Effects of the Concentration of Eu³⁺ Ions and Synthesizing Temperature on the Luminescence Properties of Sr_2−xEu_xZnMoO₆ Phosphors

Abstract

The effect of Eu₂O₃ concentration on the luminescence properties of double perovskite (cubic) Sr_2−xEu_xZnMoO₆ phosphors was thoroughly investigated using different synthesizing temperatures. Phosphors with the composition Sr_2−xEu_xZnMoO₆, where Eu₂O₃ was substituted for SrO and x was changed from 0 to 0.12, were synthesized by the solid-state method at temperatures of 900–1200 °C, respectively. Analysis of the X-ray diffraction (XRD) patterns showed that even when the synthesizing temperature was 1100 °C, secondary or unknown phases were observed in Sr_2−xEu_xZnMoO₆ ceramic powders. The effect of the concentration of Eu³⁺ ions on the luminescence properties of the Sr_2−xEu_xZnMoO₆ phosphors was readily observable because no characteristic emission peak was observed in the Sr₂ZnMoO₆ phosphor. Two characteristic emission peaks at 597 and 616 nm were observed, which correspond to the ⁵D₀–⁷F₁ and ⁵D₀–⁷F₂ transitions of Eu³⁺ ions, respectively. The two characteristic emission peaks of the Sr_2−xEu_xZnMoO₆ phosphors were apparently influenced by the synthesizing temperature and the concentration of Eu³⁺ ions. When x was larger than 0.08, a concentration quenching effect of Eu³⁺ ions in the Sr_2−xEu_xZnMoO₆ phosphors could be observed. The lifetime of the Sr_2−xEu_xZnMoO₆ phosphors decreased as the synthesizing temperature increased. A linear relation between temperature and lifetime was obtained by using a fitting curve of t = −0.0016 × T + 3.543, where t was lifetime and T was synthesizing temperature.

1. Introduction

UV light can be generated as a consequence of electronic transitions of light sources through an Hg discharge. In low-pressure Hg discharge, the main emission line is located at a wavelength of 254 nm. This light is invisible and harmful to human bodies, so it has to be converted into visible light, which can be done with a combination of luminescent materials. These luminescent materials can strongly absorb light of that wavelength and efficiently convert it into visible light. Recently, white light emitting diodes (LEDs) have become popular because they have several advantages, including high efficiency, long lifetime, and low power consumption. Red phosphors are also helpful for generating white light when they are excited by blue or near-UV lights. To obtain phosphors with highly efficient emissions, it is important to choose the right compound materials and ensure they have outstanding physical and chemical stability. Numerous studies have explored different luminescent materials to enable the development of suitable phosphors. When lanthanide contraction ions are introduced into host materials, unfilled 4f^N electron orbitals result, and these have attracted considerable attention. The resulting phosphors emit very luminescent emissions with specific light wavelengths because of variations in the energy level of some free electrons [1,2,3,4]. These phosphors have been the most promising candidates for applications in fluorescent lamps and flat panel display devices, such as electroluminescence panels, plasma display panels, and field emission displays.

A large number of isotropic compounds with perovskite structures, such as double perovskite oxides, have the general formula A₂BB’O₆, in which BO₆ and B’O₆ octahedra are corner-shared, alternately. The great flexibility of A and B(B’) sites in A₂BB’O₆ allows very rich substitutions, and this framework forms cube-octahedral cavities filled by A-site cations [5,6]. Double perovskites with the formula A₂BB’O₆, where A uses an alkaline earth, B and B’ are metal transition magnetic and nonmagnetic ions, and O is oxygen, have been investigated as magnetic materials for many years. For example, Sr₂CrMoO₆ has been studied as a half-metallic system [7]. Kobayashi et al. recently reported room-temperature low-field magnetic resistance in the ordered double perovskite Sr₂FeMoO₆ [8]. Complete ordering of Fe and Mo on the B and B’ sites of this metallic A₂BB’O₆ double perovskite is predicted to give half-metallic ferromagnetism with localized majority-spin electrons on the Fe atoms [9,10]. Recently, the study of A₂BB’O₆-based materials has increased due to various technological applications, such as inorganic oxide luminescent materials.

The emitting materials are usually composed of activators and a host lattice. Some host lattice materials can produce light themselves, and some can produce light when doped with rare-earth activators (ions). Rare-earth ions are known to exist in various valence states, although the trivalent state is the most prevalent. Rare-earth ions can be applied in lighting devices and display panels due to their abundant energy levels across a wide spectrum range, from ultraviolet to near infrared. Sm- and Eu-based ions are the most commonly used dopants because they are stable in trivalent (Sm³⁺ and Eu³⁺), as well as divalent (Eu²⁺), states. The luminescence of rare-earth ions doped in perovskite-type ceramics was actively investigated in the 1960s and 1970s because of interest in their ferroelectricity, phase transitions, and semiconducting properties [11]. Recently, many studies have shown that the double perovskite structure with a composition of A₂BMO₆ (A = Ba, Sr; B = Ca, Zn; M = Mo, W) is activated by trivalent europium ions (Eu³⁺) [12,13,14,15]. Phosphors activated by Eu³⁺ are considered ideal red sources because of their sharp emission lines in the red region [12,13,14,15,16]. Eu³⁺-doped double-perovskite materials have a broad excitation band ranging from UV to visible light, and they also show highly efficient red luminescence. For that, Eu³⁺-doped double molybdenum (Mo)-based double perovskite oxides have attracted significant attention for their possible application as luminescent materials, such as Sr₂MgMo_xW_1−xO₆: Eu³⁺ [17], Sr₂Ca(Mo/W)O₆: Eu³⁺ [18], Sr₂CaMoO₆: Eu³⁺ [19], (Ba,Sr)₂CaMoO₆: Eu³⁺, Yb³⁺ [20], Ca₂LaMO₆: Eu³⁺ (M = Sb, Nb, Ta) [21], and A₂CaMoO₆ (A = Sr, Ba) [6], respectively.

To the best of our knowledge, the luminescent characteristics of concentrated Eu³⁺ ions in Sr_2−xEu_xZnMoO₆ phosphors, which are molybdenum-based double-perovskite oxides, have not been reported. In this study, the red-emitting phosphors of Eu³⁺-doped Mo-based double-perovskite Sr_2−xEu_xZnMoO₆ oxides were synthesized by the conventional high-temperature solid state reaction method. We found that Eu³⁺ concentration and the synthesizing temperature of the Sr_2−xEu_xZnMoO₆ phosphors had a large effect on their luminescent characteristics. The concentration quenching effect of Eu³⁺ ions in Sr_2−xEu_xZnMoO₆ phosphors was found and would be well discussed [22]. In this study, the first important novelty is that we have not found any similar studies about Sr_2−xEu_xZnMoO₆ phosphors. The second important novelty is that we found a linear relationship between the synthesizing temperature and lifetime of Sr_2−xEu_xZnMoO₆ phosphors. We also investigated the relationship between the synthesized temperature and lifetime of Sr_2−xEu_xZnMoO₆ phosphors.

2. Materials and Methods

Sr_2−xEu_xZnMoO₆ powders were synthesized through the solid-state reaction method. Stoichiometric amounts of SrCO₃, ZnO, MoO₃, and Eu₂O₃ were weighed according to the composition formula of (2 − x) SrCO₃ + ZnO + MoO₃ + (0.5 x) Eu₂O₃, where x = 0, 0.04, 0.06, 0.08, 0.10, and 0.12, respectively. After being mixed in acetone, dried, and ground, the solid-state reaction method was used to heat the Sr_2−xEu_xZnMoO₆ compositions in an air atmosphere. The Sr_2−xEu_xZnMoO₆ powders were heated to 900 °C, 1000 °C, 1100 °C, and 1200 °C for 4 h. When the synthesizing temperature was equal to 1300 °C, the Sr_2−xEu_xZnMoO₆ was melted and gathered together to not be used as part of the phosphors. The crystalline structures of the synthesized Sr_2−xEu_xZnMoO₆ powders were measured using X-ray diffraction (XRD) (Bruker, Boston, MA, USA ) patterns with Cu Kα radiation (λ = 1.5418 Å) and with a scanning speed of 2° per minute. Photoluminescence (PL) properties were recorded at room temperature in the wavelength range of 450–800 nm on a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan). In the past, we had found that 271 nm had a better excitation effect on BaZr_1−xEu_xO₃ powders [16]. This result suggested that we would also need to find the optimum optical wavelength for exciting the Sr_2−xEu_xZnMoO₆ powders. In this study, the three-dimensional (3D) scanning process using a spectrophotometer (Hitach, Tokyo, Japan) was used to find the optimum optical wavelength, and this value was dependent on the compositions of the Sr_2−xEu_xZnMoO₆ phosphors and the synthesizing temperature. We found that the optimum exciting optical wavelength for all of the Sr_2−xEu_xZnMoO₆ powders was 350 nm, and the Sr_2−xEu_xZnMoO₆ powders excited by other wavelengths had the weaker PL intensities.

3. Results and Discussion

To achieve high PL properties, the preparation of Sr_2−xEu_xZnMoO₆ powders forming the double-perovskite phase is very important, because crystallization of Sr_2−xEu_xZnMoO₆ powders influences their photoluminescent properties. Figure 1 shows the XRD patterns of our Sr_2−xEu_xZnMoO₆ powders as a function of the synthesizing temperature. The strong peaks occurred at around 31.9° for the (220) diffraction peak of the six host lattices. Those results suggest that the XRD patterns showed stable double-perovskite features regardless of the synthesizing temperature and Eu³⁺ concentration.

As Figure 1a shows, when the synthesizing temperature of the Sr_2−xEu_xZnMoO₆ powders was 900 °C and as the x value increased from 0 to 0.12, the 2θ value of the (220) diffraction peak shifted from 31.85 to 31.87 and the full width at half maximum (FWHM) values for the (220) diffraction peak were in the range of 2θ = 0.25°–0.27°. When the synthesizing temperature was 1200 °C, the 2θ value of the (220) diffraction peak shifted from 31.88 to 31.90, and the FWHM values for the (220) diffraction peak of the Sr_2−xEu_xZnMoO₆ powders were in the range of 2θ = 0.19–0.20°, as the x value of the Sr_2−xEu_xZnMoO₆ powders increased from 0 to 0.12. The results in Figure 1a–d show that the 2θ of the (220) diffraction peak shifted to a higher value and the FWHM values for the (220) diffraction peaks of the Sr_2−xEu_xZnMoO₆ powders decreased as the synthesizing temperature increased.

The ideal cubic double-perovskite structure (with the same space group Pm$\overline{3}$m (221)) can be described by a faced-centered cubic (fcc) lattice with lattice constant 2a [23]. The B(B’) ion is coordinated by the B’(B) ion using an O ion as an intermediate in the middle, and the lengths of B–O and B’–O are considered to be equal. After relaxation, both lattice constants and atomic positions reduce the ideal cubic structure (space group Fm$\overline{3}$m) to a tetragonal structure (space group I4/mmm). There are two O₁ atoms located on the z-axis with B and B’ atoms sitting between, and the four O₂ atoms are located on the xy-plane; the same as the B and B’ atoms. The angle of the B–O–B’ remains at 180° during structural optimization, whereas the lattice constant and bond length change. The lattice a can be calculated by using (1) the reflection peaks (011), (111), (200), and (220) from the XRD patterns in Figure 1; and (2) the closeness of the c/a ratio to the ideal value of $\sqrt{2}$. As the synthesizing temperature was 900 °C, the Sr_2−xEu_xZnMoO₆ powders exhibited a cubic crystal structure with the cell parameters changing from a = b = c/$\sqrt{2}$ = 0.3968 nm to a = b = c/$\sqrt{2}$ = 0.3971 nm as the concentration of Eu³⁺ ions increased from 0 to 0.12. The cell parameters for Sr_2−xEu_xZnMoO₆ powders synthesized at 1200 °C were also calculated from the XRD patterns shown in Figure 1, and the cell parameters changed from a = b = c/$\sqrt{2}$ = 0.3971 nm to a = b = c/$\sqrt{2}$ = 0.3974 nm as the concentration of Eu³⁺ ions increased from 0 to 0.12. These results were in good agreement with those from the Joint Committee on Powder Diffraction Standards (JCPDS) file number 742474.

As the results of the X-ray diffraction (XRD) patterns showed in Figure 1 were compared, these results indicated that the diffraction intensities of unknown phases for peaks located at around 2θ = 28.44° and 33.09° increased as the concentration of Eu³⁺ ions increased and decreased as the synthesizing temperature increased. Those peaks are in good agreement with the (222) and (400) peaks of JCPDS file number 120393 for cubic Eu₂O₃. The decrease in the diffraction intensities of peaks located at around 2θ = 28.44° and 33.09° prove that more Eu³⁺ ions will substitute the sites of Sr²⁺ ions as the synthesizing temperature is raised. Even when the synthesizing temperature was 1100 °C, secondary or unknown phases were observed in the Sr_2−xEu_xZnMoO₆ ceramic powders. These Eu₂O₃ and secondary or unknown phases were not observed when the synthesizing temperature was 1200 °C. These results suggest that when the same synthesizing temperature is used, the concentration of Eu³⁺ ions has no apparent effect on the crystallization of Sr_2−xEu_xZnMoO₆ powders; hence, the synthesizing temperature is an important factor in determining the crystalline properties of Sr_2−xEu_xZnMoO₆ powders. Additionally, the concentration of Eu³⁺ ions and the synthesizing temperature affect the photoluminescent properties of Sr_2−xEu_xZnMoO₆ phosphors.

XRD patterns for Sr_2−xEu_xZnMoO₆ phosphors synthesized at 1200 °C for 4 h in the narrow range of 29–35° are shown in Figure 2. These results are significant. Initially, the splitting of the (220) diffraction peak was observed in the Sr₂ZnMoO₆ and Sr_1.98Eu_0.02ZnMoO₆ phosphors, but it was not observed in other Sr_2−xEu_xZnMoO₆ phosphors. The two split peaks of the Sr₂ZnMoO₆ phosphors were located at 2θ = 31.83° and 31.88°, and the two split peaks of the Sr_1.98Eu_0.02ZnMoO₆ phosphors were located at 2θ = 31.81° and 31.89°. These results suggest that the Sr₂ZnMoO₆ and Sr_1.98Eu_0.02ZnMoO₆ phosphors revealed a perovskite structure with the tetragonal phase. As the concentration of Eu³⁺ was more than 0.02, the Sr_2−xEu_xZnMoO₆ phosphors would have transformed from the tetragonal phase to the (pseudo-)cubic phase, since the splitting of the (220) diffraction peak was not observed. Since the Sr_2−xEu_xZnMoO₆ phosphors were tetragonal phase or (pseudo-)cubic phase, even when the concentration of Eu³⁺ ions was increased to 0.12, this result also proves that the Eu³⁺ ions would have substituted into the sites of Ba²⁺ ions. If the Eu³⁺ ions had substituted into the sites of Zn²⁺ (or Mo) ions, the Sr_2−xEu_xZnMoO₆ phosphors would have revealed other crystalline phases, or more secondary phases would have been revealed in the Sr_2−xEu_xZnMoO₆ phosphors rather than in the double-perovskite features.

Previously, Lin et al. successfully synthesized novel near-UV and blue-excited Eu³⁺, Tb³⁺-co-doped one-dimensional strontium germanate full-color nanophosphors by a simple sol-hydrothermal method. They found that incorporation of the Eu³⁺ and Tb³⁺ ions into strontium germanate resulted in a slight shrinkage of the lattice constants and the unit cell volume because Eu³⁺ and Tb³⁺ had smaller radii than Sr²⁺, indicating the Eu³⁺ and Tb³⁺ ions had been incorporated into the host lattice of SrGe₄O₉ and did not change the crystal structure [24]. Those results also suggested that the Eu³⁺ ions substituted into the sites of Sr²⁺ ions. The 2θ value of the (220) diffraction peak was shifted to a higher value, as the concentration of Eu³⁺ was equal to, or greater than, 0.02. The ion radius of Sr²⁺ is 0.130 nm and the ion radius of Eu³⁺ is 0.1087 nm. Hence, as more Sr²⁺ would have been substituted by Eu³⁺, the ionic radius of the Sr_2−xEu_xZnMoO₆ phosphors would have decreased, increasing the 2θ value of the (220) diffraction peak. The ion radius of Eu²⁺ is 0.131 nm, which is thought to be the same as the ion radius of Sr²⁺. If Eu₂O₃ exists as Eu²⁺ ions, the cell parameters of the Sr_2−xEu_xZnMoO₆ phosphors would not have been changed. The shift of the (220) diffraction peak to a higher 2θ value or the decrease in the cell parameters can prove that Eu₂O₃ existed in the Sr_2−xEu_xZnMoO₆ phosphors and in the Eu³⁺ state.

In order to find the best doping concentration of Eu³⁺, we synthesized a series of Sr_2−xEu_xZnMoO₆ phosphors (x = 0 to 0.12) and measured their emission spectra. The PL emission spectra of Sr_2−xEu_xZnMoO₆ powders excited at a wavelength of 350 nm are shown in Figure 3 for the light wavelength range of 400–650 nm. The spectra in Figure 3 show that that all the excitation spectra of Sr_2−xEu_xZnMoO₆ phosphors (except the Sr₂ZnMoO₆ powder) consisted of two parts: one broad band in the 400–575 nm region and two sharp peaks located at 597 nm and 616 nm. However, for the Sr₂ZnMoO₆ powder, even when the synthesizing temperature was 1200 °C, the two sharp peaks of Sr_2−xEu_xZnMoO₆ phosphors located at 597 nm and 616 nm were not found in the emission spectra. This means that no characteristic peaks were observed in Sr₂ZnMoO₆ powder even when the synthesizing temperature was 1200 °C. Obviously, the broad band in the 400–575 nm region is assignable to the well-known O²–Mo⁶⁺ charge transfer band (CTB) [25]. As Eu₂O₃ was substituted for SrO, the Sr_2−xEu_xZnMoO₆ phosphors had the same spectral profile, but with a different concentration of Eu³⁺ ions, the characteristic peaks were observed.

When the synthesizing temperature was changed from 900 to 1200 °C, the emission spectra of the Sr_2−xEu_xZnMoO₆ phosphors consisted of sharp peaks in two strong bands at 597 and 616 nm, corresponding to the ⁵D₀–⁷F₁ (597 nm) and ⁵D₀–⁷F₂ (616 nm) transitions of Eu³⁺ ions. These results prove that the two emission peaks of the Sr_2−xEu_xZnMoO₆ phosphors, ⁵D₀–⁷F₁ at 597 nm and ⁵D₀–⁷F₂ at 616 nm, were excited by the addition of Eu³⁺ ions. When the synthesizing temperature was raised to 1100 and 1200 °C, the intensity of the broadened emission went from 400 nm to 575 nm, increasing with the increase in the Eu³⁺ ion concentration. However, the emission intensities of the Sr_2−xEu_xZnMoO₆ phosphors were influenced by the synthesizing temperature and the concentration of the Eu³⁺ ions.

In a previous study, the spectra of BaZrO₃ doped with Eu³⁺ powders consisted of a series of resolved emission peaks located at 576 nm, 597 nm, 616 nm, 623 nm, 651 nm, 673 nm, 696 nm, and 704 nm, which are assignable to the ⁵D₀–⁷F_J (J = 0, 1, 2, 3, 4) transitions of Eu³⁺ ions, namely, ⁵D₀–⁷F₀ (576 nm), ⁵D₀–⁷F₁ (597 nm), ⁵D₀–⁷F₂ (616 nm, 623 nm), ⁵D₀–⁷F₃ (651 nm), and ⁵D₀–⁷F₄ (673 nm, 696 nm, and 704 nm) [16]. Liu and Wang’s research also showed that the emission intensities of ⁵D₀–⁷F₀ (576 nm), ⁵D₀–⁷F₁ (597 nm), and ⁵D₀–⁷F₂ (616, 623 nm) had almost the same values, even if the Eu concentration in the BaZr_1−xEu_xO₃ phosphor powders was different [26]. These previous results [6,16], and the results this study, suggest that the transitions of Eu³⁺ ions between different energy bands are affected by the host materials of the prepared phosphors.

As the synthesizing temperature was changed from 900 °C to 1100 °C, the intensities of the two emission peaks of the Sr_2−xEu_xZnMoO₆ phosphors was enhanced by increasing the Eu³⁺ doping concentration and reached a maximum value at x = 0.08. In contrast, the intensities of the two emission peaks of the Sr_2−xEu_xZnMoO₆ phosphors decreased when the Eu³⁺ doping ratio was more than 0.08. This proves that the concentration-quenching effect of Eu³⁺ doping happened in the Sr_2−xEu_xZnMoO₆ phosphors. When 1200 °C was used as the sintering temperature, the intensities of the two emission peaks of the Sr_2−xEu_xZnMoO₆ phosphors increased as the concentration of Eu³⁺ ions rose. The results in Figure 3 also show that when the temperature changed from 1100 °C to 1200 °C, the emission peak varied significantly. For the Sr_2−xEu_xZnMoO₆ phosphors, when the synthesizing temperature was changed from 1100 °C to 1300 °C, the emission peak with the maximum intensity shifted from ⁵D₀–⁷F₁ (597 nm) to ⁵D₀–⁷F₂ (616 nm). Nevertheless, the strong band at 576 nm corresponding to the ⁵D₀–⁷F₀ transition was not observed in the emission spectra.

The maximum emission intensities (PL_max values) of the Sr_2−xEu_xZnMoO₆ phosphors in the transitions of the ⁵D₀–⁷F₁ (597 nm) and ⁵D₀–⁷F₂ (616 nm) peaks are presented in Figure 4 as a function of the synthesizing temperature and Eu³⁺ ion concentration. Those results suggest again that the PL characteristics of the Sr_2−xEu_xZnMoO₆ phosphors were strongly affected by the synthesizing temperature and the concentration of Eu³⁺ ions. As the x value of the Sr_2−xEu_xZnMoO₆ phosphors was smaller than 0.10, the emission intensities of the ⁵D₀–⁷F₁ (597 nm) and ⁵D₀–⁷F₂ (616 nm) peaks first increased, reached a maximum, and then decreased as the synthesizing temperature increased. When the x values of Sr_2−xEu_xZnMoO₆ phosphors were 0.10 and 0.12, the emission intensity of the ⁵D₀–⁷F₁ (597 nm) first increased, then decreased at 1100 °C, and then increased at 1200 °C. The emission intensity of the ⁵D₀–⁷F₂ (616 nm) peaks increased with the increase in synthesizing temperature. These results suggest that 1100 °C is an important synthesizing temperature for Sr_2−xEu_xZnMoO₆ phosphors because the transition of PL properties happens at this temperature, but the reasons for this are not known.

The results in Figure 3 and Figure 4 present an important result regarding Sr_2−xEu_xZnMoO₆ phosphors: the PL_max values of the transition of the ⁵D₀–⁷F₁ (597 nm) critically increased as the synthesizing temperature increased from 900 °C to 1000 °C, then they did not apparently increase as the synthesizing temperature increased from 1000 °C to 1100 °C, as Figure 4a shows. However, for Sr_2−xEu_xZnMoO₆ phosphors with x = 0.04, 0.06, and 0.08, the PL_max values of the transition of the ⁵D₀–⁷F₂ (616 nm) linearly increased as the synthesizing temperature increased from 900 °C to 1100 °C. For Sr_2−xEu_xZnMoO₆ phosphors with x = 0.10 and 0.12, the PL_max values of the transition of the ⁵D₀–⁷F₂ (616 nm) linearly increased as the synthesizing temperature increased from 900 °C to 1200 °C, as Figure 4b shows. When the synthesizing temperature was 1200 °C, the emission intensity of the transition of ⁵D₀–⁷F₂ (616 nm) for Sr_1.9Eu_0.1ZnMoO₆ and Sr_1.88Eu_0.12ZnMoO₆ phosphors was higher than that of ⁵D₀–⁷F₁ (597 nm), as Figure 4 shows.

The transitions of ⁵D₀–⁷F₁ (588 nm), ⁵D₀–⁷F₂ (612 nm), ⁵D₀–⁷F₃ (649 nm), and ⁵D₀–⁷F₄ (695 nm) for the Eu³⁺ ions can be simultaneously observed in the emission spectrum of the Eu³⁺, Tb³⁺-co-doped one-dimensional strontium germinate, full-color nano-phosphors [24]. However, introduction of the Eu³⁺ ions in the Sr_2−xEu_xZnMoO₆ lattice results in disorder. The disorder in the structure will cause transitions in ⁵D₀–⁷F₂ and point defects in the lattice because of differences in the chemical valence and in the ionic radius between Eu³⁺ ions and Sr²⁺ ions. The Eu³⁺ ions could occupy either of the sites of Sr²⁺ or the sites of Zn²⁺ (Mo²⁺) in the Sr_2−xEu_xZnMoO₆ phosphor’s double-perovskite structure. If Eu³⁺ ions occupy a lattice site with a strict center of symmetry, the odd terms of the static crystal field vanish. This will lead to electric dipole transitions being strictly forbidden for purely electric transitions and the transitions of the ⁵D₀–⁷F₁ (597 nm) will predominantly produce the emission.

The Eu³⁺ ions could occupy either the tetrahedral sites of Sr²⁺ or the octahedral sites of Zn²⁺ or Mo⁶⁺ in the Sr_2−xEu_xZnMoO₆ double-perovskite crystal structure. Consequently, as both of these sites are centrosymmetric, electric dipole transitions (⁵D₀–⁷F_j=0,2, only the ⁵D₀–⁷F₂ transition is observed in this study) are forbidden and, as such, should not appear in the spectra. As the synthesizing temperature is 900 °C, the fact that the ⁵D₀–⁷F₂ electric dipole transition at 615 nm prevails over the ⁵D₀–⁷F₁ magnetic dipole transition at 595 nm in the case of Sr_2−xEu_xZnMoO₆ phosphors leads us to the conclusion that the Eu³⁺ ions are generally located in a disordered manner in the Sr_2−xEu_xZnMoO₆ powders [27]. The charge-compensating oxygen vacancies surrounding the Eu³⁺ ions will lead to the deviation from the point symmetry and relaxation of electric dipole transitions selection rules, with the appearance of the ⁵D₀–⁷F₂ transition lines in the spectra [27]. Additionally, the Eu³⁺ luminescent centers contributing to the ⁵D₀–⁷F₂ transition line are probably located at the Sr_2−xEu_xZnMoO₆ particle surface or subsurface and will dominate the emission of Sr_2−xEu_xZnMoO₆ phosphors when the synthesizing temperature is 1200 °C [27]. If Eu³⁺ ions occupy a non-centrosymmetric lattice site, both electric and magnetic transitions are possible, and the non-symmetric site occupancy will be a dominant reason for causing the emission of ⁵D₀–⁷F₂ (612 nm) transition [28].

The definition of fluorescence (or phosphorescence) lifetime is the time for the intensity of a single emission peak to decrease to 1/e (approximately 37%) of its original intensity. The decay curves of Sr_2−xEu_xZnMoO₆ phosphors were obtained at different concentrations of Eu³⁺ (x = 0.04, 0.06, 0.08, 0.010, and 0.12), and Figure 5 shows the intensity decay of the luminescence of Sr_2−xEu_xZnMoO₆ phosphors as a function of the synthesizing temperature, which can be used to calculate the lifetime. The wavelength of the exciting light was 271 nm, and the measured wavelength for the intensity decay was 597 nm (⁵D₀–⁷F₁), because it had the maximum emission intensity, as the synthesizing temperature was 900–1100 °C. Typically, the standard form of the single exponential decay function is:

A(t) = A₀ exp[-(t/τ)]

(1)

where A₀ is the initial population, τ is constant for the decay time , and t is time. Figure 5 shows that the all curves of decay time can be fitted well by a single-exponential function. Figure 5 also shows that if the same synthesizing temperature is used, the observed decay time is almost unchanged even the x value in Sr_2−xEu_xZnMoO₆ phosphor increases from 0.04 to 0.12. Moreover, the decay time is a single exponential function for Sr_2−xEu_xZnMoO₆ phosphors with different concentrations of Eu³⁺ ions and synthesized at different temperatures, because the activator lies in the same coordination environment [29].

From the results shown in Figure 5, the lifetimes of Sr_2−xEu_xZnMoO₆ phosphors, which are compared in

Table 1. Luminescence lifetimes of Sr_2−xEu_xZnMoO₆ phosphors as a function of the synthesizing temperature and concentration of Eu³⁺ ions.

Synthesizing Temperature	X = 0.04	X = 0.06	X = 0.08	X = 0.10	X = 0.12
900 °C	2.05	2.1	2.04	2.04	2.05
1000 °C	2.03	2.03	1.93	1.97	1.99
1100 °C	1.81	1.83	1.78	1.82	1.82
1200 °C	1.65	1.65	1.66	1.66	1.67

, were measured at 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms, when the synthesizing temperature was 900 °C, 1000 °C, 1100 °C, and 1200 °C, respectively. The lifetimes of Sr_2−xEu_xZnMoO₆ phosphors apparently decreased with the increase in the synthesizing temperature. When the synthesizing temperature was 900 °C and 1000 °C, the lifetimes of the Sr_2−xEu_xZnMoO₆ phosphors varied more. When the synthesizing temperature was 1100 °C and 1200 °C, the Sr_2−xEu_xZnMoO₆ phosphors had less variation in their lifetimes. In Figure 6 we use a fitting curve to find the relationship between lifetime and synthesizing temperature; the equation is:

t = −0.0016 × T + 3.543

(2)

where t represents the emitting lifetime of the luminescent material and T represents the synthesizing temperature (°C). In other words, the luminescence lifetime of Sr_2−xEu_xZnMoO₆ phosphors can be obtained by using Equation (2), as the synthesizing temperature is changed from 900 °C to 1200 °C. These results suggest that the luminescence lifetime of Sr_2−xEu_xZnMoO₆ phosphors can be a function of synthesizing temperature, and that the concentration of Eu³⁺ ions not only can affect their emitting intensity but also can affect their luminescence lifetime.

4. Conclusions

For Sr_2−xEu_xZnMoO₆ powders, the 2θ value of the (220) diffraction peak shifted to a higher value and the FWHM value for the (220) diffraction peak decreased as the synthesizing temperature increased. When the x value (the concentration of Eu³⁺ ions) increased from 0 to 0.12 and the synthesizing temperatures rose from 900 °C to 1200 °C, the cell parameters of the Sr_2−xEu_xZnMoO₆ powders were changed from a = b = c/$\sqrt{2}$ = 0.3968 nm to a = b = c/$\sqrt{2}$ = 0.3971 nm and from a = b = c/$\sqrt{2}$= 0.3971 nm to a = b = c/$\sqrt{2}$ = 0.3974 nm (cubic crystal structure). When the synthesizing temperature was 900–1100 °C, the emission intensities of the Sr_2−xEu_xZnMoO₆ phosphors peaked at x = 0.08 and decreased as the concentration of Eu⁺³ ions increased. There was an observable concentration-quenching effect of Eu³⁺ ions in the Sr_2−xEu_xZnMoO₆ phosphors. The emission spectrum of Sr₂ZnMoO₆ powder consisted of one broad band in the 400–575 nm region, and the emission spectra of the Sr_2−xEu_xZnMoO₆ phosphors (except for the Sr₂ZnMoO₆ powder) consisted of two parts: one broad band in the 400–575 nm region and two sharp peaks located at 597 nm and 616 nm. The lifetimes of the Sr_2−xEu_xZnMoO₆ phosphors, which depended on synthesizing temperature and were independent of the concentration of Eu³⁺ ions, were 2.04–2.10 ms, 1.93–2.03 ms, 1.78–1.82 ms, and 1.65–1.67 ms when the synthesizing temperature was 900 °C, 1000 °C, 1100 °C, and 1200 °C, respectively. We found that when the synthesizing temperature was in the range of 900–1200 °C, the relationship between the lifetime and the synthesizing temperature could be determined by the following equation: t = −0.0016 × T + 3.543, where t and T represent the emitting lifetime of the luminescent material and the synthesizing temperature (°C).