Photoluminescence Properties of Eu³⁺-activated Silicate Phosphors

Thermal analysis

The thermal behaviors of the phosphor systems, which were basically composed of 4MgCO₃·Mg(OH)₂·5H₂O and SrCO_3, were obtained between 50 °C and 1,300 °C (Figures 1–3).

The DTA/TG/DTG analysis results of Sr₃SiO₅: Eu³⁺, Dy³⁺ are shown in Figure 1. Figure 1 shows that there is a significant weight loss between 800 and 1,150 °C which is due to the decomposition of SrCO₃ and removal of CO₂ in the system. The decomposition of SrCO₃ under heating can be given as follows:

Figure 1:

DTA/TG/DTG curves of Sr₃SiO₅: Eu³⁺, Dy³⁺.

The endothermic peaks at 944 and 1,040 °C are attributed to an orthorhombic to rhombohedral transition and then the decomposition of SrCO₃ to SrO [5]. The TG curve exhibits a mass loss equal to 25.0 %, which is almost similar to the calculated mass loss (~25.0 %).

Figure 2 shows that the decomposition and dehydration of 4MgCO₃·Mg(OH)₂·5H₂O and SrCO₃ for the Sr₃SiO₅: Eu³⁺, Dy³⁺, Mg²⁺ system are similar to the DTA/TG/DTG curves of Sr₃SiO₅: Eu³⁺, Dy³⁺, as expected.

Figure 2:

DTA/TG/DTG curves of Sr₃SiO₅: Eu³⁺, Dy³⁺, Mg²⁺.

The Sr₃SiO₅: Eu³⁺, Dy³⁺, Mg²⁺ system has almost similar DTA/TG/DTG results. Thus, this system exhibits the decomposition of SrCO₃ and also the decomposition of a small amount of 4MgCO₃·Mg(OH)₂·5H₂O as the MgO source in the system. The decomposition of SrCO₃ under heating is given for the previous system.

Therefore, the endothermic peaks at 944 and 1,040 °C were attributed to an orthorhombic to rhombohedral transition followed by the decomposition of SrCO₃ to SrO [5], and that of 4MgCO₃·Mg(OH)₂·5H₂O to MgO. The TG curve exhibits a mass loss equal to 25.3 %, which is almost similar to the calculated mass loss (~25.0 %).

Figure 3, which is shown below, shows the thermal analysis results of Mg₃SiO₅: Eu³⁺, Dy³⁺ phosphor which is mainly composed of 4MgCO₃·Mg(OH)₂·5H₂O.

Figure 3:

DTA/TG/DTG curves of Mg₃SiO₅: Eu³⁺, Dy³⁺.

When subjected to thermal analysis, hydrated magnesium carbonates are decomposed by endothermic reactions and result in the departure of H₂O and CO₂ from the compound due to decomposition of 4MgCO₃·Mg(OH)₂·5H₂O to MgO. Much research has been conducted and it was found that the thermal decomposition of hydromagnesite proceeds via dehydration at 100–300 °C and decarbonation at 350–650 °C toward the end product, namely MgO [6]. Therefore, in our analysis, the weight losses starting from 150 to 600 °C are related to the decomposition of 4MgCO₃·Mg(OH)₂·5H₂O in the system (Figure 3). The decomposition of 4MgCO₃·Mg(OH)₂·5H₂O under heating can be summarize as follows:

The first endothermic peak (at 267 °C) is attributed to the partition of the hydroxyl group from Mg(OH)_2. The second endothermic peak to (at 450 °C) belongs the decomposition of MgCO₃ to MgO. The TG curve exhibits a total mass loss equal to 47.2 %, which is similar to calculated mass loss (~48.8 %).

X-ray diffraction (XRD) analysis

The heat treatment temperatures of each phosphor system were determined according to thermal analysis results. The sintering processes of the phosphor systems were applied as a pre-sintering stage at 800 °C for 2 h followed by main sintering processes. The base sintering temperatures are described below:

After high temperature sintering processes, the XRD analysis was applied. Figure 4 shows the XRD pattern of Sr₃SiO₅: Eu³⁺, Dy³⁺.

Figure 4:

The XRD patterns at different sintering temperatures of Sr₃SiO₅:Eu³⁺, Dy³⁺.

While the XRD patterns of the Sr₃SiO₅:Eu³⁺, Dy³⁺ phosphors for different sintering temperatures were achieved according to the DTA/TG/DTG results, several reactions occurred simultaneously, including the grain growth of SrO and SiO₂, the diffusion of Si⁴⁺ into the SrO lattice, and a phase formation reaction of Sr₂SiO₄-Sr₃SiO₅ [7]. Therefore, the crystallinity and base phase of the Sr₃SiO₅:Eu³⁺, Dy³⁺ powders improved and were well indexed with increasing sintering temperature. The Sr₃SiO₅:Eu³⁺, Dy³⁺ powders sintered at 1,470 °C for 12 h exhibited improved crystallinity compared to powders sintered at 1,300 °C for 6 h, as proved by the relatively high and sharp peaks in the XRD pattern. The phosphor sample XRD results proved that the expected crystal system could be indexed by increasing sintering temperature. Therefore the XRD pattern of Sr₃SiO₅:Eu³⁺, Dy³⁺ matched well with the PDF 00-026-0984 Sr₃SiO₅ card [8] owing to at sintering conditions 1,470 °C for 12 h. Almost all the peaks showed a single phase except that β-Sr₂SiO₄ secondary phases were observed despite the higher sintering temperature and holding time at that temperature. The sample crystallized in the tetragonal structure, the lattice parameters of which were a=6.9476 Å b=6.9476 Å, c=10.753 Å, (alpha) α=90°, (beta) β=90° and (gamma) γ=90°.

The XRD patterns of the Sr₃SiO₅:Eu³⁺, Dy³⁺, Mg²⁺ powders (Figure 5) were observed to be similar to Sr₃SiO₅: Eu³⁺, Dy³⁺, as expected. The obtained products were mainly composed of Sr₃SiO₅ in the 1,460 °C sintered sample except that some β-Sr₂SiO₄ phase existed in the system. Furthermore the result clearly suggests that trace amounts of Eu³⁺, Dy³⁺ or Mg²⁺ were incorporated into the lattice and caused no change in the lattice structure, as can be seen by the unchanged diffraction patterns. At lower sintering conditions, for example 1,300 °C for 6 h, β-Sr₂SiO₄ presented as the major phase, but this situation was changed by the higher temperatures and holding times, then the β-Sr₂SiO₄ phase was minor and the expected phase was Sr₃SiO₅. To obtain a pure Sr₃SiO₅ structure, it is necessary to increase the sintering temperature and holding time. The XRD pattern of Sr₃SiO₅:Eu³⁺, Dy³⁺, Mg²⁺ matched well with the PDF 00-026-0984 Sr₃SiO₅ card [8] at sintering conditions of 1,460 °C for 12 h. The sample crystallized in the tetragonal structure similar to Sr₃SiO₅:Eu³⁺, Dy³⁺, the lattice parameters of which were a=6.9476 Å, b=6.9476 Å, c=10.753 Å, (alpha) α=90°, (beta) β=90° and (gamma) γ=90°.

Figure 5:

The XRD patterns at different sintering temperatures of Sr₃SiO₅: Eu³⁺, Dy³⁺, Mg²⁺.

The XRD pattern of the Mg₃SiO₅:Eu³⁺, Dy³⁺ phosphor that was sintered at different temperatures to obtain a pure structure is shown in Figure 6.

Figure 6:

The XRD patterns at different sintering temperatures of Mg₃SiO₅: Eu³⁺, Dy³⁺.

It was determined that the base phase is Mg₂SiO₄; also the major first two phases of the peaks indicate those of MgO although the sintering temperature was repeated and increased as 1,250 °C. Despite the fact that a higher thermal treatment was applied for this type of phosphor according to DTA/TG results, the expected crystal system could not be indexed in XRD analysis except for Mg₂SiO₄ and MgO. Also, it is well known that the formation of the Mg₃SiO₅ phase occurs at very high temperatures, namely above 1,850 °C, according to the MgO-SiO₂ phase diagram [9].

Photoluminescence properties

All of the photoluminescence studies gave effective results with excitation and emission spectra. Although not all of the objective phosphor systems could be synthesized as pure structures, they still have fascinating PL properties. Therefore, the impressive excitation and emission bands are thanks to activated Eu³⁺, rare-earth ion.

Figure 7 shows the PL spectra of the Sr₃SiO₅:Eu³⁺, Dy³⁺ phosphor system upon excitation at 311 nm in the UV region and the obtained sharp emission with a maximum at 618 nm in the red region.

Figure 7:

The PL spectra of Sr₃SiO₅: Eu³⁺, Dy³⁺ system phosphor.

It appeared that the PL result of this system indicates maximum emission bands at 617 nm, 579 nm and 698 nm which are attributed to ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₁ [10, 11, 12] and ⁵D₀ → ⁷F₄ [12, 13] transitions of the Eu³⁺ ion, respectively. Furthermore, the sharp excitation peaks generally between 250 nm and 500 nm are associated with typical intra-4f transitions of the Eu³⁺ ion. The minör excitation bands at 396 and 465 nm are attributed to ⁷F₀ → ⁵L₆ [12, 14, 15, 16] and ⁷F₀ → ⁵D₂ [12, 15, 16, 17] transitions of the Eu³⁺ ion, respectively.

The PL spectra of the Sr₃SiO₅: Eu³⁺, Dy³⁺, Mg²⁺ phosphor powders are shown in Figure 8. The sample was excited at 287 nm in the UV region and presented maximum emission at 586 nm in the yellow-red region.

Figure 8:

The PL spectra of Sr₃SiO₅:Eu³⁺, Dy³⁺, Mg²⁺ system phosphor.

It appears that the PL result of this system indicates maximum emission bands at 586 nm, 613 nm and 701 nm, correlatively with the Sr₃SiO₅:Eu³⁺, Dy³⁺ phosphor, which are attributed to the ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ [10, 11, 12], and ⁵D₀ → ⁷F₄ [12, 13] transitions of the Eu³⁺ ion, respectively. Moreover, the sharp excitation peak at 287 nm and very little peaks at 394 nm and 463 nm are associated with typical intra-4f, ⁷F₀ → ⁵L₆ [12, 14, 15, 16] and ⁷F₀ → ⁵D₂ [12, 15, 16, 17] transitions of the Eu³⁺ ion, respectively. Recently, Jee et al. reported that the incorporation of Mg²⁺ into SrSi₂O₂Ni₂:Eu²⁺ improves the luminescence intensity and thermal quenching [18]. Therefore, in this study, the aim was to achieve more intense luminescence properties with the Sr₃SiO₅:Eu³⁺, Dy³⁺ phosphor. By combining the effects of replacing Sr²⁺ with Mg²⁺, significant enhancement of emission intensity was obtained while the spectral distribution of the emission was maintained, as can be seen from Figures 7 and 8. In Figure 8, the remarkable almost 75 % improvement in emission intensity is highlighted, with an excitation maximum in the range of 500 nm to 750 nm, which is quite attractive for different types of phosphor applications.

The last PL analysis results of Mg₃SiO₅: Eu³⁺, Dy³⁺ are also similar to the two previous studies, because the same activator, namely Eu³⁺, was used (Figure 9).

Figure 9:

The PL spectra of Mg₃SiO₅: Eu³⁺, Dy³⁺ system phosphor.

All excitation (between 250–500 nm) and emission bands (between 550–750 nm) are almost the same as the previous two Eu³⁺-doped and Dy³⁺ co-doped systems. So the transitions of Eu³⁺ ion appeared.

The important and prominent point here is that, apart from having the same PL results, these the intensities change according to the hosts. The Mg₃SiO₅: Eu³⁺, Dy³⁺ system phosphor has the most intense PL results among them.