Elsevier

Journal of Molecular Structure

Volume 1029, 12 December 2012, Pages 53-59
Journal of Molecular Structure

Structural and optical studies of Sm3+ ions doped niobium borotellurite glasses

https://doi.org/10.1016/j.molstruc.2012.06.059Get rights and content

Abstract

Trivalent samarium ions doped borotellurite glasses (TRZNB) with the composition of 10TeO2 + 15RO + 5ZnO + 10Nb2O5 + 59B2O3 (where R = Mg, Ca and Sr) have been prepared by the melt quenching technique and investigated by the FT-IR, FT-Raman, optical absorption, photoluminescence and decay curve analysis. The FT-IR spectra and FT-Raman studies reveal the presence of [BO3], [BO4] and [TeO3], [TeO4] bridging and non-bridging oxygen as well as strong OH bonds in the prepared glasses. The experimental oscillator strengths were determined from the absorption spectra and have been used to determine J–O intensity parameters Ωλ (λ = 2, 4 and 6). Using the J–O intensity parameters as well as from the emission and lifetime measurements, various radiative parameters such as calculated branching ratios (βR), measured branching ratios (βm), transition probabilities (AR), radiative lifetimes (τR), measured lifetimes (τm), effective band-width (Δλp), and stimulated emission cross sections σλp have been calculated for the excited 4G5/2 luminescent level. The decay profiles were found to be single exponential in all the three glasses. Based on these results, the effectiveness of Sm3+ ions doped borotellurite glasses as a laser active substance in the visible region is discussed.

Highlights

► In this study we prepared TRZNB glasses doped with Sm3+ ions. ► Glasses are characterized with absorption, emission and lifetime analysis. ► Structural details were investigated by FTIR and FT-Raman analysis. ► Judd–Ofelt theory is used to calculate radiative and fluorescence properties. ► TRZNB glasses have applications as lasers and photonic devices.

Introduction

Spectroscopic study of rare earth ions in glasses provides information with regard to the transition probability, lifetime, branching ratio, etc. for the excited states of Ln3+ ions which are essential in the design of various electro-optic and optical devices like lasers, color display, amplifiers, sensors, display monitors, solar concentrators, optical detectors, wave guide lasers, optical data storage and under sea optical communication [1], [2]. An extensive quantity of research work has been reported on absorption and emission properties of various Ln3+ ions doped glass hosts. The absorption and emission spectra of the rare-earth ions doped materials have broad band region from ultraviolet to infrared. The RE ions doped glasses have attracted drastically because of they can be fabricated easily in several forms [3]. The RE ions doped oxide glasses are stable hosts to obtain the efficient luminescence [4]. These glasses are also found to have extremely high optical quality and high gain, with minimum beam divergence when used as laser hosts.

The high concentrations of RE ions in borate glasses have more advantages than any other oxide glasses due to lower melting temperature and accommodation. The boric oxide (B2O3) which is used as glass former in glass system is incorporated into different kinds of glass system as a flux material in order to attain good physical and chemical properties [5]. Borate glasses show clear relationship between glass structures, physical properties, high transparency, low melting points, high thermal stability and good glass forming nature have been identified as more useful host matrices [6]. Borate glasses incorporated by heavy metal oxides can give intense fluorescence in the visible spectral region which is used as electro-optic modulators, electro-optic switches, solid state laser materials and non-linear parametric converters [7].

Tellurium oxide (TeO2) based glasses are of scientific and technological interest on account of their distinctive properties such as chemical durability, electrical conductivity, transmission capability, high dielectric constant, high refractive indices and low melting points [8]. Tellurite and heavy metal oxide glasses catch much attention in recent years because their maximum phonon energy is lower than those in borate, phosphate, silicate and germinate glasses [9]. Telluride glasses containing rare earth ions have recently gained broad attention because of their potentiality in the development of fibers and lasers covering all the main telecommunication bands and promising materials for photonic applications such as up-conversion lasers, optical fiber amplifiers and non-linear optical devices [10]. The origin of the extraordinary non-linear optical properties of TeO2 based glasses is ascribed to the high hyper polarity of a loan electron pair related to the 5 s orbital of the tellurium atom [11]. In the development of laser glasses TeO2 host glasses are more attractive and interesting for different applications [12]. In fact, pure TeO2 does not form a glass unless it is doped with other elements and formed TeO2 glasses are found to be good in transparency, resistance towards the moisture and express low phonon energies of 738 cm−1 [13]. The pure TeO2 is a conditional glass former and requires fast quenching to form glass. The presence of TeO2 in the alkali borate glass matrix decreases its hygroscopic nature, improves the glass quality and enhances the IR transmission [14]. Addition of small amount of TeO2 into borate network improves glass quality with an enhancement in transparency and refractive index. Borotellurite glasses lead to complex specification in glass structure. Addition of ZnO into borotellurite glass network constructs low rates of crystallization and increase glass forming ability [15]. The Sm3+ ion is one of the most interesting ions for analysing the fluorescence properties because of its use in high density optical storage, under sea communication, color displays and visible solid state lasers [16]. Trivalent samarium ion (Sm3+) can be used as dopants in different glass-forming crystal hosts for efficient emissions in the visible region. Sm3+ ion with 4f5 electronic configuration exhibit a strong orange–red fluorescence in the visible region. Sm3+ possesses strong fluorescence intensity, rich energy levels, large emission cross section and high quantum efficiency [17]. Further, the Sm3+ ion is well suited to analyze the energy transfer process since its lowest emitting level 4G5/2 has comparatively high quantum efficiency and shows different quenching emission channels [18]. The fluorescence intensity of Sm3+ ion depends on the concentration of RE ions as well as on the glass network.

In the present work we have studied the absorption, fluorescence and decay processes of Sm3+ ions in TRZNB glasses. From the absorption analysis the radiative lifetimes and branching ratios for different excited levels have been obtained by applying the J–O theory and are compared with the experimental results. The decay characteristics of 4G5/2 levels of Sm3+ ion has also been recorded and analyzed. An attempt was made to assess the potential of Sm3+ doped TRZNB glasses as laser active materials.

Section snippets

Experimental methods

Sm3+ doped TRZNB glasses were prepared by melt quenching method with the following chemical composition (in mol%):

  • TMZNB: 10TeO2–15MgO–5ZnO–10Nb2O5–59B2O3–1Sm2O3

  • TCZNB: 10TeO2–15CaO–5ZnO–10Nb2O5–59B2O3–1Sm2O3

  • TSZNB: 10TeO2–15SrO–5ZnO–10Nb2O5–59B2O3–1Sm2O3

All the above components were well mixed and heated for 90 min in silica crucibles at 1450 °C in an electric furnace. The glasses were obtained by pouring the melt onto a pre-heated brass mold. The glass samples were then annealed at 350 °C for 12 h

Infrared spectroscopy

The FTIR spectroscopy is one of the essential tools to explore the fundamental functional groups in crystalline and non-crystalline matrices [19]. The IR spectra of Sm3+ doped TRZNB glasses is shown in Fig. 1, and it contains several peaks broad or moderate in band width specifying the local structure. The tellurium network usually exhibit two vibrational modes, the first one around 600–640 cm−1 corresponds to Te–O vibration in trigonal bipyramid [TeO4] groups and the later one around 680–700 cm−1

Conclusions

Sm3+ ion doped telluroborate (TRZNB) glasses were prepared by conventional melt quenching method. The FTIR spectra reveal the attachment of the borate network with the B–O vibrations. The Te–O–Te linkage and Te–O bending vibrations associated with theTeO3 and TeO6 units were identified and reported. Raman results show that the present work consists of TeO3, TeO4 structural groups and Nb ions probably in octahedral coordination (Nb2O6). The ionic characters of the Sm3+ ions with its surrounding

Acknowledgements

One of the authors (O. Ravi) is highly grateful to the University Grants Commission (UGC), New Delhi, for providing financial assistance in the form of Rajiv Gandhi National Fellowship (RGNF) to carry the research work. The authors are also highly thankful to the Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology, Chennai for extending instrumental facilities.

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