Triple molybdate scheelite-type upconversion phosphor NaCaLa(MoO₄)₃:Er³⁺/Yb³⁺: structural and spectroscopic properties

Abstract

Triple molybdate NaCaLa_(1−x−y)(MoO₄)₃:xEr³⁺,yYb³⁺ (x = y = 0, x = 0.05 and y = 0.45, x = 0.1 and y = 0.2, x = 0.2 and y = 0) phosphors were successfully synthesized for the first time by the microwave sol–gel method. Well-crystallized particles formed after heat treatment at 900 °C for 16 h showed a fine and homogeneous morphology with particle sizes of 2–3 μm. The structures were refined by the Rietveld method in the space group I4₁/a. The optical properties were examined comparatively using photoluminescence emission and Raman spectroscopy. Under excitation at 980 nm, the NaCaLa_0.7(MoO₄)₃:0.1Er³⁺,0.2Yb³⁺ and NaCaLa_0.5(MoO₄)₃:0.05Er³⁺,0.45Yb³⁺ particles exhibited a strong 525 nm emission band, a weaker 550 nm emission band in the green region, and three weak 655 nm, 490 nm and 410 nm emission bands in the red, blue and violet regions. The pump power dependence and Commission Internationale de L'Eclairage chromaticity of the upconversion emission intensity were evaluated in detail.

---

1. Introduction

In recent years, rare-earth doped oxide-based upconversion (UC) particles have become of extensive interest due to their stable luminescent properties and potential applications in photonic products such as lasers, three-dimensional displays, light-emitting devices, solar cells and biological luminescent imaging media.^1–3 Previously, scheelite-type binary molybdates were reported in terms of new structures, including structure-modulation effects, promising spectroscopic characteristics and excellent upconversion (UC) photoluminescence properties.^4–9 In particular, the rare-earth binary NaLn(MoO₄)₂ (Ln = La³⁺, Gd³⁺, Y³⁺) compounds possess the tetragonal phase with the space group I4₁/a, and belong to the family of sheelite-type structures.¹⁰ The trivalent rare-earth ions in the tetragonal phase can be partially substituted by laser-active Er³⁺, Ho³⁺, Tm³⁺ and Yb³⁺ ions. These ions are efficiently doped into the crystal lattice of the tetragonal binary molybdates due to the similar radii of the trivalent rare earth ions, which results in the excellent UC photoluminescence properties.^{4–9,11–14}

Among rare-earth ions, the Er³⁺ ion is suitable for the infrared to visible light conversion through the UC process due to its appropriate electronic energy level configuration. The Yb³⁺ ion, used as a sensitizer, can be dramatically excited by incident light source energy. This energy is transferred to the activator from which radiation can be emitted. The Er³⁺ ion activator is an efficient luminescence center of the UC particles, while the sensitizer enhances the UC luminescence efficiency. Er³⁺ and Yb³⁺ ion co-doping can remarkably enhance the UC efficiency for the shift from the infrared to visible light due to the efficiency of the energy transfer from Yb³⁺ to Er³⁺.^15–17

For the preparation of the double molybdate NaLn(MoO₄)₂, several processes have been developed via specific preparation processes, including solid-state reactions,^4,8,18–21 the sol–gel method,^22,23 Czochralski growth,^24–27 the hydrothermal method,^28–32 the microwave assisted hydrothermal method³³ and pulsed laser deposition.³⁴ Nevertheless, it is necessary to create new triple molybdate compounds for the observation of the UC photoluminescence in the materials and to search for features such as a well-defined morphology and stable UC luminescent properties. However, so far, triple molybdates with the general composition NaRLn(MoO₄)₃ (R = Ca²⁺, Sr²⁺ and Ba²⁺, while Ln is a rare-earth element) have not been reported. Compared to the common technological methods, microwave synthesis has its advantages of a very short reaction time, small particle size, narrow particle size distribution, and high final polycrystalline sample purity.^35,36 The microwave heating is delivered to the material surface by radiant and/or convection heating and the heat energy is transferred to the bulk of the material via conduction.^37–39 It is an inexpensive method that provides high-homogeneity powder products and it is easy to scale up the process. Thus, the microwave method is considered a viable alternative approach for the quick synthesis of high-quality luminescent materials. In this concept, this method is optimal for the synthesis of complex oxide compounds.

In the present study, triple molybdate NaCaLa_(1−x−y)(MoO₄)₃:xEr³⁺,yYb³⁺ (NCLM:xEr³⁺,yYb³⁺) phosphors with correct doping concentrations of Er³⁺ and Yb³⁺ (x = y = 0, x = 0.05 and y = 0.45, x = 0.1 and y = 0.2, x = 0.2 and y = 0) were successfully prepared by the microwave sol–gel method followed by heat treatment in air. The synthesized particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Their optical properties were examined comparatively using photoluminescence (PL) emission and Raman spectroscopy. The pump power dependence and Commission Internationale de L'Eclairage (CIE) chromaticity parameters of the UC emission were evaluated in detail.

2. Experimental methods

In this study, precise amounts of the raw materials, the products of Sigma-Aldrich (USA) for Ca(NO₃)₂·4H₂O (99%), Na₂MoO₄·2H₂O (99%), La(NO₃)₃·6H₂O (99%), Er(NO₃)₃·5H₂O (99.9%), Yb(NO₃)₃·5H₂O (99.9%), Alfa Aesar (USA) for (NH₄)₆Mo₇O₂₄·4H₂O (99%), Daejung Chemicals (Korea) for citric acid (99.5%), NH₃·H₂O (A.R.), ethylene glycol (A.R.) and distilled water, were used to prepare NaCaLa(MoO₄)₃, NaCaLa_0.8(MoO₄)₃:0.2Er³⁺, NaCaLa_0.7(MoO₄)₃:0.1Er³⁺,0.2Yb³⁺, and NaCaLa_0.5(MoO₄)₃:0.05Er³⁺,0.45Yb³⁺ compositions. The reagents were taken in accordance with the nominal composition of the designed compounds. For the preparation of the compounds, initially 0.4 mol% Ca(NO₃)₂, 0.2 mol% Na₂MoO₄·2H₂O and 0.143 mol% (NH₄)₆Mo₇O₂₄·4H₂O were dissolved in 20 mL of ethylene glycol and 80 mL of 5 M NH₃·H₂O under vigorous stirring and heating. Subsequently, 0.4 mol% (1 − x − y)La(NO₃)₃·6H₂O with 0.4 mol% xEr(NO₃)₃·5H₂O and 0.4 mol% yYb(NO₃)₃·5H₂O (x = 0.05 and y = 0.45, x = 0.1 and y = 0.2, x = 0.2 and y = 0) were dissolved in 100 mL of distilled water under vigorous stirring and heating. At this stage, citric acid was employed with the molar ratio of citric acid to metal ions of 2 : 1. Then, the two kinds of transparent solutions were co-mixed together under vigorous stirring under heating at 80–100 °C. Finally, the mixed solutions appeared highly transparent and were treated by adjusting to pH = 7–8 using the addition of citric acid or NH₃·H₂O. The co-mixed and adjusted solutions were transferred into an oven for microwave irradiation. Microwave operations for 30 min were conducted by precise controlling. The frequency was 2.45 GHz and the maximum output-power was 1250 W. After the microwave process, the samples were treated in an ultrasonicator for 10 min, and transferred into a dry oven. The drying conditions were at 120 °C and dried gels in black color were obtained. For the crystallization of the compounds, the black dried gels were heat-treated at 900 °C for 16 h. Finally, white pure NaCaLa(MoO₄)₃ and pink particles for the Er/Yb-doped compositions were obtained. The chemical compositions of the final powder products were confirmed using EDS measurements.

The powder diffraction data of the synthesized particles for Rietveld analysis were collected over the range of 2θ = 5–90° at room temperature with a D/MAX 2200 (Rigaku, Japan) diffractometer (Cu-Kα radiation, θ–2θ geometry). The step size of 2θ was 0.02°, and the counting time was 5 s per step. The microstructure and surface morphology of the synthesized particles were examined using SEM (JSM-5600, JEOL, Japan). PL spectra were recorded using a spectrophotometer (Perkin Elmer LS55, UK) at room temperature. The pump power dependence of the UC emission intensity was measured at working power from 20 to 110 mW levels. Raman spectroscopy measurements were performed using a LabRam Aramis (Horiba Jobin-Yvon, 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; the power on the samples was kept at the 0.5 mW level to avoid decomposition of the sample.

3. Results and discussion

The XRD patterns recorded from the synthesized molybdates are shown in Fig. 1 and 1S–3S.† In general, the patterns of solutions NCLM:xEr³⁺,yYb³⁺ are similar. The difference profile plot of NCLM is shown in Fig. 1. The difference profile plots of NCLM:xEr³⁺,yYb³ are very similar to that of NCLM, as is evident from a comparison of Fig. 1 and 1S–3S.†

Fig. 1 The difference Rietveld plot of NCLM.

Rietveld refinement was performed using the TOPAS 4.2 package.⁴⁰ All diffraction peaks were indexed using the tetragonal unit cell in the space group I4₁/a with parameters close to those of CaMoO₄ ⁴¹ and Na_0.5La_0.5MoO₄.⁴² Therefore, these crystal structures were taken as a starting model for the Rietveld refinement. The site of the Ca or (Na/La) ion was taken as occupied by Ca, Na, La, Er, and Yb ions at fixed occupations according to the nominal compositions. The refinement was stable and gave low R-factors (Table 1, Fig. 1 and 1S–3S†). The obtained atomic coordinates and the main bond lengths can be found in Tables 1S and 2S,† respectively. In the compounds under consideration, Z = 4 and, from the structural point of view,⁴³ the total chemical formula calculated by summing all elements in the unit cell Na_4/3Ca_4/3La_{4(1−x−y)/3}(MoO₄)₄:4x/3Er³⁺,4y/3Yb³⁺ can be generalized as Na_1/3Ca_1/3La_{(1−x−y)/3}MoO₄:(x/3)Er³⁺,(y/3)Yb³⁺, which clearly emphasizes the relationship of the solid solutions to the CaMoO₄ structural family.⁴¹ As an example, the structure of NCLM is shown in Fig. 2.

Fig. 2 The crystal structure of NCLM. The unit cell is outlined.

Table 1 Main parameters of the processing and refinement of the NCLM:xEr³⁺,yYb³⁺ samples

Compound	NCLM	NCLM:0.2Er^3+	NCLM:0.1Er^3+,0.2Yb^3+	NCLM:0.05Er^3+,0.45Yb^3+
x	0	0.2	0.1	0.05
y	0	0	0.2	0.45
Sp. gr.	I41/a	I41/a	I41/a	I41/a
a, Å	5.2991 (1)	5.2806 (2)	5.2675 (1)	5.2421 (1)
c, Å	11.6223 (3)	11.5726 (3)	11.5377 (3)	11.4678 (2)
V, Å^3	326.37 (2)	322.70 (1)	320.13 (2)	315.17 (1)
Z	4	4	4	4
2θ-interval, °	5–90	5–90	5–90	5–90
No. of reflections	70	70	70	70
No. of refined parameters	7	7	7	7
R wp, %	19.46	16.75	15.65	15.06
R p, %	13.39	11.30	10.31	9.97
R exp, %	16.22	14.87	13.61	13.49
χ ^2	1.20	1.13	1.15	1.12
R B, %	5.29	3.15	2.09	1.65

---

The linear cell volume increase with the averaged increase of ion radii IR(Na/Ca/La/Er/Yb), as shown in Fig. 3, proves the chemical compositions of the synthesized samples.⁴⁴ On the basis of the structural results, it can be reasonably supposed that different triple molybdates with the general composition NaRLn(MoO₄)₃ should crystallize in tetragonal structures close to that of CaMoO₄ and the general formula should be rewritten as Na_1/3R_1/3Ln_1/3MoO₄. Further details of the crystal structure 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) on quoting the deposition numbers: CSD 431015–431018.

Fig. 3 Cell volume per averaged ion radii IR(Na/Ca/La/Er/Yb) of CaMoO₄ (green),⁴¹ NCLM:xEr³⁺,yYb³⁺ (red) and Na_0.5La_0.5MoO₄ (blue)⁴² compounds.

Thus, post heat-treatment plays an important role in molybdate gel crystallization. To reach a high-quality crystalline state, samples need to be heat treated at 900 °C for 16 h. It should be pointed out that this temperature is optimal for molybdate treatment in air and, earlier, other simple and complex molybdates were formed at close temperatures.^{6,7,9,37–39,45–47} Besides excellent crystallinity, the selected synthesis route provides a uniform particle morphology. The SEM images of the synthesized (a) NCLM:0.2Er³⁺ and (b) NCLM:0.05Er³⁺,0.45Yb³⁺ particles are shown in Fig. 4. The as-synthesized samples are well formed with a fine and homogeneous morphology and a particle size of 2–3 μm. The samples show no discrepancy in the aspect of morphological features, and closely agglomerated particles were induced by active grain interdiffusion.^48–50 It should be noted that doping concentrations of Er³⁺ and Yb³⁺ have no effects on the particle morphology. The recorded EDS patterns and quantitative compositions of the NCLM:0.1Er³⁺,0.2Yb³⁺ sample are shown in Fig. 4S and Table 3S.† Only constituent elements are found in the samples and the quantitative compositions are in good accord with nominal compositions. Thus, the microwave sol–gel method of triple molybdate preparation provides energy uniformly over the material bulk, and fine particles with controlled morphology can be fabricated in a short time. The method is an inexpensive way to fabricate highly homogeneous powder products with an easy scale-up and is a viable alternative for the rapid synthesis of UC particles. This suggests that the microwave sol–gel route is suitable for the creation of homogeneous NCLM:xEr³⁺,yYb³⁺ crystallites and can be successfully applied to other molybdates from this crystal family.

Fig. 4 Scanning electron microscopy images of the synthesized (a) NCLM:0.2Er³⁺ and (b) NCLM:0.05Er³⁺,0.45Yb³⁺ particles.

The UC photoluminescence emission spectra of the as-prepared NCLM, NCLM:0.2Er³⁺, NCLM:0.1Er³⁺,0.2Yb³⁺ and NCLM:0.05Er³⁺,0.45Yb³⁺ particles excited at 980 nm at room temperature are shown in Fig. 5. The UC NCLM:0.1Er³⁺,0.2Yb³⁺ and NCLM:0.05Er³⁺,0.45Yb³⁺ particles exhibited a strong 525 nm emission band, a weaker 550 nm emission band in the green region and three very weak emission bands: at 655 nm in the red region, at 490 nm in the blue region, and at 410 nm in the violet region. The strong 525 nm emission band and the weak 550 nm emission band in the green region correspond to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, respectively, while the very weak 655 nm emission band in the red region corresponds to the ⁴F_9/2 → ⁴I_15/2 transition. Another very weak band at 410 nm is due to the ²H_9/2 → ⁴I_15/2 transition, while a 490 nm peak is due to the ⁴F_9/2 → ⁴I_15/2 transition. It must be noted that in sample (d), the shortest-wavelength band at 410 nm is only two times smaller than the red one, despite the fact that a three-stage process is necessary to excite the ²H_9/2 level. The mechanism of excitation of this level might be the energy transfer from the Yb excited state to the Er ⁴F_9/2 level, since the energy difference between ⁴F_9/2 and ²H_9/2 is close to the energy of the excited Yb ion. However, since the ⁴F_9/2 population is likely to be low, the most probable excitation channel is the population of a pair of high-lying levels ⁴G_9/2 and ²K_15/2 from the well-populated ⁴S_3/2 level through the energy transfer from the Yb ion, with the subsequent decay of these high-lying levels to the ²H_9/2 one.

Fig. 5 The upconversion photoluminescence emission spectra of (a) NCLM, (b) NCLM:0.2Er³⁺, (c) NCLM:0.1Er³⁺,0.2Yb³⁺, and (d) NCLM:0.05Er³⁺,0.45Yb³⁺ particles excited at 980 nm at room temperature. The logarithmic scale is used along the vertical axis.

The UC intensities of NCLM were not detected. The UC intensities of NCLM:0.2Er³⁺ are well above the detection limit; however, they are one or two orders of magnitude smaller than those of the Yb-doped samples and, hence, the UC luminescence for an Er-doped sample is not well seen in Fig. 5. The UC luminescence is observed from all levels: ²H_11/2, ⁴S_3/2 and even from ⁴F_9/2. The intensity ratio of green and red lines is 1.4 for sample (b), 30 for sample (c) and 56 for sample (d). At the same time, the ratio of UC green band peak values for samples (d) and (b) is 92. The latter effect admits that the absorption coefficient of erbium at 980 nm is much smaller than that of ytterbium in the matrix under study. The variation of the green-to-red ratio with the ytterbium content increase is rather common and was observed earlier for several hosts.^{15–17,37–39}

The Er³⁺ ion activator is the luminescence center of these UC particles and the sensitizer Yb³⁺ effectively enhances the UC luminescence intensity because of efficient energy transfer from Yb³⁺ to Er³⁺. The concentration quenching effect can be explained by the energy transfer between the nearest Er³⁺ and Yb³⁺ ions. On increasing the Er³⁺ and Yb³⁺ ion concentrations, the distance between Er³⁺ and Yb³⁺ ions decreases, which can promote a non-radiative energy transfer, such as an exchange interaction or multipole–multipole interactions.⁵¹ As shown in Fig. 5, the higher intensity of (d) NCLM:0.05Er³⁺,0.45Yb³⁺ is caused by the ratio of Yb³⁺ : Er³⁺ = 9 : 1, while the lower intensity of (c) NCLM:0.1Er³⁺,0.2Yb³⁺ is caused by the ratio of Yb³⁺ : Er³⁺ = 2 : 1. Thus, the preferable Yb³⁺ : Er³⁺ = 9 : 1 ratio is induced by the concentration quenching effect of Er³⁺ ions. Therefore, the higher content of the Yb³⁺ ions used as a sensitizer and the lower content of the Er³⁺ ions close to the preferable ratio of Yb³⁺ : Er³⁺ = 9 : 1 can remarkably enhance the UC luminescence through the efficient energy transfer. The ratio of the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transition intensities may be influenced not only by the change in radiation probabilities from starting levels, but also by the probabilities of the non-radiative relaxation from the UC-populated ⁴F_7/2 level. Because the lifetime of the ⁴F_7/2 level is comparatively short, the excited Er³⁺ ions decay non-radiatively to the ²H_11/2 level with a higher probability, as compared to the ⁴S_3/2 level, in the case of the NCLM host matrix.

The logarithmic scale dependences of the UC emission intensities at 525, 550 and 655 nm on the working pump power over the range of 20–110 mW in the NCLM:0.05Er³⁺,0.45Yb³⁺ sample are shown in Fig. 6. In the UC process, the UC emission intensity I is proportional to the slope value n of the irradiation pumping power P. The maximum value of n is the number of pumping photons required to reach the starting energy level in the UC ion and produce UC emission:⁵²

I ∝ Pⁿ (1)

ln I ∝ n ln P (2)

Fig. 6 The logarithmic scale dependence of the upconversion emission intensity on the pump power in the range of 20–110 mW at 525, 550, 655 and 410 nm in the NCLM:0.05Er³⁺,0.45Yb³⁺ sample.

The slopes n = 1.70 and 1.68 for green emission at 525 and 550 nm, and n = 1.56 for red emission at 655 nm, respectively, are evident from Fig. 5. These results show that the UC mechanism of the green and red emissions can be explained by a two-photon UC process in the Er³⁺/Yb³⁺ co-doped phosphors.^53–55

Based on the results of the analysis of pump power dependence, the schematic energy level diagrams of Er³⁺ ions (activator) and Yb³⁺ ions (sensitizer) in the NCLM:xEr³⁺,yYb³⁺ samples and the UC mechanisms, accounting for the green and red emissions excited by the 980 nm laser wavelength, are shown in Fig. 7. The UC emissions are generated via multiple processes of ground state absorption (GSA), energy transfer upconversion (ETU), excited state absorption (ESA) and cross relaxation (CR). Under excitation at 980 nm, the Er³⁺ and Yb³⁺ ions are initially excited from the ground state to the excited state through the ground state absorption (GSA) process (Er³⁺: ⁴I_15/2 → ⁴I_11/2, Yb³⁺: ²F_7/2 → ²F_5/2) and ETU processes of ⁴I_15/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴I_11/2 (Er³⁺) + ²F_7/2 (Yb³⁺), which are responsible for the population at the ⁴I_11/2 level in the Er³⁺ ion. For the green emissions, the energy transition from the ⁴I_11/2 level to the ⁴F_7/2 level of Er³⁺ is involved in three possible processes:^53–55 (1) ESA: ⁴I_11/2 (Er³⁺) + a photon (980 nm) → ⁴F_7/2, (2) ETU: ²I_11/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴F_7/2 (Er³⁺) + ²F_7/2 (Yb³⁺) and (3) ETU: ⁴I_11/2 (Er³⁺) + ⁴I_11/2 (Er³⁺) → ⁴F_7/2 (Er³⁺) + ⁴I_15/2 (Er³⁺). These three possible processes can populate the ⁴F_7/2 level from the ⁴I_11/2 level in Er³⁺ and, then, the ⁴F_7/2 level relaxes rapidly and non-radiatively to the next lower ²H_11/2 and ⁴S_3/2 levels in Er³⁺ because of the short lifetime of the ⁴F_7/2 level. As a result, the radiative transitions of ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 processes can generate the green emission at 525 and 550 nm. The strong suppression of red UC luminescence in the host under study is similar to the effect in the earlier studied CaGd₂(MoO₄)₄:Er,Yb and CaGd₂(WO₄)₄:Er,Yb systems and differentiates this host class from a number of others. For the red emission, the ⁴F_9/2 level population is generated by non-radiative relaxation either (1) from the ⁴S_3/2 to the ⁴F_9/2 level or (2) from I_11/2 to I_13/2, and then pumping to ⁴F_9/2, as well as by cross relaxation (CR) via either (3) the ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2 transition^51–53 or (4) ⁴S_3/2 + ⁴I_15/2 = ⁴I_9/2 + ⁴I_13/2, and then pumping to ⁴F_9/2.⁵⁶ Finally, the ⁴F_9/2 level relaxes radiatively to the ground state at the ⁴I_15/2 level, and releases the red emission at 655 nm.^37–39 The radiation-free transitions (1) and (2) must not strongly vary from one oxide host to another and, then, they cannot provide such a significant difference between the red and green luminescence in the selected group of hosts. Consequently, processes (1) and (2) must be deduced to play a negligible role in the host class under study. So, intense red luminescence at the Er/Yb ratio of 3/8 is most likely, due to one of the mentioned cross-relaxation channels.⁵⁴ The (3) ⁴F_7/2 + ⁴I_11/2 = ⁴F_9/2 + ⁴F_9/2 cross-relaxation will be very weak since ⁴F_7/2 has a comparatively short lifetime and its radiationless depopulation to ⁴S_3/2 and ²H_11/2 levels is rather fast, which may explain the weak red luminescence in our host. For the (4) population channel, the weak red UC luminescence is explainable by the suggestion that, in our host, detuning between the energy of the ⁴S_3/2 state and the sum of the energies of ⁴I_9/2 and ⁴I_13/2 states is not so favorable.⁵⁴ Moreover, as Yb³⁺ concentration increases, the green emission dramatically increases, compared to the red emission. The strong 525 nm and 550 nm emission lines in the green region, as shown in Fig. 5, are assigned to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of the Er³⁺ ions, respectively, while the weak 655 nm emission band in the red region is assigned to the ⁴F_9/2 → ⁴I_15/2 transition.

Fig. 7 The schematic energy level diagrams of Yb³⁺ (sensitizer) and Er³⁺ ions (activator) in the NCLM:xEr³⁺,yYb³⁺ system and the upconversion mechanisms of the green and red emissions under 980 nm laser excitation.

The chromaticity coordinates calculated for (a) NCLM:0.1Er³⁺,0.2Yb³⁺ and (b) NCLM:0.05Er³⁺,0.45Yb³⁺ particles and the related CIE chromaticity diagram are shown in Fig. 8. The legend of Fig. 8(B) shows the chromaticity points for samples (a) and (b). When the Er³⁺/Yb³⁺ concentration ratio is varied, the chromaticity coordinate values (x, y) change. As shown in Fig. 8(A), the calculated chromaticity coordinates x = 0.227 and y = 0.686 for (a) NCLM:0.1Er³⁺,0.2Yb³⁺ and x = 0.206 and y = 0.727 for (b) NCLM:0.05Er³⁺,0.45Yb³⁺ correspond to the yellowish-green sector in the CIE diagram.

Fig. 8 (A) Calculated chromaticity coordinate (x, y) values and (B) the CIE chromaticity diagram for the NCLM:xEr³⁺,yYb³⁺ phosphors. The emission points for the sample (a) NCLM:0.1Er³⁺,0.2Yb³⁺ and (b) NCLM:0.05Er³⁺,0.45Yb³⁺ particles are shown in the inset.

The Raman spectra of the synthesized pure NCLM, NCLM:0.2Er³⁺, NCLM:0.1Er³⁺,0.2Yb³⁺ and NCLM:0.05Er³⁺,0.45Yb³⁺ particles are shown in Fig. 9. As to pure NCLM, the well-resolved sharp peaks clearly indicate a highly crystalline state of the synthesized particles. The symmetry and frequencies of all the observed modes in the Raman spectrum of pure NCLM in comparison with the Raman spectrum of isostructural CaMoO₄ ^57,58 are presented in Table 2. The decomposition of spectral regions corresponding to the bending and stretching vibrations of MoO₄ tetrahedra is shown in Fig. 10.

Fig. 9 Emission (Raman and luminescence) spectra of the synthesized (black) NCLM, (red) NCLM:0.2Er³⁺, (blue) NCLM:0.1Er³⁺,0.2Yb³⁺, and (green) NCLM:0.05Er³⁺,0.45Yb³⁺ particles excited by the 514.5 nm line of an Ar ion laser at 0.5 mW. Luminescence from ²H_11/2 and ⁴S_3/2 multiplets of the Er³⁺ ion is indicated.

Fig. 10 The decomposition of Raman spectra of NCLM in the regions of (a) bending and (b) stretching vibrations of MoO₄ tetrahedra.

Table 2 Notation and wavenumber values (cm⁻¹) of the active Raman lines in (a) NCLM, (b) NCLM:0.2Er³⁺, (c) NCLM:0.1Er³⁺,0.2Yb³⁺ and (d) NCLM:0.05Er³⁺,0.45Yb³⁺

Number	Symmetry type	Exp.	Calc.				Exp.
		(a)		(b)	(c)	(d)	CaMoO4 ^57,58	CaLa2(MoO4)4 ^7
1	Ag	885	881	907	874	895	877	906
2	Bg	829	822	849	826	841	845	828
3	Eg	777	785	804	779	798	792	762
4	Bg	390	398	420	424	430	402	393
5	Eg	380	371	378	384	388	391	377
6	Bg	332	330	328	315	328	327	333
7	Ag	318	326	348	343	347	321	317
8	Eg	255	241	257	267	272	267	251
9	Bg	228	231	219	229	229	214
10	Ag	195	202	193	200	202	204	193
11	Eg	190	183	186	192	196	189
12	Bg	141	142	149	151	152	143
13	Eg	125	127	131	134	137	111
14	Bg	96	96	90	95	95
15	Eg		79	80	83	85
16	Bg		21	20	21.4	21.4
17	Eg		18	18	18.8	19.4
18	Bg			18	19.7	19.6
19	Eg			16	17.2	17.7
20	Bg				19.1	19.1
21	Eg				16.8	17.4

---

To perform the lattice dynamics (LD) simulation of the investigated compounds, the program package LADY was used.⁵⁹ The atomic vibration frequencies were obtained using a modified random-element-isodisplacement model.⁶⁰ Only the pair-wise interactions and bond-stretching force constants A are considered. A depends on r_ij, and the A(r_ij) dependences are the same for all atom pairs – A = λ exp(–r_ij/ρ), where r_ij is the interatomic distance, and λ and ρ are the parameters characterizing the selected pair interaction. To find model parameters, a special optimization program was written and tested for several compounds from different chemical classes.^7,47,61–66 The initial parameter values were accepted as random ones and lattice stability conditions were taken into account. The resulting model parameters were obtained by minimization of residual difference values of the simulated and experimental Raman frequencies of pure NCLM using the Fletcher–Reeves method.⁶⁷ In the case of suspension of the Fletcher–Reeves algorithm because of incompatible model parameters, the initial model parameters were set randomly again. To obtain a satisfactory agreement between experimental and calculated results, O–O interatomic interactions within MoO₄ tetrahedral groups and O–O intermolecular interactions between neighboring MoO₄ groups should be described within different force constants, and the cation–cation interactions can be neglected.^68,69 The model parameters obtained for pure NCLM are shown in Table 4S.† The simulations of NCLM:0.2Er³⁺, NCLM:0.1Er³⁺,0.2Yb³⁺, and NCLM:0.05Er³⁺,0.45Yb³⁺ were carried out using the same model parameters. Uniform values of Er–O, Yb–O and La–O force constants were selected. A comparison of the observed and calculated Raman modes of pure NCLM can be found in Table 2. Calculations predict noticeable shifts of Raman frequencies in doped samples; the extent of these shifts is in general agreement with the variation of Mo–O length according to the empirical formula of Hardcastle.⁷⁰ However, the Raman signals measured for Er-doped compositions cannot be directly compared to the simulated results because of strong Er³⁺ luminescence, and the experimental check will be the subject of a separate study.

By the group theory analysis, 17 active Raman modes were predicted for the NCLM structure: Γ_raman = 3A_g + 7B_g + 7E_g; 19 active modes for NCLM:0.2Er³⁺: Γ_raman = 3A_g + 8B_g + 8E_g; 21 active modes for NCLM:0.1Er³⁺,0.2Yb³⁺ and NCLM:0.05Er³⁺,0.45Yb³⁺: Γ_raman = 3A_g + 9B_g + 9E_g. As can be seen from Table 2, additional modes, in comparison with pure molybdate, should be observed in the Raman spectra of Er,Y-doped compounds in the range below 80 cm⁻¹. The most intensive band of the NCLM Raman spectrum found at 884 cm⁻¹ corresponds to the ν₁ symmetric stretching vibration of the MoO₄ group. The lines at 829 and 777 cm⁻¹ are related to the ν₃ antisymmetric stretching vibrations. The bending ν₂ and ν₄ vibrations are situated in the wavenumber region of 300–400 cm⁻¹. The shape of the observed normal vibration modes of tetrahedral MoO₄ groups has been considered in ref. 71. The translation and rotational vibrations of MoO₄ are in the region of 150–260 cm⁻¹. The big cation vibrations are below 150 cm⁻¹. As can be seen from a comparison of Raman spectra of pure NCLM and CaLa₂(MoO₄)₄ ⁷ shown in Fig. 5S,† the positions of the bands corresponding to the symmetric stretching of MoO₄ are slightly different in these crystals. In this case, according to the LD model presented above, Mo–O bonds should have different lengths in these molybdates and this is confirmed by structural results (Table 2S†). As to Raman spectra of the doped samples recorded under excitation at 514.5 nm, the Raman lines are superimposed by strong Er³⁺ luminescence lines. To consider the vibrational spectra of these samples, it is topical to use an excitation source with a drastically longer wavelength that should avoid the excitation of Er³⁺ ion optical transitions.

4. Conclusions

Triple molybdate NCLM:xEr³⁺,yYb³⁺ phosphors were successfully synthesized by the microwave sol–gel method. Well-crystallized particles formed after heat-treatment at 900 °C for 16 h showed a fine and homogeneous morphology with particle sizes of 2–3 μm. Under excitation at 980 nm, the UC doped particles exhibited a strong 525 nm emission band and a weak 550 nm emission band in the green region, corresponding to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, and a very weak 655 nm emission band in the red region, corresponding to the ⁴F_9/2 → ⁴I_15/2 transition. The preferable Yb³⁺ : Er³⁺ ratio of 9 : 1 is controlled by the concentration quenching effect in Er³⁺ ions. The calculated slope value n indicated slopes of n = 1.70 and 1.68 for the green emission at 525 and 550 nm, respectively, and n = 1.56 for the red emission at 655 nm. The calculated chromaticity coordinates of the NCLM:xEr³⁺,yYb³⁺ phosphors correspond to the yellowish-green sector in the CIE diagram.

Acknowledgements

This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015-058813), by the Russian Foundation for Basic Research (15-52-53080) and by project no. 0358-2015-0012 of SB RAS Program no. II.2P. ASO and VVA are partially supported by the Ministry of Education and Science of the Russian Federation.

References

1. M. V. DaCosta, S. Doughan, Y. Han and U. J. Krull, Lanthanide upconversion nanoparticles and applications in bioassays and bioimaging: A review, Anal. Chim. Acta, 2014, 832, 1–33.
2. Y. J. Chen, H. M. Zhu, Y. F. Lin, X. H. Gong, Z. D. Luo and Y. D. Huang, Efficient diode-pumped continuous-wave monolithic 1.9 μm micro-laser based on Tm³⁺:BaGd₂(MoO₄)₄ cleaved plate, Opt. Mater., 2013, 35, 1422–1425.
3. C. Zhang, L. D. Sun, Y. W. Zhang and C. H. Yan, Rare earth upconversion nanophosphors: synthesis, functionalization and application as biolabels and energy transfer donors, J. Rare Earths, 2010, 28, 807–819.
4. V. V. Atuchin, O. D. Chimitova, T. A. Gavrilova, M. S. Molokeev, S.-J. Kim, N. V. Surovtsev and B. G. Bazarov, Synthesis, structural and vibrational properties of microcrystalline RbNd(MoO₄)₃, J. Cryst. Growth, 2011, 318, 683–686.
5. V. A. Morozov, A. Bertha, K. W. Meert, S. Van Rompaey, D. Batuk, G. T. Martinez, S. Van Aert, P. F. Smet, M. V. Raskina, D. Poelman, A. M. Abakumov and J. Hadermann, Incommensurate modulation and luminescence in the CaGd_2(1–x)Eu_2x(MoO₄)_4(1–y)(WO₄)_4y(0 ≤ x ≤ 1, 0 ≤ y ≤ 1) red phosphors, Chem. Mater., 2013, 25, 4387–4395.
6. P. L. Shi, Z. G. Xia, M. S. Molokeev and V. V. Atuchin, Crystal chemistry and luminescence properties of red-emitting CsGd_1−xEu_x(MoO₄)₂ solid-solution phosphors, Dalton Trans., 2014, 43, 9669–9676.
7. C. S. Lim, A. Aleksandrovsky, M. Molokeev, A. Oreshonkov and V. Atuchin, 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.
8. V. V. Atuchin, A. S. Aleksandrovsky, O. D. Chimitova, C.-P. Diao, T. A. Gavrilova, V. G. Kesler, M. S. Molokeev, A. S. Krylov, B. G. Bazarov, J. G. Bazarova and Z. S. Lin, Electronic structure of β-RbSm(MoO₄)₂ and chemical bonding in molybdates, Dalton Trans., 2015, 44, 1805–1815.
9. C. S. Lim, Highly modulated structure and upconversion photoluminescence properties of PbGd₂(MoO₄)₄:Er³⁺/Yb³⁺, Mater. Res. Bull., 2016, 75, 211–216.
10. O. D. Chimitova, V. V. Atuchin, B. G. Bazarov, M. S. Molokeev and Z. G. Bazarova, The formation and structural parameters of new double molybdates RbLn(MoO₄)₂ (Ln = Pr, Nd, Sm, Eu), Proc. SPIE-Int. Soc. Opt. Eng., 2013, 8771, 87711A.
11. C. F. Guo, H. K. Yang and J.-H. Jeong, Preparation and luminescent properties of phosphor MGd₂(MoO₄)₄: Eu³⁺ (M = Ca, Sr and Ba), J. Lumin., 2010, 130, 1390–1393.
12. J. Y. Sun, Y. J. Lan, Z. G. Xia and H. Y. Du, Sol–gel synthesis and green upconversion luminescence in BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphors, Opt. Mater., 2011, 33, 576–581.
13. J. S. Liao, D. Zhou, B. Yang, R. Q. Liu, Q. Zhang and Q. H. Zhou, Sol–gel preparation and photoluminescence properties of CaLa₂(MoO₄)₄:Eu³⁺ phosphors, J. Lumin., 2013, 134, 533–538.
14. F. R. Chen, Z. G. Xia, M. S. Molokeev and X. P. Jing, Effects of composition modulation on the luminescence properties of Eu³⁺ doped Li_1−xAg_xLu(MoO₄)₂ solid-solution phosphors, Dalton Trans., 2015, 44, 18078–18089.
15. C. S. Lim, Preparation of CaLa₂(MoO₄)₄:Er³⁺/Yb³⁺ phosphors via the microwave-modified sol-gel route and the upconversion of their photoluminescence properties, Mater. Res. Bull., 2014, 60, 537–542.
16. C. S. Lim, Upconversion photoluminescence properties of SrY₂(MoO₄)₄:Er³⁺/Yb³⁺ phosphors synthesized by a cyclic microwave-modified sol-gel method, Infrared Phys. Technol., 2014, 67, 371–376.
17. J. Y. Sun, W. Zhang, W. H. Zhang and H. Y. Du, Synthesis and two-color emission properties of BaGd₂(MoO₄)₄:Eu³⁺,Er³⁺,Yb³⁺ phosphors, Mater. Res. Bull., 2012, 47, 786–789.
18. J. F. Tang, C. H. Cheng, Y. J. Chen and Y. D. Huang, Yellow–green upconversion photoluminescence in Yb³⁺, Ho³⁺ co-doped NaLa(MoO₄)₂ phosphor, J. Alloys Compd., 2014, 609, 268–273.
19. W. T. Zhang, J. F. Li, Y. L. Wang, J. P. Long and K. H. Qiu, Synthesis and luminescence properties of NaLa(MoO₄)_2−xAG_x:Eu³⁺ (AG = SO₄²⁻, BO₃³⁻) red phosphors for white light emitting diodes, J. Alloys Compd., 2015, 635, 16–20.
20. F. W. Mo, L. Zhou, Q. Pang, F. Z. Gong and Z. J. Liang, Potential red-emitting NaGd(MO₄)₂:R (M = W, Mo, R = Eu³⁺, Sm³⁺, Bi³⁺) phosphors for white light emitting diodes applications, Ceram. Int., 2012, 38, 6289–6294.
21. G. H. Li, S. Lan, L. L. Li, M. M. Li, W. W. Bao, H. F. Zou, X. C. Xu and S. C. Gan, Tunable luminescence properties of NaLa(MoO₄)₂:Ce³⁺,Tb³⁺ phosphors for near UV-excited white light-emitting-diodes, J. Alloys Compd., 2012, 513, 145–149.
22. J. S. Liao, H. Z. Huang, H. Y. You, X. Qiu, Y. Li, B. Qiu and H.-R. Wen, Photoluminescence properties of NaGd(MoO₄)₂:Eu³⁺ nanophosphors prepared by sol–gel method, Mater. Res. Bull., 2010, 45, 1145–1149.
23. F.-B. Cao, L.-S. Li, Y.-W. Tian and X.-R. Wu, Sol–gel synthesis of red-emitting [Na_0.6La_0.8−xEu_x]₂(MoO₄)₃ phosphors and improvement of its luminescent properties by the co-doping method, Opt. Laser Technol., 2014, 55, 6–10.
24. G. M. Kuz'micheva, D. A. Lis, K. A. Subbotin, V. B. Rybakov and E. V. Zharikov, Growth and structural X-ray investigations of scheelite-like single crystals Er, Ce:NaLa(MoO₄)₂ and Yb:NaGd(WO₄)₂, J. Cryst. Growth, 2005, 275, e1835–e1842.
25. X. A. Lu, Z. N. You, J. F. Li, Z. J. Zhu, G. H. Jia, B. C. Wu and C. Y. Tu, Optical absorption and spectroscopic characteristics of Tm³⁺ ions doped NaY(MoO₄)₂ crystal, J. Alloys Compd., 2008, 458, 462–466.
26. X. Z. Li, Z. B. Lin, L. Z. Zhang and G. F. Wang, Growth, thermal and spectral properties of Nd³⁺-doped NaGd(MoO₄)₂ crystal, J. Cryst. Growth, 2006, 290, 670–673.
27. Y. K. Voron'ko, K. A. Subbotin, V. E. Shukshin, D. A. Lis, S. N. Ushakov, A. V. Popov and E. V. Zharikov, Growth and spectroscopic investigations of Yb³⁺-doped NaGd(MoO₄)₂ and NaLa(MoO₄)₂ - new promising laser crystals, Opt. Mater., 2009, 29, 246–252.
28. H. Lin, X. H. Yan and X. F. Wang, Controllable synthesis and down-conversion properties of flower-like NaY(MoO₄)₂ microcrystals via polyvinylpyrrolidone-mediated, J. Solid State Chem., 2013, 204, 266–271.
29. G. H. Li, L. L. Li, M. M. Li, W. W. Bao, Y. H. Song, S. C. Gan, H. F. Zou and X. C. Xu, Hydrothermal synthesis and luminescent properties of NaLa(MoO₄)₂:Eu³⁺,Tb³⁺ phosphors, J. Alloys Compd., 2013, 550, 1–8.
30. Y. Huang, L. Q. Zhou, L. Yang and Z. W. Tang, Self-assembled 3D flower-like NaY(MoO₄)₂:Eu³⁺ microarchitectures: Hydrothermal synthesis, formation mechanism and luminescence properties, Opt. Mater., 2011, 33, 777–782.
31. L. L. Li, W. W. Zi, G. H. Li, S. Lan, G. J. Ji, S. C. Gan, H. F. Zou and X. C. Xu, Hydrothermal synthesis and luminescent properties of NaLa(MoO₄)₂:Dy³⁺ phosphor, J. Solid State Chem., 2012, 191, 175–180.
32. Y. Tian, B. J. Chen, B. N. Tian, J. S. Sun, X. P. Li, J. S. Zhang, L. H. Cheng, H. Y. Zhong, H. Zhong, Q. G. Meng and R. N. Hua, Ionic liquid-assisted hydrothermal synthesis of dendrite-like NaY(MoO₄)₂:Tb³⁺ phosphor, Physica B, 2012, 407, 2556–2559.
33. J. C. Zhang, X. F. Wang, X. H. Zhang, X. D. Zhao, X. Y. Liu and L. P. Peng, Microwave synthesis of NaLa(MoO₄)₂ microcrystals and their near-infrared luminescent properties with lanthanide ion doping (Er³⁺, Nd³⁺, Yb³⁺), Inorg. Chem. Commun., 2011, 14, 1723–1727.
34. S. W. Park, B. K. Moon, B. C. Choi, J. H. Jeong, J. S. Bae and K. H. Kim, Red photoluminescence of pulsed laser deposited Eu:NaY(MoO₄)₂ thin film phosphors on sapphire substrates, Curr. Appl. Phys., 2012, 12, S150–S155.
35. K. I. Rybakov, E. A. Olevsky and E. V. Krikun, Microwave sintering: Fundamentals and modeling, J. Am. Ceram. Soc., 2013, 96, 1003–1020.
36. H. J. Kitchen, S. R. Vallance, J. I. Kennedy, N. Tapia-Ruiz, L. Carassiti, A. Harrison, A. G. Whittaker, T. D. Drysdale, S. W. Kingman and D. H. Gregory, Modern microwave methods in solid-state inorganic materials chemistry: From fundamentals to manufacturing, Chem. Rev., 2014, 114, 1170–1206.
37. C. S. Lim, Cyclic MAM synthesis and upconversion photoluminescence properties of CaMoO₄:Er³⁺/Yb³⁺ particles, Mater. Res. Bull., 2012, 47, 4220–4225.
38. C. S. Lim, Preparation of PbLa₂(MoO₄)₂:Er³⁺/Yb³⁺ particles via microwave sol-gel route and upconversion photoluminescence properties, Ceram. Int., 2015, 41, 12464–12470.
39. C. S. Lim, V. Atuchin, A. Aleksandrovsky, M. Molokeev and A. Oreshonkov, Microwave sol-gel synthesis of CaGd₂(MoO₄)₂:Er³⁺/Yb³⁺ phosphors and their upconversion photoluminescence properties, J. Am. Ceram. Soc., 2015, 98, 3223–3230.
40. Bruker AXS TOPAS V4: General profile and structure analysis software for powder diffraction data – User's Manual, Bruker AXS, Karlsruhe, Germany, 2008.
41. R. M. Hazen, L. W. Finger and J. W. E. Mariathasan, High-pressure crystal chemistry of scheelite-type tungstates and molybdates, J. Phys. Chem. Solids, 1985, 46, 253–263.
42. S. B. Stevens, C. A. Morrison, T. H. Allik, A. L. Rheingold and B. S. Haggerty, NaLa(MoO₄)₂ as a laser host material, Phys. Rev. B: Condens. Matter, 1991, 43, 7386–7394.
43. http://www.iucr.org/__data/iucr/cifdic_html/1/cif_core.dic/Cchemical_formula.html .
44. R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Cryst., 1976, 32, 751–767.
45. V. V. Atuchin, V. G. Grossman, S. V. Adichtchev, N. V. Surovtsev, T. A. Gavrilova and B. G. Bazarov, Structural and vibrational properties of microcrystalline TlM(MoO₄)₂ (M = Nd, Pr) molybdates, Opt. Mater., 2012, 34, 812–816.
46. V. V. Atuchin, A. S. Aleksandrovsky, O. D. Chimitova, A. S. Krylov, M. S. Molokeev, B. G. Bazarov, J. G. Bazarova and Z. Xia, Synthesis and spectroscopic properties of multiferroic β′-Tb₂(MoO₄)₃, Opt. Mater., 2014, 36, 1631–1635.
47. V. V. Atuchin, A. S. Aleksandrovsky, O. D. Chimitova, T. A. Gavrilova, A. S. Krylov, M. S. Molokeev, A. S. Oreshonkov, B. G. Bazarov and J. G. Bazarova, Synthesis and spectroscopic properties of monoclinic α-Eu₂(MoO₄)₃, J. Phys. Chem., 2014, C 118, 15404–15411.
48. V. V. Atuchin, T. A. Gavrilova, J.-C. Grivel and V. G. Kesler, Electronic structure of layered titanate Nd₂Ti₂O₇, Surf. Sci., 2008, 602, 3095–3099.
49. V. V. Atuchin, S. V. Adichtchev, B. G. Bazarov, Zh. G. Bazarova, T. A. Gavrilova, V. G. Grossman, V. G. Kesler, G. S. Meng, Z. S. Lin and N. V. Surovtsev, Electronic structure and vibrational properties of KRbAl₂B₂O₇, Mater. Res. Bull., 2013, 48, 929–934.
50. V. Atuchin, L. Zhu, S. H. Lee, D. H. Kim and C. S. Lim, Microwave-assisted solvothermal synthesis of Sr₃V₂O₈ nanoparticles and their spectroscopic properties, Asian J. Chem., 2014, 26(5), 1290–1292.
51. F. Auzel, G. Baldacchini, L. Laversenne and G. Boulon, Radiation trapping and self-quenching analysis in Yb³⁺, Er³⁺, and Ho³⁺ doped Y₂O₃, Opt. Mater., 2003, 24, 103–109.
52. M. Pollnau, D. R. Gamelin, S. R. Lüthi and H. U. Güdel, Power dependece of upconversion luminescence in lanthanide and transition-metal-ion systems, Phys. Rev. B: Condens. Matter, 2000, 61, 3337–3346.
53. H. Y. Du, Y. J. Lan, Z. G. Xia and J. Y. Sun, Synthesis and upconversion luminescence properties of Yb³⁺/Er³⁺ codoped BaGd₂(MoO₄)₄ powder, Mater. Res. Bull., 2009, 44, 1660–1662.
54. B. Li, B. Joshi, Y. K. Kshetri, R. Adhikari, R. Narro-Gracia and S. W. Lee, Upconversion luminescence properties of Er³⁺/Yb³⁺ in transparent a-Sialon ceramics, Opt. Mater., 2015, 39, 239–246.
55. J. H. Chung, J.-I. Lee, S.-L. Ryu and J. H. Ryu, Visible green upconversion luminescence of Er³⁺/Yb³⁺/Li⁺ co-doped CaWO₄ particles, Ceram. Int., 2013, 39, S369–S372.
56. J. Castañeda, M. A. Meneses-Nava, O. Barbosa-García, E. de la Rosa-Cruz and J. F. Mosiño, The red emission in two and three steps up-conversion process in a doped erbium SiO₂–TiO₂ sol–gel powder, J. Lumin., 2003, 102–103, 504–509.
57. E. Sarantopoulou, C. Raptis, S. Ves, D. Christofilos and G. A. Kourouklis, Temperature and pressure dependence of Raman-active phonons of CaMoO₄: an anharmonicity study, J. Phys.: Condens. Matter, 2002, 14, 8925–8938.
58. P. G. Zverev, Vibronic relaxation of Raman modes in CaMoO₄ and PbMoO₄ molecular ionic crystals, Phys. Status Solidi C, 2004, 1, 3101–3105.
59. M. B. Smirnov and V. Yu. Kazimirov, LADY: software for lattice dynamics simulations, JINR Communications, 2001, E 14-2001-159.
60. I. F. Chang and S. S. Mitra, Application of a modified random-element-isodisplacement model to long-wavelength optic phonons of mixed crystals, Phys. Rev., 1968, 172, 924–933.
61. A. N. Vtyurin, A. S. Krylov, S. N. Krylova, S. V. Goryainov, V. N. Voronov and A. S. Oreshonkov, Hydrostatic pressure-induced phase transitions in Rb₂KInF₆ and Rb₂KScF₆ crystals: Raman spectra and lattice dynamics simulations, Ferroelectrics, 2012, 440, 100–104.
62. Y. V. Gerasimova, A. S. Oreshonkov, A. N. Vtyurin, A. A. Ivanenko, L. I. Isaenko, A. A. Ershov and E. I. Pogoreltsev, Infrared absorption investigation of the role of octahedral groups upon the phase transition in the Rb₂KMoO₃F₃ crystal, Phys. Solid State, 2013, 55, 2331–2334.
63. A. S. Krylov, A. N. Vtyurin, A. S. Oreshonkov, V. N. Voronov and S. N. Krylova, Structural transformations in a single-crystal Rb₂NaYF₆: Raman scattering study, J. Raman Spectrosc., 2013, 44, 763–769.
64. Z. G. Xia, M. S. Molokeev, A. S. Oreshonkov, V. V. Atuchin, R.-S. Liu and C. Dong, Crystal and local structure refinement in Ca₂Al₃O₆F explored by X-ray diffraction and Raman spectroscopy, Phys. Chem. Chem. Phys., 2014, 16, 5952–5957.
65. A. A. Savina, V. V. Atuchin, S. F. Solodovnikov, Z. A. Solodovnikova, A. S. Krylov, E. A. Maximovskiy, M. S. Molokeev, A. S. Oreshonkov, A. M. Pugachev and E. G. Khaikina, Synthesis, structural and spectroscopic properties of acentric triple molybdate Cs₂NaBi(MoO₄)₃, J. Solid State Chem., 2015, 225, 53–58.
66. C. S. Lim, A. Aleksandrovsky, M. Molokeev, A. Oreshonkov and V. Atuchin, 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.
67. R. Fletcher, Practical Methods of Optimization, Wiley, Chichesteretc., 2nd edn, 1987.
68. A. Senyshyn, H. Kraus, V. B. Mikhailik and V. Yakovyna, Lattice dynamics and thermal properties of CaWO₄, Phys. Rev. B: Condens. Matter, 2004, 70, 214306.
69. A. Senyshyn, H. Kraus, V. B. Mikhailik, L. Vasylechko and M. Knapp, Thermal properties of CaMoO₄: Lattice dynamics and synchrotron powder diffraction studies, Phys. Rev. B: Condens. Matter, 2006, 73, 014104.
70. F. D. Hardcastle and I. E. Wachs, Determination of molybdenum-oxygen bond distances and bond orders by Raman spectroscopy, J. Raman Spectrosc., 1990, 21, 683–691.
71. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New Yorketc., 6th edn, 2009.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6dt02378a

---