1. Introduction

Conversion phosphors emitting a saturated green color are very interesting for white light-emitting diode (LED) applications but are at the same time surprisingly scarce. Both in lighting and imaging, a white phosphor converted LED (pc-LED), composed of a blue pumping (In,Ga)N LED and one (a yellow, for example Y₃Al₅O₁₂:Ce³⁺ [1]) or two (a green and red, for example SrSi₂O₂N₂:Eu²⁺ and Sr₂Si₅N₈:Eu²⁺ [2] or CaAlSiN₃:Eu²⁺ [3]) conversion phosphors are currently used devices. When a violet or a near-ultraviolet pumping LED is used, a blue emitting conversion phosphor is needed in addition.

For lighting, broad emission bands are advantageous for reaching high color rendering, while narrow emission bands are needed in displays. Indeed, displays typically use RGB primary colors which should span a wide color gamut. The RGB primaries should therefore be saturated colors, requiring relatively narrow emission bands. When broadband phosphors are used, obtaining saturated primaries necessitates filtering a significant amount of the emitted light. This considerably lowers the overall device efficiency. Using saturated phosphors for which no color filtering is needed to obtain the correct primary does not only increases the device efficiency, but also increases the attainable color gamut [4].

Saturated green phosphors are also of interest for the construction of green LEDs. The external quantum efficiency of (In,Ga)N diodes decreases considerably with increasing indium content, as required to shift the emission wavelength to the green spectral region [5]. By combining a saturated green phosphor with a violet or blue pumping LED, a more energy efficient green LED is obtained, provided a high conversion efficiency phosphor is available [6]. In a recent publication, Oh et al. described a monochromatic green pc-LED, using a dichroic band-pass filter (BPF) and a green orthosilicate conversion phosphor [7]. Since the color purity of this phosphor was rather poor, a short-pass dichroic filter (SPDF) was added to absorb a significant fraction of the generated light. This issue can be resolved with an alternative green phosphor with a superior color purity.

Recently, six requirements were outlined which a conversion phosphor should fulfill in order to be a suitable candidate for the above-mentioned applications [8]. Besides the appropriate emission color, the phosphor should be optimally excitable with the emission from the pumping LED. Furthermore, the conversion process must occur with high quantum efficiency (QE) in order to maximize the overall efficiency of the final device. Moreover, the thermal quenching should be sufficiently low to prevent a color drift of the LED when the phosphor heats during operation.

In Table 1, an overview of green phosphors in recent literature is presented. Phosphors which cannot be efficiently excited with blue or violet light, such as Tb³⁺ doped phosphors, were omitted. From this table, it is clear that the number of saturated green phosphors (with 50 nm or less full width at half maximum (FWHM) of the emission band) is very limited. Next to SrGa₂S₄:Eu²⁺ and β-SiAlON:Eu²⁺, no phosphor is known that exhibits a sufficiently narrow emission band, combined with a high QE.

Table 1. Overview of luminescent properties of green phosphors from recent literature. T_0.5 describes the thermal quenching. It is the temperature at which the emission intensity is 50% of the value at low temperature. QE (int/ext) is the internal/external quantum efficiency.

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Europium doped zinc thiogallate, ZnGa₂S₄:Eu²⁺ is, like every Eu²⁺ activated phosphor, expected to be a broadband emitter due to the 4f⁶5d → 4f⁷ transition that is responsible for the luminescence. The emission band of this material has been reported to be rather narrow, yielding a saturated green color [21–25]. In this paper, we investigate whether ZnGa₂S₄:Eu²⁺ has the same advantageous properties as the chemically similar SrGa₂S₄:Eu²⁺ (Table 1). The particular motivation for this work is the contradiction between a number of reports which have been recently published on ZnGa₂S₄:Eu²⁺ phosphors.

The photoluminescence of ZnGa₂S₄:Eu²⁺ was first described by Yuta and White [21], reporting broadband emission at 535 nm and a Stokes shift of 0.78 eV. Kim and Kim elaborated on the photoluminescence (PL) and cathodoluminescence (CL) of ZnGa₂S₄:Eu²⁺ and (Ca,Zn)Ga₂S₄:Eu²⁺ mixtures [22, 26]. The synthesis parameters of the solid-state reaction yielding the powders were optimized and a saturated green emission at 540 nm was reported. The lack of efficient CL was ascribed to the small particle size of the phosphor powders. By adding only a small amount of calcium to the mixture, the emission band at 540 nm disappeared abruptly and a new emission band was formed at longer wavelengths (> 550 nm). Recently, Yu et al. reported the thermal properties of ZnGa₂S₄:Eu²⁺. Herein, it was found that this material has a relatively low thermal quenching temperature, but it remains a potential candidate for LED applications because of the favorable excitation spectrum [24]. Reported key parameters about ZnGa₂S₄:Eu²⁺ (at room temperature) are an emission band at 540 nm (FWHM of 50 nm), a luminescence lifetime ranging from 126 to 79 ns, depending on the europium concentration and a quenching temperature T_0.5 of 407 K. T_0.5 was defined as the temperature where the emission intensity dropped to half the intensity value at 300 K, which is somewhat in contrast to the common approach to comparing the intensity to the value at low temperature, where no thermal quenching is noticeable. In [25], luminescence in ZnS-ZnGa₂S₄:Eu²⁺ mixed compounds was explained by energy transfer from ZnS:Eu²⁺ to ZnGa₂S₄:Eu²⁺. In contrast to the above reports, an anomalously broad emission at 565nm has been reported in [27]. Up to now, no value of the quantum efficiency of ZnGa₂S₄:Eu²⁺ has been reported.

Additionally, there is still discussion about the position of the Eu²⁺ ions in the zinc thiogallate lattice. Due to the size mismatch between the Eu²⁺ and Zn²⁺ ions, one can expect a difficult incorporation of the europium ions in the zinc-based host material. The ions could (1) substitute for the zinc ions, (2) occupy the tetrahedral vacancy sites in the lattice, or (3) substitute at octahedral voids of the host lattice. Yuta and White proposed an energy scheme for Eu²⁺ in ZnGa₂S₄, based on tetrahedral coordination [21]. The difficult incorporation of Eu²⁺ on small tetrahedral coordinated sites was already drawn to attention by Wickleder et al. [28]. They argued that the octahedral voids are favored, based on the 4f⁶5d → 4f⁷ transition energy and the observation that no europium doped compounds exist where the doping ion is fourfold coordinated as would be the case on the zinc or vacancy sites. However, no direct proof for any of the europium positions has been given yet [28, 29].

The hypothesis which is explored in the present paper is that the observed luminescence originates from small amounts of unintentionally formed EuGa₂S₄. This stoichiometric phosphor is known to exhibit efficient green luminescence, centered around 545 nm [12]. This is surprisingly similar to the reported values for the ZnGa₂S₄:Eu²⁺ phosphors. In EuGa₂S₄, the europium ions are eightfold coordinated by sulfur ions.

In this paper, the internal and external quantum efficiencies and the other vital properties of the luminescence of ZnGa₂S₄:Eu²⁺ are characterized and discussed, to complement the previously published data on this phosphor. Next, the suitability of ZnGa₂S₄:Eu²⁺ as a conversion phosphor for lighting and display applications is evaluated. Emphasis is put on the microscopic structure of the phosphor, both at the level of individual phosphor particles and at the atomic level, to get a thorough understanding of the doping of Eu²⁺ into ZnGa₂S₄.

2. Experimental

Zn_1-xEu_xGa₂S₄ powders with europium concentrations of x = 0.01, 0.02, 0.03, 0.04, 0.06, 0.08 and 0.10 were synthesized by a solid state reaction between ZnS (Alfa Products, 99.9%), Ga₂S₃ (Alfa Aesar, 99.99%) and EuF₃ (Alfa Aesar, 99.5%). Stoichiometric quantities of these starting materials were mixed and heat treated at 1000 °C for 2 hours in alumina crucibles under a H₂S flux. After the heat treatment, the powders cooled naturally and were lightly ground. Pure EuGa₂S₄ was synthesized in a similar fashion, however using EuS instead of EuF₃ and without ZnS.

Photoluminescence (PL) emission and excitation spectra were measured with an Edinburgh FS920 fluorescence spectrometer. Decay profiles were collected with an intensified CCD (Andor DH720), combined with a pulsed LED (peak wavelength of 400 nm, pulse frequency of 10 kHz) as excitation source. Measurements as a function of temperature were performed using an Oxford Optistat CF cryostat.

Diffuse reflectance spectra were measured with a Varian Cary 500 spectrophotometer, equipped with an internal integrating sphere. External quantum efficiencies were obtained inside the Edinburgh fluorescence spectrometer, using a phosphor with known QE as a standard (SrSi₂O₂N₂:Eu²⁺ with an internal QE of 90% [15]). Subsequently, the absorption of the phosphors was measured inside an integrating sphere. From this data, the internal quantum efficiency of the samples was calculated.

SEM-EDX-CL measurements were performed on a Hitachi S-3400N scanning electron microscope (SEM), equipped with an energy dispersive X-ray (EDX) detector (Thermo Scientific Noran 7). The cathodoluminescence (CL) light was captured in an optical fiber and analyzed by a CCD (Princeton Instruments ProEM 1600²), attached to a spectrograph (Princeton Instruments Acton SP2358).

The crystal structure of the powders was obtained with X-ray diffraction (XRD), using Cu Kα radiation on a Siemens D5000 diffractometer (40 kV, 40 mA).

EXAFS (extended x-ray absorption fine structure) measurements were performed at the Eu L_III edge and at the Zn K edge at the Dubble beamline (BM26A) at the ESRF synchrotron facility in Grenoble (France). A silicon (111) double crystal monochromator was used and all EXAFS data were recorded in fluorescence mode [30]. The EXAFS data reduction and analysis were performed by fitting the data to theoretical standards generated from the FEFF6 code 16 using the Athena and Artemis software by Ravel and Newville [31].

3. Results and discussion

3.1 Photoluminescence and diffuse reflectance spectra

The photoluminescence emission and excitation spectra of the powders with dopant concentrations up to x = 0.10 were measured at room temperature. Both the position of the maximum (at about 542 nm) and the full width at half maximum (FWHM, about 50 nm) of the emission peak show no significant dependence on the europium concentration. The spectra of Zn_0.99Eu_0.01Ga₂S₄, which are also representative for all other doping concentrations, are shown in Fig. 1. We refer to §3.2 for a comparison of the absolute intensity of the luminescence. The emission band of the phosphor corresponds with a saturated green color (CIE x = 0.31, y = 0.66) which exceeds the green color point of the EBU gamut.

 

Fig. 1 Top: Diffuse reflectance spectra for ZnGa₂S₄ (x = 0.00), Zn_0.99Eu_0.01Ga₂S₄ (x = 0.01) and EuGa₂S₄ (x = 1.00) at room temperature. Bottom: PL emission (at 450 nm excitation, solid lines) and excitation (at 540 nm emission, dashed lines) spectra for Zn_0.99Eu_0.01Ga₂S₄ (x = 0.01) and EuGa₂S₄ (x = 1.00) at room temperature, normalized for better readability. The dip in the excitation spectra around 470 nm is an artifact.

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Both the emission and excitation spectra of the powders are very similar to the spectra of the stoichiometric phosphor EuGa₂S₄ which are also included in the figure. This is surprising given the very different structure of the ZnGa₂S₄ and EuGa₂S₄ host crystals. The difference in the excitation spectra below 420 nm can be explained in terms of interband absorption in ZnGa₂S₄. The absorbed energy is not transferred to Eu²⁺ ions and is dissipated non-radiatively.

Diffuse reflectance spectra are also included in Fig. 1, for Zn_0.99Eu_0.01Ga₂S₄, EuGa₂S₄ and undoped ZnGa₂S₄. The steep increase in the ZnGa₂S₄ absorption around 400 nm is due to interband absorption of the host material. The bandgap of ZnGa₂S₄, 3.4 eV, was obtained from a fit to the Kubelka-Munk spectrum, calculated from the reflection spectrum. This is in correspondence with the reported value of 3.22 eV [28]. EuGa₂S₄ starts to absorb light from ± 520 nm due to electronic excitation towards the 5d levels of Eu²⁺.

3.2 Quantum efficiency

The internal quantum efficiency considers only the conversion process after a photon is absorbed, the external quantum efficiency also takes the absorption into account:

 

Fig. 2 Absorption (fraction of the incident photons that are absorbed), internal and external quantum efficiency for Zn_1-xEu_xGa₂S₄ at room temperature, measured at 460 nm excitation.

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3.3 Decay and thermal behavior

The decay profiles of the samples were measured at different temperatures with the pulsed excitation from an LED emitting at 400 nm. A luminescent lifetime was determined by fitting the initial decay with a single exponential function. The lifetime of 127 ± 24 ns did not change as a function of the europium concentration. This is a remarkably short luminescent lifetime for 4f⁶5d → 4f⁷ transitions in Eu²⁺. In comparison, the luminescent lifetime of other europium doped thiogallate and thioaluminate phosphors are given in Table 2. Emission maxima are also displayed because of the intrinsic dependence of the lifetime on the emission color, shorter wavelengths corresponding with shorter lifetimes (if the refractive indices are assumed to be similar) [32]. At low temperature (77 K), the lifetime increased to ± 350 ns. The other, more efficient thioaluminate and thiogallate phosphors do not show a change in lifetime as a function of temperature unless thermal quenching sets in [33].

Table 2. Emission maxima (λ_max) and full width at half maximum (FWHM), luminescent lifetimes (τ) and quenching temperatures (T_0.5) of multiple Eu²⁺ doped thiogallate and thioaluminate phosphors. The displayed parameters were recorded at room temperature.

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The integrated emission intensity of the Zn_1-xEu_xGa₂S₄ powders was monitored as a function of temperature (Fig. 3). Common LED phosphors show a more or less flat profile of the emission intensity as a function of temperature until a sudden steep drop at a well-defined activation temperature [38]. This activation temperature is associated with an energy barrier that, when crossed, leads to the non-radiative depopulation of the excited state. It is notable that the emission intensity does not show a stable region as a function of temperature, but decreases over the entire temperature range. This is a common behavior for stoichiometric phosphors [13, 39]. The quenching temperature, defined as the temperature where the emission intensity is half of the intensity at low temperature (here, 100 K), is about 240 K for x = 0.06. The obtained result is in accordance to the quenching temperature of 407 K as reported in [24], taking into account that the reference level was then taken at 300 K. No significant influence of the doping concentration on the decay or thermal quenching behavior was observed.

 

Fig. 3 Normalized PL intensity and luminescent lifetimes as function of temperature for Zn_0.94Eu_0.06Ga₂S₄ upon excitation at 400 nm.

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From these measurements, we can state that the luminescence of these phosphor powders is essentially quenched over the whole studied temperature range, and is characterized by an unusual short luminescent lifetime.

The temperature dependence of the corresponding time constant is very similar to that of the non-radiative decay path, as identified in SrGa₂S₄:Eu²⁺ [11]. For the latter phosphor, this decay path is only relevant at high doping concentrations or temperatures above 400 K. It was found that in this phosphor, which in contrast to ZnGa₂S₄ is isostructural with EuGa₂S₄, the environment of Eu locally resembles that of EuGa₂S₄ due to clustering of europium [11].

The luminescent lifetime for EuGa₂S₄ was evaluated to be 116 ns (at 300 K). Within the error range, this is the same as the decay time of the ZnGa₂S₄:Eu²⁺ powders. The thermal quenching profile of EuGa₂S₄ was measured by Iida et al. [12]. Their result is very similar to the profiles we obtained for ZnGa₂S₄:Eu²⁺.

This luminescence characterization of Zn_1-xEu_xGa₂S₄ powders indicates that the light emission most likely originates from EuGa₂S₄ impurities. The PL emission and excitation spectra, quantum efficiency, thermal behavior and decay dynamics can be explained with this conjecture. In the next part, structural analysis will be applied to confirm this finding.

3.4 Luminescence on a microscopic scale

By the combined mapping of cathodoluminescence and characteristic X-rays in a scanning electron microscope setup, local variations in the chemical composition of a powder sample can be related to differences in luminescent properties [40]. In Fig. 4, a SEM-CL-EDX mapping of the Zn_0.99Eu_0.01Ga₂S₄ powder is displayed. In the EDX map, the areas which are colored red (green) are the result from the mapping on Eu (Zn). One can clearly discern EuGa₂S₄ grains, where no Zn is detected, among the majority of ZnGa₂S₄ grains, where no Eu is detected. The CL map shows that the characteristic green luminescence is indeed originating from the EuGa₂S₄ grains, due to the perfect correlation between the CL and the EDX maps. No light output is detected from the ZnGa₂S₄ phase. This shows that the presence of EuGa₂S₄ in the synthesized phosphors cannot be ignored from a luminescence point of view.

 

Fig. 4 SEM-CL-EDX mapping of Zn_0.99Eu_0.01Ga₂S₄ powder, obtained at 250 K. Top left: phosphor morphology obtained by backscattered electron imaging. The integrated CL emission intensity is shown as an overlay in false colors. Top right: elemental distribution by EDX, where the colors are determined by color coding with green for Zn and red for Eu. Simultaneous detection would lead to a yellow color. Bottom: CL emission spectra for the points indicated in the top right figure, obtained in clockwise direction.

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There are two possible reasons for the absence of luminescence in the ZnGa₂S₄ grains. First, it is possible that no europium is incorporated in the ZnGa₂S₄ lattice at all. This is plausible due to the size mismatch between the Zn²⁺ and Eu²⁺ ions (88 pm versus 131 pm for sixfold coordination [41]).

Secondly, incorporation of europium ions in the host lattice does not necessarily cause light emission [42]. It would not be surprising if Eu in ZnGa₂S₄:Eu²⁺ is not luminescent. When ZnGa₂S₄ doped with europium would–coincidentally–have a very similar emission spectrum as EuGa₂S₄, then the absorption energy is estimated to be 2.4 eV, for the transition between the 4f ⁷ ground state and the lowest 4f ⁶5d excited state of europium. This absorption energy is the value where the PL excitation spectrum amounts to 20% of its maximum value [43]. Given the small bandgap of this host material (3.2-3.4 eV), the excited states will probably overlap with – or be in close distance of – the conduction band of the ZnGa₂S₄ host, totally quenching any europium activated luminescence [42, 43].

One could also argue that the absence of CL does not automatically imply the absence of PL. Although there can be a noticeable difference between the efficiency of CL and PL, a complete absence of cathodoluminescence for a lanthanide doped inorganic crystal is not expected [44].

3.5 X-ray diffraction

The XRD patterns show that crystalline powders are obtained, corresponding with the defect-stannite structure of ZnGa₂S₄ (Fig. 5) [45, 46]. However, only a limited amount of europium can apparently be dissolved in the ZnGa₂S₄ lattice without clustering, as diffraction peaks originating from a EuGa₂S₄, having a different crystal structure, are observed. Careful inspection of earlier reports on XRD patterns of ZnGa₂S₄:Eu²⁺, also suggests the presence of EuGa₂S₄ as an impurity phase [22, 25, 47].

 

Fig. 5 X-ray diffraction patterns for Zn_1-xEu_xGa₂S₄ powders, compared with reference patterns for ZnGa₂S₄ (ICSD 69542) and EuGa₂S₄ (ICSD 8053) [45, 47].

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The emerging of the EuGa₂S₄ (4 4 2) peak at 24.1° is studied, together with the ZnGa₂S₄ (1 1 0) peak at 23.8° to estimate the dependence of the ZnGa₂S₄ – EuGa₂S₄ fractional content on the doping concentration. The separate peak intensities, I, were calculated by a fitting procedure in the 2θ range from 23° to 25° with three pseudo-voigt peak shapes. The third peak is the less intense EuGa₂S₄ (4 4 0) peak at 24.4°. The pseudo-voigt profile is calculated as a linear combination of a Gaussian and a Lorentzian peak shape [48].

In Fig. 6, the values of

 

Fig. 6 y(x), as defined in Eq. (2).

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3.6 Incorporation of Eu²⁺ in ZnGa₂S₄

X-ray absorption spectroscopy is ideally suited to study both the valence state and dopant incorporation in polycrystalline inorganic materials. The valence state can be probed by means of XANES (x-ray absorption near edge spectroscopy) [49], while EXAFS gives details on the first coordination spheres of the element under investigation [50].

Figure 7 shows the Eu L_III edge XANES spectrum of Zn_0.96Eu_0.04Ga₂S₄ compared with those of Eu₂O₃ and EuS, measured as reference compounds for Eu³⁺ and Eu²⁺, respectively. The shape of the spectrum of Zn_0.96Eu_0.04Ga₂S₄ is almost the same as that for EuS having a sharp single peak at 6972 eV and different from that for Eu₂O₃ having a single peak 8 eV higher in energy. This result shows that most of Eu atoms in Zn_0.96Eu_0.04Ga₂S₄ are divalent. This implies that the europium, which is doped in a trivalent state through EuF₃ is effectively reduced during the heat treatment in the H₂S atmosphere.

 

Fig. 7 Eu L_III XANES spectrum of (top) Zn_0.96Eu_0.04Ga₂S₄, compared with the spectra of reference compounds for Eu²⁺ and Eu³⁺ (bottom), EuS (solid line) and Eu₂O₃ (dashed line).

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The extracted EXAFS spectra, k²χ(k) of the Eu L_III edge and the Zn K edge in ZnGa₂S₄:Eu²⁺ (4%) are shown in Fig. 8. This figure already suggests that the Eu ions do not occupy the Zn²⁺ sites in the lattice, but are more likely to be present in the powders as EuGa₂S₄.

 

Fig. 8 Top: Zn K edge extracted EXAFS spectrum, Middle: Eu L_III edge extracted EXAFS spectrum, both in Zn_0.96Eu_0.04Ga₂S_, Bottom: Eu L_III edge extracted EXAFS spectrum in EuGa₂S₄.

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To investigate the structure around dopant ions in detail, FEFF simulations were carried out. In the first step, the EuGa₂S₄ powder was fitted using the known crystallographic structure [47]. In this way the Debye-Waller parameters were determined.

First, a simulation with only EuGa₂S₄ was performed. The correspondence with the experimental spectrum was already very good. This indicates that the true composition of the powders does not deviate far from this model and the majority of the europium occurs as EuGa₂S₄, as already established with SEM-CL-EDX and XRD.

Secondly, the simulation where a small amount of the Eu²⁺ ions substitute on the tetrahedral coordinated zinc sites showed a bigger deviation from the experimental spectrum. This is not surprising considering the limited volume of the Zn first coordination sphere and the size mismatch between Zn²⁺ and Eu²⁺. This clearly confirms the hypothesis by Wickleder et al. that Eu²⁺ is not substituting for Zn²⁺ [28].

Thirdly, a simulation with a small amount of the Eu on the octahedral voids in ZnGa₂S₄ and the majority in the form of EuGa₂S₄, was performed. The simulated Fourier transforms of Eu L_III edge EXAFS are shown in Fig. 9, compared with experimental data. In this case, the Fourier transforms were performed in the k range of [2.1 Å⁻¹ – 7.8 Å⁻¹]. As can be seen in the figure, there is a very good correspondence between the two. The volume of the octahedral voids in the ZnGa₂S₄ structure is ~24 ų, being much larger than the volume of the coordination tetrahedron of the Zn site, but still significantly smaller than the volume of the coordination octahedron of other sulfide hosts for Eu²⁺ (~31 ų for CaS, ~36 ų for SrS [51]). The simulation also showed that the closest zinc neighbor of the incorporated europium ions disappears. This observation can be explained by charge compensation: the europium ions are divalent as well as the zinc ions. Furthermore it is observed that some of the sulfuric atoms move away from the dopants. This effect can be caused by the vacancy that occurs at the cationic position in the lattice. A crystal model of this simulation is displayed in Fig. 10.

 

Fig. 9 Fourier transform of$k^2\chi (k)$to radial distance (r) space for a Zn_0.92Eu_0.08Ga₂S₄ powder (solid line) and the result of the simulation, based on the structure of EuGa₂S₄ and the europium ions occupying the octahedral voids (circles).

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Fig. 10 Left: Crystal structure of ZnGa₂S₄ where an Eu²⁺ dopant occupies an octahedral void (sixfold coordinated). The neighboring zinc ion is removed to compensate for the excess positive charge [45]. Right: Crystal structure of EuGa₂S₄, with (slightly distorted) square antiprismatic coordination polyhedra for Eu²⁺ (eightfold coordinated) [47].

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4. Conclusions

In this paper, green zinc thiogallate phosphors with different doping concentrations were investigated. The luminescent properties of these materials were described in detail. Furthermore, the structure of the phosphor was investigated and the distribution and incorporation of europium in the powders was resolved with XRD, SEM-CL-EDX and XAS measurements. With these complementary techniques, we were able to explain the anomalous luminescent properties of these phosphors.

The emission color of this phosphor is saturated green. The material can be excited with a broad range of blue and violet wavelengths. However, the quantum efficiency of this material is far too low for any application. Furthermore, the thermal quenching is excessive. Since desirable photoluminescence properties should always occur simultaneously with low thermal quenching and high quantum efficiency, this material should be discarded for application as conversion phosphor, despite earlier reports.

Analysis of the XRD and EXAFS experiments proved that only a limited amount of europium – if any - can be incorporated into the ZnGa₂S₄ lattice. If these ions are incorporated, they occupy octahedral voids instead of the smaller zinc or vacancy sites with tetrahedral coordination. The neighboring zinc ion is then removed to compensate for the excess positive charge. The majority of the europium ions form the EuGa₂S₄ phase.

Scanning electron microscope measurements, combined with CL and EDX allowed to link light output of the powder particles to their composition. It was found that the green luminescence which is observed by different authors is coming from the small fraction of the stoichiometric EuGa₂S₄ phosphor that is formed. The majority of the powder particles, consisting of the ZnGa₂S₄ phase, do not show any light emission.