Luminescence properties of the Sm-doped borate glasses
Introduction
The electron and local structures of luminescence and paramagnetic impurity centres in crystals, glasses and other disordered compounds represent an interesting problem of the solid state physics and spectroscopy of functional materials. The optical and electron paramagnetic resonance (EPR) spectroscopy methods allow investigating both electron and local structures of the luminescence and paramagnetic centres in single crystals and disordered solids, including glasses.
Attractive optical properties and high luminescence efficiency of the rare-earth impurity ions in crystals and glasses opened new possibilities of practical applications in different branches e.g. biomedical, lasers, energy generation and transformation, telecommunications, display devices, sensors and other. Practical applications of the rare-earth doped glasses and crystals stimulate the search of new materials and investigation of their luminescence properties and laser characteristics.
Borate crystals and glasses represent suitable host materials for both investigation the nature and structure of the luminescence and paramagnetic centres and useful practical applications. In particular, the borate compounds, un-doped and doped with rare-earth and transition elements, are very promising materials for nonlinear optics, quantum electronics and laser technology [1], [2], [3], [4], [5], scintillators and thermoluminescent dosimeters [6], [7], [8], [9], [10], detectors and transformers of the ionising radiation [11], [12], and many other applications [13], [14], [15]. This especially concerns to the lithium tetraborate (Li2B4O7) single crystals, which are optically-transparent in the wide spectral region restricted from vacuum ultraviolet (UV) to middle infrared (IR) ranges [16] and have extremely high radiation stability [17], [18]. Presently the glassy (or vitreous) borate compounds are more promising than their crystalline analogues from the technological point of view, because the growth of borate single crystals is a difficult, long-term and consequently very expensive process. Besides this, a very low velocity of the crystal growth and a high viscosity of the melt leads to problems with doping of the borate crystals by the rare-earth and transition elements.
The Sm-doped crystals and glasses are well-known as efficient luminescent materials for developing of new colour emitting phosphors with high emission quantum yield at room temperature and high thermal and chemical stability in the air [19], [20]. The photoluminescence spectra of the Sm3+ centres in glasses and crystals reveal intense characteristic green, orange, and red emission bands, which correspond to the 4G5/2→6H5/2, 4G5/2→6H7/2 and 4G5/2→6H9/2 emission transitions that can be used in new light sources, fluorescent display devices, UV-sensor, and visible lasers [21], [22], [23], [24], [25]. Spectroscopic and luminescence properties of the Sm3+ ions already have been investigated in various hosts [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. In particular, optical and luminescent properties have been reported for Sm-doped borate crystals [26], [27], [28] and various glasses, such as lithium borate [29], [30], [31], lithium fluoroborate [32], lithium aluminoborate [21], [22], lithium magnesium borate [33], lithium zinc borate [34], lead fluoroborate [35], sodium fluoroborate [24], [36], zinc bismuth borate [37], zinc alumino-bismuth borate [38], borochromate [39], and borogermanate [40] glasses. Let us consider the main results of some papers.
Spectroscopic and laser properties of Sm3+ ions in lithium fluoroborate and aluminoborate glasses for efficient visible lasers have been reported in Refs. [21], [24]. Luminescence excitation and emission spectra of borate glasses with xCeO2–(9.5−x)Al2O3–0.5Sm2O3–10Li2CO3–80B2O3 composition and LiCaBO3:M3+ (M3+=Eu3+, Sm3+, Tb3+, Ce3+, and Dy3+) polycrystalline compounds, which represent promising phosphors for white light emitting diode (LED), have been investigated in Refs. [[22], [23]], respectively. Results of investigation of the EPR, optical absorption, luminescence excitation and emission spectra as well as luminescence kinetics of the Li2B4O7:Sm glasses have been reported in Ref. [29]. Based on the optical absorption, luminescence spectra and decay kinetics analysis it was shown that the Sm impurity is incorporated into the Li-sites of the Li2B4O7 glass network in the trivalent state, exclusively [29]. In Ref. [12] it was reported about synthesis, optical and luminescence properties, and some scintillation characteristics associated with detecting neutrons (En≤10 MeV) and 60Co γ-radiation for the un-doped and Sm-, Eu-, Ce-, Tb-, Tm-, and Yb-doped Li2B4O7 glasses. In general, the decay curves for Sm3+ ions in borate glasses are single exponential at Sm concentrations lower than 1.0 mol% and non-exponential at Sm concentrations higher than 1.0 mol% due to energy transfer through the cross-relaxation processes [41].
Conversion from Sm3+ to Sm2+ centres has significant interest due to the presence of intense emission bands in the red spectral region [20]. Luminescence properties of the Sm2+ centres in borate compounds were reported in Refs. [42], [43], [44]. Generally, the Sm2+- doped glasses can be produced in a strongly reducing Ar, H2 or H2/N2 atmospheres [45], [46]. The Sm2+ ions in borate compounds created by X- or γ-radiation [44] are unstable that is caused by spectral hole burning [44], [47].
Consideration and analysis of the available referenced data show that the features of spectroscopic and luminescence properties of the Sm-doped borate glasses are studied insufficiently. In particular, the electron and local structures of the Sm luminescence centres in the network of LiKB4O7:Sm, CaB4O7:Sm, and LiCaBO3:Sm glasses have not been investigated yet. Therefore, the main aim of this work is investigation of features of the spectroscopic and luminescence properties of a series borate glasses with Li2B4O7:Sm, LiKB4O7:Sm, CaB4O7:Sm, and LiCaBO3:Sm compositions and different Sm concentrations as well as determination of the electron and local structures of Sm luminescence centres in the investigated glasses using conventional EPR and optical spectroscopy techniques.
Section snippets
The glass synthesis and preparation of samples
The Sm-doped borate glasses with Li2B4O7 (Li2O – 2B2O3), LiKB4O7 (0.5Li2O – 0.5K2O – 2B2O3), CaB4O7 (CaO – 2B2O3), and LiCaBO3 (0.5Li2O – CaO – 0.5B2O3) were obtained in the air atmosphere from the corresponding polycrystalline compounds according to standard glass synthesis method and technological conditions, which described in Ref. [48]. For solid-state synthesis of the polycrystalline compounds Li2B4O7:Sm, LiKB4O7:Sm, CaB4O7:Sm, and LiCaBO3:Sm was used Li2CO3, K2CO3 and CaCO3 carbonates and
EPR spectroscopy of the Sm-doped borate glasses
The results of spectroscopic investigation of the Sm-doped lithium tetraborate (Li2B4O7:Sm) glasses firstly were presented in Ref. [29]. The preliminary results of EPR and optical spectroscopy of the LiKB4O7:Sm, CaB4O7:Sm, and LiCaBO3:Sm glasses have been reported in Ref. [50], but the obtained results had not been published yet.
The samarium impurity can be incorporated into the structure of different oxide compounds as paramagnetic Kramers Sm3+ (4f5, 6H5/2) or non-Kramers Sm2+ (4f6, 7F0) ions,
Conclusions
The borate glasses Li2B4O7:Sm, LiKB4O7:Sm, CaB4O7:Sm, and LiCaBO3:Sm of high optical quality are detailed investigated by EPR and optical spectroscopy techniques. Based on the obtained experimental results analysis supported by the structural data for those glasses and their crystalline analogues, one can conclude the following:
- •
The samarium impurity is incorporated in the Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 glass network exclusively as Sm3+ (4f5, 6H5/2) ions and forms the Sm3+ luminescence
Acknowledgements
The authors would like to thank Dr.Sc. V.T. Adamiv, Dr.Sc. Ya.V. Burak, and M.Sc. I.M. Teslyuk from Vlokh Institute of Physical Optics (Lviv, Ukraine) for synthesis of the glasses and samples preparation. Many thanks to Prof. N. Guskos and Dr. G. Żołnierkiewicz from West Pomeranian University of Technology (Szczecin, Poland) for registration of EPR spectra at liquid helium temperatures, Dr.Sc. R. Lisiecki and M.Sc. A. Watras from Institute of Low Temperature and Structure Research Polish
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