Cubic and Hexagonal NaGdF4 Crystals Precipitated from an

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Cubic and Hexagonal NaGdF4 Crystals Precipitated from an Aluminosilicate Glass: Preparation and Luminescence Properties Andreas Herrmann,* Maxi Tylkowski, Christian Bocker, and Christian Rüssel Otto-Schott-Institut, Jena University, Fraunhoferstrasse 6, 07743 Jena, Germany ABSTRACT: Depending on the composition of the glass and the annealing procedure supplied, thermal annealing of silicate glasses in the system SiO2/Al2O3/AlF3/Na2O/ Gd2O3/SmF3 led to the precipitation of cubic or hexagonal NaGdF4 nanocrystals. The glasses were already phase separated after casting and formed a droplet phase supposedly enriched in fluorides and rare earths. The droplets had sizes mainly in the range from 80 to 120 nm. During annealing at temperatures ≥600 °C, multicore particles of cubic or hexagonal NaGdF4 were precipitated inside the amorphous droplet phase. Completely transparent hexagonal NaGdF4 containing nano-glass-ceramics could be derived from this system. Fluorescence spectra and fluorescence decay curves of Sm3+ show crystal phase dependent effects. Sm3+ doped hexagonal NaGdF4 exhibits notably different fluorescence emission spectra and longer fluorescence lifetimes than cubic NaGdF4. KEYWORDS: NaGdF4, glass-ceramics, nanocrystals, luminescence, samarium III crystals in comparison to their cubic counterparts.7,8 For photonic applications, these nanometer sized crystallites have to be embedded into a transparent matrix, as, for example, polymers. In ref 3, polydimethylsiloxane (PDMS) has been used for this purpose. PDMS or related polymers are also the commonly used materials for fluorophor embedding in LED technology.9,10 A serious drawback of this technology, however, is that PDMS as well as most organic polymers degrade if exposed to intense short wavelength radiation (UV light).11 This situation is especially given for high-power LED-applications. The degradation of the polymer matrix is also a disadvantage in the field of solar technology. Additionally, the poor thermal conductivity of these materials is a matter of concern, especially with respect to high-power applications. Furthermore it should be noted that the composite material presented by Wang et al. is not completely transparent; that is, it shows notable light scattering in the visible wavelength range.3 If the polymer matrix is replaced by an inorganic glass matrix, most of the mentioned problems can be addressed. The respective transparent glass-ceramics provide better chemical and mechanical durability for high-power LED and solar applications. Silica based glasses and glass-ceramics in particular are very stable against intense short wavelength irradiation and possess a much higher thermal conductivity than PDMS.10 This study reports on the precipitation of Sm3+-doped cubic and hexagonal NaGdF4 nanocrystals from a silicate glass. The different phases can be crystallized by comparatively small alterations of the glass composition and by choosing

1. INTRODUCTION Glass-ceramics are interesting materials for various photonic applications. If doped with fluorescent ions they might combine the advantages of a glassy material with those of crystals. Glassceramics have a good forming ability and isotropy, as well as high chemical, photochemical, and mechanical stability, and they provide advantageous fluorescence properties, such as high fluorescence quantum efficiency, long fluorescence lifetime, and good up-conversion properties. For most photonic applications, a high transparency for light in the visible and near-infrared wavelength range is a prerequisite. Completely transparent glass-ceramics can be produced if the crystals sizes are within the nanometer range (preferably much smaller than 100 nm) and if a narrow crystal size distribution is ensured.1,2 Otherwise, light scattering occurs. While the crystal size determines the transparency of the glass-ceramics, the type of crystal widely affects the fluorescence properties. For example, less symmetric crystals are reported to show better fluorescence properties in comparison to more symmetric crystal phases.3 Recently, Wang et al. reported on the wet chemical preparation of nanometer sized Er3+, Yb3+, and Tm3+ doped Na(Y,Gd)F4 crystals.3 In analogy to other sodium rare-earth fluorides doped with a second type of rare earth fluoride,4,5 these crystals feature strong fluorescence emission and high upconversion efficiency. The cubic or hexagonal phase of Na(Y,Gd)F4 could be selectively synthesized depending on the Y/Gd-ratio. It is pointed out that hexagonal Na(Y,Gd)F4 crystals possess a higher up-conversion efficiency than the corresponding cubic modification. Enhanced up-conversion efficiency is also reported for Yb3+,Tm3+-codoped hexagonal NaYF4 in comparison to cubic Yb3+,Tm3+:NaYF4.6 A higher fluorescence intensity and a longer fluorescence lifetime are found for Eu3+-doped hexagonal NaYF4 and NaGdF4 nano© 2013 American Chemical Society

Received: May 2, 2013 Revised: June 14, 2013 Published: June 24, 2013 2878

dx.doi.org/10.1021/cm401454y | Chem. Mater. 2013, 25, 2878−2884

Chemistry of Materials

Article

HNO3. Then, the samples were coated with carbon (at low pressures: 10−4 Pa) in order to avoid charging effects of the nonconductive samples under the electron beam and to obtain a homogeneous fine structured carbon layer. For SEM studies, a field emission (Schottky emitter) SEM Jeol JSM 7001F was used. The as-casted samples A and C were studied by the TEM HITACHI H8100 at 200 kV. Therefore, a bulk sample was prepared by grinding, dimpling, and ion milling.

appropriate annealing parameters. Furthermore, the luminescence properties such as static spectra and dynamic behavior of the as prepared glasses and glass-ceramic samples are investigated in materials containing different NaGdF4 phases.

2. EXPERIMENTAL PROCEDURE Glasses in the system SiO2/Al2O3/AlF3/Na2O/Gd2O3/SmF3 were prepared by melting the raw materials SiO2, Al(OH)3, AlF3, Na2CO3, NaF, Gd2O3, and SmF3. The compositions of three glasses are given in Table 1. Depending on the sample, 0.35 mol% SmF3 or 0.18 mol%

3. RESULTS The casted glass with the composition A was colorless and showed slight light scattering. During annealing at temperatures in the range from 600 to 750 °C for 20 h, the sample changed from translucent to opaque with increasing annealing temperature. By contrast, the base glass of sample B showed a very high transparency after casting. Up to annealing temperatures of 650 °C, the glass-ceramic samples of composition B remained transparent. After annealing at higher temperatures, increasing light scattering occurs. The sample finally became opaque if annealed at a temperature of 750 °C. Samples of composition C were completely transparent after casting and showed no sign of light scattering. Annealing at temperatures up to 700 °C did not result in any light scattering; the samples were still transparent. Increasing the annealing temperature to 750 °C led to slight light scattering. Figure 1 shows the XRD-patterns of composition A before and after annealing at temperatures in the range from 600 to

Table 1. Chemical Compositions of the Studied Samples chemical composition in mol% sample

SiO2

Al2O3

AlF3

Na2O

NaF

Gd2O3

Sm2O3

SmF3

A B C

69.8 69.8 69.9

5.2 4.2 4.3

1.8 1.8 1.8

0.0 2.0 2.3

20.0 18.9 18.4

3.0 3.0 3.0

0.18 0.00 0.00

0.00 0.35 0.35

Sm2O3 have been added to the composition, which corresponds to an overall Sm3+ concentration of 8 × 1019 cm−3 for all three compositions. The raw materials were melted in a platinum−rhodium crucible in a middle-frequency induction furnace at 1550 °C for 45 min. The glass melt was cast on a brass block and then cooled down to room temperature in air. The cooled glass was milled and remelted in the induction furnace at 1550 °C, continuously stirred, and kept for 45 min at this temperature. The melt was cast on a brass block and then given to a muffle furnace preheated to 580 °C, where it was slowly cooled to room temperature. Afterward, the glass block was cut into small discs of about 20 × 10 × 4 mm3. The glass transition temperatures of the glasses were measured using a dilatometer (DIL 402 PC, Netzsch Gerätebau GmbH, Germany), applying a heating rate of 5 K/min. The glass-ceramic samples were obtained by annealing the glass discs at temperatures in the range from 600 to 750 °C for 20 h. The relatively long annealing time was chosen because of the small crystal growth velocity at 600 and 650 °C. For annealing at 700 and 750 °C, the annealing time was kept constant to achieve comparable results. For X-ray diffraction (XRD), the samples were powdered. The crystalline phases were identified using an X-ray diffractometer (D5000, Siemens, Germany) with Cu-Kα irradiation. The luminescence excitation and emission spectra and the luminescence decay curves were recorded using samples with a thickness of 1 mm. For this purpose, the glass-ceramic samples were ground and polished to optical quality. The luminescence emission spectra of the Sm3+-doped glass and glass-ceramic samples were recorded using a fluorescence spectrometer (RF-5301 PC, Shimadzu, Japan). Luminescence lifetime measurements were conducted with a self-made experimental setup. It consists of a pulsed high power LED (LED 395-66-60, Roithner Lasertechnik Vienna, Austria; wavelength 395 nm), a spectrometer (H.25, Jobin Yvon, France), a photomultiplier tube (R5929, Hamamatsu, Japan), and a digital storage oscilloscope (TDS2012, Tektronix, U.S.A.). For the fluorescence lifetime measurements, the samples were excited by an UV-light pulse of the high power LED at a wavelength of 395 nm. The resulting luminescence light was collected and focused to the entrance slit of a spectrometer by a lens array. The spectrum-sliced light is amplified by the photomultiplier tube, which is connected to the oscilloscope. The oscilloscope records the luminescence intensity as a function of time. The data obtained was transferred to a PC and later on analyzed with commercial scientific graphing software. The standard deviation of these measurements is not more than ±2%. Scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) was used in order to study the microstructure of some selected samples. Samples for SEM investigations were prepared from the annealed glass-ceramics and the as-casted glass. The samples were polished using a diamond suspension, and some were etched by using a mixture of HF and

Figure 1. XRD-patterns of sample A as obtained from casting and of samples annealed at temperatures of 600, 650, 700, and 750 °C for 20 h.

750 °C for 20 h. In the as-casted samples, only peaks of minor intensity are observed, which all are attributable to cubic NaGdF4 (JCPDS No. 027-0697). Annealing at a temperature of 600 °C results in more intense peaks of the cubic phase. For the samples annealed at temperatures of 650 and 700 °C, peaks of an additional crystalline phase appear. These peaks could be attributed to hexagonal NaGdF4 (JCPDS No. 027-0699). Annealing at temperatures up to 700 °C results in a narrowing and in an intensity increase of the XRD-lines of both phases. In the sample, annealed at 750 °C, the lines attributable to cubic NaGdF4 almost disappeared and mainly the hexagonal phase of NaGdF4 occurred. Figure 2 shows the XRD-patterns of samples with the composition B. The figure includes patterns of the as-casted glass as well as patterns of glass-ceramic samples annealed for 20 h at temperatures in the range from 600 to 750 °C. The ascasted sample shows notably broadened peaks that can be attributed to hexagonal NaGdF4. After annealing at different 2879



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of these particles are in the range 50−90 nm, as shown by both SEM and TEM. The TEM micrograph of sample A (Figure 4b) reveals a second type of spherical particles that are much smaller (around 10 nm). Furthermore, it can be seen that the larger particles possess an internal structure. The selected area diffraction of a well separated particle in sample A results in a bright ring (inset in Figure 4b) that indicates a polycrystalline character of the spherical particles. In analogy, also, sample C shows some diffraction spots in the SAD of a larger area including several particles (inset in Figure 4c). Hence, the ascasted sample C contains few crystallites, too. Some particles show a diffraction contrast and some include bright spots. The appearance of the latter depends on the observation time and hence is supposedly due to the interaction of the particles with the electron beam. The EDS point analysis (TEM) of sample A in Figure 5 shows an enrichment of gadolinium as well as of fluorine in the particles in comparison to the matrix. However, a statement whether samarium occurs in the particle or in the matrix is not very reliable. The SmLα transition at around 5.7 keV indicates a presence rather in the particle than in the matrix. The SEM-micrographs of the sample B and C with and without annealing also revealed small spherical droplets with sizes smaller than 100 nm (not shown). However, in sample B, annealing at a temperature of 750 °C led to an increase of the droplet size to around 150 nm. In Figure 6, the fluorescence emission spectra of the glassy and the annealed samples with the composition A are shown. For excitation, a wavelength of 402 nm has been used. The fluorescence intensities are smallest in the glassy sample and increase steadily with increasing annealing temperature. The intensity of the samples annealed at 650 and 700 °C are almost the same. For the sample annealed at 700 °C, a small additional peak appears at about 593 nm but disappears again in the sample annealed at 750 °C. The comparison of the spectra of the samples annealed at 650 and 700 °C reveals another effect: at around 610 nm, a shoulder is clearly visible for the 650 °C sample but not for the 700 °C sample. A closer look to all spectra shows that this shoulder is also visible for the as-casted sample as well as for the 600 °C annealed sample but not for the 700 and 750 °C samples. Figure 7 shows the fluorescence emission spectra of the glassy and the annealed samples with the composition B. In analogy to sample A (see Figure 6), the fluorescence intensity increases steadily with increasing annealing temperature, although the intensity of the as-casted sample and the samples annealed at temperatures of 600, 650, and 700 °C are comparatively small. For the sample annealed at 750 °C, the fluorescence emission is much more intense. In analogy to the spectra of sample A, an additional small peak at 593 nm is observed; however, for sample B, this peak is clearly visible for all supplied annealing temperatures. The as-casted sample does not show this additional peak. For composition C, the same effects as for the samples with composition B are observed (Figure 8). Some selected results of the fluorescence lifetime measurements are shown in Figures 9 and 10. In Figure 9, the fluorescence decay curves of the Sm3+ emission at 600 nm of the base glass sample and the samples annealed at 600, 700, and 750 °C of composition A are shown. For excitation, a wavelength of 395 nm was used. The fluorescence intensity is drawn in logarithmic scale, hence, a monoexponential decay results in a straight line in the graph. As seen, the decay curves

Figure 2. XRD-patterns of the sample B as obtained from casting and of samples annealed at temperatures of 600, 650, 700, and 750 °C for 20 h.

temperatures, all samples show narrow, high intensity peaks of hexagonal NaGdF4. Within the detection limit, no other crystal phase was observed for this composition. In Figure 3, the XRD-patterns of composition C are shown. The as-casted sample is amorphous while the annealed samples

Figure 3. XRD-patterns of the sample C as obtained from casting and of samples annealed at temperatures of 600, 650, 700, and 750 °C for 20 h.

exclusively show peaks attributable to hexagonal NaGdF4. In Table 2, the glass transition temperatures as well as the appearance and main crystal phases of the as-casted samples are summarized. Table 2. Glass Transition Temperatures and Appearance of the As-Casted Samples, Crystalline Phases Formed sample Tg in °C A B C

569 585 584

transparency (5 mm thickness)

spontaneous crystallization

slight scattering transparent transparent

cubic NaGdF4 hexagonal NaGdF4 amorphous

The microstructure of the glasses and glass-ceramics was studied by electron microscopy. Figure 4 shows the SEM micrograph as well as the TEM bright field micrograph for the as-casted glass samples with compositions A and C. The micrographs illustrate a homogeneously distributed droplet-like phase with a mean atomic number larger than that of the matrix phase. The droplets appear as bright dots in the SEM micrograph (Figure 4a) and dark round shaped particles in the TEM micrographs (Figure 4b and 4c). The measured sizes 2880



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Figure 4. Micrographs of sample A and sample C (without annealing): (a) SEM micrograph of sample A, (b) TEM-micrograph of sample A with SAD pattern as inset, (c) TEM micrograph of sample C with SAD pattern as inset.

Figure 5. TEM-EDS point analysis of the matrix and a particle of sample A without annealing.

Figure 7. Fluorescence emission spectra of the glassy sample of composition B and the samples annealed at 600, 700, and 750 °C for 20 h. Excitation wavelength: 402 nm.

Figure 6. Fluorescence emission spectra of the glassy sample of composition A and the samples annealed at 600, 700, and 750 °C for 20 h. Excitation wavelength: 402 nm.

Figure 8. Fluorescence emission spectra of the glassy sample of composition C and the samples annealed at 600, 700, and 750 °C for 20 h. Excitation wavelength: 402 nm.

slightly deviate from a straight line. The fluorescence lifetimes of the samples shown in Figure 9 are in the range from 2.2 to 2.7 ms. In the glassy sample and in the sample containing the cubic NaGdF4 phase, the fluorescence lifetimes are comparatively small (2.2 to 2.3 ms), while the lifetimes are notably larger for samples annealed at 650 to 750 °C, which contain the hexagonal phase (2.5 to 2.6 ms). In Figure 10, the fluorescence decay curves are shown for an emission wavelength of 600 nm for samples with the composition B before and after annealing at temperatures of 600, 650, 700, and 750 °C for 20 h. An excitation wavelength of

395 nm was used for these measurements. As for composition A, the fluorescence lifetime increases with increasing annealing temperature. However, in comparison to composition A, the lifetime of the as-casted glass is longer. After annealing at a temperature of 750 °C, the lifetime of compositions A and B is almost the same (2.7 and 2.6 ms, respectively). The fluorescence decay curves of samples C have almost the same shape as sample B and are not shown for this reason. 2881



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In samples with the compositions B and C solely the hexagonal modification was detected. This was not affected by the annealing temperature in the range studied. Thoma et al. reported in ref 12 that the formation of the cubic or hexagonal phase does not only depend on the (annealing) temperature, but also on the molar ratio of Na2O/ Gd2O3. A larger Na2O concentration results in the crystallization of the hexagonal phase, while a larger Gd 2 O 3 concentration favors the precipitation of the cubic phase. For the glass samples presented here, the sodium concentration increased steadily from composition A to C. Obviously, an increasing sodium concentration favors the precipitation of the hexagonal NaGdF4 modification. This effect was also described by Ptacek et al. for the wet chemical preparation of nanometer sized Na(Y,Gd)F4 crystals.7,8 Furthermore, the XRD patterns of Figures 1−3 show notably broadened XRD lines for the samples annealed at a temperature of 600 °C. The lines get continuously narrower with increasing annealing temperature, indicating an increasing crystallite size with increasing annealing temperature. The glasses as well as the respective glass-ceramics show spherical particles in the electron microscope in which the NaGdF4 crystals occur. The TEM analysis of sample A (which mainly contains spontaneously crystallized cubic NaGdF4) indicates the agglomeration of the crystals in these droplets. It can be assumed that phase separation occurs during casting of the glass melt. The formed droplets are enriched in those components from which the crystal is formed. This leads to the crystallization of NaGdF4, which occurs spontaneously during cooling and/or during annealing of the samples. Furthermore, crystals as well as the phase separation droplets grow slightly during annealing. However, the crystal growth is hindered, and hence, the crystals size is still in the nanometer range. Because network modifying components crystallize in this nonisochemical system, the residual glassy phase increases in viscosity and hence hinders the crystal growth. This system is another example for the mechanism of nanocrystallization, which was described in the past few years in refs 13−17. Especially if fluorides are crystallized, a shell enriched in silica is formed during crystal growth. This has been proven by both Anomalous Small Angle X-ray Scattering (ASAXS)18 and TEM with electron energy loss spectroscopy (EELS).19 If this shell is once formed, the crystal growth velocity decreases drastically, which may lead to a very narrow crystal size distribution.14 The presence of samarium in the crystalline phase could not be shown without any doubt by electron-microscopical methods, although the EDS-spectrum in Figure 5 gives a hint, that samarium is enriched in the particles. This would be in agreement with recent TEM-investigations in the same system with a different composition.20 Unfortunately, electron microscopy could not distinguish between the different crystals. Also, the nature of the second small phase with sizes smaller than 10 nm remains unclear. It appears in the as-casted sample with the compositions A and B as well as in all the annealed samples while none are present in the as-casted sample C. Figures 6, 7, and 8 show the fluorescence emission spectra of the Sm3+-doped samples of all compositions for different annealing temperatures. The spectra consist of narrow bands caused by f−f transitions, which are typical for trivalent rareearth ions. In comparison to crystals or aqueous solutions, the energy levels of trivalent rare-earth ions (and therefore also their fluorescence lines) are somewhat broadened in glassy samples due to the comparatively irregular short-range order

Figure 9. Fluorescence decay curves of the Sm3+ emission at 600 nm of the glassy sample of composition A and the samples annealed at 600, 700, and 750 °C for 20 h. Excitation wavelength: 395 nm.

Figure 10. Fluorescence decay curves of the Sm3+ emission at 600 nm of the glassy sample of composition B and the samples annealed at 600, 700, and 750 °C for 20 h. Excitation wavelength: 395 nm.

4. DISCUSSION According to the phase diagram NaF-GdF3, three binary compounds might occur: the cubic NaGdF4 phase, which is the high temperature modification, the hexagonal NaGdF 4 modification, which is the thermodynamically stable modification at room temperature, and, additionally, an orthorhombic modification.12 According to the phase diagram, hexagonal NaGdF4 transforms to cubic NaGdF4 at a temperature of around 700 °C. Gadolinium shows different Gd−F coordination numbers in the hexagonal and the cubic phase of NaGdF4. While in the hexagonal phase, Gd3+ has a coordination number of 9; the coordination number in the cubic phase is only 8. In principle, also, the Na5Gd9F32 phase, which should be stable at temperatures in the range from 751 to about 1070 °C might occur. The latter phase is orthorhombic. The studied as-casted samples contained minor quantities of crystallites. Depending on the sample composition and the annealing conditions supplied, the cubic as well as the hexagonal phase were formed during thermal annealing of the samples. For composition A, the cubic modification was observed in the as-casted sample as well as for the sample annealed at 600 °C. At higher annealing temperatures, the cubic modification obviously transforms to the hexagonal modification. It should be noted that, at lower annealing temperatures, the high temperature modification is formed while higher annealing temperatures result in the occurrence of the low temperature modification. 2882



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around the incorporated ions. The fluorescence emission spectrum of Sm3+ consists of several bands in the yellow to red spectral range, which all originate from the 4G5/2 level. The strongest fluorescence emission line is the bright orange appearing transition 4G5/2 → 6H7/2 at about 600 nm. Other typical transitions are 4G5/2 → 6H5/2 at about 562 nm and 4G5/2 → 6H9/2 at about 645 nm. All lines are split into several components that account for the broadening of the respective transitions. The fluorescence intensity increases with increasing annealing temperature for all compositions. However, as already noted, the samples get less transparent with increasing annealing temperature due to the crystallization (proved by the XRD patterns shown in Figures 1−3). Hence, light scattering in the samples increases, which may lead to an increased fluorescence emission. This effect has already been described previously.21 For the samples B and C, a small additional peak at about 593 nm was observed for all annealed samples. The same peak appears for the sample A after annealing at 700 °C. Interestingly, this effect coincides exactly with the appearance of the hexagonal NaGdF4 phase in the glass-ceramic samples. To shed some more light into this finding, Sm3+-doped hexagonal as well as cubic NaGdF4 was synthesized by solidstate sintering. According to the phase diagram in ref 12, two slightly different mixtures of NaF and GdF3 were prepared, pressed to pellets, and annealed at 925 or 1030 °C for 3 h in order to synthesize the hexagonal or cubic modification of NaGdF4, respectively. A quantity of 0.1 mol% of GdF3 was substituted for SmF3 in both samples, which corresponds to about 3 × 1019 Sm3+/cm3. During the sintering process, an argon atmosphere was applied to avoid oxidation of the fluorides. The XRD patterns of the annealed samples proved that, within the detection limit, exclusively hexagonal or cubic NaGdF4 were formed during the annealing process (not shown). In particular, peaks due to NaF or GdF3 were not found in the XRD-patterns. Figure 11 shows a comparison of the fluorescence emission spectra of the hexagonal and cubic Sm3+:NaGdF4 phase, the as-casted sample C and sample C annealed at 700 °C. As shown, the spectra of the hexagonal and cubic modifications of NaGdF4 differ notably, mainly in the

peak positions but also in the peak width. Obviously, the additional peak at 593 nm is due to the emission of Sm3+-doped hexagonal NaGdF4, which is precipitated during annealing. The shoulder at about 610 nm observed in sample A without annealing and after annealing at 650 and 600 °C can most likely be attributed to the emission of Sm3+-doped cubic NaGdF4. In contrast to the hexagonal phase, cubic NaGdF4 shows a shoulder at this particular position. Similar effects can also be noted for the other two emission peaks of Sm3+ at about 560 and 645 nm. These findings clearly confirm the incorporation of a notable amount of Sm3+ ions into the crystals. Figures 9 and 10 show dynamic fluorescence measurements for the samples A and B, respectively. The slight deviations from a linear slope indicate a nonmonoexponential fluorescence decay, most likely due to cross relaxation processes because of the relatively high Sm3+ doping concentration. These effects are described in more detail elsewhere.22 In general, an increasing fluorescence lifetime is found with increasing annealing temperature. While this lifetime increase for compositions B and C is about 5%, it is about 15% for composition A. It is observed that sample A without annealing shows much smaller fluorescence lifetime in comparison to samples B and C. In sample A, small amounts of cubic NaGdF4 (but no hexagonal NaGdF4) were detected by XRD measurements and for the samples with notable amounts of hexagonal NaGdF4 after annealing (at 650, 700, and 750 °C), the lifetime is notably longer. These results are in good agreement with the experiments by Wang et al. who reported a significant increase in up-conversion efficiency for the hexagonal phase of Na(Y,Gd)F4 in comparison to the cubic phase.3 Up-conversion efficiency is strongly dependent on the fluorescence lifetime of the doped rare earth ions (Yb3+ and Er3+ in this case). In this report, the hexagonal and cubic phases were prepared by a variation of the yttrium/gadolinium ratio. Samples with the composition B with and without annealing contain only the hexagonal NaGdF4 phase. This already explains the longer fluorescence lifetimes and the small effect of the annealing procedure on the fluorescence decay curves of these samples. The samples with the composition C show almost the same dynamic fluorescence behavior as the samples with the composition B (not shown). Although the XRD measurements did not reveal any crystallites in the as-casted sample C, the fluorescence lifetime is exceptionally high. It is comparable with those of sample B, which contains the long-lifetime hexagonal NaGdF4. The TEM analysis proves the presence of crystallites by the SAD pattern in Figure 4c. Obviously, the concentration of hexagonal phase is too small to be detected by the XRD (

Cubic and Hexagonal NaGdF4 Crystals Precipitated from an

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