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Article

Microwave-Employed Sol–Gel Synthesis of Scheelite-Type Microcrystalline AgGd(MoO4)2:Yb3+/Ho3+ Upconversion Yellow Phosphors and Their Spectroscopic Properties

by
Chang Sung Lim
1,*,
Aleksandr Aleksandrovsky
2,3,
Victor Atuchin
4,5,6,*,
Maxim Molokeev
7,8,9 and
Aleksandr Oreshonkov
10,11
1
Department of Aerospace Advanced Materials Engineering, Hanseo University, Seosan 31962, Korea
2
Laboratory of Coherent Optics, Kirensky Institute of Physics Federal Research Center KSC SB RAS, 660036 Krasnoyarsk, Russia
3
Institute of Nanotechnology, Spectroscopy and Quantum Chemistry, Siberian Federal University, 660041 Krasnoyarsk, Russia
4
Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, 630090 Novosibirsk, Russia
5
Laboratory of Semiconductor and Dielectric Materials, Novosibirsk State University, 630090 Novosibirsk, Russia
6
Research and Development Department, Kemerovo State University, 650000 Kemerovo, Russia
7
Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, 660036 Krasnoyarsk, Russia
8
Institute of Engineering Physics and Radioelectronics, Siberian Federal University, 660041 Krasnoyarsk, Russia
9
Department of Physics, Far Eastern State Transport University, 680021 Khabarovsk, Russia
10
Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics Federal Research Center KSC SB RAS, 660036 Krasnoyarsk, Russia
11
School of Engineering and Construction, Siberian Federal University, 660041 Krasnoyarsk, Russia
 
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Authors to whom correspondence should be addressed.
Crystals 2020, 10(11), 1000; https://doi.org/10.3390/cryst10111000
Submission received: 12 October 2020 / Revised: 27 October 2020 / Accepted: 29 October 2020 / Published: 4 November 2020
(This article belongs to the Section Inorganic Crystalline Materials)
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Abstract

:
AgGd(MoO4)2:Ho3+/Yb3+ double molybdates with five concentrations of Ho3+ and Yb3+ were synthesized by the microwave employed sol–gel based process (MES), and the crystal structure variation, concentration effects, and spectroscopic characteristics were investigated. The crystal structures of AgGd1−x−yHoxYby(MoO4)2 (x = 0, 0.05; y = 0, 0.35, 0.4, 0.45, 0.5)at room temperature were determined in space group I41/a by Rietveld analysis. Pure AgGd(MoO4)2 has a scheelite-type structure with mixed occupations of (Ag,Gd) sites and cell parameters a = 5.24782 (11) and c = 11.5107 (3) Å, V = 317.002 (17) Å3, Z = 4. In doped samples, the sites are occupied by a mixture of (Ag,Gd,Ho,Yb) ions, which provides a linear cell volume decrease with the doping level increase. Under the excitation at 980 nm, AGM:0.05Ho,yYb phosphors exhibited a yellowish green emission composed of red and green emission bands according to the strong transitions 5F5 → 5I8 and 5S2/5F4 → 5I8 of Ho3+ ions. The evaluated photoluminescence and Raman spectroscopic results were discussed in detail. The upconversion intensity behavior dependent on the Yb/Ho ratio is explained in terms of the optimal number of Yb3+ ions at the characteristic energy transfer distance around the Ho3+ ion.

Graphical Abstract

1. Introduction

In the last years, rare earth (RE) doped light emitters, based on the frequency upconversion (UC), have been extensively investigated and applied in fields such as optoelectronics as solid state laser devices, display technology, light emitting diode (LED) materials, solar energy cell compositions, and biological imaging sensors [1,2,3,4]. In UC phosphors, the conversion of near infrared photons to visible photons is reached via a multiphoton absorption process, and finely crystallized host materials are needed to decrease energy losses in multistage electronic transitions. Among such crystals, RE-containing molybdates are widely investigated in terms of searching for new structures, including structure-modulation effects, promising spectroscopic characteristics, and excellent UC photoluminescence (PL) properties [5,6,7,8,9,10,11,12]. In this aspect, binary RE-containing tetragonal molybdates of general composition ARE(MoO4)2 (A = Li, Na, K. Ag) and scheelite-type (ST) structure are of particular interest. The ST compounds crystallize in tetragonal space group I41/a and this crystal family is characterized by wide possibility for the substitution of RE activators at RE sites without structure disruption and significant defect generation. The complex ST molybdates are extensively investigated as host materials in the phosphor preparation and laser technology, respectively [13,14,15,16,17,18,19,20,21,22,23,24].
The present study is aimed at the synthesis and evaluation of AgGd(MoO4)2:Yb3+,Ho3+ phosphor materials. This molybdate is selected as a representative member of the AgRE(MoO4)2 group. Generally, ST molybdates AgRE(MoO4)2 are less studied and, as for AgGd(MoO4)2, only the space group and cell parameters were determined in the past [25] and such basic properties as its crystal structure and spectroscopic characteristics remain unknown. However, there are two reports on the application of AgGd(MoO4)2 as a host in phosphor materials [26,27]. In modern UC materials, operable under the excitation at 980 nm of compact high-power laser diodes, such trivalent RE ions as Ho3+, Tm3+, and Er3+ are commonly used as activators and, in most cases, Yb3+ is applied as a sensitizer. During the UC process, the Ho3+ ion can efficiently convert infrared light to a visible spectral range owing to an appropriate energy level configuration. As a sensitizer, the Yb3+ ion can be smoothly excited by a proper incident light source working at ~980 nm because this ion has a high absorption cross section at this wavelength. Thus, the co-doped Ho3+ and Yb3+ ions can be usefully employed pairwise and drastically enhance the UC efficiency via the consequent energy transfer (ET) process from Yb3+ to Ho3+ [28].
Regarding the methods developed for the synthesis of oxide phosphor compounds, the microwave employed sol–gel (MES) technique has the advantages of a very short process time, stable and homogenous particles, particle size range suitable for the UC process, and high purity resultant products [29,30,31,32,33,34]. It was found that the MES process can provide efficient results accompanying the highly homogeneous morphology and very stable structures. In the present study, the AgGd(MoO4)2:Ho3+,Yb3+ double molybdates with various concentration ratios Yb3+/Ho3+ = 7, 8, 9, 10 were synthesized by the MES-based process, and the crystal structure, composition effects, and spectroscopic characteristics were investigated. Scanning electron microscopy (SEM) was employed to evaluate the crystalline morphology. By the UC measurements using excitation at 980 nm, the resultant phosphors were evaluated and strong 5S2/5F4 → 5I8 transitions (in the green spectral range) and intense transitions of 5F5 → 5I8 (in the red spectral range) of Ho3+ were observed in terms of the ET process. The evaluated UC, Commission Internationale de L’Eclairage (CIE) coordinates, and Raman spectroscopic results were discussed in detail.

2. Experimental Procedure

In the present experiment, the fabrication of AGM:xHo,yYb (AgGd1−x−yHoxYby(MoO4)2) double molybdates with the precise doping of xHo,yYb (x = 0, 0.05; y = 0, 0.35, 0.4, 0.45, 0.5) was carried out by the MES process. In the preparation of AGM:HoYb materials, AgNO3 at purity 99.9% were received from Kojima Chemicals, Japan, (NH4)6Mo7O24∙4H2O at purity of 99.0%, as well as Gd(NO3)3∙6H2O, Yb(NO3)3∙5H2O and Ho(NO3)3∙5H2O at of purity 99.9%, were used as received from Sigma-Aldrich, USA. Besides, citric acid (CA) at purity 99.5% was received from Daejung Chemicals, Korea. Distilled water (DW), ethylene glycol (EG, A.R.), and NH4OH (A.R.) were used to bring about the transparent sol formation.
As the first sequence, to prepare the sol state of (a) AgGd(MoO4)2 (AGM), (NH4)6Mo7O24∙4H2O for 0.057 mol% was slowly dissolved in 80 mL of NH4OH (8M) with 20 mL of EG under a slight heat-treatment. Simultaneously, AgNO3 for 0.4 mol% and Gd(NO3)3∙5H2O for 0.4 mol% were carefully weighed and dissolved very slowly in 100 mL of DW under a slight heat treatment. Then, these two solutions were combined under a vigorous stirring. The CA molar ratio accounting to the numbers of all cation metal (CM) ions should be adjusted to 2:1 (CA/CM). The total volume of the combined solution, 180–200 mL, was heat-treated at ~80–100 °C in Pyrex glass (450 mL) before the MES processing. Finally, the consequent solution reveals a highly transparent state.
As for the doped compounds of AGM:xHo,yYb, the following variations were made for which to prepare the solutions: (b) AGM:0.05Ho,0.35Yb, Gd(NO3)3∙6H2O for 0.24 mol%, Yb(NO3)3∙5H2O for 0.14 mol%, and Ho(NO3)3∙5H2O for 0.02 mol%; (c) AGM:0.05Ho, 0.4Yb, Gd(NO3)3∙6H2O for 0.22 mol%, Yb(NO3)3∙5H2O for 0.16 mol%, and Ho(NO3)3∙5H2O for 0.02 mol%; (d) AGM:0.05Ho, 0.45Yb, Gd(NO3)3∙6H2O for 0.2 mol%, Yb(NO3)3∙5H2O for 0.18 mol%, and Ho(NO3)3∙5H2O for 0.02 mol%; and (e) AGM:0.05Ho,0.5Yb, Gd(NO3)3∙6H2O for 0.18 mol%, Yb(NO3)3∙5H2O for 0.2 mol%, and Ho(NO3)3∙5H2O for 0.02 mol%. The MES-derived process algorithm was previously reported and can be found elsewhere [7,10,12].
The structural properties of synthesized samples were evaluated by XRD analysis. The powder XRD patterns of the AGM:xHo,yYb particles for Rietveld analysis were precisely recorded over the angle range of 2θ = 5–90° at room temperature with a D/MAX 2200 (Rigaku in Japan) diffractometer with the use of Cu-Kα radiation and θ−2θ geometry. The 2θ size step was 0.02°, and the counting time was 5 s per step. The TOPAS 4.2 package was applied for the Rietveld analysis [35]. The typical microstructure and surface morphology of the obtained particles were observed using SEM (JSM−5600, JEOL in Japan) methods. The PL spectra were relatively recorded at room temperature using a spectrophotometer (Perkin Elmer LS55 in UK). The Raman spectra measurements were performed using an LabRam Aramis (Horiba Jobin-Yvon in France) with the spectral resolution of 2 cm−1. The 514.5 nm line of an Ar ion laser was used as an excitation source and, to avoid the sample decomposition, the power on the samples was kept at the 0.5 mW level.

3. Results and Discussion

The difference Rietveld plots of AGM and AGM:xHo,yYb samples are shown in Figure 1. All diffraction peaks are obviously indexed by the tetragonal cell (I41/a) with cell parameters close to those of AgEu(MoO4)2 [36] and NaGd(MoO4)2 [37]. Therefore, these crystal structures were taken as a starting model in Rietveld refinement. The site of (Ag/Eu) or (Na/Gd) ions was considered as that occupied by Ag, Gd, Ho, and Yb ions with fixed occupations according to the suggested formulas, because no loss channel of starting chemicals is expected in the synthesis. The refinement was stable and gave low R-factors (Figure 1). The main parameters of processing and refinement of the AGM and AGM:xHo,yYb samples are presented in Table 1. The crystal structure of AGM:xHo,yYb is shown in Figure 2. The fractional atom coordinates and isotropic displacement parameters (Å2) of the samples are given in Table 2. The main bond lengths (Å) in AGM and AGM:xHo,yYb structures are presented in Table 3. As seen in Figure 1f, the linear decrease of unit cell volume per decrease of average ion radii IR(Ag/Gd/Ho/Yb) on doping is observed in the AGM and AGM:xHo,yYb molybdates, which proves the suggested chemical formula AgGd1−x−yHoxYby(MoO4)2 of the solid solution. The obtained crystallographic data are deposited in Cambridge Crystallographic Data Centre (CSD # 2035192−2035196). The data can be downloaded from the site (www.ccdc.cam.ac.uk/data_request/cif).
It is interesting to consider the field of structural parameters available for ARE(MoO4)2 (A = Li, Na, K. Ag) compounds of the ST structure. All of the ARE(MoO4)2 compounds, the unit cell parameters of which are presently known, are listed in Table 4 and shown in Figure 3. As is evident from Table 4, the giant cell volume variation, by ~25%, is possible in the ARE(MoO4)2 crystal family, which indicates the high stability of this structure.
As seen in Figure 3, on the a–c plane, the ARE(MoO4)2 (A = Li, Na) crystals lie on the straight line determined by the relation c = 2.76(5) × a − 3.0(2)Å. Three known KRE(MoO4)2 compounds also determine the straight line nearly parallel to that of ARE(MoO4)2 (A = Li, Na) molybdates and this line is determined by the relation c = 3.1(1) × a − 4.5(7)Å. In KRe(MoO4)2 compounds, the boundary of the existence of the ST structure is reached and KNd(MoO4)2 crystallizes in monoclinic space group P21/n [55]. It is unexpected, but the AgRE(MoO4)2 crystals lie on the new straight line with a more shallow slope and the equation c = 1.9(3) × a + 1.83(6)Å. Thus, the structural behavior of AgRE(MoO4)2 with the ST structure is specific and is different from that of ARE(MoO4)2 (A = Li, Na, K) compounds. As for the position of AgGd(MoO4)2, this molybdate is in the middle part of the AgRE(MoO4)2 order and provides a possibility for a wide-range substitution of the Gd3+ ion by other RE3+ ions.
The SEM images of AGM and AGM:xHo,yYb samples are shown in Figure 4 and Figure S1, respectively. It can be stated that the final products are characterized by a homogeneous morphology. There are no specific morphological features that could be attributed to doping effects. The particle partial coalescence into agglomerates is observed in all samples. The particle size range is around 3–5 μm. Such a morphology is assumed to be induced by the material inter-diffusion between the heated grains at 600–850 °C.
In Figure 5, the UC emission spectra of AGM:0.05Ho,yYb samples are shown, and the intensity of UC luminescence is presented in the logarithmic scale. The known schematic energy level diagrams of Ho3+ (activator) and Yb3+ (sensitizer) ions in the AGM:0.05Ho,yYb samples and the UC mechanisms, accounting for the green and red emissions under the 980 nm laser excitation, are given in Figure 6. Under the excitation at 980 nm, the doped samples AGM:0.05Ho,yYb exhibited a yellowish green emission composed of red and green emission bands. At the red and green wavelengths, the Ho3+ions show strong transitions 5F5 → 5I8 and 5S2/5F4 → 5I8, respectively. In the UC intensity competition between the samples, the AGM:0.05Ho,0.45Yb particles provide the strongest 545 and 655 nm emission bands. Other samples in emission intensity are in the order of AGM:0.05Ho,0.35Yb, AGM:0.05Ho,0.40Yb, and AGM:0.05Ho,0.50Yb. Thus, the optimal Yb3+/Ho3+ ratio is revealed to be 9:1. The UC luminescence intensity at the main bands drastically increases as the Yb content grows from 0.35 to 0.4, then continues to increase at a slower rate as x grows from 0.4 to 0.45, and then drops as x grows to 0.5. This means that, at x approximately equal to 0.3, the critical distance condition for the energy transfer is achieved between the Ho3+ ion and a pair of simultaneously excited Yb3+ ions in the vicinity of the same Ho3+ion. At x > 0.45, the number of simultaneously excited Yb3+ ions at the range within the critical distance of the energy transfer exceeds 2, and the excitation of extra Yb3+ ion cannot contribute to the two-step UC process; therefore, an additional excited Yb3+ ion is unable to transfer its energy to Ho3+ and decays back to the ground state. One should mention that the UC luminescence intensity distribution between red and green channels in AGM:0.05Ho,yYb is featured by the domination of the red channel, like in most of the earlier studied molybdate [10,56,57] and tungstate hosts, except for NaPbLa(WO4)3 [34]. Therefore, the unique effect of Pb on the crystalline lattice in NaPbLa(WO4)3 is not reproduced in the case of AgGd(MoO4)2.
Besides two main UC luminescence bands specified above, several minor luminescent bands are observable in a logarithmic spectrum. First of all, the weakest band peaking at 490 nm should be mentioned. This peak should be ascribed to the 5F2,3→ 5I8 transition that starts from two closely lying states that have the smallest detuning for the two-photon cooperative UC process under the 980 nm pumping. Therefore, these states are expected to be primarily populated via the UC process and, in the absence of relaxation, the band at 490 nm would be the highest one. The extreme weakness of this band indicates the efficient relaxation of the population of initially excited levels that can happen via two possible channels, namely, via the decay to 5S2/5F4 and via the cross-relaxation to 5F5. Relative suppression of the cross-relaxation in the NaPbLa(WO4)3 lattice can be the origin of equalization of the red and green bands discovered earlier [34]. As we see, in AGM, the intensity of both relaxation channels specified above is just the same as in the majority of other molybdate and tungstate hosts, where Ho3+ upconversion was studied earlier [10,56,57].
Another minor peak in the UC luminescence spectra is observed at 459 nm and should be ascribed to the 3K8→ 5I8 transition. The starting level of this transition is too high to be populated directly by the two-step process or thermal excitation from 5F2,3. Therefore, the existence of the corresponding band admits the presence of some additional three-step UC process, most likely, cascading 5S2/5F4 → 5G3 excitation via the energy transfer from the Yb3+ ion. The peak at 750 nm is assigned to the transition from the highly populated 5S2/5F4 level to the excited 5I8 state, and its weakness suggests that the branching ratio for this transition is not enhanced in the AGM host, with respect to other ones. However, the last observed peak at 800 nm is not frequently observed, and it can be tentatively assigned to the transition from the 5I4 level populated from the closely lying 5F5 state.
In Figure 7, the calculated chromaticity coordinates (x, y) and CIE chromaticity diagram are shown for the compositions of UC output emission spectra of (a) AgGd0.6Ho0.05Yb0.35(MoO4)2, (b) AgGd0.55Ho0.05Yb0.40(MoO4)2, (c) AgGd0.50Ho0.05Yb0.45(MoO4)2, and (d) AgGd0.45Ho0.05Yb0.50(MoO4)2. The triangle depicted in Figure 7B indicates the standard coordinates for blue, green, and red colors. The inset in Figure 7B shows the chromaticity points for the samples (a), (b), (c), and (d). The chromaticity coordinates (x, y) are strongly dependent on the Ho3+/Yb3+ concentration ratio. As shown in Figure 7A, the calculated chromaticity coordinates x = 0.388 and y = 0.394 for (a) AgGd0.6(MoO4)2:Ho0.05Yb0.35, x = 0.407 and y = 0.432 for (b) AgGd0.55(MoO4)2:Ho0.05Yb0.40, x = 0.399 and y = 0.424 for (c) AgGd0.50(MoO4)2:Ho0.05Yb0.45, and x = 0.363 and y = 0.364 for (d) AgGd0.45(MoO4)2:Ho0.05Yb0.5 correspond to the standard equal energy point in the CIE diagram shown in Figure 7B.
The Raman spectra of AGM:xHo,yYb (x = 0, 0.05; y = 0, 0.35, 0.4, 0.45, 0.5) powder samples are presented in Figure 8. As shown above, all compounds under consideration have the tetragonal ST-type structure (space group I41/a, C4h6 symmetry) and they consist of MoO4 tetrahedra and (Ag/Gd/Ho/Yb)O8 polyhedra. The high-frequency boundary of spectral bands related to the vibrations of AGM structural units is expressed by a high-intensity peak of the ν1 MoO4 symmetric stretching vibration typical of ST molybdates [10,22,24,58]. In case of the samples doped with Ho3+/Yb3+ ions, the extra spectral bands associated with the luminescence of Ho3+ ions appear in the high wavenumber part of the spectra, as seen in Figure 8. The medium intensity spectral bands in the range of 730–835 cm−1 are attributed to the ν3 asymmetric stretching vibrations of [MoO4]2− ions. The internal stretching and bending vibrations of molybdate tetrahedra are separated by the gap over 430–730 cm−1 [9]. The ν4 MoO4 bending modes appear in the range of 365–435 cm−1. The ν2 bending vibrations can be observed in Figure 8 as the strong bands in the range from 290 to 350 cm−1. The Raman band at 201 cm−1 is identified as a free rotation of molybdate tetrahedra [24]. The remaining modes below 200 cm−1 are attributed to the translations of MoO4 and vibrations of (Ag/Gd/Ho/Yb)O8 polyhedra.
It can be seen from the group theory calculations that the primitive cell of AGM has 30 vibration modes, and the mechanical representation in the center of Brillouin zone can be written as follows: Γvibr= 3Ag + 6Bg + 6Eg + 6Au + 3Bu + 6Eu, where g-labeled modes are active in the Raman spectrum, Au and Eu—modes are active in the infrared spectrum, and Bu modes are silent. In Table 5, one can check the number of active spectral bands in Raman and infrared spectra related to the internal vibrations of [MoO4]2− ions [58]. The ST oxides with the C4h symmetry should show only one spectral band related to the ν1 MoO4 symmetric stretching vibration, but an extra band appears as the right shoulder of the high-intensity peak at 880 cm−1 (Figure 8). Such spectral characteristic can be due to the fact that the crystal can have symmetry lower than C4h, for example, S4 (I−4 space group) [11]. In this case, the crystal structure contains two crystallographically independent MoO4 tetrahedra and the Raman spectra should be more in line in the range of their vibrations. On the other hand, the local distortions of [MoO4]2− ions in the structure may occur because of the fact that the occupancy of Ag and Gd atoms in AGM is not equal to one (Table 2), and even a small difference in Mo-O bond lengths can cause the displacements of symmetric stretching vibration wavenumber. As mentioned above, Bu modes are silent in ST crystals. However, in the case of some AgRE(MoO4)2 (RE = La-Nd and Sm) [54], they become infrared active and appear, for example, as the [MoO4]2− ion symmetric stretching vibration that is in agreement with Table 5. At the same time, the appearance of such a spectral band in the infrared spectrum cannot be described in the framework of the structure with the S4 symmetry, because the vibration mode labeled as A (Table 5) should be active only in the Raman spectrum.

4. Conclusions

The AgGd1−x(MoO4)2:Ho3+/Yb3+ double molybdates with five Ho3+/Yb3+doping levels were synthesized by the MES-based process, and the refinement of their crystal structure was implemented for the first time. The effects of solid solution composition and spectroscopic characteristics were investigated. The phosphor samples heated at 850 °C for 16 h showed the fine and homogeneous morphology with particles sized 3–5 μm. The powder diffraction data of AgGd(MoO4)2:xHo,yYb (x = 0, 0.05; y = 0, 0.35, 0.4, 0.45, 0.5) for Rietveld analysis were collected at room temperature and the crystal structures were determined in the tetragonal space group I41/a with parameters close to those of AgEu(MoO4)2 and NaGd(MoO4)2. The site of (Ag/Eu) or (Na/Gd) ion was occupied by Ag+, Gd3+, Ho3+, and Yb3+ ions with fixed occupations, and led to low R-factors. The linear decrease of the cell volume was observed on the doping level increase. In the UC measurements, using the excitation at 980 nm, the resultant phosphors showed yellowish green output emissions derived from the strong 5S2/5F4 → 5I8 and 5F5 → 5I8 transitions of Ho3+ ions. The optimal concentration ratio Yb3+:Ho3+ was revealed to be 9:1. The behavior of UC intensity is dependent on the Yb/Ho ratio and is explained in terms of the optimal number of Yb3+ ions at the characteristic energy transfer distance around the Ho3+ ion. The room temperature Raman spectra were analyzed to obtain information on the AGM:xHoyYb crystal structure. The nature of extra bands was explained in the framework of the local distortions of MoO4 tetrahedra.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4352/10/11/1000/s1.

Author Contributions

Conceptualization, A.A.; Data curation, C.S.L.; Investigation, C.S.L., A.A., V.A., M.M. and A.O. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Research Program through the Campus Research Foundation funded by Hanseo University in 2020 (201Yunghap09).

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 10 01000 g001 550
Figure 1. Difference Rietveld plots of AgGd1−x−yHoxYby(MoO4)2: (a) x = 0, y = 0; (b) x = 0.05, y = 0.35; (c) x = 0.05, y = 0.4; (d) x = 0.05, y = 0.45; (e) x = 0.05, y = 0.5. (f) Cell volume per average ion radii IR(Ag/Gd/Ho/Yb) in AgGd1−x−yHoxYby(MoO4)2.
Figure 1. Difference Rietveld plots of AgGd1−x−yHoxYby(MoO4)2: (a) x = 0, y = 0; (b) x = 0.05, y = 0.35; (c) x = 0.05, y = 0.4; (d) x = 0.05, y = 0.45; (e) x = 0.05, y = 0.5. (f) Cell volume per average ion radii IR(Ag/Gd/Ho/Yb) in AgGd1−x−yHoxYby(MoO4)2.
Crystals 10 01000 g001
Crystals 10 01000 g002 550
Figure 2. Crystal structure of AgGd1−x−yHoxYby(MoO4)2. The unit cell is outlined. The lone atoms are omitted for clarity.
Figure 2. Crystal structure of AgGd1−x−yHoxYby(MoO4)2. The unit cell is outlined. The lone atoms are omitted for clarity.
Crystals 10 01000 g002
Crystals 10 01000 g003 550
Figure 3. Cell parameter field c–a of AB(MoO4)2 (A = Li, Na, Ag, K; B = RE) crystallized in space group I41/a.
Figure 3. Cell parameter field c–a of AB(MoO4)2 (A = Li, Na, Ag, K; B = RE) crystallized in space group I41/a.
Crystals 10 01000 g003
Crystals 10 01000 g004 550
Figure 4. Scanning electron microscopy image of the synthesized AgGd(MoO4)2 sample.
Figure 4. Scanning electron microscopy image of the synthesized AgGd(MoO4)2 sample.
Crystals 10 01000 g004
Crystals 10 01000 g005 550
Figure 5. The upconversion (UC) output emission spectra of (a) AgGd0.6Ho0.05Yb0.35(MoO4)2, (b) AgGd0.55Ho0.05Yb0.40(MoO4)2, (c) AgGd0.50Ho0.05Yb0.45(MoO4)2, and (d) AgGd0.45Ho0.05Yb0.50(MoO4)2 excited by 980 nm at room temperature.
Figure 5. The upconversion (UC) output emission spectra of (a) AgGd0.6Ho0.05Yb0.35(MoO4)2, (b) AgGd0.55Ho0.05Yb0.40(MoO4)2, (c) AgGd0.50Ho0.05Yb0.45(MoO4)2, and (d) AgGd0.45Ho0.05Yb0.50(MoO4)2 excited by 980 nm at room temperature.
Crystals 10 01000 g005
Crystals 10 01000 g006 550
Figure 6. The schematic energy level diagrams of Yb3+ (sensitizer) and Ho3+ (activator) ions in the AgGd0.95−yHo0.05Yby(MoO4)2 system and the upconversion mechanisms of the green and red emissions under 980 nm laser excitation: ET (energy transfer), ESA (Excited state absorption), GSA (Ground state absorption).
Figure 6. The schematic energy level diagrams of Yb3+ (sensitizer) and Ho3+ (activator) ions in the AgGd0.95−yHo0.05Yby(MoO4)2 system and the upconversion mechanisms of the green and red emissions under 980 nm laser excitation: ET (energy transfer), ESA (Excited state absorption), GSA (Ground state absorption).
Crystals 10 01000 g006
Crystals 10 01000 g007 550
Figure 7. (A) Calculated chromaticity coordinates (x, y) values and (B) CIE chromaticity diagram for AgGd(MoO4)2:Ho3+/Yb3+phosphors. In the inset are the emission points for the synthesized (a) AgGd0(MoO4)2, (b) AgGd0.6Ho0.05Yb0.35(MoO4)2, (c) AgGd0.55Ho0.05Yb0.40(MoO4)2, (d) AgGd0.50Ho0.05Yb0.45(MoO4)2, and (e) AgGd0.45Ho0.05Yb0.50(MoO4)2 samples.
Figure 7. (A) Calculated chromaticity coordinates (x, y) values and (B) CIE chromaticity diagram for AgGd(MoO4)2:Ho3+/Yb3+phosphors. In the inset are the emission points for the synthesized (a) AgGd0(MoO4)2, (b) AgGd0.6Ho0.05Yb0.35(MoO4)2, (c) AgGd0.55Ho0.05Yb0.40(MoO4)2, (d) AgGd0.50Ho0.05Yb0.45(MoO4)2, and (e) AgGd0.45Ho0.05Yb0.50(MoO4)2 samples.
Crystals 10 01000 g007
Crystals 10 01000 g008 550
Figure 8. Raman spectra of AGM and AGM:xHo,yYb (x = 0.05; y = 0.35, 0.4, 0.45, 0.5) samples.
Figure 8. Raman spectra of AGM and AGM:xHo,yYb (x = 0.05; y = 0.35, 0.4, 0.45, 0.5) samples.
Crystals 10 01000 g008
Table
Table 1. Main parameters of processing and refinement of the AgGd1−x−yHoxYby(MoO4)2 samples.
Table 1. Main parameters of processing and refinement of the AgGd1−x−yHoxYby(MoO4)2 samples.
CompoundAgGd(MoO4)2AgGd0.6Ho0.05
Yb0.35(MoO4)2		AgGd0.55Ho0.05
Yb0.4(MoO4)2		AgGd0.5Ho0.05
Yb0.45(MoO4)2		AgGd0.45Ho0.05
Yb0.5(MoO4)2
x00.050.050.050.05
y00.350.40.450.5
Sp.Gr.I41/aI41/aI41/aI41/aI41/a
a, Å5.24782 (11)5.22237 (8)5.21802 (13)5.21598 (9)5.21239 (8)
c, Å11.5107 (3)11.4581 (2)11.4498 (3)11.4455 (2)11,4394 (3)
V, Å3317.002 (17)312.497 (11)311.753 (18)311.392 (12)310,798 (12)
Z44444
2θ-interval, º5–905–905–905–905–90
Rwp, %19.2316.5021.8616.6416.06
Rp, %11.919.7115.0810.559.61
Rexp, %16.7614.9117.0014.3313.87
RB, %3.141.938.652.502.31
χ21.151.111.291.161.16
Table
Table 2. Fractional atomic coordinates and isotropic displacement parameters (Å2) of AgGd1−x−yHoxYby(MoO4)2 samples.
Table 2. Fractional atomic coordinates and isotropic displacement parameters (Å2) of AgGd1−x−yHoxYby(MoO4)2 samples.
xyzBisoOcc.
AgGd(MoO4)2
Ag00.250.6250.6 (3)0.5
Gd00.250.6250.6 (3)0.5
Mo00.250.1250.5 (3)1
O0.242 (3)0.1068 (17)0.0408 (8)0.8 (4)1
AgGd0.6Ho0.05Yb0.35(MoO4)2
Ag00.250.6250.3 (3)0.5
Gd00.250.6250.3 (3)0.3
Ho00.250.6250.3 (3)0.025
Yb00.250.6250.3 (3)0.175
Mo00.250.1250.5 (3)1
O0.237 (2)0.1015 (15)0.0394 (7)0.8 (4)1
AgGd0.55Ho0.05Yb0.4(MoO4)2
Ag00.250.6250.2 (3)0.5
Gd00.250.6250.2 (3)0.275
Ho00.250.6250.2 (3)0.025
Yb00.250.6250.2 (3)0.2
Mo00.250.1250.5 (3)1
O0.240 (3)0.1043 (19)0.0412 (9)0.5 (4)1
AgGd0.5Ho0.05Yb0.45(MoO4)2
Ag00.250.6250.5 (3)0.5
Gd00.250.6250.5 (3)0.25
Ho00.250.6250.5 (3)0.025
Yb00.250.6250.5 (3)0.225
Mo00.250.1250.8 (2)1
O0.238 (3)0.1010 (15)0.0393 (7)1.1 (4)1
AgGd0.45Ho0.05Yb0.5(MoO4)2
Ag00.250.6250.5 (3)0.5
Gd00.250.6250.5 (3)0.225
Ho00.250.6250.5 (3)0.025
Yb00.250.6250.5 (3)0.25
Mo00.250.1250.6 (3)1
O0.239 (2)0.1041 (14)0.0393 (6)0.6 (4)1
Table
Table 3. Main bond lengths (Å) of AgGd1−x−yHoxYby(MoO4)2 samples.
Table 3. Main bond lengths (Å) of AgGd1−x−yHoxYby(MoO4)2 samples.
AgGd(MoO4)2
(Ag/Gd)—Oi2.458 (12)Mo—O1.765 (13)
(Ag/Gd)—Oii2.506 (12)
AgGd0.6Ho0.05Yb0.35(MoO4)2
(Ag/Gd/Ho/Yb)—Oi2.494 (9)Mo—O1.759 (9)
(Ag/Gd/Ho/Yb)—Oii2.457 (9)
AgGd0.55Ho0.05Yb0.4(MoO4)2
(Ag/Gd/Ho/Yb)—Oi2.486 (13)Mo—O1.751 (13)
(Ag/Gd/Ho/Yb)—Oii2.458 (12)
AgGd0.5Ho0.05Yb0.45(MoO4)2
(Ag/Gd/Ho/Yb)—Oi2.486 (13)Mo—O1.763 (12)
(Ag/Gd/Ho/Yb)—Oii2.451 (11)
AgGd0.45Ho0.05Yb0.5(MoO4)2
(Ag/Gd/Ho/Yb)—Oi2.494 (9)Mo—O1.758 (9)
(Ag/Gd/Ho/Yb)—Oii2.442 (8)
Symmetry codes: (i) -x + 1/2, -y, z + 1/2; (ii) y−1/4, -x + 3/4, z + 3/4.
Table
Table 4. Cell parameters of AB(MoO4)2 compounds with I41/a space group.
Table 4. Cell parameters of AB(MoO4)2 compounds with I41/a space group.
ABa, Åc, ÅV, Å3References
LiLu5.1033211.0829288.6417[36]
LiYb5.1411.14294.31[38]
LiTm5.1411.16294.84[38]
LiY5.14811.173296.106[39]
LiEr5.1511.19296.79[38]
LiHo5.1611.22298.74[38]
LiDy5.1811.28302.67[38]
LiTb5.1911.29304.11[38]
LiGd5.19211.31304.88[40]
LiEu5.20262511.33824306.896[36]
LiSm5.2211.37309.81[40]
LiNd5.24311.44314.47[39]
LiPr5.264311.5011318.728[41]
LiCe5.28911.58323.93[42]
LiLa5.3311.69332.10[38]
NaLu5.159311.246299.350[21]
NaYb5.17064211.2454300.652[43]
NaEr5.181611.288303.07[44]
NaY5.198911.3299306.231[45]
NaGd5.24411.487315.887[37]
NaEu5.279711.5869322.988[46]
NaNd5.287111.5729323.502[47]
NaCe5.316711.66329.597[48]
NaLa5.343311.7432335.278[49]
KPr5.40512.05352.03[50]
KCe5.413412.0821354.065[51]
KLa5.4512.19362.07[49]
AgLu5.17255611.39257304.812[36]
AgYb5.181911.4317306.965[52]
AgTm5.196611.4271308.585[52]
AgHo5.216811.471312.183[52]
AgGd0.9Eu0.15.22211.476312.94[27]
AgGd5.228211.4869313.984[25]
AgDy5.229611.4883314.190[53]
AgTb5.24211.4995315.990[52]
AgEu5.2633411.54333319.782[36]
AgSm5.273911.56321.53[54]
AgNd5.309911.6352328.055[54]
AgPr5.316411.648329.220[54]
AgCe5.333311.686332.398[54]
AgLa5.36411.7588338.330[54]
Table
Table 5. Correlation diagram for the MoO4 tetrahedra in structures with C4h and S4 symmetry.
Table 5. Correlation diagram for the MoO4 tetrahedra in structures with C4h and S4 symmetry.
Free Ion
Td		Site
S4		Factor Group
C4h
ν1, (A1)AAg + Bu
ν2, (A1)A+BAg + Bu + Au + Bg
ν3, ν4 (T2)B+EAu + Bg + Eg + Eu
Free ion
Td		Site
S4		Factor Group
S4
ν1, (A1)AA
ν2, (A1)A + BA + B
ν3, ν4 (T2)B + EB + E
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Sung Lim, C.; Aleksandrovsky, A.; Atuchin, V.; Molokeev, M.; Oreshonkov, A. Microwave-Employed Sol–Gel Synthesis of Scheelite-Type Microcrystalline AgGd(MoO4)2:Yb3+/Ho3+ Upconversion Yellow Phosphors and Their Spectroscopic Properties. Crystals 2020, 10, 1000. https://doi.org/10.3390/cryst10111000

AMA Style

Sung Lim C, Aleksandrovsky A, Atuchin V, Molokeev M, Oreshonkov A. Microwave-Employed Sol–Gel Synthesis of Scheelite-Type Microcrystalline AgGd(MoO4)2:Yb3+/Ho3+ Upconversion Yellow Phosphors and Their Spectroscopic Properties. Crystals. 2020; 10(11):1000. https://doi.org/10.3390/cryst10111000

Chicago/Turabian Style

Sung Lim, Chang, Aleksandr Aleksandrovsky, Victor Atuchin, Maxim Molokeev, and Aleksandr Oreshonkov. 2020. "Microwave-Employed Sol–Gel Synthesis of Scheelite-Type Microcrystalline AgGd(MoO4)2:Yb3+/Ho3+ Upconversion Yellow Phosphors and Their Spectroscopic Properties" Crystals 10, no. 11: 1000. https://doi.org/10.3390/cryst10111000

APA Style

Sung Lim, C., Aleksandrovsky, A., Atuchin, V., Molokeev, M., & Oreshonkov, A. (2020). Microwave-Employed Sol–Gel Synthesis of Scheelite-Type Microcrystalline AgGd(MoO4)2:Yb3+/Ho3+ Upconversion Yellow Phosphors and Their Spectroscopic Properties. Crystals, 10(11), 1000. https://doi.org/10.3390/cryst10111000

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