative emission intensity under 400 nm excitation of CHSA:1%Ce³⁺,14%Mn²⁺ is indicated in Fig. 7. It can be clearly seen that with temperature increasing, the emission intensity decreases gradually and the emission band of Ce³⁺ goes from two apparent asymmetric broad peaks to one broad band definitely. Generally, the thermal quenching of emission intensity can be explained by a configurational coordinate diagram in which, through phonon interaction, the excited luminescence center is thermally activated through the crossing point between the excited state and the ground state. This non-radiative transition probability by thermal activation is strongly dependent on temperature resulting in the decrease of emission intensity.^36,37 Due to the increasing phonon interaction and non-radiative transition with increasing temperature, the spectral overlap between the excitation band (Fig. 3b) and the first emission band (5d–²F_5/2 emission band) increases. This results in more re-absorption of the first emission due to the 5d–²F_5/2 transition and thus in a relative stronger decrease of the 458 nm with respect to the 486 nm emission band. That leads to two apparent asymmetric broad peaks to one broad band definitely with temperature increasing. Furthermore, the Mn²⁺ emission spectra also take on a slight blue shift with temperature increasing. A similar phenomenon has been found and studied in other Mn²⁺ ion doped phosphors.³⁸ It was ascribed to the thermally active phonon-assisted tunnelling from the excited states of the low energy emission band to the excited states of the high-energy emission band in the configuration coordinate diagram. In order to better understand temperature dependence of the luminescence, the activation energy (E_a) was calculated using the Arrhenius equation³⁹

where I₀ is the initial PL intensity of the phosphor at room temperature, I_T is the PL intensity at different temperatures, c is a constant, ΔE is the activation energy, and k is the Boltzmann constant (8.62 × 10⁻⁵ eV). Making this function to do logarithmic transformation can get⁴⁰

Fig. 7 Temperature dependence of Ca₃Hf₂SiAl₂O₁₂:1%Ce³⁺,14%Mn²⁺ PL properties.

According to the equation, the activation energy ΔE can be calculated by plotting ln[(I₀/I_T) − 1] against 1/kT, where a straight slope equals ΔE. As shown in Fig. 8, ΔE of Ce³⁺ emission was found to be 0.1879 eV and 0.1781 eV for Mn²⁺ emission.

Fig. 8 The Arrhenius fitting of the emission intensity of Ca₃Hf₂SiAl₂O₁₂:1%Ce³⁺,14%Mn²⁺ phosphor and the calculated E_a for thermal quenching at Ce³⁺ (a) and Mn²⁺ (b) emission.

The PL decay curves of CHSA:1%Ce³⁺,yMn²⁺ excited at 400 nm and monitored the main emission of the Ce³⁺ ions at 486 nm were measured, and the lifetimes are shown in Fig. 9. It can be seen that the decay curves of the 5d–4f transition of CHSA:1%Ce³⁺,yMn²⁺ shows a second order exponential decay, which can be fitted by the equation:⁴¹

I(t) = A₁ exp(−t/τ₁) + A₂(−t/τ₂)

where I is the luminescence intensity; A₁ and A₂ are constants; t is time; and τ₁ and τ₂ are the lifetimes for the exponential components. Further, the average lifetime constant (τ*) can be calculated as:

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂)

Fig. 9 Decay curves of Ca₃Hf₂SiAl₂O₁₂:1%Ce³⁺,y%Mn²⁺ phosphors with different Mn²⁺ contents.

The calculated average lifetimes of CHSA:1%Ce³⁺,yMn²⁺ (y = 1%, 3%, 5%, 7%, 10%, 14%) are 43.6, 40.8, 36.4, 35.2, 31.0 and 28.1 ns, respectively. The decay time of CHSA:1%Ce³⁺ sample without the Mn²⁺ was determined to be 46.4 ns for the 5d–4f transition. However, when the Mn²⁺ was doped in the system of CHSA:1%Ce³⁺, the decay of Ce³⁺ ions becomes faster and faster. The decay time of the Ce³⁺ ions was found to decrease as the Mn²⁺ concentrations increases, which demonstrates that the energy transfer process occurs from sensitizer Ce³⁺ to activator Mn²⁺. Additional, the energy transfer efficiency from Ce³⁺ to Mn²⁺ in CHSA matrix can be determined using the calculation below:

where τ₀ and τ_x stand for the lifetimes of the sensitizer Ce³⁺ in the absence and the presence of activator Mn²⁺, respectively. The energy transfer efficiency was calculated to be 6%, 12%, 21%, 24%, 33% and 39%. The energy transfer efficiency is found to increase with increasing Mn²⁺ content. When the doped Mn²⁺ concentration was 14%, the value of energy transfer efficiency is estimated to be 39%. So, it indicated that the Mn²⁺ emission intensity was enhanced via energy transfer from sensitizer Ce³⁺, instead of the energy absorption by Mn²⁺ ions themselves.

The variation of the Commission International de L'Eclairage (CIE) chromaticity coordinates and color temperature of the CHSA:1%Ce³⁺,yMn²⁺ phosphors with different doping contents of Mn²⁺ are calculated based on the corresponding PL spectrum upon 400 nm excitation summarized in Fig. 10 and Table 1. Increasingly, the color tune can be modulated from cyan (0.2303, 0.3265) to white (0.3350, 0.3388) with the increasing doping content of the Mn²⁺ ions, which is due to the variation of the emission intensity of Ce³⁺ and Mn²⁺ through the energy transfer from Ce³⁺ to Mn²⁺. The correlated color temperature can be adjusted from 12 763 K to 5379 K. The emission color is tunable in the visible region from blue to white by controlling Mn²⁺ doped contents which indicates that the series of CHSA:1%Ce³⁺,yMn²⁺ phosphors are promising candidates for color tunable luminescence material used for the n-UV w-LEDs.

Fig. 10 CIE chromaticity diagram and coordinate of Ca₃Hf₂SiAl₂O₁₂:1%Ce³⁺,y%Mn²⁺ (y = 0–14) phosphors.

Table 1 Comparison of the CIE chromaticity coordinates (x, y) for CHSA:1%Ce³⁺,yMn²⁺ phosphors excited at 400 nm

Ce^3+ (x) and Mn^2+ (y) content	CIE coordinates (x, y)	Color temperature (K)
x = 1%, y = 0	(0.2303, 0.3265)	12 763 K
x = 1%, y = 1%	(0.2593, 0.3302)	10 005 K
x = 1%, y = 3%	(0.2735, 0.3431)	8628 K
x = 1%, y = 5%	(0.2882, 0.3413)	7768 K
x = 1%, y = 7%	(0.2994, 0.3332)	7224 K
x = 1%, y = 10%	(0.3148, 0.3381)	6345 K
x = 1%, y = 14%	(0.3350, 0.3388)	5379 K

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4. Conclusions

In summary, a series of color-adjustment CHSA:Ce³⁺,Mn²⁺ phosphors were prepared through a conventional solid-state reaction. It was found that three different cation sites (Ca²⁺, Hf⁴⁺ and Al³⁺/Si⁴⁺) in the Ca₃Hf₂SiAl₂O₁₂ phase were occupied evenly by Ce³⁺ and Mn²⁺ ions. Different situation of Mn²⁺ occupying sites and energy transfer from Ce³⁺ to Mn²⁺ can appear with different Mn²⁺ content and the critical distance was calculated to be R_c1 = 10.8 Å, R_c2 = 10.1 Å and R_c3 = 12.6 Å after calculation of energy transfer from the Ce³⁺ to Mn²⁺ by using the concentration quenching method. Ca₃Hf₂SiAl₂O₁₂:Ce³⁺,Mn²⁺ phosphors exhibited a broad excitation band ranging from 300 to 450 nm and two broad asymmetric emission bands upon 400 nm excitation. The emission color of the obtained phosphors can be modulated from blue-green (0.2303, 0.3265) to white (0.3350, 0.3388) by controlling the doping content of the Mn²⁺ ions with the fixed Ce³⁺ content. The correlated color temperature can be adjusted from 12 763 K to 5379 K.

Acknowledgements

This work is supported by Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120211130003), State Key Laboratory on Integrated Optoelectronics (No. IOSKL2013KF15).

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