Environment of the Eu3+ Ion within Nanocrystalline ... - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Environment of the Eu3+ Ion within Nanocrystalline Eu-Doped BaAl2O4: Correlation of X‑ray Diffraction, Mö ssbauer Spectroscopy, X‑ray Absorption Spectroscopy, and Photoluminescence Investigations Biserka Gržeta,*,† Dirk Lützenkirchen-Hecht,‡ Martina Vrankić,† Sanja Bosnar,§ Ankica Šarić,† Masashi Takahashi,∥ Dimitar Petrov,⊥ and Marijan Bišcá n# †

Division of Materials Physics and §Division of Materials Chemistry, Ruđer Bošković Institute, Bijenička cesta 54, P.O. Box 180, HR-10002 Zagreb, Croatia ‡ Fk. 4, Physik, Bergische Universität Wuppertal, Gauss-Straße 20, D-42097 Wuppertal, Germany ∥ Department of Chemistry, Faculty of Science, Toho University, Chiba 274-0072, Japan ⊥ Department of Physical Chemistry, Plovdiv University “Paisii Hilendarski”, Tsar Asen Str. 24, 4000 Plovdiv, Bulgaria # Institute of Physics, Bijenička cesta 46, P.O. Box 304, HR-10002 Zagreb, Croatia ABSTRACT: Powder samples of pure BaAl2O4 and doped with 4.9 atom % Eu in relation to Ba were prepared by a hydrothermal route. The samples were characterized by X-ray diffraction, 151Eu Mössbauer spectroscopy, synchrotron-based X-ray absorption spectroscopy at the Ba L3- and Eu L3-edges, and photoluminescence measurements. Diffraction lines were broadened, indicating that the samples were nanocrystallline. The samples possessed a hexagonal crystal structure, space group P63. 151Eu Mössbauer spectroscopy revealed the presence of Eu in the 3+ oxidation state. The same information on the Eu oxidation state was also obtained by the Eu L3-edge X-ray absorption near-edge structure of the doped sample. Extended X-ray absorption fine structure showed an Eu3+ ion substituted for Ba2+ on the Ba2 site in the BaAl2O4 host structure, with charge compensation by an interstitial O in the vicinity of the Ba2 site. That was confirmed by a Rietveld structure refinement for the Eu-doped BaAl2O4 sample. Analysis of the diffraction line broadening for the prepared samples was performed simultaneously with the structure refinement. Both the dopant Eu3+ and the interstitial O acted as defects in the host BaAl2O4 lattice, which increased the lattice strain from 0.02% for pure BaAl2O4 to 0.17% for the Eu-doped sample. Crystallite sizes in the samples increased with Eu doping from 32 nm for pure BaAl2O4 to 36 nm for Eu-doped BaAl2O4. This could likely be related to the increase in the diffusion rate of the cations in the sample when a part of the Ba2+ cation content was exchanged with smaller Eu3+ cations. The Eu-doped BaAl2O4 sample exhibited red photoluminescence under excitation with λexc = 308 nm. The observed emission spectrum indicated that Eu3+ ions occupied the Ba site with lower symmetry in the doped sample.

1. INTRODUCTION Barium aluminate, BaAl2O4, belongs to the group of alkalineearth aluminates that are known as very suitable starting compounds for the preparation of fluorescent and phosphorescent doped materials.1 Namely, these phosphors have a 10 times stronger brightness and a persistent time of up to 10 times longer than traditional sulfide phosphorescent materials.2,3 Therefore, a BaAl2O4 host matrix is commonly used for the development of different display and signalization devices.4,5 Barium aluminate possesses a stuffed tridimite-like structure6−8 based on layers of six-sided rings made of AlO4 tetrahedra, with the individual tetrahedra alternately pointing up and down. The layers are stacked so that the upward© XXXX American Chemical Society

pointing tetrahedra from one layer share O atoms with the downward-pointing tetrahedra. Large Ba2+ cations occupy sites in the hexagonal channels formed by corner-sharing AlO4 tetrahedra.7,8 Huang et al. reported the existence of two hexagonal phases of BaAl2O4 with a reversible phase transition at 123 °C.8 At room temperature (RT), the ferroelectric phase [hexagonal crystal system, space group P63, a = 10.449(1) Å, c = 8.793(1) Å] is observed, while at high temperature, the paraelectric phase [hexagonal crystal system, space group P6322, a = 10.447(2) Å, c = 8.799(1) Å] is predominating.8 In the structure of the RT barium aluminate phase, two Received: September 14, 2017

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DOI: 10.1021/acs.inorgchem.7b02322 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

undertaken by Rezende et al.14,15 These authors used computational atomistic modeling methods to make predictions for the Eu3+ doping sites and charge-compensation schemes. They took into consideration five reaction schemes for the incorporation of rare-earth ions with oxidation state 3+ (denoted as M) into the BaAl2O4 structure as follows: (i) substitution of M for Al, where no charge compensation is needed; (ii) substitution of two M for two Ba with the Ba vacancy as a charge-compensation agent (one Ba vacancy compensating a charge for two dopands); (iii) substitution of M for Ba and displacement of a Ba ion to an Al site; (iv) substitution of two M for two Ba with interstitial O as a chargecompensation agent (one interstitial ion compensating a charge for two dopands); (v) substitution of three M for three Ba with an Al vacancy as a charge-compensating agent.15 Calculations of solution energies at 0 and 293 K for these reaction schemes were provided. It was revealed that the calculated solution energy for scheme iv is the lowest (3.01 eV at 0 K and 2.01 at 293 K), i.e., that Eu3+ substitution at the Ba2+ site and a charge compensation enabled by interstitial O is the most favorable.15 In the same work, the authors successfully prepared a BaAl2O4 powder solely doped with Eu3+ using a sol−gel proteic method, and the prepared sample showed a red luminescence spectrum. The experimental transition energies for the observed spectral lines were compared with the theoretical ones obtained on the basis of atomistic modeling.15 The results indicated that Eu3+ preferentially substitutes at Ba sites with O interstitial compensation and that a site symmetry involving a C1 element is most probable. On the other hand, Brito et al. performed a theoretical study on the structure of Eu2+-doped BaAl2O4 using density functional theory (DFT) calculations.16 The change in the interionic distances upon doping, as well as differences in the total energies, showed that doped Eu2+ ions preferred the Ba1 site, although the Ba2 site also appears to be possible. That was confirmed in the same work by synchrotron luminescence spectroscopy of an Eu2+-doped sample prepared by employing a combustion synthesis in which reducing gases were released.16 The present work reports the synthesis of an Eu-doped BaAl2O4 powder sample via a hydrothermal route and the structural characterization of the prepared samples by means of several techniques: powder X-ray diffraction (XRD), 151Eu Mössbauer spectroscopy, synchrotron-based X-ray absorption spectroscopy (XAS), and photoluminescence (PL). The main emphasis is put on the determination of the oxidation state of the Eu dopant in the prepared sample and on its coordination within the BaAl2O4 host structure.

different Ba sites, Ba1 and Ba2, are situated in Wyckoff positions 2a (with C3 symmetry) and 6c (with C1 symmetry), respectively, with both sites coordinated by nine O ions. The average Ba−O distances are 2.86 Å for the Ba1 site and 2.87 Å for the Ba2 site.8 There are four different tetrahedral sites for Al3+, with average Al−O distances of 1.77, 1.74, 1.72, and 1.83 Å for the Al1−Al4 tetrahedra, respectively. Considering the ferroelectric RT phase, Huang et al. have established the presence of two different kinds of AlO4 tetrahedra: the first one contains the Al1 and Al2 atoms, forming an Al−O−Al angle close to 156°, and the second one contains the Al3 and Al4 atoms, constituting an Al−O−Al angle of 180°.8,9 Eu-doped BaAl2O4 is of special interest because it has already been used as a very good luminescent material,10 and, accordingly, the optical properties of Eu-doped BaAl2O4 have been widely examined using spectroscopic methods10,11,13 and also supported by electron paramagnetic resonance studies.11,12 It was found that BaAl2O4 doped with Eu2+ ions showed a broad blue or blue-green emission band, while doping with Eu3+ ions resulted in emission lines in the red spectral range.10,11,13 Preparation routes of Eu-doped BaAl2O4 samples mostly included an annealing/firing step at high temperature under different atmospheres: air to produce Eu3+-doped samples or ones doped with both Eu3+ and Eu2+ 13 and forming gas,10 N2,10−14 CO,10,14 or H214 for the production of Eu-doped samples. While the optical properties of Eu-doped BaAl2O4 have been relatively well investigated, the structure of this doped material has not yet been experimentally investigated. Nevertheless, the results of experimental optical and theoretical studies by several authors have provided partial insight into the structure of BaAl2O4 doped with Eu2+ or Eu3+.13−16 Respecting these studies, Peng and Hong have reported the incorporation of Eu2+ dopant ions on both Ba1 and Ba2 sites within the BaAl2O4 host and the preference of Eu3+ ions to occupy the Ba site with lower symmetry.13 They paid special attention to the reduction of Eu3+ ions to Eu2+ ions in the Eu-doped BaAl2O4 phosphor. Two powder samples of BaAl2O4 doped with 1.0 mol % Eu in relation to Ba were obtained by a high-temperature solid-state preparation route at 1400 °C from BaCO3, Al(OH)3, and Eu2O3.13 One sample was prepared in air and the other one in a thermal carbon-reducing (TRC) atmosphere. The sample prepared under TRC conditions showed a broad luminescence band with a maximum emission at about 498 nm under excitation with λexc = 340 nm. The emission band comprised two components with maxima at 495 and 530 nm characteristic for emission from Eu2+ on the Ba2 and Ba1 sites of the host BaAl2O4 structure, respectively. On the other hand, the sample prepared in air showed both broad-band luminescence features and sharp emission lines in the range of 550−750 nm, with the strongest line at 610 nm characteristic of Eu3+ ions occupying the Ba site with lower symmetry. The observed emissions indicate the simultaneous existence of both Eu3+ and Eu2+ ions in the latter sample.13 The appearance of Eu2+ ions in the sample was explained by a charge compensation model that included the formation of Ba vacancy defects in the sample.13 Namely, it was proposed that, upon substitution of every two Eu3+ ions for two Ba2+ ions, there appears one Ba vacancy defect (V′′Ba) with two negative charges in order to keep the charge balance in the sample. The Ba vacancy defect then acts as a donor for electrons, which may reduce Eu3+ to Eu2+.13 Theoretical approaches regarding the location of the Eu3+ dopant ions within the Eu3+-doped BaAl2O4 have been

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. Powder samples of pure BaAl2O4 (sample E0) and one doped with Eu in the amount of 4.9 atom % Eu in relation to Ba (sample E1) were prepared using a hydrothermal route, followed by a thermal treatment. Barium nitrate, Ba(NO3)2 (Fisher Chemical, USA), aluminum nitrate nonahydrate, Al(NO3)3· 9H2O (Fisher Chemical, USA), europium nitrate hexahydrate, Eu(NO3)3·6H2O (Sigma-Aldrich, U.K.), citric acid monohydrate, C6H8O7·H2O (Kemika, Croatia), and ammonium hydroxide, NH3·aq (25%) (Kemika, Croatia), were used for preparation of the samples. Aqueous solutions of aluminum nitrate, barium nitrate, and europium nitrate were prepared by dissolving stoichometric proportions of the salts in Milli-Q water (prepared in our own laboratory). Prepared aqueous solutions were mixed in a molar ratio and additionally homogenized by mixing and adding citric acid. Final solutions were obtained by adding ammonium hydroxide and adjusting the pH to about 10. Each of the prepared solutions was subjected to precipitation B


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Inorganic Chemistry

Figure 1. (a) PIXE spectra of samples E0 and E1. SEM images (30 kV) of (b) sample E0 and (c) sample E1. in an autoclave at 170 °C during 24 h. Thereafter, the obtained precipitates were separated from the supernate by using a centrifuge, additionaly washed with Milli-Q water and dried at 60 °C. The obtained powders were heated to 1100 °C in a furnace with static air at a heating rate of 10 °C min−1 and calcined at that temperature for 4 h. Afterward, they were slowly cooled to RT in the furnace. The prepared powder sample of pure BaAl2O4 was white, while the Eu-doped BaAl2O4 sample was light yellow. 2.2. Measurements and Characterization. The composition of the prepared samples E0 and E1 was determined by means of particleinduced X-ray emission (PIXE) spectroscopy. PIXE spectroscopy measurements were carried out using a nuclear microprobe facility17 with a 2 MeV proton beam, obtained from the 1 MV Tandetron accelerator at the Ruđer Bošković Institute. X-ray spectra were collected using two detectors: a Ketek silicon drift detector (SDD) for the detection of low-energy X-rays (1−7 keV) and a Canberra Si(Li) detector for the detection of higher-energy X-rays (>4 keV). The Al K-, Ba L-, and Eu L-series of radiation emitted from the powder samples were used for compositional analysis. The composition of the doped sample, E1, was also checked by electron-excited energydispersive X-ray (EDX) spectroscopy using a JEOL JSM 6510 scanning electron microscope equipped with an X-ray spectrometer (Noran, Thermo Scientific). Prepared powder samples E0 and E1 were characterized by powder XRD at RT using a Philips MPD 1880 counter diffractometer with monochromatized Cu Kα1α2 radiation. Two data sets were recorded for each prepared powder sample:18 (i) XRD pattern of the sample mixed with a Mo powder (99.999%, Koch-Light Lab Ltd., U.K.) as an internal standard reference material for the purpose of the precise determination of the unit-cell parameters; (ii) XRD pattern of the pure sample for the purpose of Rietveld structure refinement19 and size− strain analysis. All XRD patterns were scanned in steps of 0.02° (2θ), with a fix counting time of 7 s per step. The unit-cell parameters a and c of the prepared powder samples were determined using the UNITCELL program20 and refined by the whole-powder-pattern fitting method using the WPPF program.21 Crystal structure refinement and size−strain analysis were performed by the Rietveld method with the X’Pert HighScore Plus program22 using a pseudo-Voigt profile function and a polynomial background model. Isotropic vibration modes were assumed for all atoms. For the purpose of size−strain analysis, a Si powder (99.999%, Koch-Light Lab Ltd., U.K.; spherical particles with a diameter of 1 μm) was used as the standard for instrumental diffraction line broadening. The crystallite size in the sample and the lattice strain in the same sample were calculated simultaneously with Rietveld structure refinement. 151 Eu Mössbauer spectroscopy was used to determine the oxidation state and chemical environment of the Eu dopant within the Eu-doped

BaAl2O4 host matrix. 151Eu Mössbauer spectra of sample E1 and of Eu2O3, one of the typical Eu(III) compounds, were recorded at 77 K on a WissEl Mössbauer system (MR-260A, MA-260, or DFG-500) using a 151Sm/SmF3 source (5.8 GBq). The 21.5 keV Mössbauer γrays were detected using a proportional counter. The Doppler velocity was calibrated by measuring the 57Fe Mössbauer spectrum of an α-Fe foil. Isomer shift values were determined relative to that of EuF3 at RT. XAS measurements at the Eu L3-edge (6977 eV) for sample E1 were conducted in order to reveal the oxidation state of the absorbing Eu and to investigate the local short-range atomic arrangement around the Eu dopant in the sample. XAS measurements at the Ba L3-edge (5247 eV) were performed for both prepared samples, E0 and E1, in order to examine the changes of the atomic environment around the Ba atoms in the host BaAl2O4 structure due to Eu doping. The X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions obtained from the RT XAS measurements were analyzed. For comparison, XAS data were also collected and analyzed for several reference samples: Eu2O3 (cubic, space group Ia3̅),23 AlEuO3 (orthorhombic, space group Pbnm),24−26 and BaO (cubic, NaCl-type structure, space group Fm3̅m).27 Furthermore, an Eu-doped fluorozirconate-based glass sample served as the reference for both Eu2+ and Eu3+ species.28 XANES and EXAFS measurements were performed on the hard X-ray Beamline BL10 at the superconducting asymmetric wiggler at the 1.5 GeV Dortmund Electron Accelerator DELTA, equipped with a channel-cut Si(111) monochromator.29 Some of the samples were also measured at the Materials Science Endstation of the Rossendorf Beamline (BM20) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, under dedicated ring-operating conditions (6 GeV and 130−200 mA),30 as well as at the SuperXAS Beamline X10DA at the Swiss Light Source (SLS).31 Transmissionand fluorescence-mode data were collected using gas-filled ionization chambers as detectors for the incident and transmitted X-rays and a silicon drift diode (SDD) with a multichannel analyzer for fluorescence radiation. Structural information was extracted by Fourier filtering of the experimental EXAFS χ(k) data into a distance space, giving the radial distribution functions for the examined samples.32 For quantitative fits of the experimental data, phases and amplitude functions were calculated by FEFF 8.0,33 and the fits were performed using the Athena/Artemis software package.34 A Lambda Physik XeCl excimer laser (wavelength 308 nm, pulse energy 100 mJ, and duration 20 ns, operating at a 5 Hz repetition rate) was used for PL measurements of the Eu-doped BaAl2O4 (sample E1). The powder sample was placed inside a borosilicate test tube. The laser beam of a rectangular spatial profile (2 × 1 cm2) illuminated the sample perpendicular to its upper surface. The PL light from the sample was collected by an optical fiber (solar resistant and of 600 μm C


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Inorganic Chemistry core diameter) inclined at 45° with respect to the sample surface and fed into a Avantes AvaSpec 3648 CCD spectrometer. Sampling was performed over a time of 1 s with five averages. The raw measured spectrum was corrected for the spectral response of the optical system, using the correction function35 obtained by means of a deuterium− halogen spectral lamp.

are given in Table 1 along with the literature data for pure BaAl2O4 at RT.8,18 If we consider the sizes of the ionic radii for the 9-coordinated Ba2+ (1.47 Å), Eu2+ (1.35 Å), and Eu3+ (1.12 Å),38 there is a possibility that Eu enters the BaAl2O4 host matrix as Eu2+ or Eu3+ substituting for 9-coordinated Ba2+. On the basis of the refined unit-cell parameters of the Eu-doped BaAl2O4 sample, E1, it was not possible to conclude unambiguously which Eu ion was included in the structure of the doped sample, Eu2+ or Eu3+, or whether Eu did substitute for Ba within the BaAl2O4 structure. 3.3. 151Eu Mö ssbauer Spectroscopy. The 151Eu Mössbauer spectrum of Eu-doped BaAl2O4 (sample E1) is shown in Figure 3, together with that of Eu2O3. Both samples show

3. RESULTS AND DISCUSSION 3.1. Composition of the Samples. The obtained PIXE spectrum of sample E1 in comparison to the spectrum of sample E0 is presented in Figure 1a. Analysis of the PIXE data confirmed that sample E0 is pure BaAl2O4 and showed that sample E1 is Eu-doped BaAl2O4, which contains 4.9(2) atom % Eu in relation to Ba. EDX analysis agreed with this result within error, showing 4.8(4) atom % Eu in sample E1. The scanning electron microscopy (SEM) images (Figure 1b,c) indicated an agglomerated, fine-grained microstructure in both samples E0 and E1, with crystallite sizes in the range below 50 nm. Because of the limited resolution of the available SEM, the particles could not be fully resolved. 3.2. XRD Characterization of the Samples. The XRD patterns of the prepared samples E0 and E1 (Figure 2)

Figure 3. 151Eu Mössbauer spectra of Eu-doped BaAl2O4 (sample E1) and the Eu2O3 standard, recorded at 77 K.

absorption around 1 mm s−1, indicating clearly that the Eu dopand exists as an Eu3+ ion within the BaAl2O4 host matrix. The isomer shift (δ), quadrupole coupling constant (e2qQ), asymmetry parameter (η), and experimental line width (Γexp) for the examined samples are given in Table 2. The isomer shift values are given relative to that of EuF3. Table 2. 151Eu Mössbauer Parameters for Sample E1 and the Eu2O3 Standard

Figure 2. XRD patterns of samples E0 and E1 and graphical presentation of the ICDD powder XRD data36 for the RT BaAl2O4 phase (ICDD card no. 82-1349).

indicated that both samples possess a hexagonal structure of space group P63 characteristic of the RT BaAl2O4 phase, known as a ferroelectric phase.8,9 No crystalline impurities were detected in the samples. Diffraction lines were broadened, indicating that the samples were nanocrystalline. The unit-cell parameters a and c of the pure BaAl2O4 (sample E0) were already determined in our work on Cr-doped BaAl2O4,18 while the unit-cell parameters of Eu3+-doped BaAl2O4 (sample E1)

sample

δ (mm s−1)

|e2qQ| (mm s−1)

η

Γexp (mm s−1)

E1 Eu2O3

0.89 1.02

8.76 5.20

0.57 0.50

2.41 2.40

The absorption of sample E1 is asymmetric and broader than that of Eu2O3 (Table 2) because of large quadrupole interactions. The absolute value of the quadrupole coupling constant, |e2qQ|, of sample E1 is larger than that of Eu2O3. This indicates that the Eu3+ dopant ions in the host BaAl2O4 structure reside in a crystallographic position with a large electric-field gradient. Possible candidates for such a position

Table 1. Sample Notation, Eu Doping Level, and Refined Values of the Unit-Cell Parameters, along with the Literature Data for the Pure BaAl2O48,18 sample

Eu content (atom %)

Rpa

Rwp

a (Å)

c (Å)

V (Å3)

BaAl2O48 BaAl2O418

0 0 4.9(2)

0.078 0.061 0.052

0.170 0.081 0.069

10.449(1) 10.4488(4) 10.4473(6)

8.793(1) 8.7959(5) 8.7929(6)

831.44(1) 831.66(1) 831.13(1)

E1 a

Rp and Rwp are the discrepancy factors that characterize a quality of the fit.37 D


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Inorganic Chemistry are the Ba2+ sites. As mentioned before, there are two crystallographically different Ba sites in the BaAl2O4 host: one is on the Wycokff position 2a with a 3-fold symmetry C3, and the other is on the Wyckoff position 6c with C1 symmetry.8 The Ba2+ ion on the 2a site is 9-coordinated with an average Ba−O distance of 2.86 Å, while Ba2+ on the 6c site is 9-coordinated with an average Ba−O distance of 2.87 Å. Although the average Ba−O distances for these two sites are almost the same and identical with the sum of the ionic radii (1.47 Å for 9coordinated Ba2+ and 1.40 Å for O2−),38 there exist much longer Ba−O distances for these sites. There are three long Ba−O distances for the 2a site (2.98 Å) and five long ones for the 6c site (2.92−3.00 Å).8 This means that the resulting anion charge distributions around both the 2a and 6c sites are quite irregular, causing a high electric-field gradient. Thus, it may be expected that Eu3+ ions on the 2a and 6c sites have large e2qQ values. Unfortunately, these sites cannot be distinguished in the 151 Eu Mössbauer spectrum because of the large natural line width of 151Eu. 3.4. XAS. XANES. The background-subtracted and normalized Eu L3-edge XANES spectra of Eu-doped BaAl2O4 (sample E1) in comparison to those of Eu2O3, EuAlO3, and an Eu2+/ Eu3+-doped fluorozirconate-based glass are shown in Figure 4.

edge XANES spectrum of Eu-doped BaAl2O4 (sample E1) with the spectra of the reference compounds, we cannot give any evidence for the presence of AlEuO3 and Eu2O3 in sample E1 because of the different post-edge absorption minima and maxima. In addition, the measured Ba L3-edge XANES spectra of pure BaAl2O4 (sample E0) and Eu-doped BaAl2O4 (sample E1, not shown here) did not reveal any differences in terms of the absorption peak energies and intensities between those two samples; thus, it can be assumed that the pristine structure of BaAl2O4 is not affected by a great deal by the Eu3+ doping. EXAFS. Fourier transforms of the k2-weighted EXAFS function χ(k) at the Ba L3-edge for samples E0 and E1 are presented in Figure 5. Both the pure BaAl2O4 (sample E0) and

Figure 4. Comparison of the background-subtracted and normalized Eu L3-edge XANES spectra of Eu-doped BaAl2O4 (sample E1) with spectra of reference compounds AlEuO3, Eu2O3, and a glass sample containing Eu2+ and Eu3+ in approximately equal amounts (the spectra are vertically shifted for an easier comparison). Figure 5. Fitting results for the EXAFS data measured at the Ba L3edge: (a) pure BaAl2O4 (sample E0); (b) Eu-doped BaAl2O4 (sample E1). The EXAFS data χ(k)*k2 as well as the magnitude of the Fourier transforms |FT(χ(k)*k2)| are displayed. The k range for the fit is 1.40 Å−1 < k < 8.52 Å−1, and radial distance range is 1.40−3.80 Å. The fit residuals (R factor) are 0.018 and 0.028 for the pure and Eu-doped BaAl2O4, respectively.

From the results of those near-edge spectra, it is evident that the Eu in our prepared sample E1 is present as Eu3+, which qualitatively agrees with the results of 151Eu Mössbauer spectroscopy. Namely, the XAS spectrum of Eu3+ has a maximum absorption for a photon energy of about 6983 eV, like measured XANES spectra for Eu2O3 and EuAlO3 (samples that represent standard reference materials for Eu3+), and the XANES spectrum of sample E1 shows the maximum absorption for this energy as well. As can be concluded from the XANES spectrum of the Eu2+/Eu3+-doped glass, the edge position for Eu2+ is at a significantly lower energy compared to that of Eu3+, namely, at about 6975 eV.28 In contrast to what was found earlier for Eu-doped BaAl2O4 samples prepared in air,13 our XANES data clearly prove the absence of Eu2+ in the Eu-doped BaAl2O4 sample, which is in agreement with those of Resende et al.15 Furthermore, from a comparison of the Eu L3-

the Eu-doped BaAl2O4 (sample E1) have almost identical Fourier transforms. In general, the peak positions in those Fourier transforms are not identical with the crystallographic distances, i.e., owing to the photoelectron phase shift arising from the scattering processes, all the peaks in the Fourier transforms are generally shifted toward lower distances.39 As an example, the peak at about 2.2 Å in Figure 5a can be attributed E


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Inorganic Chemistry Table 3. Fit Parameters Obtained by Fitting the Ba L3-Edge EXAFS Data for Sample E0a sample

ΔE0 (eV)

atom site

S02

shell

Ni

σi2 (Å2)

Ri (Å)

E0

5.5(3)

Ba1

0.25(4)

Ba2

0.80(6)

Ba1−O Ba1−O Ba1−Al Ba1−Al Ba1−O Ba1−Al Ba1−Al Ba2−O Ba2−O Ba2−O Ba2−O Ba2−O Ba2−O Ba2−Al Ba2−Al Ba2−Al Ba2−O Ba2−Al Ba2−Al Ba2−O Ba2−Al

6 3 3 3 3 3 3 1 2 3 1 1 1 2 1 3 1 1 1 3 1

0.014(5) 0.014(5) 0.014(5) 0.014(5) 0.014(5) 0.014(5) 0.014(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5) 0.017(5)

2.78(2) 2.96(2) 3.45(2) 3.49(2) 3.88(2) 4.00(2) 4.04(2) 2.69(2) 2.77(2) 2.87(2) 2.97(2) 3.01(2) 3.40(2) 3.42(2) 3.47(2) 3.53(2) 3.87(2) 3.95(2) 3.99(2) 4.00(2) 4.04(2)

While the inner potential shift ΔE0 was treated as a global parameter for all of the shells of a respective sample, the individual amplitude reduction factor S02 and disorder parameter σi2 were used for the two inequivalent Ba sites, i.e., Ba1 and Ba2.

a

to the first Ba−O coordination in BaAl2O4 with an average distance of 2.8 Å. Quantitative fits of those Ba EXAFS data were performed using phases and amplitude functions calculated for the hexagonal crystal system with space group P63. Two O and two Al shells were used to model the experimental data. The distances and coordination numbers of the O and Al coordinations with a bond length of up to 4 Å were included in the fits, resulting in a cluster comprising a total of 31 O and 13 Al atoms for the Ba1 and Ba2 sites. Unfortunately, EXAFS as an experimental technique does not have the resolution to individually isolate each of these O and Al distances, which is a particularly severe problem at the Ba L3-edge; because of the onset of the L2-edge at 5624 eV, the accessible k range is limited to about 9 Å−1. The crystallographic data determined by XRD (section 3.2) were used as the starting parameters. In order to minimize the number of fit parameters, a global value for expansion of the lattice was used, i.e., all crystallographic distances were scaled by the same value. Furthermore, individual values for the disorder parameter σi2 were used for the Ba−O and Ba−Al coordinations in the respective Ba sites, Ba1 and Ba2, and individual values for the amplitude reduction factor S02 were used for all of the coordinations of these two sites. A single inner potential shift ΔE0 was used for all coordination shells in the respective sample, and the fit residuals were minimized in a least-squares fit using both k- and k2-weighted data. It is important to note that the Ba L3-edge EXAFS data cannot be fitted accurately without using both the Ba1 and Ba2 sites in the fit. The obtained fit parameters for sample E0 are listed in Table 3. According to the data presented in Table 3, the average Ba− O distances for the Ba1 and Ba2 sites are 2.84(2) and 2.91(2) Å, respectively. Calculated interatomic distances obtained for sample E1, in principle followed those for sample E0, with the exception of the Ba−O distances: the average Ba1−O distance decreased to 2.82(4) Å, while the average Ba2−O distance

increased to 2.93(4) Å. This might be indicative for some interstitial O in the vicinity of the Ba2 site in the doped sample E1 for the purpose of charge balance. Consequently, this observation may indicate that Eu3+ is located on the Ba2 site, substituting for Ba2+ and thus requiring more O. These EXAFS results give further evidence that the structure of Eu3+-doped BaAl2O4 is very similar to that of pure BaAl2O4 and that no notable amounts of impurity phases are formed in the prepared samples; otherwise, systematic discrepancies should have appeared in the fits (Figure 5). EXAFS data at the Eu L3-edge obtained from Eu-doped BaAl2O4 (sample E1) and Eu reference compounds, Eu2O3 and AlEuO3, are compiled in Figure 6. While only the magnitude of the Fourier transform for the experimental data of Eu-doped BaAl2O4 is presented (Figure 6a), fits are shown for Eu2O3 and AlEuO3 (Figures 6b,c). It is obvious that the radial distribution functions of all three samples are completely different, which allows one to discriminate between the different structures easily. From the Fourier-transformed data presented in Figure 6b,c, the peaks belonging to the first coordination shells around Eu were isolated by means of a filter function, back-transformed into k space, and fitted with phases and amplitude functions. A single Eu−O shell with a nearest-neighbor bond length R1, coordination number N1, and disorder parameter σ12 was sufficient for fitting of the Eu2O3 spectra (Figure 6b). The values obtained by the fit are N1 = 6, σ12 = 0.0096(24) Å2, R1 = 2.361(13) Å, S02 = 0.86(9), and ΔE0 = 3.2(7) eV, which are well in accordance with the recent values obtained by Fryxell et al.,40 who derived an Eu−O bond distance of 2.33(2) Å. In contrast to this simple coordination octahedron for Eu2O3, much more complicated models have to be used for AlEuO3 and Eu-doped BaAl2O4. In the case of AlEuO3 (Figure 6c), the fitting of the EXAFS data was performed using a cluster containing 68 atoms in a radius of 5 Å around the X-ray absorbing Eu atom, in the orthorhombic space group Pnma.41 The fit was performed with 25 different scattering paths, which F


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fitting results obtained for AlEuO3 are compiled in Table 4. Despite the rather simple fit model for such a complex structure of AlEuO3 with different backscattering species, the experimental data were well described by the fit, as shown in Figure 6c. According to the results of the Ba and Eu L3-edge XANES and EXAFS examinations, the BaAl2O4 material did not substantially changed its original structure upon Eu3+ doping, and possible impurity phases were not grown in measurable amounts. Thus, it is straightforward to assume that an Eu3+ dopant is incorporated at a regular lattice position in the BaAl2O4 host. Because the ionic radius of Al3+ in 4-fold coordination is about 0.39 Å only38 and thus substantially smaller compared to Eu3+ by about 0.8 Å,38 it is rather unlikely that Eu3+ ions are substituted for Al3+ in the BaAl2O4 lattice. In contrast, however, the ionic radius of 9-coordinated Ba2+ is about 1.47 Å,38 and thus the substitution of Eu3+ on Ba2+ sites in BaAl2O4 appears to be reasonable. Furthermore, the magnitude of the Fourier transform for the experimental data at the Eu L3-edge for Eu-doped BaAl2O4 (Figure 6a) at least qualitatively resembles that measured at the Ba L3-edge for Eudoped BaAl2O4 (Figure 5b). It should be noted, however, that an exact direct comparison is not possible because of the different scattering phases and amplitudes for Ba−O and Eu−O coordinations; i.e., the first peak in the Fourier transform of Eudoped BaAl2O4 at the Ba L3-edge is located at about a 2.2 Å radial distance, while for the Eu L3-edge data, it appears at a slightly smaller distance of about 2 Å. Last but not least, the magnitudes/amplitudes of the first nearest-neighbor peaks in the Fourier transform of the Eu L3-edge EXAFS data for Eu2O3 and AlEuO3 (Figures 6b, 6c) are considerably smaller than that for Eu-doped BaAl2O4 (Figure 6a), which suggests that substantially more than six nearest O atoms are located in the first coordination sphere around Eu in Eu-doped BaAl2O4. In conclusion, therefore we made a model for fitting the experimental EXAFS data of Eu-doped BaAl2O4 measured at Eu L3-edge assuming that Eu is located on the Ba2 site with a total of 10 nearest-neighbor O atoms. Namely, an extra O neighbor was assumed for the purpose of charge compensation when trivalent Eu3+ substitutes for divalent Ba2+. As in the case of the EXAFS at the Ba L3-edge for Eu-doped BaAl2O4, shorter and longer O bond distances can be anticipated for the Fourier transform of the Eu L3-edge EXAFS and also additional Al and O neighbors up to a radial distance of 3.5 Å were included in the fit. For the neighbors at bond distances larger than 3 Å, we assumed atomic positions as determined from the Ba L3-edge investigations (Table 3). In the fit of the positions, all of the O and Al atoms were varied. The mean-square relative displacements for the first two O shells were kept identical, thus modeling the close bonding in comparison to the Al and O atoms at longer bond distances, for which different disorder parameters could be expected. Furthermore, the global values for S02 and the inner potential shift ΔE0 were used for all of the considered shells, so that 10 parameters were independently varied in the fit. In total, 11 O and 3 Al atoms in a cluster of 3.5 Å radius were included in the fit. The obtained fit results for the Eu L3-edge EXAFS data of Eu3+-doped BaAl2O4 (sample E1) are shown in Figure 7, and the short-range order structure parameters are listed in Table 5. As can be seen in Table 5, Eu possesses six neighbors at a shorter distance of 2.42(1) Å and four neighbors at a longer bond distance of 2.76(2) Å. These distances are shorter compared to the related Ba−O distances in BaAl2O4. Such a

Figure 6. Magnitudes of the Fourier transforms |FT(χ(k)*k3)| at the Eu L3-edge for (a) Eu-doped BaAl2O4, (b) Eu2O3, and (c) AlEuO3. For Eu2O3 and AlEuO3, the experimental data (●) as well as the fits () using the corresponding crystal structures are displayed. Here also the back-transformed data in k space are shown. The k ranges for the Fourier transforms are 1.01 Å−1 < k < 10.10 Å−1 for Eu-doped BaAl2O4, 1.53 Å−1 < k < 10.76 Å−1 and radial distances of 1.25−2.56 Å for Eu2O3, and 1.70 Å−1 < k < 11.8 Å−1 and radial distances of 1.30− 4.80 Å for AlEuO3.

are listed in Table 4, with a single inner potential shift ΔE0. A single disorder parameter σ2 was used as a global parameter for all of the contributing scattering pathways, and a single lattice expansion factor was used to fit the radial distances around the absorbing Eu atom. Amplitude reduction factors were assumed to be different for O, Eu, and Al nearest neighbors, so that in total six parameters were varied in order to optimize the fit. The G


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Inorganic Chemistry Table 4. Fit Parameters Obtained by the Fit of the Eu L3-Edge EXAFS Data of the AlEuO3 Reference Sample sample

ΔE0 (eV)

shell

Ni

S02

σi2 (Å2)

Ri (Å)

AlEuO3

5.0(9)

Eu−O Eu−O Eu−O Eu−O Eu−O Eu−Al Eu−O Eu−Al Eu−Al Eu−Al Eu−Eu Eu−Eu Eu−Eu Eu−O Eu−O Eu−O Eu−O Eu−O Eu−O Eu−O Eu−O Eu−O Eu−O Eu−O Eu−O

3 1 2 1 1 2 2 2 2 2 2 2 2 4 2 2 4 2 4 8 2 2 8 4 2

0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.67(20) 0.68(12) 0.67(20) 0.67(20) 0.67(20) 0.64(20) 0.64(20) 0.64(20) 0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.68(12) 0.68(12)

0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1) 0.003(1)

2.328(3) 2.421(3) 2.526(3) 2.951(3) 2.997(3) 3.087(3) 3.160(3) 3.207(3) 3.276(3) 3.405(3) 3.683(3) 3.758(3) 3.803(3) 4.339(3) 4.407(3) 4.475(3) 4.529(3) 4.635(3) 4.670(3) 4.687(3) 4.738(3) 4.837(3) 4.844(3) 4.969(3) 5.003(3)

decrease in the bond distances is well in accordance with the smaller ionic radius of Eu3+ in comparison to that of Ba2+, as well as with the increased Coulomb attraction between Eu3+ and O2− in comparison to that between Ba2+ and O2−. Furthermore, also the reduced disorder parameter between Eu and O obtained from the Eu L3-edge data reflects such an increase of the attractive Eu−O interaction in comparison to Ba−O. The bond distances determined for the other O and Al atoms are close to the values obtained for pure and Eu-doped BaAl2O4 at the Ba L3-edge. 3.5. Rietveld Structure Refinement and Analysis of the Diffraction Line Broadening. Rietveld structure refinement and analysis of the diffraction line broadening for the sample of pure BaAl2O4 have been reported in our previous work.18 In this work, Rietveld refinement of the prepared Eu3+doped BaAl2O4 (sample E1) is presented. Refinement for this sample was performed with the aim of (1) confirming that sample E1 possesses a hexagonal structure, space group P63, (2) elucidating the mode of Eu3+ incorporation into barium aluminate BaAl2O4, and (3) determining the crystallite size and lattice strain in sample E1. A starting structure model was made using the structure of barium aluminate BaAl2O4 reported by Huang et al.8 (ICSD card no. 75426),42 in which Eu3+ partially substitutes for Ba2+ in the structure of the host BaAl2O4. The site occupancy parameter for Eu3+ on the Ba sites was used according to the results of PIXE and EDX analyses. Concerning the site occupancy, three possible variations of the structure model were done: Eu3+ substituting for Ba2+ on the Ba1 site only, on the Ba2 site only, or on both the Ba1 and Ba2 sites. Furthermore, attention was paid to include Ba vacancies13,15 or interstitial O15 into the structure model for the purpose of charge compensation. In total, six variations of the structure model were tested, and Rietveld structure refinements were performed for all of them. Isotropic

Figure 7. Magnitudes of the Fourier transform |FT(χ(k)*k3)| at the Eu L3-edge for Eu-doped BaAl2O4 (sample E1). The experimental data (●) and fit () using Eu on a Ba position within the BaAl2O4 crystal structure are displayed. Here also the back-transformed data in k space are shown. The k range for the Fourier transform was 1.01 Å−1 < k < 10.10 Å−1, and the fit was performed in a radial distance of 1.2−3.0 Å with a fit residual (R factor) of 0.075.

Table 5. Fit Parameters Obtained by the Fit of Eu L3-Edge EXAFS Data for Eu3+-Doped BaAl2O4 (Sample E1) shell

Ni

S02

ΔE0

σi2 (Å2)

Ri (Å)

Eu−O Eu−O Eu−Al Eu−O Eu−Al

6 4 2 1 1

0.88(24) 0.88(24) 0.88(24) 0.88(24) 0.88(24)

7.8(9) 7.8(9) 7.8(9) 7.8(9) 7.8(9)

0.005(2) 0.012(2) 0.019(2) 0.012(2) 0.019(2)

2.42(1) 2.76(2) 3.38(3) 3.41(3) 3.43(3)

H


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Inorganic Chemistry

Figure 8. Graphical result of Rietveld structure refinement and diffraction-line-broadening analysis for sample E1.

Table 6. Results of Rietveld Structure Refinement and Size−Strain Analysis for Eu3+-Doped BaAl2O4 (Sample E1)

a

doped BaAl2O4 sample

Eu content (atom %)

Rpa

Rwp

E1

4.9(3)

0.106

0.138

atom site

Wyckoff position

occupancy

x

y

Ba1 Ba2 Eu Al1 Al2 Al3 Al4 O1 O2 O3 O4 O5 O6 Oi

2a 6c 6c 6c 6c 2b 2b 6c 6c 6c 6c 6c 2b 2b

1.0000 0.9347 0.0653 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 0.0980

0 0.504(1) 0.504(1) 0.161(8) 0.155(6) 1 /3 1 /3 0.177(7) 0.679(8) 0.490(12) 0.178(13) 0.119(3) 1 /3 2 /3

0 0.002(2) 0.002(2) 0.336(11) 0.329(13) 2 /3 2 /3 0.002(13) 0.001(14) 0.171(7) 0.509(20) 0.305(12) 2 /3 1 /3

2

z

Biso (Å )

/4 0.258(2) 0.258(2) 0.060(1) 0.443(1) 0.941(3) 0.550(4) 0.986(8) 0.029(6) 1.000(13) 0.004(14) 0.249(17) 0.748(20) 0.779(20)

0.69(3) 0.69(3) 0.69(3) 0.75(7) 0.75(7) 0.75(7) 0.75(7) 1.31(11) 1.31(11) 1.31(11) 1.31(11) 1.31(11) 1.31(11) 1.31(11)

1

crystallite size (nm)

lattice strain (%)

35.7(1)

0.17(1)

Rp and Rwp are discrepancy factors that characterize a quality of the fit.37

Rietveld structure refinement for sample E1 are almost equal to those obtained in the previously reported structure refinement for pure BaAl2O4,18 ratifying a good structure model used for sample E1. The results of Rietveld structure refinement confirmed that sample E1 possesses a hexagonal structure, with space group P63, same as that for the pure BaAl2O4 sample.18 The result on the mode of Eu3+ substitution for Ba2+ is in agreement with the results obtained by EXAFS investigations of sample E1, presented in section 3.3. Therefore, the chemical formula for sample E1 may be expressed as

temperature factors (Biso) were used in the refinement; for the same kinds of atoms, they were constrained to change unanimously during refinement, and for Ba2+ and Eu3+ ions sharing the same site, they were constrained to change identically during refinement. In the refinement for sample E1, the lowest values of the discrepancy factors Rp and Rwp were achieved with the structure model in which Eu3+ substituted for Ba2+ on the Ba2 site solely, with the interstitial O present in the vicinity of the Eu3+ dopant ions. It should be pointed out here that the discrepancy factors Rp and Rwp obtained in the present I


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Table 7. Metal−O Distances (Å) along with Al−O5−Al and Al3−O6−Al4 Angles (deg) of Eu-Doped BaAl2O4 (Sample E1) M−O Distances (Å) sample E1

Ba1−O1 (×3) Ba1−O1 (×3) Ba1−O5 (×3) average:

Al1−O5−Al2 Al3−O6−Al4

2.78(8) 2.97(8) 2.78(3) 2.84(6)

Ba2/Eu−O2 Ba2/Eu−O2 Ba2/Eu−O3 Ba2/Eu−O3 Ba2/Eu−O4 Ba2/Eu−O4 Ba2/Eu−O5 Ba2/Eu−O5 Ba2/Eu−O6 Ba2/Eu−Oi average:

2.72(9) 3.04(8) 2.81(11) 2.92(11) 2.80(12) 2.90(12) 2.91(4) 3.43(11) 3.00(2) 3.04(2) 2.96(8) Selected Angles (deg)

Al1−O1 Al1−O2 Al1−O4 Al1−O5 average:

1.81(10) 1.80(15) 1.78(10) 1.71(12) 1.78(11)

Al2−O1 Al2−O2 Al2−O3 Al2−O5 average:

1.74(11) 1.75(12) 1.78(10) 1.74(10) 1.75(11)

Al3−O4 (×3) Al3−O6 average:

1.73(12) 1.70(11) 1.72(11)

Al4−O3 (×3) Al3−O6 average:

1.83(8) 1.74(12) 1.81(9)

155.9(6) 180.0

Figure 9. Laser-induced PL spectrum of Eu3+-doped BaAl2O4 (sample E1). λexc = 308 nm. Inset: Both the laser line and PL spectrum from the sample.

Ba2+0.951Eu3+0.049Al3+2O4.0245. The observed and calculated XRD patterns for sample E1 are presented in Figure 8 along with the values of full-width at half-maximum (fwhm) in the wide range of Bragg angles. Table 6 lists the refined structural parameters for sample E1 and the results of diffraction-line-broadening analysis performed simultaneously with structure refinement. The refined metal−O distances along with the values of the Al1−O5−Al2 and Al3−O6−Al4 angles in the structure of sample E1 are compiled in Table 7. A comparison of the results from Table 7 with those obtained for pure BaAl2O4 presented in Table 4 of our previous work,18 reveals that upon Eu3+ doping the (Ba2)O9 polyhedra turn into the ones in which a part of the Ba2+ content on the Ba2 site is substituted with Eu3+ and which possess more than nine coordinating O ions. The average Ba/Eu−O distance in these polyhedra is 2.96 Å, which is slightly longer in comparison to the average Ba−O distance of 2.95 Å in (Ba2)O9 polyhedra of pure BaAl2O4. Simultaneously, the

average Ba−O distance in (Ba1)O 9 polyhedra slightly decreased from 2.85 to 2.84 Å during Eu3+ doping. The average distance in (Al1)O4 tetrahedra slightly increased (from 1.77 to 1.78 Å), the average distances in (Al3)O4 tetrahedra slightly decreased upon Eu3+ doping (from 1.73 to 1.72 Å), and the average distances in (Al2)O4 and (Al4)O4 tetrahedra did not change. Furthermore, the Al1−O5−Al2 angle decreased upon Eu3+ doping, from 156.0° to 155.9°. The described changes of the interatomic distances and of the Al1−O5−Al2 angle caused a decrease of the unit-cell parameters, as well as of the unit-cell volume for Eu3+-doped BaAl2O4 in comparison to those of pure BaAl2O4. It should be noted here that the average Ba2/Eu−O distance in sample E1 as obtained by Rietveld refinement and the average Ba2−O distance resulting from EXAFS analysis of sample E1 at the Ba L3-edge agree within the standard deviation. The values of the crystallite size and lattice strain for Eudoped BaAl2O4 (sample E1) shown in Table 6 resulted from J


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Inorganic Chemistry

additionally confirms that this sample does not contain Eu2+ ions.

the diffraction-line-broadening analysis included in the Rietveld method, which was performed simultaneously with structure refinement. The diffraction-line-broadening analysis showed that sample E1 was nanocrystalline, similar to the sample of pure BaAl2O4 investigated in our previous work.18 The crystallite size increased from ∼32 to ∼36 nm upon doping the host material with 4.9 atom % Eu in relation to Ba. Simultaneously, the lattice strain increased from 0.02% to 0.17% upon doping. The rather unusual increase of the crystallite size in the doped sample may be explained by a possible increase in the diffusion rate of cations in the system when larger cations (Ba2+) are partially exchanged by smaller cations (Eu3+). On the other hand, dopant Eu3+ cations and interstitial O2− anions are strong defects that introduce irregularities in the crystal structure of the host material BaAl2O4, which manifests in increased lattice strain. 3.6. PL of an Eu-Doped BaAl2O4 Sample. The result of laser-induced PL measurement with λexc = 308 nm for sample E1 is presented in Figure 9. A red luminescence characteristic for the Eu3+ ion was observed. The emission lines appeared in the range of 550−750 nm, which is typical for the Eu3+ emission. Assignments of the emission lines (Table 8) were made after their wavelengths and with the aid of the corresponding energy levels of Eu3+ in crystals.43

4. CONCLUSIONS The preparation, structural investigations, and PL of an Eudoped BaAl2O4 powder sample have been reported. Powder samples of pure BaAl2O4 and the one doped with 4.9 atom % (in relation to Ba) were synthesized via a hydrothermal route followed by a thermal treatment at high temperature. The prepared samples were nanocrystalline. XRD showed that the samples possessed a hexagonal structure in the space group P63. Both unit-cell parameters, a and c, decreased upon Eu doping. 151 Eu Mössbauer spectroscopy evidenced that upon doping the Eu dopant entered the BaAl2O4 host as an Eu3+ ion, likely substituting for Ba2+ on Ba sites, but it could not be distinguished whether it was on site Ba1 or site Ba2 because of a large natural line width of the 151Eu maximum. XANES investigations of the doped sample confirmed that Eu was present as Eu3+ in the doped sample and clearly proved the absence of Eu2+ in that sample. EXAFS studies at the Ba L3edge for both samples gave evidence that the structure of Eu3+doped BaAl2O4 was very similar to that of pure BaAl2O4. A slight increase of the average Ba−O distance for the Ba2 site noticed for the doped sample indicated that the Eu3+ dopant was located on the Ba2 site, substituting for Ba2+, and that possibly an additional interstitial O simultaneously settled down in the vicinity of this site for the purpose of charge balance. The EXAFS study at the Eu L3-edge for the doped sample confirmed the presence of interstitial O in the neighborhood of the dopant Eu3+ ion. Both XANES and EXAFS revealed that no notable amounts of impurities were formed in the prepared samples. Rietveld structure refinement for the Eu-doped sample showed that upon doping an Eu3+ ion substituted for a Ba2+ ion on the Ba2 site of the BaAl2O4 structure and that the additional interstitial O entered this structure, being located near the Eu3+ ion, in excellent agreement with the EXAFS results. Diffractionline-broadening analysis showed that both the lattice strain and the crystallite size increased during Eu doping. The increase of the lattice strain indicated that the Eu3+ dopant ions and interstitial O acted as strong defects in the BaAl2O4 structure. On the other hand, it seems that partial exchange of large cations (Ba2+) by smaller ones (Eu3+) increased the diffusion rate of cations in the sample and increased the crystallite size, consequently. Eu-doped sample under UV excitation showed the red PL spectrum characteristic of the Eu3+ ion on the non symmetric site.

Table 8. Fluorescence Transitions of Eu3+ Ions in Sample E1 λ (nm)

ṽ (cm−1)

577 589, 599 614, 619 655 693, 704

17331 16978, 16694 16287, 16255 15267 14430, 14205

assignment 5

D0 D0 5 D0 5 D0 5 D0 5

→ → → → →

7

F0 F1 7 F2 7 F3 7 F4 7

All of the observed emission lines originated from the 5D0 level only, with transitions from the 5D0 to 7FJ (J = 0, 1, 2, 3, 4) energy level. Three transitions among them are known as hypersensitive, 5D0 → 7F0, 5D0 → 7F1, and 5D0 → 7F2,44 and they provide information on which site/sites Eu3+ occupies in the material. The two peaks appearing at about 580 nm would indicate that Eu3+ ions substitute for Ba2+ ions on two different sites.15 However, for sample E1 only a single peak appears at 577 nm, indicating that Eu3+ ions occupy only one kind of Ba2+ site in the host BaAl2O4 structure. Furthermore, the magnetic Eu3+ dipole transition 5D0 → 7F1 appears as a doublet or triplet for the lower-symmetry environment. The strong emission peak at about 610 nm originating from 5D0 → 7F2 transitions is characteristic of Eu3+ ions occupying Ba sites with lower symmetry, while it becomes very weak when Eu3+ ions occupy sites with higher symmetry.45 Also, if the intensity ratio I(5D0 → 7F2)/I(5D0 → 7F1) is large, it may be assumed that Eu3+ is not located in a crystallographic site with an inversion center.46 Relatively intense PL lines in the range of 690−710 nm based on 5D0 → 7F4 transitions also support the low symmetry of the Eu3+ site in Eu3+-doped BaAl2O4.15 In conclusion, the obtained PL data from sample E1 support the results obtained from EXAFS investigations and from Rietveld structure refinement that upon Eu doping an Eu3+ ion substitutes for a Ba2+ ion on the Ba2 site, with an interstitial O localized near an Eu3+ ion for the charge compensation. It is also important to mention here that sample E1 did not show any luminescence in the blue or blue-green region of the spectra under UV excitation, which

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.G.). Tel.: +385 1 3873075. Fax: +385 1 4680114. ORCID

Biserka Gržeta: 0000-0001-5141-6362 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Sciences and Education of the Republic of Croatia under Project 098-0982886-2893 and the Federal Ministry of Education and Research of Germany under Project 05K10PX1 is gratefully acknowledged. K


Article

Inorganic Chemistry

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We thank Ralf Wagner, Benjamin Bornmann, Jonas Kläs, Pascal Becker, and Oliver von Polheim for their help with synchrotron experiments performed at the DELTA storage ring, ESRF, and SLS. We also appreciate the provision of beamtime for our experiments by those facilities. Ivančica Bogdanović Radović of Ruđer Bošković Institute is greatly acknowledged for performing PIXE spectroscopy analysis.

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