Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1-xCaGd0.5Ho0.05Yb0.45(MoO4)3 Scheelite-Type Upconversion Phosphors

A set of new triple molybdates, LixNa1-xCaGd0.5(MoO4)3:Ho3+0.05/Yb3+0.45, was successfully manufactured by the microwave-accompanied sol–gel-based process (MAS). Yellow molybdate phosphors LixNa1-xCaGd0.5(MoO4)3:Ho3+0.05/Yb3+0.45 with variation of the LixNa1-x (x = 0, 0.05, 0.1, 0.2, 0.3) ratio under constant doping amounts of Ho3+ = 0.05 and Yb3+ = 0.45 were obtained, and the effect of Li+ on their spectroscopic features was investigated. The crystal structures of LixNa1-xCaGd0.5(MoO4)3:Ho3+0.05/Yb3+0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) at room temperature were determined in space group I41/a by Rietveld analysis. Pure NaCaGd0.5Ho0.05Yb0.45(MoO4)3 has a scheelite-type structure with cell parameters a = 5.2077 (2) and c = 11.3657 (5) Å, V = 308.24 (3) Å3, Z = 4. In Li-doped samples, big cation sites are occupied by a mixture of (Li,Na,Gd,Ho,Yb) ions, and this provides a linear cell volume decrease with increasing Li doping level. The evaluated upconversion (UC) behavior and Raman spectroscopic results of the phosphors are discussed in detail. Under excitation at 980 nm, the phosphors provide yellow color emission based on the 5S2/5F4 → 5I8 green emission and the 5F5 → 5I8 red emission. The incorporated Li+ ions gave rise to local symmetry distortion (LSD) around the cations in the substituted crystalline structure by the Ho3+ and Yb3+ ions, and they further affected the UC transition probabilities in triple molybdates LixNa1-xCaGd0.5(MoO4)3:Ho3+0.05/Yb3+0.45. The complex UC intensity dependence on the Li content is explained by the specificity of unit cell distortion in a disordered large ion system within the scheelite crystal structure. The Raman spectra of LixNa1-xCaGd0.5(MoO4)3 doped with Ho3+ and Yb3+ ions were totally superimposed with the luminescence signal of Ho3+ ions in the range of Mo–O stretching vibrations, and increasing the Li+ content resulted in a change in the Ho3+ multiplet intensity. The individual chromaticity points (ICP) for the LiNaCaGd(MoO4)3:Ho3+,Yb3+ phosphors correspond to the equal-energy point in the standard CIE (Commission Internationale de L’Eclairage) coordinates.

with variation of the Li x Na 1-x (x = 0, 0.05, 0.1, 0.2, 0.3) ratio under constant doping amounts of Ho 3+ = 0.05 and Yb 3+ = 0.45 were obtained, and the effect of Li + on their spectroscopic features was investigated. The crystal structures of Li x Na 1-x CaGd 0.5 (MoO 4 ) 3 :Ho 3+ 0.05 /Yb 3+ 0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) at room temperature were determined in space group I4 1 /a by Rietveld analysis. Pure NaCaGd 0.5 Ho 0.05 Yb 0.45 (MoO 4 ) 3 has a scheelite-type structure with cell parameters a = 5.2077 (2) and c = 11.3657 (5) Å, V = 308. 24 (3) Å 3 , Z = 4. In Li-doped samples, big cation sites are occupied by a mixture of (Li,Na,Gd,Ho,Yb) ions, and this provides a linear cell volume decrease with increasing Li doping level. The evaluated upconversion (UC) behavior and Raman spectroscopic results of the phosphors are discussed in detail. Under excitation at 980 nm, the phosphors provide yellow color emission based on the 5 S 2 / 5 F 4 → 5 I 8 green emission and the 5 F 5 → 5 I 8 red emission. The incorporated Li + ions gave rise to local symmetry distortion (LSD) around the cations in the substituted crystalline structure by the Ho 3+ and Yb 3+ ions, and they further affected the UC transition probabilities in triple molybdates Li x Na 1-x CaGd 0.5 (MoO 4 ) 3 :Ho 3+ 0.05 /Yb 3+ 0. 45 . The complex UC intensity dependence on the Li content is explained by the specificity of unit cell distortion in a disordered large ion system within the scheelite crystal structure. The Raman spectra of Li x Na 1-x CaGd 0.5 (MoO 4 ) 3 doped with Ho 3+ and Yb 3+ ions were totally superimposed with the luminescence signal of Ho 3+ ions in the range of Mo-O stretching vibrations, and increasing the Li + content resulted in a change in the Ho 3+ multiplet intensity. The individual chromaticity points (ICP) for the LiNaCaGd(MoO 4 ) 3 :Ho 3+ ,Yb 3+ phosphors correspond to the equal-energy point in the standard CIE (Commission Internationale de L'Eclairage) coordinates.
In ST solid solutions, different combinations of Ln 3+ ions can be reached without a loss of structural quality; this opens the way for the investigation of energy transfer (ET) effects in phosphor systems with binary and ternary doping [33][34][35][36][37][38][39]. In particular, the incorporation of appropriate Ln 3+ pairs provides enhanced frequency upconversion (UC) properties under near-infrared laser excitation [21,[29][30][31][33][34][35][36]38,39]. For Ho 3+ /Yb 3+doped UC phosphors, yellow emissions can be derived via a co-doping system based on the red emission bands from 5 F 5 → 5 I 8 transitions and green emission bands from 5 S 2 / 5 F 4 → 5 I 8 transitions [40][41][42][43]. Extremely wide color tunability was reported for Ho 3+ /Yb 3+ UC phosphor upon the addition of Er 3+ ions [44]. Laser active Ho 3+ ions play the role of activator, and Yb 3+ ions, as an efficient sensitizer with a high absorption cross section at 980 nm, could enhance the UC efficiency through the ET process between the activator and sensitizer induced by a unique energy level configuration.
The preparation of complex molybdates can be carried out via several specific processes [45][46][47][48][49][50][51][52][53][54][55]. Among the different methods, microwave accompanied sol-gel-based synthesis (MAS) can provide high-quality crystalline materials in a short processing period. The powder products fabricated by the MAS route are commonly characterized by a homogeneous microstructure and high chemical reproducibility, in reference to the nominal starting reagent ratio that is governed by the sol preparation step with high atom intermixing [56,57]. Compared with the usual methods, microwave synthesis has the advantages of shortening the reaction time and resulting in products with a small particle size, narrow particle size distribution, and high purity [58][59][60][61][62][63]. At the final step, the hightemperature microwave treatment efficiently stimulates crystal lattice formation [64,65].
Previously, a new division of triple ST molybdates NaTLn(MoO 4 ) 3 (T = Ca, Pb; Ln = La, Gd) was designed and prepared by the MAS process, and their properties, as promising phosphor hosts, were reported [21,31,[64][65][66]. Also, the Li x Na 1-x CaLa 0.5 Ho 0.05 Yb 0.45 (MoO 4 ) 3 solid solution was evaluated to determine the effect of Li + substitution for Na + on the frequency upconversion, and the ST structure was observed in the range of x = 0.05−0.4 [67]. To extend the capabilities of ST crystal engineering, it is interesting to reveal ions and ion combinations appropriate for incorporation at the mixed A position in complex ST crystals. In relation to this, the triple ST solid solutions (Li,Na)MLn(MoO 4 ) 3 (M = Ca, Sr, Pb) are of particular interest because the presence of Li + ions can affect the photoluminescence properties of Ln 3+ activators [67]. Thus, the present contribution is aimed at the preparation of Li x Na 1-x CaLa(MoO 4 ) 3 ;Yb 3+ ,Ho 3+ compounds by the MAS-based method and evaluation of their spectroscopic properties. The influence of incorporated Li + ions was investigated-to enhance the UC transition probabilities by the Ho 3+ and Yb 3+ ions in the ST molybdate Li x Na 1-x CaGd 0.5 (MoO 4 ) 3 -and is discussed in terms of the local symmetry distortion (LSD) around the cations in the substituted crystalline structure, The crystal structures and morphologies of the synthesized particles were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The spectroscopic characteristics were investigated under the consideration of efficient UC emissions, individual chromaticity points (ICP) according to Commission Internationale de L'Eclairage (CIE), and Raman scattering.
The sample notation introduced according to the nominal compositions is given in Table 1 at 0.14 mol% and LiNO 3 at 0.06 mol % were applied. Subsequently, Gd(NO 3 ) 3 ·6H 2 O at 0.2 mol%, Yb(NO 3 ) 3 ·5H 2 O at 0.18 mol%, and Ho(NO 3 ) 3 ·5H 2 O at 0.02 mol% were carefully weighed and dissolved very slowly in 100 mL of DW under slight heat treatment. Then, these two kinds of prepared solutions were slowly co-mixed and vigorously stirred. The CA/CM (molar ratio of CA to the total cation metal (CM) ions) was adjusted to 2:1. The intermixed solutions were 180-200 mL in volume, and they were heated slowly to~80-100 • C in a 450 mL Pyrex glass. At this stage, the solutions were in a highly transparent state. Then, the solutions were subjected to the MAS-derived treatment. The typical procedure applied at this stage can be found elsewhere [21,31,[64][65][66][67]. Then, the obtained dried black gels were ground and annealed at 800 • C for 16 h in air, with intervals of 100 • C between 600 and 800 • C. After the annealing process, pink-colored particles were obtained for the samples.
The structural properties of synthesized samples were evaluated by XRD analysis. The powder XRD patterns of the Li x Na 1-x CaGd 0.5 (MoO 4 ) 3 :Ho 3+ 0.05 /Yb 3+ 0.45 particles for Rietveld analysis were precisely recorded over the angle range of 2θ = 5−90 • at room temperature using a D/MAX 2200 (Rigaku in Japan) diffractometer with 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 [68]. The typical microstructure and surface morphology of the obtained particles were observed using SEM (JSM-5600, JEOL in Japan) methods. The PL spectra were recorded at room temperature using a spectrophotometer (Perkin Elmer LS55 in UK). The Raman spectral measurements were performed using a LabRam Aramis (Horiba Jobin-Yvon in France) device with 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 sample decomposition, the power on the samples was kept at the 0.5 mW level.

Results and Discussion
The XRD patterns measured for the samples listed in Table 1 are shown in Figure 1 and Figures S1-S3 (Supporting Information). In general, the patterns are similar. In Figure 1, for comparison, the XRD patterns of NCGM:HY (x = 0) and LiNCGM:HY-0.3 (x = 0.3) are presented. As can be seen, there is no significant difference even for the highest Li content. All peaks in all patterns were successfully indexed by the tetragonal cell (I4 1 /a) with cell parameters close to those of CaMoO 4 [69]. Therefore, the crystal structure of CaMoO 4 was taken as a starting model for Rietveld refinement. The Ca 2+ ion site was considered as the one occupied by a mixture of Li + , Ca 2+ , Na + , Gd 3+ , Ho 3+ , and Yb 3+ ions ( Figure 2) with fixed partial occupations according to the nominal sample composition. The refinements were stable and gave low R-factors (Table 1, Figure 1 and Figures S1-S3). The atom coordinates and the main bond lengths are summarized in Tables S1 and S2, respectively.   As is known, the effective radius of the Li + ion is noticeably lower than that of the Na + ion; therefore, substitution of Li + for Na + in the LiNCGM:HY compounds should induce a decrease in the average ion radius IR(Li/Na/Ca/Gd/Ho/Yb) of the A position and a related unit cell volume decrease in the ST structures. The dependencies of cell parameters and unit cell volume on IR(Li/Na/Ca/Gd/Ho/Yb) in the LiNCGM:HY compounds are shown in Figure 3. The IR values were calculated on the basis of the nominal compositions and the known system of ion radii [32]. For comparison, the same dependencies of two previously studied Gd-containing systems, NaCaGd(MoO 4 ) 3 :Er,Yb and NaCaGd(MoO 4 ) 3 :Ho,Yb [64,65], are also given in Figure 3. It should be pointed out that only the ST structure type was obtained in Refs. [50,51], and the structural parameters of NaCaGd(MoO 4 ) 3 :Er,Yb and NaCaGd(MoO 4 ) 3 :Ho,Yb remain unknown. For this reason, in the present work, the Rietveld refinement of the structures was implemented on the basis of the previously recorded XRD data [64,65]. The refinements were stable and gave low R-factors (Tables S3 and S4). From the observation of Figure 3, it is evident that, in the LiNCGM:HY compounds, the cell parameters and cell volume continuously decrease with decreasing IR(Li/Na/Ca/Gd/Ho/Yb) value or increasing Li content. This clearly proves the suggested chemical formulas of the LiNCGM:HY solid solutions. As to NaCaGd(MoO 4 ) 3 :Er,Yb and NaCaGd(MoO 4 ) 3 :Ho,Yb, with increasing doping level, the cell parameters and cell volume decrease proportionally to the IR value, according to the general trend observed in ST molybdates [66]. However, the decrease rate due to Li + incorporation, instead of Na + , in LiNCGM:HY is strongly lower than that in NaCaGd(MoO 4 ) 3 :Er,Yb and NaCaGd(MoO 4 ) 3 :Ho,Yb due to the rare earth ion incorporation instead of Gd 3+ . Moreover, with increasing Li content, the points related to the LiNCGM:HY system move away from the point of CdMoO 4 , which is an ST molybdate with the lowest unit cell volume [70]. To verify this effect, it would be valuable to see the behavior in other related systems, and this is possible for Li x Na 1-x CaLa 0.5 (MoO 4 ) 3 :Ho 0.05 Yb 0.45 (LiNCLM:HY) [67]. To determine the structural parameters of the LiNCLM:HY solid solutions, Rietveld refinement was carried out for the LiNCLM:HY compounds on the basis of the previously reported XRD data [67]. The refinements were stable and gave low R-factors (Table S5). The obtained results are shown in Figure 4 in association with other known La-containing systems NaTLa(MoO 4 ) 3 :Er,Yb (T = Ca, Sr, Pb) [21,31,66]. As can be seen, the variation in the unit cell volume with increasing Li + content in the LiNCLM:HY crystals is nearly the same as that in LiNCGM:HY. Comparatively, NaTLa(MoO 4 ) 3 :Er,Yb (T = Ca, Sr, Pb) solutions strongly follow the general trend of ST molybdates [66]. Thus, the substitution of Li + for Na + in LiNCGM:HY and LiNCLM:HY crystals generates a specific effect of unit cell compression that is not governed by the general trend of ST structures. It can be supposed that similar effects could be observed in other Li x Na 1-x TLn(MoO 4 ) 3 solid solutions. Further details of the crystal structures of the compounds listed in Table 2 and Tables S3-S5 may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; E-mail: crystdata@fiz-karlsruhe.de; http://www.fiz-karlsruhe.de/request_for_deposited_data.html) by quoting the deposition number: 2117686-2117704.    The  Figure 5. As can be seen, the particle morphology is similar for all five samples. The samples contain uniform partly coalescent particles of 3-10 µm in size. However, the Li-containing samples presented in Figure 5b-e show a slightly smaller characteristic particle size of 3-5 µm, compared to that of the NCGM:HY sample (5-10 µm). Faceted forms were not detected, and this may be due to the comparatively short time of hightemperature treatment used in the MAS processing. In particular, agglomerated grains can be observed, and these could be induced by the material interdiffusion between the grains. Previously, similar grain agglomeration was observed in many other oxide materials subjected to high-temperature annealing [71][72][73]. Thus, MAS-derived synthesis, when applied to LiNCGM doped with Yb 3+ /Ho 3+ , provides powder products with an uniform micrograin morphology.
The Raman spectra recorded for Li x Na 1-x CaGd 0.5 (MoO 4 ) 3 :Ho 3+ 0.05/Yb 3+ 0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) are shown in Figure 6a. As can be seen in Table S1, all of the investigated compounds contain only one crystallographically independent MoO 4 tetrahedron, and these units occupy the sites with S 4 point symmetry. It follows from the factor group analysis that Raman-active [MoO 4 ] 2− ion vibrations can be listed as: A g , symmetric stretching; B g + E g , antisymmetric stretching; A g + B g , symmetric bending; and B g + E g , antisymmetric bending [28,74]. The strongest spectral band in Figure 6a at 884 cm −1 is related to the ν 1 A g symmetric stretching vibration, and the ν 3 antisymmetric stretching vibrations are located in the range of 720-860 cm −1 . Part of the spectra between 820 and 1350 cm −1 is covered by the luminescence of Ho 3+ ions, and the determination of exact positions of the peaks related to antisymmetric stretching vibrations is therefore impossible [3]. The Raman spectrum decomposition in the range of free MoO 4 rotation and bending vibrations for Li 0.05 Na 0.95 CaGd 0.5 (MoO 4 ) 3 :Ho 3+ 0.05/Yb 3+ 0.45 is presented in Figure 6b. The spectral contour in the range of 300-360 cm −1 consists of two lines related to symmetric stretching, and two lines were found in the range of 380-425 cm −1 (antisymmetric bending). The spectral peak at 208 cm −1 is shown with a single line and corresponds to the free rotation of MoO 4 tetrahedra [75]. Thus, the number of observed peaks in this region is in accordance with the group theoretical analysis, confirming the high crystallinity of the samples.  It is interesting to note that the Raman profiles of Li x Na 1-x CaGd 0.5 (MoO 4 ) 3 :Ho 3+ 0.05/Yb 3+ 0.45 (x= 0, 0.05, 0.1, 0.2, 0.3) in the range of free rotation and bending modes of MoO 4 tetrahedra are sensitive to the Li content, as seen in Figure 7. The range of MoO 4 stretching vibrations is beyond the scope of this discussion because of the overlapping of the Raman signal with the Ho 3+ luminescence. As was mentioned earlier, the substitution of Li + for Na + should induce a decrease in the average ion radius IR(Li/Na/Ca/Gd/Ho/Yb) of the A position and a related unit cell volume decrease. In turn, this should lead to minor changes in the geometry of molybdenum-oxygen tetra-hedra, namely, changes in bond lengths (see Table S2) and angles that should affect the Raman shift of vibration modes. The graphical representation of MoO 4 bending vibrations is presented in Figure 8, and we can suppose that the angle variation in MoO 4 units should affect the symmetric bending, while the bond length variation should lead to changes in the Raman shift of antisymmetric bending vibrations. Thus, we can summarize that the Li content variation in Li x Na 1-x CaGd 0.5 (MoO 4 ) 3 :Ho 3+ 0.05/Yb 3+ 0.45 leads to minor changes in the MoO 4 bond length that are consistent with the changes in the Raman profiles in the range of antisymmetric bending vibrations. For the free MoO 4 rotation vibration, the Raman line position variation is less than 1 cm −1 .    Figure 9. Under the excitation at 980 nm, the samples exhibited yellow emission composed of red and green emission bands of the Ho 3+ ion, namely, the 5 S 2 / 5 F 4 → 5 I 8 band is in green and the 5 F 5 → 5 I 8 band is in red [42]. The peak intensity variation of the green and red bands, as well as the integral intensity of UC luminescence over the whole spectrum, shows its complex behavior, as seen in Figure S4, maximizing at two different Li contents of x = 0.1 and 0.3. The incorporation of Li + ions into the lattice of hosts instead of ions with a larger radius is known to be efficient for controlling the luminescence of doping ions via the crystal field variation affecting these luminescing ions [76]. Li + ions commonly do not destroy or alter the local symmetry of the rare earth ions in the lattice, but they can provide more appropriate UC intensities [76,77]. This is the case for the crystal structure under study. All Na, Li Ca, Gd, Ho, and Yb ions occupy the same site in the scheelite crystal structure. The introduction of Li instead of Na leads to a decrease in the tetragonal unit cell parameters a and c. However, it is a specific feature of the scheelite-type crystal structure under study that this decrease is rather tiny for x = 0.05 − 0.2, and at x = 0.3, a much stronger decrease in the unit cell size along the c axis is observed, indicating a more prominent distortion of the cell and of local structural elements within it. Specifically, all large ions mentioned above in the scheelite structure are coordinated by eight oxygen ions forming polyhedra with the local symmetry S 4 . The distances between a large cation and oxygen are not all equal, but take two unequal values, However, for the crystal under study with Li content x = 0, the difference between these values (2.484 Å) is below the accuracy of measurement by XRD. At x = 0.3, the Me-O distance values are noticeably different, being 2.44 and 2.53 Å, indicating a distortion of the large cation polyhedron. An additional factor that may be at work in the case of the scheelite structure is the noticeable electric field due to the stochastic cation distribution over equivalent sites. This factor may lead to variability of the cation distribution characteristics upon the introduction of Li ions. The existence of several possible mechanisms working as the Li content is varied produces, in our opinion, the complex dependence of the UC luminescence in Figure 9. The similarity of this dependency for both green and red bands means, as we suggest, that the introduction of Li influences the two-stage UC excitation channel, while the de-excitation channel probability variations play a less important role, resulting in minor changes in the relative intensities of red and green bands. It is interesting to compare the behavior of LiNCGM:HY studied in this paper and that of LiNCLM:HY [67] at a different Li content. In the case of LiNCLM:HY, the green upconversion band experiences two maxima, while the red upconversion band experiences a single maximum at the Li content x = 0.2. The explanation for the different behavior of the two hosts arises from the different ionic radii of La and Gd. Ho and Yb ions experience, in general, a stronger crystal field when they occupy the Gd position than when they occupy the La position in the crystal structure of the host. As a result, the LiNCGM:HY behavior is more complex in the case of varied Li content.
The CIE diagram and individual chromaticity points (ICP) for (x, y) of LiNCGM:HY phosphors are shown in Figure 10

Conclusions
New MAS-derived triple molybdate LiNCGM:HY phosphors under the variation of Li x Na 1-x (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) were successfully manufactured. The resultant particles after annealing at 800 • C for 16 h provided well-crystallized ST tetragonal phases with particles of size 3-10 µm. The crystal structures of LiNCGM:HY phosphors at room temperature were determined in space group I4 1 /a by Rietveld analysis. NCGM:HY has a scheelite-type structure with cell parameters a = 5.24782 (11) and c = 11.5107 (3) Å, V = 317.002 (17) Å 3 , Z = 4. In doped samples, the sites are occupied by a mixture of (Li,Na,Gd,Ho,Yb) ions, and this provides a linear cell volume decrease with increasing doping level. Under the excitation derived from 980 nm, the final particles led to the formation of yellow emissions based on the 5 S 2 / 5 F 4 → 5 I 8 green emission and the 5 F 5 → 5 I 8 red emission. The incorporated Li + ions gave rise to local symmetry distortion around the cations in the substituted crystal structure by the Ho 3+ and Yb 3+ ions, and further affected the UC transition probabilities in the quadruple molybdate of LiNCGM:HY. The Raman spectra of LiNCGM doped with Ho 3+ and Yb 3+ ions were totally covered by the luminescence signal of Ho 3+ ions, and increasing the Li content resulted in a difference in the multiplet Ho 3+ intensity. Variation of Raman line positions was observed in the range of MoO 4 bending vibrations depending on the Li content.
Supplementary Materials: The following are available online, Figure S1: Difference Rietveld plots