Luminescence properties of La₂W_2−xMo_xO₉ (x = 0–2):Eu³⁺ materials and their Judd–Ofelt analysis: novel red line emitting phosphors for pcLEDs

Abstract

A series of novel red emitting La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) and La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, insteps of 0.2) phosphors were prepared by a conventional solid state reaction. Powder XRD analysis of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) reveals a phase transition from a triclinic to a cubic structure with increasing Mo⁶⁺ concentration (x ≥ 0.2). All the compositions show a broad charge transfer (CT) band due to CT from O to W/Mo and red emission (due to Eu³⁺ ions). In order to obtain the strongest red emission, concentration variation of Eu³⁺ was performed in La_2−yEu_yW_1.6Mo_0.4O₉ (x = 0–2, insteps of 0.2). The Eu_1.6La_0.4W_1.6Mo_0.4O₉ composition shows a high red emission intensity (∼8.2 times) compared to a commercial red phosphor with very good CIE chromaticity coordinates (x = 0.67, y = 0.33). A selected composition has also been synthesized by simple co-precipitation method. The emission intensity of the Eu_1.6La_0.4W_1.6Mo_0.4O₉ phosphor synthesized by co-precipitation technique is ∼1.78 times higher than that of the phosphor synthesized by the solid state reaction. Phonon sideband spectrum analysis has been carried out for Eu³⁺ activated La₂M₂O₉ (M = W and Mo). The temperature dependent photoluminescence studies reveal that the Eu_1.4La_0.6W_1.6Mo_0.4O₉ compositions loses 30% of its efficiency up to 400 K (compared to ∼80% for CaS:Eu²⁺) and the Judd–Ofelt parameters were also calculated for the Eu³⁺ ions.

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Introduction

There is an ever-increasing petition to improve the efficiency of lighting devices, because ∼20% of generated electricity is currently devoted to lighting. Solid-state lighting (SSL, including white LEDs) based on semiconductor InGaN LED chips is an attractive alternative to replace conventional fluorescent lamps and incandescent bulbs due to its extraordinary features such as high emission quantum efficiencies, prolonged lifetime, and harmless constituent atoms.¹ The reason behind the development of SSL lies in dynamic research on down conversion phosphor materials.^2a,b There are abundant methods that are available to create white light from LEDs, one of the straightforward ways to generate white light is from primary color LEDs (InGaN based blue, green LEDs and red AlGaInP LED).³ Commercialization of these is difficult since it requires independent output power control on each LED since the operating voltage is different between InGaN-based LEDs and AlGaInP-based LEDs. The first white LED based on a blue InGaN LED and a yellow emitting (Y₃Al₅O₁₂:Ce³⁺, YAG:Ce³⁺) phosphor has been commercialized by Nichia, Japan.⁴ The yellow YAG:Ce³⁺ phosphor’s efficiency was improved by crystal chemical substitution in the host lattice YAG:Ce³⁺ (Y_1−aGd_a)₃(Al_1−bGa_b)₅O₁₂:Ce³⁺.^5,6 The spectral composition of the white light which is obtained from the white LED (blue LED + yellow phosphor) differs from that of natural white light. It is mainly due to the halo effect of blue/yellow color separation and poor color rendering index (CRI) caused by the lack of red component in the emission region. An alternative way to mimic efficient white light with a better CRI is the combination of a blue-emitting LED with green-emitting or red-emitting phosphors, or red-, green- and blue-(RGB) emitting phosphors with a near ultraviolet (NUV) LED.⁷ Currently, lanthanide activated sulfide and nitride [Y₂O₂S:Eu³⁺, SrGa₂S₄:Eu²⁺, CaS:Eu²⁺, Sr₃Si₅N₈:Eu²⁺, CaSiAlN₃:Eu²⁺] red phosphors are widely being used for SSL applications.^8a,b The synthesis of sulfide based phosphors requires sulfur containing gases (H₂S and CS₂). These have a severe environmental impact due to their toxic nature towards aspects of human health. Meanwhile, there are practical difficulties in the synthesis of nitride based phosphors since a very high temperature (>1800 °C), an ammonia atmosphere and high pressure (10 MPa) are required. So the use of sulfide and nitride based phosphors is ruled out and the search for non-toxic highly efficient host materials has attracted researchers, and this is how the oxide based phosphor came into the scenario. In order to achieve a high luminous output, the emission band should have a full width at half-maximum (FWHM) which is as small as possible.^8b However, the emission spectrum of Eu²⁺ activated nitride or oxynitride is very broad (FWHM = ∼80 to 90 nm). The overall luminous efficiency of radiation (LER) decreases due to the fact that the larger part of the spectrum is beyond 650 nm in wavelength and insensitive to the human eye.^9a–d On the other hand, the use of oxide based phosphor materials is free of the complications incurred from sulfide and nitride based phosphors and hence research and development of highly efficient oxide based phosphors is advantageous.

Among all the red emitting phosphors, Eu³⁺ substituted oxide based phosphors have attracted much interest due to their simple electronic energy level scheme and hypersensitive transitions (pure red emission). Generally, Eu³⁺ emission arises mainly from the ⁵D₀ level to the ⁷F_J (J = 0, 1, 2, 3, 4) manifolds. The intensity of the magnetic dipole transitions (⁵D₀ → ⁷F₁, ∼590 nm) and the electric dipole transition (⁵D₀ → ⁷F₂, ∼615 nm) is dependent on the local site symmetry of the Eu³⁺ ion present in the host lattice. The larger amplitude of the ⁵D₀ → ⁷F₂ transition than that of the ⁵D₀ → ⁷F₁ transition is an indication that the symmetry around Eu³⁺ does not contain an inversion center.^8a

Trivalent Eu substituted phosphor materials with either MO₄ tetrahedra or MO₆ octahedra [M = W or Mo] are a choice of interest due to their exceptional absorption from the near UV to blue region where the LED emission occurs, in addition these structures show decent chemical stability.^8a,b,10a–c Scheelite based compounds have been widely studied as efficient red phosphors and demonstrate potential for application in phosphor converted white LEDs.^11a–c Recently, we have explored the tungstate and molybdate based narrow-band red emitting phosphor La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.3) as a possible candidate for white light emitting diodes, due to its high absorption in the blue region and emission intensity (∼3.8 times that of the commercial red phosphor, Y₂O₂S:Eu³⁺, Nichia).¹² In the present study, we have systematically investigated the phase transition, thermal quenching and Eu³⁺ luminescence in La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) phosphors in detail. In order to improve the absorption strength (excitation spectrum) and emission intensity of the La_1.95Eu_0.05W_1.4Mo_0.6O₉ phosphor, concentration variation of Eu³⁺ has been carried out in La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2). The red emission intensities of the presently studied compositions are compared with that of the commercial red phosphor (Y₂O₂S:Eu³⁺, Nichia). A phonon sideband (PSB) was also observed in the Eu³⁺ doped La₂M₂O₉ phosphors. The Judd–Ofelt parameters were also calculated for the Eu³⁺ substituted compositions.

The high temperature solid state reaction method had certain disadvantages, such as inhomogeneous mixing, and difficulty in controlling the product’s morphology, crystalline/particle size and distribution.¹³ The use of a nonconventional method such as a co-precipitation method could facilitate overcoming the problem of inhomogeneity and higher particle size. Many of the commercially available phosphors have been synthesized by the co-precipitation technique and have found enhancement in luminescence intensity. L.-T. Chen et al., synthesized the BAM phosphor by the TEA co-precipitation method as well as by a solid state reaction. The phosphor which was synthesized by the TEA co-precipitation method showed a stronger relative luminescence intensity of Eu²⁺.¹⁴ Ce-doped YAG phosphor powders were prepared by co-precipitation and heterogeneous precipitation methods and the phosphors showed a better intensity compared to that of the heterogeneous precipitation technique.¹⁵ In addition, a comparative investigation into the synthesis and photoluminescence of the YAG:Ce phosphor has been reported. The phosphor obtained by the co-precipitation method is more homogeneous due to mixing of the starting materials at the molecular level.¹⁶ Red phosphor [Y₂(MoO₄)₃:Eu³⁺] with a flowerlike shape was prepared via a co-precipitation method. The emission quantum efficiency was found to be 22.38% for the ⁵D₀ level of the Eu³⁺ ion and the CIE color coordinates are (x = 0.656 and y = 0.344). Microwave-assisted hydrothermal synthesis, the growth mechanism and luminescent properties of 3D flower-shaped NaY(WO₄)₂:Eu³⁺ microarchitectures have also been reported.¹⁷ In the present study, efforts have been made to synthesize a phosphor through a method other than conventional synthetic methods (via co-precipitation), in order to further improve the efficiency of the selected phosphor Eu_1.6La_0.4W_1.6Mo_0.4O₉ (synthesized by a solid state reaction).

Experimental

Solid state synthesis

A series of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1), and La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2) phosphors were prepared from the appropriate mixtures of La₂O₃ (99.9%, Sigma Aldrich), Eu₂O₃ (99.99%, Sigma Aldrich), MoO₃ (99.99%, Sigma Aldrich), and WO₃ (99.99%, Sigma Aldrich), by a high temperature solid state reaction. La₂O₃ was heated at 1200 °C for 12 h prior to use to ensure decarbonation. The oxides were well mixed with agate mortar and heated in air at 500 °C for 6 h to decompose the carbonates. The powder samples were ground again and heated in air at 1100 °C for 6 h.

Co-precipitation technique

The selected phosphor was synthesized by a co-precipitation technique. Eu(NO₃)₃·6H₂O and La(NO₃)₃·6H₂O were prepared through dissolving Eu₂O₃ and La₂O₃ in nitrate acid (the volume ratio between nitric acid and water is 1 : 1). Then, the resultant solutions were dried and white powders of Eu(NO₃)₃·6H₂O and La(NO₃)₃·6H₂O were obtained. The white powders were dissolved in 10 ml of deionized water (labelled solution A), and MoO₃ (99.99%, Sigma Aldrich) and WO₃ (99.99%, Sigma Aldrich) were dissolved in liquid ammonia (labelled solution B). Solution A was slowly dropped into solution B under magnetic stirring, and subsequently a greenish white precipitate was formed. After reacting for 1 hour, the precursor solution was filtered and the resultant precipitate was washed several times with deionized water and acetone. Then the powder was dried under vacuum overnight and finally, the La_0.6Eu_1.4W_1.6Mo_0.4O₉ red phosphor was obtained and calcined at 1000 °C for 2 h.

The compositions were checked for phase formation by X-ray powder diffraction (XRD) using Cu-K_α1 radiation (Rigaku, ULTIMA IV). The UV-Vis spectrum was recorded using a Perkin Elmer Lambda 55 spectrophotometer. The excitation and emission spectra were recorded on a Horiba Jobin Yvon, USA/Fluoromax 4P spectrophotometer, which utilizes a 150 W Xe lamp as the excitation source. Temperature-dependent PL spectra were obtained using a DARSA PRO 5100 PL System (PSI Scientific Co. Ltd, Korea) equipped with a sample holder connected to a temperature controller. The morphology and elemental compositions were measured using a field emission scanning electron microscope equipped with an energy dispersive X-ray system FE-SEM-EDX (Nova NanoSEM 450/FEI). All the measurements were carried out at room temperature (RT). The Fourier transform infrared (FTIR) spectra were measured using a Bruker Alpha-E FTIR Spectroscope over a wavelength ranging from 400 to 4000 cm⁻¹, with the KBr pellet technique.

Results and discussion

Crystal structure and phase formation

α-La₂W₂O₉ and β-La₂W₂O₉ are the two different (low and high temperature polymorphs, respectively) structures of La₂W₂O₉. β-La₂W₂O₉ crystallizes in a cubic structure, whereas α-La₂W₂O₉ crystallizes in a triclinic structure.¹⁸ Similarly La₂Mo₂O₉ also exhibits two different structures, its low temperature polymorph α-La₂Mo₂O₉ crystallizes in a monoclinic structure (exhibiting a 2 × 3 × 4 superstructure of the cubic form), whereas its high temperature polymorph, β-La₂Mo₂O₉, crystallizes in cubic structure. In the α-La₂W₂O₉ structure, W1 is surrounded by five oxygen atoms which form a trigonal bipyramid, whereas W2 is in a six-fold octahedral coordination. Two octahedra share two corners with two trigonal bipyramids in order to form a four-membered ring with a [W₄O₁₈]¹²⁻ formulation. All of the nine oxygen atom sites are involved in one isolated cluster. The Mo atom in β-La₂Mo₂O₉ is approximatively in a trigonal bipyramidal surrounding with statistical occupancy for two on the three oxygen sites. The X-ray powder diffraction patterns of the selected compositions of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) with its parent compounds are shown in Fig. 1. In the La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) compositions, for x ≤ 0.2 all of the compositions crystallize in a triclinic structure and on further increasing x, the system undergoes a compositionally induced phase transition from a triclinic structure to a cubic structure (x ≥ 0.2). No impurity line was found in the XRD patterns clearly indicating that the compounds formed are of a single phase.

Fig. 1 X-ray powder diffraction patterns of select compositions of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) with its parent compounds.

Morphology of the powder samples

The microstructures and particle sizes of the synthesized powders were inspected by FE-SEM (Fig. 2a–d). All the micrographs show nearly round or elliptical fashioned particles with some kinds of aggregation in the case of the phosphor synthesized via a typical solid state reaction method (Fig. 2c and d). The size distribution of the particles is 0.4–3 μm, which is appropriate for the fabrication of SSL (solid state light) devices. Meanwhile, the case of the phosphor synthesized via the co-precipitation procedure (Fig. 2a and b) shows an irregular shape with more agglomeration of the particles compared to the previous method. The effect of the particles’ size and agglomeration on the photoluminescent properties is quite significant and is discussed in the following section. Fig. 2e represents the EDX spectrum of Eu_1.4La_0.6W_1.6Mo_0.4O₉, which confirms the presence of La, Eu, Mo, W and O atoms in the compound and provides supporting evidence to confirm the composition and purity. The molar ratio of La to Eu was found to be 1 : 2.33.

Fig. 2 Comparison of FE-SEM images of Eu_1.4La_0.6W_1.6Mo_0.4O₉ solid state (b and d) versus co-precipitation (a and c), (e) EDX spectrum of Eu_1.4La_0.6W_1.6Mo_0.4O₉.

FT-IR studies of La_0.6Eu_1.4W_1.6Mo_0.4O₉

The FT-IR spectra of the corresponding phosphor is shown in Fig. 3. The absorption bands observed in the lower wavenumber region i.e. less than 950 cm⁻¹ were assigned to the metal oxygen (M–O) bonds. So the peaks at 427 cm⁻¹, 878 cm⁻¹ and 1019 cm⁻¹ were attributed to the ν1, ν3 and ν2 modes of the MO₄ group (M–O, M = Mo/W) bond stretching vibration. The main absorption bands were detected at 427 cm⁻¹, 878 cm⁻¹ and 1019 cm⁻¹, and were assigned to La₂M₂O₉ (M = Mo/W). In addition, the appeared additional bands at 3559 and 1651 cm⁻¹ correspond to surface-absorbed water and hydroxyl groups, respectively. The band located at 2367 cm⁻¹ arises from a small quantity of CO₂ in the air. The observed band at 2925 cm⁻¹ belongs to a C–H stretching frequency, this could be a possible solvent molecule present on the surface of the phosphor.¹⁹

Fig. 3 FT-IR spectra of La_0.6Eu_1.4W_1.6Mo_0.4O₉ synthesized via solid state and co-precipitation methods.

DRS study of LaM₂O₉:Eu³⁺

The diffuse reflectance spectra of La₂M₂O₉ [M = W and Mo] and La_1.95Eu_0.05Mo_1.8W_0.2O₉ are shown in Fig. 4. The DRS spectra show a broad absorption band which is attributed to the charge transfer transition from oxygen to tungsten or molybdenum. In addition to this, the Eu substituted composition shows additional absorption at around 395 and 465 nm which is attributed to the Eu f–f electronic transitions.

Fig. 4 DRS spectra of La₂M₂O₉ [M = W and Mo] and La_1.95Eu_0.05Mo_1.8W_0.2O₉.

Luminescence of La_1.95Eu_0.05W₂O₉ and La_1.95Eu_0.05Mo₂O₉

The room temperature photoluminescence excitation and emission spectrum and the corresponding energy level diagram of La_1.95Eu_0.05W₂O₉ are shown in Fig. 5. The excitation spectrum shows a broad absorption band as well as line absorption. The broad absorption band (250 to 350 nm) is due to charge transfer from oxygen to tungsten (Eu–O is not observed clearly, this may be due to the overlap with the O to W CT band), whereas the line absorptions are due to 4f–4f electronic transitions of the Eu³⁺ ion. The emission spectrum contains sharp lines at 615 nm and 592 nm which are due to ⁵D₀–⁷F₂ electric dipole (ED) and ⁵D₀–⁷F₁ magnetic dipole (MD) transitions of the Eu³⁺ ion, respectively. However, no emission corresponding to tungstate is observed. The photoluminescence excitation and emission spectrum of La_1.95Eu_0.05Mo₂O₉ is shown in Fig. 6. The excitation spectrum shows a broad absorption band as well as line absorption. The broad absorption band (250 to 450 nm) is due to charge transfer from oxygen to molybdenum, whereas the line absorptions are due to the Eu³⁺ ion (4f–4f electronic transition). The width of the absorption band is extended from 350 to 450 nm, compared to that of the tungsten analogue and this clearly indicates that the band gap of tungstate is different from that of molybdate. The LMCT band position can be estimated using the empirical formula proposed by Reisfeld and Jørgensen:²⁰

E_ct (cm⁻¹) = [χ_opt(X) − χ_opt(M)] × 30 000 cm⁻¹ (1)

where χ_opt(X) is the optical electronegativity of the anion, which is equal to the Pauling electronegativity value, 3.2; χ_opt(M) is the optical electronegativity of the central metal ion (Mo = 2.101²¹ and Eu = 1.74).²² From expression (1), the LMCT of O²⁻–Mo⁶⁺ is around ∼303 nm and the O²⁻–Eu³⁺ LMCT is at ∼228 nm. The calculation clearly indicates that the estimations agree well with the experimental results.

Fig. 5 PLE and PL spectra and the corresponding energy level diagram of La_1.95Eu_0.05W₂O₉.

Fig. 6 PLE and PL spectra of La_1.95Eu_0.05Mo₂O₉.

The emission spectrum contains sharp lines at 615 nm and 592 nm which are due to ⁵D₀–⁷F₂ electric dipole (ED) and ⁵D₀–⁷F₁ magnetic dipole (MD) transitions of Eu³⁺ ion, respectively. However, no emission corresponding to any transition of molybdate is observed. The ED and MD emission wavelengths (due to 4f–4f transitions) are only moderately influenced by the crystal chemical environment of the lanthanide ions since the partially filled 4f shell is well shielded by the filled 5s and 5p orbitals. Nevertheless, it is possible to correlate the spectrum with the local site symmetry of the Eu site.

Luminescence of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–1.9, in steps of 0.1)

The photoluminescence excitation spectrum of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) under excitation at 615 nm is shown in Fig. 7. The prominent broad absorption band ranging from ∼250–400 nm is attributed to an oxygen to metal (tungsten/molybdenum) charge transfer (LMCT) transition which is visualized in the corresponding spectrum. In other words, the observed LMCT band is due to an electronic transition from a valence band (primarily oxygen 2p non-bonding character) to a conduction band (arising from the π* interaction between the tungsten metal ion t_2g orbital and oxygen). In general, in the Eu³⁺ substituted red phosphors, the LMCT band is stronger than that of the 4f–4f electronic transitions, because the former is a spin and parity allowed transition and the latter is a forbidden transition. However, in the tungstate/molybdate phosphors, the band intensity of the oxygen to europium charge transfer (LMCT) O²⁻ to Eu³⁺ was relatively weak in the excitation spectrum, because the contribution of the O²⁻ to M⁶⁺ (M = W/Mo) transition to the excitation was large. Hexavalent molybdenum and tungsten are interchangeable due to their almost identical ionic radii (Mo⁶⁺ = 0.41 Å or 0.59 Å and W⁶⁺ = 0.42 Å or 0.6 Å for coordination numbers 4 and 6, respectively).²³ A shift in the LMCT band is observed from 360 to 430 nm, by successive replacement of tungsten by molybdenum in the host lattice, which can be explained based on the effective cation electronegativity and the covalency of the d orbitals. In general, a decrease in band gap is observed, by replacing the 5d ion (W⁶⁺) with the less electropositive and more covalent 4d ion (Mo⁶⁺).

Fig. 7 The photoluminescence excitation spectrum of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) under excitation at 615 nm.

In the present study, the highly electropositive and less covalent W⁶⁺ is replaced by Mo⁶⁺ (La_1.95Eu_0.05W_2−xMo_xO₉) and the result shows that the absorption edge is shifted towards the near UV region (a decrease in band gap is observed). Similar observations have been made in perovskite and its related structures (for example Sr₂CaMoO₆ = 2.7 eV whereas Sr₂CaWO₆ = 3.6 eV).²⁴ The charge transfer band of Eu³⁺–O²⁻ is not clearly observed in the excitation spectra, this could be due to a possible overlap of the LMCT band with that of the tungstate or molybdate group.^8b,22,25 In addition, characteristic Eu³⁺ excitation lines (⁵L₆–⁷F₀, ⁷F₀–⁵D₂ and ⁷F₁–⁵D₁) are also observed in the spectrum. However, the relative intensity of the LMCT band and the excitation lines of ⁷F₀–⁵L₆ and ⁷F₁–⁵D₁ decreased, whereas the intensity of the ⁷F₀–⁵D₂ line slightly increased. This can be explained based on the crystal structure of the host lattice. The presently studied composition crystallizes in a triclinic structure when x = 0, whereas when x = 0.3 it crystallizes in a cubic structure. So the observed results clearly indicate that the minor distortion in the host lattice crystal structure plays a vital role in determining the luminescence properties of these systems. Similar observations have been made in one of the present authors’ previous luminescence studies on AgGd_0.95Eu_0.05(WO₄)_2−x(MoO₄)_x (x = 0–2, in steps of 0.25)^8a,b and AgLa_0.95Eu_0.05(WO₄)_2−x(MoO₄)_x (x = 0–2, in steps of 0.25).^10a It is worth noting that the crystal structure of the phosphor is very important for determining luminescence properties. In the AgGd_0.95Eu_0.05(WO₄)_2−x(MoO₄)_x (x = 0–2) phosphor, it has been shown that in the excitation spectra the 394 nm absorption line is intense for tungstates (monoclinic), whereas for molybdates (tetragonal) the 465 nm absorption line is intense. In contrast, the ⁷F₀–⁵D₂ excitation line is found to be of much higher intensity compared to any other lines in the AgLa_0.95Eu_0.05(WO₄)_2−x(MoO₄)_x (x = 0–2, in steps of 0.25) solid solution and interestingly both tungstate and molybdate crystallize in a tetragonal structure. The photoluminescence emission spectrum of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) is shown in Fig. 8. The spectrum contains sharp lines at 615 nm and 592 nm which are due to electric dipole (ED) and magnetic dipole (MD) transitions of the Eu³⁺ ion, respectively. However, no emission corresponding to the host lattice (tungstate or molybdate) is observed. This implies that there is energy transferred from the tungstate and molybdate host lattices to the Eu³⁺ activator ions. In the same way, there is energy migration in the host lattices, which competes against the energy transfer process. Red emission from Eu³⁺ is observed under charge transfer excitation, which clearly indicates the energy transfer from the tungstate or molybdate group to the Eu³⁺ levels. However, the relative PL intensity of Eu³⁺ is lower with CT band excitation when compared to that from Eu³⁺ excitation. In other words, the presence of an absorption band due to the tungstate or molybdate group in the excitation spectrum of Eu³⁺, when monitored for Eu³⁺ emission (615 nm), clearly suggests that the energy absorbed by the tungstate or molybdate group (Fig. 7) is transferred to the Eu³⁺ levels non-radiatively.^8b This reveals that the energy transfer from the host lattice to Eu³⁺ is not competent. The variation of the electric dipole emission intensity of Eu³⁺vs. x value (increase of Mo concentration) is plotted in Fig. S1 (ESI†).

Fig. 8 The photoluminescence emission spectrum of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) under excitation at 350 nm.

Generally the Eu³⁺ emission lines are hypersensitive, i.e., they are highly sensitive to the crystal chemical environment.²⁶ Electric dipole transition is dominant when Eu³⁺ occupies a non-centrosymmetric site in the lattice.²⁷ In the present study, the relative intensity of ED transition is found to be high. This clearly indicates that the Eu³⁺ ion occupies a non-centrosymmetric site in the presently studied compositions. All compositions show red emission under a CT band, at 394 nm (Fig. 9) and 465 nm (Fig. 10) excitation; however the relative emission intensity varies with composition.

Fig. 9 The photoluminescence emission spectrum of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) under excitation at 394 nm.

Fig. 10 The photoluminescence emission spectrum of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) under excitation at 465 nm.

In the absence of a commercial narrow-band red emitter excitable at 465 nm, we have compared our phosphors with a UV excitable commercial narrow-band red emitter Y₂O₂S:Eu³⁺ (Nichia).^8b,10a,b The emission is recorded under the same experimental measurement conditions. The intensities of the ⁵D₀ → ⁷F₂ emission lines of all of the compositions (Table ST1, ESI†) were compared with that of the Y₂O₂S:Eu³⁺ commercial red phosphor (Nichia) and the relative emission intensity of the La_1.95Eu_0.05W_1.6Mo_0.4O₉ composition was found to be ∼4.92 times higher than that of the commercial one under 465 nm excitation. The CIE chromaticity coordinates (x, y) of the La_1.95Eu_0.05W_1.6Mo_0.4O₉ compound are x = 0.6482 and y = 0.3495.

Luminescence of La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2)

The excitation spectrum of La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2) is shown in Fig. 11. The spectrum contains both a broad absorption band as well as line absorption. The former is due to the charge transfer absorption from oxygen to tungsten or molybdenum whereas the latter is due to characteristic absorption lines attributed to the Eu³⁺ ion (⁷F₀–⁵L₆, ⁷F₀–⁵D₂ and ⁷F₁–⁵D₁). The absorption intensity (charge transfer band and ⁷F₀–⁵L₆) is found to be at its maximum when y = 1.8, whereas the ⁷F₀–⁵D₂ and ⁷F₀–⁵D₁ transition shows its maximum intensity when y = 1.4 composition (as shown in Fig. 12). The emission spectrum of La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2) is shown in Fig. 12. The spectrum contains sharp lines at 615 nm and 596 nm which are due to electric dipole (ED) and magnetic dipole (MD) transitions of the Eu³⁺ ion. All the compositions show red emission at around 615 nm, which is due to the ⁵D₀–⁷F₂ transition. Whatever the excitation value may be (350, 395 and 465 nm), red emission is observed; however the relative emission intensity is found to be different. The emission spectra of La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2) under 395 and 465 nm excitation are shown in Fig. 13 and 14. The red emission intensity is found to be at its maximum when x = 1.8 under a CT band and 394 nm excitation, whereas x = 1.4 shows the maximum intensity under 465 nm excitation (similar to the excitation spectrum shown in Fig. 11). This indicates that the synthesized red phosphors have a high critical value of Eu³⁺ quenching concentration.

Fig. 11 The excitation spectra of La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2).

Fig. 12 The emission spectra of La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2) under 350 nm excitation.

Fig. 13 The emission spectra of La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2) under 394 nm excitation.

Fig. 14 The emission spectra of La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2) under 465 nm excitation.

The various 4f–4f electronic transitions of Eu³⁺ emission intensity are highly dependent on the local environment of the europium ion. As mentioned earlier, the electric dipole transition is a hypersensitive transition. Therefore, it is possible to gain more insight into the change of the Eu³⁺ structural surroundings by calculating the ratio of integrated emission intensities of ED and MD transitions.²⁸ The ratio between the ED and MD [(⁵D₀/⁷F₂)/(⁵D₀/⁷F₁)] is called the asymmetric ratio and gives a measure of the degree of distortion from the inversion symmetry of the local environment of the Eu³⁺ ion in the lattice.²⁹ The asymmetry ratios as a function of Eu³⁺ concentration are shown in Fig. 15 (tabulated values available in the ESI,† ST1 and ST2). The asymmetry ratio increased from 5.4 to 7.0, when the Eu concentration increases from 0.2 to 2, in steps of 0.2.

Fig. 15 The asymmetry ratios as a function of Eu³⁺ concentration (x = 0.2 to 2, in steps of 0.2).

In general, there are two types of energy transfer process which are expected and lead to concentration quenching; energy migration between the same ions (activator) to the killer sites (such as crystalline defects) and resonant or phonon-assisted cross-relaxations via matched energy levels between two adjacent centers in the host lattice.^30a,b There are no matched energy levels available to quench the emissions from ⁵D₀ level of Eu³⁺. Hence, the concentration quenching cannot be attributed to cross relaxation. As the concentration of Eu³⁺ (the luminescent centers) increases, the distance between the Eu³⁺ ions becomes small enough to allow a resonant energy transfer. The energy from one Eu³⁺ to another Eu³⁺ ion can easily be transferred and these energy-transfer chains trigger energy migration to crystalline defects. Therefore, beyond the optimum concentration of the activator ion (Eu³⁺), one can expect concentration quenching.

A rough estimate of the critical distance of energy transfer (R_c), the distance at which the probability of transfer is equal to the probability of radiative emission, can be calculated using the relation:^31a,b

where x_c is the critical concentration, N is the number of host cations in the unit cell and V is the volume of the unit cell (in this case 364.12 ų). Since a high emission intensity is observed for the x = 1.4 concentration, this value is used as the critical concentration, x_c. Using the above eqn (2), the critical distance is determined to be ∼6.21 Å, at which point the energy transfer among the adjacent ions becomes so significant that the radiative transitions become dominant over the non-radiative transitions.

A theoretical description was developed by Huang for the doping concentration vs. the luminescence intensity³² and this has been verified by many researchers by their experimental results.^33a–c The luminescence intensity (I) vs. doping concentration (C) could be expressed by the following eqn (3) and (4) (according to Huang's description)

where A and X₀ are constants, and Γ(1 + s/d) is a Γ function, γ is the intrinsic transition probability of the sensitizer, s is the index of electric multipole, for the electric dipole–dipole (D–D), electric dipole–quadrupole (D–Q), and electric quadrupole–quadrupole (Q–Q) interactions; s = 6, 8 and 10, respectively. If s = 3, the interaction type is an exchange interaction. d is a dimension of the sample; here d = 3, since the energy transfer between Eu³⁺ ions inside the particles is considered. From the above equation one can derive the following eqn (5)

In the above equation, f is independent of the doping concentration. Fig. 16 shows the log(I/C) vs. log(C) plot for the electric dipole transitions (⁵D₀–⁷F₂) of the Eu³⁺ ions in the Eu_1.4La_0.6W_1.6Mo_0.4O₉ red phosphors. The experimental data was fitted using linear fittings and the slope parameter −s/d was obtained to be −0.66 which is close to 1 (corresponding to s = 3). The obtained slope parameter and its corresponding s value clearly indicate that the energy transfer among activator (Eu³⁺) ions occurs in the Eu_1.4La_0.6W_1.6Mo_0.4O₉ phosphor through a dominant exchange interaction mechanism. The intensities of the ⁵D₀ → ⁷F₂ emission lines of all the compositions (Table ST2, ESI†) were compared with that of the Y₂O₂S:Eu³⁺ commercial red phosphor (Nichia) and the relative emission intensity of the La_0.6Eu_1.4W_1.6Mo_0.4O₉ composition is found to be ∼8.24 times higher than that of the commercial one under 465 nm excitation.

Fig. 16 The relationship between the doping concentration (Eu³⁺) (log C) and log(I/C) for the ⁵D₀–⁷F₂ transition in Eu_1.4La_0.6W_1.6Mo_0.4O₉.

Temperature dependent luminescence and thermal quenching

Thermal quenching is one of the most important parameters for phosphors used in white LEDs, because it has a great impact on the light output and CRI. The temperature-dependent photoluminescence properties were measured at temperatures in the range of 25–200 °C under 465 nm excitation and the results are shown in Fig. 17. The relative PL intensity vs. temperature plots of the Eu_1.6La_0.4W_1.6Mo_0.4O₉ and CaS:Eu²⁺ phosphors are shown in Fig. 18. Temperature dependent photoluminescence studies reveal that the Eu_1.4La_0.6W_1.6Mo_0.4O₉ composition loses 30% of its efficiency up to 400 K (compared to ∼80% for CaS:Eu²⁺) and the results demonstrate that the investigated phosphor has a relatively good thermal quenching effect. Although a decrease in the PL intensity of the phosphor was observed as the temperature rose, the position of the emission band did not shift, which indicates that the color coordinates of the phosphor are stable with increases in temperature. Indeed, there was little change in the CIE color coordinates of the phosphor while varying the temperature. The phenomenon of thermal quenching can be explained based on a configurational coordinate diagram (Fig. 19). The electron–phonon interaction is expected to be dominant in the high temperature environment. The luminescence center [Eu³⁺] in the excited state is activated by phonon communications initially, which are released through a crossover between the excited state [⁵D₀] and ground state [⁷F_J (J = 1, 2, 3, 4)],³⁴ as a result, the emission intensity decreases as the temperature increases. In order to elucidate the thermal quenching phenomena and to determine the activation energy for thermal quenching, the Arrhenius equation was fitted to the thermal quenching data of the red emitting LaEuMoWO₉ phosphor and CaS:Eu²⁺ commercial phosphor as shown in Fig. 18.

Fig. 17 Temperature dependent emission spectrum of Eu_1.4La_0.6W_1.6Mo_0.4O₉ under 465 nm excitation.

Fig. 18 Relative PL intensity vs. temperature and plots of ln[(I₀/I) − 1] vs. 1/kT for the Eu_1.4La_0.6W_1.6Mo_0.4O₉ and CaS:Eu²⁺ phosphors.

Fig. 19 A configurational diagram given to explain the crossover process.

According to the classical theory of thermal quenching, the temperature dependent luminescence intensity can be described by the following expression (eqn (6)), where I₀ is the initial PL intensity, I_T is the PL intensity at a given temperature T, A is a constant and k is Boltzmann's constant.^35a–dFig. 18 shows the plot of ln[(I₀/I) − 1] vs. 1/kT and gives straight line. The best fit was observed for the activation energies E = 0.27 and 0.33 eV for the LaEuMoWO₉ phosphor and CaS:Eu²⁺ commercial phosphor, respectively.

CIE color coordinates

The Commission International del'Eclairage (CIE) chromaticity coordinates of the synthesized phosphor and the corresponding emission intensity are tabulated in Tables ST1 and ST2, ESI.† The CIE chromaticity coordinates (x and y) of the optimized red phosphor, Eu_1.6La_0.4W_1.6Mo_0.4O₉, are (0.67 and 0.33) and are very close to the NTSC (National Television System Committee) standard color coordinate values (0.67, 0.33). The CIE color coordinates of a commercial yellow phosphor (YAG:Ce³⁺) are also included with the synthesized red phosphor in Fig. 20. The CIE color coordinates of the currently synthesized phosphor show higher color saturation, which is better than that of the commercial Eu³⁺ doped oxysulfide phosphor (x = 0.64, y = 0.34). In addition, as evidenced by the excitation spectrum, the currently studied phosphor can absorb not only the emissions from LED based UV radiation but also blue LEDs. Hence it can be used to compensate for the red emission in the white LED based on the blue LED + YAG:Ce³⁺ phosphor or combined with the blue LED + green phosphor to create white light.

Fig. 20 CIE color coordinate diagram (inset shows the Eu_1.4La_0.6W_1.6Mo_0.4O₉ phosphor in daylight and under 365 nm).

Comparative photoluminescence study of the Eu_1.4La_0.6W_1.6Mo_0.4O₉ phosphor (solid state (SS) vs. co-precipitation (COP) technique)

In general, heating at high temperature and the wet reaction times are vital factors which affect crystallization and luminescence intensity. In case of the co-precipitation method with an increase of reaction time which involves nothing but stirring the reactants for a prolonged period, a promising phosphor with high intensity is acquired. In order to have a comparative study of the luminescence behaviour, two different methods were adopted to synthesise the Eu_1.4La_0.6W_1.6Mo_0.4O₉ phosphors and PL studies were performed. The obtained results shows that there is a prominent increase in the luminescence intensity of the phosphor synthesized via the co-precipitation technique compared with the solid state one. The comparative excitation spectrum of Eu_1.4La_0.6W_1.6Mo_0.4O₉ (prepared by SS versus COP) is shown in Fig. 21. The excitation spectrum shows both LMCT (due to M–O charge transfer band M = W/Mo) and Eu³⁺ f–f electronic transitions. However, the absorption strength is greater for the phosphor which is synthesized by the COP method, which clearly indicates that the synthesis technique plays a role in improving the absorption. Similarly the emission spectrum also shows an electric dipole transition which is due to the Eu³⁺ ion (Fig. 22 and 23). However the emission intensity varies and it is found to be higher for the phosphors that are synthesized by the COP method. We can clearly observe that the intensity has almost doubled. The reason behind this is due to the proper mixing of the starting materials at the molecular level when stirred for a long time (homogenous product formation). It is noted that the synthesis process for obtaining particles with a non-agglomerated shape as well as high crystallization plays an important role in the enhancement of luminescence efficiency. The calculated CIE color chromaticity values are well matched with NTSC standard values.

Fig. 21 The comparative excitation spectra of Eu_1.4La_0.6W_1.6Mo_0.4O₉ under 615 nm excitation.

Fig. 22 The comparative emission spectra of Eu_1.4La_0.6W_1.6Mo_0.4O₉ under 393 nm excitation.

Fig. 23 The comparative emission spectra of Eu_1.4La_0.6W_1.6Mo_0.4O₉ under 463 nm excitation.

Phonon sideband spectrum and calculation of the Huang–Rhys factor

Fig. 24a and b shows the phonon sideband spectrum (PSB) of Eu³⁺ activated La₂M₂O₉ (M = W and Mo) under an emission wavelength of 616 nm (⁵D₀–⁷F₂). From the spectrum it is clearly shown that the PSB lies in the spectral range from 420 to 480 nm is linked with the ⁷F₀–⁵D₂ transition. The highest two peaks are around 427.4 nm and 449.6 nm (corresponding phonon energy of 1901 and 746 cm⁻¹) for La₂W₂O₉ and 427 and 447 nm (corresponding phonon energy of 1867 and 819 cm⁻¹) for La₂Mo₂O₉ were experiential. They are attributed to the symmetric vibration of O–M–O and asymmetric vibration of O–M where M = W and Mo respectively. Based on the above results it can be concluded that the presently studied host La₂M₂O₉ has a higher phonon energy than other oxides. In addition, the Huang–Rhys factor can be calculated according to eqn (7).where I_1P and I_ZP are the integral intensity of the one-phonon line and zero-phonon line, respectively. S is the Huang–Rhys factor.

Fig. 24 (a and b) Phonon sideband spectrum of the La_1.95Eu_0.05M₂O₉ (M = W or Mo) phosphor monitoring the ⁵D₀–⁷F₂ emission at 616 nm excitation.

The transition selection rules state that, for the ⁷F₀–⁵D₂ transition, the zerophonon line is purely an electric dipole transition and the multiphonon sideband is located on the higher energy side of the zero-phonon line. Through this process a phonon can be produced and therefore p ≥ 0 in eqn (7). Raman scattering spectra of the sample have a similar spectral outline to the phonon sideband spectra, so I_PSB = I_1P. Furthermore, 〈1 + m〉 can also be approximately assumed as 1 because of the higher phonon energy at room temperature. According to the above statement, the derived Huang–Rhys factor can be rewritten as

S = I_PSB/I_ZP (8)

where I_PSB is the integral intensity of the one-phonon line and I_ZP is zero-phonon line. Based on eqn (8), the calculated Huang–Rhys factor is found to be 0.031 and 0.023 for La₂W₂O₉ and La₂Mo₂O₉, respectively.

Judd–Ofelt parameters calculation

The effect of the chemical environment has a strong and phenomenal influence on the intensity of the Eu³⁺ ion. To study this change and the variation in intensity, the Judd–Ofelt theory is one best tools ever used.^36a,b By using the photoluminescence emission spectra of the Eu³⁺ ion, it is easy to determine the Ω_λ values where λ = 2, 4, 6. Calculation of the experimental intensity parameters Ω_λ (J = 2 and 4) was done for the ⁵D₀ →⁷F₂ and ⁵D₀ → ⁷F₄ transitions by keeping ⁵D₀ → ⁷F₁ as a reference. The Ω₆ intensity parameter could not be projected for the ⁵D₀ → ⁷F₆ transition, since this emission was not being observed due to instrumental limitations. The relation between the radiative emission rate and integrated emission intensity is given bywhere I_0–J is the area under the spectral curve and hϑ_0–J is the energy corresponding to the ⁵D₀ → ⁷F_J (J = 1, 2, 4) transition of Eu³⁺. The magnetic dipole radiative transition rate, A_0–1, was taken to be 50 s⁻¹. Hence the electric dipole transition probability can be obtained from the above relation. The forced electric dipole transitions ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ as a function of the Judd–Ofelt intensity parameter are formulated aswhere ϑ is the Lorentz local field correction factor which is a function of the refractive index, n = 2.02, of the host and is given by the relation χ = n(n₂ + 9)²/9. 〈⁵D₀|U_J|⁷F₂〉² are the square reduced matrix elements, independent of the chemical environment of the Eu³⁺ ion. Values of the non zero square reduced matrix, 〈⁵D₀|U_J|⁷F₂〉² = 0.0032 for the ⁵D₀ → ⁷F₂ transition and 〈⁵D₀|U_J|⁷F₄〉² = 0.0023 for the ⁵D₀ → ⁷F₄ transition, were taken for the present calculations. The calculated Judd–Ofelt parameters were tabulated in Table 1. The uncertainty for this calculation is estimated to be around 5%. It is also noteworthy that in most of the molybdate/tungstate phosphors the Ω₆ values are unknown due to the lack of ⁵D₀–⁷F₆ transition data; the Ω₂ value changes greatly from host lattice to host lattice since Ω₂ is very sensitive to the crystal field around the central metal ion (Eu³⁺).³⁷

Table 1 Judd–Ofelt parameters for La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2)

Concentration of Eu^3+ ion	Judd–Ofelt intensity parameter Ω2 (10^−20 cm^2)	A 0−1 in S^−1	A 0−2
0.2	3.9241	50	381.286
0.4	3.3231	50	270.98
0.6	3.4176	50	278.970
0.8	3.0331	50	247.432
1	2.9551	50	241.020
1.2	3.6677	50	299.94
1.4	3.8306	50	314.095
1.6	3.9640	50	325.103
1.8	4.0926	50	334.689
2	4.1950	50	342.988

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Conclusions

In summary, we have successfully synthesized the red emitting phosphors La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) and La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, in steps of 0.2) and studied their optical properties. In La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1), when x ≥ 0.2 it crystallizes in a triclinic structure, and upon further increasing x, the system undergoes a compositionally induced phase transition from a triclinic to cubic structure (x > 0.2). All the compositions of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) and La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2 in steps of 0.2) show red emission under NUV or blue ray excitation. The relative emission intensity was found to be at its maximum when y = 0.3 and these were compared with a commercial red phosphor (showing an emission intensity ∼ 8.24 times higher than that of the commercial one) under blue ray excitation. In addition, the selected compositions which were prepared by the co-precipitation technique show a much better emission intensity (almost two times) than that of the phosphor synthesized by a solid state reaction. Calculation of the spectral parameters was done through Judd–Ofelt theory from the emission spectra. The phonon sideband was also observed in the systems La₂W₂O₉ and La₂Mo₂O₉ and this indicates that these phosphors have a higher phonon energy. Therefore, the newly developed red phosphors can combine with a yellow phosphor and a blue LED to generate warm white LEDs.

Acknowledgements

The authors thank NRB, DRDO for the financial support and Prof. Duk Young Jeon, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Deajeon 305-701, Republic of Korea for the temperature dependent PL emission study. S. Kasturi acknowledges the DST for the research fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6qm00118a

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