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Article

Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 Upconversion Scheelite-Type Phosphors

by
Chang Sung Lim
1,*,
Aleksandr Aleksandrovsky
2,3,
Maxim Molokeev
4,5,6,
Aleksandr Oreshonkov
7,8,* and
Victor Atuchin
9,10,11,12
1
Department of Aerospace Advanced Materials and Chemical Engineering, Hanseo University, Seosan 31962, Republic of 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 Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, 660036 Krasnoyarsk, Russia
5
Institute of Engineering Physics and Radioelectronics, Siberian Federal University, 660041 Krasnoyarsk, Russia
6
Department of Physics, Far Eastern State Transport University, 680021 Khabarovsk, Russia
7
Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, 660036 Krasnoyarsk, Russia
8
School of Engineering and Construction, Siberian Federal University, 660041 Krasnoyarsk, Russia
9
Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, 630090 Novosibirsk, Russia
10
Research and Development Department, Kemerovo State University, 650000 Kemerovo, Russia
11
Department of Industrial Machinery Design, Novosibirsk State Technical University, 630073 Novosibirsk, Russia
12
R&D Center “Advanced Electronic Technologies”, Tomsk State University, 634034 Tomsk, Russia
 
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Authors to whom correspondence should be addressed.
Crystals 2023, 13(2), 362; https://doi.org/10.3390/cryst13020362
Submission received: 6 February 2023 / Revised: 16 February 2023 / Accepted: 17 February 2023 / Published: 20 February 2023
(This article belongs to the Special Issue Rare Earths-Doped Materials (Volume II))
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Abstract

:
New triple molybdates LixNa1−xCaLa0.5(MoO4)3:Er3+0.05/Yb3+0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) were manufactured successfully using the microwave-assisted sol-gel-based technique (MAS). Their room-temperature crystal structures were determined in space group I41/a by Rietveld analysis. The compounds were found to have a scheelite-type structure. In Li-substituted samples, the sites of big cations were occupied by a mixture of (Li, Na, La, Er, Yb) ions, which provided a linear cell volume decrease with the Li content increase. The increased upconversion (UC) efficiency and Raman spectroscopic properties of the phosphors were discussed in detail. The mechanism of optimization of upconversion luminescence upon Li content variation was shown to be due to the control of excitation/energy transfer channel, while the control of luminescence channels played a minor role. The UC luminescence maximized at lithium content x = 0.05. The mechanism of UC optimization was shown to be due to the control of excitation/energy transfer channel, while the control of luminescence channels played a minor role. Over the whole spectral range, the Raman spectra of LixNa1−xCaLa0.5(MoO4)3 doped with Er3+ and Yb3+ ions were totally superimposed with the luminescence signal of Er3+ ions, and increasing the Li+ content resulted in the difference of Er3+ multiple intensity. The density functional theory calculations with the account for the structural disorder in the system of Li, Na, Ca, La, Er and Yb ions revealed the bandgap variation from 3.99 to 4.137 eV due to the changing of Li content. It was found that the direct electronic transition energy was close to the indirect one for all compounds. The determined chromaticity points (ICP) of the LiNaCaLa(MoO4)3:Er3+,Yb3+ phosphors were in good relation to the equal-energy point in the standard CIE (Commission Internationale de L’Eclairage) coordinates.

1. Introduction

Complex molybdates are among the most widely studied oxide materials in modern electronics and photonics owing to their excellent performances [1,2,3,4,5,6]. It was noted that over the past few decades that special research attention was paid to the use of molybdate hosts in the design of new optical and electronic materials [7,8,9,10,11,12], and the syntheses of efficient rare-earth-containing molybdate-based phosphor materials working in the visible range became a hot topic of solid-state technology because of the possible wide-range cation substitution and related tuning of physical properties [6,8,11,12,13,14,15,16].
Recently, the compounds with a scheelite-type structure were proposed as efficient phosphor hosts [3,10,11,12,17,18,19,20]. As it can be reasonably assumed, in the disordered scheelite-type structure, the trivalent rare-earth ions could be substituted, at least partially, by different Ln3+ ions, and the crystal lattice efficiently could incorporate the ions due to similar radii of Ln3+ ions, and that would increase the limit at the appropriate doping level. Among rare-earth ions, the Er3+ ion can be used for the infrared to visible light conversion through the frequency up-conversion (UC) process according to its appropriate electronic energy level configuration. The Yb3+ ions co-doping can remarkably enhance the UC yield, and it is provided by the efficient Yb3+ → Er3+energy transfer. In this cation pair, the Er3+ ion activator is the luminescence center in the UC particles, and the Yb3+ sensitizer increases the UC luminescence efficiency [21,22,23,24,25].
The simple and double molybdate crystals are widely applied in photonic and laser technologies because of their specific structural, thermal and electronic characteristics, high chemical stability and excellent spectroscopic properties [3,4,10,26,27,28,29,30,31,32,33]. However, ternary molybdates are less studied, and only several ternary compounds, including scheelite-type ones, have been recently considered [12,18,34,35,36,37,38,39,40]. For the practical application of UC photoluminescence in devices and systems, such as three-dimensional displays, lasers, light-emitting elements and biological detectors, the characteristics, such as the homogeneous UC particle size distribution and uniform morphology, need to be reached. Usually, complex molybdates are prepared by a multistep solid-state reaction method, and it requires high temperatures, long heating process and subsequent grinding. The stages may occasionally result in a loss of the emission intensity. In comparison, the sol-gel-based process has some advantages, including high starting atom intermixing, lower calcination temperature, small particle size and a narrow particle size distribution, which are promising for good phosphor characteristics. However, a disadvantage of long gelation time is common for sol-gel process. In comparison to the traditional methods, a very short reaction time, small-sized particles, narrow particle size distribution and high purity of the final polycrystalline product are the characteristics of microwave synthesis. Microwave heating is delivered to the material surface through the convection and/or radiant heating which is transferred to the material bulk via conduction [3,12,14,41,42,43,44]. Thus, the microwave sol-gel process is an efficient method that yields high-homogeneity powder products, and it is emerging as a viable alternative approach to the quick synthesis of high-quality crystalline luminescent materials.
In the present study, the LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 compounds with the fixed Er3+ and Yb3+ contents and x = 0–0.3 were synthesized by the microwave sol-gel method, and the structural and spectroscopic properties were evaluated to determine the effects of Li+ substitution for Na+. Previously, the scheelite-type NaCaLa(MoO4)3:Er,Yb solid solutions were observed and the rare earth element ratio (La0.5Er0.05Yb0.45) was determined as optimal to reach strong UC emission [45]. Thus, in the present work, only this rare earth element ratio was selected, and a wide variation of the Li/Na ratio was implemented to elucidate the solid solution range and related spectroscopic effects. The synthesized products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and energy-dispersive X-ray spectroscopy (EDS). The spectroscopic properties were comparatively evaluated using Raman spectroscopy and photoluminescence (PL) measurements.

2. Experimental Methods

2.1. Preparation of Chemical Solutions and Phosphor Products

LiNO3, La(NO3)3∙6H2O, Ca(NO3)2∙4H2O, Na2MoO4∙2H2O of purity 99.0% (Sigma-Aldrich, St. Louis, MO, USA) and (NH4)6Mo7O24∙4H2O (Alfa Aesar, Haverhill, MA, USA) of purity 99.0% were employed as the starting reagents. Additionally, for the precise doping levels, Er(NO3)3∙5H2O and Yb(NO3)3∙5H2O in purity 99.9% (Sigma-Aldrich, USA) were applied. Normal citric acid (CA) at purity 99.5% was used as received from Daejung Chemicals Company, Republic of Korea. As to other chemical reagents, NH4OH (A.R.), ethylene glycol (EG, A.R.) and distilled water (DW) were available to approach the transparent chemical solutions.
The nominal compositions of the LixNa1−xCaLa(MoO4)3:Er3+0.05/Yb3+0.45 samples synthesized in the experiment are listed in Table 1. For the Li-free composition, NaCaLa0.5(MoO4)3:Er3+0.05/Yb3+0.45 ((a) NCLM:EY), 0.4 mol% Ca(NO3)2∙4H2O, 0.2 mol% Na2MoO4∙2H2O and 0.171 mol% (NH4)6Mo7O24∙4H2O were dissolved in one 250 mL. For the second compound, Li0.05Na0.95CaLa0.5Er0.05Yb0.45(MoO4)3 ((b) LiNCLM:EY-0.05), 0.4 mol% Ca(NO3)2∙4H2O, 0.19 mol% Na2MoO4∙2H2O, 0.02 mol% LiNO3, and 0.171 mol% (NH4)6Mo7O24∙4H2O were used. For the third composition, Li0.1Na0.9CaLa0.5Er0.05Yb0.45(MoO4)3 ((c) LiNCLM:EY-0.1), 0.4 mol% Ca(NO3)2∙4H2O, 0.18 mol% Na2MoO4∙2H2O, 0.04 mol LiNO3 and 0.171 mol% (NH4)6Mo7O24∙4H2O were employed. For the fourth compound, Li0.2Na0.8CaLa0.5Er0.05Yb0.45(MoO4)3 ((d) LiNCLM:EY-0.2), 0.4 mol% Ca(NO3)2∙4H2O, 0.16 mol% Na2MoO4∙2H2O, 0.08 mol% LiNO3 and 0.171 mol% (NH4)6Mo7O24∙4H2O were applied. For the fifth compound, Li0.2Na0.8CaLa0.5Er0.05Yb0.45(MoO4)3 ((e) LiNCLM:EY-0.3), 0.4 mol% Ca(NO3)2∙4H2O, 0.14 mol% Na2MoO4∙2H2O, 0.12 mol% LiNO3 and 0.171 mol% (NH4)6Mo7O24∙4H2O were mixed. Each reported reagent mixture was dissolved in a Pyrex glass with the addition of 80 mL 8M NH4OH and 20 mL EG. In another 250 mL Pyrex glass, the solution of the rare-earth compounds in a fixed stoichiometry of 0.2 mol% La(NO3)3∙6H2O, 0.02 mol% Er(NO3)3∙5H2O and 0.18 mol% Yb(NO3)3∙5H2O was prepared by the addition of 100 mL distilled water. Subsequently, the individual two chemical solutions in each sequence for (a)–(e) were co-mixed slowly in a 450 mL Pyrex glass.
The co-mixed solutions were vigorously stirred and adjusted to pH = 7–8 using CA and 8M NH4OH. At this time, the molar ratio of CA to the full employed cation metal ions (CM) would be recommended to reach at 2:1. The solutions were slowly heated up to 80–100 °C before microwave treatments. At this stage, the final solutions of about 20–30 mL in volume appeared to be highly transparent sol formations in 450 mL Pyrex glasses. Then, the resultant solutions were moved into a microwave oven (Samsung Company, Republic of Korea, frequency of 2.45 GHz and maximum power of 1250 W). The parameters of the microwave process applied in the present experiment can be found in previous studies [36,38,45]. After the microwave treatment, the black gels dried at 120 °C were ground and annealed at 850 °C for 16 h in the air atmosphere. The main synthesis steps are shown in Scheme 1. After the annealing stage, the obtained (a) NCLM:EY, (b) LiNCLM:EY-0.05, (c) LiNCLM:EY-0.1, (d) LiNCLM:EY-0.2 and (e) LiNCLM:EY-0.3 powder samples exhibited light pink colors.

2.2. Characterization

The structural properties of the synthesized powder products LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 were determined using an X-ray (D/MAX 2200, Rigaku, Tokyo, Japan) diffractometer equipped with the Cu Kα radiation (λ = 1.5406 Å) source. The scans were carried out over the diffraction angle range of 2θ = 5–90° at room temperature. The 2θ size step was 0.02°, and the counting time was as long as 5 s per step. For the Rietveld analysis, the TOPAS 4.2 software package was used [46]. The micromorphology of the synthesized particles was observed using scanning electron microscopy (SEM) (JSM-5600, JEOL, Tokyo, Japan). The photoluminescence spectra were acquired at room temperature using a spectrophotometer (Perkin Elmer LS55, Beaconsfield, UK). In the measurements, the samples were excited at 980 nm. The Raman scattering measurements were implemented using a LabRam Aramis (Horiba Jobin-Yvon, Palaiseau, France) device with the spectral resolution of 2 cm−1. The 514.5 nm line of an Ar ion laser was exploited as an excitation source and the power on the sample surface was kept at the 0.5 mW level to avoid the possible sample decomposition.

2.3. Quantum-Chemical Calculations

Quantum-chemical calculations were carried out within the framework of density functional theory (DFT) [47,48,49] using the CASTEP (Cambridge Serial Total Energy Package) code [50]. The meta-generalized gradient approximation (meta-GGA) and RSCAN [51] (improved version of SCAN [52]) functional were chosen. The on-the-fly generated norm-conserving pseudopotentials [53] with a cutoff energy equal to 800 eV were used for all investigated compounds. The Brillouin zone (BZ) was sampled according to the Monkhorst–Pack scheme [54] with the 4 × 4 × 5 k-point mesh that provided separation of k-points equal to 0.05 Å−1. Considering that the structural site of Ca ion was occupied by a mixture of Na, Li, Ca, La, Er and Yb ions with the total occupancy equal to 1, the mixed ion was simulated within the virtual crystal approximation [55]. The tolerance for ground-state wavefunctions founding was chosen to be 2 × 10−6 eV. Pulay density mixing scheme was chosen for electronic minimization.

3. Results and Discussion

3.1. Particle Morphology and Phase Composition

The SEM patterns of the synthesized powder samples are shown in Figure 1 and Figure S1 (see Supplementary Materials). In general, the particle morphologies are similar for all samples. The particles are partly coalescent due to the material diffusion between the grains, and this is typical of oxides subjected to a high temperature annealing [9,56,57]. In all SEM patterns, the biggest grains were ~5–7 μm in size. For comparison, in Figure 1, the SEM patterns are shown for pure NCLM:EY and LiNCLM:EY-0.3 with the highest Li content. In both samples, big grains were mixed with low-sized grains. However, in Figure 1b, the presence of partly faceted big grains is evident and the quantity of low-sized particles was lower. Thus, the presence of Li in the intermediate gels stimulated the grain growth and faceting.
The XRD patterns recorded for the samples are presented in Figure 2 and Figure S2. All peaks were well indexed by the tetragonal cell (space group I41/a) with the cell parameters close to CaMoO4 (COD 9009632) [58]. There were no alien peaks detected in the patterns, and it meant that all samples were in the pure phase state. Therefore, the CaMoO4 crystal structure was taken as a starting model for the Rietveld refinement. The Ca ion site was considered as that occupied by Na, Li, Ca, La, Er and Yb ions (Figure 3) with fixed occupancies according to nominal compositions. The refinement was stable and gives low R-factors (Table 2, Figure 2 and Figure S2). The coordinates of atoms and main bond lengths are listed in Tables S1 and S2, respectively. The linear increase in cell volume per average ion radii IR (Na/Li/Ca/La/Er/Yb) proves the suggested chemical compositions (Figure 2c).
Further details of the crystal structures may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; E-mail: [email protected]; http://www.fiz-karlsruhe.de/request_for_deposited_data.html, accessed on 5 February 2023) on quoting the deposition numbers: CSD 2117700-2117704.
The dependence of the unit cell volume on the average ion radius of the cations mixed in the Ca2+ site of the LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 solid solutions is shown in Figure 4. For comparison, the cell volume values in CaMoO4 [58], Na0.5La0.5MoO4 [59] and Na1/3Ca1/3La(1−x−y)/3Erx/3Yby/3MoO4 [45] scheelites are also shown. As can be seen, the previously obtained points of CaMoO4, Na1/3Ca1/3La(1−x−y)/3Erx/3Yby/3MoO4 and Na0.5La0.5MoO4 were well fitted by the linear function general for the scheelite-type structures. Additionally, the points related to the Na1/3Ca1/3La0.5/3Er0.05/3Yb0.45/3MoO4 composition, as obtained in [45] and in this study, practically coincided, and that verifies high reproducibility of sol-gel microwave synthesis in the application to complex molybdates. However, the points related to Li-containing compositions Lix/3Na(1−x)/3CaLa0.5/3Er0.05/3Yb0.45/3MoO4 form a new branch going strongly away from the straight line general for Li-free scheelites. For the first time, this structural effect was detected and considered in detail in [39], and it was attributed to a very small radius of the Li+ ion in reference to the radius of Na+ ion. Thus, in the present work, this unusual structural effect was observed in one more molybdate system.

3.2. Upconversion Properties

The UC emission spectra of Na1−xLixCaLa(MoO4)3:Er,Yb (x = 0, 0,05, 0.1, 0.2 and 0.3) phosphors excited at 980 nm at room temperature are shown in Figure 5. Under the excitation at 980 nm, Yb3+ ions were excited to the 2F5/2 state. A pair of excited Yb3+ ions transferred their excitation to a single neighboring Er3+ ion, thus exciting that Er3+ ion to the 2I11/2 state. Furthermore, this excitation can be distributed to lower-lying 4S3/2 and 4F7/2 states. As a result, the upconversion emission of the samples under study exhibited the emission composed of two strong green bands and one weaker red band of the Er3+ ion, namely the 2I11/2 → 4I15/2, and 4S3/2 → 4I15/2 bands in green and 4F7/2 → 4I15/2 band in red. The variation of peak intensities of green and red bands, as well as the variation of integral intensity of UC luminescence over the whole spectrum, showed similar behavior on the variation of Li/Na ratio, as seen in Figure 6, all of these dependencies maximizing at comparatively low Li content x = 0.05. The Li+ ion incorporation into the lattice of hosts instead of larger ions was efficient for manipulating the crystal field affecting the ions positioned within the second coordination sphere of the Li+ ion or even onto more distant ions. This crystal field manipulation had led to the variation of luminescence of doping ions positioned in the second coordination sphere of the Li+ ion (see, e.g., [39]. It is highly likely that Li+ ions did not severely alter the local symmetry of the rare-earth ions in the lattice, but they enabled useful crystal field variations. According to Figure 2c, the cell volume of the solid solution under study monotonically decreases upon the Li+ content. Thus, in the first approximation, we can expect the monotonical increase in crystal field strengths on Er3+ and Yb3+ ions. This increase can affect either oscillator strengths in the radiative recombination channels or the efficiency of upconversion excitation process. Hence, there are two ways to optimize the upconversion luminescence, namely (1) to control the full probability of energy transfer from Yb ions to Er ions in the excitation channel or (2) to control the oscillator strengths in the emission channel in order to increase radiative recombination rate against non-radiative one. The similarity of the concentration dependence of upconversion luminescence (Figure 6) shows that the effect of Li+ incorporation acts through the efficiency of excitation channel, while changes in the oscillator strengths at luminescence channels lead to a practically unobservable effect.

3.3. Raman Spectroscopy

The spectral signals recorded from Na1−xLixCaLa(MoO4)3:Er,Yb (x = 0, 0,05, 0.1, 0.2 and 0.3) samples under the excitation of 514.5 nm in the range of Raman-active vibrations are shown in Figure 7. In scheelite-type molybdates, the Raman bands are typically located from 200 to 900 cm−1 and they correspond to rotation (around 200 cm−1), bending (from 300 to 450 cm−1) and stretching (700–900 cm−1) vibrations of MoO4 tetrahedra [3,12,14,18,22,36,38,39,45]. At the same time, the most intense Raman band of scheelite-type molybdates is associated with the symmetrical stretching of [MoO4]2− ions, and this band is commonly located around 880 cm−1. As can be seen in Figure 7, only a very weak peak was observed in this spectral range in the Raman spectra of Na1−xLixCaLa(MoO4)3:Er,Yb (x = 0, 0,05, 0.1, 0.2 and 0.3) compounds. Such behavior is associated with the fact that the Raman signal is almost totally superimposed by the 2H11/2 and 4S3/2 luminescence lines of Er3+ ions. According to the spectra shown in Figure 7, the variation of peak intensities of 2H11/2 bands was different from the UC intensity dependence presented in Figure 5 and Figure 6.

3.4. Calculation of Band Structure

The primitive cell of scheelite-type LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 (x = 0–0.3) compounds is shown in Figure 8a and the corresponding Brillouin zone is presented in Figure 8b. The path along the high symmetry points of the BZ should be written as: Γ–X–P–N–Γ–M–S|S0–Γ|X–R|G–M. The coordinates of these points are: Γ (0, 0, 0), X (0,0,0.5), P (0.25, 0.25, 0.25), N (0, 0.5, 0), M (0.5, 0.5, −0.5), S (0.302, 0.697, −0.302), S0 (−0.302, 0.302, 0.302), R (−0.104, 0.104, 0.5), G (0.5, 0.5, −0.104).
The dominant part of the DFT simulations of scheelite-type structures is related to CaMoO4 [60,61,62]. Additionally, the calculation of the band structure for CaMoO4 doped with one ion can be found in several contributions [62,63]. In this work, we implemented the electronic band structure simulation of LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 (x = 0–0.3) complex compounds using VCA (virtual crystal approximation). The result of the calculation for NCLM:EY is presented in Figure 9a. The band gap value for the direct (optical) electronic transition was 4.0 eV. It should be noted that the obtained values for direct and indirect band gaps differed in the second decimal place only. According to Figure 9b, the valence band top of NCLM:EY was formed by the p-electrons of O, while the conduction band bottom was dominated by the Mo d-electrons. The effects of other cations were minor. Therefore, the transitions that govern the fundamental absorption edge were of charge transfer type and occurred due to the transfer of the O 2p electron to the Mo6+ ion within MoO4 tetrahedra. The energy states of f orbitals of Yb and Er ions that are responsible for intraconfigurational f-f transitions and upconversion processes, according to the multiple experimental studies, were all within the bandgap. The band gap values calculated for LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 crystals are presented in Table 3. As is evident, the band gap energy only slightly increased on the Li content increase. An exception was observed for the LiNCLM:EY-0.2 sample. As for this fluctuation, it was revealed that the specific band gap energy value was induced by a slight variation of the O atom coordinates, as determined by the Rietveld refinement.

4. Conclusions

In the present study, the microwave sol-gel method was successfully employed for the preparation of the LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3, x = 0–0.3, solid solutions. The compounds were crystallized in tetragonal space group I41/a, and they are typical representatives of the scheelite crystal family. However, the specific behavior of the structural parameters was observed as a function of the Li content. As the similar effect was previously observed in several related solid solutions with the partial substitution of Li for Na, it can be concluded that an unusual variation of the structural parameters is a general feature of complex Li-containing scheelite-type crystals. Evidently, it could be interesting to search for this effect in other complex scheelites containing big alkaline ions instead of Na.
The upconversion luminescence intensity of LixNa1−xCaLa0.5(MoO4)3:Er3+0.05/Yb3+0.45 disordered scheelite is optimized via the variation of Li content. It was demonstrated that, in contrast to LixNa1−xCaGd0.5Ho0.05Yb0.45(MoO4)3, which exhibited its non-monotonic variation of the UC intensity upon the Li content, the material under study was featured by a single pronounced maximum of UCL at the Li content x = 0.05. The mechanism that controls the UC intensity was shown to be the variation of probability of energy transfer from Yb ions to Er ions, while the control of the emission channels played an insignificant role. Consequently, these Li-containing phosphors doped with Er3+/Yb3+ lead to higher UC emitting efficiency with superior thermal and chemical stabilities overcoming the current limitations of traditional UC materials. It is emphasized that these new UC phosphors can be considered as potentially active optical switching and solar cell devices in practical optoelectronic fields.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst13020362/s1, Table S1: Fractional atomic coordinates and isotropic displacement parameters (Å2) of the (Na1−xLix)Ca(La0.5Er0.05Yb0.45)(MoO4)3 samples; Table S2: Main bond lengths (Å) of the (Na1−xLix)Ca(La0.5Er0.05Yb0.45)(MoO4)3 samples; Figure S1: SEM patterns recorded for (a) LiNCLM:EY-0.05, (b) LiNCLM:EY-0.1 and (c) LiNCLM:EY-0.2; Figure S2: Rietveld difference patterns obtained for (a) LiNCLM:EY-0.05, (b) LiNCLM:EY-0.1 and (c) LiNCLM:EY-0.2. Measured points are given in red, calculated profile—in black, difference profile—in grey, and calculated peak positions are shown by segments in green.

Author Contributions

Conceptualization, C.S.L.; methodology, C.S.L.; software, M.M. and A.O.; validation, C.S.L., A.A., M.M., A.O. and V.A.; formal analysis, C.S.L., A.A., M.M., A.O. and V.A.; investigation, C.S.L., A.A., M.M., A.O. and V.A.; resources, C.S.L.; data curation, C.S.L.; writing—original draft preparation, C.S.L., A.A., M.M., A.O. and V.A.; writing—review and editing, C.S.L., A.A., M.M., A.O. and V.A.; visualization, C.S.L., A.A., M.M. and A.O.; supervision, C.S.L.; project administration, C.S.L.; funding acquisition, C.S.L. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the Research Program through the Campus Research Foundation funded by Hanseo University in 2022 (2022046).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are available on request.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Jendoubi, I.; Smail, R.B.; Maczka, M.; Zid, M.F. Optical and electrical properties of the yavapaiite-like molybdate NaAl(MoO4)2. Ionics 2018, 24, 3515–3533. [Google Scholar] [CrossRef]
  2. Goulas, A.; Chi-Tangyie, G.; Wang, D.; Zhang, S.; Ketharam, A.; Vaidhyanathan, B.; Reaney, I.M.; Cadman, D.A.; Whittow, W.G.; Vardaxoglou, C.; et al. Microstructure and microwave dielectric properties of 3D printed low loss Bi2Mo2O9 ceramics for LTCC applications. Appl. Mater. Today 2020, 21, 100862. [Google Scholar] [CrossRef]
  3. Lim, C.S.; 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. [Google Scholar]
  4. Sha, X.; Chen, B.; Zhang, X.; Zhang, J.; Xu, S.; Li, X.; Sun, J.; Zhang, Y.; Wang, X.; Zhang, Y.; et al. Pre-assessments of optical transition, gain performance and temperature sensing of Er3+ in NaLn(MoO4)2 (Ln = Y, La, Gd and Lu) single crystals by using their powder-formed samples derived from traditional solid state reaction. Opt. Laser Technol. 2021, 140, 107012. [Google Scholar] [CrossRef]
  5. Chimitova, O.D.; Bazarov, B.G.; Bazarova, J.G.; Atuchin, V.V.; Azmi, R.; Sarapulova, A.E.; Mikhailova, D.; Balachandran, G.; Fiedler, A.; Geckle, U.; et al. The crystal growth and properties of novel magnetic double molybdate RbFe5(MoO4)7 with mixed Fe3+/Fe2+ states and 1D negative thermal expansion. CrystEngComm 2021, 23, 3297–3307. [Google Scholar] [CrossRef]
  6. Wu, F.; Zhou, D.; Du, C.; Xu, D.-M.; Li, R.-T.; Shi, Z.-Q.; Darwish, M.A.; Zhou, T.; Jantunen, H. Design and fabrication of a satellite communication dielectric resonator antenna with novel low loss and temperature-stabilized (Sm1−xCax) (Nb1−xMox)O4 (x = 0.15–0.7) microwave ceramics. Chem. Mater. 2023, 35, 104–115. [Google Scholar] [CrossRef]
  7. Atuchin, V.V.; Grossman, V.G.; Adichtchev, S.V.; Surovtsev, N.V.; Gavrilova, T.A.; Bazarov, B.G. Structural and vibrational properties of microcrystalline TlM(MoO4)2 (M = Nd, Pr) molybdates. Opt. Mater. 2012, 34, 812–816. [Google Scholar] [CrossRef]
  8. Li, T.; Guo, C.; Wua, Y.; Li, L.; Jeong, J.H. Green upconversion luminescence in Yb3+/Er3+ co-doped ALn(MoO4)2 (A = Li, Na and K, Ln = La, Gd and Y). J. Alloys Compd. 2012, 540, 107–112. [Google Scholar] [CrossRef]
  9. Atuchin, V.V.; Chimitova, O.D.; Adichtchev, S.V.; Bazarov, J.G.; Gavrilova, T.A.; Molokeev, M.S.; Surovtsev, N.V.; Bazarova, Z.G. Synthesis, structural and vibrational properties of microcrystalline β-RbSm(MoO4)2. Mater. Lett. 2013, 106, 26–29. [Google Scholar] [CrossRef]
  10. 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. [Google Scholar] [CrossRef]
  11. Hao, S.-Z.; Zhou, D.; Hussain, F.; Su, J.-Z.; Liu, W.-F.; Wang, D.-W.; Wang, Q.-P.; Qi, Z.-M. Novel scheelite-type [Ca0.55(Nd1−xBix)0.3]MoO4 (0.2 ≤ x ≤ 0.95) microwave dielectric ceramics with low sintering temperature. J. Am. Ceram. Soc. 2020, 103, 7259–7266. [Google Scholar] [CrossRef]
  12. Lim, C.S.; Aleksandrovsky, A.S.; Atuchin, V.V.; Molokeev, M.S.; Oreshonkov, A.S. Microwave sol-gel synthesis, microstructural and spectroscopic properties of scheelite-type ternary molybdate upconversion phosphor NaPbLa(MoO4)3:Er3+/Yb3+. J. Alloys Compd. 2020, 826, 152095. [Google Scholar] [CrossRef] [Green Version]
  13. Cheng, F.; Xia, Z.; Molokeev, M.S.; Jing, X. Effect of composition modulation on the luminescence properties of Eu3+ doped Li1−xAgxLu(MoO4)2 solid-solution phosphors. Dalton Trans. 2015, 44, 18078–18089. [Google Scholar] [CrossRef]
  14. Lim, C.S.; Aleksandrovsky, A.; Molokeev, M.; Oreshonkov, A.; Atuchin, V. The modulated structure and frequency upconversion properties of CaLa2(MoO4)4:Ho3+/Yb3+ phosphors prepared by microwave synthesis. Phys. Chem. Chem. Phys. 2015, 17, 19278–19287. [Google Scholar] [CrossRef]
  15. Xiao, J.; Zhang, W.; Wang, T.; Zhang, J.; Du, H. Photoluminescence enhancement in a Na5Y(MoO4)4:Dy3+ white-emitting phosphor by partial replacement of MoO42− with WO42− or VO43−. Ceram. Int. 2021, 47, 12028–12037. [Google Scholar] [CrossRef]
  16. Yu, Y.; Shao, K.; Niu, C.; Liu, L.; Zhu, X.; Wang, Z.; Ma, Z.; Wang, G. Crystal growth, first-principle calculations, optical properties and laser performances toward a molybdate Er+3:KBaGd(MoO4)3 crystal. J. Lumin. 2022, 252, 119324. [Google Scholar] [CrossRef]
  17. Lim, C.S.; Atuchin, V.V.; Aleksandrovsky, A.S.; Molokeev, M.S. Preparation of NaSrLa(WO4)3:Ho3+/Yb3+ ternary tungstates and their upconversion photoluminescence properties. Mater. Lett. 2016, 181, 38–41. [Google Scholar] [CrossRef]
  18. Lim, C.S.; Aleksandrovsky, A.S.; Molokeev, M.S.; Oreshonkov, A.S.; Atuchin, V.V. Microwave synthesis and spectroscopic properties of ternary scheelite-type molybdate phosphors NaSrLa(MoO4)3:Er3+, Yb3+. J. Alloys Compd. 2017, 713, 156–163. [Google Scholar] [CrossRef]
  19. Wang, H.; Yamg, T.; Feng, L.; Ning, Z.; Liu, M.; Lai, X.; Gao, D.; Bi, J. Energy transfer and multicolor tunable luminescence properties of NaGd0.5Tb0.5−xEux(MoO4)2 phosphors for UV-LED. J. Electron. Mater. 2018, 47, 6494–6506. [Google Scholar] [CrossRef]
  20. Xie, J.; Cheng, L.; Tang, H.; Wang, Z.; Sun, H.; Lu, L.; Mi, X.; Liu, Q.; Zhang, X. Wide range color tunability and efficient energy transfer of novel NaCaGd(WO4)3:Tb3+, Eu3+ phosphors with excellent thermal stability for pc-WLEDs. Inorg. Chem. Front. 2021, 8, 4517–4527. [Google Scholar] [CrossRef]
  21. Lim, C.S.; Aleksandrovsky, A.; Molokeev, M.; Oreshonkov, A.; Atuchin, V. Microwave sol–gel synthesis and upconversion photoluminescence properties of CaGd2(WO4)4:Er3+/Yb3+ phosphors with incommensurately modulated structure. J. Solid State Chem. 2015, 228, 160–166. [Google Scholar] [CrossRef]
  22. Lim, C.S.; Atuchin, V.; Aleksandrovsky, A.; Molokeev, M.; Oreshonkov, A. Microwave sol–gel synthesis of CaGd2(MoO4)4:Er3+/Yb3+ phosphors and their upconversion photoluminescence properties. J. Am. Ceram. Soc. 2015, 98, 3223–3230. [Google Scholar] [CrossRef] [Green Version]
  23. Nascimento, J.P.C.; Sales, A.J.M.; Sousa, D.G.; da Silva, M.A.S.; Moreira, S.G.C.; Pavani, K.; Soares, M.J.; Graça, M.P.F.; Kumar, J.S.; Sombra, A.S.B. Temperature-, power-, and concentration-dependent two and three photon upconversion in Er3+/Yb3+ co-doped lanthanum ortho-niobate phosphors. RSC Adv. 2016, 6, 68160–68169. [Google Scholar] [CrossRef]
  24. Krut’ko, V.A.; Komova, M.G.; Pominova, D.V. Synthesis and luminescence of Gd2−x−yYbxEr(Ho)yGeMoO8 germanate-molybdates with scheelite structure. J. Solid State Chem. 2020, 292, 121704. [Google Scholar] [CrossRef]
  25. Singh, P.; Jain, N.; Shukla, S.; Tiwari, A.K.; Kumar, K.; Singh, J.; Pandey, A.C. Luminescence nanothermometry using a trivalent lanthanide co-doped perovskite. RSC Adv. 2023, 13, 2939–2948. [Google Scholar] [CrossRef]
  26. Zhang, J.; Gao, Z.; Liu, S.; Zhang, S.; Zhang, W.; Feng, X.; Zhao, P.; He, J.; Tao, X. Simultaneous dual-wavelength operation of β-BaTeMo2O9 Raman laser at 1320 and 1500 nm. Appl. Phys. Express 2013, 6, 072702. [Google Scholar] [CrossRef]
  27. Shi, P.; Xia, Z.; Molokeev, M.S.; Atuchin, V.V. Crystal chemistry and luminescence properties of red-emitting CsGd1−xEux(MoO4)2 solid-solution phosphors. Dalton Trans. 2014, 43, 9669–9676. [Google Scholar] [CrossRef]
  28. Zhang, Y.; Cong, H.; Jiang, H.; Li, J.; Wang, J. Flux growth, structure, and physical characterization of new disordered laser crystal LiNd(MoO4)2. J. Cryst. Growth 2015, 423, 1–8. [Google Scholar] [CrossRef]
  29. Reshak, A.H.; Alahmed, Z.A.; Bila, J.; Atuchin, V.V.; Bazarov, B.G.; Chimitova, O.D.; Molokeev, M.S.; Prosvirin, I.P.; Yelisseyev, A.P. Exploration of the electronic structure of monoclinic α-Eu2(MoO4)3: DFT-based study and X-ray photoelectron spectroscopy. J. Phys. Chem. C 2016, 120, 10559–10568. [Google Scholar] [CrossRef]
  30. Khyzhun, O.Y.; Bekenev, V.L.; Atuchinb, V.V.; Pokrovsky, L.D.; Shlegel, V.N.; Ivannikova, N.V. The electronic structure of Pb2MoO5: First-principles DFT calculations and X-ray spectroscopy measurements. Mater. Des. 2016, 105, 315–322. [Google Scholar] [CrossRef]
  31. Atuchin, V.V.; Aleksandrovsky, A.S.; Molokeev, M.S.; Krylov, A.S.; Oreshonkov, A.S.; Zhou, D. Structural and spectroscopic properties of self-activated monoclinic molybdate BaSm2(MoO4)4. J. Alloys Compd. 2017, 729, 843–849. [Google Scholar] [CrossRef] [Green Version]
  32. Solodovnikov, S.F.; Atuchin, V.V.; Solodovnikova, Z.A.; Khyzhun, O.Y.; Danylenko, M.I.; Pishchur, D.P.; Plyusnin, P.E.; Pugachev, A.M.; Gavrilova, T.A.; Yelisseyev, A.P.; et al. Synthesis, structural, thermal, and electronic properties of palmierite-related double molybdate α-Cs2Pb(MoO4)2. Inorg. Chem. 2017, 56, 3276–3286. [Google Scholar] [CrossRef]
  33. Ren, H.; Li, H.; Zou, Y.; Deng, H.; Peng, Z.; Ma, T.; Ding, S. Growth and properties of Pr3+-doped NaGd(MoO4)2 single crystal: A promising InGaN laser-diode pumped orange-red laser crystal. J. Lumin. 2022, 249, 119034. [Google Scholar] [CrossRef]
  34. Savina, A.A.; Atuchin, V.V.; Solodovnikov, S.F.; Solodovnikova, Z.A.; Krylov, A.S.; Maximovskiy, E.A.; Molokeev, M.S.; Oreshonkov, A.S.; Pugachev, A.M.; Khaikina, E.G. Synthesis, structural and spectroscopic properties of acentric triple molybdate Cs2NaBi(MoO4)3. J. Solid State Chem. 2015, 225, 53–58. [Google Scholar] [CrossRef]
  35. Mhiri, M.; Badri, A.; Amara, M.B. Synthesis and crystal structure of NaMgFe(MoO4)3. Acta Crystallogr. Sect. E Crystallogr. Commun. 2016, E72, 864–867. [Google Scholar] [CrossRef] [Green Version]
  36. Lim, C.S. Synthesis of NaCaLa(MoO4)3:Ho3+/Yb3+ phosphors via microwave sol-gel route and their upconversion photoluminescence properties. Korean J. Mater. Res. 2016, 26, 363–369. [Google Scholar] [CrossRef]
  37. Rajendran, M.; Vaidyanathan, S. New red emitting phosphors NaSrLa(MO4)3:Eu3+ [M = Mo and W] for white LEDs: Synthesis, structural and optical study. J. Alloys Compd. 2019, 789, 919–931. [Google Scholar] [CrossRef]
  38. Lim, C.S. Synthesis of microcrystalline LiNaCaLa(MoO4)3:Yb3+/Ho3+ upconversion phosphors and effect of Li+ on their spectroscopic properties. Trans. Electr. Electron. Mater. 2019, 20, 60–66. [Google Scholar] [CrossRef]
  39. Lim, C.-S.; Aleksandrovsky, A.; Molokeev, M.; Oreshonkov, A.; Atuchin, V. Structural and spectroscopic effects of Li+ substitution for Na+ in LixNa1−xCaGd0.5Ho0.05Yb0.45(MoO4)3 scheelite-type upconversion phosphors. Molecules 2021, 26, 7357. [Google Scholar] [CrossRef]
  40. Yu, Y.; Shao, K.; Zhu, X.; Zhang, X.; Qiu, H.; Wu, S.; Wang, G. Research on a novel molybdate Er3+:KBaY(MoO4)3 crystal as a prominent 1.55 μm laser medium. J. Lumin. 2021, 237, 118194. [Google Scholar] [CrossRef]
  41. Ryu, J.H.; Yoon, J.-W.; Lim, C.S.; Shim, K.B. Microwave-assisted synthesis of barium molybdate by a citrate complex method and oriented aggregation. Mater. Res. Bull. 2005, 40, 1468–1476. [Google Scholar] [CrossRef]
  42. Rybakov, K.I.; Olevsky, E.A.; Krikun, E.V. Microwave sintering: Fundamentals and modeling. J. Am. Ceram. Soc. 2013, 96, 1003–1020. [Google Scholar] [CrossRef]
  43. Kitchen, H.J.; Vallance, S.R.; Kennedy, J.L.; Tapia-Ruiz, N.; Carassiti, L.; Harrison, A.; Whittaker, A.G.; Drysdale, T.D.; Kingman, S.W.; Gregory, D.H. Modern microwave methods in solid-state inorganic materials chemistry: From fundamentals to manufacturing. Chem. Rev. 2014, 114, 1170–1206. [Google Scholar] [CrossRef]
  44. Perera, S.S.; Munasinghe, H.N.; Yatooma, E.N.; Rabuffetti, F.A. Microwave-assisted solid-state synthesis of NaRE(MO4)2 phosphors (RE = La, Pr, Eu, Dy; M = Mo, W). Dalton Trans. 2020, 49, 7914–7919. [Google Scholar] [CrossRef]
  45. Lim, C.S.; Aleksandrovsky, A.S.; Molokeev, M.S.; Oreshonkov, A.S.; Ikonnikovg, D.A.; Atuchin, V.V. Triple molybdate scheelite-type upconversion phosphor NaCaLa(MoO4)3:Er3+/Yb3+: Structural and spectroscopic properties. Dalton Trans. 2016, 45, 15541–15551. [Google Scholar] [CrossRef] [Green Version]
  46. Bruker. Bruker AXS TOPAS V4: General Profile and Structure Analysis Software for Powder Diffraction Data—User’s Manual; Bruker AXS: Karlsruhe, Germany, 2008. [Google Scholar]
  47. Hohenberg, P.; Kohn, W. Inhomogeneous electron gas. Phys. Rev. 1964, 136, B864–B871. [Google Scholar] [CrossRef] [Green Version]
  48. Kohn, W.; Sham, L. Self-consistent equations including exchange and correlation effects. Phys. Rev. 1965, 140, A1133–A1138. [Google Scholar] [CrossRef] [Green Version]
  49. Oreshonkov, A.S. SI: Advances in density functional theory (DFT) studies of solids. Materials 2022, 15, 2099. [Google Scholar] [CrossRef]
  50. Clark, S.J.; Segall, M.D.; Pickard, C.J.; Hasnip, P.J.; Probert, M.I.J.; Refson, K.; Payne, M.C. First principles methods using CASTEP. Z. Für Krist. Cryst. Mater. 2005, 220, 567–570. [Google Scholar] [CrossRef] [Green Version]
  51. Bartók, A.P.; Yates, J.R. Regularized SCAN functional. J. Chem. Phys. 2019, 150, 161101. [Google Scholar] [CrossRef]
  52. Sun, J.; Remsing, R.C.; Zhang, Y.; Sun, Z.; Ruzsinszky, A.; Peng, H.; Yang, Z.; Paul, A.; Waghmare, U.; Wu, X.; et al. Accurate first-principles structures and energies of diversely bonded systems from an efficient density functional. Nat. Chem. 2016, 8, 831–836. [Google Scholar] [CrossRef]
  53. Lee, M.H. Improved Optimised Pseudopotentials and Application to Disorder in γ-Al2O3. Ph.D. Thesis, Cambridge University, Cambridge, UK, 1996. [Google Scholar]
  54. Monkhorst, H.J.; Pack, J.D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188–5192. [Google Scholar] [CrossRef]
  55. Bellaiche, L.; Vanderbilt, D. Virtual crystal approximation revisited: Application to dielectric and piezoelectric properties of perovskites. Phys. Rev. B 2000, 61, 7877. [Google Scholar] [CrossRef] [Green Version]
  56. Atuchin, V.V.; Gavrilova, T.A.; Grivel, J.-C.; Kesler, V.G. Electronic structure of layered titanate Nd2Ti2O7. Surf. Sci. 2008, 602, 3095–3099. [Google Scholar] [CrossRef]
  57. Atuchin, V.V.; Bazarov, B.G.; Gavrilova, T.A.; Grossman, V.G.; Molokeev, M.S.; Bazarova, Z.G. Preparation and structural properties of nonlinear optical borates K2(1−x)Rb2xAl2B2O7, 0 < x < 0.75. J. Alloys Compd. 2012, 515, 119–122. [Google Scholar]
  58. Hazen, R.M.; Finger, L.W.; Mariathasan, J.W.E. High pressure crystal chemistry of scheelite-type tungstates and molybdates. J. Phys. Chem. Solids 1985, 46, 253–263. [Google Scholar] [CrossRef]
  59. Stevens, S.B.; Morrison, C.A.; Allik, T.H.; Rheingold, A.L.; Haggerty, B.S. NaLa(MoO4)2 as a laser host material. Phys. Rev. B Condens. Matter. 1991, 43, 7386–7394. [Google Scholar] [CrossRef] [PubMed]
  60. Bouzidi, C.; Horchani-Naifer, K.; Khadraoui, Z.; Elhouichet, H.; Ferid, M. Synthesis, characterization and DFT calculations of electronic and optical properties of CaMoO4. Phys. B Condens. Matter 2016, 497, 34–38. [Google Scholar] [CrossRef]
  61. Marques, V.S.; Cavalcante, L.S.; Sczancoski, J.C.; Alcantara, A.F.P.; Orlandi, M.O.; Moraes, E.; Longo, E.; Varela, J.A.; Li, M.S.; Santos, M.R.M.C. Effect of Different Solvent Ratios(Water/Ethylene Glycol) on the Growth Process of CaMoO4 Crystals and Their Optical Properties. Cryst. Growth Des. 2010, 10, 4752–4768. [Google Scholar] [CrossRef]
  62. Gupta, S.K.; Sahu, M.; Ghosh, P.S.; Tyagi, D.; Saxena, M.K.; Kadam, R.M. Energy transfer dynamics and luminescence properties of Eu3+ in CaMoO4 and SrMoO4. Dalton Trans. 2015, 44, 18957–18969. [Google Scholar] [CrossRef]
  63. Tranquilin, R.L.; Oliveira, M.C.; Santiago, A.A.G.; Lovisa, L.X.; Ribeiro, R.A.P.; Longo, E.; de Lazaro, S.R.; Almeida, C.R.R.; Paskocimas, C.A.; Motta, F.V.; et al. Presence of excited electronic states on terbium incorporation in CaMoO4: Insights from experimental synthesis and first-principles calculations. J. Phys. Chem. Solids 2021, 149, 109790. [Google Scholar] [CrossRef]
Crystals 13 00362 sch001 550
Scheme 1. Flow chart for the synthesis of LixNa1−xCaLa(MoO4)3:Er3+/Yb3+ UC phosphors by the microwave sol-gel method.
Scheme 1. Flow chart for the synthesis of LixNa1−xCaLa(MoO4)3:Er3+/Yb3+ UC phosphors by the microwave sol-gel method.
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Crystals 13 00362 g001 550
Figure 1. SEM patterns recorded for (a) NCLM:EY and (b) LiNCLM:EY-0.3.
Figure 1. SEM patterns recorded for (a) NCLM:EY and (b) LiNCLM:EY-0.3.
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Crystals 13 00362 g002a 550Crystals 13 00362 g002b 550
Figure 2. Rietveld difference patterns obtained for (a) NCLM:EY and (b) LiNCLM:EY-0.3, (c) cell volume dependence on x in the LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 solid solutions. Measured points are given in red, calculated profile—in black, difference profile—in grey, and calculated peak positions are shown by segments in green.
Figure 2. Rietveld difference patterns obtained for (a) NCLM:EY and (b) LiNCLM:EY-0.3, (c) cell volume dependence on x in the LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 solid solutions. Measured points are given in red, calculated profile—in black, difference profile—in grey, and calculated peak positions are shown by segments in green.
Crystals 13 00362 g002aCrystals 13 00362 g002b
Crystals 13 00362 g003 550
Figure 3. Crystal structure of scheelite-type LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 crystals.
Figure 3. Crystal structure of scheelite-type LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 crystals.
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Crystals 13 00362 g004 550
Figure 4. Unit cell volume per averaged ion radii IR(Na/Li/Ca/La/Er/Yb) of CaMoO4 (green) [58], Na1/3Ca1/3La(1−x−y)/3Erx/3Yby/3MoO4 (red) [45], Na0.5La0.5MoO4 (blue) [59] and Lix/3Na(1−x)/3CaLa0.5/3Er0.05/3Yb0.45/3MoO4 compounds.
Figure 4. Unit cell volume per averaged ion radii IR(Na/Li/Ca/La/Er/Yb) of CaMoO4 (green) [58], Na1/3Ca1/3La(1−x−y)/3Erx/3Yby/3MoO4 (red) [45], Na0.5La0.5MoO4 (blue) [59] and Lix/3Na(1−x)/3CaLa0.5/3Er0.05/3Yb0.45/3MoO4 compounds.
Crystals 13 00362 g004
Crystals 13 00362 g005 550
Figure 5. Upconversion luminescence spectra of Na1−xLixCaLa(MoO4)3:Er, Yb. The starting levels for the transitions to the 4I15/2 ground state are indicated.
Figure 5. Upconversion luminescence spectra of Na1−xLixCaLa(MoO4)3:Er, Yb. The starting levels for the transitions to the 4I15/2 ground state are indicated.
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Crystals 13 00362 g006 550
Figure 6. Dependences of intensities of individual UC lines and the integral UC intensity on the Li content in Na1−xLixCaLa(MoO4)3:Er,Yb. The starting levels for the transitions to the 4I15/2 ground state are indicated.
Figure 6. Dependences of intensities of individual UC lines and the integral UC intensity on the Li content in Na1−xLixCaLa(MoO4)3:Er,Yb. The starting levels for the transitions to the 4I15/2 ground state are indicated.
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Crystals 13 00362 g007 550
Figure 7. Spectral signal from the LixNa1−xCaLa0.5(MoO4)3: Er3+0.05/Yb3+0.45 samples excited by a 514.5 nm laser line. A very weak Raman peak is marked with the arrow.
Figure 7. Spectral signal from the LixNa1−xCaLa0.5(MoO4)3: Er3+0.05/Yb3+0.45 samples excited by a 514.5 nm laser line. A very weak Raman peak is marked with the arrow.
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Crystals 13 00362 g008 550
Figure 8. Primitive cell (a) and Brillouin zone (b) of the scheelite-type LixNa1−xCaLa0.5Er0.05Yb0.45 crystals.
Figure 8. Primitive cell (a) and Brillouin zone (b) of the scheelite-type LixNa1−xCaLa0.5Er0.05Yb0.45 crystals.
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Crystals 13 00362 g009 550
Figure 9. Electronic band structure (a) and NCLM:EY density of states (b).
Figure 9. Electronic band structure (a) and NCLM:EY density of states (b).
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Table
Table 1. Abbreviations and chemical compositions for the LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 samples.
Table 1. Abbreviations and chemical compositions for the LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 samples.
AbbreviationChemical Composition
NCLM:EYNaCaLa0.5Er0.05Yb0.45(MoO4)3
LiNCLM:EY-0.05Li0.05Na0.95CaLa0.5Er0.05Yb0.45(MoO4)3
LiNCLM:EY-0.1Li0.1Na0.9CaLa0.5Er0.05Yb0.45(MoO4)3
LiNCLM:EY-0.2Li0.2Na0.8CaLa0.5Er0.05Yb0.45(MoO4)3
LiNCLM:EY-0.3Li0.3Na0.7CaLa0.5Er0.05Yb0.45(MoO4)3
Table
Table 2. Main processing and refinement parameters of the (Na1-xLix)Ca(La0.5Er0.05Yb0.45)(MoO4)3 samples.
Table 2. Main processing and refinement parameters of the (Na1-xLix)Ca(La0.5Er0.05Yb0.45)(MoO4)3 samples.
CompoundSpace GroupZCell Parameters (Å), Cell Volume (Å3)Rp, RB (%)
χ2
NCLMI41/a4a = 5.2452 (1)
c = 11.4731 (4)
V = 315.65 (2)		10.94, 2.04
1.11
Na0.95Li0.05CLM:
0.05Er, 0.45Yb		I41/a4a = 5.2432 (1)
c = 11.4690 (3)
V = 315.30 (2)		9.26, 1.93
1.10
Na0.9Li0.1CLM:
0.05Er, 0.45Yb		I41/a4a = 5.2418 (2)
c = 11.4646 (4)
V = 315.01 (2)		11.10, 2.95
1.12
Na0.8Li0.2CLM:
0.05Er, 0.45Yb		I41/a4a = 5.2405 (1)
c = 11.4609 (3)
V = 314.75 (1)		11.22, 2.82
1.10
Na0.7Li0.3CLM:
0.05Er, 0.45Yb		I41/a4a = 5.2364 (1)
c = 11.4494 (3)
V = 313.94 (2)		10.11, 2.64
1.11
Table
Table 3. Comparison of calculated band gap values for LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 with data for several scheelite-type structures.
Table 3. Comparison of calculated band gap values for LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 with data for several scheelite-type structures.
CompoundSourceCalc./Exp.Band Gap, eV
NCLM:EYthis workcalc.4.00
LiNCLM:EY-0.05this workcalc.4.02
LiNCLM:EY-0.1this workcalc.4.12
LiNCLM:EY-0.2this workcalc.3.99
LiNCLM:EY-0.3this workcalc.4.137
CaMoO4[62]calc2.95
CaMoO4[60]calc.3.2
CaMoO4:Eu3+[62]calc3.35
CaMoO4:Tb3+[63]calc3.96
CaMoO4[61]calc.4.64
CaMoO4[63]exp.3.87
CaMoO4[60]exp.3.9
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Lim, C.S.; Aleksandrovsky, A.; Molokeev, M.; Oreshonkov, A.; Atuchin, V. Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 Upconversion Scheelite-Type Phosphors. Crystals 2023, 13, 362. https://doi.org/10.3390/cryst13020362

AMA Style

Lim CS, Aleksandrovsky A, Molokeev M, Oreshonkov A, Atuchin V. Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 Upconversion Scheelite-Type Phosphors. Crystals. 2023; 13(2):362. https://doi.org/10.3390/cryst13020362

Chicago/Turabian Style

Lim, Chang Sung, Aleksandr Aleksandrovsky, Maxim Molokeev, Aleksandr Oreshonkov, and Victor Atuchin. 2023. "Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 Upconversion Scheelite-Type Phosphors" Crystals 13, no. 2: 362. https://doi.org/10.3390/cryst13020362

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