Crystal field studies in Eu3+ doped Bi12SiO20 and Bi12SiO20:V5+ single crystals

doi:10.1016/S0925-8388(01)01026-X

Journal of Alloys and Compounds

Volumes 323–324, 12 July 2001, Pages 260-266

https://doi.org/10.1016/S0925-8388(01)01026-X Get rights and content

Abstract

Site-selective laser spectroscopy investigations of Eu³⁺ in Bi₁₂SiO₂₀ (BSO) and Bi₁₂SiO₂₀:V⁵⁺ (BSO:V) single crystals have been carried out. The line narrowed fluorescence shows the existence of a single crystal field site for the rare earth in BSO whereas in BSO:V five different crystal field sites for Eu³⁺ may exist, which depend on the vanadium concentration. The number of Stark components observed in ⁵D₀→⁷F_J transitions corresponds to a complete absence of degeneracy in ⁷F_J multiplets and indicates a symmetry C_2v or lower for these optically active centres. Simulations of the sequence of Eu³⁺ energy levels for each of the five observed sites have been worked out by using the single-particle crystal-field theory. The nature of the new active optical sites has also been investigated by comparing the phenomenological crystal-field parameters with those calculated by a semi-empirical model which takes into account the distortions of the oxygen bonds produced by the presence of V⁵⁺.

Introduction

A growing interest is focused on Bi₁₂SiO₂₀ (BSO) due to its optical properties and technological applications, most of them derived from its photorefractive behaviour [1].

Bismuth oxide compounds of the Bi₁₂SiO₂₀ type have a body-centered cubic structure (sillenite structure) which is able to accommodate a variety of different M atoms (M=Si in this compound). This is due to the fact that the oxygen tetrahedron surrounding the M atom can change its size without a major effect on the remaining atomic arrangement [2]. This peculiarity has promoted an effort to establish the mechanisms of isomorphous replacement in sillenites. From a fundamental point of view the knowledge of the defects responsible for the photorefractive effect still remains as an unsolved question, in particular as regards to the absorption bands associated with those defects. Up to now most of the works adscribe the absorption shoulder, located at energies close to the absorption edge, to the photoconductive and photorefractive behaviour. However, no definite conclusions have been reached about which defects are responsible for these behaviours.

BSO crystals doped with several transition metal ions have been studied [3], [4], [5], [6] in order to ascertain their influence on the absorption shoulder appearing in pure BSO crystals. Among them, V⁵⁺ has shown very interesting features. In particular, it has been demonstrated [7] that at small concentrations V⁵⁺ gives an increase of the absorption coefficient in the shoulder region whereas at higher concentrations a bleaching of the BSO crystals occurs. Moreover, the presence of V⁵⁺ in the sillenite structure strongly affects the type of Bi–O bondings, and therefore, the Bi site symmetry [8], [9]. On the other hand, only a few studies are known on the optical properties of BSO doped with rare earths [10], [11], [12], [13], [14], [15], [16]. It is well established that spectroscopical studies of rare earths may give valuable information about lattice site, lattice distortion, degree of covalency to ligand ions, and many other related dopant-host properties.

In recent works [14], [15] some of the present authors reported, for the first time in pure and vanadium codoped BSO crystals, a preliminary identification of the spectroscopic active sites of Nd³⁺ and Eu³⁺ ions by using site-selective laser spectroscopy. Three different crystal-field sites for Nd³⁺ and five different crystal-field sites for Eu³⁺, which depend on vanadium concentration, were found and optically isolated. However, neither a clear explanation was found about the nature and characteristic features presented by the emission spectra corresponding to the optical active sites, nor a plausible argument permitting to relate the rare earth optical behaviour with the presence of vanadium in the BSO crystal structure.

The work reported here comprises the spectroscopic results on site-selective emission spectra and a detailed crystal-field (cf) simulation of the Eu³⁺ energy level scheme for the five different optical centres in the BSO host. Calculations of cf effects have been carried out by using the single-particle cf theory. The phenomenological simulation of each Eu³⁺ energy level scheme is conducted on the basis of the strongly reduced ⁷F_JM set alone, i.e. only considering 49∣SLJM_J〉 levels. A descending symmetry method, from C_2v to C₂ symmetries, has been used in the fitting procedure. The appearance and relative intensity of these centres are discussed in terms of plausible crystallographic distortions of the EuO₅ polyhedron when compared with the BiO₅ in the non-substituted BSO host. Simulations of corresponding cf parameters for these changes in the Bi environment were performed through a semi-empirical model which considers the crystallographic positions of the oxygen atoms around Bi.

Section snippets

Experimental

Pure and doped BSO crystals were grown by the Czochralski method from grade A1 Johnson-Matthey powder. Several crystals were grown with rare earth concentrations up to 10 000 ppm in the melt. The absorption spectra of these samples revealed that a Eu³⁺ saturation is attained at concentrations greater than 5000 ppm. For this reason the codoped vanadium crystals (with vanadium contents of 100, 200, 500, and 999 ppm) were obtained while keeping 5000 ppm for rare earths.

The samples temperature was

Eu³⁺ spectroscopy

Time-resolved line-narrowed fluorescence spectra of the ⁵D₀→⁷F_J transitions of Eu³⁺ in singly doped and vanadium codoped crystals were obtained at 4.2 K by using different resonant excitation wavelengths into the ⁷F₀→⁵D₀ transition. These spectra were obtained at different time delays after the laser pulse. As an example, Fig. 1a shows the ⁵D₀→⁷F_0,1,2 spectra at 4.2 K for a BSO crystal doped with 5000 ppm of Eu³⁺ obtained at 1 ms after the laser pulse by exciting at 579.1 nm. As can be observed

Structural considerations

The existence of several possible sites for rare earths in BSO can be understood on the basis of its structural framework. As shown in Fig. 2, the ideal sillenite structure is built up by Bi-polyhedra connected via common edges to form dimers that link translationally identical [MO(3)₄] tetrahedra. Recent neutron diffraction studies [8], [9] have shown the existence of sillenites with M cations having an effective valence lower than four and with a structure being to a great extent O(3) atom

Crystal-field analysis and simulation of the energy level schemes

The phenomenological cf simulation of the Eu³⁺ energy level scheme can be conducted on the basis of the ⁷F_JM set alone by using only 49 levels out of a total of 3003 of the 4f⁶ Eu³⁺ configuration. The use of this truncation is enabled by two facts: firstly, the cf operator mixes only the levels with the same multiplicity, and secondly, the ground ⁷F_J (J=0 – 6) term is well isolated from the rest of the configuration (about 12 000 cm⁻¹ between ⁷F₆ and ⁵D₀), which renders the mixing of the

Discussion

The five energy level schemes for each of the optically active sites are characterized by strong splittings of the ⁷F_1–6 levels under the cf effect, and in accordance very large cf parameters can be expected to be obtained. The close resemblance of these schemes from B, C, and D active sites in BSO is evidently due to the rather similar coordination of the Eu³⁺ in each case. From this point of view, differences observed in optical spectra corresponding to the two remaining A and E sites clearly

Conclusions

On the basis of the above arguments and account taken of the great flexibility of the BSO structure, the difficulty to assign particular sites to a rare earth in the BSO structure becomes clear. Nevertheless, regarding the spectroscopic results of Eu³⁺ ions in BSO it is worthwhile noticing the extreme sensitivity of the Bi polyhedra geometry to the presence of V⁵⁺ ions. Moreover, these geometrical changes are very sharply defined, and as the Eu³⁺ spectroscopy shows, they do not seem to present

Acknowledgements

This work has been financially supported by the Spanish Government CICYT (Ref. MAT97-1009) and Basque Country University (ref: G21/98).

References (29)

W Wardzynski et al.
J. Phys. Chem. Sol.
(1984)
Y Nagao et al.
Mater. Res. Bull.
(1989)
D Nesheva et al.
J. Phys. Chem. Solids
(1995)
T Katsumata et al.
Mat. Res. Bull.
(1995)
A Oleaga et al.
J. Lumin.
(1997)
J Fernández et al.
Opt. Mater.
(1997)
C Cascales et al.
J. Sol. State Chem.
(1990)
C Cascales et al.
J. Alloys Comp.
(1992)
O.L Malta
Chem. Phys. Lett.
(1982)
A.J Freeman et al.
J. Magn. Magn. Mat.
(1979)

L Arizmendi et al.

Int. J. Optoelectron.

(1992)

S.C Abrahams et al.

J. Chem. Phys.

(1979)

T.V Panchenko et al.

Ferroelectrics

(1991)

V.M Orlov et al.

Izv. Akad. Nauk. SSSR, Neorg. Mater.

(1986)

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Er³⁺-doped Bi₂O₃ thin films were sputter-deposited on SiO₂ substrates under various process conditions. After post annealing in an O₂ atmosphere, near-infrared photoluminescence (PL) spectra from Er³⁺ ions in specific product oxides were observed. For sufficiently oxidized Bi₂O₃:Er films with Er contents less than 1.5 at%, α-Bi₂O₃:Er having a monoclinic structure nucleated with negligible intermixing of the elements across the interface. Its PL spectra exhibited Er³⁺ luminescence signals consisting of eight Stark splitting peaks. Deposition at temperatures higher than 400 °C and/or with high Er content produced a somewhat reduced Bi₂O₃ network. Post annealing triggered diffusion of Si into the film as well as diffusion of Bi and Er into the SiO₂ substrate, creating Bi₂O₃ −SiO₂ −Er₂O₃ compound oxides. The original film body crystallized into a δ-Bi₂O₃ structure, which was stabilized by the presence of a large amount of Er. The emission-active product at Er contents of 2 at% was Bi₂O₃:Er−SiO₂ (Bi₂SiO₅:Er), which exhibited four-line emission spectra and the intensity was lower than that of α-Bi₂O₃:Er. For Er³⁺ contents higher than 4 at%, interdiffusion of Si and O atoms was enhanced and SiO_x:Er domains were created in the Bi₂O₃:Er film. Intense and broad emission peaks at 1530 and 1560 nm were observed in the PL spectra. The emission-active species in the Bi₂O₃ −SiO₂ −Er₂O₃ compound oxide will be either Er³⁺ doped in the strongly disordered δ-Bi₂O₃ lattice involving oxygen deficiencies or Er³⁺ attached to SiO_x domains formed inside the δ-Bi₂O₃ network.
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Nevertheless, to the best of our knowledge, no accurate attempts have been made to provide a full analysis of different synthetic procedures that studies their influence on the luminescence features of these materials. Among all the lanthanides, we have chosen Eu3+ as a dopant ion in view of its adequacy as a site-sensitive structural probe [41–43], which can be useful to discuss the optical response of the materials. In addition, the ionic radius for Y3+ (coordination number, CN = 8) is 1.019 Å and for Eu3+ (CN = 8), it is 1.066 Å [44].
Eu³⁺-doped KY₃F₁₀ materials with a dopant content between 0 and 5 mol% were prepared based on the nominal composition K(Y_3-_xEu_x)F₁₀ using different synthetic routes. The reaction conditions have been proven to be critical factors for the characteristics of the final products: morphology, size and crystallinity. As a result, noticeable changes in their photoluminescence spectra and lifetimes were observed. Quantum cutting processes or similar energy transfers between Eu³⁺ ions allowed obtaining high quantum efficiencies, while the analysis of the $Ω_{2}$ Judd-Ofelt parameter suggested that the crystal field of Eu³⁺ was very similar in all the compositions. A well-designed synthesis of Ln³⁺-doped fluorides can provide a full range of opportunities to explore new phenomena. Thus, this study highlights the complexity of the fluoride-based systems, which are exceptional candidates for doping with luminescent lanthanide ions and have very important characteristics for their future application in bioanalytics, biomedics or photonics. Indeed, the color-tunable emissions of the phosphors, which vary from orangish to yellow, could be interesting for their application in white light-emitting diodes through their combination with blue chips.
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Bismuth silicon oxides, obtained by crystallisation of the Bi2O3–SiO2 system, such as Bi12SiO20 and Bi4Si3O12, have attracted much interest due to their technological applications [1–3] in medicine, geological exploration, nuclear physics and high energy physics [4,5]. In particular, Bi12SiO20 is used in scintillators and photorefractive materials, in lasers [6,7] and in materials with applications in electrooptics, acoustics, and piezotechnics, where physical properties such as photorefractivity, photoconductivity, optical activity, reflection holography, lensometry, the velocity of ultrasound wave propagation are important [8–16]. Bi4Si3O12 is used as a highly efficient scintillator in gamma ray spectroscopy and high energy physics, and in non-linear optical devices, nuclear medicine, and photorefractive materials [5].
Defect properties of Bi₁₂SiO₁₂ and Bi₄Si₃O₁₂ compounds were investigated using atomistic computer modelling techniques based on energy minimisation. Interatomic potentials obtained by empirical fitting reproduce the lattice parameters for both materials and the available elastic and dielectric constants with reasonable accuracy. The relative stability of the phases and the intrinsic defects of both phases are predicted. A new methodology is used for calculating solution energies for rare earth doping which takes doping concentration into account.
Influence of annealing temperature on luminescent properties of Eu3+/V5+ co-doped nanocrystalline Gd<inf>2</inf>Ti<inf>2</inf>O<inf>7</inf> powders
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Nanosized Gd_2(1–x)Eu_2xTi₂O₇: yV⁵⁺ phosphors were prepared via sol-gel method and characterized with X-ray diffraction, Raman spectroscopy, diffuse reflectance spectra and photoluminescence spectra. Their PL properties were investigated as functions of the Eu³⁺ doping concentration and annealing temperature. The results indicated that the as-prepared samples showed a strong emission of Eu³⁺ under the irradiation of 303 nm. For Eu³⁺-doped Gd₂Ti₂O₇, the orange emission at 586 nm was the strongest, which was corresponding to magnetic dipole transition ⁵D₀–⁷F₁ due to an energy transfer from host, and the optimum concentration for Eu³⁺ was determined to be 4 mol.%. While for Eu³⁺/V⁵⁺ co-doped Gd₂Ti₂O₇ annealed at 1000 °C, the spectrum was dominated by 615 nm red emissions corresponding to electric dipole transition ⁵D₀–⁷F₂. Additionally, the PL intensity of Eu³⁺ was enhanced greatly by increasing the annealing temperature. These results are very encouraging for its application in polychromatic displays.
The enhanced and color-tunable photoluminescence of Eu3+/V5+ co-doped Gd<inf>2</inf>Ti<inf>2</inf>O<inf>7</inf> nanocrystals
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Eu³⁺-doped and Eu³⁺/V⁵⁺ co-doped pyrochlore-type gadolinium dititanate (Gd₂Ti₂O₇) nanoparticles were synthesized by sol–gel method. X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and photoluminescence (PL) spectroscopy were employed to characterize the phosphors obtained, respectively. In Eu³⁺-doped Gd₂Ti₂O₇ nanocrystals, only the bright orange emissions which can be assigned to the ⁵D₀–⁷F₁ magnetic dipole transition of Eu³⁺ had been noticeable because of the existence of inversion symmetry for Gd³⁺ sites occupied by Eu³⁺ ions. Interestingly, upon co-doping V⁵⁺ in Gd₂Ti₂O₇:Eu³⁺ phosphor, the bright red emissions which can be assigned to the ⁵D₀–⁷F₂ electric dipole transition of Eu³⁺ were observed, due to the absence of inversion symmetry induced by the steric effect and charge effect. The ratio of the intensity of red luminescence to that of orange luminescence increased with increasing the doping concentration of V⁵⁺, which is encouraging for its application in optical devices.
Crystal-field analysis of Eu3+ energy levels in the new rare-earth R BiY<inf>1-x</inf>R<inf>x</inf>GeO<inf>5</inf> oxide
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Journal of Alloys and Compounds

Abstract

Introduction

Section snippets

Experimental

Eu³⁺ spectroscopy

Structural considerations

Crystal-field analysis and simulation of the energy level schemes

Discussion

Conclusions

Acknowledgements

References (29)

J. Phys. Chem. Sol.

Mater. Res. Bull.

J. Phys. Chem. Solids

Mat. Res. Bull.

J. Lumin.

Opt. Mater.

J. Sol. State Chem.

J. Alloys Comp.

Chem. Phys. Lett.

J. Magn. Magn. Mat.

Int. J. Optoelectron.

J. Chem. Phys.

Ferroelectrics

Izv. Akad. Nauk. SSSR, Neorg. Mater.

Cited by (11)

Er<sup>3+</sup>-catalyzed interdiffusion of elements across the interface between Bi<inf>2</inf>O<inf>3</inf> film and SiO<inf>2</inf> substrate resulting in host-material-specific photoluminescence spectra of Er<sup>3+</sup>

Tuning the optical and photoluminescence properties of high efficient Eu<sup>3+</sup>-doped KY<inf>3</inf>F<inf>10</inf> phosphors by different synthetic approaches

Computer modelling of Bi<inf>12</inf>SiO<inf>20</inf> and Bi<inf>4</inf>Si<inf>3</inf>O<inf>12</inf>: Intrinsic defects and rare earth ion incorporation

Influence of annealing temperature on luminescent properties of Eu<sup>3+</sup>/V<sup>5+</sup> co-doped nanocrystalline Gd<inf>2</inf>Ti<inf>2</inf>O<inf>7</inf> powders

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Crystal-field analysis of Eu<sup>3+</sup> energy levels in the new rare-earth R BiY<inf>1-x</inf>R<inf>x</inf>GeO<inf>5</inf> oxide

Crystal field studies in Eu3+ doped Bi12SiO20 and Bi12SiO20:V5+ single crystals

Abstract

Introduction

Section snippets

Experimental

Eu3+ spectroscopy

Structural considerations

Crystal-field analysis and simulation of the energy level schemes

Discussion

Conclusions

Acknowledgements

J. Phys. Chem. Sol.

Mater. Res. Bull.

J. Phys. Chem. Solids

Mat. Res. Bull.

J. Lumin.

Opt. Mater.

J. Sol. State Chem.

J. Alloys Comp.

Chem. Phys. Lett.

J. Magn. Magn. Mat.

Int. J. Optoelectron.

J. Chem. Phys.

Ferroelectrics

Izv. Akad. Nauk. SSSR, Neorg. Mater.

Crystal field studies in Eu³⁺ doped Bi₁₂SiO₂₀ and Bi₁₂SiO₂₀:V⁵⁺ single crystals

Eu³⁺ spectroscopy