Incommensurately Modulated Structures and Luminescence

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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02029 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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Chemistry of Materials

Incommensurately Modulated Structures and Luminescence Properties of the AgxSm(2-x)/3WO4 (x = 0.286, 0.2) Scheelites as Thermographic Phosphors

Vladimir Morozov,1 Dina Deyneko,1 Olga Basovich,2 Elena G. Khaikina,2,3 Dmitry Spassky,4 Anatolii Morozov,5 Vladimir Chernyshev,1,6 Artem Abakumov,7 Joke Hadermann*,8

1

Chemistry Department, Moscow State University, 119991, Moscow, Russia

2

Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Science,

670047, Ulan-Ude, Russia 3

Buryat State University, 670000, Ulan-Ude, Russia

4

Skobeltsyn Institute of Nuclear Physics, Moscow State University, 119991, Moscow, Russia

5

Department of Material Science, Moscow State University, 119991, Moscow, Russia

6

A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, 31 Leninsky prosp.,

119991, Moscow, Russia 7

Skolkovo Institute of Science and Technology, Nobel str. 3, 143026, Moscow, Russia

8

EMAT, University of Antwerp, Groenenborgerlaan 171, Belgium B-2020

*

Corresponding author:

Joke Hadermann Tel.: +32-32653245 Fax.: +32-32653257 E-mail: [email protected] EMAT laboratory, University of Antwerp, B-2020, Antwerp, Belgium

1 ACS Paragon Plus Environment

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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 of 550– 720 nm , with the J = 9/2 transition at the ~648 nm region being dominant for all PL spectra. The intensities of the 4G5/2 → 6H9/2 and 4G5/2 → 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.


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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 times and 5.0 times those (~626 nm) of the commercial 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 drawbacks of existing techniques like thermocouples, thermochromic liquid crystals and non-invasive 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, alkali-earth or rare-earth elements, and B’ and B’’ being W, Mo. The scheelitetype ABO4 (CaWO4) structure consists of a 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. 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 order, frequently forming incommensurately modulated structures.31,33 Earlier, we studied the structure and the luminescence properties of the Eu-based scheeliterelated 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 n-UVemitting LED chips. Lowering x from 0.5 to 0 reduced the intensity of the 5D0 → 7F2 emission 3 ACS Paragon Plus Environment


for the AgxEu(2-x)/3(1-2x)/3WO4 samples 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. 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 Ag0.5Gd0.2Eu0.30WO4 to Ag0.2Gd0.3Eu0.30.2WO4. In the current paper, we focus on the non-stoichiometric AgxSm(2-x)/3(1-2x)/3WO4 scheelites with x = 0.286 and 0.2, to study the relation between the Sm3+ content, amount of cation vacancies, crystal structure and luminescence properties. 2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. AgxSm(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 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 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 of 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 4 ACS Paragon Plus Environment

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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 4 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 150W Xe arc as an excitation source. All measurements were performed in the temperature range from 80 K 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 luminesce under 405 nm irradiation was checked before the measurements of the temperature dependences. All samples were measured under the same conditions. 3. RESULTS 3.1. Elemental Composition. Using SEM-EDX, the Ag:Sm:W ratios in AgxSm(2-x)/3(12x)/3WO4

(x = 0.286, 0.2) were found to be 0.302:0.579:1 (16.07 ± 0.03 at% Ag, 30.79 ± 0.05 at%

Sm, 53.15 ± 0.02 at% W) and 0.204:0.597:1 (11.35 ± 0.31 at% Ag, 33.13 ± 0.28 at% Sm, 55.53 ± 0.59 at% W), respectively, close to the 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 at% Ag, 33.08±0.47 at% Sm, 56.43±0.61 at% 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 5 ACS Paragon Plus Environment


collected in the 2θ range of 0.3-43º with step of 0.003º. We used the unit cell parameters, modulation 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 compounds31,33,38,39, we tested different models during the Rietveld refinement. Annex 1 of 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 and refined the x 40 coordinates (except x 40 (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(2nd order satellites) = 4.77 % (x = 0.286) and 4.95 % (x = 0.2)) (Table S1 and Table S2 of Supporting information). According to the 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). Comparing the refinement results of the models with one (Crenel-2H (ii) and Crenel2L(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 (Table S1 and Table S2 of Supporting information). Model (iv) with the step-like 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 identical reliability factors for the main reflections as 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(1st order satellites) = 4.00 % (iv) and 4.32 % (v) and RF(2nd order satellites) = 4.38 % (iv) and 4.56 %(v). Therefore, model (iv) with the step-like occupational modulation and three crenel domains of Sm was adopted. Refining the coordinates and lengths of the 3 atomic domains results in the compositions Ag0.268Sm0.5770.155WO4 and Ag0.196Sm0.6010.203WO4, for the x = 0.286 and x = 0.2 phases as, 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 6 ACS Paragon Plus Environment

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of the AgxSm(2-x)/3(1-2x)/3WO4 phases after the Rietveld refinement. 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 a part of the incommensurately modulated Ag0.268Sm0.5770.175WO4 and Ag0.196Sm0.6010.203WO4 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 с-axis (Figure 4), however, the distribution of the Ag and Sm cations and vacancies is different (Figure 5). The cation order in 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) (Table S3 and Table 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 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 neighbours of the [SmO8]n chains (6.50-6.97 Å) and practically do not change with the vacancy content (Figures 5, Table S4 and Table S6). Ordering of [Sm2O14]-dimers along the [210] direction is clearly observed in the Ag0.196Sm0.6010.203WO4 structure while in the Ag0.268Sm0.5770.155WO4 structure the formation of [Sm2O14]-dimers is fragmented. The widhth of the sections between the dimers differs for different compositions (Figure 5), resulting indifferent amounts of [Sm2O14] dimers for the Ag0.268Sm0.5770.155WO4 and Ag0.196Sm0.6010.203WO4 structures. Figure 5 shows that the Ag, Sm and vacancies are distributed in the ab plane in an incommensurately modulated manner. The Ag0.196Sm0.6010.203WO4 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 Ag0.269Sm0.5770.175WO4 (Figure 7a), there is only occasionally such 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 of 310–500 nm. 7 ACS Paragon Plus Environment


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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 Ag0.268Sm0.5770.155WO4 and Ag0.196Sm0.6010.203WO4 are similar. Figure 10 shows the temperature dependencies of the AgxSm(2-x)/3(1-2x)/3WO4 PL spectra in the range from 80 K 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. 4. DISCUSSION We investigated the Ag0.268Sm0.5770.155WO4 and Ag0.196Sm0.6010.203WO4 scheelite-related phases and revealed that 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 1.709-1.876 Å and 1.693-1.947 Å for Ag0.268Sm0.5770.155WO4 and Ag0.196Sm0.6010.203WO4, 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 x (2.382 - 2.609 Å and 2.329 - 2.627 Å for x = 0.268 and 0.196 resp.) than they did in AgxEu(2-x)/3(1-2x)/3WO4 (2.399 - 2.626 Å and 2.425 to 2.631 Å for x = 0.238 and 0.157 resp.).33 The differences and similarities between the structures with different x are more clear when considering the Sm arrangement (Figures 4, 5 and 7; Figure S3, Table 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)/3WO433.

The

ab

projections

of

Ag0.268Sm0.5770.155WO4

and

Ag0.196Sm0.6010.203WO4 show different A cation ordering, which results in different Sm-Sm interactions. The Sm – Sm distances vary between 3.869–3.965 Å (x = 0.268) and 3.832–4.022 Å (x = 0.196) within the [SmO8]n chains. Between the [Sm2O14] dimers and [SmO8]n chains, the shortest Sm-Sm distance is 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 Smaggregates in Ag0.268Sm0.5770.155WO4 (Figure 5 top, encircled grey Sm atoms and similar pairs in the [SmO8]n chains) but occur in Ag0.196Sm0.6010.203WO4 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 8 ACS Paragon Plus Environment

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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- group31 and the excitation band is also known as charge transfer (CT) band. The WO42group 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 Ag0.268Sm0.5770.175WO4 PLE spectra in the range from 80 K to 500 K are shown in Figure 8b. The increase of the temperature from 80 K to 500 K leads to broadening of the 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 (Figure 9 and Figure 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 → 6H5/2 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 4

G5/2 → 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 transitions can therefore be used to measure the symmetry of the local environment of the Sm3+ ions. The 4

G5/2 → 6H9/2/4G5/2 → 6H5/2

integral 4

intensity

ratio

lies

in

the

range

5.9-6.7

for

6

AgxSm(2-x)/3(1-2x)/3WO4. Thus the G5/2 → H9/2 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 Eu2/31/3MoO4 and Eu2/31/3WO4, 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 9 ACS Paragon Plus Environment


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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

5

D0 → 7F2 intensity from

Ag0.5Gd0.2Eu0.30WO4 to Ag0.2Gd0.3Eu0.30.2WO4. A small decrease in the integral intensity of the 4G5/2 → 6H9/2 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 non-radiative 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 Ref.46,47, the energy gap between the levels 4G5/2 and 6F9/2 is nearly the same as that between the 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 6F9/2 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

F9/2. 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 multi-phonon relaxation process. The luminescence spectrum was monitored as a function of temperature in the range from 80 K 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 4

G5/2 → 6H9/2 and 4G5/2 → 6H7/2 transitions (Figure 11). The intensity of the emission bands

centered at ~ 605 nm and ~ 651 nm decreases gradually with increasing temperature while the 10 ACS Paragon Plus Environment

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maximum of the emission lines centered at ~ 600 nm and ~ 646 nm is observed at 300 K. The intensity of the bands at ~ 600 nm and ~ 646 nm is higher at 500 K than at 80 K. The difference in the fluorescence intensity variation with temperature for the 4G5/2 → 6H9/2 and 4G5/2 → 6H7/2 bands

transitions

can

be

used

to

measure

temperature,

allowing

the

use

of

AgxSm(2-x)/3(1-2x)/3WO4 as thermographic phosphor. Figure 13 shows the variation of the emission intensity ratios (R) for bands at 600 nm and 646 nm to the bands at 605 nm 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 lnT versus R for both intensity ratios can be used for the temperature determination (Figure 14). 4. 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 Ag0.268Sm0.5770.175WO4 and Ag0.196Sm0.6010.229WO4 from high resolution synchrotron powder X-ray diffraction data, supported by transmission electron microscopy. The structures have a different distribution 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 4

G5/2 → 6H7/2 transitions depend differently on temperature, allowing the use of these materials

as thermographic phosphors. ACKNOWLEDGEMENTS 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 0339-20160007), V.A.M. thanks the Russian Foundation for Basic Research (Grant 18-03-00611) for financial support. E.G.K. and O.M.B. acknowledge financial support from the Russian Foundation for Basic Research (Grant 16-03-00510). D.V.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.V.C. is grateful for 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). Supporting Information Available: EDX spectra of AgxSm(2-x)/3(1-2x)/3WO4 (x = 0.286, 0.2) from selected area and a using element 11 ACS Paragon Plus Environment


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mapping technique (Figure S1). Models tested during the Rietveld refinement using the SXPD data (Annex 1). Two schemes of the occupation modulation function of the A position of scheelite-type AgxSm(2-x)/3(1-2x)/3WO4 structures (Diagram S1). Characteristics of the different model refinements x = 0.286 (Table S1). Characteristics of the different model refinements x = 0.200 (Table S2). Atomic coordinates, amplitudes of Fourier components for the occupational and displacive modulation functions and isotropic atomic displacement parameters for

Ag0.268Sm0.5770.155WO4

structure

(Table

S3).

Main

interatomic

distances

for

Ag0.268Sm0.5770.155WO4 (Table S4). Atomic coordinates, amplitudes of Fourier components for the occupational and displacive modulation functions and isotropic atomic displacement parameters for Ag0.196Sm0.6010.203WO4 structure (Table S5). Main interatomic distances for Ag0.196Sm0.6010.203WO4 (Table S6). x4-plots of Ag−O, Sm−O and W−O bond lengths for the Ag0.268Sm0.5770.155WO4 and Ag0.196Sm0.6010.203WO4 structures (Figure S2). x4-plots of Sm−Sm distances for the Ag0.268Sm0.5770.155WO4 and Ag0.196Sm0.6010.203WO4 structures (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Shur, M. S.; Zukauskas, A. Solid-state lighting: Toward superior illumination, Proc. IEEE 2005, 93, 1691-1703. (2) Yam, F. K.; Hassan, Z. Innovative advances in LED technology, Microelectr J. 2005, 36, 129-137. (3) Kim, T.; Kang, S. Potential red phosphor for UV-white LED device, J. Lumin. 2007, 122– 123, 964–966. (4) 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. Applicata 2009, 39, 375-383. (5) Md. Haque, M.; Lee, H.-I.; Kim, D.-K. Luminescent properties of Eu3+-activated molybdatebased novel red-emitting phosphors for LEDs, J. Alloys and Compd. 2009, 481, 792–796. (6) Shao, Q.; Li, H.; Wu, K.; Dong, Y.; Jiang J. Photoluminescence studies of red-emitting NaEu(WO4)2 as a near-UV or blue convertible phosphor, J. Lumin. 2009, 129, 879-883. (7) 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. (8) Thangaraju, D.; Durairajan, A.; Balaji, D.; Moorthy Babu, S.; Hayakawa, Y. Novel KGd1-(x+y)EuxBiy(W1-zMozO4)2 nanocrystalline red phosphors for tricolor white LEDs, J. Lumin. 2013, 134, 244-250.


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(9) Wang, Z.; Zhong, J.; Jiang, H.; Wang, J.; Liang, H. Controllable synthesis of NaLu(WO4)2:Eu3+ microcrystal and luminescence properties for LEDs, Cryst. Growth Des. 2014, 14, 3767−3773. (10) Zhang, W.; Li, J.; Wang, Y.; Long, J.; Qiu, K. Synthesis and luminescence properties of NaLa(MoO4)2-xAGx:Eu3+ (AG = SO42-, BO33-) red phosphors for white light emitting diodes, J. Alloys and Compd. 2015, 635, 16-20. (11) Li, G.; Wei, Y.; Li, Z.; Xu, G. Synthesis and photoluminescence of Eu3+ doped CaGd2(WO4)4 novel red phosphors for white LEDs applications, Opt. Mater. 2017, 66, 253-260. (12) Guo, W.; Chen, Y.; Lin, Y.; Gong, X.; Luo, Z.; Huang, Y., Spectroscopic analysis and laser performance of Tm3+ : NaGd(MoO4)2 crystal, J. Phys. D: Appl. Phys. 2008, 41, 115409. (13) Chen, Y.J.; Lin, Y.F.; Guo, W.J.; Gong, X.H.; Huang, J.H.; Luo, Z.D.; Huang, Y.D. Efficient 1.9µm monolithic Tm3+:NaLa(MoO4)2 micro-laser, Laser Phys. Lett. 2012, 9, 141–144. (14) Yu, Y.; Zhang, L.; Huang, Y.; Lin, Z.; Wang G. Growth, crystal structure, spectral properties and laser performance of Yb3+:NaLu(MoO4)2 crystal, Laser Phys. 2013, 23, 105807 (6pp). (15) Feng, J.; Xu, J.; Zhu, Z.; Wang, Y.; You, Z.; Li, J.; Wang, H.; Tu, C. Spectroscopic properties and orthogonally polarized dual-wavelength laser of Yb3+:NaY(WO4)2 crystals with high Yb3+ concentrations, J. Alloys and Compd. 2013, 566, 229-234. (16) 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, Optic Express 2014, 22, A961−A972. (17) Wang, J.; Bu, Y.; Xiangfu Wang, X.; Seo H.J. A novel optical thermometry based on the energy transfer from charge transfer band to Eu3+-Dy3+ ions, Sci. Rep. 2017, 7, 6023. (18) Zhou, X.; Wang, R.; Xiang, G.; Jiang, S.; Li, L.; Luo, X.; Pang, Y.; Tian, Y. Multiparametric thermal sensing based on NIR emission of Ho(III) doped CaWO4 phosphors, Opt. Mater. 2017, 66, 12-16. (19) Khalid, A.; Kontis, K. Thermographic Phosphors for High Temperature Measurements: Principles, Current State of the Art and Recent Applications, Sensors 2008, 8(9), 5673-5744 (20) Chambers, M.D.; Clarke, D.R. Doped Oxides for High-Temperature Luminescence and Lifetime Thermometry, Annu. Rev. Mater. Res. 2009, 39, 325–359 (21) Wang, X.; Wolfbeis, O.S.; Meier, R.J. Luminescent probes and sensors for temperature, Chem. Soc. Rev. 2013, 42, 7834-7869 (22) Dramicanin, M.D. Sensing temperature via downshifting emissions of lanthanide-doped metal oxides and salts. A review, Methods Appl. Fluoresc. 2016, 4, 042001



(23) Nuebert, P. Device for indicating the temperature distribution of hot bodies - US Patent No. 2,071,471. 1937 (24) Bradley, L.C. A temperature sensitive phosphor used to measure surface temperatures in aerodynamics. Rev. Sci. Instrum. 1953, 24, 219–220 (25) Brubach, J.; Kissel, T.; Frotscher, M.; Euler, M.; Albert, B.; Dreizler, A. A survey of phosphors novel for thermography, J. Lumin. 2011, 131, 559-564. (26) Litterscheid, C.; Kruger, S.; Euler, M.; Dreizler, A.; Wickleder C.; B. Albert, B. Solid solution between lithium-rich yttrium and europium molybdate as new efficient red-emitting phosphors, J. Mater. Chem. C, 2016, 4, 596-602 (27) Feist, J.P.; Heyes, A.L. The characterization of Y2O2S:Sm powder as a thermographic phosphor for high temperature applications, Meas. Sci. Technol. 2000, 11, 942–947 (28) Rai, V.K. Sm3+ as a fluorescence lifetime temperature sensing, IEEE Sens. J. 2007, 7, 1110– 1111 (29) Nikolic, M.G.; Jovanovic, D.J.; Dordevic, V.; Antic, Z.; Krsmanovic, R.M.; Dramicanin, M.D. Thermographic properties of Sm3+-doped GdVO4 phosphor, Phys. Scr. 2012, T149, 014063 (30) Kaczkan, M.; Boruc, Z.; Turczynski, S.; Malinowski, M. Effect of temperature on the luminescence of Sm3+ ions in YAM crystals, J. Alloys and Compd. 2014, 612, 149-153 (31) 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 Euclusters occurring in their incommensurately modulated structure, Chem. Sci. 2012, 3, 384-390. (32) 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. (33) Morozov, V. A.; Batuk, D.; Batuk, M.; Basovich, O. M.; Khaikina, E. G.; Deyneko, D. V.; Lazoryak, B. I.; Leonidov, I. I.; Abakumov, A. M.; Hadermann, J. Luminescence properties upgrading via the structure and cation changing in AgxEu(2-x)/3WO4 and AgxGd(2-x)/3-0.3Eu0.3WO4, Chem. Mater. 2017, 29, 8811-8823. (34) Le Bail, A.; Duroy, H.; Fourquet, J. L. Ab-initio structure determination of LiSbWO6 by Xray powder diffraction, Mater. Res. Bull. 1988, 23, 447-452. (35) Petricek, V.; Dusek, M.; Palatinus, L. Crystallographic Computing System JANA2006: General features, Z. Kristallogr., 2014, 229, 345–352. (36) Brandenburg, K. DIAMOND, Version. 2.1c. Crystal Impact GbR, Bonn, Germany, 1999. (37) Van Smaalen, S.; Campbell, B.J.; Stokes, H.T. Equivalence of superspace groups, Acta Crystallogr. Sect. A 2013, 69, 75-90. 14 ACS Paragon Plus Environment

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(38) 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. (39) Morozov, V.A.; Bertha, A.; Meert, K.W.; Van Rompaey, S.; Batuk, D.; Martinez, G.T.; Van Aert, S.; Smet, P.F.; 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. (40) Yao, K.; Wang, M.W.; Liu, S.X.; Zhang, L.D.; Li, W.J. Effects of host doping on spectral and long-lasting properties of Sm3+-doped Y2O2S, J. Rare Earths 2006, 24, 524-528. (41) Song, K. S.; Williams, R. T. Self-Trapped Excitons, 2nd ed. Springer Series in Solid-State Sciences, 1996. V. 105. Springer-Verlag, Berlin. (42) Wang, Y.; Lin, C.; Zheng , H.; Sun, D.; Li, L.; Chen, B. Fluorescent and chromatic properties of visible-emitting phosphor KLa(MoO4)2:Sm3+, J. Alloys and Compd. 2013, 559, 123-128. (43) Dexter, D.L. A theory of sensitized luminescence in solids, J. Chem. Phys. 1953, 21, 836850. (44) Dexter, D.L. Theory of concentration quenching in inorganic phosphors, J. Chem. Phys. 1954, 22, 1063-1070. (45) Li, Y.; Liu X. Photoluminescence properties and energy transfer of KY1-xLnx(MoO4)2 (Ln= Sm3+, Eu3+) red phosphors, J. Lumin. 2014, 151, 52–56. (46) Van Uitert, L.G.; Johnson, L.F. Energy transfer between rare‐earth ions, J. Chem. Phys. 1966, 44, 3514-3522. (47) Blasse, G.; Dirksen, G.J. A simple luminescence experiment suggesting rare earth ion pairing in the fluorite structure, J. Electrochem. Soc. 1980, 127, 978-979.



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Table 1. Selected crystallographic data and refinement parameters for x = 0.286 and x = 0.200 compounds.

x = 0.286 Ag0.268Sm0.5770.155WO4

Formula Crystal system Superspace group Lattice parameters: a (Å) b (Å) c (Å) γ (°) V (Å3) q vector Formula units, Z Color density, g/cm3 Data collection Diffractometer Radiation/ Wavelength (λ, Å)

x = 0.200 Ag0.196Sm0.6010.203WO4 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 7.40(3)

7.47(3)

ID22 Beamline Synchrotron / 0.399927(4)

2θ range (o) Step scan (2θ) Imax Number of points Refinement

0.3-42.999 0.002 31363 14234

0.3-43.002 0.003 34440 14235

878

778

165 351 360 84/39

150 309 319 81/39

3.31 and 2.82

3.37 and 3.31

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

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

The number of reflections Among them: Main The 1st order satellites The 2nd order satellites No. of refined parameters/ refined atomic parameters R and Rw (%) for Bragg reflections Among them: Main The 1st order satellites The 2nd order satellites RP; RwP ; Rexp Goodness of fit (ChiQ) Max./min. residual density (e×Å-3)


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Figure captions Figure 1. (a) [001] ED pattern of Ag0.2Sm0.60.2WO4 and (b) the indexation schema. Figure 2. (Color online). Illustration of the different model refinements performed from SXPD data for Ag0.196Sm0.6010.229WO4: (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. Figure 3. (Color online). 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 a low-angle part of the profile. Black and green bars mark the positions of the main and satellite reflections, respectively. Figure 4. (Color online). (a) A portion of the 10a×11b×1c supercell in the ab projection of the Ag0.268Sm0.5770.175WO4 aperiodic structure. (b) A portion of the 10a×10b×1c supercell in the ab projection of the Ag0.196Sm0.6010.229WO4 aperiodic structure. The W and O atoms are shown as yellow and red spheres. WO4 tetrahedra are not shown. Figure 5. (Color online). (a) A-cation subset of the Ag0.269Sm0.5770.175WO4 aperiodic structure. (b) A-cation subset of the Ag0.196Sm0.6010.229WO4 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. Figure 6. The single [Sm2O14] dimer surrounded by WO4 tetrahedra. O atoms are shown as red spheres. Calculated values of inter-cationic distances are indicated by arrows. Figure 7. (Color online). (a) One layer (along z, 0≤z≤0.25) of the 10a×11b supercell of Ag0.269Sm0.5770.175WO4; (b) One layer (along z, 0≤z≤0.25) of the 10a×10b supercell of Ag0.196Sm0.6010.229WO4. WO4 tetrahedra, AgO8 and SmO8 polyhedra are shown as yellow, green and grey colour, respectively. Blue squares select vacancies in the structure. The O atoms are shown as red spheres. Figure 8. Excitation (λem = 650 nm) spectra: (a) Ag0.268Sm0.5770.175WO4 (1) and Ag0.196Sm0.6010.229WO4 (2) at 300 K; (b) Ag0.268Sm0.5770.175WO4 at 80 (1), 300 (2) and 500 K (3). 17 ACS Paragon Plus Environment


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. Figure 10. The temperature dependence of AgxSm(2-x)/3(1-2x)/3WO4 (x = 0.268 (a), 0.196 (b)) emission spectra in the range from 100 K to 500 K under excitation at λex = 405 nm. Figure 11. The 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. Figure 12. (a) Excitation (λem = 615 and 650 nm) and (b) emission (λem = 395 and 405 nm) spectra of Ag0.268Sm0.5770.175WO4 at 80 K. Figure 13. The temperature dependence for 600 nm/605 nm bands intensity ratio (1) and 646 nm/651 nm bands 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 14. lnT versus R for 600 nm/605 nm bands intensity ratio (1) and 646 nm/651 nm bands 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)).


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Table of Contents

Vladimir A. Morozov, Dina V. Deyneko, Olga M. Basovich, Elena G. Khaikina, Dmitry A. Spassky, Anatolii V. Morozov, Vladimir V. Chernyshev, Artem M. Abakumov, Joke Hadermann Chem. Mater. Incommensurately Modulated Structures and Luminescence Properties of the AgxSm(2-x)/3WO4 (x = 0.286, 0.2) Scheelites as Thermographic Phosphors

In this paper, Ag+ for Sm3+ substitution in the scheelite-type AgxSm(2-x)/3(1-2x)/3WO4 tungstates has been investigated for its influence on the cationvacancy ordering and luminescence properties. Transmission electron microscopy revealed the (3+1)D incommensurately modulated structures, which were refined from high resolution synchrotron powder X-ray diffraction data. Different temperature dependencies were found for the intensity of the 4 G5/2 → 6H9/2 and 4G5/2 → 6H7/2 transitions of Sm3+ ions. The emission intensity ratios (R) for these bands vary reproducibly with temperature, allowing the use of these materials as thermographic phosphors.



Figure 1. (a) [001] ED pattern of Ag0.2Sm0.6WO4 and (b) the indexation schema 160x320mm (300 x 300 DPI)

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Figure 2. (Color online). Illustration of the different model refinements performed from SXPD data for Ag0.196Sm0.601WO4: (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.



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


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Figure 4. (Color online). (a) A portion of the 10a×11b×1c supercell in the ab projection of the Ag0.268Sm0.577WO4 aperiodic structure. (b) A portion of the 10a×10b×1c supercell in the ab projection of the Ag0.196Sm0.601WO4 aperiodic structure. The W and O atoms are shown as yellow and red spheres. WO4 tetrahedra are not shown. 78x177mm (300 x 300 DPI)



Figure 5. (Color online). (a) A-cation subset of the Ag0.268Sm0.577WO4 aperiodic structure. (b) A-cation subset of the Ag0.196Sm0.601WO4 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|. [Sm12O14] dimers are marked by red ellipses. 81x178mm (300 x 300 DPI)


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Figure 6. The single [Sm12O14] dimer surrounded by WO4 tetrahedra. O atoms are shown as red spheres. Calculated values of inter-cationic distances are indicated by arrows. 50x29mm (300 x 300 DPI)



Figure 7. (Color online). (a) one layer (along z, 0 ≤ z ≤ 0.25) of the 10a×11b supercell of Ag0.268Sm0.577WO4; (b) one layer (along z, 0 ≤ z ≤ 0.25) of the 10a×10b supercell of Ag0.196Sm0.601WO4. WO4 tetrahedra, AgO8 and SmO8 polyhedra are shown as yellow, green and grey colour, respectively. Blue squares select vacancies in the structure. The O atoms are shown as red spheres. 80x170mm (300 x 300 DPI)


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Figure 8. Excitation (λem = 650 nm) spectra: (a) Ag0.268Sm0.577WO4 (1) and Ag0.196Sm0.601WO4 (2) at 300 K; (b) Ag0.268Sm0.577WO4 at 80 (1), 300 (2) and 500 K (3). 126x201mm (300 x 300 DPI)



Figure 9. Emission spectra of AgxSm(2-x)/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. 125x196mm (300 x 300 DPI)


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Figure 10. The temperature dependence of AgxSm(2-x)/3WO4 (x = 0.268 (a), 0.196 (b)) emission spectra in the range from 100 K to 500 K under excitation at λex = 405 nm. 116x168mm (300 x 300 DPI)



Figure 11. The temperature dependence of the emission band intensities under excitation at λex = 405 nm of AgxSm(2-x)/3WO4 (x = 0.268 (a), 0.196 (b)). The heating rate was 10 K/min and the measurement step time was 6 s. 118x176mm (300 x 300 DPI)


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Figure 12. (a) Excitation (λem = 615 and 650 nm) and (b) emission (λem = 395 and 405 nm) spectra of Ag0.268Sm0.577WO4 at 80 K. 115x166mm (300 x 300 DPI)



Figure 13. The temperature dependence for 600 nm/605 nm bands intensity ratio (1) and 646 nm/651 nm bands intensity ratio (2) in the excitation spectra (λex = 405 nm) of AgxSm(2-x)/3WO4 (x = 0.268 (a), 0.196 (b)). 104x137mm (300 x 300 DPI)


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Figure 14. lnT versus R for 600 nm/605 nm bands intensity ratio (1) and 646 nm/651 nm bands intensity ratio (2) in the emission spectra (λex = 405 nm) of AgxSm(2-x)/3WO4 (x = 0.268 (a), 0.196 (b)). 104x135mm (300 x 300 DPI)


Incommensurately Modulated Structures and Luminescence

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