Crystal Structure and Luminescent Properties of Eu3+-Doped A

Article pubs.acs.org/JPCC

Crystal Structure and Luminescent Properties of Eu3+-Doped A‑La2Si2O7 Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis Alberto J. Fernández-Carrión,†,‡ Manuel Ocaña,† Pierre Florian,§,⊥ Jorge García-Sevillano,∥ Eugenio Cantelar,∥ Andrew N. Fitch,¶ Matthew R. Suchomel,# and Ana I. Becerro*,† †

Instituto de Ciencia de Materiales de Sevilla (CSIC-US), 49-41092Seville, Spain Inorganic Chemistry Department, University of Seville, Seville, Spain § CNRS, UPR3079 CEMHTI, Orleans, France ⊥ University of Orleans, 6 Avenue du Parc Floral, 45100Orleans, France ∥ Departmento Física de Materiales, C-04, Universidad Autónoma de Madrid, 28049 Madrid, Spain ¶ ESRF, BP 220, F-38043 Grenoble, Cedex, France # Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: Pure A-La2Si2O7 powder has been synthesized through a spray pyrolysis method followed by calcination at 1100 °C for 15 h. The crystallographic structure, refined from the synchrotron powder diffraction pattern of the sample, showed tetragonal symmetry with space group P41, a = 6.83565(1) Å, and c = 24.84133(1) Å. The 29Si and 139La NMR spectra have been described here for the first time in the literature and could be simulated with four Si and four La resonances, respectively, in good agreement with the presence of four Si and four La crystallographic sites in the unit cell. The same synthesis method was successful for the synthesis of Eu3+-doped A-La2Si2O7 (%Eu = 3− 40). The analysis of the unit cell volumes indicated that Eu3+ replaces La3+ in the unit cell for all Eu3+ substitution levels investigated. However, anomalous diffraction data indicated that the La/Eu substitution mechanism was not homogeneous, but Eu much prefers to occupy the RE3 sites. The Eu-doped A-La2Si2O7 phosphors thus synthesized exhibited a strong orange-red luminescence after excitation at 393 nm. Lifetime measurements indicated that the optimum phosphor was that with an Eu3+ content of 20%, which showed a lifetime of 2.3 ms. The quantum yield of the latter was found to be 12% at 393 nm excitation. These experimental observations together with the high purity of the phase obtained by the proposed spray pyrolysis method make this material an excellent phosphor for optoelectronic applications.

1. INTRODUCTION Rare earth (RE) silicate-based phosphors have attracted much attention because of their excellent thermal and chemical stability and high luminescent efficiency. The study of the luminescent properties of RE silicates has been mainly based on oxyorthosilicate (RE2SiO5) matrices,1−3 while less attention has been paid to pyrosilicates (also known as disilicates, RE2Si2O7). Of particular interest for photoluminescence applications are the pyrosilicates of Y, La, Lu, and Gd. La3+ and Y3+ ions possess no electrons in the 4f shell while the shell is half and fully filled in Gd3+ and Lu3+, respectively. Therefore, the four of them are optically inert in the visible and near-ultraviolet regions of the spectrum. Most of the luminescent studies on RE2Si2O7 matrices are based on Y2Si2O7 doped with different active lanthanide ions (Eu3+, Tb3+, Ce3+, and Dy3+),4−8 and to a lesser extent on Gd2Si2O79−11 and Lu2Si2O7.12−14 The use of La2Si2O7 as a host matrix is, however, much rarer. In fact only two experimental studies have been published on the photoluminescent properties of RE-doped La2Si2O7.15,16 This compound is dimorphic, showing a low-temperature tetragonal © 2013 American Chemical Society

modification (A-La2Si2O7) and a high-temperature monoclinic phase (G-La2Si2O7).17 Both photoluminescent studies reported in the literature are based on the high-temperature G-La2Si2O7 polymorph, doped with Tm3+(15) and Eu3+.16 To the best of our knowledge the synthesis of A-La2Si2O7 has only been reported in two different works: Dago et al.18 obtained A-La2Si2O7 after hydrothermal reaction of La2O3 and SiO2 using K2CO3, but the purity of the product cannot be assessed from the results described in the paper. Some single crystals of A-La2Si2O7 were also found as side products after thermal decomposition of La3F3[Si3O9].19 In the present study we show that pure A-La 2Si 2O 7 crystalline powder can be obtained via a spray pyrolysis synthesis method from TEOS (tetraethyl orthosilicate) and La(NO3)3 in H2O/ethanol solutions. The purity of the phase has allowed reliable structural parameters to be obtained from Received: July 19, 2013 Revised: September 10, 2013 Published: September 11, 2013 20876

dx.doi.org/10.1021/jp407172z | J. Phys. Chem. C 2013, 117, 20876−20886

The Journal of Physical Chemistry C

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1100 °C for 15 h was further calcined at 1150 and 1200 °C for 48 h. 2.2. Characterization Techniques. 2.2.1. X-Ray Powder Diffraction Patterns. X-ray powder diffraction patterns were recorded on three types of diffractometers. (i) Laboratory X-ray powder diffraction (XRPD) data were measured on a PANalytical X’Pert Pro Diffractometer (CuKα) with an X-Celerator detector over an angular range of 10° < 2θ < 120°, 2θ step width of 0.02°, and 10 s counting time. The purpose of these measurements was twofold. First, the XRPD patterns were used to identify crystalline phases during the synthesis process. The ICDD powder diffraction database22 was used to help identify known phases in the powder patterns. Second, the XRPD patterns of the Eu-doped A-La2Si2O7 samples were analyzed using the Rietveld method23 with the TOPAS software24 to obtain unit cell parameters and prove the substitution of La3+ for Eu3+ in the unit cell. (ii) High-resolution synchrotron powder diffraction (SPD) data for the undoped A-La2Si2O7 sample were measured at beamline 11-BM of the Advanced Photon Source (APS) at Argonne National Laboratory. Data were collected over the 1.0−60° 2θ range with a 0.001° step size at room temperature using a wavelength of λ = 0.413 96 Å. The sample was contained in a 0.7 mm capillary and was spun at 60 Hz during data collection. The pattern was analyzed using the Rietveld method23 with the TOPAS software.24 Refined parameters were the following: background coefficients, zero error, scale factors, unit cell parameters, anisotropic atomic displacement parameters, and atomic positions. The Stephens approach25 combined with a Voigt function was selected to model the diffraction peaks that exhibited evident anisotropic peak broadening (see Supporting Information and Figures S1 and S2 for further description of the anisotropic broadening). In order to find an efficient way of describing the two different [Si2O7]6− dihedral units in the A-La2Si2O7 structure, they were described by a flexible rigid body (FRB) model. The SiO bridging distances and Si−O−Si angles were refined along with the translation and rotation of these two rigid bodies. (iii) Anomalous high-resolution powder synchrotron diffraction (ASPD) was used to differentiate La from Eu in the 8, 12, 16, 20, 30, and 40% Eu3+-doped A-La2Si2O7 samples. Data were measured at the ID31 diffractometer at the European Synchrotron Radiation Facilities (ESRF) and collected over the 2θ range 0−35 in a continuous-scanning mode sampling data every 0.0005°. The data from the nine counters were combined, normalized, and rebinned to a step of 0.002 using the standard routines available on the instrument. Measurements were made at room temperature using wavelengths of λ1 = 0.3185 Å, λ2 = 0.3187 Å, and λ3 = 0.3542 Å. The samples were contained in a 0.4 mm borosilicate capillary spun at 50 Hz during data collection with the aim to maximize powder averaging. The three patterns of each composition were analyzed simultaneously using the Rietveld method23 with Topas.24 Refined parameters were the following: background coefficients, zero error, scale factors, unit cell parameters, isotropic atomic displacement parameters, atomic positions, and La/Eu occupancies. As noted above, the Stephen’s model25 was used to accurately describe the peak shape, while the [Si2O7]6− dihedral unit was described with the FRB model. 2.2.2. 29Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) Spectroscopy. 29Si MAS NMR spectroscopy was carried out in a Bruker model Avance III WB 600 MHz spectrometer equipped with a multinuclear

the Rietveld analysis of its powder synchrotron diffraction pattern. Likewise, the local environments of silicon and lanthanum have been analyzed, for the first time in the literature, by means of 29Si and 139La nuclear magnetic resonance (NMR). We have extended this synthesis method to the preparation of Eu3+-doped A-La2Si2O7 solid solutions and analyzed the La/Eu distribution by means of anomalous diffraction. This technique allows differentiation of two elements whose X-ray scattering factors are very similar and are therefore indistinguishable by conventional X-ray diffraction. The anomalous dispersion occurs when a wavelength is selected that is in the vicinity of an absorption edge of one of the elements. At such a wavelength, the scattering factor of the specific element undergoes a change due to anomalous dispersion and can therefore be distinguished from the other element, whose scattering factor remains unchanged. Finally, we demonstrate the effectiveness of using Eu3+-doped ALa2Si2O7 as a red phosphor, which has not been reported previously.

2. EXPERIMENTAL SECTION 2.1. Synthesis of A-La2Si2O7 and Eu3+-Doped ALa2Si2O7. Pure A-La2Si2O7 was synthesized by pyrolysis of aerosols, also known as spray pyrolysis (SP). The liquid aerosols were generated in an apparatus previously described20 from the mixture of a 0.028 M aqueous solution of La(NO3)3· 6H2O (99.9% Sigma) with a 0.067 M solution of Si(OC2H5)4 (TEOS, 99% solution Sigma) in absolute ethanol with a volume ratio of 7:3. The mixture was atomized using a glass nozzle and air (0.5 kg·cm−2) as a carrier gas. The resulting aerosols were introduced into a first furnace heated at 250 °C to evaporate the solvent and in a second furnace heated at 600 °C for precursor decomposition. The resulting solid particles were collected on a glass filter with a very high efficiency. The powdered sample obtained was placed in a platinum crucible and calcined at 1100 °C for 15 h using a heating rate of 5 °C min−1. The Eu3+-doped A-La2Si2O7 samples (Eu3+ mol % = 3, 5, 8, 12, 16, 20, 30, and 40) were synthesized using the same method, but introducing the required amount of Eu(NO3)3· 5H2O into the La(NO3)3·6H2O starting solution. Two other synthesis routes were tested to synthesize ALa2Si2O7, with comparative purposes, namely, sol−gel (SG) and solid state (SS) route. The sol−gel route used for this study was derived from the previously reported synthesis of a wellhomogenized gel of (Lu,Y)2Si2O7 which can be summarized as follows.21 The starting materials were La(NO3)3·6H2O (99.9% Sigma), Si(OC2H5)4 (TEOS, 99% solution, Sigma), and CH3CH2OH (pure ethanol, min. 99.8%, Aldrich Chemical Co.). A TEOS solution in ethanol (1:3 in volume) was added over appropriate amounts of La(NO3)3·6H2O and stirred at 40 °C for 7 h. The transparent gel obtained was dried at 60 °C for 24 h in air. Nitrates were eliminated by calcination at 500 °C for 1 h at a heating rate of 1 °C·min−1. The white powder obtained was subsequently calcined at a heating rate of 5 °C· min−1 up to 1100 °C for 15, 24, and 48 h. The sample calcined at 1100 °C for 15 h was further calcined at 1150 and 1200 °C for 48 h. Finally, the solid state route consisted of the thorough mixture of La2O3 (Sigma Aldrich, >99.9%) previously calcined at 900 °C and SiO2 (Umicore, 99.99%, 0.2−0.7 mm) in an agate mortar with isopropyl alcohol. The dried mixture was then pressed at 7 T into a 10 mm diameter cylinder and calcined at 1100 °C for 15, 24, and 48 h. The sample calcined at 20877



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3. RESULTS 3.1. Synthesis of A-La2Si2O7. A previous study about structural phase transitions in La1.0Y1.0Si2O7 showed that ALa1.0Y1.0Si2O7 was obtained following a sol−gel route plus calcination of the xerogel at 1100 °C for 4 days.34 Following this study, we have applied the same synthesis method to the La2Si2O7 composition, which led to the formation of lanthanum oxyapatite (La9.33(SiO4)6O2, PDF 00-049-0443) as the main phase after 15 h of calcination (Figure 1a); some low-intensity

probe, using 4 mm zirconia rotors spinning at 12 kHz. A single pulse sequence was used, with an observation frequency for 29Si of 119.23 MHz, a pulse width of 2.0 μs (π/2 pulse length = 6.0 μs), and a delay time of 300 s. Chemical shifts are reported in parts per million from tetramethylsilane (TMS). The experimental 29Si NMR spectra were fitted using a modified version of the Bruker Winfit program, which handles the finite spinning speed in MAS experiments.26 2.2.3. 139La NMR Spectroscopy. The 139La static spectrum was acquired on a Bruker 20.0 T Advanced III spectrometer operating at 120.1 MHz. To increase the signal-to-noise ratio we used a CPMG acquisition27 with π/2 pulse of 1.4 μs (υrf = 120 kHz), an echo spacing of 100 μs, and 640 echos acquired with the accumulation of scans. Due to a very large spanning of the signal, a variable offset cumulative spectra (VOCS28) procedure was also used using an offset step of 100 kHz, leading to the acquisition of 41 spectra (+1.5 MHz ↔ −2.0 MHz). For each individual spectrum, the CPMG echos were separated, apodized with a Gaussian function, Fourier transformed, and summed; this ensemble of spectrum obtained as a function of offset was then coadded to lead to the final spectrum. First-principles calculations with periodic boundary conditions were performed using the CASTEP code,29 which employs the planewave pseudopotential formalism of DFT. The electron correlation effects are modeled using the Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation (GGA).30 For geometry optimizations we employed a planewave cutoff energy of 610 eV, a 4 × 4 × 1 k point set, and the default “ultrasoft”31 pseudopotentials of CASTEP 5.5. The La pseudopotential was adjusted to account for the treatment of the 4f cationic localized empty orbitals.32 Convergence thresholds were set to 5 × 10−7 eV/atom for the total energy. The NMR calculations were performed using the gauge including projector augmented wave approach (GIPAW)33 at the same cutoff energy of 610 eV. 2.2.4. Excitation and Emission Spectra of the Powder Phosphors. The excitation and emission spectra of the powder phosphors were recorded in a Horiba Jobin-Yvon Fluorolog3 spectrofluorometer operating in the front face mode. The CIE color coordinates of the emitted light were calculated from the emission spectra considering a 2° observer. The photoluminescence quantum yield (QY) of optimum emitting Eu3+-doped A-La2Si2O7 phosphor, defined as the ratio between photons emitted and absorbed by the powders, was determined by an absolute method upon direct excitation of the Eu3+ energy levels at 393 nm. The setup used consisted of an integrating sphere (Labsphere) with its inner face coated with Spectralon, attached to the spectrofluorimeter. A Spectralon block situated in the sample holder was used as a blank. Spectral correction curves for sphere and emission detector were provided by Horiba Jobin-Yvon. Lifetime measurements were obtained under pulsed excitation at 266 nm by using the fourth harmonics of a Nd:YAG laser (Spectra Physics model DCR 2/2A 3378) with a pulse width of 10 ns and a repetition rate of 10 Hz. The fluorescence was analyzed through a Princeton Instruments monochromator (Acton SP2500) and then detected synchronously with an EMI-9558QB photomultiplier and recorded by a Tektronix TDS420 digital oscilloscope.

Figure 1. Selected portions of the XRPD patterns of the different precursors (from sol−gel method (SG), mixed oxides (SS), and spray pyrolysis (SP), annealed at 1100 °C for 15 h. Tick marks correspond to A-La2Si2O7 (black), La9.33(SiO4)6O2 (blue), La2SiO5 (green), and La2O3 (red).

reflections of A-La2Si2O7 (PDF 01-072-2456), lanthanum oxyorthosilicate (La2SiO5, PDF 00-040-0234), and lanthanum oxide (La2O3, PDF 01-076-7398) could also be observed in the pattern. Continued calcination led only to the practical disappearance of the La2O3 and La2SiO5 reflections, but the rest of the pattern remained unchanged (see Supporting Information, Figure S3). Likewise, increasing calcination temperature to 1150 and 1200 °C for 48 h led to the increase in the A-phase content but the high-temperature G-La2Si2O7 started to form as well, and the lanthanum oxyapatite phase was still present at any temperature (see Supporting Information, Figure S4). As an alternative method, we prepared a mixture of stoichiometric amounts of the lanthanum and silicon oxides and calcined them at 1100 °C for 15 h, as described in the Experimental section (solid state route). La9.33(SiO4)6O2 was obtained here as well as the main phase, together with smaller amounts of La2SiO5 and La2O3. Some very low intensity reflections of A-La2Si2O7 could also be observed in the pattern (Figure 1b). Continued calcination did not produce any appreciable change in the XRD pattern of the sample (see Supporting Information, Figure S5), except for the disappearance of the La2O3 and La2SiO5 reflections. Likewise, increasing calcination temperature to 1150 and 1200 °C produced the simultaneous formation of A-La2Si2O7 and G-La2Si2O7 while the lanthanum oxyapatite was still the main phase at both temperatures (see Supporting Information, Figure S6). None of 20878



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A-Pr2Si2O7. All the reflections could be fitted on the basis of a tetragonal cell with space group P41. The CIF file can be found in the Supporting Information. Figure 3a shows the

these synthesis methods were therefore appropriate for the synthesis of A-La2Si2O7. It has been shown in the literature that the method known as pyrolysis of liquid aerosols (or spray pyrolysis, SP) is a very convenient procedure for the preparation of ceramic powders, which offers several advantages over other synthesis techniques such as high purity, excellent control of composition and stoichiometry in multicomponent systems, simplicity (powders are produced from a solution in a single step), easy control of particle size and shape (always spherical since each droplet act as an independent reactor), and continuous character, which makes it suitable for industrial production.35,36 To the best of our knowledge, the spray pyrolysis method has never been used before for the synthesis of rare earth silicates. Given the disadvantages shown by the sol−gel and solid state routes described above, we decided to apply the spray pyrolysis method to synthesize A-La2Si2O7, following the methodology described in the Experimental section. The powder obtained from the pyrolysis process was submitted to calcination at 1100 °C for 15 h. The XRD pattern of the sample unexpectedly indicated the formation of crystalline A-La2Si2O7 as the unique phase (Figure 1c) and demonstrated, for the first time, the success of this technique for the synthesis of rare earth pyrosilicates. A plausible explanation for this result is that the SP route promotes a high degree of mixing of the precursors at the particle level, which favors the diffusion processes required for the crystallization of A-La2Si2O7. Increasing calcination temperature (Figure 2) to 1150 °C increased the crystallization

Figure 3. (a) Experimental (circles) and fitted (lines) SPD patterns of A-La2Si2O7. The difference curve is also included. (b) View of the ALa2Si2O7 refined structure along the [010] direction.

Table 1. Refined Atomic Coordinates for A-La2Si2O7 from SPD Data Collected at Room Temperaturea

Figure 2. Selected portions of the XRPD patterns of the SP precursor annealed at (a) 1100 °C for 15 h, (b) 1150 °C for 48 h, and (c) 1200 °C for 48 h.

degree of the A-La2Si2O7 phase, as inferred from the narrowing of the reflections, while a higher calcination temperature (1200 °C) led to the transition to the high-temperature G-La2Si2O7 phase, in good agreement with Felsche’s phase diagram.17 3.2. Structural Characterization of A-La2Si2O7 at Longand Short-Range Orders. The sample obtained from the calcination at 1150 °C of the SP precursor has been subsequently characterized at both long- and short-range order, by means of powder synchrotron diffraction and 139La and 29Si nuclear magnetic resonance, respectively. The crystal structure of A-La2Si2O7 was refined from highresolution powder synchrotron diffraction using the Rietveld method23 with the TOPAS software.24 This compound is isostructural with A-Pr2Si2O7, and therefore, we have taken as starting structural parameters those reported by Felsche17 for

atom

site

x

y

z

occ.

La1 La2 La3 La4 Si1 Si2 Si3 Si4 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14

4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a

0.75164(4) 0.52063(3) 0.33193(3) 0.11461(7) 0.8448(2) 0.59908(7) 0.2557(2) 0.0034(2) 0.8901(2) 0.7141(4) 1.0348(2) 0.7161(3) 0.4895(4) 0.4292(4) 0.7438(2) 0.3303(5) 0.4369(2) 0.1188(2) 0.1219(5) −0.0463(3) 0.1501(6) −0.1902(4)

0.31106(4) 0.17354(5) 0.92274(4) 0.77110(5) 0.7738(2) 0.7075(4) 0.3817(3) 0.3024(2) 0.6285(2) 0.9512(2) 0.8535(4) 0.6399(4) 0.5274(7) 0.8640(6) 0.8265(4) 0.5707(3) 0.2398(3) 0.2499(4) 0.4398(6) 0.4621(6) 0.1418(2) 0.2135(5)

0 0.14940(3) 0.00114(2) 0.14194(2) 0.01789(4) 0.11460(8) 0.02569(4) 0.12211(3) −0.03243(5) −0.00697(7) 0.04927(8) 0.0596(1) 0.1430(1) 0.0957(1) 0.15427(7) −0.0066(1) 0.04463(9) −0.01578(9) 0.0780(2) 0.16842(8) 0.1499(1) 0.09276(7)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

a

Space group P41, a = 6.835 65(1) Å, c = 24.841 33(1) Å. Rwp = 3.80%, Rp= 3.05%, χ2 = 2.22. 20879



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Table 2. Anisotropic Displacement Parameters for A-La2Si2O7 from SPD Data (Å2) atom

U11

U22

U33

U12

U13

U23

La1 La2 La3 La4 Si1 Si2 Si3 Si4 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14

0.0095(1) 0.0056(1) 0.0064(2) 0.0065(1) 0.010(1) 0.0055(8) 0.0042(9) 0.0039(9) 0.012(3) 0.011(2) 0.007(1) 0.003(2) 0.007(2) 0.0000(4) 0.003(2) 0.0000(2) 0.009(2) 0.005(2) 0.005(2) 0.009(2) 0.0016(5) 0.03(3)

0.0062(1) 0.0055(2) 0.0058(1) 0.0077(2) 0.0035(6) 0.0026(6) 0.0052(3) 0.0055(3) 0.004(2) 0.008(2) 0.0000(8) 0.00000(9) 0.015(3) 0.005(3) 0.002(2) 0.025(3) 0.009(2) 0.021(2) 0.0000(5) 0.004(2) 0.015(1) 0.0000(5)

0.00726(6) 0.0065(2) 0.0049(2) 0.0047(1) 0.0000(4) 0.0090(6) 0.0056(7) 0.0041(3) 0.030(3) 0.0000(5) 0.003(1) 0.025(1) 0.009(2) 0.0126(8) 0.00000(0) 0.0000(6) 0.0000(7) 0.004(1) 0.017(2) 0.0084(6) 0.005(1) 0.007(1)

−0.0021(2) −0.0005(1) −0.0016(1) −0.0020(1) −0.0015(7) −0.0033(7) 0.0017(4) 0.0031(9) −0.007(1) 0.0057(9) 0.005(1) −0.0001(6) 0.024(3) 0.007(2) 0.015(2) 0.000(2) −0.003(2) 0.006(2) 0.005(1) 0.001(1) 0.001(1) 0.004(2)

0.0036(1) 0.0015(1) 0.00030(5) 0.0012(1) 0.0021(4) 0.0008(9) −0.0006(6) 0.0006(9) 0.007(3) −0.003(1) −0.0015(7) −0.0153(9) 0.006(1) 0.003(1) −0.005(1) −0.012(1) −0.004(1) 0.004(2) −0.006(1) 0.003(2) 0.006(1) 0.005(1)

−0.0007(1) −0.0007(1) 0.0005(1) 0.0006(2) −0.0025(6) −0.0038(8) −0.0032(2) 0.0008(8) 0.0162(9) 0.001(1) 0.0038(9) 0.008(1) 0.011(2) −0.011(2) −0.0046(7) −0.012(2) −0.005(1) −0.000(2) 0.000(1) 0.010(2) 0.003(1) 0.009(1)

Table 3. Main Atomic Distances and Angles for A-La2Si2O7 Obtained from SPD Data La−O Distances (Å) La1−O14 La1−O7 La1−O9 La1−O2 La1−O1 La1−O10 La1−O4

O3−Si1−O1 O3−Si1−O2 O3−Si1−O4 O1−Si1−O2 O1−Si1−O4 O2−Si1−O4 Si1−O4−Si2

2.431(2) 2.461(2) 2.469(2) 2.479(2) 2.501(2) 2.575(2) 2.703(3)

La2−O5 La2−O14 La2−O13 La2−O6 La2−O8 La2−O10 La2−O9 La2−O1 La2−O7

115.1 112.0 108.4 105.5 104.3 111.2 128.6

05−Si2−O7 05−Si2−O6 05−Si2−O4 O7−Si2−O6 O7−Si1−O4 O6−Si1−O4

2.434(5) La3−O12 2.441(2) La3−O3 2.542(4) La3−O8 2.578(4) La3−O6 2.644(3) La3−O9 2.651(2) La3−O2 2.703(2) La3−O10 2.768(2) La3−O13 2.823(3) Si−O Angles (deg) 113.8 107.1 112.1 106.2 110.4 106.8

O8−Si3−O11 O8−Si3−O9 O8−Si3−O10 O11−Si3−O9 O11−Si3−O10 Si9−O11−O10 Si3−O11−Si4

experimental and fitted patterns, while Tables 1, 2, and 3 show the related refined atomic parameters, the corresponding anisotropic atomic displacement parameters, and main bond distances and angles, respectively. Figure 3b is a view of the ALa2Si2O7 structure calculated from the refinement. The unit cell contains eight formula units (Z = 8), with La and Si in four different crystallographic sites each. None of the sites is centrosymmetric. The crystal structure is characterized by four different sheets of isolated dihedral units [Si2O7]6− perpendicular to [001], which are related through the 90° rotation of the 41 axis. The bridging atoms of the two different dihedral units are O4 and O11, with SiOSi angles of 128.6(1)° and 131.4(1)°, respectively. The heavy La atoms are attached to each side of the sheets, providing a connection to the adjacent sheet. They exhibit irregular coordination polyhedra: La2 and La4 are nine-

2.393(2) 2.404(2) 2.414(2) 2.475(3) 2.526(2) 2.627(3) 2.702(3) 2.872(3)

112.2 112.3 107.8 109.7 108.0 106.4 131.4

La4−O3 La4−O12 La4−O6 La4−O13 La4−O2 La4−O7 La4−O1 La4−O11 La4−O5 O12−Si4−O14 O12−Si4−O13 O12−Si4−O11 O14−Si4−O13 O14−Si4−O11 O13−Si4−O11

2.432(2) 2.471(4) 2.519(3) 2.554(2) 2.581(2) 2.581(2) 2.697(1) 2.767(4) 3.056(4) 113.5 106.2 101.0 115.9 108.2 111.1

coordinated to oxygen while La1 and La3 are seven- and eightcoordinated to oxygen, respectively. In order to analyze the local environment of the cations in the A-La2Si2O7 structure and to carry out a comparative study with other RE2Si2O7 phases, we have recorded the corresponding 29Si and 139La NMR spectra. The experimental 29Si MAS NMR spectrum of A-La2Si2O7 is shown in Figure 4a. It consists of three resonances in the chemical shift range −80 to −84 ppm, the middle one showing an asymmetry toward lower frequencies. The spectrum could be successfully simulated with four Si resonances which showed very similar areas under the curve (fitting parameters are given in Table 4). This is in agreement with the diffraction data, which indicated the presence of four equally populated Si sites (Table 1). The full widths at half-maximum values were found to be very similar to those reported for the G-La2Si2O7 form.34 Although 20880



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Lu2Si2O7.47 It can be observed that, except for δ-Y2Si2O7, all the RE2Si2O7 compounds show 29Si chemical shift values that shift toward lower frequencies (more negative parts per million values) with increasing SiOSi angle. Based on this trend and on the values of the SiOSi angles calculated from diffraction in the above section, it is tempting to suggest that the highest frequency signals in our A-La2Si2O7 spectrum (−83.9 and −82.4 ppm) arise from the Si nuclei in the (Si1, Si2) dihedral unit (SiOSi angle = 128.6°), while the lowest frequency ones (−81.8 and −80.5 ppm) can be assigned to the Si nuclei in the (Si3, Si4) dihedral unit (SiOSi angle = 131.4°). Using this assumption, the 29Si chemical shift values reported in Table 4 for A-La2Si2O7 have also been plotted in Figure 4b. The data fit reasonably well to the linear trend, which proves our assignation of chemical shifts to each dihedral unit. However, although the SiOSi angle seems to be the main factor governing the resonance frequency values, the situation is probably complicated by cation effects, as observed for the β-RE2Si2O7 compounds (Figure 4b). This is, to the best of our knowledge, the first 29Si MAS NMR spectrum of A-La2Si2O7 reported and interpreted in the literature. The static 139La spectrum of A-La2Si2O7 is shown in Figure 5 and clearly displays an extensive broadening due to second-

Figure 4. (a) Experimental (crosses) and simulated (solid line) 29Si MAS NMR spectrum of A-La2Si2O7. Individual contributions are also shown. (b) Plot of the SiOSi angle of the [Si2O7]6− unit versus the corresponding 29Si chemical shift values for different RE2Si2O7 compounds reported in the literature (see text for references). The values obtained for A-La2Si2O7 in the present work have also been plotted. The dashed line is a guide to the eye.

Table 4. 29Si Chemical Shift Values, Full Width at HalfMaximum (fwhm), and Area under the Curves of the Different Contributions Resulting from the Simulation of the 29 Si NMR Spectrum of A-La2Si2O7 29

Si ch. sh. (ppm) −80.5 −81.8 −82.4 −83.9

fwhm (Hz)

area under the curve

106 96 130 130

25.1 24.8 25.0 24.9

Figure 5. 139La static experimental (dots) NMR spectrum of ALa2Si2O7 obtained using a VOCS−CPMG procedure at 20.0 T. Simulation is shown as a continuous red line, and the four individual components (“Sim” in Table 5) corresponding to the four crystallographic sites of the A-La2Si2O7 unit cell are shown below.

the spectrum is quite well resolved, the four different Si environments of A-La2Si2O7 gave signals nearly at the same chemical shifts, and it is difficult to assign the peaks to each Si crystallographic site. In order to shed some light on this aspect, the 29Si chemical shift values reported in the literature for different RE2Si2O7 compounds (RE = Y,37,38 La,34 LaY,34 Sc,39 and Lu40) have been plotted in Figure 4b versus the corresponding SiOSi angle of the dihedral [Si2O7]6− unit reported for β-Y2Si2O7,41 γ-Y2Si2O7,42 δ-Y2Si2O7,43 y-Y2Si2O7,44 G-La 2 Si 2 O 7 , 45 G-La 1.0 Y 1.0 Si 2 O 7 , 34 β-Sc 2 Si 2 O 7 , 46 and β-

order quadrupolar interaction spanning over a range bigger than 2 MHz. Knowing that four distinct crystallographic sites are present in this compound, it is impossible to extract a priori the NMR parameters for each of them from this complex line shape. We hence decided to start the simulation from the parameters derived from the DFT first-principles calculation, imposing on the four lines to be in the ratio 1:1:1:1, as derived from the crystal structure refinement described above. This led to the set of “Sim” parameters listed in Table 5, which cannot 20881



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Table 5. 139La NMR Parameters Derived from the DFT First-Principles Calculation (DFT) and Obtained from the Simulation of the Experimental Spectrum (Sim). Isotropic shielding (σiso), isotropic chemical shift (δiso), chemical shift anisotropy (ΔCS), chemical shift asymmetry parameter (ηCS), quadrupolar coupling constant (CQ), quadrupolar asymmetry parameter (ηQ), and euler angles between the CSA and quadrupolar tensor (ϕ, ξ, ψ). σiso (ppm)

DFT

La1 La2 La3 La4

Sim

La1 La2 La3 La4

δiso (ppm)

4824.3 4819.4 4638.6 4784.6 260 155 505 −178

ΔCS (ppm)

ηCS

CQ (MHz)

ηQ

ϕ (deg)

ξ (deg)

ψ (deg)

−195.7 121.4 254.6 −92.4

0.65 0.23 0.71 0.92

−95.5 36.3 −79.7 −65.7

0.79 0.63 0.37 0.97

158.6 150.6 −8.0 −67.0

39.1 69.3 115.3 68.1

−109.6 12.5 −51.5 9.7

−230 392 443 −178

0.38 0.77 0.92 1.00

105.0 29.8 94.5 82.5

0.86 0.87 0.45 0.63

4 105 28 91

−14 −167 33 45

be considered as the unique set being able to account for the NMR spectrum, but rather a “refinement” of the DFT-derived set. A very good linear correlation can be found between the experimental and calculated CQ, and the one obtained for δiso and σiso appears very reasonable. The La2 crystallographic site shows interestingly a strong sensitivity of its calculated CQ to the optimization of the structure as performed by CASTEP (optimized, 36.3 MHz; nonoptimized −21.8 MHz), possibly pointing to residual local strain (released upon optimization) on this site which, conversely, shows the markedly smallest experimental CQ value. 3.3. Structural Study of Eu3+-Doped A-La2Si2O7. Different Eu3+-containing samples were synthesized, with nominal Eu3+ concentrations of 3, 5, 8, 12, 16, 20, 30, and 40% using the same protocol previously described for the synthesis of the undoped material. 3.3.1. Unit Cell Parameters Evolution: Laboratory X-ray Powder Diffraction (XRPD) Study. Figure 6a shows the XRPD patterns of representative compositions of the Eu-doped ALa2Si2O7 samples. The patterns are extremely similar to each other, which demonstrate the success of the synthesis method for the Eu-doped samples. The only difference among them is the position of the reflections, which shift toward high 2θ angles with increasing Eu3+ content. This is consistent with the smaller ionic size of Eu3+ compared to La3+ in whatever coordination.48 In order to obtain unit cell parameters values and prove the substitution of La3+ for Eu3+ in the unit cell, the XRPD patterns were analyzed using the Rietveld method with the TOPAS software. Figure 6b plots unit cell volumes as a function of Eu3+ content, and a linear decrease is clearly observed in the full compositional range (0−40%), in good agreement with Vergard’s law for solid solutions. 3.3.2. Substitution Mechanism of La3+ and Eu3+ in the A(La,Eu)2Si2O7 Compositions: Anomalous High-Resolution Synchrotron Powder Diffraction (ASPD) Study. The ALa2Si2O7 unit cell contains four different La crystallographic sites (RE1, RE2, RE3, and RE4), as described in section 3.2. Unfortunately, the close X-ray scattering factors of Eu3+ and La3+ did not allow determination of the La3+ and Eu3+ occupancies on the different RE sites. In order to analyze precisely the substitution mechanism of La3+ and Eu3 on the four crystallographic sites, ASPD patterns have then been recorded on several compositions (LaEu8, LaEu12, LaEu16, LaEu20, LaEu30, and LaEu40) of the A-(La,Eu)2Si2O7 solid solution using three different wavelengths (see details in the Experimental section). The three patterns have been analyzed simultaneously for each composition, using P41 as the space

102 153 −116 −148

Figure 6. (a) XRPD patterns of different Eu3+-doped A-La2Si2O7 phosphors. (b) Unit cell volume as a function of Eu3+ content in the Eu3+-doped A-La2Si2O7 phosphors (The line is a linear fit to the data R2 = 0.998).

group and as starting parameters those obtained for A-La2Si2O7 in our previous refinement (section 3.2.). CIF files for all the compositions analyzed, with information on the RE sites occupancies, are supplied as Supporting Information. Figure 7a shows, as an example, the experimental and fitted patterns for LaEu40 (A-La1.20Eu0.80Si2O7) at a wavelength of λ1 = 0.3185 Å. Figure 7b shows the behavior of the Eu3+ occupation of the four RE sites versus the nominal Eu3+ content in all compositions analyzed. Analysis of the anomalous diffraction data clearly demonstrated that Eu3+ preferentially occupies the site RE3. 20882



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Figure 8. (a) Excitation spectrum of 3% Eu3+-doped A-La2Si2O7 recorded at λem= 617 nm. (b) Emission spectrum of the same sample recorded after exciting at 393 nm. Inset: photograph showing the red luminescence of the 3% Eu3+-doped A-La2Si2O7 powdered phosphor under UV illumination.

Figure 7. (a) Experimental (crosses) and fitted (solid line) ASD patterns for LaEu40 (A-La1.2Eu0.8Si2O7) registered at a wavelength of λ1 = 0.3185 Å. The fit has been carried out simultaneously with that of the patterns registered at λ2= 0.3187 Å and λ3= 0.3542 Å. The difference curve is also included. Space group P41, a = 6.777 98(4), c = 24.676 2(1). Rwp = 5.835%, Rp= 4.15%, χ2 = 1.26. (b) Eu3+ content of the four RE sites vs the nominal Eu3+ content in the Eu3+-doped ALa2Si2O7.

direct excitation of the Eu3+ ground state to higher levels of the 4f-manifold, the most intense appearing at 393 nm. The emission spectrum of the same sample (Figure 8b), recorded at λexc = 393 nm, shows the emission lines corresponding to the well-known 5D0−7FJ (J = 0, 1, 2, 3, 4) transitions, which are responsible for the strong orange-red luminescence of the sample (inset of Figure 8b) which shows CIE coordinates of x = 0.63 and y = 0.37. The emission around 617 nm, due to the electric dipole transition (5D0−7F2), appears stronger than that of the magnetic dipole transition at 586 nm (5D0−7F1), as expected for Eu3+ ions located in noninversion symmetry sites.49 This is in good agreement with the crystal structure of A-La2Si2O7 reported above (Table 1). The excitation and emission spectra of the Eu3+-doped ALa2Si2O7 phosphors having different Eu3+ contents were very similar to those described above for the 3% Eu3+-containing sample, although differences in the intensity of the bands could be observed, which were analyzed by luminescence lifetime measurements. The decay curves shown in Figure 9a were recorded at an emission wavelength of 617 nm (5D0 → 7F2 transition). The decays can be fitted by using a biexponential temporal dependence

Therefore, the La3+ for Eu3+ substitution mechanism in the A polymorph does not obey a homogeneous distribution evolution. The preference of Eu3+ (of smaller ionic radius than La3+)48 to occupy the RE3 site can be explained by the fact that the RE3−O distances are shorter than the other RE−O distances (average distances calculated on the basis of the first seven RE−O distances for each site) (Table 3). However, if this were the only factor governing the La/Eu distribution, a preferential occupation of site 1 versus site 4 should be expected, as the RE1−O distances are shorter than the RE4−O ones. Therefore, in addition to the La3+−Eu3+ ionic radii differences, other driving forces must push the cations to occupy the different sites and minimize the total energy of the system. 3.4. Luminescent Properties of the Eu3+-Doped ALa2Si2O7 Phosphors. In order to evaluate the luminescent properties of the new Eu3+-doped A-La2Si2O7 phosphors, we have recorded the corresponding excitation and emission spectra. Figure 8a shows the excitation spectrum recorded on the 3% Eu3+-containing A-La2Si2O7 phosphor by monitoring the typical Eu3+ emission at 617 nm. The excitation spectrum displayed a set of features in the 300−400 nm range, due to the

I(t ) = I01exp( −t /τ1) + I02exp(−t /τ2)

(1)

where I(t) is the luminescence intensity, t is the time after excitation, and τi (i = 1,2) is the decay time of the ith 20883



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and the distances between Eu3+ occupied sites decreases, thus leading to the luminescence concentration quenching.

4. CONCLUSIONS A phase pure A-La2Si2O7 material has been synthesized successfully by spray pyrolysis. Its structure has been described by high-resolution synchrotron powder diffraction and 29Si and 139 La NMR techniques. The crystal structure is characterized by a large unit cell with four different La3+ crystallographic sites and two crystallographically different [Si2O7]6− dihedral units. The experimental 29Si MAS NMR spectrum has been simulated with four resonances of very similar areas under the curve, in good agreement with the diffraction data. The positions of the bands appearing in the 29Si chemical shift range from −80 to −84 ppm, and they have been related to each dihedral unit. On the other hand, the 139La NMR spectrum of A-La2Si2O7 clearly displayed an extensive broadening due to second-order quadrupolar interaction spanning a range bigger than 2 MHz, and it could also be fitted to four La resonances. The synthesis procedure turned out to be a very useful method for the synthesis of pure Eu3+-doped A-La2Si2O7 powders (Eu3+ concentration = 3−40%). Laboratory X-ray diffraction measurements indicated the formation of a complete solution in the (La, Eu)2Si2O7 binary system. The La/Eu substitution mechanism is not homogeneous, but Eu3+ prefers to occupy the RE3 site, as demonstrated by analysis of anomalous powder diffraction. Finally, the phosphors obtained following this method showed an intense orange-red luminescence under UV excitation, and the optimum Eu3+ content was 20%. The quantum yield (QY) of the latter was found to be 12% at 393 nm excitation. The purity of the phase as well as the photoluminescent properties of the phosphor make it an excellent candidate material for optoelectronic applications.

■

Figure 9. (a) Normalized luminescence decay curves of the 5D0 → 7F2 transition of Eu3+ ions in the Eu3+-doped A-La2Si2O7 phosphors recorded after excitation at 532 nm. Continuous lines represent the calculated least-squares fittings by using a biexponential temporal dependence, as explained in the text. (b) Evolution of the average lifetimes with Eu3+ content for the Eu-doped A-La2Si2O7 phosphors.

Anisotropic peak broadening (including Figures S1 and S2); figures corresponding to the XRPD patterns of the SG and SS precursors calcined at 1100 °C with increasing time (Figures S3 and S4, respectively); figures corresponding to the XRPD patterns of the SG and SS precursors calcined at increasing temperatures for 48 h (Figures S5 and S6, respectively); CIFs of A-La2Si2O7 and of Eu-doped A-La2Si2O7 samples (Eu contents = 8%, 12%, 16%, 20%, 30%, and 40%). This material is available free of charge via the Internet at http://pubs.acs.org.

component, with intensity I0i. The average decay times, ⟨τ⟩, calculated as ∞

⟨τ ⟩ =

∫0 tI(t ) dt ∞

∫0 I(t ) dt

ASSOCIATED CONTENT

S Supporting Information *

= (τ12I01 + τ22I02)/(τ1I01 + τ2I02)

■

have been plotted in Figure 9b. As observed, ⟨τ⟩ remained almost constant (∼2.3 ms) when increasing the Eu3+ content from 2% to 20%, whereas a decrease of this value to 1.84 ms was detected when the Eu3+ content was further increased to 30%, indicating the presence of concentration quenching in the latter sample. Consequently, the most efficient Eu-doped ALa2Si2O7 phosphors are those with an Eu3+ content of 3−20%, the latter showing the highest emission intensity due to the higher Eu3+ content. The quantum yield (QY) of the latter was found to be 12% at 393 nm excitation. The absence of concentration quenching at Eu3+ contents as high as 20%, compared to the values reported in the literature for other matrices,50,51 could be explained on the basis of the heterogeneous La/Eu substitution mechanism. The fact that Eu prefers site RE3 versus the rest of RE sites implies a bigger distance between Eu3+ ions than in the case of a homogeneous distribution. At high Eu3+ contents, the other sites start to fill up

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel. +34 954489545. Fax: +34 954460665. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.J.F.-C. gratefully acknowledges an F.P.D.I. grant from Junta de Andaluciá and J.G.-S. an F.P.I. grant from MICINN. Supported by DGICYT (Project No. CTQ2010-14874/BQU), MEC (Project. MAT2012-34919), and Junta de Andaluciá (JA FQM 06090). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We thank the 20884



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ESRF for provision of time on beamline ID31. M. Calvo is gratefully acknowledged for help with quantum yield determination.

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