Influence of Doping and Excitation Powers on Optical Thermometry in Yb³⁺-Er³⁺ doped CaWO₄

Abstract

Optical thermometry has been widely studied to achieve an inaccessible temperature measurement in submicron scale and it has been reported that the temperature sensitivity depends mainly on host types. In this work, we propose a new method to improve the optical temperature sensitivity of Yb³⁺-Er³⁺ co-doped CaWO₄ phosphors by doping with Li⁺, Sr²⁺, and Mg²⁺ ions and by controlling excitation powers of 980 nm laser. It is found that the thermometric parameters such as upconversion emission intensity, intensity ratio of green-to-red emission, fluorescence color, emission intensity ratios of thermally coupled levels (²H_11/2/⁴S_3/2), and relative and absolute temperature sensitivity can be effectively controlled by doping with Li⁺, Sr²⁺, and Mg²⁺ ions in the Yb³⁺-Er³⁺ co-doped CaWO₄ system. Moreover, the relative sensitivity S_R and the absolute sensitivity S_A are proved to be dependent on the pump power of 980 nm laser. The sensitivities of S_R and S_A in Yb³⁺-Er³⁺ co-doped CaWO₄ increase about 31.5% and 12%, respectively, by doping with 1 mol% Sr²⁺.

Introduction

In recent years, more and more research works have been focused on detecting temperature in submicron scale through optical thermometry, due to wide application in nano-devices^1,2,3. Optical thermometry is a method to calculate the temperature of phosphors by fitting the fluorescence intensity ratios (FIR) of thermally coupled levels at different temperate points⁴. Compared with conventional temperature measurement contact methods, the non-contact FIR technique overcomes some limitations of spatial resolution and accuracy of detection. The thermally coupled levels ²H_11/2 and ⁴S_3/2 of Er³⁺ ion were reported as one of preferable choice to achieve optical thermometry, due to their large energy gap and less overlap of two emission peaks between the ²H_11/2 and ⁴S_3/2 levels^1,5,6. Three comprehensive reviews summarized the optical thermometry based on upconversion emissions of Er³⁺ doped phosphors, such as oxide crystals, glasses, core-shell heterojunction nanoparticles, and transparent glass ceramics containing fluoride nanocrystals^7,8,9. It was reported that the optical temperature sensitivity of Er³⁺ doped phosphors depends mainly on host types irrespective of the other conditions, such as excitation powers, doping concentrations, and sizes and shapes of samples. The main weakness of the previous research works is the failure to address the question: If the power of excitation source changes from low to high, how the changes in power affect the optical temperature sensitivity? Rakov and Maciel observed the optical temperature sensitivity is inconsistent between continuous wave and pulsed (~5 ns) laser excitations in the Er³⁺-Yb³⁺ doped Y₂SiO₅ system¹⁰. Chen et al. reported that the intensity ratio of green and red emissions depends strongly on the excitation power density¹¹. Thus, it is necessary to study the influence of pump power on optical temperature sensitivity.

If the excitation power is fixed, how to enhance the optical temperature sensitivity without changing the category of hosts? According to the crystal-field theory, the emissions from the ²H_11/2 and ⁴S_3/2 thermally coupled levels are inevitably affected by the crystal field around Er³⁺ ions in hosts^12,13,14. In some works, the tuning of the upconversion emissions of Er³⁺ were realized by tailoring the crystal field around Er³⁺ ions. For examples, the enhancement of the upconversion luminescence was achieved in Yb³⁺/Er³⁺-codoped Y₂O₃ nano-crystals by tri-doping with Li⁺ ions¹⁵. Qin et al. reported and discussed a strategy to enhance upconversion emission in β-NaLuF₄:Yb³⁺, Er³⁺ nano-crystals by tri-doping with La³⁺ ions¹⁶. The lattice distortion of hosts is induced by dopant ions with small ionic radius and affects strongly the crystal-field environment around Er³⁺ ions leading to increase in radiative transition probability from the excited state to the ground state of ⁴I_15/2¹⁷. The doping of the ions with small ionic radius may be a preferable method to improve the optical temperature sensitivity.

Recently, the Yb³⁺-Er³⁺ co-doped CaWO₄ phosphors were reported as one of excellent materials to achieve the optical thermometry based on the green emissions from the thermally coupled ²H_11/2 and ⁴S_3/2 levels of Er³⁺ ions^18,19,20. The CaWO₄ crystal has tetragonal structure with distorted [WO6] octahedral clusters containing two quadrilateral biconical units²¹. This asymmetric crystal structure can be efficiently distorted by doping with small ions giving rise to change in fluorescence intensity ratio of the ²H_11/2 and ⁴S_3/2 levels. It means that the optical temperature sensitivity can be improved by changing crystal-field environment around Er³⁺ ions. In this work, we study the influences of the tri-doping with Li⁺, Sr²⁺, and Mg²⁺ ions and excitation powers on optical thermometry in Yb³⁺-Er³⁺ co-doped CaWO₄. The relative and absolute emission sensitivities can be controlled effectively by doping with Li⁺, Sr²⁺, and Mg²⁺ ions in Yb³⁺-Er³⁺ co-doped CaWO₄ and by changing the pump power of 980 nm laser.

Results

Figure 1 presents the XRD patterns of as-synthesized CaWO₄ doped with Yb³⁺-Er³⁺, Yb³⁺-Er³⁺-0.1%Li⁺, Yb³⁺-Er³⁺-1%Sr²⁺, and Yb³⁺-Er³⁺-0.5%Mg²⁺. The position and relative intensity of all the diffraction peaks can be readily indexed to pure Scheelite-type tetragonal CaWO₄ according to the PDF card (No. 77-2236). The sharp and well defined diffraction peaks in the patterns suggest that the synthesized phosphors are well crystallized. The raw materials could not be detected in the as-synthesized samples confirming that Yb³⁺, Er³⁺, Li⁺, Sr²⁺ and Mg²⁺ are doped into the CaWO₄ lattices. Figure 1(b) shows the crystal structure and atomic arrangement along the a-axis direction in CaWO₄ which has tetragonal symmetry (ICSD 60550) with octahedral quadrilateral biconical crystals²¹. The tetragonal CaWO₄ belongs the I41/aZ space group, and contains 4 formula units in each unit cell. In the unit cell of CaWO₄, there are 4b Ca and 4a W cation sites. The Sr and Mg ions occupy the same 4b Ca cation sites, and the Li ion is located at an interstitial site in the CaWO₄ lattice. In order to study the influence of the Li⁺, Sr²⁺ and Mg²⁺ doping on the morphology change, the scanning electron microscope (SEM) images are given in Figs S1, S2 and S3. One can find that the average size of Yb³⁺-Er³⁺ doped CaWO₄ increases with the increase of Li⁺, Sr²⁺ and Mg²⁺ concentration, and its morphology becomes more irregular and agglomerated. The shape and size modifications are attributed to the surface charge redistribution of the crystal nucleus induced by inner electron charge transfer between the dopant ions and lattice cations²².

Figure 1

(a) XRD patterns of CaWO₄ doped with Yb³⁺-Er³⁺, Yb³⁺-Er³⁺-0.1 mol%Li⁺, Yb³⁺-Er³⁺-1 mol%Sr²⁺, and Yb³⁺-Er³⁺-0.5 mol%Mg²⁺ and (b) the schematic views of unit cell of CaWO₄ structure along a-direction.

The upconversion emission spectra of CaWO₄:Yb³⁺, Er³⁺, Li⁺ excited by 980 nm infrared radiation are shown in Fig. 2(a). Three emission bands of Er³⁺ are observed at 528, 550, and 660 nm due to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 transitions, respectively. No spectral shifts of the emission bands occur for various Li⁺ concentrations in CaWO₄:Yb³⁺, Er³⁺, Li⁺. Compared with CaWO₄:Yb³⁺, Er³⁺, the red emission intensity of CaWO₄:Yb³⁺, Er³⁺, Li⁺ increases sharply with increasing Li⁺ concentration, while the green emission intensity changes irregularly with increasing Li⁺ concentration, as shown in Fig. 2(b). The total emission intensity increases sharply with increasing Li⁺ concentration from 0 to 0.5 mol%, as shown in Fig. 2(c). Figure 2(d) shows the intensity ratio of the red/green emissions (I_R/I_G) which is changed by changing Li⁺ concentration and the ratio reaches a maximum at 0.1 mol%. The CIE chromaticity in the inset of Fig. 2(d) shows that the emission color is tunable from greenish-yellow to green with increasing Li⁺ concentration. The spectrum modulation of CaWO₄:Yb³⁺, Er³⁺ can also be achieved by doping with Sr²⁺ and Mg²⁺ ions, as shown in Figs S4 and S5. Similar to the Li⁺ ions doped in CaWO₄:Yb³⁺, Er³⁺, the codoping with Sr²⁺ ions increases the total emission intensity of Er³⁺ ions (Fig. S4(c)). But for the Mg²⁺ ions, the total emission intensity of Er³⁺ ions rather decreases at higher concentration of 1.5 mol% (Fig. S5(c)). Consequently, the changes in total intensity and the intensity ratio of red to green emissions are not consistent in three kind of dopant ions (Li⁺, Sr²⁺ and Mg²⁺). Nevertheless, the results show that the spectrum modulation can be induced by doping of Li⁺, Sr²⁺ and Mg²⁺ as previously reported in Li⁺-doped Y₂O₃:Er^3+ 15.

Figure 2

Li⁺ concentration dependent (a) upconversion spectra, (b) intensities of green and red emissions, (c) total emission intensity, and (d) the intensity ratios of the red to green emissions of CaWO₄:Yb³⁺, Er³⁺, Li⁺.

The enhancement of visible emissions induced by doping can be explained as follows: CaWO₄ has octahedral quadrilateral biconical structure for which the Ca ion is coordinated with eight oxygen ions in the CaWO₄ lattice, as shown in Fig. 1(b). Er³⁺ (eight coordination r = 1.004 Å) as an optical active center has a local structure with the ErO₈ unit when the Er³⁺ is doped in CaWO₄ lattice (eight coordination Ca²⁺, r = 1.12 Å). When the Li⁺ ions (eight coordination r = 0.92 Å) are doped into CaWO₄:Er³⁺, they compensate the excess charge of 1⁺ between Er³⁺ and Ca²⁺. The Li⁺ ions may occupy the sites in or near the ErO₈ units or diffuse to various interstitial sites in the CaWO₄ lattice. The Sr²⁺ and Mg²⁺ ions (eight coordination r_Sr = 1.26 Å, r_Mg = 0.89 Å) substitute for the Ca²⁺ sites in CaWO₄:Er³⁺ leading to the distortion of the Er-O bonds. As a result, the local symmetry of the crystal field around Er³⁺ lowers with the change of Coulomb interaction¹⁷. The forbidden intra-4f electronic transitions are partially allowed with increase of the intra-4f electronic transitions probability of the Er³⁺ ions^12,17. Thus, the visible emissions can be enhanced by tailoring the local environments of the Er³⁺ ions by doping with Li⁺, Sr²⁺ and Mg²⁺ ions.

Based on upconversion emissions centered at 528 nm (²H_11/2 → ⁴I_15/2) and 550 nm (⁴S_3/2 → ⁴I_15/2) of Er³⁺ ions, the optical temperature sensing of Yb³⁺-Er³⁺ co-doped CaWO₄ was studied by Li et al. by analyzing the temperature dependent fluoresce intensity ratio (FIR)¹⁹. Figure 3 shows the influence of doping with Li⁺, Sr²⁺, and Mg²⁺ on optical temperature sensing of Yb³⁺-Er³⁺ doped CaWO₄. According to previous works^4,19, the FIR of two thermally coupled levels of ²H_11/2 and ⁴S_3/2 can be fitted by the Boltzmann distributing law

Figure 3

Temperature dependent FIR of 528 nm and 550 nm emissions of (a) Yb³⁺-Er³⁺co-doped CaWO₄, (b) Yb³⁺-Er³⁺-Li⁺ tri-doped CaWO₄, (c) Yb³⁺-Er³⁺-Sr²⁺ tri-doped CaWO₄, and (d) Yb³⁺-Er³⁺-Mg²⁺ tri-doped CaWO₄.

where A is a constant, ΔE is the energy difference between thermally coupled levels, k is the Boltzmann constant, and T is the absolute temperature. The relative sensitivity S_R and the absolute sensitivity S_A are defined as

Figure 3 shows the FIR as a function of temperature for various doping systems. The solid lines in Fig. 3 are best fit results obtained using Eq. (1). All the data are well fitted to Eq. (1). It is obvious that the fit results depend strongly on the types of dopant ions. Compared with the fit result for Yb³⁺-Er³⁺ co-doped CaWO₄, the fit results vary much more for Yb³⁺-Er³⁺-Mⁿ⁺ (Mⁿ⁺ = Li⁺, Sr²⁺, and Mg²⁺) tri-doped CaWO₄. The values of ΔE can be calculated from the data points of luminescence intensity ratios at different temperatures using Eq. (1). This indicates that the different types of dopant ions cause different values of ΔE. The peaks of green emissions at 528 and 550 nm do not shift with temperature for all the samples. However, the experimental value of ΔE should be unique for thermometry. As observed in Fig. 4 the changes in error also occur between the best-fit and experimental values of ΔE depending on the types of dopants and their concentrations. The error δ between ΔE_f and ΔE_m is expressed as:

Figure 4

Doping dependent δ values in (a) Yb³⁺-Er³⁺ doped CaWO₄, (b, c, d, e) Yb³⁺-Er³⁺-xLi⁺ (x = 0.1, 0.2, 0.3, and 0.5 mol%) doped CaWO₄, (f,g,h,i) Yb³⁺-Er³⁺-ySr²⁺ (y = 0.5, 1.0, 1.5 and 2.0 mol%) doped CaWO₄, and (j,k,l,m) Yb³⁺-Er³⁺-zMg²⁺ (z = 0.25, 0.5, 1.0, and 1.5 mol%) doped CaWO₄.

where ΔE_f is the fit value of ΔE, and ΔE_m is experimental value of ΔE from the spectrum⁹. The experimental value of ΔE is 757.6 cm⁻¹ from the spectra in Fig. 2. The errors δ for Yb³⁺-Er³⁺ co-doped and Yb³⁺-Er³⁺-Mⁿ⁺ (Mⁿ⁺ = Li⁺, Sr²⁺, and Mg²⁺) tri-doped CaWO₄ are calculated using Eq. (4) and the results are shown in Fig. 4. One can find that the values of error δ depend strongly on the types of dopant ions. All the values of error δ are more than 20%, and are too large to be ignored. Moreover, the value of error δ for Yb³⁺-Er³⁺ co-doped CaWO₄ exceeds 30%. The large values of δ are attributed to the inconsistency between the restriction of Eq. (1) and luminescence mechanism at high temperature. As a theoretical model, Eq. (1) was proposed only by considering the Boltzmann distribution and the radiative transitions in the thermally coupled levels²³. The temperature dependent nonradiative relaxation and energy transfer between the host and the rare-earth ions were not be considered at all. In fact, the nonradiative relaxation and energy transfer become active at high temperature²⁴ giving rise to the large errors of ΔE in the optical temperature sensing process. The large errors of ΔE were also observed in more than fifty kinds of materials⁹ as well as Yb³⁺-Er³⁺ co-doped CaWO. This means that the ΔE is an unreasonable parameter to evaluate the optical temperature sensitivity S_R and S_A. Thus, Eqs (2) and (3) should be modified to be reasonable for optical thermometry.

Considering the nonradiative relaxation and energy transfer at high temperature, Eq. (1) is modified as

where a is constant dependent on the materials¹⁰. The b is a correction term for the comprehensive population of thermally coupled energy levels induced by not only the thermal population but also the nonradiative relaxation and energy transfer. The relative sensitivity S_R and the absolute sensitivity S_A are defined as

where a and b are from Eq. (5). Figure 5 shows the temperature dependent FIR of 528 and 550 nm green emissions in CaWO₄ doped with different types of dopant ions. One can find that the experimental points can be fitted well with a linear mode. The slopes of the fit results depend on the types of dopant ions and concentrations. It means that the FIR of thermally coupled levels (²H_11/2/⁴S_3/2) are susceptible to the ion doping. Notably, all the a values in Fig. 5 are not consistent with those values of ΔE/k from Eq. (1) as shown in Fig. 3. In general, no difference in the a value is found between in Eqs (1) and (5), if considering only the thermal population. This deviation proves that it is unreasonable to use the ΔE to evaluate the optical temperature sensitivities S_R and S_A. Figure 6 shows temperature dependent sensitivities S_R and S_A of CaWO₄ doped with different types of dopant ions. All the sensitivity curves exhibit maxima with increasing temperature. The maximum S_R value of all the tri-doped CaWO₄ systems changes non-monotonically with increasing dopant concentration, but the maximum S_R value of the Yb³⁺-Er³⁺ co-doped CaWO4 system are enhanced obviously by doping with Li⁺, Sr²⁺ and Mg²⁺ ions in optimum dopant concentrations. The maximum S_R value of Yb³⁺-Er³⁺ co-doped CaWO₄ is estimated to be 0.0073 K⁻¹ at 380 K, while 0.0076 K⁻¹ at 380 K for Yb³⁺-Er³⁺-Li⁺ (0.1 mol%), 0.0096 K⁻¹ at 429 K for Yb³⁺-Er³⁺-Sr²⁺ (1.0 mol%), and 0.0093 K⁻¹ at 386 K for Yb³⁺-Er³⁺-Mg²⁺ (0.5 mol%) systems. The corresponding S_A values decrease with increasing temperature as shown in Fig. 6(d). The S_A value of Yb³⁺-Er³⁺ co-doped CaWO₄ is enhanced by doping with Sr²⁺ ions. The values of S_R and S_A indicate that the resulting Yb³⁺-Er³⁺-Sr²⁺ tri-doped CaWO₄ might be a promising candidate for optical temperature sensors.

Figure 5

Temperature dependent relationship between the LnFIR and 1/T of (a) Yb³⁺-Er³⁺ doped CaWO₄, (b) Yb³⁺-Er³⁺-Li⁺ doped CaWO₄, (c) Yb³⁺-Er³⁺-Sr²⁺ doped CaWO₄, and (d) Yb³⁺-Er³⁺-Mg²⁺ doped CaWO₄.

Figure 6: Optical temperature sensitivities S_R and S_A as a function of temperature for various dopant ions.

(a) Yb³⁺-Er³⁺-Li⁺, (b) Yb³⁺-Er³⁺-Sr²⁺, and (c) Yb³⁺-Er³⁺-Mg²⁺ doped CaWO₄. (d) Temperature dependent S_A.

The thermal stability of emission bands plays a key role in the optical temperature sensors. In order to explore the thermal stability, the temperature dependent log–log plots of emission intensity and pump power need to be investigated. The upconversion emission intensity I and excitation power P is expressed as follows

where n is the number of photons absorbed to pump the population in particular level in upconversion process¹³. Theoretically, the n values of green and red emissions of Er³⁺ ions excited by 980 nm are close to 2²⁴. The temperature dependent log–log plots of emission intensity and pump power for green and red emissions are shown in Fig. S6. The slopes of the fit results for green and red emissions of Yb³⁺-Er³⁺ co-doped CaWO₄ depend on temperature, and show that the upconversion process involves two photons to contribute green and red emissions. The slopes of the red emissions are much smaller than 2, which means that the red emission is not stable at high temperature²⁴. Compared with the slopes of Yb³⁺-Er³⁺ co-doped CaWO₄, those of the best-fit results for green and red emissions of Yb³⁺-Er³⁺-1.0%Sr²⁺ tri-doped CaWO₄ are closer to 2. It means that Yb³⁺-Er³⁺-1.0%Sr²⁺ tri-doped CaWO₄ has high thermal stability in emission bands than Yb³⁺-Er³⁺ co-doped CaWO₄.

Together with the thermal stability of emission bands, the excitation power dependence of the luminescence spectra was investigated in the temperature range from 298 to 573 K by using a 980 nm laser with pump powers of 85.2 and 322.4 mW/mm², as shown in Fig. S7. The luminescence spectra contain green and red emission bands, which are assigned to the ⁴I_15/2 → ²H_11/2 (528 nm), ⁴I_15/2 → ⁴S_3/2 (550 nm), and ⁴I_15/2 → ⁴F_9/2 (660 nm) transitions of Er³⁺ ion. One can find that the intensities of 550 and 660 nm emission bands greatly decrease as temperature increases, while the intensity of 528 nm emission bands greatly increase as temperature increases. No peak shift of the emission bands induced by the doping and excitation powers is observed. The LnFIR as a function of 1/T for various excitation powers are illustrated in Fig. 7. The slopes of the fit results at low excitation powers are different from those at high excitation powers. Moreover, the slopes of Yb³⁺-Er³⁺ co-doped CaWO₄ are different from those of Yb³⁺-Er³⁺-1%Sr²⁺ tri-doped CaWO₄ at the same excitation power. It means that the FIR of thermally coupled levels (²H_11/2/⁴S_3/2) are also susceptible to excitation powers like the dopant ions described above. The excitation power dependences of S_A and S_R are illustrated in Fig. 8. One can find that the values of S_A and S_R change non-monotonically with increasing excitation powers. The large value of S_A and S_R appear at the excitation power of 322.4 mW/mm². The values of S_A and S_R of Yb³⁺-Er³⁺ co-doped CaWO₄ are more susceptible to excitation powers than those of Yb³⁺-Er³⁺-1%Sr²⁺ tri-doped CaWO₄. Thus, the Yb³⁺-Er³⁺-1%Sr²⁺ tri-doped CaWO₄ is a better candidate for optical temperature sensors than Yb³⁺-Er³⁺ co-doped CaWO₄ by considering the stabilities induced by temperature and excitation powers.

Figure 7

The LnFIR as a function of 1/T for various excitation powers for (a) Yb³⁺-Er³⁺ doped CaWO₄ and (b) Yb³⁺-Er³⁺-1.0%Sr²⁺ doped CaWO₄.

Figure 8

Excitation powers dependent S_A and S_R of Yb³⁺-Er³⁺ doped CaWO₄ and Yb³⁺-Er³⁺-1.0%Sr²⁺ doped CaWO₄.

Conclusions

In this work, a series of Yb³⁺-Er³⁺ co-doped and Yb³⁺-Er³⁺-Mⁿ⁺ (Mⁿ⁺ = Li⁺, Sr²⁺, and Mg²⁺) tri-doped CaWO₄ phosphors were synthesized by using the high-temperature solid-state reaction method. The structural property of resulting powder is investigated by the X-ray diffraction. The upconversion emission intensity, green-to-red emission intensity ratio, fluorescence color of the Yb³⁺/Er³⁺ co-doped sample are controlled by doping Li⁺, Sr²⁺ and Mg²⁺ ions. The fluorescence intensity ratios of thermally coupled levels (²H_11/2/⁴S_3/2) and optical temperature sensitivity are observed to be dependent on dopant ions and excitation powers. It is found that the temperature sensitivity of CaWO₄:5%Yb³⁺, 0.3%Er³⁺, 1.0%Sr²⁺ can reach a maximum relative sensitivity of 0.0111 K⁻¹ at 456 K and absolute sensitivity of 949.8/T², which is much higher than the reported temperature sensors based on Er³⁺ green luminescence. This work opens a new method to increase the optical temperature sensitivity of rare earth ions doped materials.

Methods

All starting materials are CaCO₃ (AR), WO₃ (99.99%), Er₂O₃ (99.99%), Yb₂O₃ (99.99%), Li₂CO₃ (AR), SrO (AR) and MgO (AR). All the chemicals were used as received without any further purification.

The solid-state reaction method is used to synthesize CaWO₄:Er³⁺/Yb³⁺ and CaWO₄:Er³⁺/Yb³⁺/Mⁿ⁺ (Mⁿ⁺ = Li⁺, Sr²⁺ and Mg²⁺) phosphors. The molar ratio of CaWO₄:Er³⁺/Yb³⁺ sample is as follows: 48.7CaCO₃-48.7WO₃-0.15Er₂O₃-2.45Yb₂O₃. The molar ratios of CaWO₄:Er³⁺/Yb³⁺/Li⁺ samples are as follows: 48.7CaCO₃-48.7WO₃-0.15Er₂O₃-2.45Yb₂O₃-xLi₂CO₃, x = 0.05, 0.10, 0.15, and 0.25; The molar ratios of CaWO₄:Er³⁺/Yb³⁺/Sr²⁺ samples are as follows: (48.7-y)CaCO₃-48.7WO₃-0.15Er₂O₃-2.45Yb₂O₃-ySrO, y = 0.50, 1.00, 1.50 and 2.00. The molar ratios of CaWO₄: Er³⁺/Yb³⁺/Ca²⁺ samples are as follows: (48.7-z)CaCO₃-48.7WO₃-0.15Er₂O₃-2.45Yb₂O₃-zMgO, z = 0.25, 0.50, 1.00, and 1.50. The starting materials are entirely mixed and ground with alcohol for 1 hour. The powder is subsequently sintered in a furnace for 10 h at 1350 °C. Finally, a series of the required samples in the form of white powder are obtained after cooling down naturally.

Structures of the samples were investigated by X-ray diffraction (XRD) using a X’TRA (Switzerland ARL) equipment provided with Cu tube with K_α radiation at 1.54056 Å. The size and shape of the samples were observed using a scanning electron microscope (LEO-1550). Luminescence spectra were obtained by the Acton SpectraPro Sp-2300 Spectrophotometer with a photomultiplier tube equipped with 980 nm laser as the excitation sources. Different temperature spectra were obtained by using an INSTEC HCS302 Hot and Cold System.