Synthesis and characterization of SrSnO₃ doped with Er³⁺ for up-conversion luminescence temperature sensors

Abstract

The present work shows the results of the synthesis of SrSnO₃:Er³⁺ (doped from 0% to 7% with Er³⁺) prepared using a sol–gel method. Structural studies based on powder X-ray diffraction (XRD) showed that the compounds crystallize in an orthorhombic perovskite type structure (space group: Pbnm). The infrared spectra (IR) of Er₂O₃, SrSnO₃ and SrSnO₃:Er (1%) were measured in the range of 400–1000 cm⁻¹, which confirmed the octahedral tilting of SnO₆. The dependence of the green and red up-conversion (UC) emission bands on the pump power have been investigated using NIR (975 nm) laser irradiation. The decay curve was collected to determine the plausible mechanisms of the UC green emission, which showed that a Ground-State Absorption/Excited-State Absorption (GSA/ESA) process involving two photons was the indicated mechanism for this emission. The optical temperature sensing properties were investigated using the fluorescence intensity ratio technique (FIR) for the green UC emission. The temperature dependence of the green emissions at 528 nm (²H_11/2 → ⁴I_15/2) and 549 nm (⁴S_3/2 → ⁴I_15/2) were investigated. The maximum temperature sensitivity over the range of 294–372 K was 7.91 × 10⁻³ K⁻¹ at 368 K, and the sensor sensitivity was 9.97 × 10⁻³ K⁻¹ at 294 K. The results suggest that Er³⁺ doped SrSnO₃ is a promising material for optical temperature sensors for biological processes.

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1. Introduction

Recently, the temperature-dependent up-conversion luminescence of materials doped with lanthanide ions has drawn considerable attention because of the potential application in thermal imaging, cancer treatment, temperature sensors and optical heaters.^1,2 Non-invasive optical temperature sensors based on up-conversion emission are one of the most important applications. In these sensors, the temperature is measured using the fluorescence intensity ratio (FIR) technique. This technique involves a comparison of the fluorescence intensities from two thermally coupled energy levels (TCEL).³ The FIR technique plays a key role because of its high resolution and the independence of the measurement compared to other optical-based techniques, such as fluorescence lifetime and amplified spontaneous emission.^1,4 Many materials doped/co-doped with rare earth ions, such as Pr³⁺, Dy³⁺, Nd³⁺, Sm³⁺, Ho³⁺, Tm³⁺, Yb³⁺ and Eu³⁺, have been studied for temperature sensing based on the change in the FIR of two thermally coupled levels as a function of temperature.^5,6 TCELs with large energy gaps are required for high temperature sensitivity. However, when the gap between these two close levels increases to a certain point, the population of ions between these two levels, by inter-relaxation, tends to slow down exponentially with the gap, in consequence the thermal-coupling begins to fade when the rates are no longer much faster than the relaxation rates to outside levels.⁷ Among the trivalent rare earth ions, temperature sensors based on the green up-converted luminescence of Er³⁺ are the most popular because they exhibit up-conversion fluorescence emissions upon excitation using infrared (IR) radiation and the erbium ions have a thermally coupled pair of energy levels, ²H_11/2 and ⁴S_3/2,⁸ whose populations follow the Boltzmann distribution law. The temperature can be obtained using the fluorescence intensity ratio (FIR) technique. The fluorescence intensity ratio of luminescence bands corresponding to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ fulfill a temperature-dependent function, because the energy gap between the two thermally coupled excited states, ²H_11/2 and ⁴S_3/2, is quite small (approximately 800 cm⁻¹). The relative fluorescence intensity of the two green bands of the Er³⁺ ion varies substantially with temperature, whereas their spectral positions remain the same. This feature is often used for temperature measurements.⁹

To the best of our knowledge, SnSrO₃ has not been used as a host structure for the Er³⁺ cation for use as temperature sensor. In this paper, we report on the synthesis of Sr_1−xEr_xSnO_3−δ via a modified sol–gel process and its application as a temperature sensor. The up-conversion mechanism is also investigated.

2. Experimental

2.1. Synthesis

The starting materials SrCO₃ (99.99%, Aldrich), SnCl₂·2H₂O (98%, Aldrich) and Er₂O₃ (99.99%, Aldrich) were purchased from Aldrich Chemical Co. and used as received.

Solid solutions of the Sr_1−xEr_xSnO_3−δ (x = 0.01, 0.03, 0.04, 0.05 and 0.07) system were prepared using the method described by Ouni et al.¹⁰ Stoichiometric quantities of Er₂O₃, SrCO₃, and SnCl₂·2H₂O were dissolved in 150 ml of a 1 : 2 solution of hydrochloric acid and distilled water. Then, 5 ml of ethylene glycol (99.99%, Aldrich) and one equivalent of citric acid per mole of M³⁺ cation (99.99%, Aldrich) were added and the solution was heated on a hot plate at 353 K with constant stirring for approximately 6 h.

The obtained transparent gel was decomposed by heating at 523 K for 3 h. The resulting fine dark powder was ground up and then placed in a muffle furnace in air at 1173 K for 6 h resulting in the formation of the final white powder.

2.2. Characterization

Powder X-ray diffraction (XRD) was performed using the Bruker AXS D8 Advance diffractometer in the range from 10° to 60°, with a graphite monochromator using CuKα radiation (λ = 1.54057 Å) operating at 40 kV and 30 mA. The sample images were obtained using a TESCAN VEGA3 SB model scanning electron microscope. Infrared spectroscopy was performed using a FRONTIER IR spectrophotometer from Perkin Elmer. The up-conversion spectra were obtained by exciting the samples using a titanium-sapphire laser (λ = 975 nm) pumped by a Millennia laser (λ = 532 nm) with a maximum power of 15 W. The laser was focused on the sample using a lens with a focal length of 30 mm. The emitted light was focused on the slit door of the CCD spectrograph (ANDOR SHAMROCK 303i) using a lens with a focal length of 50 mm. The luminescence decay curves were obtained by exciting the samples using an OPO pulsed laser (EKSPLA/NT342/3/UVE). The emission was focused on the input of a spectrograph coupled to a photomultiplier (R928 HAMAMATSU in the visible range) connected to a digital oscilloscope (TEKTRONIX 2430). The temperature-dependent luminescence measurements were performed in an electric tube furnace (GERO RES-E 230/3) from 294 K to 374 K. The sample was placed in the center of the furnace and heated at a rate of 1 K min⁻¹. The sample was excited with laser radiation at 488 nm from argon laser.

3. Results and discussion

3.1. Powder X-ray diffraction

The X-ray diffraction patterns for the series of SrSnO₃ powders doped with Er³⁺ (1%, 3%, 5% and 7%) were analyzed and are shown in Fig. 1. All samples were indexed to the distorted orthorhombic perovskite with the space group Pbnm. Inspection of the X-ray diffraction patterns at intermediate phases of the synthesis appeared to indicate the formation of not only the SrSnO₃ perovskite type structure but also minor impurity phases of SnO₂ and Er₂O₃, as shown in Fig. 1. It is important to note that according to the literature, the synthesis of SrSnO₃ is never pure and always contains some impurities, as shown in other studies.^10,11 In the structure of SrSnO₃, the Sr²⁺ and Sn⁴⁺ occupy only one crystallographic site, respectively. The Sn⁴⁺ is six-fold coordinated by oxygen atoms, whereas the Sr²⁺ is twelve-fold coordinated by oxygen atoms. Upon doping, the trivalent erbium ions are expected to occupy the Sn⁴⁺ sites, since the ionic radius of Er³⁺ r = 0.89 Å (CN = 6) is slightly larger than the ionic radius of Sn⁴⁺ r = 0.69 Å (CN = 6). Due to the ionic radius of Sr²⁺ r = 1.44 Å (CN = 12) is larger than that of Er³⁺, the replacement of the Sr²⁺ ions by Er³⁺ can results in a strong contraction of the unit cell.¹² Our results show that the unit cell parameters of SrSnO₃:Er³⁺ increase with the increasing of Er³⁺ concentration, this fact should be explained by the replacement of the Sn⁴⁺ cations by the rare-earth ions. Although the Goldschmidt's tolerance factor is compatible with the replacement of Sr²⁺ or Sn⁴⁺ in order to retain the orthorhombic structure of the host matrix.¹³ The value of the cell parameters, confirmed by Le Bail refinement^14,15 for all compounds, using the program JANA2006,¹⁶ are given in Table 1. When increasing the Er concentration to 5%, the previously named impurities began to appear as shown in Fig. 1, this implies that the maximum solubility of the Er ion in SrSnO₃ is approximately 3%.¹⁰ We can conclude then, that due to the impurities formation, Er ions does not occupied a place in the structure but rather begins to form these new compounds.

Fig. 1 XRD patterns of the SrSnO₃ doped with different content of Er³⁺.

Table 1 Unit cell volume and lattice parameters for SrSnO₃ doped with Er³ ions up to 7%

Samples	a (Å)	b (Å)	c (Å)	Vol. (Å^3)
Er1%	5.7212(5)	5.7056(1)	8.0681(2)	263.37(0)
Er3%	5.7228(5)	5.7069(1)	8.0721(2)	263.63(0)
Er5%	5.7285(7)	5.7042(1)	8.0680(2)	263.63(0)
Er7%	5.7412(7)	5.7061(2)	8.0699(3)	264.37(0)

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3.2. Infrared spectra

The IR spectra of the doped and undoped samples of SrSnO₃ are shown in Fig. 2a and b, respectively, along with the spectra of the Er₂O₃. The obtained powders were pulverized, mixed with KBr and then pressed into fine pellets.

Fig. 2 IR spectrum of (a) SrSnO₃ un-doped and doped with 1% and 7% of Er³⁺ and (b) Er₂O₃.

The IR bands below 1000 cm⁻¹ are generally attributed to the deformation modes of Sn–O bonds in SnO₆ octahedra or the deformation of Sn–O–Sn bridges.¹⁷ The vibration of the stannate (SnO₃²⁻) group produced high intensity bands in the ranges of 300–400 cm⁻¹ and 600–700 cm⁻¹.¹⁸ The IR spectrum of Er₂O₃ shows a band at 563 cm⁻¹, which corresponds to Er–O vibrations originating from the octahedral coordination ErO₆. By comparing this spectrum with the spectra of the different synthesized phases, it is obvious that this band is absent. Due to the concentrations of Er³⁺ ions are so low we are unable to detect the band corresponding to the Er–O vibrations, although the Er³⁺ ions replace the Sn⁴⁺ ions in the host structure.

3.3. Scanning electron microscopy

The SEM image of the SrSnO₃:Er³⁺ 1% doping, is shown in Fig. 3. The image was generated using backscattered electrons (BSE). The main observation from these images is the formation of micron-sized agglomerates which lack a defined morphology.

Fig. 3 SEM micrography of SrSnO₃ doped up to 1% of Er³⁺.

3.4. Up-conversion emission

The room temperature UC spectra of the doped samples with different concentrations of the Er³⁺ ions are shown in Fig. 4. All spectra were obtained using a 975 nm excitation. The emission spectra of the SrSnO₃:Er³⁺ phosphors exhibit strong emission bands at 528 and 549 nm and a weak emission at 665 nm, which correspond to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 electronic transitions of the Er³⁺ ion, respectively. The UC emission intensity decreases as the concentration of the Er³⁺ ion increases. The intensity of the emission band at 528 nm is higher than the emission band at 549 nm at high concentration of Er³⁺ (7%) in the SrSnO₃ host structure. This could be explained because of, at high concentration of Er³⁺ ions (7%), Er₂O₃ impurities became more important, resulting in a higher emission band at 528 nm. This could be explained because the ²H_11/2 → ⁴I_15/2 transition of the Er₂O₃ begins to interfere with the emission spectra of the SrSnO₃:Er³⁺ doped structure. The dominance of the ²H_11/2 → ⁴I_15/2 transition in the Er₂O₃ structure was reported recently by Tabanli et al.¹⁹

Fig. 4 Up-conversion spectra emission of the SrSnO₃ doped with Er³⁺ from 1% to 7%, exciting at 975 nm.

To determine the mechanism behind photon UC in SrSnO₃:Er³⁺, we have also studied the intensity of the emissions at 549 nm and 665 nm as a function of the pump power of the incident laser. It is well-known that the up-conversion intensity is directly related to the intensity of the infrared excitation by the following expression:

I_UC ∝ I_IRⁿ (1)

where n is the number of photons involved in the up-conversion process.^20,21 In Fig. 5, the logarithm UC emission intensities is plotted as a function of the logarithm of the laser pumping power for the 549 nm and 665 nm. By fitting the logarithmic curves according to eqn (1), the slope, n, obtained for the SrSnO₃:Er³⁺ 1% sample was 1.4 for both emissions, indicating that two photons are required in each case.

Fig. 5 Logarithm plot for the dependence of up-conversion emission intensities of the 549 and 665 nm bands on pump power at 975 nm.

The discrepancy between the actual values of the slope (1.4) and the number of photons involved in the process (2) is due to a phenomenon described by Pollnau et al. as a saturation in the up-conversion process at high power, which causes the slope of the curve to gradually decrease as the laser power increases.²² According to this interpretation, a high pump power would increase the competition between linear decay and up-conversion for the depletion of the intermediate excited states, resulting in a substantially reduced slope.

Finally, the dynamics of the 549 nm emission was studied by measuring the corresponding temporal evolution under excitation at 975 nm (see Fig. 6). In this case, a rapid rise time is observed after excitation with the laser, followed by a rapid decline. This behavior is indicative of the GSA/ESA mechanism for the UC process in which, after excitation by a pulsed laser at 975 nm, the Er³⁺ ion is immediately excited to the ⁴I_11/2 level by absorbing a photon and then to the ⁴F_7/2 level by absorbing a second 975 nm photon. In this process, the population of the ⁴F_7/2 level increases rapidly as a result of successive two-photon absorption, similar to the population that could be observed, if this level was directly excited. Subsequently, a non-radiative relaxation occurs from the ⁴F_7/2 level to the ²H_11/2 and ⁴S_3/2 levels, which generates the emissions at 528 nm and 549 nm. Therefore, as shown in Fig. 6, there is an immediate rise time after the pulse excitation at 975 nm, confirming the proposed UC mechanism. On the other hand, due to their small energy gap, Er³⁺ ions in the ⁴S_3/2 state rapidly relax to the ⁴F_9/2 levels through a non-radiative process, from which they radiatively relax to the ⁴I_15/2 ground state emitting a red photon (665 nm). The mechanism is shown in Fig. 7.

Fig. 6 Temporal evolution of the up-conversion emission at 549 nm of the SrSnO₃ doped with 1% of Er³⁺ for excitation at 975 nm.

Fig. 7 Energy level diagram of Er³⁺ ion, and the possible up-conversion process.

3.5. Temperature sensing study

To investigate the optical temperature sensing behavior of the SrSnO₃ phosphor doped with Er³⁺, the UC spectra, as shown in Fig. 8, were recorded using a 975 nm excitation over the range of 510–580 nm while increasing the temperature from 294 to 372 K. This temperature range is interesting for biological processes. The spectra show that the integrated intensity of both green emission bands, assigned to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of the Er³⁺ ion at 528 nm and 549 nm, respectively, do not change position as a function of temperature. Furthermore, the emission intensity of both bands shows an obvious dependence on the temperature. The relative intensity of the ²H_11/2 → ⁴I_15/2 transition increases with an increase in temperature, respect to the ⁴S_3/2 → ⁴I_15/2 transition. It is well-known that the energy gap between the ²H_11/2 and ⁴S_3/2 levels is quite small (200–2000 cm⁻¹).²³ Because of this small separation, the ²H_11/2 level can be easily populated from the ⁴S_3/2 level by thermal agitation, resulting in the variation of the emission intensity of the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions as the temperature increases.⁴

Fig. 8 Green up-conversion spectrum of SrSnO₃:Er³⁺ phosphor at different temperatures.

3.5.1. Fluorescence intensity ratio (FIR) and sensor sensitivity (S). As stated previously, the ²H_11/2 and ⁴S_3/2 levels are thermally coupled and the transition from these two levels to the ground state can be used for optical thermometry using the FIR or R technique⁸ and, according to the Boltzmann distribution theory, the FIR can be expressed by the following equation:where I₅₂₈ and I₅₄₉ are the integrated intensities of the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, respectively, ΔE is the energy gap between the ²H_11/2 and ⁴S_3/2 levels, A is a constant, k_B is the Boltzmann constant and T is the absolute temperature.³

Following eqn (2), the ln(I₅₂₈/I₅₄₉) was plotted against the inverse absolute temperature (1/T), as shown in Fig. 9. All the experimental data points were fit to a line, which had a slope of −836.07 ± 4.49 and an intercept of 2.52 ± 0.01, to determine the energy gap, ΔE, of the two thermally coupled levels and the coefficient, A, which were 581.09 cm⁻¹ and 12.4, respectively. For a better understanding of the performance and the applicability of this material, it is also important to consider the detection sensitivity (S), which is calculated using the following formula:²⁴

Fig. 9 Plot of the logarithmic of FIR (I₅₂₈/I₅₄₉) versus inverse of absolute temperature.

Alternatively, the relative temperature sensitivity (S_R) is another key parameter for sensing applications. S_R is used to compare broad categories of temperature sensors and can be expressed as:^9,25

The calculated values obtained from eqn (3) and (4) were plotted as a function of absolute temperature as shown in Fig. 10. The maximum detection sensitivity (S) has a value of 7.91 × 10⁻³ K⁻¹ at 368 K, whereas the maximum relative sensitivity (S_R) is 9.97 × 10⁻³ K⁻¹ at 294 K. Table 2 shows a comparison of the maximum relative sensor sensitivities, used in optical thermometry, from different synthesized materials. As shown, SrSnO₃ has not yet been reported for optical thermometry, but SrSnO₃ is comparable to other synthesized materials, with a sensitivity that is similar to the previously reported values.

Fig. 10 Variation of sensor sensitivity and relative sensor sensitivity as a function of temperature.

Table 2 Relative sensitivity for FIR-based optical temperature sensors in different host materials

Sensing materials	Relative sensitivity (K^−1)	Temperature range (K)	Reference
Er^3+:SrSnO3	836.7/T^2	294–372	This work
Er^3+/Yb^3+:SrMoO4	979.8/T^2	93–773	4
Er^3+:germanate glass	918.6/T^2	300–600	9
ZnO:Er^3+ nanocrystals	880/T^2	273–573	21
Er^3+/Yb^3+:silicate glass	592.6/T^2	296–723	26
Er^3+/Yb^3+:La2O3 phosphor	814.4/T^2	303–600	27
Er-Mo:Yb2Ti2O7 nanophoshor	679.2/T^2	290–610	28
Er^3+:fluorotellurite glass	755/T^2	100–573	29
Er^3+/Yb^3+:Gd2O3 nanophosphor	746.4/T^2	300–900	30
YF:Er^3+/Yb^3+ nano/micro crystals	997.4/T^2	260–490	31

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The as-prepared up-conversion phosphors could be used as temperature sensors in biological systems. This is in good agreement with the results reported by Suo et al.³¹ where similar systems were used as temperature sensors in ex vivo experiments at sub-tissue level. It is know that's micron-sized agglomerates are unappropriated for biological applications. More work in order to obtain particles of nanometric-size is in progress.

4. Conclusions

SrSnO₃ doped with Er³⁺ was successfully synthesized using a sol–gel method. Powder XRD characterization showed that the structure crystalized as a distorted orthorhombic perovskite of space group Pbnm. The cell volume and the lattice parameters showed that the substitution of the Er³⁺ ions into the Sn⁴⁺ sites increase with an increase in the Er³⁺ concentration, which is in good agreement with previously reported results.

The upconversion emission spectra are dominated by the emissions at 528 nm and 549 nm. These emissions originate from the thermally coupled levels (²H_11/2, ⁴S_3/2). A detailed analysis shows that the emission at 549 nm is the result of a successive, two-photon absorption of the 975 nm radiation. Furthermore, the decay curve of the emission at 549 nm from excitation at 975 nm is in good agreement with the GSA/ESA mechanism.

The synthetized phosphors can be used in the fabrication of a temperature sensor probe to monitor the temperature within the range of 294–372 K interesting for ex vivo biological experiments in cell cultures or tissues. The maximum sensitivity is 7.91 × 10⁻³ K⁻¹ at 368 K and the relative sensor sensitivity is 9.97 × 10⁻³ K⁻¹ at 294 K.

Conflicts of interest

The are no conflicts to declare.

Acknowledgements

The authors acknowledge Conicyt-Chile and Fondecyt (Grant 1130248) for their financial support. I. R. Martin acknowledges MINECO and EU-FEDER (projects MAT2015-71070-REDC and MAT2016-75586-C4-4-P). M. V. acknowledges Conicyt-Chile for a doctoral fellowship. The authors acknowledge Dr Rodrigo Castillo for helpful discussion and UCN for financial support.

References

1. J. Cao, F. Hu, L. Chen, H. Guo, C. Duan and M. Yin, Optical thermometry based of up-conversion luminescence behavior of Er³⁺-doped KYb₂F₇ nano-crystals in bulk glass ceramics, J. Alloys Compd., 2017, 693, 326.
2. C. D. S. Brites, A. Millán and L. D. Carlos, Lanthanides in Luminescent Thermometry, in Handbook on the Physics and Chemistry of Rare Earths, ed. J. C. Bunzli and V. K. Pecharsky, North Holeng, 2016, Vol. 49, p. 339.
3. X. Cheng, K. Yang, J. Wang, L. Yang and X. Cheng, Up-conversion luminescence and optical temperature sensing behavior of Yb³⁺/Er³⁺ codoped CaWO₄ material, Opt. Mater., 2016, 58, 449.
4. P. Du, L. Luo and J. S. Yu, Infrared-to-visible upconversion emission of Er³⁺/Yb³⁺-codoped SrMoO₄ phosphors as wide-range temperature sensor, Curr. Appl. Phys., 2015, 15, 1576.
5. S. Jiang, P. Zeng, L. Liao, S. Tian, H. Guo, Y. Chen, C. Duan and M. Yin, Optical thermometry based on upconverted luminescence in transparent glass ceramics containing NaYF₄:Yb³⁺/Er³⁺ nanocrystals, J. Alloys Compd., 2014, 617, 538.
6. X. Wang, J. Zheng, Y. Xuan and X. Yan, Optical temperature sensing of NaYbF₄:Tm³⁺ SiO₂ core-shell micro-particles induced by infrared excitation, Opt. Express, 2013, 21, 18.
7. X. Tian, X. Wei, Y. Chen, C. Duan and M. Yin, Temperature sensor based on ladder-level assisted thermal coupling and thermal-enhanced luminescence in NaYF₄:Nd³⁺, Opt. Express, 2014, 22, 24.
8. D. Manzani, J. F. S. Petruci, K. Nigoghossian, A. A. Cardoso and S. J. L. Ribeiro, A portable luminescent thermometer based on green up-conversion emission of Er³⁺/Yb³⁺ co-doped tellurite glass, Sci. Rep., 2017, 7, 41596.
9. W. A. Pisarski, J. Pisarska, R. Lisiecki and W. Ryba-Romanowski, Er³⁺/Yb³⁺ co-doped lead germanate glasses for up-conversion luminescence temperature sensors, Sens. Actuators, A, 2016, 252, 54.
10. S. Ouni, S. Nouri, J. Rohlicek and R. Ben Hassen, Structural and electrical properties of the sol-gel prepared Sr_1−xEr_xSnO_3−δ compounds, J. Solid State Chem., 2012, 192, 132.
11. C. P. Udawatte, M. Kakihana and M. Yoshimura, Low temperature synthesis of pure SrSnO₃ and the (Ba_xSr_1−x)SnO₃ solid solution by the polymerized complex method, Solid State Ionics, 2000, 128, 217.
12. S. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in alloys and chalcogenides, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751.
13. V. M. Goldschmidt, Gesetze der Krystallochemie, Die Naturwissenschaften, 1926, 21, 477.
14. A. Le Bail, Whole powder patter decomposition methods and application: a retrospection, Powder Diffr., 2005, 20, 316.
15. A. Le Bail, H. Duroy and J. L. Fourquet, Ab-initio structure determination of LiSbWO₆ by X-ray powder diffraction, Mater. Res. Bull., 1988, 23, 447.
16. V. Petricek, M. Dusek and L. Palatinus, Crystallographic computing system JANA2006: general features, J. Crystallogr., 2014, 229, 345.
17. M. Muralidharan, V. Anbarasu, A. E. Perumal and K. Sivakumar, Room temperature ferromagnetism in Cr doped SrSnO₃ perovskite system, J. Mater. Sci.: Mater. Electron., 2017, 28, 4125.
18. M. C. F. Alves, S. C. Souza, M. R. S. Silva, E. C. Paris, S. J. G. Lima, R. M. Gomes, E. Longo, A. G. de Souza and I. M. G. dos Santos, Thermal analysis applied in the crystallization study of SrSnO₃, J. Therm. Anal. Calorim., 2009, 97, 179.
19. S. Tabanli, G. Eryurek and B. D. Bartolo, White light emission from Er₂O₃ nano-powder excited by infrared radiation, Opt. Mater., 2017, 69, 207.
20. B. M. van der Ende, L. Aarts and A. Meijerink, Lanthanide ions as spectral converters for solar cells, Phys. Chem. Chem. Phys., 2009, 11, 11081.
21. W. Yu, Y. Tian, M. Xing, Y. Fu, H. Zhang and X. Luo, Up-conversion luminescence of NaY(WO₄)₂: Yb, Er under 1550 and 980 nm excitation, Mater. Res. Bull., 2016, 80, 223.
22. M. Pollnau, D. R. Gameli, S. R. Lüthi, H. U. Güdel and M. P. Helen, Power dependence of upconversion luminescence in lanthanide and transition-metal-ions systems, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 61, 3337.
23. A. Pandey, V. K. Rai, V. Kumar, V. K. Kumar and H. C. Swart, Upconversion based temperature sensing ability of Er³⁺-Yb³⁺ codoped SrWO₄: an optical heating phosphor, Sens. Actuators, B, 2015, 209, 352.
24. A. K. Soni and V. K. Rai, Thermal and pump power effect in SrMoO₄:Er³⁺-Yb³⁺ phosphors for thermometry and optical heating, Chem. Phys. Lett., 2017, 667, 226.
25. X. Wang, X. Kong, Y. Yu, Y. Sun and H. Zhang, Effect of Annealing on Upconversion Luminescence of ZnO:Er³⁺ Nanocrystals and High Thermal Sensitivity, J. Phys. Chem. C, 2007, 41, 111.
26. C. Li, B. Dong, S. Li and C. Song, Er³⁺-Yb³⁺ co-doped silicate glass for optical temperature sensor, Chem. Phys. Lett., 2007, 443, 426.
27. R. Dey and V. K. Rai, Yb³⁺ sensitized Er³⁺ doped La₂O₃ phosphor in temperature sensors and display devices, Dalton Trans., 2014, 43, 111.
28. B. S. Cao, Y. Y. He, Z. Q. Feng, Y. S. Li and B. Dong, Optical temperature sensing behavior of enhanced green upconversion emissions from Er-Mo:Yb₂Ti₂O₇ nanophosphor, Sens. Actuators, B, 2011, 159, 8.
29. S. F. L. Luis, U. R. R. Mendoza, I. R. Martín, E. Lalla and V. Lavín, Effects of Er³⁺ concentration on thermal sensitivity in optical temperature fluorotellurite glass sensors, Sens. Actuators, B, 2013, 176, 1167.
30. S. K. Singh, K. Kumar and S. B. Rai, Er³⁺/Yb³⁺ codoped Gd₂O₃ nano-phosphor for optical thermometry, Sens. Actuators, A, 2009, 149, 16.
31. H. Suo, X. Zhao, Z. Zhang, T. Li, E. M. Goldys and C. Guo, Constructing Multiform Morphologies of YF:Er³⁺/Yb³⁺ Up-conversion Nano/Micro-crystals towards Sub-tissue Thermometry, Chem. Eng. J., 2017, 313, 65.

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