We report on photoluminescence from Eu3+-activated Na,Y-silicates of the Na3YSi3O9 type. The effect of structural environment on Eu3+-related emission is considered by comparing a glassy reference host to its crystalline correspondents which are obtained through crystallization of the super-cooled melt. Crystallization is accompanied by a quantitative increase in quantum yield and external efficiency of photoluminescence. At the same time, strong Stark splitting of the emission bands and significant changes in the spectral symmetry of photoemission with a notable relative enhancement of the 5D0 → 7F4 relaxation are observed. The results indicate that Eu3+ ions partition on Y3+ lattice sites which undergo changes in coordination, volume and symmetry when moving from glassy to crystalline environment. Potential application of such Eu3+-activated Na3YSi3O9 silicate glass ceramics in light converters is considered.
© 2014 Optical Society of America
Photoluminescence (PL) from Eu3+-activated glasses and glass ceramics is characterized by various sharp PL bands which result from the intra-configurational 4f ↔ 4f parity-forbidden electronic transitions from 5D0 to 7FJ (J = 0, 1, 2, 3, 4) [1–6]. The local environment around Eu3+ does not affect 5D0 →7F1 PL, but the probability of 5D0 →7F2 relaxation is strongly affected by the ligand symmetry. As a consequence, Eu3+ dopants can also be used as sensitive probes of the structural symmetry in the vicinity of Eu3+ [1,2]. Technical applications rely on the broad absorption band of Eu3+ in the spectral region of 350–420 nm, associated with a relatively high conversion efficiency to the red spectral range, which have made Eu3+ (and its divalent counterpart) one of the most prominent optical activators in phosphors, e.g., for light conversion [7–14]. In this context, glasses as host materials offer the advantages of very high compositional flexibility, simple and low-cost manufacture and versatile forming technologies, superior thermal stability and an epoxy-free assembly process [3,4, 15–20]. This, however, usually goes at the expense of absorption cross section and quantum efficiency of luminescence, what results in potentially low quantum yield. Controlled crystallization of the supercooled liquid to generate a glass ceramic material where one or more crystalline phases are deposited in a matrix of residual glass may be employed to overcome this problem: the presence of crystallite species may, on the one side, provide interfaces for multiple scattering of the incoming light which improves luminescence yield. On the other side, the crystallization procedure and the precipitated crystalline phase can be chosen in such a way that during crystallization, the active ion at least partially partitions into the crystalline matrix to attain the spectral properties of a polycrystalline phosphor. The type of the precipitated crystallite species is typically adjusted by choosing the appropriate chemical composition of the precursor glass. The partitioning of Eu3+ ions is determined primarily through the relation of ionic radii, valence states and coordination numbers of available lattice sites. An increase in internal quantum efficiency (ηQE) can be obtained when the lifetime of the excited state is increased and the probability of non-radiative energy transfer is decreased [11,12]. However, such glass ceramics which are suitable as a host material for Eu3+ can be obtained only from a rather limited number of chemical systems.
Here, we report on Eu3+-activated Na3YSi3O9. Polycrystalline materials of this type have previously been demonstrated as efficient red-emitting phosphors by various authors [21–23]. We now discuss an Eu3+-activated Na,Y - silicate glass and the corresponding glass ceramic, focusing on the changes in spectroscopic properties which are associated with the transition from glass to glass ceramic. We then consider this material as a promising alternative for photoluminescent light conversion to the deep red spectral range.
Precursor glass samples with nominal compositions (in mol%) 30Na2O-8Y2O3-53SiO2-5Al2O3-2TiO2-2P2O5-0.5Eu2O3 (NYSA) were prepared by conventional melting of 50 g batches of analytical grade reagents Na2CO3, Y2O3, SiO2, TiO2, NH4H2PO4 and Eu2O3, where NH4H2PO4 acts as an oxidizing agent to prevent the formation of divalent europium. The thoroughly mixed raw material mixtures were melted at 1600 °C in ambient air for 2 h, using alumina crucibles. Glass slabs were obtained after pouring the melt into a graphite mould which was preheated to 500 °C. The molded glass was subsequently annealed at 600 °C for 2 h to relax residual thermal stress. From the slabs, disks of 15 × 15 × 1.5 mm3 were cut and polished on both sides for further investigation. Crystallization experiments were conducted on individual disks which were placed on alumina substrates and isothermally annealed at 800 °C for time spans between 4 and 16 h (in ambient air). The selection of this crystallization temperature was done in accordance with the results of non-isothermal differential scanning calorimetry (DSC, Netzsch, Ar atmosphere) which was conducted at a heating rate of 20 K/min. To identify and further describe the crystalline phases which occurred during crystallization, X-ray diffractometry (XRD, Siemens Kristalloflex D500, Bragg-Brentano, 30 kV/30 mA, Cu Kα) was employed (using a step width of 0.02°/s over 2θ range of 10-60°). Static and dynamic PL were studied with a high-resolution spectrofluorometer and through time correlated single photon counting (TCSPC, Horiba Jobin Yvon Fluorolog FL3-22) using a static Xe lamp (450 W) and a Xe flashlamp (75 W) as excitation sources. Photoluminescence excitation (PLE) spectra were corrected over the lamp intensity with a silicon photodiode and PL spectra were corrected by the spectral response of the employed PMT. CIE color coordinates were subsequently determined from the integrated PL spectra. The external quantum efficiency ηeQE, i.e., the number of PL photons divided by the number of PLE photons  was obtained from three individual measurements using a BaSO4-coated integration sphere,
3. Results and discussion
Figure 1(a) shows a DSC curve of the Eu3+-activated NYSA glass after baseline correction. The onset temperature of glass transition Tg and two non-isothermal crystallization peak temperatures Tc1 and Tc2 were identified at 635.5 ± 0.5, 818.6 ± 0.5 and 905.7 ± 0.5 °C, respectively. The appearance of two crystallization peaks indicates that at least two crystalline phases precipitate at elevated temperature. Based on this temperature, we have chosen the annealing temperature of 800 °C which lies close to Tc1 and only in the onset of the second crystallization step. We may readily assume that for different time spans of treatment (4-16h), the crystallization step which corresponds to Tc1 is always completed, and the step which corresponds to Tc2 is partially completed for the lower annealing times, and fully completed for the 16 h treatment. The difference in the glass ceramics which is obtained through the different annealing times therefore arises from different phase assemblage and different stages of crystal growth (i.e., different crystallite sizes).
Figure 1(b) represents XRD patterns of the NYSA precursor glass and the corresponding glass ceramic after 16 h of annealing at 800 °C. The purely glassy state of the precursor material to within the limitations of laboratory XRD is confirmed by the absence of any sharp diffraction peaks. Thermal annealing leads to the appearance of multiple diffraction peaks. Due to the structural similarity within the system of NaxYSiyO1/2x + 3/2 + 2y, accurate phase assignment is rather complicated. The following indexing was therefore done to obtain a simplified experimental overview. That is, we assigned the observed diffraction peaks to two primary NaxYSiyO1/2x + 3/2 + 2y-type-crystalline species, (x = 1;y = 1) and (x = 3; y = 3). The corresponding structures are described on JCPDS card no. 00-022-1049 (NaYSiO4) and JCPDS card no. 00-036-0127 (Na3YSi3O9). We further observe another minor crystalline phase, NaAlSiO4 (JCPDS card no. 00-011-0220). The crystal structures of all three phases are depicted in Fig. 1(c)-(e). Orthorhombic NaYSiO4 belongs to the space group Pbn21 (33) and is composed of [YO6] and [NaO6] octahedra and [SiO4] tetrahedra . Orthorhombic Na3YSi3O9 belongs to space group P212121 (19), where Na+ and Y3+ again occupy octahedral lattice sites, while Si4+ is fourfold coordinated . Hexagonal NaAlSiO4 has a nepheline-type structure which belongs to space group P63 (173) and comprises [NaO6] octahedra and [AlO4] and [SiO4] tetrahedra .
Duffy’s optical basicity Λ of the glass provides a simplistic means to evaluate the oxygen field strength and to estimate the prevailing redox equilibrium between Eu2+ and Eu3+ [28–30]. It is obtained through linear mixing of the partial molar basicity Λi of each component i according to its molar fraction Xi,31]. We may hence assume here that europium exists predominantly in its trivalent form. This assumption will be confirmed in the following consideration of luminescence data.
Figure 2(a) shows PLE spectra (monitoring the electric-dipole transition 5D0 →7F2 of the Eu3+ at 612 nm in the range of 300–500 nm) of the as-melted Eu3+-activated NYSA glass and corresponding glass ceramics. The inset of Fig. 2(a) is the energy diagram of Eu3+ ions, shown to illustrate the origin of the PLE and PL lines. The obtained PLE spectra cover a broad spectral region, i.e., 360–470 nm and consist of five sharp peaks centered at 361, 382, 394, 415 and 464 nm which are readily attributed to the intrinsic intra-configurational 4f ↔ 4f transitions from the ground state (7F0) to the labeled excited states of Eu3 . The position of the excitation peaks remains unaffected by crystallization. However, with thermal annealing of the sample, the bands sharpen notably and their intensity generally increases. Furthermore, the transition of 7F0 →5L6 (with a full width at half maximum FWHM ~3 nm) exhibits the highest excitation intensity of all bands and for all samples, regardless of annealing time. Consequently, the peak of this band was selected to record the Eu3+-related PL spectra.
In Fig. 2(b), the corresponding PL spectra are shown. They range from orange to the far red spectral region (i.e., from 575 to 720 nm) and consist of the five lines which are typical for Eu3+-related photoluminescence, i.e., the intra-configurational 4f ↔ 4f transitions from the first excited level 5D0 to the indicated crystal field split levels 7FJ (J = 0, 1, 2, 3 and 4) (also shown in the inset of Fig. 2(a). The three major emission bands are located at 591 nm (magnetic-dipole transition 5D0 → 7F1), 612 nm (electric-dipole transition 5D0 →7F2) and 702 nm (5D0 →7F4). The broad PL band which would be typical for Eu2+ is not detected in any of the samples, confirming the absence of Eu2+ within detection limits .
As expected, thermal annealing of NYSA glass samples results in significant changes of PLE as well as PL spectra. Firstly, for short annealing time, i.e., ≤ 4 h, the red PL line at 612 nm (5D0 →7F2) is dominating the emission spectrum. For longer annealing (≥ 8 h), the bands at 591 and 702 nm strongly increase in intensity relative to the band at 612 nm (Fig. 2(b) and 3(a)). A second observation is that with increasing annealing time, also the absolute intensity of all PL bands increases notably, which is consistent with the PLE observation. That is, the peak PL intensity at 612 nm and the integrated PL intensity over the whole spectrum increase ~11-fold and ~20-fold, respectively, after crystallization (Fig. 2(c)). As mentioned before, this enhancement of PL as well as PLE intensity may be caused by either multiple scattering, increased absorption cross-section or enhanced lifetime of the excited state (increased internal quantum efficiency and reduced probability of non-radiative energy transfer as a result of the incorporation of Eu3+ into the crystalline environment). Thirdly, as a result of crystallization, all PL lines become sharper and components are better resolved. For example, the FWHM of PL line at 712 nm is narrowed from 10 to 4 nm after crystallization. Likewise, the other PL lines show a similar change as a function of annealing time.
In the glass ceramic samples, all PL lines exhibit strong Stark-splitting (702 nm into 702 and 688 nm contributions, 652 nm into 652 and 657 nm, 612 nm into 614 and 624 nm and 591 nm into 589 and 593 nm, see Fig. 2(b) and 3(a)). This observation of general sharpening and well-resolved splitting is a strong indication for the incorporation of Eu3+ species into a crystalline environment.
Besides PL from the first excited Eu3+:5D0 level, only very weak PL lines from the higher levels of 5DJ (J = 1, 2, 3) are observed in the glass sample. This is related to strong non-radiative relaxation of the 5DJ (J = 1, 2, 3) levels through a fast phonon-assisted process (inset of Fig. 2(b)) . However, after crystallization, the PL lines which are attributed to the higher lying excited levels (with peaks at 511, 537 and 553 nm) are strongly enhanced (inset of Fig. 2(b)).
Chromaticity coordinates of all samples were calculated from the PL spectra. For the glass sample, the emission color is (0.655, 0.345), i.e., pure red. These coordinates do not change notably upon crystallization, what is surprising on the first view. That is, the photoemission occurs in pure red color for the glass as well as for the glass ceramics, and can readily be observed with the naked eye.
As mentioned in the introduction section, the electronic-dipole-allowed transition of 5D0 →7F2 depends strongly on the structural symmetry of the Eu3+ ligands. At the same time, the magnetic-dipole allowed transition of 5D0 →7F1 is independent on local environment. Thus, the PL intensity ratio of both transitions can be used as a measure to evaluate the ligand symmetry of the Eu3+ sites. In general, a low value of R represents a high ligand symmetry and a low bond covalency of Eu3+ sites. As for the present case, a value of R = 4.8 is obtained for the NYSA glass sample (Fig. 3(b)). This indicates that in the glass, the Eu3+ ions occupy highly non-centrosymmetric lattice sites. With increasing degree of crystallization, the value of R decreases down to about 1.4 (Fig. 3(a)-(b)), what indicates that the Eu3+ ions locate in a more and more symmetric and less covalent environment.
Figure 4 shows the normalized PL decay curves of the NYSA glass and glass ceramics, monitoring the PL lines at 591 nm (5D0 →7F1), 612 nm (5D0 →7F2) and 702 nm (5D0 →7F4). All decay curves can be well-fit to a single exponential decay function, with the lifetime τ after which the emission intensity reached 1/e of its initial value. The lifetime of the first excited state 5D0 of Eu3+ increases with increasing annealing time, i.e., the lifetimes of Eu3+ PL lines at 591, 612 and 702 nm increase from 2.04 to 3.22 ms, from 1.93 to 2.87 ms and from 2.12 to 3.22 ms, respectively (insets of Fig. 4(a)-(c)). This enhancement further reflects the lower probability of non-radiative energy transfer and, thus, higher internal quantum efficiency. Consistent with this observation, also the directly measured value of ηeQE increases, i.e., from 19.5 to 36.2% after crystallization.
The PL intensity of Eu3+, the observation of Stark splitting, the asymmetry ratio R, PL lifetimes and the directly observed external quantum efficiency ηeQE are strongly dependent on partitioning of the Eu3+ species. All present results support the conclusion that during crystallization, Eu3+ ions are incorporated into a crystalline lattice. Although four different crystalline lattice sites (VIY3+, VINa3+, IVSi4+ and IVAl3+) are available, VIEu3+ ions are most probably incorporated on VIY3+ sites in NaYSiO4 and Na3YSi3O9 due to the similarity of ionic radii (the ionic radius mismatch between VIY3+ and VIEu3+ is about 5%) and equivalent valence state, as compared to VINa+, IVSi4+ and IVAl3+ sites (Table 1 ,). In sodium disilicate glass, Eu3+ is reported to be (6 ± 0.5)-fold coordinated in a highly distorted oxygen environment . While corresponding data are not available for exactly the present glass composition, we assume that we face a very similar situation also in the present case with 6-fold coordinated Eu3+ in a highly-distorted coordination shell. This explains the high asymmetry ratio of 4.8 which we observed here and leads to spectral dominance of electric-dipole allowed red emission in the NYTS glass host. In NaYSiO4 and Na3YSi3O9 crystalline phases, octahedral [YO6] groups are located in a distorted octahedral environment without centrosymmetry [25,26]. The standard deviation of Y3+-O2˗ distance in [YO6] groups is used to evaluate the degree of distortion for octahedral [YO6] groups, yielding values of 0.07 and 0.09 Å in NaYSiO4 and Na3YSi3O9, respectively, which indicates that the degree of local distortion of the [YO6] groups is similar in NaYSiO4 and Na3YSi3O9 crystals. After crystallization, Eu3+ ions are embedded in the NaYSiO4 and Na3YSi3O9 crystalline phases, resulting in a less asymmetric environment. The degree of crystallization can be controlled by annealing time at 800 °C, i.e., the crystalline volume fraction increases with annealing time resulting in more Eu3+ ions incorporating into the NaYSiO4 and Na3YSi3O9 crystalline phases with increasing annealing time. Consequently, the asymmetry ratio decreases with increasing annealing time from 4.8 to 1.4, emission lines of Eu3+ are more and more Stark split, and the emission intensity, lifetime and ηeQE are enhanced. Further consideration of the evolution of the asymmetry ratio with increasing annealing time reveals that the major part of gain in symmetry occurs already after relatively short annealing, i.e., after 4 h at 800 °C. Considering the previous discussion of the presence of two crystallization steps, we may assume that in this regime, the first crystallization step is well completed while the second has occurred, if at all, to an unknown extend. Considering further the composition of the precursor glass, we have a ratio of Y3+:Si4+ of about 1:3, a ratio of Na+:Si4+ of about 1:1 and a ratio of Na+:Y3+ of > 3:1. Crystallization occurs towards the general join of NaxYSiyO1/2x + 3/2 + 2y where the coordination numbers in the glass phase are already close to the coordination numbers of the individual species in the crystalline phase. The most mobile species is the monovalent Na+ ion which is, in addition, present in relative abundance with respect to the general stoichiometry of NaxYSiyO1/2x + 3/2 + 2y when x = y. The formation of a phase with specific stoichiometry is therefore determined by the abundance and mobility of Y3+ and Si4+, where the stoichiometry of the precursor glass dictates the formation of Na3YSi3O9. Any phase with higher or lower Y3+:Si4+ ratio would require a certain degree of Y3+ diffusion which becomes an increasing barrier with increasing degree of crystallization (through increasing consumption of available Y3+). We therefore conclude that crystallization occurs first into Na3YSi3O9, characterized by Tc1. During this process, especially Al2O3 and Na2O enrich in the residual liquid phase, what results in further precipitation of NaAlSiO4, characterized by Tc2. However, as NaAlSiO4 does not provide suitable lattice sites for the incorporation of Eu3+, its contribution to the emission characteristics is only through affecting the scattering behavior of incoming light. Controlled precipitation of this phase through prolonged thermal treatment or a second annealing step around Tc2 can therefore be employed to tune light scattering without affecting Eu3+ partitioning.
In conclusion, we reported on photoluminescence from Eu3+-activated Na,Y-silicate glasses and glass ceramics of the Na3YSi3O9 type. Glass ceramics were obtained through controlled crystallization of the super-cooled precursor liquid. We demonstrated that during crystallization, a substantial amount of Eu3+ partitions on octahedral Y3+ lattice sites. An evaluation of the asymmetry ratio of the PL band intensities of 5D0 →7F2 and 5D0 →7F1 indicates that this incorporation occurs primarily in Na3YSi3O9, which is also the first phase to precipitate. Controlled precipitation of NaAlSiO4 through prolonged thermal treatment or a second annealing step can be employed to tune light scattering and, hence, absorption cross section and light extraction without affecting Eu3+ partitioning. As a result of crystallization, we obtain a 20-fold gain in absolute emission intensity, depending on crystallization conditions, and a 50%-gain in external quantum efficiency. Due to a strong shift of the spectral symmetry to the 5D0 →7F4 emission band at 702 nm with increasing thermal annealing time, the pure red emission color is kept unaffected by glass crystallization. The material may therefore be considered as an alternative red-emitting phosphor for light conversion.
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