Incommensurately Modulated Structures and Luminescence

Article Cite This: Chem. Mater. 2018, 30, 4788−4798

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Incommensurately Modulated Structures and Luminescence Properties of the AgxSm(2−x)/3WO4 (x = 0.286, 0.2) Scheelites as Thermographic Phosphors Vladimir Morozov,† Dina Deyneko,† Olga Basovich,‡ Elena G. Khaikina,‡,§ Dmitry Spassky,∥ Anatolii Morozov,⊥ Vladimir Chernyshev,†,# Artem Abakumov,∇ and Joke Hadermann*,○

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†

Chemistry Department, ∥Skobeltsyn Institute of Nuclear Physics, and ⊥Department of Material Science, 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 # A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, 31 Leninsky prospect, 119991, Moscow, Russia ∇ Skolkovo Institute of Science and Technology, Nobel street 3, 143026, Moscow, Russia ○ EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium S Supporting Information *

ABSTRACT: Ag+ for Sm3+ substitution in the scheelite-type AgxSm(2−x)/3□(1−2x)/3WO4 tungstates has been investigated for its influence on the cation-vacancy ordering and luminescence properties. A solid state method was used to synthesize the x = 0.286 and x = 0.2 compounds, which exhibited (3 + 1)D incommensurately modulated structures in the transmission electron microscopy study. Their structures were refined using high resolution synchrotron powder X-ray diffraction data. Under near-ultraviolet light, both compounds show the characteristic emission lines for 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2, and 11/2) transitions of the Sm3+ ions in the range 550−720 nm, with the J = 9/2 transition at the ∼648 nm region being dominant for all photoluminescence spectra. The intensities of the 4G5/2 → 6H9/2 and 4 G5/2 → 6H7/2 bands have different temperature dependencies. The emission intensity ratios (R) for these bands vary reproducibly with temperature, allowing the use of these materials as thermographic phosphors.

1. INTRODUCTION White light emitting diodes (WLEDs) are promising solid-state lighting sources that have already replaced for a large part conventional incandescent and fluorescent lamps. They are reliable, have a long lifetime, consume only minor amounts of energy, and are environmentally friendly.1,2 Among the materials with high potential for WLED applications (and also for solid state lasers) are the molybdate and tungstate phosphors with a scheelite-type (CaWO4) structure, doped by rare earth elements.3−15 They have broad, intense absorption bands in the near-UV region originating from a charge transfer from oxygen to Mo/W. For example, for NaEu(WO4)2 the emission peak (∼615 nm) intensity and the integral emission intensity (proportional to the quantum efficiency) are, respectively, about 8.5 and 5.0 times those (∼626 nm) of the com mercial Y2O2S:Eu3+ phosphor.6 Recently, the possibility to use them to visualize temperature gradients with high spatial resolution and accuracy has opened a new possibility to use them as thermographic phosphors.16−18 Thermographic phosphors exhibit temperature-dependent luminescence features, such as a variation in the location of excitation/emission peaks, or in the fluorescence lifetime, emission intensity, or anisotropy.19−26 They overcome some © 2018 American Chemical Society

drawbacks of existing techniques such as thermocouples, thermochromic liquid crystals, and noninvasive pyrometry.19 One of the major advantages over electronic methods is that the temperature can be analyzed instantaneously with high sensitivity and accuracy. Several materials containing Sm3+ cations have already been proposed as thermographic phosphors.27−30 In this paper, we investigate the temperature dependence of the luminescence of the scheelite-type AgxSm(2−x)/3□(1−2x)/3WO4 in correlation with a varying Ag:Sm ratio and the corresponding structural changes. Scheelite-related compounds conform to the formula (A′,A″)n[(B′,B″)O4]m, with A′ and A″ being alkali, alkaline earth, or rare earth elements, and B′ and B″ being W and Mo. The scheelite-type ABO4 (CaWO4) structure consists of a three-dimensional (3D) framework with, along the c-axis, columns of vertex sharing AO8 polyhedra and BO4 tetrahedra. CaWO4 has tetragonal symmetry, with space group I41/a. The scheelite symmetry can be lower, depending on the nature of cations or the presence of ordered vacancies. Received: May 14, 2018 Revised: June 20, 2018 Published: June 21, 2018 4788

DOI: 10.1021/acs.chemmater.8b02029 Chem. Mater. 2018, 30, 4788−4798

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Chemistry of Materials

Figure 1. (a) [001] ED pattern of Ag0.2Sm0.6□0.2WO4 and (b) indexation schema.

The scheelite-type structure can accommodate a high amount of vacancies at the A-cation position, leading to a cation ratio with (A′ + A″) < (B′ + B″), i.e., n < m. The A cations and vacancies can be ordered, frequently forming incommensurately modulated structures.31−33 Earlier, we studied the structures and the luminescence properties of the Eu-based scheelite-related phases AgxEu(2−x)/3 □(1−2x)/3WO4 and AgxGd(2−x)/3−0.3Eu0.3□(1−2x)/3WO4.33 The strongest absorption occurred at 395 nm, which provides a good math with commercial GaN-based near-UV-emitting LED chips. Lowering x from 0.5 to 0 reduced the intensity of the 5D0 → 7F2 emission for the AgxEu(2−x)/3□(1−2x)/3WO4 samples by almost 7 times due to the concentration quenching effect, while it remained almost constant in the interval from x = 0.238 to x = 0.157. We proposed that the higher number of [Eu3+2O14] dimers in the structure caused the higher 5D0 → 7F2 emission intensity for x = 0.157 compared to x = 0.238. The substitution of Ag+ by Gd3+ removed the concentration quenching effect.

Figure 2. Illustration of different model refinements performed from SXPD data for Ag0.196Sm0.601□0.229WO4. (a) Wave occupation function approximation (harmonic model). (b) Crenel-2H approximation. (c) Crenel-3H approximation. The lower-angle parts of the experimental, calculated, and difference SXPD profiles with the indexation of reflections are shown. Black and green bars mark the positions of the main and satellite reflections, respectively.

At the optimal Eu3+ concentration, we observed improvement in the luminescence properties, and the 5D0 → 7F2 Eu3+ emission increased more than 2.5 times from Ag0.5Gd0.2Eu0.3□0WO4 to Ag0.2Gd0.3Eu0.3□0.2WO4. In the current paper, we focus on the nonstoichiometric AgxSm(2−x)/3□(1−2x)/3WO4 scheelites with x = 0.286 and 0.2, to study the relation between the Sm3+ content, amount 4789


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Chemistry of Materials of cation vacancies, crystal structure, and luminescence properties.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Ag x Sm (2−x)/3 □(1−2x)/3WO4 (x = 0.286, 0.2) samples were synthesized using a solid state reaction from a stoichiometric mixture of Ag2WO4 and Sm2/3WO4 at 823 K for 10 h, followed by annealing at 1273 K for 96 h. Ag2WO4 was prepared using a solid state reaction from stoichiometric amounts of AgNO3 (99.99%) and WO3 (99.99%) at 623− 823 K for 60 h. The Sm2/3WO4 precursor was synthesized using a solid state reaction from Sm2O3 (99.99%) and WO3 at 873 K for 10 h followed by annealing at 1123 K for 80 h. 2.2. Characterization. The scanning electron microscopy− energy dispersive X-ray (SEM−EDX) analysis of the Ag:Sm:W ratio was performed for both samples using a Jeol JSN-6490LV scanning electron microscope equipped with an EDX spectrometer (Oxford Instruments). 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 a 2θ range 4−100° with steps of 0.01°. To determine the lattice parameters, Le Bail decomposition34 was applied using the JANA2006 software.35 High-resolution synchrotron X-ray powder diffraction (SXPD) data were collected at the ID22 Beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) using a constant wavelength of λ = 0.399927(4) Å and eight scintillation detectors, each preceded by a Si(111) analyzer crystal. The powder sample was ground and deposited in a thin-walled borosilicate glass capillary with a diameter of ∼0.3 mm. The capillary was spun during the experiment. The SXPD patterns were recorded at 300 K. The crystal structures were refined using the Rietveld method as implemented in the JANA2006 package.35 Illustrations were created with JANA2006 and DIAMOND.36 Samples for transmission electron microscopy (TEM) were made by crushing powders in an agate mortar and dispersing them 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 electron diffraction (ED) patterns were recorded with a Philips CM20 transmission electron microscope operating at 200 kV. The composition of the AgxSm(2−x)/3 □(1−2x)/3WO4 (x = 0.2) sample was also confirmed using energy dispersive X-ray (EDX) analysis on the Philips CM20 microscope with an Oxford INCA attachment (TEM−EDX), using the AgL, SmL, and WL lines. TEM−EDX analyses at four points for 10 different crystallites of each sample were linked with the ED analysis of the crystallites. Photoluminescence emission and photoluminescence excitation spectra were measured using a Lot-Oriel MS-257 spectrograph equipped with a Marconi CCD detector and 150 W Xe arc as an excitation source. All measurements were performed in the temperature range from 80 to 500 K. The temperature dependent measurements were performed using a vacuum optical Cryotrade LN-120 cryostat. The temperature dependences of the band intensities were performed at the excitation 405 nm with a heating rate of 10 K/min and a measurement step time of 6 s. Absence of the thermostimulated luminescence under 405 nm irradiation was checked before the measurements of the temperature dependences. All samples were measured under the same conditions.

Figure 3. Experimental, calculated, and difference SXPD profiles after Rietveld refinement of AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.268 (a), 0.196 (b)). Insets show low-angle parts of the profile. Black and green bars mark the positions of the main and satellite reflections, respectively.

expected composition. Figure S1 shows representative SEM− EDX spectra for both samples. The more local composition by TEM−EDX performed for AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.2) agreed with the results by SEM−EDX, and gave the Ag:Sm:W ratio of 0.186:0.586:1 (10.46 ± 0.63 atom % Ag, 33.08 ± 0.47 atom % Sm, 56.43 ± 0.61 atom % W). 3.2. Electron Diffraction Study. The ED pattern and indexation scheme for the most informative [001] zone axis of AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.2) are shown in Figure 1. Weak satellite reflections are visible, indicating ordering between Ag, Sm, and the cation vacancies. The patterns were indexed using hklm indexes and (3 + 1)D monoclinic symmetry. The reflection conditions hklm: h + k + l = 2n and hk0m: h, k = 2n agree with the (3 + 1)D superspace group I2/b(αβ0)00 (unique axis c) (15.1.4.1 in the notation proposed by Stokes, Campbell, and van Smaalen, B2/b(αβ0)00 in a standard setting)37 and the modulation vector q ≈ 0.60a* + 0.81b*. 3.3. Refinement of the Crystal Structure of AgxSm(2−x)/3□(1−2x)/3WO4. SXPD data was used to refine the crystal structures of the x = 0.286 and 0.2 phases. The SXPD data were collected in the 2θ range 0.3−43° with a step of 0.003°. We used the unit cell parameters, modulation

3. RESULTS 3.1. Elemental Composition. Using SEM−EDX, the Ag:Sm:W ratios in AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.286, 0.2) were found to be 0.302:0.579:1 (16.07 ± 0.03 atom % Ag, 30.79 ± 0.05 atom % Sm, 53.15 ± 0.02 atom % W) and 0.204:0.597:1 (11.35 ± 0.31 atom % Ag, 33.13 ± 0.28 atom % Sm, 55.53 ± 0.59 atom % W), respectively, close to the 4790


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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 crystal system superspace group lattice parameters a (Å) b (Å) c (Å) γ (deg) V (Å3) q vector formula units, Z color density (g/cm3) diffractometer radiation/wavelength (λ, Å) 2θ range (deg) step scan (2θ) Imax no. points no. reflections main first order satellites second order satellites no. refined params/refined atomic params R and Rw for Bragg reflections (%) main first order satellites second order satellites RP; RwP; Rexp goodness of fit (ChiQ) max/min residual density (e Å−3)

x = 0.200

Ag0.268Sm0.577□0.155WO4

Ag0.196Sm0.601□0.203WO4 monoclinic I2/b(αβ0)00

5.25405(1) 5.30863(1) 11.58917(3) 91.2390(2) 323.167(2) 0.59200(4)a* + 0.82989(4)b*

5.24204(1) 5.31091(1) 11.58685(2) 92.0175(2) 322.378(1) 0.60016(3)a* + 0.80804(4)b* 4 white

7.47(3) Data Collection

0.3−42.999 0.002 31363 14234 Refinement 878 165 351 360 84/39 3.31 and 2.82 2.38 and 2.60 4.00 and 2.72 4.38 and 3.02 5.52; 7.99; 2.71 2.95 2.34/−2.88

vectors, and superspace symmetry determined from the electron diffraction data to index the SXPD patterns of the x = 0.286 and 0.2 phases. The incommensurately modulated structures were refined from the powder diffraction intensities using superspace group I2/b(αβ0)00. Based on refined structures of other scheelite-related compounds,31,33,38,39 we tested different models during the Rietveld refinement. Annex 1 in the Supporting Information gives detailed descriptions of the models. The five models (i−v) differ by the function used for the occupancy of the A position and by the function used for the displacive modulation. We constrained all A site cations constrained to the same coordinates, displacive modulation functions, and isotropic atomic displacement parameters (ADPs), and refined the x04 coordinates (except x04(Sm1) = 0.5) and the lengths (δ) of the atomic domains. Tables S1 and S2 list the results of the Rietveld refinement of the structures in all models. The first model (i) demonstrates the best reliability factors for main reflections (RF(main reflections) = 2.28% (x = 0.286) and 2.30% (x = 0.2)) but worse reliability factors than the other models for the second order satellites (RF(second order satellites) = 4.77% (x = 0.286) and 4.95% (x = 0.2)) (Tables S1 and S2 of the Supporting Information). According to model i, the second order satellites should have noticeable intensities, which is not in agreement with the experimental data (Figure 2). Therefore, we discarded model i.

7.40(3) ID22 Beamline synchrotron/0.399927(4) 0.3−43.002 0.003 34440 14235 778 150 309 319 81/39 3.37 and 3.31 2.48 and 2.47 3.97 and 3.54 4.06 and 3.75 5.85; 8.55; 2.39 3.58 4.00/−4.27

Comparing the refinement results of the models with one (crenel-2H (ii) and crenel-2L (iii)) and three crenel domains of Sm (crenel-3H (iv) and crenel-3L (v)) shows that models iv and ii have better reliability factors for all reflections (Tables S1 and S2 of the Supporting Information). Model iv with the steplike occupational modulation and three crenel domains of Sm has the best agreement with the experimental data, showing discernible intensities for the second order satellites (Figure 2). Model iv has reliability factors for the main reflections identical with those of model v for x = 0.286 (RF(main reflections) = 2.38% (iv) and 2.37% (v)), but when the intensities of the satellites are compared, there is a clear difference between the models: RF(first order satellites) = 4.00 (iv) and 4.32% (v) and RF(second order satellites) = 4.38 (iv) and 4.56% (v). Therefore, model iv with the steplike occupational modulation and three crenel domains of Sm was adopted. Refining the coordinates and lengths of the three atomic domains results in the compositions Ag0.268Sm0.577□0.155WO4 and Ag0.196Sm0.601□0.203WO4, for the x = 0.286 and x = 0.2 phases, respectively. The reliability factors Rall (3.31% (x = 0.286), 3.37% (x = 0.2)) and RP (5.52% (x = 0.286), 5.85% (x = 0.2)) show a good agreement between the calculated and the experimental profiles. Figure 3 shows the experimental, calculated, and difference SXPD profiles of the AgxSm (2−x)/3□(1−2x)/3WO4 phases after the Rietveld refinement. 4791


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Figure 4. (a) Portion of the 10a × 11b × 1c supercell in the ab projection of the Ag0.268Sm0.577□0.175WO4 aperiodic structure. (b) Portion of the 10a × 10b × 1c supercell in the ab projection of the Ag0.196 Sm0.601□0.229WO4 aperiodic structure. The W and O atoms are shown as yellow and red spheres. WO4 tetrahedra are not shown.

Figure 5. (a) A-cation subset of Ag0.269Sm0.577□0.175WO4 aperiodic structure. (b) A-cation subset of Ag0.196Sm0.601□0.229WO4 aperiodic structure. WO4 tetrahedra are not shown. The gray-and-white wave indicates the continuously changing chemical composition. The direction and length of this wave (arrow) is parallel to the q vector with length 1/|q|. [Sm2O14] dimers are marked by red ellipses.

Table 1 lists the crystallographic information. Tables S3 and S5 list the atomic parameters and the coefficients of the modulation functions; Tables S4 and S6 give the main interatomic distances. 3.4. Specific Features of the AgxSm(2−x)/3□(1−2x)/3WO4 Framework. Figures 4 and 5 show parts of the incommensurately modulated Ag0.268Sm0.577□0.175WO4 and Ag0.196Sm0.601 □0.203WO4 structures and of the subset of the A cations in the projection [001] . In both structures, columns of [...−(SmO8/ AgO8)−WO4− ...] and [...−□−WO4−...] run along the c-axis (Figure 4); however, the distributions of the Ag and Sm cations and vacancies are different (Figure 5).

The cation order is defined by the coefficients α and β of the modulation vector q = αa* + βb* (Table 1) and the parameters of the three atomic domains for the same cation position (Sm1, Sm2, and Ag) (Tables S3 and S5). Sm aggregates exist in two forms in the AgxSm(2−x)/3□(1−2x)/3WO4 structures: [Sm2O14] dimers and infinite chains of SmO8 polyhedra parallel with the c-axis (Figure 6). The [Sm2O14] dimers contain only Sm1 cations, while both Sm1 and Sm2 are present in the infinite [SmO8]n chains (Figure 5). This is 4792


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Figure 6. Single [Sm2O14] dimer surrounded by WO4 tetrahedra. O atoms are shown as red spheres. Calculated values of intercationic distances are indicated by arrows.

similar to the structure of the NaxEu3+(2−x)/3□(1−2x)/3MoO429 and AgxEu(2−x)/3□(1−2x)/3WO431 phases. In Figure 5, The [Sm2O14] dimers show up as isolated Sm1 pairs, separated from other Sm3+ cations by Ag+ or vacancies. Figure 6 shows a single [Sm2O14] dimer surrounded by WO4 tetrahedra. The distances between Sm1 atoms in [Sm2O14] dimers (∼3.96 Å) are shorter than the Sm1−Sm1 distances to the four nearest Sm neighbors of the [SmO8]n chains (6.50−6.97 Å) and practically do not change with the vacancy content (Figure 5, Tables S4 and S6). Ordering of [Sm2O14] dimers along the [210] direction is clearly observed in the Ag0.196Sm0.601□0.203WO4 structure, while in the Ag0.268 Sm0.577□0.155WO4 structure the formation of [Sm2O14] dimers is fragmented. The width of the sections between the dimers differs for different compositions (Figure 5), resulting in different amounts of [Sm2O14] dimers for the Ag0.268Sm0.577□0.155 WO4 and Ag0.196Sm0.601□0.203WO4 structures. Figure 5 shows that Ag, Sm, and vacancies are distributed in the ab plane in an incommensurately modulated manner. The Ag0.196Sm0.601□0.203WO4 framework consists of four AO8 polyhedra alternating with a cation vacancy, i.e., 2SmO8− AgO8−SmO8−□ along the a-axis and AgO8−3SmO8−□ along b (Figure 7b). In Ag0.269Sm0.577□0.175WO4 (Figure 7a), there is only occasionally such a set of four AO8 polyhedra alternating with a vacancy along the a-axis. 3.5. Luminescence Properties. Figures 8 and 9 show the photoluminescence excitation (PLE) and emission (PL) spectra for Sm3+ in AgxSm(2−x)/3□(1−2x)/3WO4 phosphors. PLE spectra for both samples measured by monitoring the 648 nm emission of Sm3+ are shown in Figure 8. The PLE spectra show a broad excitation band in the range 250−310 nm as well as a group of sharp lines in the range 310−500 nm. PL spectra at 300 K under excitation of the WO42− anion at λex = 268 nm and excitation into the 6H5/2 → 4F7/2 transition of Sm3+ at λex = 405 nm are shown in Figure 9. The emission spectra of Ag0.268Sm0.577□0.155WO4 and Ag0.196Sm0.601□0.203 WO4 are similar. Figure 10 shows the temperature dependencies of the AgxSm(2−x)/3□(1−2x)/3WO4 PL spectra in the range from 80 to 500 K under excitation at λex = 405 nm. For all PL spectra, the 4G5/2 → 6H9/2 transition centered at the ∼646 nm region dominates. Figure 11 shows the temperature dependent intensity of some lines in the PL spectra of AgxSm(2−x)/3□(1−2x)/3WO4.

Figure 7. (a) One layer (along z, 0 ≤ z ≤ 0.25) of the 10a × 11b supercell of Ag0.269Sm0.577□0.175WO4. (b) One layer (along z, 0 ≤ z ≤ 0.25) of the 10a × 10b supercell of Ag0.196Sm0.601□0.229WO4. WO4 tetrahedra, AgO8 polyhedra, and SmO8 polyhedra are shown in yellow, green and gray, respectively. Blue squares select vacancies in the structure. The O atoms are shown as red spheres.

the difference between their structures lies in the exact spreading of Ag+/Sm3+ cations and vacancies (Figures 4, 5, and 7). Figure S2 shows the bond lengths Ag−O, Sm−O, and W−O in x4-plots. There is a significant distortion of the WO4 tetrahedra, with W−O distances varying between 1.709 and 1.876 Å and between 1.693 and 1.947 Å for Ag0.268 Sm0.577□0.155WO4 and Ag0.196Sm0.601□0.203WO4, respectively (Tables S4 and S6). The distortion of the SmO8 and AgO8 polyhedra increases with x. The Sm−O bond distances are higher for Sm2 (2.523−2.705 Å for x = 0.268; 2.446−2.680 Å for x = 0.196) than for Sm1 (2.343−2.560 Å for x = 0.268; 2.273−2.543 Å for x = 0.196). The Ag−O distances change more with

4. DISCUSSION We investigated the Ag0.268Sm0.577□0.155WO4 and Ag0.196 Sm0.601□0.203WO4 scheelite-related phases and revealed that 4793


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Figure 8. Excitation (λem = 650 nm) spectra: (a) Ag0.268Sm0.577 □0.175WO4 (1) and Ag0.196Sm0.601□0.229WO4 (2) at 300 K; (b) Ag0.268 Sm0.577□0.175WO4 at 80 (1), 300 (2), and 500 K (3).

Figure 9. Emission spectra of AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.268 (1), 0.196 (2)) at 300 K under excitation at λex = 268 nm (a) and λex = 405 nm (b). The electronic transitions for the main emission peaks are indicated. All samples are measured under the same conditions.

x (2.382−2.609 and 2.329−2.627 Å for x = 0.268 and 0.196, respectively) than they did in AgxEu(2−x)/3□(1−2x)/3WO4 (2.399−2.626 and 2.425−2.631 Å for x = 0.238 and 0.157, respectively).33 The differences and similarities between the structures with different x values are more clear when considering the Sm arrangement (Figures 4, 5, and 7; Figure S3, Tables S4 and S6). There are two forms of Sm aggregates: [Sm2O14] dimers and infinite [SmO8]n chains running along the c-axis. Earlier, similar aggregates of Eu were found in NaxEu3+(2−x)/3 □(1−2x)/3MoO431 and AgxEu3+(2−x)/3□(1−2x)/3WO4.33 The ab projections of Ag0.268Sm0.577□0.155WO4 and Ag0.196Sm0.601 □0.203WO4 show different A cation ordering, which results in different Sm−Sm interactions. The Sm−Sm distances vary between 3.869 and 3.965 Å (x = 0.268) and between 3.832 and 4.022 Å (x = 0.196) within the [SmO8]n chains. Between the [Sm2O14] dimers and [SmO8]n chains, the shortest Sm−Sm distances are 5.14 Å (x = 0.268) and 5.15 Å (x = 0.196). Sm1−Sm1 interactions at distances of about 3.96 Å exist within both forms of Sm aggregates in Ag 0.268 Sm 0.577 □0.155WO4 (Figure 5 top, circled gray Sm atoms and similar pairs in the [SmO8]n chains) but occur in Ag0.196Sm0.601 □0.203WO4 only in [Sm2O14] dimers that are isolated by Ag+ cations and vacancies from other Sm atoms (Figure 5, bottom; no similar pairs exist in the [SmO8]n chains). The Sm1−Sm1 distances in the [Sm2O14] dimers remain approximately the

same when changing x (3.959−3.965 Å for x = 0.268 and 3.956−3.958 Å for x = 0.196). The PLE spectra show a broad excitation band in the 250− 320 nm range and a group of sharp lines in the 320−500 nm range at 300 K (Figure 8a). The sharp lines between 340 and 550 nm are f−f transitions of Sm3+, and the main absorption is the 6H5/2 → 4F7/2 transition of Sm3+ at 405 nm. The broad excitation band centered at ∼270 nm is assigned to electron transitions from the top of the valence band formed by the 2p orbital of oxygen to the low-energy electron states of the conduction band formed by the 5d orbital of tungsten. The transition occurs within the WO42− group,31 and the excitation band is also known as the charge transfer (CT) band. The WO42− group CT band may also overlap with the O2− → Sm3+ CT27,40 band. The presence of the CT broad band in the PLE spectrum of Sm3+ emission points to an energy transfer from the host to the luminescent ions. With the decrease of temperature from 300 to 80 K, the long wavelength onset of the CT band shifts to shorter wavelengths. The onset follows the fundamental absorption edge, whose behavior is determined by the exciton−phonon interaction and is usually described by the Urbach rule.41 The temperature dependencies of the Ag0.268Sm0.577□0.175WO4 PLE spectra in the range from 80 to 500 K are shown in Figure 8b. The increase of the temperature from 80 to 500 K leads to broadening of the 4794


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Figure 10. Temperature dependence of AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.268 (a), 0.196 (b)) emission spectra in the range from 100 to 500 K under excitation at λex = 405 nm.

Figure 11. Temperature dependence of the emission band intensities under excitation at λex = 405 nm of AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.268 (a), 0.196 (b)). The heating rate was 10 K/min, and the measurement step time was 6 s.

bands in the spectrum and a shift in the center of the CT band to the long wavelength region. PL spectra of the AgxSm(2−x)/3□(1−2x)/3WO4 (Figures 9 and 10) are composed of four major groups of emission peaks centered at 562, 600, 646, and 705 nm, which originate from 4 G5/2 → 6HJ (J = 5/2, 7/2, 9/2, and 11/2) transitions of Sm3+ ions.27,40,42 The intensity ratio between the transitions 4G5/2 → 6 H5/2 and 4G5/2 → 6H9/2 depends on the local symmetry of the crystal field of Sm3+ ions. When Sm3+ ions occupy a lattice site with no inversion center, the 4G5/2 → 6H9/2 transition (electric dipole transition) is predominant and hypersensitive to the crystal field environment, while the 4G5/2 → 6H5/2 transition (magnetic dipole transition) is insensitive to the site symmetry. The intensity ratio of 4G5/2 → 6H9/2 to 4G5/2 → 6H5/2 transi tions can therefore be used to measure the symmetry of the local environment of the Sm3+ ions. The 4G5/2 → 6H9/2/4G5/2 → 6H5/2 integral intensity ratio lies in the range 5.9−6.7 for AgxSm(2−x)/3□(1−2x)/3WO4. Thus, the 4G5/2 → 6H9/2 electric dipole transition at ∼646 nm dominates for all PL spectra and indicates that the site symmetry of the Sm3+ position possesses no inversion center. For Eu-based scheelites, the lowest values of the 5D0 → 7F2 emission intensity of the Eu3+-centered luminescence were observed for Eu2/3□1/3MoO4 and Eu2/3□1/3WO4, whose structures contain Eu3+ cations in [EuO8]n chains only, without any isolated [Eu3+2O14] dimers. The intensity of the 5D0 → 7F2 emission becomes almost 7 times lower for AgxEu3+(2−x)/3 □(1−2x)/3WO4 phases when decreasing x from 0.5 to 0, except for a small increase in intensity from x = 0.238 to x = 0.157.

The decrease was related to a concentration quenching effect and the local increase with a higher number of [Eu3+2O14] dimers. Concentration quenching occurs when there is energy transfer between luminescent centers, triggering the energy migration to energy sinks such as crystalline defects or trace ions. Thus, two competing processes are observed: a decrease in intensity due to concentration quenching and an increase in intensity due to an increase in the number of [Eu3+2O14] dimers. Removing the concentration quenching effect by substituting Ag+ by Gd3+ led to an increase of more than 2.5 times of the 5D0 → 7F2 intensity from Ag0.5Gd0.2Eu0.3 □0WO4 to Ag0.2Gd0.3Eu0.3□0.2WO4. A small decrease in the integral intensity of the 4G5/2 → 6 H9/2 emission is also observed on the PL spectra of the present Sm-based scheelites AgxSm(2−x)/3□(1−2x)/3WO4 from x = 0.268 to x = 0.196, i.e., with increasing Sm3+ content (Figure 9). The integral intensity of 4G5/2 → 6H9/2 emission for x = 0.196 is ∼87% and ∼73% intensity for x = 0.268 at λex = 405 nm and λex = 268 nm, respectively. The nonradiative energy transfer from one Sm3+ to another Sm3+ ion usually takes place by exchange interaction, radiation reabsorption, or multipole−multipole interaction.43,44 For Sm3+ ions, the typical critical distance of exchange interactions is around 5 Å.45 The Sm−Sm distances in the AgxSm(2−x)/3□(1−2x)/3WO4 structures are much shorter. According to refs 46 and 47, the energy gap between the levels 4G5/2 and 6F9/2 is nearly the same as that between the 4795


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Figure 13. Temperature dependence for 600 nm/605 nm band intensity ratio (1) and 646 nm/651 nm band intensity ratio (2) in the excitation spectra (λex = 405 nm) of AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.268 (a), 0.196 (b)).

Figure 12. (a) Excitation (λem = 615 and 650 nm) and (b) emission (λex = 395 and 405 nm) spectra of Ag0.268Sm0.577□0.175WO4 at 80 K.

levels 6H5/2 and 6F9/2. Therefore, cross-relaxation from resonance energy transfer is most likely between neighboring Sm3+ ions; i.e., excitation energy from a Sm3+ ion decaying from the 4G5/2 level promotes a neighboring Sm3+ ion from the 6 F9/2 to the 6H5/2 state. The cross-relaxation channel in the AgxSm(2−x)/3□(1−2x)/3WO4 phosphor is 4G5/2 + 6H5/2 → 6F9/2 + 6 F 9/2 . Therefore, the concentration quenching for AgxSm(2−x)/3□(1−2x)/3WO4 phosphors can be ascribed to the cross-relaxation of Sm3+ pairs. The luminescence from 4G5/2 levels can also be quenched by high-energy lattice phonons, leading to a multiphonon relaxation process. The luminescence spectrum was monitored as a function of temperature in the range from 80 to 500 K to evaluate the thermal quenching behavior (Figure 10). The emission profile of Sm3+ changes with the temperature, and the dependence differs for different emission bands. The emission line at ∼616 nm can be associated with the 5D0 → 7F2 transition of trace Eu3+ ions, which could indeed be seen on some of the EDX spectra (Figure S1). The intrinsic emission of the WO4 group has not been observed in the whole temperature range 80−500 K probably due to the efficient energy transfer from WO4 to Sm3+. It should be noted that a different temperature dependence of the intensity is observed for the bands of the 4G5/2 → 6H9/2 and 4G5/2 → 6H7/2 transitions (Figure 11). The intensity of the emission bands centered at ∼605 and ∼651 nm decreases gradually with increasing temperature, while the maximum of the emission lines centered at ∼600 and ∼646 nm is observed at 300 K. The intensity of the bands at ∼600 and ∼646 nm is

Figure 14. ln T versus R for 600 nm/605 nm band intensity ratio (1) and 646 nm/651 nm band intensity ratio (2) in the emission spectra (λex = 405 nm) of AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.268 (a), 0.196 (b)). 4796



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higher at 500 K than at 80 K. The difference in the fluorescence intensity variation with temperature for the 4G5/2 → 6 H9/2 and 4G5/2 → 6H7/2 band transitions can be used to measure temperature, allowing the use of AgxSm(2−x)/3□(1−2x)/3 WO4 as a thermographic phosphor. Figure 13 shows the variation of the emission intensity ratios (R) for bands at 600 and 646 nm to the bands at 605 and 651 nm as a function of temperature. The form of all R versus T dependencies indicates a logarithmic dependence and can be approximated by the functions inserted in Figure 13. The linear dependencies of ln T versus R for both intensity ratios can be used for the temperature determination (Figure 14).

ACKNOWLEDGMENTS This research was supported by FWO (Project G039211N), Flanders Research Foundation. The research was carried out within the state assignment of FASO of Russia (Themes No. 0339-2016-0007). V.M. thanks the Russian Foundation for Basic Research (Grant 18-03-00611) for financial support. E.G.K. and O.B. acknowledge financial support from the Russian Foundation for Basic Research (Grant 16-03-00510). D.D. thanks the Foundation of the Russian Federation President (Grant MK-3502.2018.5) for financial support. We are grateful to the ESRF for granting the beamtime. V.C. is grateful for the financial support of the Russian Ministry of Science and Education (Project No. RFMEFI61616X0069). We are grateful to the ESRF for the access to ID22 station (experiment MA-3313).

5. CONCLUSION We investigated the effect of cation substitution and the ordering among A cations and vacancies in the AgxSm(2−x)/3 □(1−2x)/3WO4 system in order to control the luminescence properties. We refined the crystal structures of Ag0.268Sm0.577 □0.175WO4 and Ag0.196Sm0.601□0.229WO4 from high resolution synchrotron powder X-ray diffraction data, supported by transmission electron microscopy. The structures have different distributions of the Ag and Sm cations and vacancies. There are two types of Sm3+ aggregates in the structures: infinite chains of [SmO8] polyhedra and [Sm2O14] dimers. The lower x, the more Sm3+ ions form [Sm3+2O14] dimers. The excitation spectra exhibit strongest absorption at 405 nm. The intensities of the bands of 4G5/2 → 6H9/2 and 4G5/2 → 6H7/2 transitions depend differently on temperature, allowing the use of these materials as thermographic phosphors.

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REFERENCES

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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.8b02029. EDX spectra of AgxSm(2−x)/3□(1−2x)/3WO4 (x = 0.286, 0.2); models tested during Rietveld refinement using SXPD data; schemes of occupation modulation function of the A position of scheelite-type AgxSm(2−x)/3□(1−2x)/3 WO4 structures; characteristics of different model refinements (x = 0.286 and 0.2); atomic coordinates, amplitudes of Fourier components for occupational and displacive modulation functions, and isotropic atomic displacement parameters for Ag0.268Sm0.577□0.155WO4 and Ag0.196Sm0.601□0.203WO4 structures; main interatomic distances for Ag 0.268Sm 0.577□ 0.155 WO 4 and Ag0.196Sm0.601□0.203WO4; x4-plots of Ag−O, Sm−O, and W−O bond lengths for Ag0.268Sm0.577□0.155WO4 and Ag0.196Sm0.601□0.203WO4 structures; x4-plots of Sm−Sm distances for Ag0.268Sm0.577□0.155WO4 and Ag0.196Sm0.601□0.203WO4 structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Vladimir Morozov: 0000-0002-0674-2449 Artem Abakumov: 0000-0002-7135-4629 Notes

The authors declare no competing financial interest. 4797


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Incommensurately Modulated Structures and Luminescence

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