Novel Red Phosphor of Gd³⁺, Sm³⁺ co-Activated Ag_xGd_{((2−x)/3)−0.3−y}Sm_yEu³⁺_0.30☐_{(1−2x−2y)/3}WO₄ Scheelites for LED Lighting

Abstract

Gd³⁺ and Sm³⁺ co-activation, the effect of cation substitutions and the creation of cation vacancies in the scheelite-type framework are investigated as factors influencing luminescence properties. Ag_xGd_{((2−x)/3)−0.3−y}Sm_yEu³⁺_0.3☐_(1−2x)/3WO₄ (x = 0.50, 0.286, 0.20; y = 0.01, 0.02, 0.03, 0.3) scheelite-type phases (A_xGS_yE) have been synthesized by a solid-state method. A powder X-ray diffraction study of A_xGS_yE (x = 0.286, 0.2; y = 0.01, 0.02, 0.03) shows that the crystal structures have an incommensurately modulated character similar to other cation-deficient scheelite-related phases. Luminescence properties have been evaluated under near-ultraviolet (n–UV) light. The photoluminescence excitation spectra of A_xGS_yE demonstrate the strongest absorption at 395 nm, which matches well with commercially available UV-emitting GaN-based LED chips. Gd³⁺ and Sm³⁺ co-activation leads to a notable decreasing intensity of the charge transfer band in comparison with Gd³⁺ single-doped phases. The main absorption is the ⁷F₀ → ⁵L₆ transition of Eu³⁺ at 395 nm and the ⁶H_5/2 → ⁴F_7/2 transition of Sm³⁺ at 405 nm. The photoluminescence emission spectra of all the samples indicate intense red emission due to the ⁵D₀ → ⁷F₂ transition of Eu³⁺. The intensity of the ⁵D₀ → ⁷F₂ emission increases from ~2 times (x = 0.2, y = 0.01 and x = 0.286, y = 0.02) to ~4 times (x = 0.5, y = 0.01) in the Gd³⁺ and Sm³⁺ co-doped samples. The integral emission intensity of Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄ in the red visible spectral range (the ⁵D₀ → ⁷F₂ transition) is higher by ~20% than that of the commercially used red phosphor of Gd₂O₂S:Eu³⁺. A thermal quenching study of the luminescence of the Eu³⁺ emission reveals the influence of the structure of compounds and the Sm³⁺ concentration on the temperature dependence and behavior of the synthesized crystals. Ag_0.286Gd_0.252Sm_0.02Eu_0.30WO₄ and Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄, with the incommensurately modulated (3 + 1)D monoclinic structure, are very attractive as near-UV converting phosphors applied as red-emitting phosphors for LEDs.

1. Introduction

Light-emitting diodes (LEDs) have now mostly replaced the other light sources such as incandescent or fluorescent lamps. These new light sources are more reliable, last longer and are more environmentally friendly [1]. New light sources find interesting areas of application, for example, for architectural lighting, lighting in electronic technology, etc. [2].

Phosphors based on tungstates and molybdates with broad and intense absorption bands in the near UV region are widely used. Scheelite-type tungstates and molybdates doped with rare-earth elements are promising materials for WLED [3,4,5,6,7,8,9,10,11] and solid-state lasers [12,13,14,15]. For example, for a phosphor based on NaEu(WO₄)₂, the emission intensity at ~615 nm and the integral intensity are 8.5 and 5.0 times higher, respectively, than for commercial Y₂O₂S:Eu³⁺ phosphor (~626 nm) [4]. Such materials can determine temperature gradients with high accuracy, which is important for their application as thermographic phosphors [16,17,18,19].

In the scheelite-related phases Ag_xEu_(2−x)/3☐_(1−2x)/3WO₄ and Ag_xGd_{(2−x)/3−0.3}Eu_0.3☐_(1−2x)/3WO₄, strong absorption occurs at 395 nm [20]. This absorption region is in good agreement with the absorption region of a commercial GaN-based LED chip emitting n–UV. The good agreement between the absorption regions of the studied tungstates and the GaN-based chip makes it possible to use these phosphors as emitters. Phosphor Ag_xGd_{((2−x)/3)−0.3}Eu³⁺_0.3☐_(1−2x)/3WO₄ is a more efficient emitter than Ag_xEu_(2−x)/3☐_(1−2x)/3WO₄ [20].

In the present research, we have focused on Ag_xGd_{((2−x)/3)−0.3−y}Sm_yEu³⁺_0.30☐_{(1−2x−2y)/3}WO₄ scheelites to analyze the influence of Gd³⁺ and Sm³⁺ co-activation and the number of cation vacancies on the luminescent properties and structure. Sm³⁺ cation is usually used as a sensitizer to enhance the luminescence of Eu³⁺ cations [4,21,22,23]. By changing the concentration of Sm³⁺ cations, it is possible to increase the width of the excitation band and thereby increase the luminescence intensity of Eu³⁺ cations upon excitation of the blue LED or near UV.

2. Materials and Methods

2.1. Sample Preparation

Ag_xGd_{((2−x)/3)−0.3−y}Sm_yEu³⁺_0.30☐_{(1−2x−2y)/3}WO₄ (x = 0.5, 0.286, 0.2; y = 0.01–0.3) (A_xGS_yE—short entry) was synthesized from a stoichiometric mixture of R_2/3☐_1/3WO₄ (R = Sm, Eu, Gd) at 833 K for 10 h, followed by annealing at 1283 K for 95 h by the solid phase method. Ag₂WO₄ was synthesized at 633 K for 12 h from stoichiometric mixtures of WO₃ (99.99%) and AgNO₃ (99.99%), followed by annealing at 833 K for 42 h. R_2/3☐_1/3WO₄ tungstates were synthesized from stoichiometric mixtures WO₃ and R₂O₃ (99.99%) at 883 K for 12 h, followed by annealing at 1133 K for 82 h. To study the luminescent properties of Ag_xR_{((2−x)/3)−0.3}Eu³⁺_0.30☐_(1−2x)/3WO₄ (R = Eu, Gd; x = 0.5, 0.286, 0.2), we used samples obtained earlier [19,20].

2.2. Characterization

X-ray diffraction (XRD) patterns of A_xGS_yE (x = 0.5, 0.286, 0.2; y = 0.01–0.3) were obtained on a Thermo ARL X’TRA powder diffractometer (Waltham, MA, USA) (CuK_α radiation, λ = 1.5418 Å, Bragg–Brentano geometry, scintillation detector, at T_R, the 5–70° 2θ range, steps 0.02°). Moreover, XRD patterns of Ag_xR_{((2−x)/3)−0.3}Eu³⁺_0.30☐_(1−2x)/3WO₄ (R = Eu, Gd; x = 0.5, 0.286, 0.2) were obtained on a Huber G670 Guinier diffractometer (Rimsting, Germany) (Cu K_α1 radiation, transmission mode, curved Ge(111) monochromator, image plate detector, at T_R, HUBER Diffraktionstechnik GmbH & Co. (Rimsting, Germany), 4–100° 2θ range, steps 0.01°). The lattice parameters were refined by the Le Bail method [24] using JANA2006 software (version Jana2006 for Windows) [25].

Photoluminescence excitation (PLE) and emission (PL) spectra were obtained on a Varian Cary Eclipse fluorescence spectrometer (75 kW xenon light source, pulse τ = 2 μs, frequency ν = 80 Hz, resolution 0.5 nm; R928 PMT Hamamatsu, Hamamatsu Photonics, Hamamatsu, Japan). All spectrum measurements were performed at room temperature in a copper cell (∅ 10 mm × 20 mm) and corrected for the sensitivity of the spectrometer. The integral intensity (I_int) of the ⁵D₀ → ⁷F₂ transition in polycrystalline Ag_0.50Eu_0.50WO₄ was taken as 100%. The integral intensities of the ⁵D₀ → ⁷F₂ transition for other samples were normalized to the integral intensity of this transition for Ag_0.50Eu_0.50WO₄. The Perkin Elmer LS–55 fluorescence spectrometer (PerkinElmer, Inc., Waltham, MA, USA) [26] was used to study the effect of temperature (300 K–700 K) on luminescent properties. The PL spectra were obtained with excitation at 395 nm, measurement step 20 s, heating rate 4 K/min. Before measuring the temperature, the presence of thermostimulated luminescence was checked upon excitation at 395 nm. All measurements of the samples were carried out under the same conditions.

3. Results

3.1. Crystallography

Figure 1 shows the XRD patterns of the four investigated phases. The x = 0.5 compounds, Ag_0.5Gd_0.2−ySm_yEu³⁺_0.30WO₄ and Ag_0.5Eu_0.2Eu_0.3WO₄ belong to the tetragonal structure (space group (SG) I4₁/a). The XRD patterns of A_xGS_yE (x = 0.286, 0.2; y = 0.01–0.3) and Ag_0.2Eu_0.3Eu³⁺_0.3☐_0.2WO₄ show intense reflections of a scheelite-type superstructure and satellite reflections with low intensity. The patterns can be attributed to monoclinic incommensurately modulated (3 + 1)D symmetry (superspace group (SSG) I2/b(αβ0)00). The unit cell parameters of the A_xGS_yE samples, determined from X-ray diffraction patterns using the Le Bail decomposition in the SSG I2/b(αβ0)00 and the modulation vector q, are given in

Table 1. Unit cell parameters and symmetry of Ag_xR_{((2−x)/3)−0.3}Eu³⁺_0.30☐_(1−2x)/3WO₄ (x = 0.5, 0.286, 0.2) solid solutions were determined from X-ray diffraction data.

R((2−x)/3)−0.3	a, Å	b, Å	c, Å	γ, Deg.	V, Å3	q	Ref.
x = 0.50 (SG I41/a)
Sm0.20	5.2780(1)		11.5340(6)		321.31(6)
Eu0.20	5.2787(1)		11.50026(7)		320.482(3)		[20]
Gd0.20	5.2740(1)		11.49348(6)		319.689(2)		[20]
x = 0.286 (SSG I2/b(αβ0)00)
Sm0.271	5.2311(2)	5.2950(2)	11.5501(5)	91.460(3)	319.82(3)	0.602a* + 0.817b*
Eu0.271	5.2309(2)	5.2897(2)	11.52995(4)	91.3121(3)	318.948(1)	0.586a* + 0.819b*	[20]
Gd0.24Sm0.03	5.2188(3)	5.2841(1)	11.5220(5)	91.570(3)	317.62(4)	0.608a* + 0.806b*
Gd0.25Sm0.02	5.2153(1)	5.2872(1)	11.5262(2)	91.561(2)	317.71(1)	0.598a* + 0.814b*
Gd0.26Sm0.01	5.2169(1)	5.2902(1)	11.5292(2)	91.689(2)	318.05(1)	0.601a* + 0.808b*
Gd0.271	5.2218(2)	5.2880(2)	11.52406(4)	91.4677(4)	318.106(4)	0.585a* + 0.820b*	[20]
x = 0.20 (SSG I2/b(αβ0)00)
Sm0.30	5.2307(1)	5.3003(1)	11.55624(2)	92.0647(2)	320.180(1)	0.591a* + 0.802b*
Eu0.30	5.2193(2)	5.2897(2)	11.52562(4)	92.1118(3)	317.993(1)	0.591a* + 0.800b*	[20]
Gd0.27Sm0.03	5.2074(2)	5.2858(1)	11.5164(2)	92.345(3)	316.73(2)	0.590a* + 0.801b*
Gd0.28Sm0.02	5.2058(1)	5.2856(1)	11.5189(4)	92.345(2)	316.69(2)	0.590a* + 0.805b*
Gd0.29Sm0.01	5.2049(1)	5.2839(1)	11.5181(2)	92.374(2)	316.50(1)	0.592a* + 0.803b*
Gd0.30	5.2097(2)	5.2872(3)	11.51483(6)	92.2489(3)	316.929(3)	0.592a* + 0.803b*	[20]

Note: a* and b* are reciprocal parameter vectors.

together with reference data.

3.2. Luminescence Properties

The photoluminescence emission (PL) and excitation (PLE) spectra of A_xGS_yE (x = 0.5, 0.286, 0.2; y = 0–0.03) tungstates are shown in Figure 2 and Figure 3. Figure 2a shows PLE spectra for the Eu³⁺ in Ag_0.20Gd_0.30−ySm_yEu_0.30☐_0.20WO₄ (y = 0–0.03) upon emission at 615 nm. The excitation spectra (PLE) contain a broad band (220–320 nm) and a group of narrow lines (320–550 nm). The luminescence spectra of A_xGS_yE (x = 0.5, 0.286, 0.2; y = 0–0.03) (λ_ex = 395 nm) are shown in Figure 2b and Figure 3.

The comparative integral intensity of the ⁵D₀ → ⁷F₂ luminescence for Ag_xEu³⁺_(2−x)/3☐_(1−2x)/3WO₄ and A_xGS_yE scheelite-type phases is presented in Figure 4. The electric dipole transition ⁵D₀ →⁷F₂ at ~615 nm is the most intensive for all the luminescence spectra. The data on the effect of Gd³⁺ and Sm³⁺ co-activation on the ⁵D₀ →⁷F₂ transition luminescence intensity in Ag_xR_{((2−x)/3)−0.3−y}Sm_yEu³⁺_0.3☐_(1−2x)/3WO₄ (R = Eu, Gd, Sm; x = 0.50, 0.286, 0.20; y = 0–0.03) are summarized in Table S1 (Supporting Information). The comparison of the PL spectra and integral ⁵D₀ → ⁷F₂ emission intensity of Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄ and the commercially used Gd₂O₂S:Eu³⁺ red phosphor is shown in Figure 5.

To evaluate the behavior of thermal quenching, the PL spectra were studied at temperatures ranging from 300 to 700 K. Figure 6 and Figure 7 show the temperature-dependent PL spectra and the intensity of the ⁵D₀ → ⁷F₂ transition at ~615 nm.

4. Discussion

The present work is focused on the scheelite-type phases of A_xGS_yE (x = 0.5, 0.286, 0.2; y = 0.01, 0.02, 0.03, 0.3) with a variable composition. One can establish that the substitution of Ag⁺ by R³⁺ (R = Eu, Gd, Sm) leads to a noticeable distortion of the scheelite-like framework. The phase with x = 0.5 crystallizes in a tetragonal scheelite-like structure (I4₁/a). From the high-resolution synchrotron XRD data [20] for a sample with x = 0.5, it is found that there is no Ag⁺ and R³⁺ cations ordering.

We have previously analyzed that the formation of cationic vacancies in Ag_xGd_{((2−x)/3)−0.3}Eu³⁺_0.3☐_(1−2x)/3WO₄ [20] and Ag_xSm_(2−x)/3☐_(1−2x)/3WO₄ [19] structures is accompanied by ordering of cations and vacancies with the formation of (3 + 1) D incommensurably modulated structures for Ag_xR_(2−x)/3☐_(1−2x)/3WO₄ (R = Eu, Sm; x = 0.286, 0.2) phases. As revealed earlier for Ag_xGd_{((2−x)/3)−0.3}Eu³⁺_0.3☐_(1−2x)/3WO₄ (x = 0.286, 0.2) [20] and shown here for A_xGS_yE (x = 0.286, 0.2; y = 0.01, 0.02, 0.03), the structures of these scheelite-related phases have an incommensurately modulated character too. The Ag_xR_(2−x)/3☐_(1−2x)/3WO₄ (R = Eu, Sm; x = 0.286, 0.2) aperiodic structures are built from two types of columns [… – (RO₈/AgO₈)–WO₄ – …] and [… – ☐ – WO₄ – …], which are extended along the c-axis. The structures of these phases are characterized by different distributions of Ag⁺/R³⁺ cations and vacancies in the A-cationic column of the scheelite structure. The specific coefficients α and β of the modulation vector q = αa* + βb* in incommensurately modulated structures, and the individual parameters of the atomic domains, define specific cations ordering for each modulated structure. In Ag_xR_(2−x)/3☐_(1−2x)/3WO₄ (R = Eu, Sm; x = 0.286, 0.2) structures, two types of R–aggregates have been described: [R₂O₁₄] dimers and infinite [RO₈]_n chains from EuO₈ polyhedra [19,20].

As an example, a portion of the Ag_0.157Eu_0.614☐_0.229WO₄ disordered structure is shown in the [001] projection in Figure 8a. [Eu₂O₁₄]-dimers are Eu-pairs surrounded by only WO₄ tetrahedra and isolated from all other Eu³⁺ in the infinite [EuO₈]_n chains by vacancies and Ag⁺ cations (Figure 8b,c). The amount of [Eu₂O₁₄]-dimers in the Ag_xEu_(2−x)/3☐_(1−2x)/3WO₄ framework depends on x. In the structures of Ag_xEu_(2−x)/3☐_(1−2x)/3WO₄, the number of Eu³⁺ cations involved in the formation of [Eu₂O₁₄]-dimers increases from 15.56% to 28.33%, with decreasing x from 0.238 to 0.157 [20]. There is a relationship between the number of [Eu₂O₁₄]-dimers in the structure and the luminescence characteristics (total ($Q_L^{Eu}$) and intrinsic ($Q_{Eu}^{Eu}$) quantum yields and lifetimes of Eu³⁺ (⁵D₀) (τ_obs)) [27]. The values of the luminescent characteristics increase with an increase in the number of Eu³⁺ dimers. However, the maximal of the luminescence characteristics is observed in the disordered structure of Na_0.5Eu_0.5MoO₄.

The phosphors Ag_xGd_{((2−x)/3)−0.3}Eu³⁺_0.3☐_(1−2x)/3WO₄ at the values of x (0.5–0) have better characteristics for emitters than Ag_xEu_(2−x)/3☐_(1−2x)/3WO₄ [20]. In Ag_xGd_{((2−x)/3)−0.3}Eu³⁺_0.3☐_(1−2x)/3WO₄ the emission of Eu³⁺ (λ_ex = 395 nm) increases with the Gd³⁺ content from 0.2 (x = 0.5) to 0.3 (x = 0.2) by a factor of 3 compared to the phosphors Ag_xEu_(2−x)/3☐_(1−2x)/3WO₄ (Figure 4). The replacement of Eu³⁺ cations by Gd³⁺ is accompanied by the destruction of infinite [EuO₈]_n chains and the formation of [Eu₂O₁₄]-dimers and/or isolated EuO₈ polyhedra inside them (Figure 8d).

The PLE spectra of Ag_xGd_{((2−x)/3)−0.3}Eu³⁺_0.3☐_(1−2x)/3WO₄ and Ag_xEu_(2−x)/3☐_(1−2x)/3WO₄ are characterized by broad (220–320 nm) and narrow (320–500 nm) excitation bands. The broad excitation line peaking at ~240 nm is due to charge transfer (CT) from the 2p orbital of oxygen to the 3d orbital of tungsten in the WO₄²⁻ tetrahedron [28,29] and coincides with the O²⁻ → Eu³⁺ CT band. Co-activation with Gd³⁺ and Sm³⁺ ions leads to a noticeable decrease in intensity of CT bands in comparison with Gd³⁺ single-doped phosphors (Figure 2a). All the phases reveal characteristic lines in the spectral region from 320 to 500 nm, which originate from 4f–4f transitions of Eu³⁺ and Sm³⁺ (for Sm-containing compounds). The main absorption bands in the PLE spectra are associated with the ⁷F₀ → ⁵L₆ Eu³⁺ transition at 395 nm and the ⁶H_5/2 → ⁴F_7/2 Sm³⁺ transition at 405 nm.

In the PL spectra of Ag_xR_{((2−x)/3)−0.3}Eu³⁺_0.30☐_(1−2x)/3WO₄ (R = Eu, Gd; x = 0.5, 0.286, 0.2) and A_xGS_yE (x = 0.5, 0.286, 0.2; y = 0.01, 0.02, 0.03) in the region of 570–650 nm, the characteristic lines are observed for Eu³⁺ cations due to the ⁵D₀ → ⁷F_J (J = 0, 1, 2) transitions (Figure 2b and Figure 3). In the luminescence spectra of all phases, the most intense electric dipole transition ⁵D₀ → ⁷F₂ is observed at ~615 nm, which indicates the absence of a center of symmetry in the polyhedra of Eu³⁺ cations [30,31]. The magnetic dipole transition ⁵D₀ → ⁷F₁ is observed at 590 nm. The intensity of this transition depends a little on the environment of the luminescent cation, since it has a magnetic dipole nature. The asymmetry coefficient (R/O), I_int(⁵D₀ → ⁷F₂)/I_int(⁵D₀ → ⁷F₁) [32] is usually used to estimate the symmetry of the Eu³⁺ polyhedron. The high values of the R/O ratio for the studied tungstates (Figure 2 and Figure 3) indicate a non-centrosymmetric local environment of Eu³⁺ in these phases. Further evidence of the non-centrosymmetric environment of europium cations in the PL structure of A_xGS_yE (x = 0.5, 0.286, 0.2; y = 0.01, 0.02, 0.03) is the presence of the ⁵D₀ → ⁷F₀ transition band at ~580 nm (insert in Figure 2b and Figure 3). In europium-containing phases, the ⁵D₀ → ⁷F₀ transition is forbidden for electric and magnetic dipole interactions. For this reason, the intensity of this transition is very low or not observed. The number of peaks in the region of the ⁵D₀ → ⁷F₀ transition indicates often the number of europium polyhedra in the compound. In the studied scheelites for all the compositions, only one peak is found in the region of the ⁵D₀ → ⁷F₀ transition (insert in Figure 2b and Figure 3). This is because the same environment of Eu³⁺ cations in the studied phases, and because europium cations occupy only one polyhedron [33].

In accordance with Table S1, the change of the symmetry from tetragonal for Ag_0.5R_0.2−ySm_yEu³⁺_0.3WO₄ (R = Eu, Gd, Sm) to monoclinic for A_xGS_yE (x = 0.286, 0.20; y= 0.01, 0.02, 0.03) practically does not change the value of the R/O ratio and the position of the peaks in the region of ⁵D₀ → ⁷F₀ transitions (Figure 2b and Figure 3). Figure 4 shows the comparative integral luminescence intensity of the ⁵D₀ → ⁷F₂ transition (Eu³⁺) for Ag_xEu³⁺_(2−x)/3☐_(1−2x)/3WO₄, Ag_xSm_{(2−x)/3−0.3}Eu_0.3☐_(1−2x)/3WO₄ and A_xGS_yE (x = 0.50, 0.286, 0.20; y = 0.01, 0.02, 0.03) (λ_ex = 395 nm). According to Figure 4 and Table S1, the luminescence intensity of the ⁵D₀ → ⁷F₂ transition is increased from ~2 times (x = 0.2, y = 0.01 and x = 0.286, y = 0.02) to ~4 times (x = 0.5, y = 0.01) during Gd³⁺ and Sm³⁺ co-activation. The integrated emission intensity of Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄ in the ⁵D₀ → ⁷F₂ transition is ~20% higher than that of the Gd₂O₂S:Eu³⁺ phosphor, which is used in industry (Figure 5). This confirms that Ag_0.286Gd_0.252Sm_0.02Eu_0.30WO₄ and Ag_0.20Gd_0.29Eu_0.30Sm_0.01WO₄ with a (3 + 1)D monoclinic incommensurately modulated structure are more attractive for use as red-emitting phosphors for LEDs.

However, an increase in Sm³⁺ concentration leads to a noticeable decrease in Eu³⁺ luminescence intensity at the ⁵D₀ → ⁷F₂ transition. The intensity of the ⁵D₀ → ⁷F₂ transition of the A_xGS_yE (x = 0.5, 0.286, 0.2) phases is reduced almost 24 and 73 times with increasing y from 0.03 to 0.20 (x = 0.5) and 0.30 (x = 0.20), respectively. A further increase in the concentration of Sm³⁺ cations leads to lower europium luminescence intensity (Figure 4 and Table S1). The phosphors with a high concentration of Sm³⁺ are not related to the materials with a high luminescence efficiency [34,35] due to the large number of Sm³⁺ energy levels in the visible region of the spectrum (Figure 9). The presence of a large number of energy levels in the Sm³⁺ cation promotes energy transfer between the Sm³⁺ and Eu³⁺ cations. However, the opposite effect of energy transfer to the Sm³⁺ cation is also possible, with further luminescence quenching at defects or due to cross relaxation. On the contrary, when Gd³⁺ cations are introduced into the matrix, the reverse energy transfer to the Gd³⁺ cation is impossible due to the large energy difference (3.96 eV) between the ground and first excited states (Figure 9). Thus, when energy is transferred from Gd³⁺ to Eu³⁺, reverse transfer to the Gd³⁺ cation is impossible. For this reason, the introduction of Gd³⁺ cations into a matrix with europium is more efficient than the introduction of Sm³⁺.

Figure 6 shows the luminescence spectra of Ag_0.20Gd_0.29Sm_0.01Eu_0.30☐_0.20WO₄ as a function of temperature. The luminescence intensity of the ⁵D₀ → ⁷F₂ transition in the region of ~615 nm decreases with increasing temperature and completely disappears at T = 473 K. As the temperature increases, the rapid quenching of the luminescence seems to be due to the nonradiative population of the excited states. The temperature dependencies of the luminescence intensity of the ⁵D₀ → ⁷F₂ transition at ~615 nm can be compared for the Ag_0.20Gd_0.29Sm_0.01Eu_0.30☐_0.20WO₄ and Ag_xR_{((2−x)/3)−0.3−y}Sm_yEu³⁺_0.3☐_(1−2x)/3WO₄ phases (Figure 7). The observed thermal quenching depends on both the crystal structure of the compounds and the Sm³⁺ concentration. The thermal behavior of PL in Ag_0.20Gd_{((2−x)/3)−0.3−y}Sm_yEu³⁺_0.30☐_0.20WO₄ (y = 0, 0.01, 0.30) with the incommensurately modulated (3 + 1) D monoclinic structure differs from one of the Ag_0.5R_0.20Eu_0.30WO₄ (R = Gd, Sm) phases with the tetragonal I4₁/a scheelite structure. The red luminescence of Eu³⁺ (transition ⁵D₀ → ⁷F₂) in the monoclinic phases with y = 0 and y = 0.01 is more thermally stable than that of the tetragonal phases. However, the full substitution of Gd³⁺ by Sm³⁺ (y = 0.30) in Ag_0.20R_0.30Eu_0.30WO₄ leads to lower thermal stability of the ⁵D₀ → ⁷F₂ luminescence of Eu³⁺ cation.

The use of synthesized phosphors offers advantages over commercially available Gd₂O₂S:Eu³⁺ phosphor. The PL spectrum of Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄ slightly shifted towards a smaller wavelength (Figure 5). The calculated CIE coordinates (Figure 10) for Gd₂O₂S:Eu³⁺ are (0.6312; 0.3562), whereas for Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄ they are (0.6402; 0.3528). It can be observed from the CIE coordinates that the synthesized phosphor is closer to the red standard of the National Television Standard Committee (NTSC) system (0.67; 0.33). The color purity, calculated by the formula reported in [36], shows the values as 89% for Gd₂O₂S:Eu³⁺ and 92% for Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄. Based on these results, the synthesized phosphors have potential for use in UV-excited LED packages.

5. Conclusions

Gd³⁺ and Sm³⁺ co-activation, the effect of cation substitutions and the formation of cation vacancies in Ag_xGd_{((2−x)/3)−0.3−y}Sm_yEu³⁺_0.30☐_{(1−2x−2y)/3}WO₄ (x = 0.5, 0.286, 0.2; y = 0.01, 0.02, 0.03) with a scheelite-type structure have been studied here as a factor for managing the luminescence properties. The XRD study of Ag_xGd_{((2−x)/3)−0.3−y}Eu³⁺_0.30Sm_y☐_{(1−2x−2y)/3}WO₄ (x = 0.286, 0.2; y = 0.01, 0.02, 0.03) shows that its structures have incommensurately modulated character, as well as the structures of the other cation deficient scheelite-related phases. The strongest line at 395 nm in the PLE spectra of the synthesized phosphors is in good agreement with GaN LED chips emitting in the near-ultraviolet range. The electric dipole transition ⁵D₀ → ⁷F₂ at ~615 nm has the maximum intensity for all the PL spectra, and its intensity increases from ~2 times (x = 0.2, y = 0.01 and x = 0.286, y = 0.02) to ~4 times (x = 0.5, y = 0.01) when co-activation with Gd³⁺ and Sm³⁺ is implemented. Ag_0.286Gd₀.₂₅₂Sm_0.02Eu₀.₃₀WO₄ and Ag₀.₂₀Gd₀.₂₉Sm_0.01Eu₀.₃₀WO₄ with a monoclinic incommensurately modulated (3 + 1)D monoclinic structure are very attractive as phosphor materials for converting near-UV radiation in LEDs. A considerable thermal quenching of the observed luminescence is revealed. The temperature dependence and behavior of the considered phases are influenced by the crystal structure of scheelite-type tungstates and the concentration of Sm³⁺ ions.