Elsevier

Journal of Luminescence

Volume 169, Part B, January 2016, Pages 669-673
Journal of Luminescence

Local site symmetry of Sm3+ in sol–gel derived α′-Sr2SiO4: Probed by emission and fluorescence lifetime spectroscopy

https://doi.org/10.1016/j.jlumin.2014.10.009Get rights and content

Highlights

  • Sol–gel synthesis.

  • Sm3+ used as a structural probe.

  • Sm3+ occupies 9-coordinated Sr2+.

  • Site symmetry around Sm3+ is C2v.

  • Mechanism of concentration quenching.

Abstract

Trivalent samarium-doped strontium silicate (Sr2SiO4) phosphors were prepared by sol–gel synthesis using tetra ethyl orthosilicate (TEOS) as precursor. The synthesis temperature could be brought down to 600 °C for the formation of a single phase sample. The emission and excitation spectra, and decay curves were employed to study the luminescence properties. The calcined powders of the Sm3+ ions doped in the Sr2SiO4 emit reddish orange light. In our present study, the 4G5/26H5/2 (MD) transition of Sm3+ ions is less intense than 4G5/26H9/2 (ED) transition. This indicates that Sm3+ ions preferentially occupy asymmetric site in α-Sr2SiO4. Fluorescence life time measurement has shown monoexponential behavior for Sm3+ in strontium silicate with lifetime value of 2.23 ms. Based on lifetime and emission spectra, it can be inferred that Sm3+ occupies 9-coordinated Sr2+ site in strontium silicate. Based on stark splitting pattern it was inferred that site symmetry around Sm3+ is C3v which further justifies it׳s occupancy in 9-coordinated sites. It was observed that, the emission intensity of Sm3+ increases with the increase in concentration initially, reaching maxima at 1.0 mol % and then decreases with the increasing concentration due to concentration quenching. At higher concentrations (x>0.01), however, the observed decay curves were bi-exponential.

Introduction

Among the materials currently evaluated as host for lanthanide ions, in this work our attention was focused on alkaline earth silicates. These materials are characterized by good transmission properties in the visible part of the electromagnetic spectrum and by relatively low phonon energies. They can be efficiently doped with lanthanide ions, due to the similarity between the ionic radius of the alkaline earth and the lanthanide ions. Therefore these materials are prospective high efficiency luminophors and are attracting increasing interest for photonics and optoelectronics applications. Indeed alkaline earth silicates are resistant to many chemicals and air exposure and can also be grown with low-cost techniques.

Oxide-based hosts have received considerable attention for use in flat-panel displays due to their luminescent characteristics, stability in high vacuum, and the absence of corrosive gas emission under electron bombardment, as compared to currently used sulfide-based phosphors [1]. Therefore, oxide-based phosphors are likely to emerge as the choice for field emission diodes (FED) green or red phosphors. Among these, strontium silicate is an excellent matrix due to its stable crystal structure, good mechanical strength and high thermal stability provided by the tetrahedral silicate (SiO4)2− group [2]. Sr2SiO4 has attracted interest due to its special structural features and potential application in developing white light-emitting-diodes (LEDs), because GaN (400 nm chip) coated with Sr2SiO4: Eu2+ exhibits better luminous efficiency than that of industrially available products such as InGaN (460 nm chip) coated with YAG: Ce [3]. The optical band gap of alkaline earth silicate is in the range of 4–7 eV and therefore these materials are characterized by good transmission properties in the visible part of the electromagnetic spectrum.

When an active dopant is introduced into structures with multiple sites (A and B), their optical and magnetic properties are dramatically changed depending on its distribution in the ceramic. Studies of dopant ion distribution among different sites have attracted much attention because they may allow better understanding of the correlations between structure and properties such as color, magnetic behavior, catalytic activity, and optical properties, etc., which are strongly dependent on the occupation of these two sites by metals.

Eu3+ is a well known structural probe [2], [4], [5], [6], [7] because of its non-degenerate ground state 7F0 and non-overlapping 2S+1LJ multiplets. Local environment of Eu3+ and Dy3+ in Sr2SiO4 has been reported by our group in our previous work [2], [8]. Due to their sharp emission lines; the Sm3+ ions could also be used as a luminescent probe [9].

Although reports do exist in the literature on luminescence properties of Sm3+ ions in Sr2SiO4 [10], [11], none of them explain the site occupancy of Sm3+ and their related effect on luminescence properties in these matrices. TRES is extensively used to understand such phenomena. In Sr2SiO4, there are two types of sites for Sr2+, one is a more symmetric 10 coordinated site Sr (1) and the other is a less symmetric 9 coordinated site Sr (2). We have also tried to investigate the effect of concentration of Sm3+ and annealing temperature on its luminescence properties.

Section snippets

Synthesis

All the chemicals used in the sample preparation were of AR grade and procured from Sigma-Aldrich. The alkaline earth silicate samples were prepared via a sol–gel route using tetraethyl orthosilicate (TEOS) and strontium nitrate adopting the standard procedure [12].

Instrumentation

The phase purity of the prepared phosphors were confirmed by X-ray diffraction (XRD). The measurements were carried out on a STOE X-ray diffractometer equipped with Ni filter, scintillation counter and graphite monochromator. The

Phase purity: X-ray diffraction

Fig. 1 shows the PXRD patterns of undoped and 0.5 mol% Sm3+ doped Sr2SiO4. The PXRD patterns were in agreement with standard JCPDS no. 39–1256 corresponding to orthorhombic phase. Further no additional peaks can be found in doped samples. Since ionic radius of Sm3+ was relatively less when compared to Sr, it was expected that the Sm3+ ion can easily enter the Sr lattice without disturbing the crystal structure. It is natural to assume that Sm3+ occupies the Sr2+ position though cation vacancies

Conclusions

Sr2SiO4:Sm3+ phosphor were synthesized at 600 °C by the sol–gel route using TEOS as a precursor and characterized by X-ray diffraction (XRD) and photoluminescence (PL) techniques. From the emission spectroscopy the 4G5/26H9/2 (ED) transitions of Sm3+ ions were more intense than 4G5/26H5/2 (MD) transition, indicating the asymmetric nature of Sm3+ ion in Sr2SiO4 host matrix. Fluorescence life time measurement showed monoexponential behavior for Sm3+ in strontium silicate with a lifetime value of

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