Enhanced red emission in Ca_0.5La(MoO₄)₂:Eu³⁺ phosphors for UV LEDs

Abstract

La³⁺ was introduced into CaMoO₄ to form a novel compound with the chemical formula Ca_0.5La(MoO₄)₂ and Eu³⁺ activated the new compound was synthesized by a solid-state reaction method. An enhanced red emission at 615 nm originating from the ⁵D₀–⁷F₂ transition of Eu³⁺ was reported in the Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor. XRD patterns indicate a structural similarity to CaMoO₄. The red emission intensity of the Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor is 5.92 times higher than that of CaMoO₄:Eu³⁺. Photoluminescent spectra, diffuse reflection spectra and lifetime measurements indicate that the red emission enhancement is due to the increase of the ⁷F₀–⁵L₆ absorption intensity of Eu³⁺ in the distorted crystal field of the Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor. Enhanced red emission, stronger ⁷F₀–⁵L₆ absorption intensity, and high color purity indicate that the Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor is a promising red phosphor for UV LEDs. The luminescence properties of the Bi³⁺ co-doped Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor was also investigated. Although Bi³⁺ co-doped into the Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor broadens PLE spectra in the UV range and makes it more suitable for UV pumped LEDs, it decreases the distortion and induces a new quenching path, and thus makes the emission intensity decrease.

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1. Introduction

With the development of photoelectric technology and materials science, white-light emitting diodes (w-LED) have been regarded as a fourth generation lighting source because of their important benefits including energy saving, safety, reliability, long operation time and environmentally friendly. Among the many ways of achieving white light, phosphor-converted (pc) white LEDs are the main method due to their high efficiency, low cost and easy preparation.¹ Usually, there are two methods to obtain white light in pc-LEDs. One is using a blue LED chip combined with orange or yellow-red phosphors.^2,3 The other is using a (near) ultraviolet (NUV/UV) LED chip combined with blue-green-red phosphors.^4,5 The yellow, blue and green phosphors are commercially available and could meet the requirements. However, the red phosphors have many drawbacks, such as unstable,^6,7 high cost,^8,9 hard to prepare, re-absorption and low efficiency.¹⁰ Lacking of red phosphors limits the scope of w-LEDs applications, such as light sources in offices, schools, medicals, hospitals and hotels. No matter what kind of method to achieving white light, exploring red phosphors are necessary. Thus, it is important to obtain red phosphors with high quantum efficiency and good color saturation for blue and UV LEDs.

Eu³⁺ activated materials are of particular interest because the lowest excited level ⁵D₀ of the 4f⁶ configuration is situated below the 4f⁵5d configuration for Eu³⁺.¹¹ Eu³⁺ activated luminescence materials mainly show sharp line at red region peaked around 612 nm. Though Eu³⁺ shows high ⁷F₀–⁵L₆ transitions at about 395 nm in most hosts, the transition is parity-forbidden, so it is a narrow line and cannot absorb the excitation energy efficiently, especially for absorption LED chip's emission.

Among of many inorganic hosts, molybdate compounds have attracted many attentions owing to their peculiar spectroscopic properties and potential application as laser and luminescent hosts.^12,13 Molybdate (MoO₄²⁻) has a tetrahedral symmetry (T_d) with the central Mo⁶⁺ coordinated by four O²⁻ ions. Molybdate phosphors have broad and intense absorption bands due to charge transfer (CT) from O²⁻ to Mo⁶⁺ in the UV region,^14,15 which may increase the absorption for UV LED chip. Besides, molybdates are chemically stable, which is better than oxysulfide and sulfide red-emitting phosphors, such as Y₂O₂S:Eu³⁺,⁶ CaS:Eu²⁺.⁷ Molybdates are also easier synthesized with low cost than nitride and oxynitride, such as Sr₂Si₅N₈:Eu²⁺,⁸ CaSiAlN₃:Eu²⁺,⁹ which require critical preparation conditions like high temperature, high pressure and expensive raw materials, and so on.

Eu³⁺ activated molybdates have been widely prepared as red phosphors for UV LEDs.^16–18 The most popular molybdate host is CaMoO₄. Eu³⁺ activated CaMoO₄ can be excited by UV light and emit highly red light. However, there is charge mismatch between Eu³⁺ and Ca²⁺ in CaMoO₄, causing lower solubility saturation of activator and luminescence intensity. Thus, it is better to use charge compensators to enhance the intensity. For example, Li⁺, Na⁺ or K⁺ was included into CaMoO₄ host, and the emission intensity was greatly enhanced.¹⁹ Besides, Bi³⁺ was also used to enhance the red emission by increasing transition probability from the ground state to ⁵L₆ and ⁵D₂ states of Eu³⁺, as the addition of Bi³⁺ broke the symmetry of CaMoO₄ and distorted its crystal structure.¹¹

In this paper, La³⁺ is introduced into CaMoO₄ to form a novel molybdate host as Ca_0.5La(MoO₄)₂. On the one hand, La³⁺ can provide a proper crystal site for Eu³⁺ doping, on the other hand, the addition of La³⁺ will distort the crystal and decrease the symmetry. Thus, enhanced red emission can be expected in Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor. Besides, the effect of Bi³⁺ co-doped Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor is also studied.

2. Experimental

Ca_0.5La_1−xEu_x(MoO₄)₂ (0.1 ≤ x ≤ 1) (Ca_0.5La(MoO₄)₂:xEu³⁺) and Ca_0.5La_0.4−yBi_yEu_0.6(MoO₄)₂ (0 ≤ y ≤ 0.2) (Ca_0.5La(MoO₄)₂:0.6Eu³⁺, yBi³⁺) phosphors were synthesized by solid state reaction. Starting materials were CaCO₃, La₂O₃, Eu₂O₃, MoO₃ and Bi(NO₃)₃ with a purity of A. R. The stoichiometric raw materials were ground in an agate mortar and heated to 950 °C in air for 4 h. Then the as-synthesized samples were slowly cooled to room temperature. CaMoO₄:0.24Eu³⁺ (best doping concentration^11,20) was synthesized using CaCO₃, Eu₂O₃ and (NH₄)₆Mo₇O₂₄·4H₂O as raw materials and calculated at 800 °C in air for 6 h. The commercial Y₂O₃:Eu³⁺ (com-Y₂O₃:Eu³⁺) red phosphor (GS-Y-R) was purchased from Gansu Rare Earth New Materials Co., Ltd.

The phase structure of the obtained samples was characterized by a Bruker D8 Advance X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.15406 nm). The accelerating voltage and emission current were 40 kV and 40 mA, respectively. The excitation (PLE)/emission (PL) spectra and lifetime were recorded using a FLS920P Edinburgh Analytical Instrument apparatus equipped with a 450 W xenon lamp and a μF900H high-energy micro-second flash lamp as the excitation sources. The diffuse reflection spectra of as-synthesized samples were obtained by an UV-vis spectrophotometer (PE lambda 950) using BaSO₄ as a standard reference in the measurements. All of the measurements were done at room temperature.

3. Results and discussion

3.1 X-ray diffraction analysis of Ca_0.5La(MoO₄)₂:xEu³⁺ phosphors

Fig. 1 shows XRD patterns of Ca_0.5La(MoO₄)₂:xEu³⁺ and synthesized CaMoO₄:0.24Eu³⁺ phosphors. All XRD profiles are well matched with the PDF card no. 29-0351 (CaMoO₄). No detectable impurity phase is observed in the obtained samples even in Ca_0.5Eu(MoO₄)₂ (x = 1). This indicates that all obtained samples have a scheelite (CaMoO₄) structure and all of them are single phased with the space group of I41/a(88). In Fig. 1a, we observe the highest intensity peak around 28° (2θ) marked in green. It is obviously observed in the expanded version of the XRD patterns in Fig. 1b. The peak of Ca_0.5La(MoO₄)₂:0.2Eu³⁺ shifts to smaller 2θ value compared with that of CaMoO₄:0.24Eu³⁺ due to larger ionic radii of La³⁺ (r = 1.16 Å when the coordination number (CN) = 8) than that of Ca²⁺ (r = 1.12 Å when CN = 8).²¹ Based on the above phenomena and analysis, it is expected that the new scheelite structure is most probably formed by one Ca²⁺, two La³⁺ and one Ca²⁺ vacancy (V_Ca) in Ca_0.5La(MoO₄)₂ substitution four Ca²⁺ sites in CaMoO₄, described by:

Ca²⁺ + 2La³⁺ + V_Ca → 4Ca²⁺

Fig. 1 XRD patterns of Ca_0.5La(MoO₄)₂:xEu³⁺ and synthesized CaMoO₄:0.24Eu³⁺ phosphors.

In addition, Fig. 1b shows that as the content of Eu³⁺ increase the peaks shift to higher 2θ value. This is due to the smaller ionic radii of Eu³⁺ (r = 1.07 Å when CN = 8) than La³⁺ (r = 1.16 Å when CN = 8),²¹ which proves that Eu³⁺ ions have been successfully occupied the La³⁺ sites in the Ca_0.5La(MoO₄)₂ host lattice.

3.2 Luminescence properties of Ca_0.5La(MoO₄)₂:xEu³⁺ phosphors

Fig. 2a shows a typical PLE spectra of Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor by monitoring at 615 nm and 612 nm. Both excitation spectra show the same profile including two broad bands range in 233–353 nm and several peaks ranged from 360–434 nm, which can be assigned to the charge transfer bands (CTB) of O–Eu^22,23 and O–Mo,^24,25 and the characteristic intra-configurational transition 4f–4f transitions of Eu³⁺,²² respectively. Similar profiles indicate that the emission peaks centered at 615 nm and 612 nm belong to the same 4f–4f transition of Eu³⁺. In addition, the PLE spectra indicate that Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor can be efficiently excited by UV-LED chips. Fig. 2b gives the PL spectra of Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor under different excitation wavelength. There is no perceptible difference between three PL spectra except the emission intensity. All PL spectra consist of a number of sharp lines ranging from 577 to 710 nm, associated with the transitions ⁵D₀–⁷F_J (J = 1, 2, 3, 4) of Eu³⁺. The strongest peak is at 615 nm (⁵D₀–⁷F₂, electric dipole transition), which is much higher than that of 591 nm (⁵D₀–⁷F₁, magnetic dipole transition).²⁶ The electric dipole transition is allowed in Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor indicating good CIE (Commission Internationale de I'Eclairage) chromaticity coordinates. Besides, there is no host emission in Ca_0.5La(MoO₄)₂, as shown in the insert of Fig. 2b. Under 395 nm excitation, the emission intensity was the highest. Thus, we use 395 nm to excite all synthesized phosphors for the following.

Fig. 2 Typical PLE (a), PL (b) spectra and the x value dependent on the emission intensity change (c) of Ca_0.5La(MoO₄)₂:xEu³⁺ phosphors, (d) log plot for the emission intensity at 395 nm per activator ions as a function of the activator concentration.

A series of Ca_0.5La(MoO₄)₂:xEu³⁺ phosphors were synthesized and the luminescence properties were measured as shown in Fig. 2c. Under 395 nm excitation, with increasing Eu³⁺ contents, the intensity of Eu³⁺ emission increases rapidly and reaches a maximum at x = 0.6 and then slowly decreases. Luminescence quenching is relation with energy transfer. Energy transfer is generally associated with multipolar interactions, radiation reabsorption, or exchange interaction.^27–30 Among them, multipolar interactions are usually prevalent which have several types, such as dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions. Exchange interaction is generally limited to interactions between RE ions in nearest or next nearest neighbour. If migration is rapid compared to direct transfer, quenching tends to be proportional to quenching-ions concentration.^{27,28,31–34} And the exchange interaction is generally for the energy transfer of forbidden transition.³⁵ For a better understanding of energy transfer in the Ca_0.5La(MoO₄)₂:xEu³⁺ phosphors, the relationship of emission intensity and activator concentration is discussed. As the reports of L. G. van Uitert and Ozawa, the type of energy transfer can be determined from the change in the emission intensity from the emitting level.^27,36 The emission intensity (I) per activator ion follows the equation:

where x is the activator concentration, I/x is the emission intensity (I) per activator concentration (x), and K and β are constants for the same excitation condition for a given host crystal. According to eqn (1), θ = 3 for exchange interaction, namely the energy transfer among the nearest-neighbour ions, while θ = 6, 8, 10 for d–d, d–q, q–q interactions, respectively. Fig. 2d plots the dependence of log(I/x) on log(x) and a slope of (−θ/3) is obtained to be −0.885. Then the value of θ can be determined to be 2.66, close to 3, which means that the quenching is directly proportional to the ion concentration. The result indicates that the energy transfer among nearest-neighbour ions is the main mechanism of concentration quenching for the Eu³⁺-site emission centers in the Ca_0.5La(MoO₄)₂:xEu³⁺ phosphors.

3.3 Bi³⁺ co-doped Ca_0.5La(MoO₄)₂:0.6Eu³⁺ phosphors

Usually, Bi³⁺ was utilized to enhance the red emission of Eu³⁺ in lots of luminescence materials.^11,37,38 In this paper, Bi³⁺ co-doped Ca_0.5La(MoO₄)₂:0.6Eu³⁺ phosphors were also synthesized, and their XRD patterns are all consistent with PDF card no. 29-0351, shown in Fig. 3. This suggests that all Ca_0.5La(MoO₄)₂:0.6Eu³⁺, yBi³⁺ (0 ≤ y ≤ 0.2) phosphors are still single phase. The ionic radii of Bi³⁺ (r = 1.17 Å when CN = 8) are close to that of La³⁺ (r = 1.16 Å when CN = 8) and Ca²⁺ (r = 1.12 Å when CN = 8), therefore we believe that Bi³⁺ may occupy the La³⁺ sites or V_Ca in Ca_0.5La(MoO₄)₂.

Fig. 3 XRD patterns of Ca_0.5La(MoO₄)₂:0.6Eu³⁺, yBi³⁺ phosphors.

Fig. 4 shows the PLE spectra of Ca_0.5La(MoO₄)₂:0.6Eu³⁺, yBi³⁺ phosphors. The profiles of the PLE spectra are similar to Fig. 2a, to clearly understand the effect of introducing Bi³⁺, the excitation spectra were normalized with ⁵D₀–⁷F₂ transition and were given in Fig. 4a. A quite broader shoulder band appears at the longer-wavelength side of the O–Eu and O–Mo CTBs. And by increasing the content of Bi³⁺, the broad band intensity increases. The broad shoulder band should be ascribed to the absorption of Bi³⁺.³⁹ Broaden PLE spectra in UV range make Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor more suitable for UV pumped LEDs. The shape of PL spectra of Ca_0.5La(MoO₄)₂:0.6Eu³⁺, yBi³⁺ phosphors are similar, while the intensity raises with increasing Bi³⁺ concentration until its concentration exceeds 0.03, then the intensity decrease, shown in Fig. 4b.

Fig. 4 PLE spectra monitored by 615 nm (a) and the y value dependent on the emission intensity change under 395 nm excitation (b) of Ca_0.5La(MoO₄)₂:0.6Eu³⁺, yBi³⁺ phosphors.

3.4 The analysis of the abnormal phenomenon

PLE/PL spectra and lifetimes of ⁵D₀ state of Eu³⁺ in CaMoO₄:0.24Eu³⁺, Ca_0.5La(MoO₄)₂:0.6Eu³⁺ and Ca_0.5La(MoO₄)₂:0.6Eu³⁺, 0.03Bi³⁺ phosphors were measured and shown in Fig. 5a–c. As all the transitions include ⁵D₀–⁷F_J (J = 1, 2, 3, 4) are contributed to the characteristic intra-configurational transition 4f–4f transitions of Eu³⁺. And in Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor, all of them are enhanced compared with CaMoO₄:0.24Eu³⁺ and com-Y₂O₃:Eu³⁺ phosphors. In order to facilitate analysis, only the ⁵D₀–⁷F₂ transition (615 nm) is discussed. Fig. 5a shows the PLE spectra of three phosphors normalized by O–Eu CTB. It is clearly seen, the ⁷F₀–⁵D₂ intensity in Ca_0.5La(MoO₄)₂:0.6Eu³⁺ is about 1.3 times stronger than that of Ca_0.5La(MoO₄)₂:0.6Eu³⁺, 0.03Bi³⁺, and 5.9 times stronger than that of CaMoO₄:0.24Eu³⁺. In Fig. 5b, the ⁵D₀–⁷F₂ intensity of Ca_0.5La(MoO₄)₂:0.6Eu³⁺ is about 1.3 times stronger than that of Ca_0.5La(MoO₄)₂:0.6Eu³⁺, 0.03Bi³⁺, and 5.92 times stronger than that of CaMoO₄:0.24Eu³⁺, similar with PLE spectra. It is clearly seen that the red emission is greatly enhanced by La³⁺ introduced into CaMoO₄ host. Besides, Bi³⁺ co-doped into Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor decreases the emission intensity. This is different with previous reports.

Fig. 5 PLE (a), PL (b) spectra, luminescence decay traces recorded upon excitation at 395 nm (c) and diffuse reflection spectra (d) of CaMoO₄:0.24Eu³⁺, Ca_0.5La(MoO₄)₂:0.6Eu³⁺, 0.03Bi³⁺ and Ca_0.5La(MoO₄)₂:0.6Eu³⁺ phosphors.

PL decay curves of three phosphors upon excitation at 395 nm (Fig. 5c) can be well fitted into a single exponential function as:

I(t) = I₀ exp(−t/τ) (2)

where I₀ is the initial intensity at t = 0. The lifetime (τ) of three phosphors is determined and given in Fig. 5c. The lifetime of ⁵D₀ reduces following this order CaMoO₄:0.24Eu³⁺ > Ca_0.5La(MoO₄)₂:0.6Eu³⁺, 0.03Bi³⁺ > Ca_0.5La(MoO₄)₂:0.6Eu³⁺, indicating that the quantum efficiency of ⁷F₀–⁵D₂ emission is hardly affected by La³⁺ or Bi³⁺ addition. Therefore, according to Fig. 5, it hints that the red emission enhancement of Ca_0.5La(MoO₄)₂:0.6Eu³⁺ phosphor should result from the increase of the ⁷F₀–⁵L₆ absorption intensity.

Fig. 5d shows diffuse reflection spectra of three phosphors. In Fig. 5d, the absorption peaks of intra-4f transitions of Eu³⁺ at 395 nm and 465 nm are presented. The absorbance increases from following order, CaMoO₄:0.24Eu³⁺ > Ca_0.5La(MoO₄)₂:0.6Eu³⁺, 0.03Bi³⁺ > Ca_0.5La(MoO₄)₂:0.6Eu³⁺, which was the same as PLE, PL and decay curves. This is direct evidence that the red emission enhancement is caused mainly by the increase of absorption strength of ⁷F₀–⁵L₆ absorption.

In view of the same phase structure in CaMoO₄:0.24Eu³⁺, Ca_0.5La(MoO₄)₂:0.6Eu³⁺ and Ca_0.5La(MoO₄)₂:0.6Eu³⁺, 0.03Bi³⁺ phosphors, combined with above analysis, the change of the ⁷F₀–⁵L₆ absorption intensity of Eu³⁺ should be caused by crystal symmetry destroying. For CaMoO₄:0.24Eu³⁺ phosphor, the central Mo⁶⁺ metal ion is coordinated with four oxygen atoms in tetrahedral symmetry, and Ca²⁺ ion, with eight oxygen atoms from a different tetrahedron, shown in Fig. 6a. For Ca_0.5La(MoO₄)₂:0.6Eu³⁺ phosphor, as described in Fig. 1, is probably formed by one Ca²⁺, two La³⁺ and one Ca²⁺ vacancy (V_Ca) substitution four Ca²⁺ sites in CaMoO₄. As the radius of La³⁺ (r = 1.16 Å when CN = 8) is larger than that of Ca²⁺ (Ca²⁺ (r = 1.12 Å when CN = 8)),²¹ the [Ca–LaO₈] lattice will get slightly expand compared with [Ca–CaO₈]; in the meanwhile, the [La–V_CaO₈] lattice must shrink greatly, seen in Fig. 6b. These distort the crystal field around Eu³⁺, and make the symmetry decrease. As the difference between La³⁺ radius and Ca²⁺ radius is very little, it is easily understand that the [La–V_CaO₈] lattice shrink is the main reason for crystal symmetry destroying. It is therefore speculated that more opposite parity components, namely, 4f⁵5d states, mixed into the 4f⁶ transitional levels of Eu³⁺. This will cause enhancement of the intra 4f⁶ transition probability of the ⁷F₀–⁵L₆ of Eu³⁺. While for Bi³⁺ co-doped Ca_0.5La(MoO₄)₂:0.6Eu³⁺ phosphor, as the ionic radii of Bi³⁺ (r = 1.17 Å, when CN = 8) and are close to that of Ca²⁺ (r = 1.12 Å when CN = 8) and La³⁺ (r = 1.16 Å when CN = 8).²¹ If Bi³⁺ occupies La³⁺ sites, the larger radius of Bi³⁺ will decrease the [Bi–V_CaO₈] lattice shrinkage, shown in Fig. 6c. If Bi³⁺ occupies V_Ca sites, was shown in Fig. 6d, it will restore the shrinkage. Namely, no matter which site is substituted, the crystal symmetry will be restored. And thus the transition probability will decrease. Meanwhile, if Bi³⁺ occupies V_Ca site, due to short distance, the energy of Eu³⁺ will easily transfer to Bi³⁺ on higher Eu³⁺ content. All above reason will cause the red emission intensity decrease compared with Ca_0.5La(MoO₄)₂:0.6Eu³⁺ phosphor, but higher than CaMoO₄:0.24Eu³⁺ phosphor.

Fig. 6 Crystal structure of CaMoO₄:0.24Eu³⁺ (a), Ca_0.5La(MoO₄)₂:0.6Eu³⁺ (b), Ca_0.5La(MoO₄)₂:0.6Eu³⁺, 0.03Bi³⁺ (c and d) phosphors.

The emission intensity of the optimized Ca_0.5La(MoO₄)₂:0.6Eu³⁺ phosphor is compared with the com-Y₂O₃:Eu³⁺ phosphor and CaMoO₄:0.24Eu³⁺ shown in Fig. 7, colors are filled with photos of each phosphors under 365 nm light. The results show that the intensity of synthesized phosphor is 2.67 and 5.92 times than that of the com-Y₂O₃:Eu³⁺ phosphor and CaMoO₄:0.24Eu³⁺, respectively. The CIE chromaticity coordinates of Ca_0.5La(MoO₄)₂:0.6Eu³⁺ phosphor is also calculated chromaticity coordinates (0.667, 0.330) compared with (0.629, 0.340) for com-Y₂O₃:Eu³⁺ phosphor. The CIE chromaticity coordinates are on the margin of the NTSC (National Television Standard Committee) standard values (0.67, 0.33).⁴⁰ Enhanced red emission, stronger ⁷F₀–⁵L₆ absorption intensity, and high color purity of Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor indicate that it is a promising red phosphor for white LEDs.

Fig. 7 The PL intensity of Ca_0.5La(MoO₄)₂:0.6Eu³⁺, com-Y₂O₃:Eu³⁺ and CaMoO₄:0.24Eu³⁺ phosphors, colors are filled with photos of each phosphors under 365 nm light.

4. Conclusion

Novel Ca_0.5La(MoO₄)₂ molybdate host and red emitting Ca_0.5La(MoO₄)₂:Eu³⁺ phosphors were synthesized by solid state reaction. The structure of Ca_0.5La(MoO₄)₂ molybdate host is similar to CaMoO₄ with space group of I41/a(88). Eu³⁺ activated this molybdate shows intense absorption in the UV region and exhibits intense red emission. The excitation spectra indicate novel molybdate phosphor is suitable for UV pumped LEDs. For optimized Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor, the intensity is 2.67 and 5.92 times than that of the com-Y₂O₃:Eu³⁺ phosphor and CaMoO₄:0.24Eu³⁺, respectively. The red emission enhancement is due to the increase of the ⁷F₀–⁵L₆ absorption intensity of Eu³⁺ in the distorted crystal field in Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor. The CIE chromaticity coordinates (0.667, 0.330) of the synthesized phosphor are on the margin of the NTSC standard values. Enhanced emission and high color purity indicate that Ca_0.5La(MoO₄)₂:Eu³⁺ phosphor could be a good candidate for the red component of UV white LEDs. The introduction of Bi³⁺ into optimized phosphor can broaden excitation spectra in the UV range, but decrease the emission intensity slightly due to restoring the distortion and providing new quenching path.

Acknowledgements

Project supported by Program for Scientific Research Foundation of the Higher Education Institutions of He'nan Province (15A430054), High level personnel fund of Zhoukou Normal University (ZKNU2014106), Open project of Zhoukou Normal University (K201501), Innovative Research Team (in Science and Technology) in University of Henan Province (14IRTSTHN009), Excellent Youth Foundation of He'nan Scientific Committee (134100510018). Henan Province Key Discipline of Applied Chemistry (201218692), and Innovation Scientists and Technicians Troop Construction Projects of Henan Province (2013259).

References

1. Z. Ci, Q. Sun, S. Qin, M. Sun, X. Jiang, X. Zhang and Y. Wang, Phys. Chem. Chem. Phys., 2014, 16, 11597.
2. Y. Shi, G. Zhu, M. Mikami, Y. Shimomura and Y. Wang, Dalton Trans., 2015, 44, 1775.
3. J. R. Oh, S. H. Cho, Y. H. Lee and Y. R. Do, Opt. Express, 2009, 17, 7450.
4. L. Yi, J. Zhang, Z. Qiu, W. Zhou, L. Yu and S. Lian, RSC Adv., 2015, 5, 67125.
5. J. K. Park, K. J. Choi, C. H. Kim, H. D. Park and S. Y. Choi, Electrochem. Solid-State Lett., 2004, 7, H15.
6. S. Neeraj, N. Kijima and A. K. Cheetham, Chem. Phys. Lett., 2004, 387, 2.
7. J. Lin, Y. Huang, Y. Bando, C. Tang and D. Golberg, Chem. Commun., 2009, 66313.
8. R. J. Xie, N. Hirosaki, T. Suehiro, F. F. Xu and M. Mitomo, Chem. Mater., 2006, 18, 5578.
9. X. Piao, K. Machida, T. Horikawa, H. Hanzawa, Y. Shimomura and N. Kijima, Chem. Mater., 2007, 19, 4592.
10. Y. Cao, G. Zhu and Y. Wang, RSC Adv., 2015, 5, 65710.
11. S. X. Yan, J. H. Zhang, X. Zhang, S. Z. Lu, X. G. Ren, Z. G. Nie and X. J. Wang, J. Phys. Chem. C, 2007, 111, 13256.
12. Y. Zhou and B. Yan, CrystEngComm, 2013, 15, 5694.
13. Y. C. Chang, C. H. Liang, S. A. Yan and Y. S. Chang, J. Phys. Chem. C, 2010, 114, 3645.
14. A. Xie, X. M. Yuan, S. J. Hai, J. J. Wang, F. X. Wang and L. Li, J. Phys. D: Appl. Phys., 2009, 42, 105107.
15. H. Y. Li, H. M. Noh, B. K. Moon, B. C. Choi, J. H. Jeong, K. Jang, H. S. Lee and S. S. Yi, Inorg. Chem., 2013, 52, 11210.
16. C. L. Zhao, Y. S. Hu, W. D. Zhuang, X. W. Huang and T. He, J. Rare Earths, 2009, 27, 758.
17. H. Y. Li, H. K. Yang, B. K. Moon, B. C. Choi, J. H. Jeong, K. Jang, H. S. Lee and S. S. Yi, Inorg. Chem., 2011, 50, 12522.
18. X. Q. Liu, L. Li, H. M. Noh, B. K. Moon, B. C. Choi, S. K. Neeraj and J. H. Jeong, Dalton Trans., 2014, 43, 8814.
19. J. Liu, H. Lian and C. Shi, Opt. Mater., 2007, 29, 1591.
20. Y. Hu, W. Zhuang, H. Ye, D. Wang, S. Zhang and X. Huang, J. Alloys Compd., 2005, 390, 226.
21. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751.
22. G. Blasse and A. Bril, J. Inorg. Nucl. Chem., 1967, 29, 2231.
23. V. R. Bandi, M. Jayasimhadri, J. Jeong, K. Jang, H. S. Lee, S. S. Yi and J. H. Jeong, J. Phys. D: Appl. Phys., 2010, 43, 395103.
24. A. K. Parchur, R. S. Ningthoujam, S. B. Rai, G. S. Okram, R. A. Singh, M. Tyagi, S. C. Gadkari, R. Tewari and R. K. Vatsa, Dalton Trans., 2011, 40, 7595.
25. Z. Lu and T. Wanjun, Ceram. Int., 2012, 38, 837.
26. M. Thomas, P. P. Rao, S. P. K. Mahesh, L. S. Kumari and P. Koshy, Phys. Status Solidi A, 2011, 208, 2170.
27. L. G. van Uitert, J. Electrochem. Soc., 1967, 114, 1048.
28. L. G. van Uitert, E. F. Dearborn and J. J. Rubin, J. Chem. Phys., 1967, 47, 547.
29. H. Liu, Y. Hao, H. Wang, J. Zhao, P. Huang and B. Xu, J. Lumin., 2011, 131, 2422.
30. S. Dutta, S. Som and S. K. Sharma, Dalton Trans., 2013, 42, 9654.
31. L. G. van Uitert, E. F. Dearborn and J. J. Rubin, J. Chem. Phys., 1966, 45, 1578.
32. L. G. van Uitert, E. F. Dearborn and H. M. Marcos, Appl. Phys. Lett., 1966, 9, 255.
33. L. G. van Uitert, E. F. Dearborn and J. J. Rubin, J. Chem. Phys., 1967, 46, 3551.
34. L. G. van Uitert, E. F. Dearborn and J. J. Rubin, J. Chem. Phys., 1967, 46, 420.
35. X. Zhang, L. Zhou and M. Gong, Opt. Mater., 2013, 35, 993.
36. L. Ozawa and P. M. Jaffe, J. Electrochem. Soc., 1971, 118, 1678.
37. L. L. Han, Y. H. Wang, J. Zhang and Y. Z. Wang, Mater. Chem. Phys., 2013, 139, 87.
38. L. Chen, K. J. Chen, C. C. Lin, C. Chu, S. F. Hu, M. H. Lee and R. S. Liu, J. Comb. Chem., 2010, 12, 587.
39. A. Xie, X. M. Yuan, Y. Shi, F. X. Wang and J. J. Wang, J. Am. Ceram. Soc., 2009, 92, 2254.
40. Z. L. Wang, H. B. Liang, J. Wang and M. L. Gong, Appl. Phys. Lett., 2006, 89, 071921.

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