Luminescence Property Upgrading via the Structure and Cation

Sep 26, 2017 - The WO4 tetrahedra are significantly distorted, and W–O distances vary as 1.677–1.847 Å and 1.689–1.900 Å for Ag0.238Eu0.587□...
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Cite This: Chem. Mater. 2017, 29, 8811-8823

Luminescence Property Upgrading via the Structure and Cation Changing in AgxEu(2−x)/3WO4 and AgxGd(2−x)/3−0.3Eu0.3WO4

Vladimir A. Morozov,†,‡ Dmitry Batuk,† Maria Batuk,† Olga M. Basovich,§ Elena G. Khaikina,§,∥ Dina V. Deyneko,‡ Bogdan I. Lazoryak,‡ Ivan I. Leonidov,⊥ Artem M. Abakumov,# and Joke Hadermann*,† †

EMAT, University of Antwerp, Groenenborgerlaan 171, Antwerp, Belgium B-2020 Chemistry Department, Moscow State University, 119991, Moscow, Russia § Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Science, 670047, Ulan-Ude, Russia ∥ Buryat State University, 670000, Ulan-Ude, Russia ⊥ Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences, 620990, Ekaterinburg, Russia # Skolkovo Institute of Science and Technology, 143026, Moscow, Russia ‡

S Supporting Information *

ABSTRACT: The creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescence properties. AgxEu3+(2−x)/3□(1−2x)/3WO4 and AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5−0) scheelite-type phases were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder synchrotron X-ray diffraction. Transmission electron microscopy also revealed the (3 + 1)D incommensurately modulated character of AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.286, 0.2) phases. The crystal structures of the scheelite-based AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2) red phosphors have been refined from high resolution synchrotron powder X-ray diffraction data. The luminescence properties of all phases under near-ultraviolet (n-UV) light have been investigated. The excitation spectra of AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2) phosphors show the strongest absorption at 395 nm, which matches well with the commercially available n-UV-emitting GaN-based LED chip. The excitation spectra of the Eu2/3□1/3WO4 and Gd0.367Eu0.30□1/3WO4 phases exhibit the highest contribution of the charge transfer band at 250 nm and thus the most efficient energy transfer mechanism between the host and the luminescent ion as compared to direct excitation. The emission spectra of all samples indicate an intense red emission due to the 5D0 → 7F2 transition of Eu3+. Concentration dependence of the 5D0 → 7F2 emission for AgxEu(2−x)/3□(1−2x)/3WO4 samples differs from the same dependence for the earlier studied NaxEu3+(2−x)/3□(1−2x)/3MoO4 (0 ≤ x ≤ 0.5) phases. The intensity of the 5D0 → 7F2 emission is reduced almost 7 times with decreasing x from 0.5 to 0, but it practically does not change in the range from x = 0.286 to x = 0.200. The emission spectra of Gd-containing samples show a completely different trend as compared to only Eu-containing samples. The Eu3+ emission under excitation of Eu3+(5L6) level (λex = 395 nm) increases more than 2.5 times with the increasing Gd3+ concentration from 0.2 (x = 0.5) to 0.3 (x = 0.2) in the AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4, after which it remains almost constant for higher Gd3+ concentrations.

1. INTRODUCTION

along the c-axis. The AO8 polyhedra and BO4 tetrahedra share common vertices and form a 3D framework. The basic scheelite structure has a tetragonal I41/a symmetry. However, depending on the nature of the A and B cations and the n/m ratio, the symmetry can be lower. Such deviation from the 1:1 ratio between the (A′ + A′′) and (B′ + B′′) cations is possible (n < m) because the A sublattice can accommodate a large amount of vacancies (the extreme case is Eu2/3□1/3WO4 with

Scheelite related compounds (SRC) are compounds that can be described with a general formula (A′,A′′)n[(B′,B′′)O4]m, where A′, A′′ = alkali, alkali-earth, or rare-earth elements; B′, B′′ = W, Mo. SRC are attractive for many industrial applications, including solid oxide fuel cells, photocatalysts, and optical materials (white-light-emitting diodes (WLEDs)).1−5 Recently a new application field has emerged for these materials due to their ability to visualize temperature gradients with high accuracy and spatial resolution, making them excellent thermographic phosphors. 6 The scheelite-type ABO 4 (CaWO4) structure is built up by [...−AO8−BO4− ...] columns © 2017 American Chemical Society

Received: July 26, 2017 Revised: September 26, 2017 Published: September 26, 2017 8811

DOI: 10.1021/acs.chemmater.7b03155 Chem. Mater. 2017, 29, 8811−8823

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Chemistry of Materials 33% of vacancies (designated as □) in the A sublattice). The creation of vacancies in the A cation subset during the cation substitution in CaWO4 of Ca2+ by Eu3+ or of Ca2+ by the combination of Na+ and Eu3+/Gd3+ are shown in the numerous publications. The A cations can order with the vacancies, forming an incommensurately modulated structure.7−12 Recently Zhang et al.13 studied the luminescence properties in the stoichiometric scheelites (n = m) Ag0.5Gd0.5−xEux(W1−yMoy)O4 (x = 0.0−0.5, y = 0.0−1.0). They found that the materials preserve the tetragonal structure within the whole compositional range. Among the tungstates, the maximum emission intensity corresponds to the Eu3+ content of x = 0.3. In our research, we focused on the nonstoichiometric scheelites in the Ag−(Gd/Eu)−W−O system, to analyze the influence of the concentrations of Eu3+ and cation vacancies on the crystal structure and luminescence properties. Earlier we studied the concentration dependence of the 5D0 → 7F2 emission for NaxEu3+(2−x)/3□(1−2x)/3MoO4 (0 ≤ x ≤ 0.5) phases.7,8 The size of Ag+ (rVIII = 1.28 Å14) is closer to the size of Na+ (rVIII = 1.18 Å14) than the size of Li+ (rVIII = 0.92 Å14) and K+ (rVIII = 1.51 Å14) cations. Thus, the ordering between the Ag+ and the Eu3+ cations in AgxEu(2−x)/3□(1−2x)/3WO4 could be similar to the ordering between the Na+ and the Eu3+ cations in NaxEu3+(2−x)/3□(1−2x)/3MoO4 phases. First, we studied the AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5−0) phases with variable Eu3+ as well as variable □ concentration. Afterward, we fixed the Eu3+ concentration at x = 0.313 (using a varying Gd content) to study separately the influence of the cation vacancies on the luminescence, by investigating the series AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5−0), where the amount of vacancies decreases with increasing Ag:R ratio (R = Gd, Eu).

electron diffraction (ED) patterns were recorded on a CCD camera with a Philips CM20 transmission electron microscope operating at 200 kV. The elemental composition of AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2) samples was confirmed by energy dispersion X-ray (EDX) analysis performed with a Philips CM20 microscope with an Oxford INCA attachment. For the EDX analysis, the results were based on the AgL, EuL, and WL lines. High angle annular dark field scanning transmission electron microscopy (HAADF STEM) images were obtained at 300 kV on a FEI Titan 50−80 microscope equipped with a probe aberration corrector. Theoretical HAADF-STEM images were calculated using the QSTEM 2.0 software.18 Photoluminescence emission (PL) and excitation (PLE) spectra and decay curves were recorded on a Varian Cary Eclipse fluorescence spectrometer with a 75 kW xenon light source (pulse length τ = 2 μs, pulse frequency ν = 80 Hz, wavelength resolution 0.5 nm; PMT Hamamatsu R928). The powder was placed in a cell (⌀ 10 mm × 1 mm) with a light-reflecting coat. Photoluminescence spectra of all samples were measured under the same conditions. All measurements were performed at room temperature and corrected for the sensitivity of the spectrometer.

3. RESULTS 3.1. Elemental Composition. The elemental composition of AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2) samples was confirmed by EDX analysis performed inside the transmission electron microscope and linked with the ED analysis for each crystallite. The EDX study was performed at 4 points for 10 different crystallites for each sample. Results of the EDX analysis are summarized in Table S1 of the Supporting Information. Cation ratios found by EDX analysis are close to the intended AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2) compositions. 3.2. Preliminary Characterization. Figure 1 represents the PXRD patterns of the eight phases under investigation. The x = 0.5 compounds, Ag0.5Eu0.5WO4 and Ag0.5Gd0.3Eu0.2WO4, have tetragonal I41/a structure. The patterns of the x = 0.286 and x = 0.2 samples contain intense reflections corresponding to the scheelite superstructure and weaker satellite reflections. The patterns can be indexed in the monoclinic incommensurately modulated (3 + 1)D symmetry (superspace group I2/b(αβ0)00). At x = 0 the phases have monoclinic symmetry (space group C2/c) with the Eu2/3□1/3WO4-type structure.19 Unit cell parameters of the AgxEu3+(2−x)/3□(1−2x)/3WO4 and AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 samples determined from PXRD patterns using Le Bail decomposition in the C2/c space group and I2/b(αβ0)00 superspace group are listed in Table S2 of Supporting Information. The Gd-containing samples are isostructural to their Eu-containing analogues, and the substitution of Eu3+ (rVIII = 1.066 Å14) by Gd3+ (rVIII = 1.053 Å14) leads to a decrease in unit cell volume. As shown in Figure 2, decreasing x from 0.2 to 0.154 in AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (and thus further increasing the amount of cation vacancies) results in a two-phased sample. Increasing of R3+ concentration leads to a mixture of AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 phase with monoclinic incommensurately modulated (3 + 1)D structure and (Gd,Eu)2/3WO4 phase with Eu2/3WO4-type structure (Figure 2, Table S2 of Supporting Information). The unit cell parameters of the phase with incommensurately modulated structure observed on the PXRD patterns of the AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 samples with x = 0.154 and x = 0.125 slightly differ from the parameters for the phase with x = 0.2. Thus, in W-containing systems the formation of

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Ag x Eu 3+ (2−x)/3 □ (1−2x)/3 WO 4 (x = 0.5, 0.286, 0.2, 0) and AgxGd(2−x)/3−0.3Eu0.3WO4 (x = 0.5, 0.286, 0.2, 0.154, 0.125, 0) samples were synthesized by a solid state reaction from a stoichiometric mixture of Ag2WO4 and R2/3□1/3WO4 (R = Gd, Eu) at 823 K for 10 h followed by annealing at 1273 K for 96 h. Ag2WO4 was prepared by a solid state reaction from stoichiometric amounts of AgNO3 (99.99%) and WO3 (99.99%) at 623 K for 10 h followed by annealing at 823 K for 40 h. R2/3□1/3WO4 precursors were synthesized by a solid state reaction from R2O3 (99.99%) and WO3 at 873 K for 10 h followed by annealing at 1123 K for 80 h. 2.2. Characterization. Powder X-ray diffraction (PXRD) patterns were collected on a Huber G670 Guinier diffractometer (Cu Kα1 radiation, curved Ge(111) monochromator, transmission mode, image plate). PXRD data were collected over the 4−100° 2θ range with steps of 0.01°. To determine the lattice parameters, Le Bail decomposition15 was applied using the JANA2006 software.16 High-resolution synchrotron X-ray powder diffraction (SXPD) data were collected at the ID31 Beamline of European Synchrotron Radiation Facility (ESRF, Grenoble, France) at wavelength 0.400073 Å in the 2θ range 1.002−40°. The powder sample was thoroughly ground and placed in a thin-walled borosilicate glass capillary with a diameter of ∼0.3 mm. It was spun during the experiment. The SXPD patterns were recorded at 300 K. The crystal structures were refined by the Rietveld method in the JANA2006 package.16 Illustrations were produced with this package in combination with the program DIAMOND.17 Samples for electron microscopy were made by crushing the powder in an agate mortar and dispersing it in methanol. After treatment in an ultrasonic bath to disperse the crystallites, a few drops of the solution were put on a copper grid with a holey carbon film. Selected area 8812

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Figure 1. PXRD patterns of the AgxEu3+(2−x)/3□(1−2x)/3WO4 (a) and AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (b) samples. On the x = 0.286 and 0.2 patterns, the satellite reflections are indicated with red diamonds. Figure 2. Parts of PXRD patterns of AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 samples in 2θ ranges of 10−25.5° (a) and 26.5−34.5° (b). Red stars indicate the reflections of (Gd,Eu)2/3□1/3WO4 phase on the PXRD patterns of the samples with x = 0.154 and 0.125.

Ag0.125R0.625□0.25WO4 phases is not observed, unlike in Mocontaining systems, where Ag0.125R0.625□0.25MoO4 is formed.9 3.3. Electron Diffraction Study. The electron diffraction patterns taken along the most informative [001] zone axis are shown in Figure 3. The pattern of the x = 0.5 sample consists of very bright main reflections and weak satellites. The pattern can be indexed using five hklmn indexes given by diffraction vector H = ha* + kb* + lc* + mq1+ nq2, with two symmetrically dependent modulation vectors q1 ≈ 0.62a* + 0.82b* and q2 ≈ −0.82a* + 0.62b*. The hklmn, h + k + l = 2n, and hk0mn, h, k = 2n, reflection conditions are in agreement with the space group I41/a for the basic structure. No reflection conditions are imposed on the m and n indexes suggesting the (3 + 2)D superspace group I41/a(α,β,0)00(−β,α,0)00 (88.2.59.1 in the Stokes−Campbell−van Smaalen notations20). The presence of weak satellite reflections indicates ordering between the Ag and the Eu cations. However, the absence of the satellite reflections on the PXRD and SXPD patterns means that the order is very local. The patterns of x = 0.286 and x = 0.2 samples look similar. They can be indexed in the (3 + 1)D monoclinic symmetry using four hklm indexes. There are reflection conditions for the basic reflections, hklm: h + k + l = 2n, and hk0m, h, k = 2n, and no conditions for the satellite reflections suggesting the I2/b(α,β,0)00 superspace group and the modulation vector q ≈ 0.59a* + 0.82b*. 3.4. Refinement of the Ag 0.286 Eu 0.572 WO 4 and Ag0.2Eu0.6WO4 Crystal Structures. The crystal structures of the AgxEu3+(2−x)/3□(1−2x)/3WO4 phases (x = 0.5, 0.286, 0.2) were refined from SXPD data. On the SXPD patterns of the

x = 0.5 compound, only reflections of the basic scheelite subcell were present. Therefore, this structure was refined in the I41/a space group with random distribution of Ag+ and Eu3+ cations (Figure 4). The SXPD patterns of AgxEu3+(2−x)/3□(1−2x)/3WO4 phases (x = 0.286, 0.2) were indexed with the unit cell parameters, modulation vectors, and superspace symmetry determined from electron diffraction. According to the NaxEu3+(2−x)/3□(1−2x)/3MoO47 model of the occupation modulation in the A site, the crenel functions have been applied for the description of three atomic domains, Eu1, Eu2, and Ag, in the atomic position A (Diagram S1 of Supporting Information). All cations distributed on the single A position were restricted to the same coordinates (x, y, z), identical isotropic atomic displacement parameters (ADPs), and displacive modulation functions. The x40 coordinates (besides x40(Eu1) = 0.5) and lengths of atomic domains (δ) have been refined for each compound. The refinement of the coordinates and lengths of three atomic domains allow us to determine the composition of AgxEu3+(2−x)/3□(1−2x)/3WO4 phases (x = 0.286, 0.2) as Ag0.238Eu0.587□0.175WO4 and Ag0.157Eu0.614□0.229WO4, respectively. The experimental, calculated, and difference SXPD profiles of the AgxEu3+(2−x)/3□(1−2x)/3WO4 phases after the Rietveld refinement are shown in Figure 4. The crystallographic 8813

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Figure 3. [001] ED patterns of Ag x Eu 3+ (2−x)/3 □ (1−2x)/3 WO 4 (x = 0.5 (a), 0.286 (b), 0.2 (c)).

information is given in Table 1, and the atomic parameters and coefficients of the modulation functions are listed in Table S3 and Table S5 of Supporting Information. The main interatomic distances are given in Table S4 and Table S6 of Supporting Information. 3.5. Specific Features of the AgxEu(2−x)/3□(1−2x)/3WO4 Framework. A portion of the incommensurately modulated Ag0.238Eu0.587□0.175WO4 and Ag0.157Eu0.614□0.229WO4 structures and A-cation subset are shown in the [001] projection in Figures 5 and 6. The incommensurately modulated Ag0.238Eu0.587□0.175WO4 and Ag0.157Eu0.614□0.229WO4 structures both consist of two types of columns running along the c-axis: [...−(EuO8/AgO8)−WO4−...] and [...−□−WO4−...] (Figures 5a and 6a). However, the two structures differ by the distributions of the Ag/Eu cations and vacancies on the Acationic subset (Figures 5b and 6b). The specific coefficients α and β of the modulation vector q = αa* + βb* (Table 1) and the individual parameters of the three atomic domains (Eu1, Eu2, and Ag) constituting one

Figure 4. Experimental, calculated, and difference SXPD profiles after Rietveld refinement of AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5(a), 0.238(b), 0.157(c)). Insets show a low-angle part of the profile. Black and green bars mark the positions of the main and satellite reflections, respectively.

cationic position (Table S3 and Table S5 of Supporting Information) define the specific ordering of the cations for each modulated structure. Similar to the Na xEu3+(2−x)/3□(1−2x)/3MoO4 phases,7 two types of Eu aggregates can be distinguished in the AgxEu(2−x)/3□(1−2x)/3WO4 structures: [Eu3+2O14] dimers and the infinite [EuO8]n chains of EuO8 polyhedra which are parallel to the c axis. The [Eu3+2O14] dimers or diatomic Eu3+ clusters7 are formed only by Eu1 cations while both Eu cations (Eu1 and Eu2) participate in the formation of the infinite [EuO8]n chains. 8814

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Chemistry of Materials Table 1. Selected Crystallographic Data and Refinement Parameters for x = 0.286 and x = 0.200 Compounds x = 0.286 formula cell setting super space group lattice parameters: a (Å) b (Å) c (Å) γ (deg) V (Å3) q vector formula units, Z color density, g/cm3 Data Collection diffractometer radiation/wavelength (λ, Å) absorption coefficient, μ (mm−1) F(000) 2θ range (deg) step scan (2θ) Imax number of points Refinement refinement background function number of reflections (all/observed) among them: main 1st order satellites 2nd order satellites no. of refined parameters/refined atomic parameters R and Rw (%) for Bragg reflections (Rall/Robs) among them: main 1st order satellites 2nd order satellites RP and RwP; Rexp goodness of fit (ChiQ) max/min residual density (e × Å−3)

x = 0.200

Ag0.238Eu0.587□0.175WO4

Ag0.157Eu0.614□0.229WO4 monoclinic I2/b(αβ0)00

5.23488(1) 5.29271(1) 11.54043(2) 91.2928(1) 319.6358(9) 0.58320(2)a* + 0.82144(3)b* 4 white 7.5367

5.22517(1) 5.29395(1) 11.54137(1) 92.0154(1) 319.0575(9) 0.59223(2)a* + 0.79928(3)b*

7.4546

ID31 Beamline synchrotron/0.40073 12.371 617 0.5−40 0.002 62870 19750

11.893 608 0.5−40 0.001 60291 39500

Rietveld Legendre polynomials, 16 terms 632/624

579/576

128/128 255/250 249/246 81/39 4.23/4.17 and 3.55/3.55

113/113 241/240 225/223 81/39 3.77/3.72 and 3.05/3.05

2.98/2.98 and 2.80/2.80 5.18/5.03 and 3.82/3.81 6.24/6.19 and 4.18/4.17 6.86 and 9.18; 3.73 2.46 3.48/−3.69

2.86/2.86 and 2.38/2.38 4.64/4.57 and 4.03/4.03 4.39/4.24 and 3.14/3.14 7.54 and 10.23; 5.00 2.05 3.72/−3.29

In A-cationic subsets (Figures 5b and 6b), the [Eu12O14]dimers can be recognized as Eu1 pairs isolated from all other Eu3+ by Ag+ cations and vacancies. A single [Eu12O14] dimer surrounded by WO4 tetrahedra is shown in Figure 7a. The sections in between the dimers have a different width for different compositions (Figures 5b and 6b), and as a result, a different amount of [Eu12O14] dimers is observed in the Ag0.238Eu0.587□0.175WO4 and Ag0.157Eu0.614□0.229WO4 aperiodic structures. The ordering of the Ag/Eu cations and vacancies is different for Ag0.238Eu0.587□0.175WO4 and Ag0.157Eu0.614□0.229WO4 (Figure 7b,c). As can be deduced from Figure 7, the distribution of Ag/Eu cations and vacancies in the A position is periodic along the c axis but aperiodic (modulated) in the ab plane. Basically, the Ag0.157Eu0.614□0.229WO4 framework can be considered as consisting of groups of four AO8 polyhedra (EuO8−AgO8− 2EuO8 along the a axis and 3EuO8−AgO8 along the b axis) alternating with a cation vacancy. This ordering (four AO8 polyhedra + vacancy) is interrupted by the occasional formation of two vacancies along the b axis and less AO8

polyhedra. Similar fragments with alternation of four AO8 polyhedra and a vacancy are observed in Ag0.238Eu0.587□0.175WO4 (Figure 7c). However, this alternation is not violated by the formation of vacancy dimers along the b axis but by the occurrence of extra AO8 polyhedra. 3.6. HAADF-STEM Observations. HAADF-STEM images of the tetragonal Ag 0 . 5 Eu 0 . 5 WO 4 and monoclinic Ag0.2Eu0.6□0.2WO4 phases taken along the [001] direction are shown in Figure 8. The images consist of the rectangular pattern of bright dots corresponding to the A and B cation positions projected onto each other. The occupational modulation appears as a set of dark stripes. In the case of Ag0.5Eu0.5WO4 all dots have almost the same brightness, indicating the uniform distribution between Ag (ZAg = 47) and Eu (ZEu = 63). It indicates there is Ag+/Eu3+ ordering on a local scale and agrees with the observations on the electron diffraction patterns. 3.7. Luminescence Properties. Photoluminescence excitation (PLE) and emission (PL) spectra of the and AgxEu(2−x)/3□(1−2x)/3WO4 8815

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Figure 6. Portion of the 10a × 10b × 1c supercell in the ab projection (a) and A-cation subset (b) of the Ag0.157Eu0.614□0.229WO4 aperiodic structure. The W and O atoms are shown as yellow and red spheres. WO4 tetrahedra are not shown. The gray-and-white wave indicates the continuously changing chemical composition. [Eu12O14] dimers are marked by red ellipses.

AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4 phosphors are shown in Figures 9−11. PLE spectra for the Eu3+ emission in and AgxEu(2−x)/3□(1−2x)/3WO4 AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4 samples are shown in Figure 9a and Figure 10 upon monitoring the emission at 615 nm. PLE spectra for both series of phosphors show a broad excitation band in the range 220−320 nm as well as a group of sharp lines in the range 320−500 nm. The corresponding emission spectra of AgxEu(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.237, 0.156, 0) after excitation into the 7F0 → 5L6 transition of Eu3+ at λex = 395 nm and the dependence of the integral intensity of the 5D0 → 7F2 emission as a function of Ag+ content (x) are shown in Figure 9b. PL spectra of AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2, 0) under excitation of the WO42− anion at λex = 250 nm and excitation of the Eu3+(5L6) level at λex = 395 nm are shown in Figure 11. 5D0 →7F2 forced electric dipole transition at ∼615 nm region is dominant for all PL spectra.

Figure 5. Portion of the 7a × 11b × 1c supercell in the ab projection (a) and A-cation subset (b) of the Ag0.238Eu0.587□0.175WO4 aperiodic structure. The W and O atoms are shown as yellow and red spheres. WO4 tetrahedra are not shown. The gray-and-white wave indicates the continuously changing chemical composition. [Eu12O14] dimers are marked by red ellipses. 8816

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Figure 8. [001] HAADF-STEM image of tetragonal Ag0.5Eu0.5WO4 (top) and monoclinic Ag0.157Eu0.614□0.229WO4 (bottom) .

4. DISCUSSION Two types of scheelite-related phases with variable composition have been investigated: Ag x Eu (2−x)/3 □ (1−2x)/3 WO 4 and AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4. The substitution of Ag+ by R3+ (R = Eu, Gd) leads to significant structural distortions. The x = 0.5 samples have tetragonal I41/a scheelite structure. According to the SXPD data, there is no ordering between the Ag+ and R3+ cations. It agrees with the criterion published in ref 12, that cation ordering requires an ionic size difference of ∼0.35 Å (in the case of Ag0.5Eu0.5WO4, Δr ∼ 0.21 Å). However, the microscopic study (ED patterns and HAADF-STEM images) indicates local cation ordering in the structure. Introduction of cation vacancies in the AgxEu(2−x)/3□(1−2x)/3WO4 structures causes ordering between the cations and vacancies. Ag0.238Eu0.587□0.175WO4 and Ag0.157Eu0.614□0.229WO4 differ by the exact distributions of the Ag/Eu cations and vacancies on the cationic subset. The variation of different structural parameters can be illustrated using x4-plots (Figure S1 of Supporting Information). Figure S2 represents the x4‑plots of the Ag−O, Eu−O, and W−O bond lengths. The WO4 tetrahedra are significantly distorted, and W−O distances

Figure 7. (a) Single [Eu12O14] dimer surrounded by WO4 tetrahedra; (b) one layer (along z, 0 ≤ z ≤ 0.25) of the 7a × 11b supercell of Ag0.238Eu0.587□0.175WO4; (c) one layer (along z, 0 ≤ z ≤ 0.25) of the 10a × 10b supercell of Ag0.157Eu0.614□0.229WO4. WO4 tetrahedra and AgO8 and EuO8 polyhedra are shown as yellow, green, and red color, respectively. Blue squares indicate some of the vacancies in the structure. The O atoms are shown as red spheres.

The influence of the structure via Ag+ concentration on the D0 →7F2 emission intensity in AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2, 0) after excitation into the 5L6 excited state and excitation of WO42− anion has been investigated and is summarized in Figure 12. Luminescence decay times of the Eu3+ emission of AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4 phosphors have been recorded after excitation at 395 nm and by monitoring the 5D0 to 7 F2 transition intensity (Figure 13). 5

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Figure 11. Emission spectra for AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5, 0.237, 0.156, 0) upon excitation into the 5L6 level (λex = 395 nm) of Eu3+ (a) and CT band (λex = 250 nm) (b) at room temperature. All samples are measured under the same conditions.

Figure 9. Excitation (a) (λem = 615 nm) and emission (b) (λex = 395 nm) spectra of AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5 (1), 0.237 (2), 0.156 (3), 0 (4)) at room temperature and concentration dependence of the integral intensity of the 5D0 →7F2 emission on x (c). The inset shows the 5D0 →7F0 emission. The electronic transitions for the main excitation and emission peaks are indicated. All samples are measured under the same conditions.

Figure 12. Concentration dependence of the 5D0−7F2 integral emission intensity in AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.200, 0) upon excitation into the 5L6 level (λex = 395 nm) of Eu3+ and CT band (λex = 250 nm) at room temperature.

Figure 10. Excitation spectra (λem = 615 nm) AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5, 0.237, 0.156, 0) at room temperature. All samples are measured under the same conditions.

vary as 1.677−1.847 Å and 1.689−1.900 Å for Ag0.238Eu0.587□0.175WO4 and Ag0.156Eu0.615□0.229WO4, respectively (Table S4 and Table S6 of Supporting Information). Eu−O distances are larger for the Eu2 position (from 2.504 to 2.673 Å for x = 0.238; from 2.400 to 2.694 Å for x = 0.157) than that for the Eu1 position (from 2.360 to 2.536 Å; from 2.289 to 2.533 Å for x = 0.157) while the Ag−O distances practically do not change with x. The difference and similarity between the structures can be better understood by considering

Figure 13. Decay curves of AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5, 0.237, 0.156, 0) phosphors at room temperature shown on a logarithmic intensity scale.

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Chemistry of Materials the arrangement of Eu3+ cations (Figure S2, Table S4, and Table S6 of Supporting Information). The structure refinement reveals two types of Eu-aggregates in the AgxEu(2−x)/3□(1−2x)/3WO4 structures: [Eu3+2O14] dimers and infinite [EuO8]n chains of EuO8 polyhedra, parallel to the c axis and normal to the modulation vector q. In Ag0.238Eu0.587□0.175WO4 and Ag0.157Eu0.614□0.229WO4, the A cation ordering is different, clearly observed in the ab projections (Figure 14), resulting in different Eu−Eu

isolated from all other Eu-atoms by Ag+ cations and vacancies (Figure 14b). The vacancy concentration increasing with changing x = 0.238 to x = 0.157 in the AgxEu(2−x)/3□(1−2x)/3WO4 phases results in an increase of Eu1−Eu1 distances in [Eu12O14] dimers from 3.92 Å to 3.947− 3.959 Å (Figure 14b, Table S6 of the Supporting Information). Eu and Gd are very similar cations (r(Eu3+)VIII = 1.066 Å, r(Gd3+)VIII = 1.053 Å), and therefore the structures of the Gdcontaining compounds are isostructural to their Eu-containing analogous. PLE spectra for both series of phosphors show a broad excitation band in the range 220−320 nm as well as a group of sharp lines in the range 320−500 nm. The broad excitation bands centered at ∼250 nm are assigned to a charge transfer (CT) from the 2p orbital of oxygen to the 3d orbital of tungsten inside the WO42− group21,22 and overlap with the O2− → Eu3+ CT band. In the spectral region from 320 to 500 nm all phases show characteristic lines which originate from intraconfigurational 4f−4f transitions of Eu3+. In accordance with Figure 9a and Figure 10, the change of the crystal structure from tetragonal for Ag0.5(Gd,Eu)0.5WO4 to monoclinic for (Gd,Eu)2/3□1/3WO4 practically does not change the positions of the CT band and the bands of 7F0 → 5L6 (395 nm) and 7 F0 → 5D2 (465 nm) transitions but leads to a change in the intensity of these bands and in the CT/7F0 → 5L6 (or CT/7F0 → 5D2) ratio. The CT broad band is in general associated with the host and its charge transfer states and points to an energy transfer between the host and the luminescent ion. The intensity of the CT band in the PLE spectra of AgxEu(2−x)/3□(1−2x)/3WO4 and AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4 increases with decreasing x. In line with PXRD and TEM studies, the substitution of Ag+ by Eu 3 + or Gd 3 + in Ag x Eu ( 2 − x ) / 3 □ ( 1 − 2 x ) / 3 WO 4 and AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4 promotes the transformation of the structure from tetragonal Ag0.5(Gd,Eu)0.5WO4 to monoclinic for (Gd,Eu)2/3□1/3WO4 through the formation of phases with incommensurately modulated structures. Apparently, the structure has an influence on the efficiency of this energy transfer. The excitation spectra of the Eu2/3□1/3WO4 (Figure 9a) and Gd0.367Eu0.30□1/3WO4 phases exhibit the highest contribution of the charge transfer band and thus the most efficient energy transfer mechanism between the host and the luminescent ion as compared to direct excitation. Two reasons are possible for the intensity increase of the CT band: (1) increasing number of Eu3+/Gd3+ and (2) increasing number of cation vacancies. On the one hand, a similar dependence of the intensity of the CT band from Eu3+ number and A-cations size is found for Eu3+-doped AWO4 (A = Ca, Sr, Ba).23,24 Cation substitution in AWO4 of A2+ by Eu3+ leads to the creation of vacancies in the A cation subset too. On the other hand, the intensity of the CT band increases with increasing Eu3+ concentration for NaGd1−xEux(WO4)2 solid solutions.25 Thus, apparently the increase of the CT band intensity is due to the increase in the Eu3+/Gd3+ concentration. Changing the crystal structure from tetragonal for Ag0.5Eu0.5WO4 to monoclinic for Eu2/3□1/3WO4 with the different ordering of cations and vacancies the A cation subset leads to the distortions of the EuO8 and WO4 polyhedra. The W−O and Eu−O bond lengths change from 1.69 Å and 2.51− 2.58 Å for Ag0.5Eu0.5WO4 to 1.70−1.81 Å and 2.36−2.47 Å for Eu2/3□1/3WO4, respectively. This affects the effectiveness of O2− → W and O2− → Eu3+ charge transfers.

Figure 14. Schematic presentation of the A-cationic subset (3a × 5b × 1c supercell) in the Ag0.238Eu0.587□0.175WO4 (a) and Ag0.157Eu0.614□0.229WO4 (b). Calculated values of intercationic distances are indicated by arrows. [Eu12O14] dimers are marked by red ellipses.

interactions. The Eu−Eu distances in the AgxEu(2−x)/3□(1−2x)/3WO4 structures vary as 3.851−3.964 Å (x = 0.238) and 3.857−4.013 Å (x = 0.157) within the infinite [EuO8]n chains, whereas the shortest Eu3+−Eu3+ distances between [Eu12O14] dimers and the [EuO8]n chains is longer and is equal to about 5.22 Å (x = 0.238) and 5.12 Å (x = 0.157). Eu1−Eu1 interactions with Eu3+−Eu3+ distances of about 3.92 Å are observed within both types of Eu-aggregates in Ag0.238Eu0.587□0.175WO4 (Figure 14a) while these interactions are found in Ag0.157Eu0.614□0.229WO4 only in [Eu3+2O14] dimers 8819

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Chemistry of Materials PL spectra of the Ag x Eu (2−x)/3 □ (1−2x)/3 WO 4 and AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4 phases show the typical red emitting features of Eu3+ including 5D0 → 7FJ (J = 0, 1, 2) emissions in the spectral range from 570 to 650 nm (Figure 9b and Figure 11). For all PL spectra, the 5D0 → 7F2 forced electric dipole transition at ∼615 nm is dominant and indicates that the site symmetry of the Eu3+ position possesses no inversion center.26,27 The transition at 590 nm is the 5D0 → 7F1 magnetic dipole transition. Emission wavelengths of these 4f−4f transitions are only moderately influenced by the 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 symmetry of the site. CaWO4 with the tetragonal symmetry (space group I41/a) has C4h as 3D point group. The substitution of Ca2+ in CaWO4 by Ag+ and R3+ (R = Ln, Y, Bi) leads to the formation of double tungstates Ag0.5R0.5WO4 with random distribution of Ag+ and R3+ cations. Due to a distortion of the EuO8 polyhedron for AgxEu(2−x)/3□(1−2x)/3WO4 and AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4 (0 ≤ x ≤ 0.5) structures (Figure 9b and Figure 11), the Ag+ and R3+ cations occupy a site with maximally C2 site symmetry. The lack of inversion symmetry induces a high intensity of the hypersensitive 5D0 → 7F2 transition. Another consequence is the appearance of a band of the 5 D0 → 7F0 transition at 580 nm on the PL spectra of the AgxEu(2−x)/3□(1−2x)/3WO4 phases (inset in Figure 9b). Since the 5D0 → 7F0 transition is forbidden for both electric and magnetic dipole interactions, the intensity can be very low or even nonobservable. Yet, for C2 symmetry the transition is induced, so a peak can be expected at that position. As splitting of the initial and final levels, both characterized by J = 0, is not possible, observing more than one transition would be an indication of the presence of more than one nonequivalent site for the luminescent Eu3+ ions. Since we observe only one peak for all AgxEu3+(2−x)/3□(1−2x)/3WO4 phases, the local environment of the Eu3+ions probably remains the same over the whole crystal.28 The position of the 5D0 → 7F0 band (580 nm for x = 0, 0.286; 580.28 nm for x = 0.2) and the 5 D0 → 7F2/5D0 → 7F0 intensity ratio does not change practically with the changing the crystal structure from tetragonal for Ag 0 . 5 Eu 0 . 5 WO 4 to monoclinic for Eu2/3□1/3WO4 (Figure 9b). The intensity decreasing of the 5 D0 → 7F0 band is related with total decreasing of the Eu3+ emission efficiency. Figure 9c shows the concentration dependence of the 5 D 0 → 7 F 2 emission on the PL spectra of the AgxEu(2−x)/3□(1−2x)/3WO4 phases as a function of Eu3+ content in the range from x = 0.5 to x = 0. Concentration dependence of the 5D0 →7F2 emission for AgxEu(2−x)/3□(1−2x)/3WO4 samples differs from the same dependence for the early studied NaxEu3+(2−x)/3□(1−2x)/3MoO4 (0 ≤ x ≤ 0.5) phases.8 The intensity of the 5D0 →7F2 emission is reduced almost 7 times with decreasing x from 0.5 to 0 with the small local increasing intensity in the range from x = 0.238 to x = 0.157. There are two possible reasons for the decreasing 5D0 → 7F2 emission intensity of AgxEu(2−x)/3□(1−2x)/3WO4 red phosphors as x varies from 0.5 to 0 (Figure 9c): (1) a quenching effect because usually the luminescence intensity decreases with increasing concentration of the luminescent centers and (2) changing the tetragonal structure with a random distribution of Eu3+ and Ag+ cations (Eu:W = 1:2) to a monoclinic structure with an ordered distribution of Eu3+ and cation vacancies (Eu:W = 2:3) via a

(3 + 1)D incommensurately modulated structure with an ordered distribution of vacancies and Eu3+ and Ag+ cations. Concentration quenching is caused by the energy transfer between luminescent centers, and these energy-transfer chains trigger the energy migration to the energy sink such as crystalline defects or trace ions. The concentration quenching effect is the cause of the overall trend of decreasing intensity in the range x = 0.5−0. However, an intensity increase for x = 0.157 in comparison with x = 0.238 is an anomaly for this trend and is only due to specific features of the AgxEu(2−x)/3□(1−2x)/3WO4 framework. Substitution of Ag+ (rVIII(Ag+) = 1.28 Å14) by Eu3+ (rVIII (Eu3+) = 1.066 Å14) leads to a decreasing unit cell volume, whereas the Eu−Eu distances in the AgxEu(2−x)/3□(1−2x)/3WO4 structures increase in both types of Eu aggregates (Figure 14b, Table S6 of the Supporting Information) and reduce the nonradiative energy transfer among Eu3+ ions. Earlier, a correlation was proposed between the relative amounts of [Eu3+2O14] dimers and the characteristic parameters of the Eu3+-centered luminescence, based on six (3 + 1)D incommensurately modulated phases NaxEu3+(2−x)/3□(1−2x)/3MoO4 (0.015 ≤ x ≤ 0.25) with ordered distribution of vacancies and Na and Eu cations.7 Luminescence Eu ) quantum yields, parameters (overall (Q LEu ) and intrinsic (Q Eu Eu(5D0) lifetimes (τobs)) increase with growing relative amount of Eu3+ dimers. The maximal values correspond to disordered Na0.5Eu0.5MoO4, while the lowest value of both parameters is observed for Eu2/3□1/3MoO4. The lowest value of the 5 D0 → 7F2 emission intensity (Figure 9c) is observed for Eu2/3WO4 whose structure contains Eu cations only in infinite [EuO8]n chains, and the isolated [Eu3+2O14] dimers are absent (Figure 15). The number of Eu3+ ions forming [Eu3+2O14] dimers in the AgxEu(2−x)/3□(1−2x)/3WO4 framework increases with decreasing x from x = 0.238 to x = 0.157, from 15.56% (28Eu1 out of a total of 180Eu = 129Eu1 + 51Eu2 in the

Figure 15. Portion of the Eu2/3□1/3WO4 scheelite-type structure in the ab projection; (b) A-cation subset in the Eu2/3□1/3WO4 structure. The W and O atoms are shown as yellow and red spheres. WO4 tetrahedra are missing. as and cs refer to the scheelite-type unit cell. 8820

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Chemistry of Materials 7a × 11b supercell) to 28.33% (68Eu1 out of a total of 240Eu = 164Eu1 + 76Eu2 in the 10a × 10b supercell). Thus, the local increase of 5D0 → 7F2 emission intensity for x = 0.157 in comparison with that for x = 0.238 can be associated with a larger amount of [Eu3+2O14] dimers. Figure 11 shows the PL spectra of AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2, 0) samples under excitation of WO42− anion at λex = 250 nm and excitation of Eu3+(5L6) level at λex = 395 nm. As it can be seen under both excitations (λex = 250 nm and λex = 395 nm), a completely different trend is observed for Gd-containing samples as compared to only Eu-containing samples. The Eu3+ emission under excitation of Eu3+(5L6) level (λex = 395 nm) increases more than 2.5 times with the Gd3+ concentration from 0.2 (x = 0.5) to 0.3 (x = 0.2) in AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4, after which it remains almost constant for higher Gd3+ concentrations (Figure 12a). The 5D0 → 7F2 emission intensity under excitation of the WO42− anion (λex = 250 nm) also increases upon increasing Gd3+ concentration and with the changing of the crystal structure (Figure 12b). However, a maximum of 5D0 → 7F2 emission intensity at λex = 250 nm is observed for the Gd0.367Eu0.3□1/3WO4 monoclinic structure and is close to same intensity at λex = 395 nm. Earlier Zhang et al.13 found that the maximum 5D0 → 7F2 emission intensity for Ag0.5Gd0.5−xEuxWO4 solid solutions corresponds to the Eu3+ content of x = 0.3. A similar value of x = 0.3 (45 mol % Eu3+) for maximum emission intensity was found for La2/3□1/3WO4:Eu3+ solid solutions.29 Gdcontaining AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 turn out to be more efficient emitters than AgxEu(2−x)/3□(1−2x)/3WO4 in the whole x range. This confirms that Ag0.2Gd0.3Eu0.3□0.2WO4 with the (3 + 1)D monoclinic structure and Gd0.367Eu0.3WO4 with 3D monoclinic structure are exceptionally attractive as near-UV converting phosphors applied as a red-emitting phosphor for LEDs. Thus, the removal of the concentration quenching effect through the substitution of Ag+ by Gd3+ with the optimal concentration of Eu3+ results in a positive effect on the luminescence properties. The lifetime of the Eu3+ 5D0 → 7 F2 emission for AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 was measured at different Gd3+ concentrations. The normalized decay curves recorded for the AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 phosphors under excitation at 395 nm are shown in Figure 13. All samples show a monoexponential decay, and fitting the expression t I(t ) = I0 exp( − τ ) to the experimental data yields decay constants in the range from τ = 329 μs (x = 0.367) to τ = 384 μs (x = 0.2). The decay curves indicate that all Eu3+ ions possess the same atom environment. The lifetime values for AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 reach a maximum at the Gd3+ content equal to 0.3 (x = 0.2). Then, the values rapidly decrease upon increasing the Gd3+ content. Maximum lifetime value is observed for Ag0.2Gd0.3Eu0.3□0.2WO4 with the (3 + 1)D monoclinic structure while the minimum lifetime value is characterized of 3D Gd0.367Eu0.3WO4 monoclinic structure. Apparently, the structure influences the lifetime of the 5D0 → 7F2 emission in AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 phosphors.

investigated as a factor for controlling the scheelite-type structure and luminescence properties. Transmission electron microscopy revealed the (3 + 1)D incommensurately modulated character of Ag x Eu 3 + ( 2 − x ) / 3 □ ( 1 − 2 x ) / 3 WO 4 (x = 0.286, 0.2) phases. The crystal structures of the scheelite-based AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2) red phosphors have been refined from high resolution synchrotron powder X-ray diffraction data. Introduction of cation vacancies in the AgxEu(2−x)/3□(1−2x)/3WO4 structures causes the ordering between the cations and the vacancies. Ag0.238Eu0.587□0.175WO4 and Ag0.157Eu0.614□0.229WO4 aperiodic structures differ by the distribution of the Ag/Eu cations and vacancies on the cationic subset. Two types of Eu aggregates can be distinguished in the Ag x Eu (2−x)/3 □ (1−2x)/3 WO 4 structures: [Eu3+2O14] dimers and infinite [EuO8]n chains of EuO8 polyhedra. The number of Eu3+ ions forming [Eu3+2O14] dimers in the AgxEu(2−x)/3□(1−2x)/3WO4 framework increases with decreasing x. The luminescence properties of scheelite related compounds under near-ultraviolet (n-UV) light are essentially affected by the presence of Eu3+ aggregates. The excitation spectra of AgxGd(2−x)/3−0.3Eu 3+0.3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2) phosphors show the strongest absorption at 395 nm, which matches well with the commercially available n-UV-emitting GaN-based LED chip. The Eu3+ emission under excitation of the Eu3+(5L6) level (λex = 395 nm) increases more than 2.5 times when increasing the Gd3+ concentration from 0.2 (x = 0.5) to 0.3 (x = 0.2) in AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4, after which it remains almost constant for higher Gd3+ concentrations. The emission spectra of all samples indicate an intense red emission due to the 5D0 → 7F2 transition of Eu3+. The intensity of the 5D0 → 7 F2 emission is reduced almost 7 times with decreasing x from 0.5 to 0. The local increase of 5D0 → 7F2 emission intensity for x = 0.156 in comparison with x = 0.237 can be associated with a large amount of [Eu3+2O14] dimers. The emission spectra of the Gd-containing samples show a completely different trend as compared to only Eu-containing samples.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03155. EDX analysis results of AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2) compounds (Table S1). Schema of the occupation modulation function of the A position of scheelite-type AgxEu3+(2−x)/3□(1−2x)/3WO4 structures (Diagram S1). Space symmetry and unit cell parameters for AgxEu(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2, 0) and AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2, 0.154, 0.125, 0) determined from PXRD patterns (Table S2). Atomic coordinates, amplitudes of Fourier components for the occupational and displacive modulation functions, and isotropic atomic displacement parameters for Ag0.237Eu0.588□0.175WO4 structure (Table S3). Main interatomic distances for Ag0.237Eu0.588□0.175WO4 (Table S4). Atomic coordinates, amplitudes of Fourier components for the occupational and displacive modulation functions, and isotropic atomic displacement parameters for Ag0.156Eu0.615□0.229WO4 structure (Table S5). Main interatomic distances for Ag0.156Eu0.615□0.229WO4 (Table S6). x4‑plots of Ag−O, Eu−O, and W−O bond lengths

5. CONCLUSION The creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are 8821

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for the Ag0.237Eu0.588□0.175WO4 (a) and Ag0.156Eu0.615□0.229WO4 (b) structures (Figure S1). x4‑plots of Eu−Eu distances for the Ag0.237Eu0.588□0.175WO4 (a) and Ag0.156Eu0.615□0.229WO4 (b) structures (Figure S2) (PDF)

an incommensurately modulated scheelite-type structure. J. Solid State Chem. 2006, 179, 1183−1191. (10) Morozov, V.; Arakcheeva, A.; Redkin, B.; Sinitsyn, V.; Khasanov, S.; Kudrenko, E.; Raskina, M.; Lebedev, O.; Van Tendeloo, G. Na2/7Gd4/7MoO4: a Modulated Scheelite-Type Structure and Conductivity Properties. Inorg. Chem. 2012, 51, 5313−5324. (11) Morozov, V.; Bertha, A.; Meert, K.; Van Rompaey, S.; Batuk, D.; Martinez, G. T.; Van Aert, S.; Smet, P. E.; Raskina, M. V.; Poelman, D.; Abakumov, A. M.; Hadermann, J. Incommensurate modulation and luminescence in the CaGd2(1‑x)Eu2x(MoO4)4(1‑y)(WO4)4y(0 ≤ x ≤ 1, 0 ≤ y ≤ 1) red phosphors. Chem. Mater. 2013, 25, 4387−4395. (12) Abakumov, A. M.; Morozov, V. A.; Tsirlin, A. A.; Verbeeck, J.; Hadermann, J. Cation ordering and flexibility of the BO42− tetrahedra in incommensurately modulated CaEu2(BO4)4 (B = Mo, W) Scheelites. Inorg. Chem. 2014, 53, 9407−9415. (13) Zhang, Y.; Jiao, H.; Du, Y. Luminescent properties of HTP AgGd1‑xW2O8:Eux3+ and AgGd1‑x(W1‑yMoy)2O8:Eux3+ phosphor for white LED. J. Lumin. 2011, 131, 861−865. (14) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (15) Le Bail, A.; Duroy, H.; Fourquet, J. L. Ab-initio structure determination of LiSbWO6 by X-ray powder diffraction. Mater. Res. Bull. 1988, 23, 447−452. (16) Petricek, V.; Dusek, M.; Palatinus, L. Crystallographic Computing System JANA2006: General features. Z. Kristallogr. Cryst. Mater. 2014, 229, 345−352. (17) Brandenburg, K. DIAMOND, Version. 2.1c; Crystal Impact GbR: Bonn, Germany, 1999. (18) Koch, C. Determination of core structure periodicity and point defect density along dislocations; Arizona State University: Phoenix, AZ, 2002. (19) Templeton, D. H.; Zalkin, A. Crystal structure of europium tungstate. Acta Crystallogr. 1963, 16, 762−766. (20) Van Smaalen, S.; Campbell, B. J.; Stokes, H. T. Equivalence of superspace groups. Acta Crystallogr., Sect. A: Found. Crystallogr. 2013, 69, 75−90. (21) Shi, S.; Liu, X.; Gao, J.; Zhou, J. Spectroscopic properties and intense red-light emission of (Ca,Eu,M)WO4 (M = Mg, Zn, Li). Spectrochim. Acta, Part A 2008, 69, 396. (22) Blasse, G. The luminescence of Closed-shell transition-metal complexes. New developments.Structure and bonding; Springer-Verlag: 1980; pp 1−41. (23) Jiang, P.; Gao, W.; Cong, R.; Yang, T. Structural investigation of the A-site vacancy in scheelites and the luminescence behavior of two continuous solid solutions A1−1.5xEux□0.5xWO4 and A0.64−0.5yEu0.24Liy□0.12−0.5yWO4 (A = Ca, Sr; □ = vacancy). Dalton Trans 2015, 44, 6175−6183. (24) Barbosa, H. P.; Kai, J.; Silva, I. G. N.; Rodrigues, L. C. V.; Felinto, M. C. F. C.; Hölsä, J.; Malta, O. L.; Brito, H. F. Luminescence investigation of R3+-doped alkaline earth tungstates prepared by a soft chemistry method. J. Lumin. 2016, 170, 736−742. (25) Durairajan, A.; Suresh Kumar, J.; Thangaraju, D.; Valente, M. A.; Moorthy Babu, S. Photoluminescence properties of sub-micron NaGd1‑xEux(WO4)2 red phosphor for solid state lightings application: Derived by different synthesis routes. Superlattices Microstruct. 2016, 93, 308−321. (26) Wiglusz, R. J.; Bednarkiewicz, A.; Strek, W. Role of the Sintering Temperature and Doping Level in the Structural and Spectral Properties of Eu-Doped Nanocrystalline YVO4. Inorg. Chem. 2012, 51, 1180−1186. (27) Tanner, P. Some misconceptions concerning the electronic spectra of tri-positive europium and cerium. Chem. Soc. Rev. 2013, 42, 5090−5101. (28) Blasse, G.; Bril, A.; Nieuwpoort, W. C. On the Eu3+ fluorescence in mixed metal oxides: Part IThe crystal structure sensitivity of thr intensity ratio of electric and magnetic dipole emission. J. Phys. Chem. Solids 1966, 27, 1587−1592.

AUTHOR INFORMATION

Corresponding Author

*Joke Hadermann. Tel.: +32-32653245. Fax.: +32-32653257. E-mail: [email protected]. ORCID

Vladimir A. Morozov: 0000-0002-0674-2449 Dmitry Batuk: 0000-0002-6384-6690 Ivan I. Leonidov: 0000-0002-6635-4747 Artem M. Abakumov: 0000-0002-7135-4629 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by FWO (project G039211N), Flanders Research Foundation. V.A.M. is grateful for financial support of the Russian Foundation for Basic Research (Grant 15-03-07741). E.G.K. and O.M.B. are grateful for financial support of the Russian Foundation for Basic Research (Grants 13-03-01020 and 16-03-00510). D.V.D. is grateful for financial support of the Russian Foundation for Basic Research (Grant 16-33-00197) and the Foundation of the President of the Russian Federation (Grant MK-7926.2016.5.). We are grateful to the ESRF for granting the beamtime. Experimental support of Andy Fitch at the ID31 beamline of ESRF is kindly acknowledged.

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REFERENCES

(1) Kim, T.; Kang, S. Potential red phosphor for UV-white LED device. J. Lumin. 2007, 122−123, 964−966. (2) Hwang, K.-S.; Jeon, Y.-S.; Hwangbo, S.; Kim, J.-T. Red-emitting LiEuW2O8 phosphor for white emitting diodes prepared by sol−gel process. Opt. Appl. 2009, 39, 375−383. (3) Yan, B.; Wu, J.-H. NaY(MoO4)2:Eu3+ and NaY0.9Bi0.1(MoO4)2:Eu3+ submicrometer phosphors: Hydrothermal synthesis assisted by room temperature-solid state reaction, microstructure and photoluminescence. Mater. Chem. Phys. 2009, 116, 67− 71. (4) Haque, M. M.; Lee, H.-I.; Kim, D.-K. Luminescent properties of Eu3+-activated molybdate-based novel red-emitting phosphors for LEDs. J. Alloys Compd. 2009, 481, 792−796. (5) Guo, C.; Gao, F.; Xu, Y.; Liang, L.; Shi, F. G.; Yan, B. Efficient red phosphors Na5Ln(MoO4)4:Eu3+ (Ln = La, Gd and Y) for white LEDs. J. Phys. D: Appl. Phys. 2009, 42, 095407. (6) Meert, K. W.; Morozov, V. A.; Abakumov, A. M.; Hadermann, J.; Poelman, D.; Smet, P. F. Energy transfer in Eu3+ doped scheelites: use as thermographic phosphor. Opt. Express 2014, 22, A961−A972. (7) Arakcheeva, A.; Logvinovich, D.; Chapuis, G.; Morozov, V.; Eliseeva, S. V.; Bunzli, J.-C. G.; Pattison, P. The luminescence of NaxEu3+(2−x)/3MoO4 scheelites depends on the number of Eu-clusters occurring in their incommensurately modulated structure. Chem. Sci. 2012, 3, 384−390. (8) Morozov, V. A.; Lazoryak, B. I.; Shmurak, S. Z.; Kiselev, A. P.; Lebedev, O. I.; Gauquelin, N.; Verbeeck, J.; Hadermann, J.; Van Tendeloo, G. Influence of the structure on the properties of NaxEuy(MoO4)z red phosphors. Chem. Mater. 2014, 26, 3238−3248. (9) Morozov, V. A.; Mironov, A. V.; Lazoryak, B. I.; Khaikina, E. G.; Basovich, O. M.; Rossell, M. D.; Van Tendeloo, G. Ag1/8Pr5/8MoO4: 8822

DOI: 10.1021/acs.chemmater.7b03155 Chem. Mater. 2017, 29, 8811−8823

Article

Chemistry of Materials (29) Liu, X.; Hou, W.; Yang, X.; Shen, Q. Pure-phase La2(WO4)3:Eu3+ nanocrystals and spindle-like NaLa(WO4)2:Yb3+/ Er3+ nano/microcrystals: selective synthesis, morphologies and photoluminescent properties. Dalton Trans 2013, 42, 11445−11454.

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DOI: 10.1021/acs.chemmater.7b03155 Chem. Mater. 2017, 29, 8811−8823