Solid-state synthesis and luminescence study of Bi or Eu-doped LiAlGeO₄

INTRODUCTION

Among all of the luminescent materials, rare-earth doped oxides have been in the center of attention for a very long time. Germanates, however, are not as popular as most other oxides. Most of the research done on germanate luminescence was on bismuth doped germanate based glasses. However, they have an outstanding advantage over the much researched silicate based glasses, because GeO₂ has more similar melting and boiling temperatures to those of Bi₂O₃ than SiO₂, which means that less Bi₂O₃ evaporates during synthesis. Germanates also have similar properties to those of silicates, since germanium is in the same element group as silicon [1]. There are a  handful of different germanium based materials; however, there were barely any studies done on LiYGeO₄ and LiAlGeO₄. Luminescence studies on these materials include LiYGeO₄: Bi, LiAlGeO₄: Cr and LiYGeO₄: Eu. In this work, bismuth was chosen as an activator for LiYGeO₄ and LiAlGeO₄ so that their optical properties can be compared to one another as well as to those of LiYGeO₄ synthesised by J. Shi and the others [2]. Eu³⁺ was also chosen as an activator for LiAlGeO₄ to analyse optical properties of the material more thoroughly. Eu³⁺-doped materials usually act as red phosphors and are suitable for various applications [3–6]. Therefore, LiAlGeO₄:Eu³⁺ samples were prepared by solid-state synthesis and luminescent properties were analysed. The obtained results are discussed in this paper.

EXPERIMENTAL

All samples were synthesised using a  conventional solid-state synthesis method. Stoichiometric amounts of analytical grade starting materials (Li₂CO₃, GeO₂, Eu₂O₃, Al₂O₃, Y₂O₃ and Bi₂O₃) were mixed and ground in agate mortar. The  obtained mixtures were transferred to a crucible and heated. LiAlGeO₄ was synthesised by heating samples the  first time at 800°C for 6  h and afterwards at 1050°C for 6  h. The  obtained products were reground in agate mortar and used for further analysis.

The purity of synthesised compounds was assessed by X-ray powder diffraction employing a MiniFlex II (Rigaku) diffractometer with Cu Kα radiation (λ  =  1.5406  °A). Measurements were performed using a  Bragg–Brentano geometry at a rate of 10 °/min. Excitation and emission spectra were recorded using an Edinburgh Instruments FLS980 spectrometer equipped with a 450 W Xe arc discharge lamp, a photomultiplier (Hamamatsu R928) and mirror optics for powder analysis. Excitation spectra were corrected with a reference detector.

RESULTS AND DISCUSSION

XRD analysis of LiAlGeO₄ samples

A series of LiAlGeO₄ samples was prepared and their structure and purity were recorded and evaluated using X-ray powder diffraction analysis. From Fig. 1 we can see that all samples except LiAlGeO₄:Eu 16% are monophasic and match the standard LiAlGeO₄ peaks (PDF#04-007-7636). The diffraction pattern of LiAlGeO₄:Eu 16% also mostly coincides with the standard; however, additional peaks at 31 and 34 degrees can be identified and the signal-to-noise ratio becomes much worse likely due to additional phases forming from excess europium.

Luminescence properties of the synthesised LiAlGeO₄:Eu powder

To determine optical properties of LiAlGeO₄:Eu samples, excitation and emission studies were performed, the results of which are shown in Fig. 2. As can be seen from the measurements presented below, the highest excitation and emission intensities were found to be in the LiAlGeO₄:Eu 16% sample.

The most intensive transition observed in the  excitation (left) spectrum is ⁵L₆  ←  ⁷F₀ (at 393  nm). Other excitation transitions with lower intensities are visible and marked in Fig.  2. A  broad excitation peak around 250–300  nm is due to charge transfer (oxygen to europium). The emission spectra consist of four ⁵D₀ → ⁷FJ=1, 2, 3, 4 transitions. Emissions from 5D₂ and 5D₁ are quenched due to cross-relaxation. ⁵D₀ → ⁷F₁ emission is observed due to magnetic dipole transitions and ⁵D₀ → ⁷F_2, 3, 4 due to electric dipole transitions [7]. ⁵D₀ → ⁷F₃ transition is forbidden thus of much weaker intensity. The strongest emission is at 702.5 nm (⁵D₀ → ⁷F₄ transition). Moreover, in the  emission (Fig.  2, right) spectrum, the  splitting of emission peaks is visible. The likely reason for these splittings is that the structure contains more than one crystallographic node that can be occupied by europium. Normally, the emission intensity should increase to a certain value and then decrease (due to concentration quenching), but in this case the mentioned decrease is not observed. During the preparation of the 32% Eu-doped sample, the desired phase could not be obtained, so we could not compare the emission.

From emission spectra CIE 1931 colour coordinates were calculated and are depicted in Fig. 3. We can see that the LiAlGeO₄:Eu samples are orange-red in colour and the emission colour coordinates vary only slightly with the  Eu concentration in the  compound. However, the  colour coordinates shift slightly to the  red side (to the right) with the increase of Eu concentration in the sample.

Luminescence kinetics were evaluated as well. As can be seen from Fig.  4, the  longest luminescence lifetime is observed in the LiAlGeO₄: Eu 1% sample. To determine these figures more accurately, the luminescence lifetimes were fitted using the bicomponent exponential decay function and their average values (combined from t₁ and t₂) were calculated.

The calculated LiAlGeO₄:Eu luminescence life time values show the  same results as in Fig.  4  – the  longest luminescence is observed in LiAlGeO₄: Eu 1%, and the shortest by LiAlGeO₄:Eu 4% sample.

Studies of the luminescence quantum yields of the samples were carried out, the results of which are presented in Table 2. Measurements of quantum yields were performed using the  integrated sphere method.

These measurements reveal which concentration of Eu the  LiAlGeO₄:Eu compound has the  highest rate of emitted electrons to absorbed photons, which basically shows the luminescence efficiency of the  compound. LiAlGeO₄:Eu has the  highest quantum efficiency at 16% and the  lowest at 4%. The  emission intensity of the  sample doped with 0.5% Eu was too low, so the quantum yield could not be determined.

Investigation of the thermal quenching properties of the synthesised LiAlGeO₄:Eu 1% powder

For phosphors actual application, the  temperature stability is also one of the  most important parameters because it greatly affects the  output of the light and CRI in LEDs [8]. Figure 5 shows the temperature-dependent PL spectra. The highest emission peak of LiAlGeO₄:Eu 1% sample is due to the ⁵D₀ → ⁷F₃ transition at 610.5 nm. The highest luminescence excitation intensity is observed at room temperature, but the highest emission intensity at 77 K temperature. The likely reason for this is photoionization.

In Fig. 6 we can see that colour coordinates also vary very slightly with changes in temperature. As the temperature increases, the colour coordinates shift slightly to the left side of the spectrum.

As can be seen in Fig. 7, change in temperature also has an effect on the luminescence lifetime of a  sample. As the  temperature increases, luminescence lifetimes decrease.

Normalised total emission intensities were calculated ant plotted in Fig. 8. In order to calculate TQ_1/2 (temperature at which emission loses half of its intensity) the exponential decay fit was performed to the data. The obtained results revealed that the  emission reached half of its maximum value at 323 K.

Investigation of the luminescence properties of the synthesised LiYGeO₄:Bi and LiAlGeO₄:Bi powders

Luminescence excitation and emission studies of the samples were conducted, the results of which are shown in Fig. 9. As can be seen from Fig. 9, the LiYGeO₄:Bi samples have a much higher intensity excitation and emission spectrum compared to those of LiAlGeO₄:Bi. One of the possible reasons for this is the fact that Bi³⁺ is of a more similar size to Y³⁺ than it is to Al³⁺.

Afterglow times were measured by monitoring emission at 375 nm and exciting with 254 or 295  nm light. After 60 s, the  excitement source was shut and the emission intensity was continued to measure, as can be seen in Fig. 10. The luminescence lifetime of LiAlGeO₄:Bi was not measured because compared to LiYGeO₄:Bi the emission was much weaker, thus reliable data could not be obtained. LiAlGeO₄:Bi and LiYGeO4:Bi were prepared because the previously published research [2] stated that LiYGeO₄:Bi has a very long luminescence lifetime. The source states that the luminescence of this compound is still bright after 72 h and is still observed 300 h after excitation has passed. Our results showed that LiAlGeO₄ is much less suitable host for bismuth doping compared to LiYGeO₄. On the other hand, our LiYGeO₄:Bi sample showed afterglow lasting for a few minutes – no way near super-long persistent luminescence reported by Shi et al. One of possible explanations of such discrepancy might be use of different synthesis and measurement approaches.

CONCLUSIONS

In summary, LiAlGeO₄: Eu phosphors were synthesised using the  solid state synthesis method with 1% Bi and 0.5, 1, 2, 4, 8 as well as 16% Eu, and their phase purity was confirmed by X-ray diffraction analysis. After studying the  luminescence of bismuth in LiYGeO₄ and LiAlGeO₄ compounds, it was found that LiYGeO₄:Bi exhibits a significantly more intensive emission. In addition, this compound was characterised by emission afterglow measurement; however, the  obtained afterglow times were much shorter than reported in other studies. The comparison of emission intensities of different LiAlGeO₄:Eu samples revealed that LiAlGeO₄:Eu 16% has the  highest excitation and emission intensity. After performing studies on the luminescence kinetics of LiAlGeO₄:Eu, it was found that the LiAlGeO₄:Eu 1% sample has the longest emission decay times (τ = 1047.66 µs), while increasing the europium concentration decreases the quenching times. The highest quantum yield was found in the LiAlGeO₄:Eu 16% sample and reached 6.31%. Thermal quenching studies were also carried out on the LiAlGeO₄:Eu 1% sample. The highest excitation intensity was observed at room temperature, on the other hand, the highest emission intensity was determined at 77 K. The increase in temperature also reduces the luminescence afterglow time, with the longest one observed at 77 K. The calculated temperature at which phosphor loses half of its maximum emission was 323 K.

ACKNOWLEDGEMENTS

This article is dedicated to the anniversary of Prof. Rimantas Ramanauskas.

This project has received funding from the European Social Fund (Project No.  09.3.3-LMTK-712-19-0119) under Grant Agreement with the Research Council of Lithuania (LMTLT).

References

1. M. Peng, N. Zhang, L. Wondraczek, J. Qiu, Z. Yang, Q.  Zhang, Opt. Express, 19(21), 20799 (2011) [https://doi.org/10.1364/OE.19.020799].

2. J.  Shi, X.  Sun, S.  Zheng, et al., Adv. Opt. Mater., 7(19), 1900526 (2019)  [https://doi.org/10.1002/adom.201900526].

3. M.  Saraf, P.  Kumar, G.  Kadewat, et al., Inorg. Chem., 54(6), 2616 (2015) [https://doi.org/10.1021/ic5027784].

4. T. Dai, G. Ju, Y. Lv, Y. Jin, H. Wu, Y. Hu, J. Lumin., 237, 118193 (2021)  [https://doi.org/10.1016/j.jlumin.2021.118193].

5. D.  K.  Patel, B.  Vishwanadh, V.  Sudarsan, S. K. Kulshreshtha, J. Am. Ceram. Soc., 96(12), 3857 (2013) [https://doi.org/10.1111/jace.12596].

6. F. A. Rabuffetti, S. P. Culver, J. S. Lee, R. L. Brutchey, Nanoscale, 6, 2909 (2014) [https://doi.org/10.1039/C3NR06610J].

7. S.  Shionoya, W.  M.  Yen, H.  Yamamoto, Phosphor Handbook, 2nd edn., CRC Press (2006).

8. B. Fan, W. Zhou, S. Qi, W. Zhao, J. Solid State Chem., 283, 121158 (2020)  [https://doi.org/10.1016/j.jssc.2019.121158].

Artūras Harnik, Martynas Misevičius

Bi ARBA Eu LEGIRUOTO LiAlGeO₄ SINTEZĖ KIETAFAZIŲ REAKCIJŲ METODU BEI LIUMINESCENCIJOS TYRIMAI