Synthesis, Thermal Evolution, and Luminescence Properties of Yttrium

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Chem. Mater. 2005, 17, 1774-1782

Synthesis, Thermal Evolution, and Luminescence Properties of Yttrium Disilicate Host Matrix M. Dı´az,† C. Pecharroma´n,† F. del Monte,† J. Sanz,† J. E. Iglesias,*,† Jose´ S. Moya,† C. Yamagata,‡ and S. Mello-Castanho‡ Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco 28049, Madrid, Spain, and Instituto de Pesquisas Energe´ ticas e Nucleares, TraVessa R, 400-05508-900 Sao Paulo, Brazil ReceiVed NoVember 22, 2004. ReVised Manuscript ReceiVed January 14, 2005

A new pressureless hydrothermal method for the preparation of yttrium disilicate is presented. The obtained amorphous precursor was calcined at different temperatures to form the Y-, R-, β-, and γ-phases of Y2Si2O7, which have been characterized by DTA, XRD, TEM, and 29Si-MAS RMN and IR spectroscopy. It is shown that the proposed method allows the strictly controlled doping of yttrium disilicate using RE elements such as dysprosium (2.5 and 5 at. %). The luminescent properties, in terms of emission efficiency, of different polymorphs of Dy-doped Y2Si2O7 have been investigated by fluorescence measurements at room temperature. The results indicate that the β-phase doped with 2.5 at.% has the maximum efficiency. It is important to note that the efficiency of this phase is approximately 40% of that measured for the commercial phosphor Eu-Y2O3.

1. Introduction The binary disilicates, and specially the rare earth disilicates, have been widely studied for their magnetic, electrical, and optical properties.1 In particular, these materials exhibit an adequate behavior for luminescent applications as plasma displays, laser materials, and high-energy phosphors when doped with rare-earth elements. The luminescence of these phosphors is very efficient under both cathode ray and UV excitation.2-8 In the case of Y2Si2O7, only the luminescence of Ce3+ and Tb3+ ions has been extensively studied.2,9-11 Y2Si2O7, which occurs naturally as yttrialite, displays interesting structural properties because of its high refractoriness (mp ) 1775 °C) and stability in oxidizing environments. Y2Si2O7 possesses five polymorphs (Y, R, β, γ, δ) that have been studied and revised in the past by several authors.12-16 However, some controversy remains with regard * To whom correspondence should be addressed. Tel: (+34) 91 334 9000. Fax: (+34) 91 3720623. E-mail: [email protected]. † Instituto de Ciencia de Materiales de Madrid (CSIC). ‡ Instituto de Pesquisas Energe ´ ticas e Nucleares.

(1) Felsche, J. Struct. Bonding 1973, 13, 100. (2) Peters, T. E. J. Electrochem. Soc. 1969, 116 (7), 985. (3) Tsujimoto, Y.; Fukuda, Y.; Sugai, S.; Fukai, M. J. Lumin. 1975, 9, 475. (4) Reichardt, J.; Sttiebler, M.; Hirrle, R.; Kemmler-Sack, S. Phys. Status Solidi A 1990, 119, 631. (5) Chateau, C.; Ho¨lsa, J.; Porcher, P. Z. Phys. Chem. Neue Folge 1990, 166, 211-222. (6) Lin, J.; Su, Q. High Temp. Mater. Sci. 1996, 36, 117-125. (7) Choi, Y. Y.; Sohn, K.-S.; Park, H. D.; Choi, S. Y. J. Mater. Res. 2001, 16 (3), 881-889. (8) Marsh, P. J.; Silver, J.; Vecht, A.; Newport, A. J. Lumin. 2002, 97, 229-236. (9) Gomes de Mesquita, A. H.; Bril, A. Mater. Res. Bull. 1969, 4, 643650. (10) Meiss, D.; Wischert, W.; Kemmler-Sack, S. Phys. Status Solidi A 1992, 134, 539. (11) Kepinski, L.; Hreniak, D.; Strek, W. J. Alloys Compd. 2002, 341 (1-2), 203-207.

to the stability of the different polymorphs. In particular, it has been pointed out that the crystallization mechanism is strongly related to the synthesis route employed and the thermal history.17 The synthesis methods commonly used to prepare yttrium disilicate are the conventional solid-state reaction of mixed oxides Y2O3 and SiO2, and the calcination of yttrium disilicate precursors synthesized by sol-gel and hydrothermal processing. The first route involves the interdiffusion of the oxide powders and requires long reaction times, around 100 h, and temperatures in the range 9001800 °C.18 Much lower temperatures (300 °C) are needed to form single-phase crystals of yttrium disilicate from Y2O3 and SiO2 precursors via the hydrothermal method by Nekrasov and Kashirtseva,19 but long holding times (100 days) are necessary: even using an autoclave and a processing temperature of 650 °C, 11 days are required. Such long synthesis times, together with a tendency to retain some amount of unreacted precursor components, are the main drawbacks for preparation of yttrium disilicate phases from a practical point of view. Parmentier et al.20 synthesized yttrium disilicate by calcination of gels obtained from TEOS and Y(NO3)3, but in this case the hydrolysis reaction was performed by the water of crystallization in the Y(NO3)3, leading to a very long gelation time (∼3 weeks). It was concluded that, although it is possible to obtain pure single(12) (13) (14) (15) (16) (17)

Bondar, I. A.; Toropov, N. A. Mater. Res. Bull. 1967, 2, 479. Ito, J.; Johnson, H. Am. Mineral. 1968, 53, 1940. Felsche, J.; Hirsiger, W. J. Less-Common Met. 1969, 18, 131-137. Batalieva, N.; Pyatenko, Yu, A. SoV. Phys. Crystallogr. 1972, 16, 786. Felsche, J. J. Less-Common Met. 1970, 21, 1-14. Trusty, P. A.; Chan, K. C.; Ponton, C. B. J. Mater. Res. 1998, 13 (11), 3135-3143. (18) Liddell, K.; Thompson, D. P. Br. Ceram. Trans. J. 1986, 85, 17-22. (19) Nekrasov I. Ya; Kashirstseva, G. A. Dokl. Akad. Nauk SSSR 1976, 231, 166-169. (20) Parmentier, J.; Bodart, P. R.; Audoin, L.; Massouras, G.; Thompson, D. P.; Harris R. K.; Goursat, P.; Besson, J.-L. J. Solid State Chem. 2000, 149, 16-20.

10.1021/cm047957b CCC: $30.25 © 2005 American Chemical Society Published on Web 03/11/2005

Synthesis of Yttrium Disilicate Host Matrix

phases of yttrium disilicate, the polymorphs obtained showed different temperature ranges of stability than those reported in previous papers. Recently, different sol-gel and hydrothermal methods, which allow the mixing of the chemical precursors at the nanoscale, have been proposed to improve the homogeneity and to reduce both the time and the temperature required to form yttrium disilicate. Moya et al.21 developed a sol-gel method to synthesize nanocrystalline yttrium disilicate powder by calcination of a precursor obtained from an aqueous solution of TEOS and Y(NO3)3, using a slight excess of oxalic acid as a precipitating agent. The formation of a crystalline single-phase of yttrium disilicate at 1065 °C in short calcination times (2 h) was reported. However, the amount of oxalic acid added to the solution must be strictly controlled in order to obtain pure Y2Si2O7 single-phases, because small changes in its initial concentration can give rise to compositional heterogeneities in the precursor that lead to an excess of unreacted components, such as Y2O3, in the final product. For that reason, Dı´az et al.22 carried out a modification of that method, using hydrochloric acid as an acid catalyst for the hydrolysis of TEOS. The amorphous precursor obtained yielded pure nanocrystalline R-Y2Si2O7 at 1030 °C, irrespective of the HCl concentration in the initial TEOS/Y(NO3)3 aqueous solution. Trusty et al.17 have also investigated the hydrothermal processing for the synthesis of nanosized single-phase yttrium disilicate precursor particles from TEOS and yttrium acetate, using autoclaving techniques. The precursor synthesis was carried out under acidic and basic conditions, with the one prepared at basic pH giving the most homogeneous compositions. As in the previously described sol-gel methods by Dı´az et al., the calcination times required to transform the precursor into crystalline yttrium disilicate are short, of the order of 1 h, and the temperature at which this phase occurs is similar, 1050 °C. However, further TEM and XRD studies23 revealed that the powders obtained under alkaline conditions and heated at 1038 and 1100 °C did not form a yttrium disilicate single-phase, but a mixture of phases constituted by R-Y2Si2O7, as the main phase, and Y-Y2Si2O7. The Y-polymorph, also called “low yttrialite”, is stable up to 1200 °C, and can be obtained by heating of the natural metamictic mineral yttrialite. According to the literature13,15 this phase can be stabilized in the presence of impurities such as H+, Na+, Mg2+, Mn2+, Fe2+, Fe3+, and Al3+, and by doping with rare-earth cations such as Th4+. The only structural data corresponding to this phase are those reported by Batalieva et al.15 from XRD of a Th-doped Y2Si2O7 crystal. Recently, Becerro et al.24 have obtained Y-Y2Si2O7 via hydrothermal synthesis from smectites. The results indicate that the presence of Al3+ in the starting material aids the stabilization of the Y-phase. (21) Moya, J. S.; Dı´az, M.; Serna, C. J.; Mello-Castanho, S. J. Eur. Ceram. Soc. 1998, 18, 1381-1384. (22) Dı´az, M.; Garcı´a-Cano, I.; Mello-Castanho, S.; Moya, J. S.; Rodrı´guez, M. A. J. Non-Cryst. Solids 2001, 289, 151-154. (23) MacLaren, I.; Trusty, P. A.; Ponton, C. B. Acta Mater. 1999, 47 (3), 779-791. (24) Becerro, A. I.; Naranjo, M.; Alba, M. D.; Trillo, J. M. J. Mater. Chem. 2003, 13, 1835-1842.

Chem. Mater., Vol. 17, No. 7, 2005 1775 Table 1. Crystalline Phases Obtained from YSIO1, YSIO2, and DYSIO Precursors Using Different Heat Treatments heat treatment precursor

temp (°C)

time (hours)

crystalline phases

YSIO1

900 925 975 1000 1200 1300 1400 1600 1200 1400 1600 1200

2 2 2 2 2; Qa 2; Q 2; Q 2; Q 2; Q 2; Q 2; Q 2; Q

amorphous idem Y + R*b idem R + Y* R + Y* + β* β γ R β γ R

DYSIO YSIO2 a

Q denotes quenching in air.

b

* Indicates small amounts.

In the present work a simple, reliable, low-cost, pressureless hydrothermal route for the synthesis of Y-Y2Si2O7 is reported. The method consists of the formation of an amorphous precursor via hydrothermal treatment of a coprecipitated mixture composed of yttrium hydroxide and silica gel, which is formed from an aqueous solution of Y(NO3)3 and TEOS by addition of a concentrated ammonia solution. The thermal evolution of Y-Y2Si2O7 has been studied using X-ray diffraction, TEM, IR, and 29Si MAS NMR spectroscopies. In particular, 29Si NMR is a very useful technique to analyze different Y2Si2O7 phases or mixtures of them.20,24 Finally, the luminescent properties of yttrium disilicate doped with dysprosium as phosphor, have been studied since this method allows the strict control of doping in a simple manner. The luminescent properties, in terms of emission efficiency, of the different polymorphs have also been investigated. 2. Experimental Procedure 2.1. Materials. The following reagents were used as starting materials: yttrium nitrate tetrahydrate (99.99%, Aldrich Chemical Co.), tetraethyl orthosilicate (TEOS) (99%, Aldrich Chemical Co.), ammonium hydroxide (30% in water, Aldrich Chemical Co.), ethyl alcohol (96%), and dysprosium nitrate pentahydride (99.9%, Aldrich Chemical Co.). All of them were used without further purification. 2.2. Synthesis. A solution of TEOS (3.28 g, 14 mmol) in ethyl alcohol (volume ratio 1:5) was added to an aqueous solution of Y(NO3)3‚4H2O (5 g, 14 mmol) and stirred to ensure a thorough mixing. Upon addition of an ammonia solution (6 N) the precipitation of a white compound was produced at pH 10. The suspension was refluxed at 80 °C for 6 h under atmospheric pressure. Finally, the resulting Y2Si2O7 precursor was dried at 60 °C for 12 h. This powder precursor, labeled YSIO1, was used for further hightemperature treatments in the range 900-1600 °C. The same procedure was followed for the synthesis of Y2Si2O7 doped with 2.5 and 5 at. % of Dy3+. The amount of Dy3+ was added to the starting solutions as nitrate (Dy(NO3)3‚4H2O). This precursor was labeled as DYSIO, and was subsequently heated between 1200 and 1600 °C to obtain the different polymorphs. For comparative purposes, a third precursor, labeled YSIO2, was prepared via sol-gel, using the method described in a previous paper.22 The results of the different heat treatments applied to the precursors are summarized in Table 1. 2.3. Experimental Techniques. Yttrium disilicate precursors were analyzed by DTA up to 1200 °C in a static air atmosphere, using a heating rate of 10 °C/min and alumina as reference material.

1776 Chem. Mater., Vol. 17, No. 7, 2005 The thermal evolution of yttrium disilicate phases was studied by means of XRD, TEM, 29Si MAS NMR, and infrared spectroscopy. The X-ray diffraction powder patterns were collected in a Philips X’pert diffractometer operated in the Bragg-Brentano configuration, which was fitted with an incident-beam Ge(111) monochromator of the Johansson symmetric type, aligned to select the Cu KR1 emission, λ ) 1.5405981 Å. The patterns were calibrated by repeating the runs with a small amount of intermixed Si powder, whose lattice parameter was taken as a ) 5.430940 Å for the wavelength mentioned above (NBS standard reference material 640b). TEM analysis of the obtained precursors was carried out using a JEOL transmission electron microscopy, model JEM 2000 FXII, at 200 kV. The samples were prepared by dispersing them in acetone with the aid of ultrasonic agitation, and then allowing a drop of this suspension to dry over a carbon film on a supporting copper grid. 29Si MAS NMR spectra were obtained with a MSL 400 Avance, working at 79.4 MHz (9.4 T). To analyze the central components of the spectrum, the sample was rotated at spinning rates higher than 4 kHz. In all cases, the spectra were taken after irradiation of the sample with a π/2 pulse (4 µs). To avoid saturation effects, the recycling time used was 30 s. The number of scans used in all cases was in the range 40-100. The chemical shifts of the NMR components were referred to the TMS standard. For IR reflectance analyses, the powders were compacted in a uniaxial press under a nominal pressure of 1 GPa. A very smooth die, made in zircona partially stabilized with yttria, was used to obtain self-supporting powder pellets with a good quality specular surface. The IR spectra were taken in a Bruker IFS 66V vacuum spectrophotometer with a resolution of 1 cm-1. Two different beam splitters were used, CsI from 4000 to 220 cm - 1, and Mylar from 600 to 50 cm-1, to cover a wide spectral range. The fluorescence measurements of Dy-doped yttrium disilicate samples were carried out at 25 °C on a 48000s SLM-Aminco (T-Optics) spectrofluorometer. Emission and excitation spectra were recorded by reflection (front face mode) in the backward direction on pills of ∼100 µm thickness, obtained through packing of the finely powdered samples. The specular reflection of the excitation light was minimized by fixing the angle of incidence on the pill surface at 79°. Appropriate filters were also used to eliminate Rayleigh and Raman scattering. Excitation and emission spectra were corrected for the wavelength dependence of the 450-W xenon arc excitation, but not for the wavelength dependence of the detection system.

Dı´az et al.

Figure 1. DTA curves of precursors obtained via hydrothermal route, YSIO1 (pure) and DYSIO (doped with dysprosium), and by sol-gel synthesis, YSIO2.

3. Results 3.1. DTA and TEM Analysis. The DTA curves for hydrothermal samples, YSIO1 and DYSIO, and sol-gel specimen, YSIO2, Figure 1, show exothermic activity in the temperature range of 900-1200 °C. The formation of the Y-Y2Si2O7 phase takes place at 1031 °C for the YSIO1 precursor; however, the R-Y2Si2O7 phase was detected at 1054 and 1058 °C in the case of the DYSIO and YSIO2 precursors. The area of the exothermic peaks corresponding to YSIO1 and DYSIO are quite similar and larger than that of YSIO2 sample. The TEM micrographs and the corresponding SAD patterns of the YSIO1 precursor as prepared and after calcination at 975 °C are shown in Figure 2. It can be observed that after the exothermic peak a pure and well-crystallized Y-phase is obtained from the amorphous precursor.

Figure 2. (a) TEM micrograph of a precursor YSIO1 particle and the corresponding SAD pattern showing its amorphous character. (b) TEM image of a single crystal obtained after calcination of the amorphous precursor at 975 °C. The SAD pattern (right) has been assigned to the Y-Y2Si2O7 phase.

3.2. X-ray Diffraction. The X-ray diffraction pattern of the R-phase, obtained by heating of the YSIO2 precursor at 1200 °C, is given in Figure 3. This pattern could not be

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Figure 4. (a) Sequence of 29Si-MAS RMN spectra corresponding to YSIO1 precursor after calcination at different temperatures. (b) 29Si-MAS RMN spectrum of YSIO2 precursor calcined at 1200 °C.

Figure 3. XRD patterns obtained for the different phases of Y2Si2O7.

indexed with the triclinic cell proposed by Ito and Johnson.13,18 The best fit was obtained with the triclinic cell: a ) 6.594(2) Å, b ) 6.629 (4) Å, c ) 12.295(6) Å, R ) 94.27(6)°, β ) 89.36(5)°, γ ) 92.76(6)°, V ) 535.27 Å3. However, deviations between observed and calculated values of 2θ in some cases exceeded the error limit (about 0.025°) expected from the quality of the data. The above least squares refinement was carried out with 21 reflections to 2θ ) 42°, and out of these, only 12 behaved well. Ito’s cell is hence doubtful, and moreover the single character of the phase is difficult to postulate. The figure of merit25 is M(20) ) 8 (0.000078, 104). X-ray diffraction patterns of Y-, β-, and γ-phases obtained during thermal treatments of the YSIO1 precursor are shown in Figure 3. The Y-phase pattern was indexed by starting from the lattice parameters originally reported by Battalieva and Pyatenko (Liddell and Thompson).18 Least squares refinement with 37 reflections up to 2θ ) 60.5° converged to the final monoclinic parameters a ) 7.4471(9) Å, b ) 8.0688(4) Å, c ) 5.0472(5) Å, β ) 111.64(1)°, V ) 281.90 Å3 , and no calculated value of 2θ deviated by more than 0.025° from the observed value. The figure of merit is M(20) ) 52 (0.000032, 44). The sample was slightly impurified by a small amount of a second phase whose peaks were identifiable as belonging to the R-phase (see Figure 3). (25) deWolff, P. M. J. Appl. Crystallogr. 1968, 1, 108-113.

The β-phase pattern was indexed in a straightforward fashion from the monoclinic cell quoted in Liddell and Thompson.18 The refinement was carried out with 55 reflections up to 2θ ) 67°, M(20) ) 150 (0.000021, 21), and |2θobs - 2θcalc| < 0.02 for all reflections. The refined lattice parameters are a ) 6.8766(3) Å, b ) 8.9743(3) Å, c ) 4.7183(2) Å, β ) 101.759(4)°, V ) 285.07 Å3. The systematic absences agree with the reported space group C2/m (no. 12). However, the phase was not totally pure, and at least six visible peaks, the strongest one having an intensity of 2% that of the most intense peak in the pattern, were unindexable with our refined cell parameters (Figure 3). The pattern of the γ-phase can be easily indexed by starting with the lattice parameters reported in the paper by Liddell and Thompson.18 A least squares refinement was carried out with 81 reflections to 68.5° in 2θ, and for all of them |2θobs - 2θcalc| < 0.02 was satisfied (Figure 3). The figure of merit was M(20) ) 79 (0.000030, 24). The final monoclinic lattice parameters obtained were a ) 5.5839(2) Å, b ) 10.8431(4) Å, c ) 4.6893(2) Å, β ) 96.028(4)°, V ) 282.35 Å3. The systematic absences, h0l, h odd and 0k0, k odd are consistent with space group P21/a (no. 14) for the chosen axes; the orientation (P21/n) chosen by Liddell and Thompson18 is inconsistent with our data, and is inconsistent with the Miller indices and cell parameters they quote in their paper as well. 3.3. 29Si MAS NMR Spectroscopy. 29Si MAS NMR spectra recorded at room temperature in samples prepared by the hydrothermal route (YSIO1 precursor) and previously heated between 925 and 1600 °C, are given in Figure 4. The spectrum of the amorphous compound, obtained after heating the disilicate precursor at 925 °C, is formed by an asymmetric, broad component, centered at -82 ppm.

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Dı´az et al.

Figure 5. Plot of relative Si content for each yttrium disilicate phase calculated by deconvolution of experimental 29Si-MAS RMN spectra.

When the sample is heated at increasing temperatures, the intensity of the broad component decreases considerably and two new components at -83.4 and -85.4 ppm are detected. The presence of these two components indicates the formation of the Y-Y2Si2O7 phase at the expense of the amorphous phase. At 975 °C the 29Si MAS NMR spectrum is mainly formed by the two mentioned components. When the samples are heated at 1000 °C, two new components at -80.7 and -82.1 ppm are detected. The intensities of the four components at -80.7, -82.1, -83.2, and -85.4 ppm change with temperature, suggesting the presence of two phases (Y-Y2Si2O7 and R-Y2Si2O7) in samples heated between 1000 and 1200 °C. In this range of temperature, the intensity of the four components decreases, and a new component is detected at -93 ppm that has been ascribed to the formation of the β-Y2Si2O7 phase. In samples heated between 1400 and 1600 °C, the four peaks associated with the previous phases are not found. Finally, in samples heated above 1500 °C, a new peak at -94 ppm is detected that corresponds to the formation of the γ-Y2Si2O7 phase. A calculation of the relative percentage of the different phases is plotted in Figure 5. In the case of the sample prepared from the precursor YSIO2 by the sol-gel technique (Figure 4), heated previously at 1200 °C, the 29Si MAS NMR spectrum is formed by four components at -80.7, -82.1, -83, and -84.7 ppm of the same intensity. The purity of this phase was considerably higher than that of samples prepared through the hydrolysis technique at the same temperature. From these observations it can be concluded that the R-phase is preferentially formed by the sol-gel technique. 3.4. Infrared Spectroscopy. Near-normal specular reflectance infrared spectra of Y, R, β, and γ phases of Y2SiO7 appear in Figure 6. Normal specular reflectance is related to the dielectric constant through one of the Fresnel equations:

x(〈〉 - 1) 2 | x(〈〉 + 1)

R)|

(1)

where <  > is the effective dielectric constant of the aggregate. This magnitude can be related to the dielectric

Figure 6. Near-normal specular reflectance infrared spectra of Y, R, β, and γ phases of Y2Si2O7. The dashes on the horizontal axes represent the calculated vibrational modes.

constant of the analyzed material, p, by using an effectivemedium formalism.26-29 For p, a superposition of Lorentzian harmonic oscillators has been assumed. 4πFωTk2

N

 p ) ∞ +

∑

2

k)1ω Tk

- ω2 - iγTkω

(2)

where k runs over the N vibrational modes, ∞ is the dielectric constant at optical frequencies, ωTk is the transverse frequency of the k-th mode, and γk and 4πFk are its damping constant and oscillation strength, respectively. The maximum error in all fits was ∼4%. To obtain precise mode assignations, lattice dynamic simulations were made by VIBRAT software.30 To determine the frequencies of the modes, some hypotheses about the atomic potentials inside the crystal were made. Y-O potential information was deduced from experimental data of both Raman and IR spectra31 of Y2O3. Si-O potential constants were taken from the literature.32 (26) Bruggeman, D. A. G. Ann. Phys. 1935 (Leipzig) 24, 636. (27) Landauer, R. In Proceedings of the Fist Conference on the Electrical Transport and Optical Properties of Inhomogeneous Media; Garland, J. C., Tanner, D. B., Eds.; AIP Conf. Proc. Nο. 40; AIP: New York, 1978; p 2. (28) Pecharroma´n, C.; Iglesias, J. E. Phys. ReV., B. 1994, 49, 7137. (29) Pecharroma´n, C.; Iglesias, J. E. J. Phys. Condens. Matter 1994, 6, 7125. (30) Dowty, E. Phys. Chem. Miner. 1987, 14, 67.

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Figure 7. Room-temperature emission (dashed line) and excitation (solid line) spectra for the R-phase of yttrium disilicate doped with dysprosium.

Group-theoretical analysis of the different disilicate structures has determined the following irreducible representations: Γ ) 16Ag +11Bg + 17Au + 22Bu, for the Y-phase Γ ) 66Ag +66Bg + 66Au + 66Bu, for the R-phase Γ ) 8Ag + 7Bg + 7Au + 11Bu, for the β-phase Γ ) 15Ag +15Bg + 18Au + 18Bu, for the γ-phase In all cases, Ag and Bg stand for Raman modes, while Au and Bu stand for IR modes. Once the three acoustic modes are subtracted, the total number of IR active modes for each phase is: 36, 129, 15, and 33 for Y-, R-, β-, and γ- phases, respectively. 3.5. Luminescence Properties. The transitions 4F9/2-6H15/2 dominate the emission spectrum of R-DYSIO, though the transitions 4F9/2-6H13/2 are almost of equal intensity (Figure 7). The excitation spectrum of R-DYSIO shows incompletely resolved bands (Figure 7), corresponding to transitions from the ground state (6H15/2) to different excited states (Figure 8): 350 nm for [6P7/2 + 4M15/2] and 4I(3)11/2; 365 nm for 6P 4 4 4 5/2 and D(2)3/2; 380-400 nm for M19/2 and [ M21/2 + 4 4 4 4 K(1)17/2 + I(3)13/2 + F(3)7/2]; 425 nm for G(4)11/2; 450 nm for 4I(3)15/2; and 470 nm for 4F(3)9/2.33 On the other hand, the transitions 4F9/2-6H13/2 dominate over 4F9/2-6H15/2 in the emission spectra of γ-DYSIO and β-DYSIO (Figure 9). These compounds display also the transition 4F9/2-6H11/2, although with low intensity. The excitation spectra of these materials show bands which are better resolved than they are in the case of R-DYSIO (Figure 9). 4. Discussion Crystalline Phases. In the pressureless hydrothermal route, the Y-Y2Si2O7 phase was formed at 975 °C but in the case of the sol-gel route, the R-Y2Si2O7 phase was preferentially obtained at 1200 °C. In the first case, addition of dysprosium as a dopant stabilized the R-Y2Si2O7 phase in (31) Repelin, Y.; Proust, C.; Husson E.; Beny, J. M. J. Solid State Chem. 1995, 118, 163. (32) Dowty, E. Phys. Chem. Miner. 1987, 14, 67. (33) Kaminskii, A. A.; Gruber, J. B.; Bagaev, S. N.; Ueda, K.; Ho¨mmerich, U.; Seo, J. T.; Temple, D.; Zandi, B.; Kornienko, A. A.; Dunina, E. B.; Palyuk, A. A.; Klevtsova, R. F.; Kuznetsov, F. A. Phys. ReV. B 2002, 65, 125108.

Figure 8. Diagram of 2s+1LJ states that guide the spectroscopic studies for Dy3+ ions in R and β phases of Y2Si2O7. The spectral ranges of intermanifold 4f 9-4f 9 transitions are given in µm.

Figure 9. Room-temperature emission (dashed line) and excitation (solid line) spectra for β-phase of yttrium disilicate doped with dysprosium.

the range of temperatures 975-1200 °C. Both phases display different arrangements of the SiO4 tetrahedra and hence the structural transformation relating both phases should have a reconstructive character. Above 1200 °C, β- and γ-Y2Si2O7 were formed, displaying the same domains of stability in both preparations. In the case of the precursor YSIO2, where R-Y2Si2O7 is preponderant, the β-Y2Si2O7 phase was detected at 1400 °C and γ-Y2Si2O7 was detected at 1600 °C. In the case of the precursor YSIO1, both R- and Y-phases are formed simultaneously at 975 °C (see Figure 5), with the Y-Y2Si2O7 being the main

1780 Chem. Mater., Vol. 17, No. 7, 2005

phase (∼80%); as it can be observed in Figure 4, small amounts of the starting precursor and some quantity of β-phase, which begins to form, are present together with the Y- and R-phases. In the temperature range between 975 and 1300 °C the amount of Y-phase gradually decreases, while that of the β-phase increases rapidly from 1200 °C, at the expense of R- and Y-phases, up to a percentage slightly higher than that of the Y-phase at 1300 °C. From this temperature, whereas the fraction of the Y-phase decreases drastically, that of the β-phase increases almost at the same rate up to 1400 °C. This result shows that the Y-phase is stable up to 1300 °C. At 1400 °C the β-phase is the only phase present, as shown in the X-ray diffraction pattern and the 29Si NMR spectra (Figures 3 and 4, respectively). Finally, the β-phase transforms to γ-phase between 1400 and 1600 °C. Similar results have been recently reported by Becerro et al.24 Y-Y2Si2O7 Phase. Although precursors synthesized by hydrothermal methods did not yield single-phase materials at temperatures below 1400 °C, in our case it has been possible to obtain the Y-Y2Si2O7 phase as the main product at 975 °C starting from the YSIO1 precursor, without the aid of any stabilizing cation. Consistent with the purity of the sample, the 29Si NMR spectrum of this phase is formed by just two components at -83.4 and -85.4 ppm, that correspond to the two independent crystal Si sites in this phase, as it has been reported by Becerro et al.24 The IR near-normal reflectance spectrum of Y-Y2Si2O7 was fitted by 29 modes (group theory predicts 36). In the IR spectrum of this phase two specific features are worth pointing out: (1) the absence of modes in the spectral region from 1050 to 1100 cm-1, and (2) the presence of three modes at 644, 682, and 716 cm-1. Computing runs of the program VIBRAT have led us to assign these observations to bending modes of Si-O-Si bonds in Y-Y2Si2O7 phase. It must also be noted the similitude between amorphous and Y-Y2Si2O7 IR reflectance spectra (data not shown). This observation, also made in other compounds,34,35 indicates a resemblance in the short-range order of amorphous and crystalline compounds. r-Y2Si2O7 Phase. 29Si NMR spectra of samples heated above 1000 °C display the presence of two additional peaks at -80.7 and -81.8 ppm, that, in agreement with XRD analysis, have been ascribed to the formation of the R-Y2Si2O7 phase. In the case of the YSIO2 precursor (prepared via sol-gel), where the R-phase is preponderant at 1200 °C, the 29Si NMR spectrum is formed by four peaks of equal intensity at -80.7, -81.9, -82.5, and -85 ppm, from which the formation of some amount of R-Y2Si2O7 during heating of the YSIO1 precursor can be confirmed. The doping with small amounts of Dy3+ of the precursor YSIO1 gives also rise to pure R-Y2Si2O7 at 1200 °C, as in the case of the solgel precursor YSIO2. To analyze the structural evolution of the Y-phase, a dynamic sintering up to 1500 °C (data not shown), with a (34) Pecharroma´n, C.; Gonza´lez-Carren˜o, T.; Iglesias, J. E. Phys. Chem. Miner. 1995, 22, 21-29. (35) Pecharroma´n, C.; Sobrados, I.; Iglesias, J. E.; Gonza´lez-Carren˜o T.; Sanz, J. J. Phys. Chem. B 1999, 103, 6160-6170.

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heating rate of 5 °C/min, was performed in a dilatometer (Adamel-Lhomargy DI-24) using an axially pressed green bar of the YSIO1 precursor (prepared via hydrothermal route). A slope change takes place in the range 1350-1430 °C, indicating an expansion that can be justified only by the formation of some amount of R-Y2Si2O7, with lower density than the Y-Y2Si2O7 phase. In R-Ho2Si2O7, the structure1,36 is formed by isolated tetrahedra and linear chains of three tetrahedra. The spectral features detected in the 29Si NMR spectrum are compatible with the structural information published for this phase. The four components detected in 29Si NMR could be assigned to Si atoms in three types of environments: isolated tetrahedra (Q0 environment), external tetrahedra of linear chains (Q1 environments), and central tetrahedra of linear chains (Q2 environments). The similar intensity of all the four NMR components is in agreement with this assignment. A similar assignment has been reported by Parmentier et al.,20 and recently, the 89Y-NMR study37 has confirmed the existence of four crystallographic sites for yttrium ions in this phase. The possible similarity between the structures of R-Y2Si2O7 and R-Ho2Si2O7 was tested by the calculation of the powder pattern with the lattice parameters we find, assuming Ito’s cell as correct, and the structural parameters reported for the holmium compound,1,36 but using the atomic scattering factor of yttrium instead of that of holmium. Although the pattern thus calculated bore a general similarity to the observed one (see Figure 3), it seems difficult to accept the equivalence of both structures, since the calculated pattern was considerably more complex than the observed one. However, the spectroscopic evidence reported above suggests that we may be dealing, indeed, with a single phase. Some trials were made at indexing the pattern (under the assumption of its representing a single phase) from scratch by the use of the program TREOR,38 but the cells obtained left several lines unindexed, which probably should be taken as the presence of a second minor phase accompanying the main one. The cell that best fits experimental results was monoclinic, a ) 17.931(7) Å, b ) 6.794(2) Å, c ) 13.299(5) Å, β) 96.29(1)°, V ) 1610.29 Å3 , M(20) ) 13. Given the fact that the search for indexing solutions is guided by the figure of merit M(20), designed to strongly favor small cell volumes (the quality of the fit being equal), a possibility exists that we are grappling here with a single phase with a derivative structure (superstructure) for which it would be difficult to find a satisfactory indexing scheme from scratch. The IR spectra of this phase support similar conclusions. The spectral features deduced from the theoretical model agree with the experimental data, although the number of modes is different: according to group theory, 129 modes should be expected for the R-Y2Si2O7 polymorph, assuming it to be isostructural with holmium disilicate, but only 37 modes were found. However, the R-Ho2Si2O7 model cannot (36) Felsche, J. Naturwissenschaften 1972, 59, 35. (37) Becerro, A. I.; Escudero, A.; Florian, P.; Massiot, D.; Alba, M. D. J. Solid State Chem. 2004, 177, 2783-2789. (38) Werner, P.-E.; Eriksson, L.; Westdahl, M. J. Appl. Crystallogr. 1985, 18 (5), 367-370.

Synthesis of Yttrium Disilicate Host Matrix

be discarded because a considerable number of modes could be very weak, or not be well resolved, or even have too low a frequency to be detected, as frequently happens when interpreting IR spectra in systems with large lattice parameters and low symmetry.34 β-Y2Si2O7 Phase. The β-Y2Si2O7 phase, isostructural with the mineral thortveitite,39 Sc2Si2O7, is apparently well characterized, although we have been unable to locate any structure refinement from diffraction data. Our lattice parameters (see above) are in good agreement with the literature:39 a ) 6.5304(4) Å, b ) 8.5208(4) Å, c ) 4.6806(5) Å, β ) 102.630(7)°. In agreement with the structural data, the 29Si NMR spectrum displays one component at -93 ppm that corresponds to the single crystallographic site. In this phase, the existence of linear Si-O-Si bonds explains that the values detected for the isotropic chemical shift are more negative than those in the spectra of the Y- and R-Y2Si2O7 phases.20 The near-normal infrared reflectance spectrum of β-Y2Si2O7 was successfully fitted by 18 modes (group theory predicts 15). The agreement between calculated and observed modes is quite good, and it has been possible to assign the majority of the modes. The 1100-1077 cm-1 band corresponds to stretching vibrations of silicon atoms against oxygen atoms in Si-O-Si bonds (Figure 10). The remaining high-frequency modes correspond to Si-O stretching motion involving apical (nonbridging) oxygens. In particular, we have assigned the mode located at 974 cm-1 to stretching modes involving an oxygen atom coordinated to two yttrium atoms defining a line normal to the Si-O-Si chain, and 902 cm-1 to similar modes involving two yttrium atoms in a line subparallel to the Si-O-Si chain. The mode located at 830 cm-1 corresponds to multiple Si-O stretching in which all oxygen and silicon atoms of disilicate groups participate in the motion. The lower frequency modes are more difficult to characterize, because single Y-O and Si-O vibrations coexist in similar spectral ranges.31,40 Grossly speaking, it can be stated that the modes from 600 to 500 cm-1 are mainly due to Y-O stretching, and those from 500 to 400 cm-1 are modes with a large component of Si-O bending; from 450 to 350 cm-1, Si-O bending modes coexist with Y-O bending modes, and below this frequency, yttrium octahedral and silicon tetrahedral mutual motion can be found. A difference between β- on one hand, and Y- and Rphases on the other hand, is the absence of modes in the spectral region 600-800 cm-1, ascribed to Si-O-Si bonds in the disilicate group. In general, it has been accepted that the Si-O-Si chain is always bent, except in the case of rare earth disilicates, where bonds seem to be linear. Thus, IR spectroscopy corroborates this exception in some polymorphs (β and γ) of Yttrium disilicate. γ-Y2Si2O7 Phase. The γ phase is well characterized, but the space group symbol here deduced differs18 from that reported. As said above, there is inconsistency between their (39) Foord, E. E.; Birmingham, S. D.; Demartin, F.; Pilati, T.; Gramaccioli, C. M.; Lichte, F. Can. Mineral. 1993, 31, 337-346. (40) Hofmeister A. M.; Chopelas, A. Phys. Chem. Miner. 1991, 17, 503526.

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Figure 10. Ball-and-stick representations of the structures of Y-, β-, and γ-phases of Y2Si2O7. In the case of the β-phase, an estimation of the structure has been carried out from that of the isostructural Yb2Si2O7.

chosen symbol and the reported Miller indices. The symbols P21/a, P21/c, and P21/n are different Hermann-Mauguin representations for the same space group C2h5(Scho¨nflies notation), uniquely determined by its serial number (no. 14) in the International Tables for X-ray Crystallography.41 The distinction is important, because different Hermann-Mauguin symbols denote different orientation of the symmetry operations of the group with respect to the chosen labels for the cell axes. It must be emphasized that the observed powder pattern is completely different from the pattern that can be calculated from the structural data of Battalieva,15 but matches quite well the pattern which can be calculated on the basis of the structural parameters reported by Leonyuk et al.42 It would appear to be quite improbable that two structurally different polymorphs of yttrium pyrosilicate would turn out to have the same space group and lattice parameters; consequently the structural model from Battalieva et al. should be looked at with suspicion. The 29Si NMR spectrum is formed by a single component at -94 ppm that has been ascribed to the unique crystal(41) International Tables for Crystallography; D. Reidel Publishing Co.: Dordrecht, 1983; vol. A. (42) Leonyuk, N. I.; Belokoneva, E. L.; Bocelli, G.; Righi, L.; Shvanskii, E. V.; Henrykhson, R. V.; Kulman, N. V.; Kozhbakhteeva, D. E. Cryst. Res. Technol. 1999, 34, 1175-1182.

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lographic site of Si atoms in γ-Y2Si2O7.20 The chemical shift value detected confirms again the linear character of the Si-O-Si bonds in this phase. The IR spectrum of γ-Y2Si2O7 was fitted by 22 modes (group theory predicts 33). The structure of the γ-phase is similar to that of the β-phase, but with some distortion, especially in the yttrium coordination (Figure 10). In the case of the β-phase, all these cations lie in planes, while in the γ-phase the Y atoms are all no longer coplanar. As a result, the spectra of the β- and γ-phases look very similar. Differences are apparently due to the symmetry reduction which implies an increase in the IR active number of modes, most of them arising from the splitting of the former β-phase modes. Optical Properties. The luminescent properties of the different phases of yttrium disilicate doped with different amounts of dysprosium (2.5 and 5 at. %) have been investigated. The results indicate that for similar Dy concentrations, the luminescence efficiency is higher in the β-Y2Si2O7 than it is in the R-Y2Si2O7 phase, in good concordance with previous reports on yttrium silicates doped with RE elements.10,43 Differences in luminescence efficiency must be ascribed to quenching effects resulting from Dy-Dy interactions. The R-phase is characterized by the existence of four crystallographically nonequivalent Y3+ positions with strong variations in the shape of the coordination polyhedra. Thus, the Y-Y average distance is shorter for the R- than for the β-phase, which should ultimately favor quenching effects in the former case. The relevance of quenching effects in luminescence efficiency is further corroborated when measurements are performed on samples with different Dy3+ concentrations. Thus, the highest efficiency has always been obtained for the samples with minimum activator-ion concentration (e.g., 2.5 at. %), irrespective of the Dy-Y2Si2O7 phase studied. The fluorescence emission of the commercial phosphor Eu-Y2O3 used in plasma display panels (supplied by DGTec-Technologies, France) was measured for comparative purposes. The excitation wavelengths selected for the estimation of fluorescence efficiencies were 350 and 345 nm, which are the wavelengths of the most intense absorption peaks in (43) Liu, Y.; Xu, C. N.; Chen, H.; Tateyama, H. J. Lumin. 1998, 97, 135140.

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the UV range of β-(Dy,Y)2Si2O7 and the commercial phosphor, respectively. The fluorescence emission of the β-phase, doped with 2.5 at. %, was found to be approximately 40% of that measured for Eu-Y2O3. This result is remarkable, since it places RE-doped yttrium disilicate among the most efficient inorganic phosphors ever found. It is interesting to note that the 6H13/2 is situated about 3500 cm-1 above the ground level (6H15/2), which could make this system favorable for a four level laser action in the visible range.44 5. Conclusions In this work, the formation of different crystallographic phases from two different Y2Si2O7 precursors has been analyzed. By the proposed pressureless hydrothermal route, an almost pure Y-Y2Si2O7 phase was formed without the presence of any stabilizing impurity at 975 °C. In the case of the sol-gel route, the R-Y2Si2O7 was the phase obtained at 1200 °C. X-ray diffraction results and spectroscopic data suggest that the so-called R-phase could be a superstructure of the R-Ho2Si2O7 structure. In the first case (hydrothermal precursor), doping with dysprosium (2.5 at. %) stabilized the R-Y2Si2O7 phase. The fluorescence emission of the Dy-doped β-phase is clearly higher than that of the R-phase. In the β-phase, the fluorescence is approximately 40% of that measured for a phosphor such Eu-Y2O3, which is currently commercially used in plasma display panels. This experimental observation together with the high purity of the different phases obtained by the proposed hydrothermal method make this material an excellent candidate for optical applications. Acknowledgment. We thank Dr. I. Sobrados for technical assistance with NMR recording. This research has been supported by the Spanish Ministry of Education and Science under project MAT2003-04199-C02 and by CNPq and Fapesp Brazilian Institutions. CM047957B (44) Macalik, L.; Hanuza, J.; Macalik, B.; Ryba-Romanowski, W.; Goła¸ b, S.; Pietraszko, A. J. Lumin. 1998, 79, 9-19.