CHEMIJA. 2022. Vol. 33. No. 4. P. 120–126
© Lietuvos mokslų akademija, 2022
In this research pristine, Bi-doped and Eu-doped LiAlGeO4 were prepared using a solid-state synthesis method. All samples were analysed by X-ray powder diffraction (XRD) and luminescence measurements. The highest Eu3+ concentration yielding monophasic samples in LiAl1-xGeO4: Eux was x = 0.08, while higher doping concentrations resulted in the formation of additional phases. The luminescence measurements revealed that the highest emission intensity was observed in the 16% Eu sample. Furthermore, the same sample demonstrated the highest quantum yield, while the longest luminescence decay was observed in the LiAlGeO4:Eu 1% sample. The temperature-dependent luminescence measurements revealed that phosphor lost half of its efficiency at 323 K.
Keywords: luminescence, LiAlGeO4, X-ray powder diffraction, solidstate synthesis
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 . 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 . 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.
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.
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.
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 → 7F2, 3, 4 due to electric dipole transitions . 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.
|Eu concentration in the compound||Luminescence lifetime|
|Eu concentration in the compound||Quantum efficiency|
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.
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 . 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 TQ1/2 (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.
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  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.
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.
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).
Received 20 September 2022
Accepted 28 September 2022
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Šio tyrimo metu, panaudojus kietafazių reakcijų sintezės metodą, buvo paruošti gryni, bismutu arba europiu legiruoti LiAlGeO4 mėginiai. Visi mėginiai ištirti rentgeno difrakcijos bei liuminescencinės spektroskopijos tyrimų metodais. Didžiausia Eu3+ koncentracija, kai gaunami vienfaziai LiAl1-xGeO4:Eux mėginiai, buvo x = 0,08, o didesnės legiravimo koncentracijos lėmė papildomų fazių formavimąsi. Liuminescencijos tyrimai atskleidė, kad didžiausiu emisijos intensyvumu pasižymėjo 16 % Eu legiruotas mėginys. Be to, pastarasis mėginys išsiskyrė didžiausia emisijos kvantine išeiga, tačiau liuminescencijos gesimo trukmė ilgiausia užfiksuota LiAlGeO4:Eu 1 % mėginyje. Emisijos priklausomybės nuo temperatūros matavimai atskleidė, kad šio fosforo emisijos efektyvumas sumažėja pusiau pasiekus 323 K temperatūrą.