PAPER www.rsc.org/dalton | Dalton Transactions Modifications induced by acetylacetone in properties of sol–gel derived Y3 Al5 O12 : Tb3+ – I: structural and morphological organizations† Audrey Potdevin,*a,b Geneviève Chadeyron,a,b Valérie Briois,c Fabrice Leroux,b,d Celso V. Santilli,e Marc Dubois,b,d Damien Boyera,b and Rachid Mahioub,d Received 3rd December 2009, Accepted 24th June 2010 DOI: 10.1039/c005452f Acetylacetone has been used as a chemical modifier for the synthesis of undoped and Tb3+ -doped Y3 Al5 O12 powders. A systematic investigation concerning its influence on the structural and morphological properties of amorphous and crystallized samples has been carried out. These properties have been comparatively studied by means of X-ray diffraction, infrared spectroscopy, SEM, XAS and SAXS. 27 Al NMR and EPR experiments have been performed to complete the study. The combined results have evidenced that acetylacetone promotes organic groups departure during calcination, entailing a better structural organization at lower temperatures compared with unmodified powders. Structuration has been proven to occur at short-scale range until a 600 ◦ C heating treatment before being extended by coalescence at higher temperatures. Finally, the presence of acac ligands on the alkoxides leads to a monomer-cluster aggregation process, and thus to a more open network. Introduction Most lighting devices contain mercury as a primary component of light generation.1 This chemical element is toxic and has to be replaced soon. The main alternative technological solutions foreseen by the lighting industry consist in combining a Xe– Ne plasma1–2 or UV/blue emitting GaN LEDS3 as excitation sources with appropriate phosphors. These phosphors must be characterized by strong absorption in the VUV or UV/blue wavelength domains in addition to high conversion efficiency into the visible range. Rare-earth doped oxides or hybrid materials1,4 are generally used for this purpose. Among extensively studied luminescent materials, Tb3+ and Ce3+ activated Y3 Al5 O12 (YAG) represent promising candidates for such devices since they fulfil the aforementioned conditions.3 YAG matrix exhibits excellent chemical stability and mechanical resistance as well as an important optical damage threshold allowing it to withstand harsh conditions. Besides, when doped with rare-earth ions, it shows emissions lying from the near infrared to the UV ranges. a Clermont Université, Ecole Nationale Supérieure de Chimie de ClermontFerrand, LMI, BP 10448, F-63000, Clermont-Ferrand b CNRS, UMR 6002, LMI, F-63173, AUBIERE. E-mail: audrey.potdevin@ univ-bpclermont.fr; Fax: 33(0)4 73 40 71 08 c Synchrotron SOLEIL, L’Orme des Merisiers, Saint–Aubin-BP48-91192, Gif sur Yvette, (France) d Clermont Université, Université Blaise Pascal, LMI, BP 10448, F-63000, Clermont-Ferrand e Instituto de Quimica, CP14800-14900, Araraquara SP, (Brazil) † Electronic supplementary information (ESI) available: Fig. S1: XRD patterns of undoped Y3 Al5 O12 powders synthesized with (a) RC = 0 and (b) RC = 1 and sintered for 4 h at different temperatures; Fig. S2: (a) experimental and linear least-squares fittings of k3 c(k) EXAFS oscillations and (b) experimental and fitted filtered EXAFS spectra for YAG reference powder; Table S1: structural parameters determined from the EXAFS data recorded at Y K edge. Data between brackets correspond to the literature.27 See DOI: 10.1039/c005452f 8706 | Dalton Trans., 2010, 39, 8706–8717 Phosphors such as YAG were usually prepared by solid-state reaction5 at temperatures higher than 1500 ◦ C. This process does not allow particle morphology control and leads to byproducts that can hamper luminescence. Furthermore, as optical properties are greatly dependent on particles morphology, samples with narrow size distribution are generally required for the aimed applications. As a result, wet-chemical processes have been developed over the last few years to prepare high-quality and tunable YAG: spray pyrolysis,6 hydrothermal synthesis,7 Pechini method8 or sol–gel.9 The latter is among the most convenient methods to achieve hetero-metallic oxides like YAG.10 Indeed, this process leads to high purity samples at lower temperatures than that required by solid-state reactions, giving rise to more eco-energetic synthesis conditions. Besides, rheological properties of the sols can be controlled using chemical additives during the synthesis, which allows preparing samples of versatile shapes (monoliths, fibres, coatings etc.). The most frequently applied additives are organic acids and b-diketones or derivatives.11 They are used to prevent or at least reduce the hydrolysis stage during the sol–gel process. This complexation results in changes in gelation time or particle morphology for instance.11 This paper deals with the use of acetylacetone (acacH) as a chemical modifier during the sol–gel synthesis of YAG powders. AcacH was chosen in order to obtain stable and homogeneous YAG sols suitable for coating processes. This work completes a previously published study12 which had evidenced an earlier crystallization for acac-modified YAG powders as well as different optical properties upon 485 nm-excitation between YAG:Tb3+ sols and powders with respect to the acacH modification. In order to better apprehend these behaviors, further analyses have been carried out on both kinds of samples: sols and powders sintered at different temperatures have been studied by means of X-ray diffraction (XRD), infra-red (IR) spectroscopy, scanning electron microscopy (SEM), 27 Al nuclear magnetic resonance (NMR), This journal is © The Royal Society of Chemistry 2010 electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS) and small angle X-ray scattering (SAXS). A better comprehension of the mechanisms involved in the sol–gel process should allow to optimize the optical properties of the resulting luminescent materials. Contrary to the previous paper which compared sols, dried xerogels and xerogels heated at 1100 ◦ C, the present investigation concerns new batches of undoped and Tb3+ activated YAG xerogels and powders calcined between 200 ◦ C and 1400 ◦ C. This is divided into two parts: the first (presented here) is relative to the influence of acetylacetone on YAG:Tb structural and morphological properties while the second part concerns YAG:Tb3+ optical features with regard to terbium local environment and oxidation state. Even though it represents a complementary study of this work, we have chosen to separate these two parts for sake of clarity. For the same reason, results related to sols will be published in another paper. Experimental Materials Anhydrous yttrium chloride YCl3 (99.99% pure, Aldrich), and terbium chloride TbCl3 (99.99% pure, Aldrich), metallic potassium (98% pure, Aldrich), aluminium isopropoxide (99.99+% pure, Aldrich), acetylacetone (2,4-pentanedione, ≥99%, Aldrich) and anhydrous isopropanol (i PrOH, 99.8+% pure, Acros) have been used as starting materials. Synthesis The sol–gel synthesis procedure used for YAG has been detailed in previous studies.12–13 It consists in preparing separately a solution A of anhydrous yttrium and terbium chlorides dissolved in anhydrous isopropanol and a solution B of potassium isopropoxide. Solution B is slowly added to solution A under vigorous stirring: a precipitate of KCl appears immediately. The mixed solution is maintained at 85 ◦ C during 1 h, then aluminium isopropoxide powder is poured directly into the solution. After further reflux for 4 h at 85 ◦ C, a clear and homogeneous solution (solution C) is obtained together with the KCl precipitate. The latter is eliminated by centrifugation subsequent to cooling. All the synthesis steps must take place under dry argon atmosphere since alkoxides are very sensitive to moisture. Solution C is either hydrolyzed by introducing an excess of water, then dried at 80 ◦ C and heat-treated to form crystallized YAG powders, or stabilized by adding acetylacetone 2 h after the introduction of aluminium isopropoxide. After the addition of acacH into the reaction medium, the solution is held under reflux for a further 2 h. This stabilized sol is usually used to elaborate thin films by dip-coating for example or could be hydrolyzed with the procedure aforementioned to provide stabilized powders. Xerogels obtained after sols drying were then heat-treated in a muffle furnace for 4 h at a temperature comprised between 200 ◦ C and 1400 ◦ C. In this work, in order to unravel the influence of acetylacetone on the structural and morphological properties of YAG powders, two sets of syntheses were carried out with different complexation ratios: RC = 0 or 1, RC being defined as the ratio between This journal is © The Royal Society of Chemistry 2010 molar quantities of aluminium isopropoxide and acetylacetone. The complexation ratio RC = 1 has been optimized to lead to homogeneous stable solutions and well-crystallized pure YAG phases. Undoped and Tb3+ -activated (20 mol%) powders were prepared via the synthesis procedure described above. The doping rate of 20 mol% Tb3+ was chosen since it has been determined to be the optimal concentration in YAG under blue excitation (485 nm).13 Characterization First of all, it must be specified that several characterizations as NMR and EPR investigations have been carried out on undoped samples since the presence of lanthanide ions is a nuisance for these techniques. Other studies, such as IR spectroscopy, have been performed on both undoped and terbium-doped powders and have shown similar evolutions. Consequently, we can assume that results obtained from undoped powders are identical to those concerning terbium-doped samples. XRD measurements were performed on a Siemens D501 diffractometer operating with the Cu-Ka radiation. IR spectra were recorded on a Thermo-Nicolet 5700 spectrometer equipped with the Smart Orbit diamond ATR (Attenuated Total Reflection) accessory. Thirty-two scans were collected at 4 cm-1 resolution. EPR spectra were recorded with a Bruker EMX digital X-band (u = 9.649 GHz) spectrometer. Diphenylpicrylhydrazil (DPPH) was used as the calibration reference to determine the resonance frequency. SAXS measurements were performed using the SAXS beamline of the National Synchrotron Light Laboratory (LNLS, Campinas, Brazil). The beamline is equipped with an asymmetrically cut and bent silicon (111) monochromator that yields a monochromatic (l = 1.6083 Å, E = 7709 eV) and horizontally focused beam. A vertical position-sensitive X-ray gas detector (PSD) and a multichannel analyzer were used to record the scattering intensity, I(q), as a function of the modulus of scattering vector defined as q = (4p/l)sin q, q being half the scattering angle. The samples characterized by SAXS in transmission mode were YAG-based powders placed between two thin mica discs. Two SAXS patterns were collected for each sample using detector slits of 0.5 and 2 mm, respectively. The use of a 0.5 mm slit is better for minimizing smearing effects on the measured scattering intensity at very small q, while a 2 mm slit is better for enhancing the counting rate at high q. The parasitic scattering from the slits and mica windows were then subtracted and the resulting curves were normalized to equivalent intensity of the direct X-ray beam and same sample thickness. SAXS spectra were also corrected for the non-constant sensitivity of the PSD, and for the time varying intensity due to the decreasing lifetime of synchrotron radiation. Finally, both of the patterns collected with 0.5 and 2 mm detector slits were properly merged. Because of the mentioned normalization procedure, the SAXS intensity was determined for all samples using the same arbitrary units, so that they can be directly compared. Micrographs were recorded by means of a ZEISS Supra 55VP scanning electron microscope operating in high vacuum between 4 and 15 kV, using secondary electron detector (Everhart-Thornley detector). Specimens were prepared by sticking powder on the surface of an adhesive carbon film. Dalton Trans., 2010, 39, 8706–8717 | 8707 Yttrium K-edge EXAFS spectra of references and samples were collected at the Elettra Sincrotrone (Trieste, Italy) on the beamline BL-11-1 of the storage ring operating at 2/2.4 GeV with an optimal current around 300 mA. A Pt-coated mirror at fixed grazing incidence was used to collimate the X-rays at the entrance of the monochromator. Measurements were performed at the Y K-edge (17080 eV) using a monochromator equipped with Si(111) crystals. All data were recorded at liquid nitrogen temperature (77 K) in transmission mode using two ionization chambers filled with a mixture of gas (N2 , Ar) optimized in pressure at each edge in order to achieve absorptions of 20% and 80% for incident I 0 and transmitted I 1 photon flux, respectively. EXAFS spectra were collected with a step of 2 eV each 2 s. Typically, three or four scans were recorded for each sample to get a good signal over noise ratio. The powdered samples, finely grinded with boron nitride, were prepared in shape of pellets whose thickness was chosen so that the absorption jump at the edge was close to 1. The pellets R adhesives tapes. were sandwiched between two Kapton XAFS analysis was performed using the software code Athena.14 First, three or four spectra were averaged, before being converted from energy to k space by setting E 0 (using the first derivative of the raw spectra to determine the inflection point in the edge). Pre-edge background was first removed, using a linear polynomial function. Post-edge background using the Autobk algorithm was applied with a cut-off Rbkg = 1 in order to isolate the EXAFS oscillations c(k). Then, the EXAFS data were Fourier transformed using k3 ponderation and an appropriate Kaiser Bessel window (3.4–15 Å) leading to the pseudo-radial structural functions (PRDF). Besides, the software package ATOMS15 and FEFF616 were employed to generate ab initio phases and amplitude functions for single and multiple scattering paths in YAG,17 using the ARTEMIS graphical interface. Finally, coordination shells EXAFS fitting was performed with the ARTEMIS interface to IFEFFIT18 using least-square refinements. The S0 2 and enot were calibrated at 1.35 ± 0.05 and -2.4 ± 0.4 eV respectively by fitting the YAG crystalline reference17 and these values were used in all fits. In order to minimize the number of parameters, the fit of crystallized samples were done using the same Debye–Waller factors for the two first coordination shells and the same Debye–Waller factors for the yttrium, terbium and aluminium coordination shells located at 3.70 Å. The fit leads to the determination of the structural parameters R (average atomic distance from the absorbing atom), N (number of atoms in the coordination shell) and s (Debye–Waller factor). The reliability of the fit could be assessed by a residual factor RF that had to be minimized.19 High resolution 27 Al MAS NMR spectra were collected on a Bruker 300 instrument operating at 7.04 T, the Larmor frequency being equal to 78.20 MHz. 4 mm diameter zirconia rotors were spinned at 10 kHz during the MAS conditions, therefore the central transition (+1/2, -1/2) was recorded only. Single pulse experiment was applied and the quadrupolar nature of 27 Al (5/2) nuclei necessitated that the flip angle should satisfy the condition: ⎛ ⎞ ⎜⎜ I + 1 ⎟⎟ ⋅ w ⋅ t ≤ p R. F . p ⎜⎝ 2 ⎟⎠ 6 where w R.F (rad s-1 ) is the Larmor frequency of the corresponding quadrupolar nuclei and tp (s) the pulse time. Small pulse angle 8708 | Dalton Trans., 2010, 39, 8706–8717 of about 10◦ corresponding to about 0.8 to 1.2 ms pulses were used in the MAS sequence. This was associated to a recycling time of 500 ms. Calibration was adjusted with the resonance line of AlCl3 at 0 ppm. A collection of 2000 transients to get a proper signal to noise response was necessary. Single pulse experiment is quantitative and therefore the relative nuclei site population was accessible. However for non-integer quadrupolar nuclei, the central transition is not perturbed by the first order quadrupolar interaction, while the second order is known to broaden the resonance line as well as to shift its position from the isotropic chemical shift. In the following, the chemical shifts are neither corrected for second-order quadrupolar effects and nor reported as isotropic values. Indeed, the maximum intensity positions are comparatively discussed for the two YAG series. Results and discussion Structural organization has been investigated at different levels, from local atomic environment to long-range structure. Consequently, results will be divided in three sub-sections depending on the studied scale range. Long-range structural organization Long-range structural organization has been studied by means of XRD and IR spectroscopy. EPR measurements have been carried out to evidence organic residues departure during the crystallization process. XRD patterns obtained for unmodified and acac-modified Tb3+ powders heat-treated for 4 h at temperatures lying between 200 ◦ C and 1400 ◦ C are gathered in Fig. 1. Those for undoped samples are presented as ESI in Fig. S1.† The overall effect of acacH addition, observed in both undoped and Tb3+ -doped YAG powders is to lower the crystallization temperature of the YAG matrix by at least 100 ◦ C. Namely, the crystallization occurs at 600 ◦ C for doped modified powders (Fig. 1(b)) and at 700 ◦ C for doped unmodified ones (Fig. 1(a)). It is noteworthy that, in the absence of Tb3+ , YAG crystallization starts at 700 ◦ C and 800 ◦ C for modified and unmodified powders respectively (Fig. S1, ESI†). This difference in crystallization temperature between doped and undoped powders suggests that, like acacH, Tb3+ assists as well the matrix crystallization. Below 800 ◦ C, a large and weak band centered at 28◦ (2q), characteristic of an amorphous phase is observed for both sets of samples. All the diffraction peaks are assigned to the YAG structure (JCPDSfile 33-0040) and no intermediate phase such as YAlO3 or Y4 Al2 O9 can be observed. Furthermore, the crystallization process becomes much more significant at higher temperature since diffraction peaks are much narrower and well-defined. Besides, mean crystallite sizes <D> have been determined for doped and undoped YAG powders from XRD patterns using Scherrer’s equation:20 D = 0.9l b cos q (1) where l is the X-ray wavelength, b the corrected half-width of the strongest diffraction peak located at about 33.3◦ (2q). This journal is © The Royal Society of Chemistry 2010 Table 1 Observed IR frequencies and band assignments of xerogels and crystallized powders Wavenumbers/cm-1 Sample RC = 0 Xerogels RC = 1 Assignments 1593 1517 u(C O) : bonded acac u(C C) : bonded acac u(C–H) : i PrOH, i PrO, acac d(C–H) + u(C C): acac 1508 1400 1448 (shoulder) 1389 1357 (shoulder) 1290 1265 849 781 781 716 681 563 511 717 681 563 509 u (Y–O) u (Al–O) u (Y–O) u (Y–O) 762 656 528 Fig. 1 XRD patterns of Y3 Al5 O12 : Tb(20%) powders synthesized with (a) RC = 0 and (b) RC = 1 and sintered for 4 h at different temperatures. Mean crystallite size evolution according to the heatingtreatment temperature, calculated for both sets of samples, is presented in Fig. 2. The general effect of acacH addition, observed Fig. 2 Temperature evolution of mean crystallite size determined from XRD patterns for unmodified and acac-modified YAG powders heated for 4 h at different temperatures. This journal is © The Royal Society of Chemistry 2010 u(C–CH3 ) + u(C C) : acac g(C–H) : acac g(C–H) : i PrO p(C–H): acac p (CH3 –C(CO)) : acac ring deformation + u (M–O) u (Al–O) 839 Crystallized powders d(CH3 ) : i PrO, acac d(CH3 ) : acac on samples heated until 1100 ◦ C, is to decrease the crystallite size, with a more marked effect for undoped samples. This could be the result of the presence of acac-groups at the surface of colloidal particles in the modified sol precursor,11–12,21 which slow down the condensation process and prevent the particles from aggregation by steric hindrance. At higher temperature, distinct behaviours are observed for undoped and doped samples since, for these latter, crystallite size of modified powders is about twice smaller than that of unmodified whereas similar crystallite sizes are obtained for the two sets of undoped samples. No explanation is proposed up to now for these differences. In order to unravel how acetylacetone promotes structural organization in modified powders, IR spectroscopy investigation has been carried out. Fig. 3 shows ATR spectra of unmodified (Fig. 3(a) and 3(b)) and acac-modified (Fig. 3(c) and 3(d)) YAG:Tb(20%) powders. For clarity, results are presented in two sets: the first one, in the 1700–1200 cm-1 range (Fig. 3(a) and 3(c)), is mainly sensitive to the vibrations relative to organic residues whereas the second one, within the 900–400 cm-1 range (Fig. 3(b) and 3(d)), is more sensitive to the M–O (M = Y/Tb or Al) vibrations. All assignments22–23 concerning xerogels and crystallized powders are gathered in Table 1. The presence of characteristic bands21 located at 1517 and 1593 cm-1 for the modified xerogel (Fig. 3(c)) clearly evidences that acac-groups are chelated to metal atoms and remain chelated until 200 ◦ C. Namely, for this sample, the two broad bands observed between 1350 and 1600 cm-1 appear like an envelop of the chelatedacac groups characteristic bands with a clear shoulder at about 1517 cm-1 . IR results show that acac departure occurs between 200 and 600 ◦ C, in agreement with the TG/DTA results reported elsewhere.12 Considering other organic residue departure (Fig. 3(a) and Fig. 3(c)), one can notice that bands relative to i PrO-groups Dalton Trans., 2010, 39, 8706–8717 | 8709 Fig. 3 Infrared spectra of (a,b) unmodified and (c,d) acac-modified YAG:Tb (20%) powders unheated (xerogel) and sintered for 4 h at different temperatures. Several curves are dot-lines for clarity. (Fig. 3(c)) vanish after a 700 to 800 ◦ C heating treatment. Meanwhile, typical Al–O and Y–O vibrations in YAG appear at 700 ◦ C and 800 ◦ C for acac-modified and unmodified powders respectively (Fig. 3(b) and 3(d)). This is in accordance with the XRD study. The black or grayish color of samples calcined at temperatures between 600 ◦ C and 800 ◦ C suggests the presence of disordered carbonaceous impurities, resulting from the organic precursor pyrolysis. The presence of pyrolytic carbons has been confirmed in unmodified samples by Raman spectroscopy.12 Since EPR evidences paramagnetic substances such as localized unpaired electron, i.e. dangling bond, involved in pyrolysis of organic materials,24 it has been used to prove organic residue removal. It will allow us to know if YAG samples contain pyrolitic carbons by highlighting dangling bonds. EPR spectra obtained for unmodified and acac-modified powders heat-treated at 600, 700 and 800 ◦ C are shown in Fig. 4. Table 2 reports the EPR parameters deduced from these measurements: the Landé factor g, the peak-to-peak linewidth DH pp and the asymmetry ratio X1 /X2 . Regarding the g factor close to 2.003, recorded spectra are characteristic of carbonaceous radicals. 8710 | Dalton Trans., 2010, 39, 8706–8717 Unmodified samples are characterized by strong signals till a 800 ◦ C thermal treatment, for which very few radicals remain (Fig. 4(a), curve (C)). At higher sintering temperatures, no trace of organic residues has been detected. For acac-modified samples, a heating temperature of 600 ◦ C leads to a very intense signal (Fig. 4(b) curve (A)) whereas no more pyrolytic carbon can be identified at 800 ◦ C. Acac-modified samples are characterized by an asymmetric signal (see ratio X1 /X2 in Table 2). Particularly, the EPR spectrum recorded for the 600 ◦ C-sintered acac-modified powder is completely different from its unmodified counterpart (Fig. 4, curves (A)): it is narrower and asymmetric. It can be concluded that these samples contain dangling bonds of different natures. Furthermore, EPR signal obtained for the 700 ◦ C-sintered acac-modified powder can be decomposed into two distinct Lorentzian contributions (as shown in Fig. 5), corresponding either to two types of free radicals, derivate from the acac and isopropoxy groups or to different carbon sheet extend, which results in different localization for the unpaired electrons.25 The first ones could entail the sharp signal obtained at 600 ◦ C (contribution 1 in Fig. 5) whereas the second lead to the broader contribution added at 700 ◦ C (contribution 2 in Fig. 5). This This journal is © The Royal Society of Chemistry 2010 Table 2 EPR parameters of unmodified and acac-modified YAG powder sintered for 4 h at 600 ◦ C, 700 ◦ C and 800 ◦ C Parameter RC = 0 RC = 1 DH pp (G) X1 /X2 N S /N 0 Landé factor g 600 ◦ C/4 h 2.9 0.8 1 2.0036 ± 0.0005 700 ◦ C/4 h 2.7 0.9 0.75 800 ◦ C/4 h 3.5 1.0 not assessed 600 ◦ C/4 h 0.6 0.8 1.2 700 ◦ C/4 h 1.3 0.7 0.22 2.8 Fig. 5 EPR spectrum of acac-modified YAG powder sintered at 700 ◦ C for 4 h and corresponding simulation. Fig. 4 EPR spectra of (a) unmodified and (b) acac-modified YAG powder sintered for 4 h at (A) 600 ◦ C, (B) 700 ◦ C and (C) 800 ◦ C. X1 and X2 represent parameters of the asymmetry ratio (see Table 2). assumption, consistent with the IR spectroscopy study detailed below, is strengthened by DH pp values: DH pp1 = 1.3 G and DH pp2 = 2.8 G, close to values obtained for unmodified samples (see Table 2). Finally, the relative quantity of spin carriers has been assessed using the following equation: N S ⎛⎜ S ⎞⎟ ⎛⎜ B ⎞⎟ ⎛⎜ G0 ⎞⎟ = ⎜ ⎟⎟×⎜ ⎟⎟ ×⎜ ⎟ N 0 ⎜⎜⎝ S0 ⎟⎠ ⎜⎜⎝ B0 ⎟⎠ ⎝⎜ G ⎟⎠ 2 This journal is © The Royal Society of Chemistry 2010 (2) where N S and N 0 are the numbers of spin carriers of the sample and the reference respectively (the reference is the unmodified powder sintered at 600 ◦ C); S and S0 represent the integral surfaces, B and B0 the spectral windows and G, G0 the received gains. After a heating-treatment at 600 ◦ C, the acac-modified sample contains more spin carriers than the unmodified specimen due to the acac-groups (see Table 2). At 700 ◦ C, the amount of spin carriers is sensitively lower in acac-modified sample. This confirms a better elimination of organic residues. These results are in good accordance with powders appearance since unmodified samples remain grayish till a calcination temperature of 800 ◦ C. This behavior is of importance for phosphors since dark powders are less luminescent than white ones as it will be detailed in the second part of this investigation. It represents an original way to give rise to more efficient luminescent materials with the same heating treatment. It is noteworthy to precise that long-range structural organization had been already studied by X-ray diffraction and IR spectroscopy in our previous paper12 but the investigations performed here allow us to better understand its evolution during crystallization. Indeed, the removal process of organic residue has been elucidated, thanks to the EPR study combined with IR results, and crystallite sizes have been calculated from XRD patterns. Nanostructure and morphology The bilogarithmic plot of SAXS data obtained for the two series of powders fired at different temperatures are shown in Fig. 6 and 7. Dalton Trans., 2010, 39, 8706–8717 | 8711 Fig. 6 Log–log plots of SAXS data and structure parameters for Tb-doped unmodified powders unheated (xerogel) and heat-treated for 4 h at different temperatures. Fig. 7 Log–log plots of SAXS data and structure parameters for Tb-doped acac-modified powders unheated (xerogel) and heat-treated for 4 h at different temperatures. For unmodified powders fired below 800 ◦ C, two power law decay regimes can be recognized on the wave vector (q) scattering curves, I(q) presented in Fig. 6. At high q range the curves follow an asymptotic linear behavior, with a slope approximately equal to -4, thus indicating that the Porod law (I μ q-4 ) is obeyed. This behavior indicates also that the sample nanostructure can be described by a two-electron density model; i.e., as a biphasic system with a sharp and smooth interface. However, for powders treated at 200 ◦ C or less, the slope value (-3.1) is higher than that of Porod law indicating the formation of a rough interface. The second power law decay regime, verified at low q range presents a slope value (-2.2 to -1.7) characteristic of mass fractal objects (I μ q-D ). A reduction of the extension of the fractal regime occurs by increasing the firing temperature; as a consequence, only the Porod regime is clearly observed in the sample fired at 1100 ◦ C. The position of the crossover between the Porod (qr ) and the fractal 8712 | Dalton Trans., 2010, 39, 8706–8717 regime corresponds to the reciprocal value of the minimum radius of the hard building block (r = 1/qr ) giving rise to the fractal structure. The SAXS profiles of acac-modified xerogel and powders treated at 200 and 400 ◦ C (Fig. 7) present a single power law decay regime with a slope of -3.5, indicating that this scattering is related to the rough surface of a large particle (>15 nm). The positive departure of the Porod regime observed at high q-range (q > qp ) is associated with the random electronic fluctuation in the solid matrix. The extension of this deviation became more evident as the firing temperature increases to 400 ◦ C, giving rise to a maximum in the scattered intensity. Taking into account the EPR results, we assume that this peak maximum is an interference effect in the X-ray scattering amplitude produced by the existence of a strong spatial correlation between thermodecomposition heterogeneities (nanoporosity or pyrolitic carbons rich domains). From the position of peak maximum (qmax ) we estimated that the average distance (d = 2p/qmax ) between these domains is ª7.0 nm, while their Guinier gyration radius (Rg ) is ª1.0 nm. This maximum is shifted to intermediate q-region and a second departure from Porod power law decay is observed at the low qrange of SAXS profile of powders fired at 600 ◦ C. This feature can be explained by the existence of a hierarchic nanostructure formed by two well-resolved textural levels presenting Rg close to 2.2 and 7.0 nm. The formation of each textural level can be associated to the stepped thermodecomposition involved in the elimination of acac and isopropoxyl derivatives ligands. Finally the wide and the short plateaus observed in SAXS profiles corresponding to samples fired at 800 and 1100 ◦ C, respectively, evidence the continuous coarsening of nanoporosity as the firing temperature increases. From these SAXS results, the most important noticeable differences between acac-modified and unmodified YAG-based powders are as follow: (i) While the unmodified xerogel presents a fractal structure characterized by a dimensionality close to the values expected for a reaction-limited cluster–cluster growth processes (Df = 2.2),25 acacmodified xerogels presents a compact and rough structure which is consistent with the monomer-cluster aggregation process.26 This finding evidences that the decrease of the hydrolytic reactivity of sol–gel precursors by acac-complexation favours the existence of a continual source of monomers, which is the necessary condition for monomer-cluster growth mechanism. It is important to note that this proposition is confirmed by the SEM images presented in Fig. 8. As it can be observed in Fig. 8(a), unmodified xerogel is made of an open aggregate structure whereas acacmodified xerogel is characterized by larger and dense particles (Fig. 8(b)). (ii) The second point is related to the thermo-stability of the nanostructure. Between 80 (xerogel) and 600 ◦ C, the largest modification of the nanostructure of unmodified powders is observed after treatment at 200 ◦ C, that leads to the lower D (1.7) and Rg (1.0 nm) values and to the largest deviation from the Porod regime. However, the relevant nanostructural parameters of xerogel are recovered after firing at 400 or 600 ◦ C. The relatively unusual feature at 200 ◦ C is due to residual street channels originated by the evaporation of organics condensed in the small capillary. On the opposite, for acac-modified powders, the hierarchic porosity originated from the thermodecomposition This journal is © The Royal Society of Chemistry 2010 Fig. 8 SEM images of unmodified (a) and acac-modified (b) YAG: Tb(20%) xerogels (no heating-treatment) – magnification is ¥250 000. of the organic ligands coarses continuously as the temperature increases. The progressive morphology change with temperature is also scrutinized by the SEM images displayed in Fig. 9. After a thermal treatment at 800 ◦ C, initial sintering of the intra-aggregated structure is observed in the unmodified powder (Fig. 9(a)). When acacH is used during the synthesis, the same heating-treatment results in a much denser mesoporous network with relatively smooth surface (Fig. 9(b)). Densification process is already in progress in this acac-modified powder, since YAG phase has crystallized as shown by XRD (Fig. 1(b)). A 1100 ◦ C thermal treatment leads to coalescent primary particles agglomerated into a sponge-like network for unmodified powder (Fig. 9(c)) whereas the acac-modified sample is characterized by well-clusterized roughly spherical primary particles (Fig. 9(d)) of 50 nm. Firing at 1400 ◦ C entails a pronounced densification of unmodified powder while an interconnected porous structure is obtained for acac-modified sample (Fig. 9(f)), due to the bridging of adjacent particles. It can also be noticed an increase in primary particle size with the heating temperature: from 20–30 nm for 800 ◦ C to 400–500 nm for 1400 ◦ C. Short-range structural organization Y3 Al5 O12 generally adopts a cubic garnet structure with lattice parameter of 12 Å (space group Ia3̄d).27 Its structure This journal is © The Royal Society of Chemistry 2010 consists of a 3D oxygen network where aluminium atoms reside both in octahedral and tetrahedral interstices whereas yttrium atoms occupy dodecahedral sites. These polyhedra are distorted:27 octahedra are lengthened whereas tetrahedra are compressed. The evolution of local environments of yttrium and aluminium atoms in our sol–gel derived samples during their structuration have been studied by XAS and 27 Al NMR. The pair radial distribution functions (PRDFs) around yttrium, uncorrected from phase shift, recorded for both kinds of YAG:Tb(20%) powders (RC = 0 and RC = 1) are shown in Fig. 10 and Fig. 11, respectively. Structural parameters were determined for all samples and are gathered in Table S1, ESI.† The corresponding fit, compared with the theoretical EXAFS spectrum calculated by FeFF6, is exhibited in Fig. S2, ESI.† Data and theoretical signals are in good agreement. Until 400 ◦ C, both types of powders are characterized by a primitive arrangement around yttrium atoms since only the first coordination shell can be observed: environments consisting of 3.8 ± 0.6 and 2.2 ± 0.4 oxygen atoms with distances in agreement with YAG structure (R1 = 2.33 Å and R2 = 2.46 Å) .17 For unmodified samples (Fig. 10, ESI†), the second coordination shell corresponding to YAG crystallization appears at 800 ◦ C whereas a 600 ◦ C heating-treatment is sufficient to entail the structuration onset in acac-modified powders with the appearance of the second and third coordination shells (Fig. 11) corresponding to Y–Al or Y–Y/Tb distances.17 It is noteworthy to point out the evolution of the local environment of yttrium atoms; after being octahedral in xerogels and slightly sintered powders, it changes to pentahedral (after 600 ◦ C and 700 ◦ C heating treatments for acac-modified and unmodified powders respectively), before becoming again octahedral jointly to YAG crystallization (see Table S1, ESI†). These changes could be the result of the late departure of organic groups, like isopropoxyl, which should produce oxygen vacancies in the YAG structure. Terbium coordination shells have also been studied in these samples. Details will be given in the second part of this investigation dealing with the influence of acacH on the optical properties of these powders. Finally, Al local environment has been investigated by 27 Al NMR on undoped unmodified (RC = 0) and acac-modified (RC = 1) YAG samples from sols to crystallized powders. Since the chemical shift of 27 Al NMR is sensitive to the local coordination, this technique is largely applied for checking the different phases and coordination states of Al centers in aluminates such as YAG.28 Fig. 12 shows the 27 Al NMR spectra. Given that aluminium atoms are located both in octahedral and tetrahedral sites in crystallized YAG,27 corresponding 27 Al NMR spectra should display at least two distinguishable signals:29 octahedral AlO6 sites resonate between 20 and -10 ppm, and tetrahedral AlO4 between +80 and +50 ppm. Indeed, such expected contributions, with an intense signal centered around 0 ppm, assigned to AlO6 polyhedra and broad down-field signal assigned to lower coordination number are depicted on the 27 Al MAS NMR spectra (Fig. 12). In detail, the more deshielded response is composed of two major resonance peaks, located between +70 and +60 ppm for the first, and between +35 and +20 ppm for the second, that can be ascribed to four and five-fold coordinated aluminium atoms, respectively. Dalton Trans., 2010, 39, 8706–8717 | 8713 Fig. 9 SEM images of undoped unmodified (a,c,e) and acac-modified (b,d,f) YAG powders heat-treated for 4 h at (a,b) 800 ◦ C, (c,d) 1100 ◦ C and (e,f) 1400 ◦ C – magnification are different. It can be noticed that the resonance around 30 ppm can also be consistent with a tetrahedral site distorted due to the presence of oxygen defects.30 These signals are strongly overlapped at room temperature and more and more separated in temperature. This latter phenomenon is explained by two different effects: (i) a high field shift of the intermediate response (materialized by the dashed lines in Fig. 12) and (ii) the shrinkage of all the resonance lines giving rise to well-defined contributions when the firing temperature increases. The down-field response associated to broad line shape can be explained by a distribution of electric gradients coming from a large degree of structural disorder (distribution of Al sites). Similarly, but relatively less pronounced, the high-field response characteristic of Al(Oh) is also enlarged. In addition to site distribution, one cannot discard the presence of proton in the vicinity of Al nuclei, thus enlarging the resonance lines by heteronuclear coupling 1 H–27 Al, this especially for low-temperature 8714 | Dalton Trans., 2010, 39, 8706–8717 treated materials. Indeed, above 800 ◦ C, the signal corresponding to AlO6 polyhedra shrinks into a sharp signal. This feature is characteristic of YAG matrix;31 consequently, the appearance of this sharp signal should correspond to the YAG crystallization onset. The other two bands remain for the crystallized powders, which reinforces the assumption about the presence of distorted tetrahedra in YAG matrix. These results are in disagreement with Veith et al.’s study31 in which crystallized YAG and xerogel were characterized by very different spectra. Furthermore, if compared acac-modified and unmodified samples, one can notice that spectra relative to xerogels and crystallized powders (for temperatures superior to 800 ◦ C) are similar for both kinds of powders. On the contrary, visible discrepancies can be observed on the 400–800 ◦ C range, which could be called “transitional temperature range” since most of the crystallization process occurs within this temperature range. In order to highlight these discrepancies, 27 Al NMR spectra corresponding to umodified and This journal is © The Royal Society of Chemistry 2010 Fig. 10 (a)Y K-edge kc(k) EXAFS oscillations and (b) its corresponding Fourier Transform uncorrected from phase shift for unmodified YAG powders (RC = 0) sintered at different temperatures. acac-modified powders calcined between 400 ◦ C and 800 ◦ C have been superimposed and gathered in Fig. 13. First, a shoulder can be observed around +10 ppm on spectra relative to unmodified powders sintered between 400 ◦ C and 700 ◦ C. This signal very likely corresponds to another octahedral population and does not appear for acac-modified samples since the shoulder cannot be explained by a mere broadening due to dipolar hetero coupling or quadrupolar interaction. To our knowledge, this secondary octahedral population in mixed oxides like YAG is evidenced for the first time. Besides, the sharp octahedral signal related to YAG begins to appear quite early in unmodified sample (400 ◦ C) but delayed in comparison to acac-modified powder since the YAG signature is observed directly for this latter at 600 ◦ C while non-modified sample displays a double contribution until 800 ◦ C. These results are in good accordance with XRD and IR investigations, even if unmodified samples appear “more locally structured” than XRD should have guessed, showing that structuration occurring at small scale range is limited in size and developed with coalescence at higher temperatures. Conclusion Surmised with our early work,12 the influence of acetylacetone on the structural rearrangement during the crystallization of YAG samples has been further evidenced here, using specific This journal is © The Royal Society of Chemistry 2010 Fig. 11 (a)Y K-edge kc(k) EXAFS oscillations and (b) its corresponding Fourier Transform uncorrected from phase shift for acac-modified YAG powders (RC = 1) sintered at different temperatures. characterization techniques. Namely, SAXS and SEM are both conclusive about the network aggregation process whereas the temperature of crystallization onset was determined by XAS and NMR around 600 ◦ C, that means about 200 ◦ C below the temperature at which crystallite size can be determined from XRD. These techniques, particularly suitable for studying amorphous samples, give us new insight about the local order around the two cationic species of the matrix (yttrium and aluminium). It was evidenced that the local order around yttrium (distances in the first oxygen coordination shell) and around aluminium (distribution in octahedral and tetrahedral sites) is in agreement with the neoformation of YAG crystallites. To summarize, XRD and EXAFS have confirmed an earlier crystallization in acac-modified powders whereas IR spectroscopy and EPR measurements have revealed that acac-groups are withdrawn before isopropoxy-ones. On this account, one can imagine that the pyrolysis of acac-groups, that happens between 200 ◦ C and 400 ◦ C, according to IR spectroscopy, acts as a driving force for YAG structural arrangement. This feature results in a faster organization in acac-modified samples beyond 400 ◦ C. The better organic residue removal, shown by IR and EPR spectroscopies can be ascribed to the more open morphology of the acac-modified powders related to a monomer-cluster aggregation process. This leads to less grayish powders, and as a result to better luminescence properties as it will be detailed in the second part of this article. Dalton Trans., 2010, 39, 8706–8717 | 8715 Fig. 12 27 Al NMR spectra of unmodified (RC = 0) and acac-modified (RC = 1) undoped YAG powders heat-treated for 4 h at different temperatures. 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