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Synthesis and spectroscopy of tetraborate glasses doped with copper
Journal of Non-Crystalline Solids 356 (2010) 2033–2037
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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Synthesis and spectroscopy of tetraborate glasses doped with copper B. Padlyak a,b,⁎, W. Ryba-Romanowski c, R. Lisiecki c, O. Smyrnov b, A. Drzewiecki b, Ya. Burak a, V. Adamiv a, I. Teslyuk a a b c
Institute of Physical Optics, Sector of Spectroscopy, 23 Dragomanov Str., 79-005 Lviv, Ukraine University of Zielona Góra, Institute of Physics, Division of Spectroscopy of Functional Materials, 4a Szafrana Str., 65-516 Zielona Góra, Poland Institute of Low Temperatures and Structure Research, Polish Academy of Sciences, 2 Okólna Str., 50-422 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 12 May 2010 Available online 9 June 2010 Keywords: Borate glass; Cu2+ center; Cu+ center; EPR; Optical absorption; Luminescence; Decay kinetics
a b s t r a c t Lithium and potassium–lithium tetraborate glasses doped with Cu (Li2B4O7:Cu and KLiB4O7:Cu) of high optical quality were obtained from polycrystalline compounds by fast cooling of the corresponding melt. Cu impurity was added to the Li2B4O7 and KLiB4O7 compounds in the form of a CuO oxide in the amounts of 0.4 and 1.6 mol.%. On the basis of EPR and optical spectroscopy (absorption, emission and luminescence excitation and kinetics) data analysis it was shown that the Cu impurity was incorporated into the tetraborate glass network as Cu2+ (3d9) and Cu+ (3d10) ions. The EPR spectra of Cu2+ centers were almost identical in glasses with the Li2B4O7:Cu and KLiB4O7:Cu compositions and were characteristic for glassy compounds. The Cu2+ EPR spectra parameters (g-values, hyperfine constants and peak-to-peak linewidths) in the Li2B4O7:Cu and KLiB4O7:Cu glasses were obtained at T = 300 K. The characteristic broad absorption band peaked near 750 nm was assigned to the 2B1g → 2B2g transition of the Cu2+ centers. An intense absorption in the UV region (λ b 350 nm) was related to the Cu2+ → O2− charge-transfer band. Broad emission bands with the maxima near 420 and 465 nm were observed in the luminescence spectra of the Li2B4O7:Cu and KLiB4O7:Cu glasses. The emission bands, peaked near 420 and 465 nm, were characterized by single exponential decay with lifetimes τ = 23 and 27 μs, respectively for the Li2B4O7:Cu and KLiB4O7:Cu glasses (Cu content—1.6 mol.%) at T = 300 K. Both emission bands belonged to the Cu+ centers with different local environments or their distortion. A possible local structure for two types of Cu+ centers in a tetraborate glass network is discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Borate compounds (crystals and glasses), un-doped and doped with transition and rare-earth elements are very promising for quantum electronics and non-linear optics [1,2] scintillators and tissue-equivalent materials for thermoluminescent (TL) dosimetry [3–5], as well as γ and neutron detectors [6–8]. This also concerns lithium tetraborate crystals which are characterized by extremely high radiation stability [9,10], high transparency in a wide spectral range from VUV to far IR [11] and good TL properties [4–8]. It is technologically difficult, takes a long-time, and as consequence, it is very expensive to obtain tetraborate single crystals with different compositions by the Czochralski method. In addition to that, a very low velocity of the crystals growth and high viscosity of the melt lead to problems with the doping of tetraborate crystals. Thus, from the technological point of view the glassy (or vitreous) borate compounds are most prospective in comparison with the corresponding single crystals. On the other hand, the study of the ⁎ Corresponding author. Institute of Physical Optics, 23 Dragomanov Str., 79-005 Lviv, Ukraine. Fax: +48 068 328 2920. E-mail address: [email protected] (B. Padlyak). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.027
electron and local structure of impurity and other point defects in complex oxide glasses, particularly in glasses with Li2B4O7 (LTB) and KLiB4O7 (KLTB) basic compositions, poses an interesting scientific problem. The electron paramagnetic resonance (EPR) and optical spectroscopy allow investigating the electron and local structure of point defects in crystals and corresponding glasses. Structural and corresponding spectroscopic data for crystalline analogies are needed for an interpretation of the EPR and optical spectra in complex oxide glasses [12,13]. Tetraborate compounds are good candidates to study the nature of point defects as boron-containing compounds, including tetraborates, can be obtained in both crystalline and glassy states. The optical spectra of lithium tetraborate crystals and glasses doped with Cu (LTB:Cu) have been investigated by different authors [6–8,14,15]. Particularly in [14] it has been shown by optical spectroscopy that the multivalent states of impurity transition ions for non-irradiated LTB:Cu crystals and glasses, “as-grown” in the air, obtained from melted crystals are characteristic and the copper impurity is revealed as Cu+ and Cu2+ ions. At the present time, the emission spectra and luminescence kinetics of Cu+ centers have been investigated in LTB:Cu single crystals [6,7] and glasses [8,14–17],
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however, the peculiarities of the Cu+ luminescence properties of LTB: Cu glasses have not been clearly explained to this day. The EPR spectra of the Cu2+ paramagnetic ions in the LTB:Cu crystal and glass at different temperatures, typical for crystalline and glassy compounds, have been presented in [16], however, their spin Hamiltonian parameters have not been determined. In [18] we have reported for the first time the parameters of the Cu2+ EPR spectra in Li2B4O7:Cu and KLiB4O7:Cu glasses. On the basis of the referenced data analysis it can be concluded that at the present time the EPR and optical spectra of LTB:Cu glasses doped with Cu have been studied insufficiently. The results of optical and EPR spectroscopy for new crystal and glass with KLiB4O7:Cu compositions have not been published up to now. The presented work is devoted to detailed EPR and optical investigations of the LTB:Cu and KLTB:Cu glasses and to determining the electron and local structure for paramagnetic and luminescence copper centers in their network on the basis of the obtained and referenced spectroscopic and structural data. 2. Glass synthesis, experimental equipment and characterization of samples LTB:Cu and KLTB:Cu glasses were obtained in the air from corresponding polycrystalline compounds according to a standard glass synthesis using the technological conditions developed by the authors. Carbonates (Li2CO3 and K2CO3) and boric acid (H3BO3) of high chemical purity (99.999%) in the corresponding proportion were used for a solid state synthesis of the Li2B4 O7 and KLiB4 O7 polycrystalline compounds. Cu impurity was added to the Li2B4O7 and KLiB4O7 compounds in the form of a CuO oxide of the chemical purity in the amounts of 0.4 and 1.6 mol.%. The LTB:Cu and KLTB:Cu glasses were obtained by fast cooling of the corresponding melts which were heated about 100 K higher than their melting temperature (Tmelt = 1190 and 1080 K, respectively) to exceed the glass transition points. The non-controlled and copper paramagnetic impurities in the obtained glasses were registered by the EPR technique using modernized commercial X-band spectrometers of the SE/X-2013 and SE/X-2544 types (“RADIOPAN”, Poznań, Poland), operating in the magnetic field high-frequency (100 kHz) modulation mode at room temperature. The microwave frequency was measured using a 5350 B Hewlett Packard microwave frequency counter and a DPPH g-marker (g = 2.0036 ± 0.0001). The parameters of the registered EPR spectra were obtained by a computer analysis of the experimental data. The optical absorption, luminescence excitation and emission spectra as well as luminescence kinetics were registered in the UV– VIS spectral range at room temperature. The optical absorption spectra were recorded with a Varian Model 5E UV–VIS–NIR spectrophotometer. The emission and luminescence excitation spectra were acquired with a Dongwoo (model DM711) scanning system consisting of an excitation monochromator with the 150 mm focal length and an emission monochromator with the focal length of 750 mm equipped with a photomultiplier and an InGaAs detector. The spectral response of the whole emission system was calibrated in the 400 ÷ 800 nm spectral region against the reference source. The resulting signal was analyzed by a Stanford (model SRS 250) boxcar integrator and was stored in a personal computer. The luminescence decay curves were recorded with a Tektronix (model TDS 3052) digital oscilloscope at T = 300 K. Excitation was provided by a Continuum Surelite I Optical Parametric Oscillator (OPO) pumped by a third harmonic of an Nd:YAG laser (λ = 355 nm) and the emitted light was filtered using a grating monochromator GDM with 1000 mm focal length. The obtained un-doped LTB and KLTB glasses are uncolored and characterized by a high transparency in the 330 ÷ 2500 nm spectral range. According to [8] un-doped LTB glasses are transparent in the
281 ÷ 2760 nm region, whereas nominally-pure LTB single crystals reveal high transparency in a very wide (167 ÷ 3200 nm) spectral range [11]. The obtained LTB:Cu and KLTB:Cu glasses are lightly blue (CuO—0.4 mol.%) and aquamarine (CuO—1.6 mol.%) in color and they are characterized by high optical quality. EPR and optical spectra characteristic for glassy (or vitreous) compounds were observed in all the LTB:Cu and KLTB:Cu samples. The spectra are presented in Figs. 1– 4 and discussed in Section 3. 3. Results and discussion 3.1. EPR spectroscopy of tetraborate glasses doped with Cu It should be noted that a characteristic EPR signal with geff =4.29± 0.01 was observed in all the LTB:Cu and KLTB:Cu glasses (Fig. 1, a). The same signal with less intensity was also observed in the un-doped and rare-earth doped LTB and KLTB glasses. In the investigated samples the integral intensity of the EPR signal with g≅4.29 is comparable with the Cu2+ signal intensity (Fig. 1, a). The first explanation of the signal at geff ≅4.29 in the glass network was proposed by Castner et al. [19] on the basis of a spin Hamiltonian in the form given by Bleaney and Stevens [20]: h i 2 2 2 Hˆ = β⋅ B⋅g⋅ Sˆ + D Sz −1 = 3SðS + 1Þ + E Sx −Sy
ð1Þ
where D and E are the axial and orthorhombic crystal field terms, respectively. At the present time it is generally acknowledged [21] that the signal with geff ≅ 4.29 originates from isolated 3d5-ions (Fe3+ and Mn2+) for a large second-order ligand field splitting in which the value of the ∣E/D∣ ratio lies in the vicinity of its maximum value of 1/3 (for fully rhombic symmetry ∣E/D∣ = 1/3). Several types of fully
Fig. 1. Complete (a) and central part (b) of X-band EPR spectra of LTB:Cu (a) and KLTB: Cu (b) glasses, containing 0.4 mol.% of Cu, recorded at T = 300 K.
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Fig. 2. Optical absorption spectrum of LTB:Cu glass, containing 0.4 mol.% of Cu, recorded at T = 300 K.
rhombic distortions of the sixfold (octahedral) and fourfold (tetrahedral) oxygen coordinated sites in glasses have been considered in [22]. According to [22] the EPR signal with geff ≅ 4.29 in LTB:Cu and KLTB:Cu glasses can be assigned to isolated Fe3+ (3d5, 6S5/2) ions in octahedrally- and (or) tetrahedrally-coordinated sites of a tetraborate glass network with a strong rhombic distortion. In our opinion the main source of the Fe3+ impurity in the LTB:Cu and KLTB:Cu glasses is the CuO doping compound which contains near 0.02 wt.% of iron according to the certificate. One can notice that the presence of the Fe3+ signal with g ≅ 4.29 clearly demonstrates a classical glass structure of the investigated compounds. The EPR spectra of the LTB:Cu and KLTB:Cu glasses are presented in Fig. 1. The observed EPR spectra are closely similar for glasses with the Li2B4O7:Cu and KLiB4O7:Cu compositions. Particularly, in both the tetraborate glasses the Cu EPR spectra consist of a pronounced band with a weakly-resolved four-component structure centered at g = 2.34 (Fig. 1, a) and a relatively narrow quadruplet centered at g = 2.06 (Fig. 1, b). Both the observed signals belong to paramagnetic Cu2+ (3d9, 2D5/2,) ions and are characteristic for disordered compounds and glasses, including borate glasses [16,18,21,23–26]. Four weakly-resolved components in the Cu2+ EPR spectra (Fig. 1) are related to a hyperfine structure caused by the 63Cu and 65Cu isotopes nuclei (natural abundance—69.1% of 63Cu and 30.9% of 63Cu, nuclear spin I = 3/2 for both isotopes). The EPR spectra parameters (g-values, constants of hyperfine interaction and peak-to-peak linewidth
Fig. 4. Luminescence spectra of LTB:Cu (a) and KLTB:Cu (b) glasses, containing 0.4 mol.% of Cu, recorded at λexc = 330 nm and T = 300 K.
values) for the Cu2+ centers in the LTB:Cu and KLTB:Cu glasses were measured at T = 300 K. The observed anisotropic EPR spectra of the Cu2+ centers in LTB:Cu and KLTB:Cu glasses can be satisfactory described by the spin Hamiltonian of axial symmetry, presented in the form: Ηˆ = gjj βBz Sˆz + g⊥ β Bx Sˆx + By Sˆy + Ajj Sˆz Iˆz + A⊥ Sˆx Iˆx + Sˆy Iˆy ð2Þ
Fig. 3. Luminescence excitation spectrum of LTB:Cu glass, containing 1.6 mol.% of Cu, recorded at λmon = 465 nm and T = 300 K.
with the following parameters at T = 300 K: g || = 2.34 ± 0.05, g⊥ = 2.06 ± 0.05, A|| = (14.28 ± 0.05) mT, A⊥ = (2.34 ± 0.05) mT (for LTB:Cu glass) and A|| = (14.21 ± 0.05) mT, A⊥ = (2.55 ± 0.05) mT (for KLTB:Cu glass). The peak-to-peak linewidth of hyperfine components for LTB:Cu and KLTB:Cu glasses at T = 300 K equals: ΔB||pp = (5.11 ± 0.05) mT, ΔB⊥ pp = (1.80 ± 0.05) mT. The obtained g|| and g⊥ values are characteristic for Cu2+ (3d9) Jahn–Teller ions coordinated by six O2− ligands that form an oxygen octahedron, elongated along the z-axis [27]. The ground state for the unpaired electron of the Cu2+ ion is 2B1g (dx2–y2 orbital), because g|| N g⊥ N ge = 2.0023 [24,25]. One can notice that the hyperfine components which belong to the 63Cu and 65Cu isotopes are not resolved in the observed EPR spectra (Fig. 1) because their nuclear magnetic moments are closely similar (the nuclear magnetic moment is 7.1% higher for 65Cu than that for 63Cu). Some differences in the hyperfine constants for the Cu2+ centers in the LTB and KLTB glass network can be related to slightly different local environments in their second (cationic) coordination shell.
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3.2. Optical spectra and luminescence kinetics of tetraborate glasses doped with Cu The optical spectra and luminescence kinetics of lithium and potassium–lithium tetraborate glasses doped with copper are presented, analyzed and discussed below. The optical absorption spectrum of the LTB:Cu glass, presented in Fig. 2, is typical for all the investigated tetraborate glasses doped with Cu. The characteristic broad absorption band peaked near 750 nm (Fig. 2) is assigned to the 2B1g → 2B2g transition of the Cu2+ centers in the tetraborate glass network. An intense absorption in the UV region (λ b 350 nm) belongs to the Cu2+ → O2−charge-transfer band (Fig. 2). These results show a good correlation with the Cu2+ optical absorption spectra obtained earlier in a number of oxide compounds, including borate crystals and glasses [14,25] and confirm the incorporation of Cu impurity in a divalent state into the network of lithium and potassium–lithium tetraborate glasses. Several broad bands in the 220 ÷ 380 nm spectral range, including bands around 230, 260, 330, and 370 nm are observed in the luminescence excitation spectra of both LTB:Cu and KLTB:Cu glasses (Fig. 3). Typical luminescence spectra of the LTB:Cu and KLTB:Cu glasses, obtained under excitation with λexc = 330 nm are presented in Fig. 4. The luminescence spectra of the investigated tetraborate glasses, doped with Cu are restricted to the 380 ÷ 600 nm spectral range and consist of two intense bands with the maxima close to 420 and 465 nm (Fig. 4). An emission band, peaked at 420 nm (Fig. 4, a) dominates in the LTB:Cu glass, whereas an emission band, peaked at 465 nm dominates in the KLTB:Cu glass (Fig. 4, b). It should be noted that with the increasing Cu content, the intensity of the emission band at 465 nm increases in comparison with that of the emission band at 420 nm. The two observed emission bands can be connected with different local environments for luminescence centers in the LTB:Cu and KLTB:Cu glasses. The luminescence kinetics shows that the emission with λem = 450 nm in the LTB:Cu glass (Cu content—0.4 mol.%) is characterized by single exponential decay with lifetime value τ = 27.7 μs at T = 300 K. Detailed investigations of the luminescence kinetics in LTB: Cu and KLTB:Cu glasses (Cu content—1.6 mol.%) show that the band at 420 nm is characterized by single exponential decay with lifetime τ = 23 μs (Fig. 5, a), whereas the band at 465 nm is characterized by single exponential decay with lifetime τ = 27 μs (Fig. 5, b) for both the investigated glasses. The luminescence excitation and emission spectra (Figs. 3 and 4) and luminescence kinetics (Fig. 5) are similar to the corresponding data for Cu+ centers in the LTB:Cu single crystals [6,7,14–17]. In particular, according to [6,7,14], the emission spectrum of the Cu+ centers in the LTB:Cu crystals, excited near 260 nm, consists of an intense single band centered near 365 ÷ 370 nm and characterized by lifetime values in the 24 ÷ 29 μs range, independent of the temperature in the 0 ÷ 150 °C region. The emission of LTB:Cu crystals in the 330 ÷ 430 nm range is caused by parity- and spin-forbidden 3d94s → 3d10 transition from triplet to singlet states of Cu+ ions and can be excited between 220 and 300 nm that correspond to the 3d10 → 3d94s absorption transition [28,29]. The emission spectra of LTB:Cu glasses consist of a broad complicated band ranging from 350 to 650 nm with extended maxima in the 380 ÷ 480 nm spectral range and are characterized by exponential decay with lifetimes 21 ÷ 24 μs [14–16]. According to [8,14–17] the broad emission band in LTB:Cu glasses is related to the 3d94s → 3d10 metal-centered triplet–singlet transition of numerous types of Cu+ centers with slightly different local environments in the glass network. Based on the obtained and referenced [6–8,14–17] spectroscopic data for LTB:Cu crystals and glasses the emission bands with the maxima at 420 and 465 nm in our LTB:Cu and KLTB:Cu glasses (Fig. 4) can be assigned to two types of Cu+ centers with different local environments. In comparison with the Cu+ emission spectra in LTB:Cu glasses, which have been presented in [8,14–17], the emission spectra of our LTB:Cu and KLTB:Cu glasses are
Fig. 5. Luminescence decay of LTB:Cu (a) and KLTB:Cu (b) glasses, containing 1.6 mol.% of Cu, recorded at T = 300 K for emission bands with maxima at λem = 420 nm (a) and λem = 465 nm (b). Black solid lines—results of single exponential fitting.
more similar to the Cu+ emission spectra, which were observed in LTB: Cu single crystals by different authors [6,7,14–17]. A significant red shift and inhomogeneous broadening of the Cu+ emission bands in our LTB:Cu and KLTB:Cu glasses in comparison with LTB:Cu single crystals are characteristic for glassy compounds and have been also observed for Cu+ centers in the SrB4O7:Cu glass [30]. In [31] it has been shown using the EXAFS technique that the local environments (first coordination shell) for cations in oxide glasses and crystals with the same chemical composition are closely similar. Therefore, the local structure of Cu2+ and Cu+ centers in the LTB and KLTB glasses using the local structure of corresponding crystals, in particular the Li2B4O7 crystal, are discussed below. The EPR and optical investigations show the presence of Cu2+ paramagnetic and Cu+ luminescence centers simultaneously in the tetraborate glass network. On the basis of the obtained EPR spectroscopy results and the structural data for the Li2B4O7 crystal [32] it can be supposed that the Cu2+ and Cu+ centers are incorporated into the Li (Li/K) sites of the LTB (KLTB) glass network. The charge compensation mechanism for the Cu2+ → Li+ (K+) heterovalence substitutions can be related to Li and K vacancies (VLi and VK). According to [32], the B3+ cations in the Li2B4O7 crystal lattice occupy threefold-coordinated (BO3 unit) and fourfold-coordinated (BO4 unit) sites with average B3+–O2– bonds equal to 1.373 and 1.477 Å, respectively, whereas the Li+ ions are located in fourfold-coordinated distorted tetrahedra with Li+–O2– distances in the 1.97 ÷ 2.14 Å range. Hence, the coordination number to oxygen (N) for Li cations in the LTB crystal depends on the Li–O interatomic distances (dLi–O) and results in N = 4 for dLi–O ≤ 2.14 Å and N = 7 for dLi–O ≤ 2.88 Å [32]. For the Li–O distances which are equal to 2.63, 2.85, and 2.88 Å, the coordination numbers to oxygen are equal to 5, 6, and 7, respectively [32]. The multisite nature of the Cu+ luminescence in the LTB glass and crystal can be related to statistically-
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distributed Cu–O distances (so-called “positional disorder”) that lead to a distribution of the Cu+ spectroscopic parameters and are revealed as inhomogeneous broadening of spectral lines. Additionally a glass network is characterized by continual disturbance of the short-range order that destroys the middle- and long-range order. This glassy-like disorder in the second (cationic) coordination sphere around the Cu+ and Cu2+ centers leads to additional inhomogeneous broadening of the spectral lines. The above presented discussion also allows explaining two emission bands of the Cu+ luminescence centers with different maxima and lifetime values that are observed in the LTB:Cu and KLTB:Cu glasses. In accordance with the Li2B4O7 crystal structure the extreme values of the coordination number for the Cu+ centers in the tetraborate glass network can be 4 and 6 as the real glass structure contains a large number of oxygen vacancies. Therefore, in our opinion two distinct emission bands in the LTB:Cu and KLTB glasses can be related to the isolated Cu+ centers in the sites with the extreme values of the coordination number to oxygen N = 4 (tetrahedral coordination) and N = 6 (octahedral coordination) or isolated Cu+ centers with different distortion of the local environment by threefold- and fourfoldcoordinated B3+ cations of the second coordination shell. The local structure of different types of the Cu impurity in tetraborate glasses needs more detailed investigation of the extended X-ray absorption fine structure (EXAFS) for the Cu K-edge. 4. Conclusions LTB:Cu and KLTB:Cu glasses of high optical quality were obtained from corresponding polycrystalline compounds by a standard glass synthesis using the technological conditions developed by the authors. The EPR, optical absorption, luminescence excitation and emission spectra as well as luminescence kinetics of the LTB:Cu and KLTB:Cu glasses were investigated. On the basis of an analysis of the obtained and referenced data the following was shown: 1) The Cu impurity was incorporated into the LTB and KLTB glass network as the Cu2+ (3d9) and Cu+ (3d10) ions. The EPR and optical spectra of the Cu2+ and Cu+ centers were weaklydependent on the basic chemical composition of the tetraborate glass matrix. 2) The EPR spectra of the Cu2+ centers in the LTB:Cu and KLTB:Cu glasses were characterized by the following parameters obtained at T = 300 K: g|| = 2.34 ± 0.05, g⊥ = 2.06 ± 0.05, ΔB||pp = (5.11 ± 0.05) mT, ΔB⊥ pp = (1.80 ± 0.05) mT and A|| = (14.28 ± 0.05) mT, A⊥ = (2.34 ± 0.05) mT (for LTB:Cu glass) and A|| = (14.21 ± 0.05) mT, and A⊥ = (2.55 ± 0.05) mT (for KLTB:Cu glass). 3) The broad optical absorption band peaked around 750 nm in the tetraborate glasses, doped with Cu was assigned to the 2B1g → 2B2g transition of the Cu2+ centers. An intense absorption in the UV region (λ b 350 nm) was related to the Cu2+ → O2−charge-transfer band. 4) Two intense broad emission bands with the maxima at 420 and 465 nm were observed in the luminescence spectra of the LTB:Cu and KLTB:Cu glasses. They were characterized by single exponential decay with lifetime values τ = 23 and 27 μs at T = 300 K, respectively. Both the observed emission bands were related to the parity- and spin-forbidden 3d94s → 3d10 triplet–singlet transition of the Cu+ ions. It was supposed that two distinct emission bands in the LTB:Cu and KLTB:Cu glasses belonged to the Cu+ centers in
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the tetrahedral and octahedral sites or related to the Cu+ centers with different distortion of their local environment by B3+ cations. 5) The Cu2+ paramagnetic and Cu+ luminescence centers were localized in the Li (Li/K) sites of the LTB (KLTB) glass network with different coordination numbers to O2− anions (N = 4 ÷ 7) which depend on the Li–O (K–O) distances (positional disorder in the first coordination shell). This led to statistical distribution of the Cu–O distances and spectroscopic parameters for the Cu2+ and Cu+ centers and was revealed in the inhomogeneous broadening of their spectral lines. The charge compensation mechanism for Cu2+ → Li+ (K+) heterovalence substitutions could be related to cationic (VLi and VK) vacancies in the glass network. Acknowledgements This work is supported by the Ministry of Education and Sciences of Ukraine (Research Project No. 0109U001063) and the University of Zielona Góra (Poland). References [1] T. Sasaki, Y. Mori, M. Yoshimura, Y.K. Yap, T. Kamimura, Mater. Sci. Eng., R 30 (2000) 1. [2] M. Ghotbi, M. Ebrahim-Zadeh, Optics Express 12 (2004) 6002. [3] M. Santiago, M. Lester, E. Caselli, A. Lavat, A. Ges, F. Spano, C. Kessler, J. Mater. Sci. Lett. 17 (1998) 1293. [4] M. Prokic, Radiat. Prot. Dosim. 100 (2002) 265. [5] N. Can, T. Karali, P.D. Townsend, F. Vildiz, J. Phys. D: Appl. Phys. 39 (2006) 2038. [6] N. Senguttuvan, M. Ishii, M. Shimoyama, M. Kobayashi, N. Tsutsui, M. Nikl, M. Dusek, H.M. Shimizu, T. Oku, T. Adachi, K. Sakai, J. Suzuki, Nucl. Instr. Meth. Phys. Research A 486 (2002) 264. [7] M. Ishii, Y. Kuwano, S. Asaba, T. Asai, M. Kawamura, N. Senguttuvan, T. Hayashi, M. Koboyashi, M. Nikl, S. Hosoya, K. Sakai, T. Adachi, T. Oku, H.M. Shimizu, Radiat. Measur. 38 (2004) 571. [8] B.I. Zadneprowski, N.E. Eremin, A.A. Paskhalov, Funct. Mater. 12 (2005) 261. [9] Ya.V. Burak, B.V. Padlyak, V.M. Shevel, Nucl. Instr. Meth. Phys. Research B 191 (2002) 633. [10] Ya.V. Burak, B.V. Padlyak, V.M. Shevel, Radiat. Eff. Defects Solids 157 (2002) 1101. [11] D. Podgórska, S.M. Kaczmarek, W. Drozdowski, M. Berkowski, A. Worsztynowicz, Acta Phys. Polon. A 107 (2005) 507. [12] B.V. Padlyak, A. Gutsze, Appl. Magn. Reson. 14 (1998) 59. [13] B.V. Padlyak, Radiat. Eff. Defects Solids 158 (2003) 411. [14] M. Ignatovych, V. Holovey, A. Watterich, T. Vidóczy, P. Baranyai, A. Kelemen, O. Chuiko, Radiat. Measur. 38 (2004) 567–570. [15] M. Ignatovych, V. Holovey, A. Watterich, T. Vidóczy, P. Baranyai, A. Kelemen, V. Ogenko, O. Chuiko, Radiat. Phys. Chem. 67 (2003) 587. [16] M. Ignatovych, V. Holovey, T. Vidóczy, P. Baranyai, A. Keleman, V. Laguta, O. Chujko, Func. Mater. 12 (2005) 313. [17] B.T. Huy, V.X. Quang, H.T.B. Chau, J. Lumin. 128 (2008) 1601. [18] B.V. Padlyak, W. Wojtowicz, V.T. Adamiv, Ya.V. Burak, I.M. Teslyuk, in: Y. Zhydachevskii (Ed.), Book of Abstracts: “International Scientific Workshop Oxide Materials for Electronic Engineering—Fabrication, Properties and Application (ОМЕE-2009)”, June 22–26, 2009, Lviv, Ukraine, Lviv Polytechnic National University, Lviv, 2009, p. 90. [19] T. Castner Jr., G.S. Newell, W.C. Holton, C.P. Slichter, J. Chem. Phys. 32 (1960) 668. [20] B. Bleaney, K.W.H. Stevens, Rep. Prog. Phys. 16 (1953) 108. [21] D.L. Griscom, J. Non-Cryst. Solids 40 (1980) 211. [22] C.M. Brodbeck, R.R. Bukrey, Phys. Rev. B 25 (1981) 2334. [23] J.M. Dance, J.J. Videau, J. Portier, J. Non-Cryst. Solids 86 (1986) 88. [24] B. Sreedhar, Ch. Sumalatha, Kazuo Kojima, J. Non-Cryst. Solids 192 &193 (1995) 203. [25] B. Karthikeyan, S. Mohan, Mater. Lett. 57 (2003) 3789. [26] H. Smigielska, G. Lewandowicz, J. Goslar, S.K. Hoffmann, Acta Phys. Polon. A 108 (2005) 303. [27] H. Imagawa, Phys. Status Solidi (b) 30 (1968) 469. [28] C. Pedrini, Phys. Stat. Solidi B 87 (1978) 273. [29] G. Blasse, B.C. Grabmajerm, Luminescent Materials, Springer, Berlin, 1994. [30] J.W.M. Verwey, J.M. Coronado, G. Blasse, J. Solid State Chem. 92 (1991) 531. [31] A. Witkowska, B. Padlyak, J. Rybicki, Opt. Mater. 30 (2008) 699. [32] J. Krogh-Moe, Acta Cryst. B 24 (1968) 179.
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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Synthesis and spectroscopy of tetraborate glasses doped with copper B. Padlyak a,b,⁎, W. Ryba-Romanowski c, R. Lisiecki c, O. Smyrnov b, A. Drzewiecki b, Ya. Burak a, V. Adamiv a, I. Teslyuk a a b c
Institute of Physical Optics, Sector of Spectroscopy, 23 Dragomanov Str., 79-005 Lviv, Ukraine University of Zielona Góra, Institute of Physics, Division of Spectroscopy of Functional Materials, 4a Szafrana Str., 65-516 Zielona Góra, Poland Institute of Low Temperatures and Structure Research, Polish Academy of Sciences, 2 Okólna Str., 50-422 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 12 May 2010 Available online 9 June 2010 Keywords: Borate glass; Cu2+ center; Cu+ center; EPR; Optical absorption; Luminescence; Decay kinetics
a b s t r a c t Lithium and potassium–lithium tetraborate glasses doped with Cu (Li2B4O7:Cu and KLiB4O7:Cu) of high optical quality were obtained from polycrystalline compounds by fast cooling of the corresponding melt. Cu impurity was added to the Li2B4O7 and KLiB4O7 compounds in the form of a CuO oxide in the amounts of 0.4 and 1.6 mol.%. On the basis of EPR and optical spectroscopy (absorption, emission and luminescence excitation and kinetics) data analysis it was shown that the Cu impurity was incorporated into the tetraborate glass network as Cu2+ (3d9) and Cu+ (3d10) ions. The EPR spectra of Cu2+ centers were almost identical in glasses with the Li2B4O7:Cu and KLiB4O7:Cu compositions and were characteristic for glassy compounds. The Cu2+ EPR spectra parameters (g-values, hyperfine constants and peak-to-peak linewidths) in the Li2B4O7:Cu and KLiB4O7:Cu glasses were obtained at T = 300 K. The characteristic broad absorption band peaked near 750 nm was assigned to the 2B1g → 2B2g transition of the Cu2+ centers. An intense absorption in the UV region (λ b 350 nm) was related to the Cu2+ → O2− charge-transfer band. Broad emission bands with the maxima near 420 and 465 nm were observed in the luminescence spectra of the Li2B4O7:Cu and KLiB4O7:Cu glasses. The emission bands, peaked near 420 and 465 nm, were characterized by single exponential decay with lifetimes τ = 23 and 27 μs, respectively for the Li2B4O7:Cu and KLiB4O7:Cu glasses (Cu content—1.6 mol.%) at T = 300 K. Both emission bands belonged to the Cu+ centers with different local environments or their distortion. A possible local structure for two types of Cu+ centers in a tetraborate glass network is discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Borate compounds (crystals and glasses), un-doped and doped with transition and rare-earth elements are very promising for quantum electronics and non-linear optics [1,2] scintillators and tissue-equivalent materials for thermoluminescent (TL) dosimetry [3–5], as well as γ and neutron detectors [6–8]. This also concerns lithium tetraborate crystals which are characterized by extremely high radiation stability [9,10], high transparency in a wide spectral range from VUV to far IR [11] and good TL properties [4–8]. It is technologically difficult, takes a long-time, and as consequence, it is very expensive to obtain tetraborate single crystals with different compositions by the Czochralski method. In addition to that, a very low velocity of the crystals growth and high viscosity of the melt lead to problems with the doping of tetraborate crystals. Thus, from the technological point of view the glassy (or vitreous) borate compounds are most prospective in comparison with the corresponding single crystals. On the other hand, the study of the ⁎ Corresponding author. Institute of Physical Optics, 23 Dragomanov Str., 79-005 Lviv, Ukraine. Fax: +48 068 328 2920. E-mail address: [email protected] (B. Padlyak). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.027
electron and local structure of impurity and other point defects in complex oxide glasses, particularly in glasses with Li2B4O7 (LTB) and KLiB4O7 (KLTB) basic compositions, poses an interesting scientific problem. The electron paramagnetic resonance (EPR) and optical spectroscopy allow investigating the electron and local structure of point defects in crystals and corresponding glasses. Structural and corresponding spectroscopic data for crystalline analogies are needed for an interpretation of the EPR and optical spectra in complex oxide glasses [12,13]. Tetraborate compounds are good candidates to study the nature of point defects as boron-containing compounds, including tetraborates, can be obtained in both crystalline and glassy states. The optical spectra of lithium tetraborate crystals and glasses doped with Cu (LTB:Cu) have been investigated by different authors [6–8,14,15]. Particularly in [14] it has been shown by optical spectroscopy that the multivalent states of impurity transition ions for non-irradiated LTB:Cu crystals and glasses, “as-grown” in the air, obtained from melted crystals are characteristic and the copper impurity is revealed as Cu+ and Cu2+ ions. At the present time, the emission spectra and luminescence kinetics of Cu+ centers have been investigated in LTB:Cu single crystals [6,7] and glasses [8,14–17],
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however, the peculiarities of the Cu+ luminescence properties of LTB: Cu glasses have not been clearly explained to this day. The EPR spectra of the Cu2+ paramagnetic ions in the LTB:Cu crystal and glass at different temperatures, typical for crystalline and glassy compounds, have been presented in [16], however, their spin Hamiltonian parameters have not been determined. In [18] we have reported for the first time the parameters of the Cu2+ EPR spectra in Li2B4O7:Cu and KLiB4O7:Cu glasses. On the basis of the referenced data analysis it can be concluded that at the present time the EPR and optical spectra of LTB:Cu glasses doped with Cu have been studied insufficiently. The results of optical and EPR spectroscopy for new crystal and glass with KLiB4O7:Cu compositions have not been published up to now. The presented work is devoted to detailed EPR and optical investigations of the LTB:Cu and KLTB:Cu glasses and to determining the electron and local structure for paramagnetic and luminescence copper centers in their network on the basis of the obtained and referenced spectroscopic and structural data. 2. Glass synthesis, experimental equipment and characterization of samples LTB:Cu and KLTB:Cu glasses were obtained in the air from corresponding polycrystalline compounds according to a standard glass synthesis using the technological conditions developed by the authors. Carbonates (Li2CO3 and K2CO3) and boric acid (H3BO3) of high chemical purity (99.999%) in the corresponding proportion were used for a solid state synthesis of the Li2B4 O7 and KLiB4 O7 polycrystalline compounds. Cu impurity was added to the Li2B4O7 and KLiB4O7 compounds in the form of a CuO oxide of the chemical purity in the amounts of 0.4 and 1.6 mol.%. The LTB:Cu and KLTB:Cu glasses were obtained by fast cooling of the corresponding melts which were heated about 100 K higher than their melting temperature (Tmelt = 1190 and 1080 K, respectively) to exceed the glass transition points. The non-controlled and copper paramagnetic impurities in the obtained glasses were registered by the EPR technique using modernized commercial X-band spectrometers of the SE/X-2013 and SE/X-2544 types (“RADIOPAN”, Poznań, Poland), operating in the magnetic field high-frequency (100 kHz) modulation mode at room temperature. The microwave frequency was measured using a 5350 B Hewlett Packard microwave frequency counter and a DPPH g-marker (g = 2.0036 ± 0.0001). The parameters of the registered EPR spectra were obtained by a computer analysis of the experimental data. The optical absorption, luminescence excitation and emission spectra as well as luminescence kinetics were registered in the UV– VIS spectral range at room temperature. The optical absorption spectra were recorded with a Varian Model 5E UV–VIS–NIR spectrophotometer. The emission and luminescence excitation spectra were acquired with a Dongwoo (model DM711) scanning system consisting of an excitation monochromator with the 150 mm focal length and an emission monochromator with the focal length of 750 mm equipped with a photomultiplier and an InGaAs detector. The spectral response of the whole emission system was calibrated in the 400 ÷ 800 nm spectral region against the reference source. The resulting signal was analyzed by a Stanford (model SRS 250) boxcar integrator and was stored in a personal computer. The luminescence decay curves were recorded with a Tektronix (model TDS 3052) digital oscilloscope at T = 300 K. Excitation was provided by a Continuum Surelite I Optical Parametric Oscillator (OPO) pumped by a third harmonic of an Nd:YAG laser (λ = 355 nm) and the emitted light was filtered using a grating monochromator GDM with 1000 mm focal length. The obtained un-doped LTB and KLTB glasses are uncolored and characterized by a high transparency in the 330 ÷ 2500 nm spectral range. According to [8] un-doped LTB glasses are transparent in the
281 ÷ 2760 nm region, whereas nominally-pure LTB single crystals reveal high transparency in a very wide (167 ÷ 3200 nm) spectral range [11]. The obtained LTB:Cu and KLTB:Cu glasses are lightly blue (CuO—0.4 mol.%) and aquamarine (CuO—1.6 mol.%) in color and they are characterized by high optical quality. EPR and optical spectra characteristic for glassy (or vitreous) compounds were observed in all the LTB:Cu and KLTB:Cu samples. The spectra are presented in Figs. 1– 4 and discussed in Section 3. 3. Results and discussion 3.1. EPR spectroscopy of tetraborate glasses doped with Cu It should be noted that a characteristic EPR signal with geff =4.29± 0.01 was observed in all the LTB:Cu and KLTB:Cu glasses (Fig. 1, a). The same signal with less intensity was also observed in the un-doped and rare-earth doped LTB and KLTB glasses. In the investigated samples the integral intensity of the EPR signal with g≅4.29 is comparable with the Cu2+ signal intensity (Fig. 1, a). The first explanation of the signal at geff ≅4.29 in the glass network was proposed by Castner et al. [19] on the basis of a spin Hamiltonian in the form given by Bleaney and Stevens [20]: h i 2 2 2 Hˆ = β⋅ B⋅g⋅ Sˆ + D Sz −1 = 3SðS + 1Þ + E Sx −Sy
ð1Þ
where D and E are the axial and orthorhombic crystal field terms, respectively. At the present time it is generally acknowledged [21] that the signal with geff ≅ 4.29 originates from isolated 3d5-ions (Fe3+ and Mn2+) for a large second-order ligand field splitting in which the value of the ∣E/D∣ ratio lies in the vicinity of its maximum value of 1/3 (for fully rhombic symmetry ∣E/D∣ = 1/3). Several types of fully
Fig. 1. Complete (a) and central part (b) of X-band EPR spectra of LTB:Cu (a) and KLTB: Cu (b) glasses, containing 0.4 mol.% of Cu, recorded at T = 300 K.
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Fig. 2. Optical absorption spectrum of LTB:Cu glass, containing 0.4 mol.% of Cu, recorded at T = 300 K.
rhombic distortions of the sixfold (octahedral) and fourfold (tetrahedral) oxygen coordinated sites in glasses have been considered in [22]. According to [22] the EPR signal with geff ≅ 4.29 in LTB:Cu and KLTB:Cu glasses can be assigned to isolated Fe3+ (3d5, 6S5/2) ions in octahedrally- and (or) tetrahedrally-coordinated sites of a tetraborate glass network with a strong rhombic distortion. In our opinion the main source of the Fe3+ impurity in the LTB:Cu and KLTB:Cu glasses is the CuO doping compound which contains near 0.02 wt.% of iron according to the certificate. One can notice that the presence of the Fe3+ signal with g ≅ 4.29 clearly demonstrates a classical glass structure of the investigated compounds. The EPR spectra of the LTB:Cu and KLTB:Cu glasses are presented in Fig. 1. The observed EPR spectra are closely similar for glasses with the Li2B4O7:Cu and KLiB4O7:Cu compositions. Particularly, in both the tetraborate glasses the Cu EPR spectra consist of a pronounced band with a weakly-resolved four-component structure centered at g = 2.34 (Fig. 1, a) and a relatively narrow quadruplet centered at g = 2.06 (Fig. 1, b). Both the observed signals belong to paramagnetic Cu2+ (3d9, 2D5/2,) ions and are characteristic for disordered compounds and glasses, including borate glasses [16,18,21,23–26]. Four weakly-resolved components in the Cu2+ EPR spectra (Fig. 1) are related to a hyperfine structure caused by the 63Cu and 65Cu isotopes nuclei (natural abundance—69.1% of 63Cu and 30.9% of 63Cu, nuclear spin I = 3/2 for both isotopes). The EPR spectra parameters (g-values, constants of hyperfine interaction and peak-to-peak linewidth
Fig. 4. Luminescence spectra of LTB:Cu (a) and KLTB:Cu (b) glasses, containing 0.4 mol.% of Cu, recorded at λexc = 330 nm and T = 300 K.
values) for the Cu2+ centers in the LTB:Cu and KLTB:Cu glasses were measured at T = 300 K. The observed anisotropic EPR spectra of the Cu2+ centers in LTB:Cu and KLTB:Cu glasses can be satisfactory described by the spin Hamiltonian of axial symmetry, presented in the form: Ηˆ = gjj βBz Sˆz + g⊥ β Bx Sˆx + By Sˆy + Ajj Sˆz Iˆz + A⊥ Sˆx Iˆx + Sˆy Iˆy ð2Þ
Fig. 3. Luminescence excitation spectrum of LTB:Cu glass, containing 1.6 mol.% of Cu, recorded at λmon = 465 nm and T = 300 K.
with the following parameters at T = 300 K: g || = 2.34 ± 0.05, g⊥ = 2.06 ± 0.05, A|| = (14.28 ± 0.05) mT, A⊥ = (2.34 ± 0.05) mT (for LTB:Cu glass) and A|| = (14.21 ± 0.05) mT, A⊥ = (2.55 ± 0.05) mT (for KLTB:Cu glass). The peak-to-peak linewidth of hyperfine components for LTB:Cu and KLTB:Cu glasses at T = 300 K equals: ΔB||pp = (5.11 ± 0.05) mT, ΔB⊥ pp = (1.80 ± 0.05) mT. The obtained g|| and g⊥ values are characteristic for Cu2+ (3d9) Jahn–Teller ions coordinated by six O2− ligands that form an oxygen octahedron, elongated along the z-axis [27]. The ground state for the unpaired electron of the Cu2+ ion is 2B1g (dx2–y2 orbital), because g|| N g⊥ N ge = 2.0023 [24,25]. One can notice that the hyperfine components which belong to the 63Cu and 65Cu isotopes are not resolved in the observed EPR spectra (Fig. 1) because their nuclear magnetic moments are closely similar (the nuclear magnetic moment is 7.1% higher for 65Cu than that for 63Cu). Some differences in the hyperfine constants for the Cu2+ centers in the LTB and KLTB glass network can be related to slightly different local environments in their second (cationic) coordination shell.
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3.2. Optical spectra and luminescence kinetics of tetraborate glasses doped with Cu The optical spectra and luminescence kinetics of lithium and potassium–lithium tetraborate glasses doped with copper are presented, analyzed and discussed below. The optical absorption spectrum of the LTB:Cu glass, presented in Fig. 2, is typical for all the investigated tetraborate glasses doped with Cu. The characteristic broad absorption band peaked near 750 nm (Fig. 2) is assigned to the 2B1g → 2B2g transition of the Cu2+ centers in the tetraborate glass network. An intense absorption in the UV region (λ b 350 nm) belongs to the Cu2+ → O2−charge-transfer band (Fig. 2). These results show a good correlation with the Cu2+ optical absorption spectra obtained earlier in a number of oxide compounds, including borate crystals and glasses [14,25] and confirm the incorporation of Cu impurity in a divalent state into the network of lithium and potassium–lithium tetraborate glasses. Several broad bands in the 220 ÷ 380 nm spectral range, including bands around 230, 260, 330, and 370 nm are observed in the luminescence excitation spectra of both LTB:Cu and KLTB:Cu glasses (Fig. 3). Typical luminescence spectra of the LTB:Cu and KLTB:Cu glasses, obtained under excitation with λexc = 330 nm are presented in Fig. 4. The luminescence spectra of the investigated tetraborate glasses, doped with Cu are restricted to the 380 ÷ 600 nm spectral range and consist of two intense bands with the maxima close to 420 and 465 nm (Fig. 4). An emission band, peaked at 420 nm (Fig. 4, a) dominates in the LTB:Cu glass, whereas an emission band, peaked at 465 nm dominates in the KLTB:Cu glass (Fig. 4, b). It should be noted that with the increasing Cu content, the intensity of the emission band at 465 nm increases in comparison with that of the emission band at 420 nm. The two observed emission bands can be connected with different local environments for luminescence centers in the LTB:Cu and KLTB:Cu glasses. The luminescence kinetics shows that the emission with λem = 450 nm in the LTB:Cu glass (Cu content—0.4 mol.%) is characterized by single exponential decay with lifetime value τ = 27.7 μs at T = 300 K. Detailed investigations of the luminescence kinetics in LTB: Cu and KLTB:Cu glasses (Cu content—1.6 mol.%) show that the band at 420 nm is characterized by single exponential decay with lifetime τ = 23 μs (Fig. 5, a), whereas the band at 465 nm is characterized by single exponential decay with lifetime τ = 27 μs (Fig. 5, b) for both the investigated glasses. The luminescence excitation and emission spectra (Figs. 3 and 4) and luminescence kinetics (Fig. 5) are similar to the corresponding data for Cu+ centers in the LTB:Cu single crystals [6,7,14–17]. In particular, according to [6,7,14], the emission spectrum of the Cu+ centers in the LTB:Cu crystals, excited near 260 nm, consists of an intense single band centered near 365 ÷ 370 nm and characterized by lifetime values in the 24 ÷ 29 μs range, independent of the temperature in the 0 ÷ 150 °C region. The emission of LTB:Cu crystals in the 330 ÷ 430 nm range is caused by parity- and spin-forbidden 3d94s → 3d10 transition from triplet to singlet states of Cu+ ions and can be excited between 220 and 300 nm that correspond to the 3d10 → 3d94s absorption transition [28,29]. The emission spectra of LTB:Cu glasses consist of a broad complicated band ranging from 350 to 650 nm with extended maxima in the 380 ÷ 480 nm spectral range and are characterized by exponential decay with lifetimes 21 ÷ 24 μs [14–16]. According to [8,14–17] the broad emission band in LTB:Cu glasses is related to the 3d94s → 3d10 metal-centered triplet–singlet transition of numerous types of Cu+ centers with slightly different local environments in the glass network. Based on the obtained and referenced [6–8,14–17] spectroscopic data for LTB:Cu crystals and glasses the emission bands with the maxima at 420 and 465 nm in our LTB:Cu and KLTB:Cu glasses (Fig. 4) can be assigned to two types of Cu+ centers with different local environments. In comparison with the Cu+ emission spectra in LTB:Cu glasses, which have been presented in [8,14–17], the emission spectra of our LTB:Cu and KLTB:Cu glasses are
Fig. 5. Luminescence decay of LTB:Cu (a) and KLTB:Cu (b) glasses, containing 1.6 mol.% of Cu, recorded at T = 300 K for emission bands with maxima at λem = 420 nm (a) and λem = 465 nm (b). Black solid lines—results of single exponential fitting.
more similar to the Cu+ emission spectra, which were observed in LTB: Cu single crystals by different authors [6,7,14–17]. A significant red shift and inhomogeneous broadening of the Cu+ emission bands in our LTB:Cu and KLTB:Cu glasses in comparison with LTB:Cu single crystals are characteristic for glassy compounds and have been also observed for Cu+ centers in the SrB4O7:Cu glass [30]. In [31] it has been shown using the EXAFS technique that the local environments (first coordination shell) for cations in oxide glasses and crystals with the same chemical composition are closely similar. Therefore, the local structure of Cu2+ and Cu+ centers in the LTB and KLTB glasses using the local structure of corresponding crystals, in particular the Li2B4O7 crystal, are discussed below. The EPR and optical investigations show the presence of Cu2+ paramagnetic and Cu+ luminescence centers simultaneously in the tetraborate glass network. On the basis of the obtained EPR spectroscopy results and the structural data for the Li2B4O7 crystal [32] it can be supposed that the Cu2+ and Cu+ centers are incorporated into the Li (Li/K) sites of the LTB (KLTB) glass network. The charge compensation mechanism for the Cu2+ → Li+ (K+) heterovalence substitutions can be related to Li and K vacancies (VLi and VK). According to [32], the B3+ cations in the Li2B4O7 crystal lattice occupy threefold-coordinated (BO3 unit) and fourfold-coordinated (BO4 unit) sites with average B3+–O2– bonds equal to 1.373 and 1.477 Å, respectively, whereas the Li+ ions are located in fourfold-coordinated distorted tetrahedra with Li+–O2– distances in the 1.97 ÷ 2.14 Å range. Hence, the coordination number to oxygen (N) for Li cations in the LTB crystal depends on the Li–O interatomic distances (dLi–O) and results in N = 4 for dLi–O ≤ 2.14 Å and N = 7 for dLi–O ≤ 2.88 Å [32]. For the Li–O distances which are equal to 2.63, 2.85, and 2.88 Å, the coordination numbers to oxygen are equal to 5, 6, and 7, respectively [32]. The multisite nature of the Cu+ luminescence in the LTB glass and crystal can be related to statistically-
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distributed Cu–O distances (so-called “positional disorder”) that lead to a distribution of the Cu+ spectroscopic parameters and are revealed as inhomogeneous broadening of spectral lines. Additionally a glass network is characterized by continual disturbance of the short-range order that destroys the middle- and long-range order. This glassy-like disorder in the second (cationic) coordination sphere around the Cu+ and Cu2+ centers leads to additional inhomogeneous broadening of the spectral lines. The above presented discussion also allows explaining two emission bands of the Cu+ luminescence centers with different maxima and lifetime values that are observed in the LTB:Cu and KLTB:Cu glasses. In accordance with the Li2B4O7 crystal structure the extreme values of the coordination number for the Cu+ centers in the tetraborate glass network can be 4 and 6 as the real glass structure contains a large number of oxygen vacancies. Therefore, in our opinion two distinct emission bands in the LTB:Cu and KLTB glasses can be related to the isolated Cu+ centers in the sites with the extreme values of the coordination number to oxygen N = 4 (tetrahedral coordination) and N = 6 (octahedral coordination) or isolated Cu+ centers with different distortion of the local environment by threefold- and fourfoldcoordinated B3+ cations of the second coordination shell. The local structure of different types of the Cu impurity in tetraborate glasses needs more detailed investigation of the extended X-ray absorption fine structure (EXAFS) for the Cu K-edge. 4. Conclusions LTB:Cu and KLTB:Cu glasses of high optical quality were obtained from corresponding polycrystalline compounds by a standard glass synthesis using the technological conditions developed by the authors. The EPR, optical absorption, luminescence excitation and emission spectra as well as luminescence kinetics of the LTB:Cu and KLTB:Cu glasses were investigated. On the basis of an analysis of the obtained and referenced data the following was shown: 1) The Cu impurity was incorporated into the LTB and KLTB glass network as the Cu2+ (3d9) and Cu+ (3d10) ions. The EPR and optical spectra of the Cu2+ and Cu+ centers were weaklydependent on the basic chemical composition of the tetraborate glass matrix. 2) The EPR spectra of the Cu2+ centers in the LTB:Cu and KLTB:Cu glasses were characterized by the following parameters obtained at T = 300 K: g|| = 2.34 ± 0.05, g⊥ = 2.06 ± 0.05, ΔB||pp = (5.11 ± 0.05) mT, ΔB⊥ pp = (1.80 ± 0.05) mT and A|| = (14.28 ± 0.05) mT, A⊥ = (2.34 ± 0.05) mT (for LTB:Cu glass) and A|| = (14.21 ± 0.05) mT, and A⊥ = (2.55 ± 0.05) mT (for KLTB:Cu glass). 3) The broad optical absorption band peaked around 750 nm in the tetraborate glasses, doped with Cu was assigned to the 2B1g → 2B2g transition of the Cu2+ centers. An intense absorption in the UV region (λ b 350 nm) was related to the Cu2+ → O2−charge-transfer band. 4) Two intense broad emission bands with the maxima at 420 and 465 nm were observed in the luminescence spectra of the LTB:Cu and KLTB:Cu glasses. They were characterized by single exponential decay with lifetime values τ = 23 and 27 μs at T = 300 K, respectively. Both the observed emission bands were related to the parity- and spin-forbidden 3d94s → 3d10 triplet–singlet transition of the Cu+ ions. It was supposed that two distinct emission bands in the LTB:Cu and KLTB:Cu glasses belonged to the Cu+ centers in
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the tetrahedral and octahedral sites or related to the Cu+ centers with different distortion of their local environment by B3+ cations. 5) The Cu2+ paramagnetic and Cu+ luminescence centers were localized in the Li (Li/K) sites of the LTB (KLTB) glass network with different coordination numbers to O2− anions (N = 4 ÷ 7) which depend on the Li–O (K–O) distances (positional disorder in the first coordination shell). This led to statistical distribution of the Cu–O distances and spectroscopic parameters for the Cu2+ and Cu+ centers and was revealed in the inhomogeneous broadening of their spectral lines. The charge compensation mechanism for Cu2+ → Li+ (K+) heterovalence substitutions could be related to cationic (VLi and VK) vacancies in the glass network. Acknowledgements This work is supported by the Ministry of Education and Sciences of Ukraine (Research Project No. 0109U001063) and the University of Zielona Góra (Poland). References [1] T. Sasaki, Y. Mori, M. Yoshimura, Y.K. Yap, T. Kamimura, Mater. Sci. Eng., R 30 (2000) 1. [2] M. Ghotbi, M. Ebrahim-Zadeh, Optics Express 12 (2004) 6002. [3] M. Santiago, M. Lester, E. Caselli, A. Lavat, A. Ges, F. Spano, C. Kessler, J. Mater. Sci. Lett. 17 (1998) 1293. [4] M. Prokic, Radiat. Prot. Dosim. 100 (2002) 265. [5] N. Can, T. Karali, P.D. Townsend, F. Vildiz, J. Phys. D: Appl. Phys. 39 (2006) 2038. [6] N. Senguttuvan, M. Ishii, M. Shimoyama, M. Kobayashi, N. Tsutsui, M. Nikl, M. Dusek, H.M. Shimizu, T. Oku, T. Adachi, K. Sakai, J. Suzuki, Nucl. Instr. Meth. Phys. Research A 486 (2002) 264. [7] M. Ishii, Y. Kuwano, S. Asaba, T. Asai, M. Kawamura, N. Senguttuvan, T. Hayashi, M. Koboyashi, M. Nikl, S. Hosoya, K. Sakai, T. Adachi, T. Oku, H.M. Shimizu, Radiat. Measur. 38 (2004) 571. [8] B.I. Zadneprowski, N.E. Eremin, A.A. Paskhalov, Funct. Mater. 12 (2005) 261. [9] Ya.V. Burak, B.V. Padlyak, V.M. Shevel, Nucl. Instr. Meth. Phys. Research B 191 (2002) 633. [10] Ya.V. Burak, B.V. Padlyak, V.M. Shevel, Radiat. Eff. Defects Solids 157 (2002) 1101. [11] D. Podgórska, S.M. Kaczmarek, W. Drozdowski, M. Berkowski, A. Worsztynowicz, Acta Phys. Polon. A 107 (2005) 507. [12] B.V. Padlyak, A. Gutsze, Appl. Magn. Reson. 14 (1998) 59. [13] B.V. Padlyak, Radiat. Eff. Defects Solids 158 (2003) 411. [14] M. Ignatovych, V. Holovey, A. Watterich, T. Vidóczy, P. Baranyai, A. Kelemen, O. Chuiko, Radiat. Measur. 38 (2004) 567–570. [15] M. Ignatovych, V. Holovey, A. Watterich, T. Vidóczy, P. Baranyai, A. Kelemen, V. Ogenko, O. Chuiko, Radiat. Phys. Chem. 67 (2003) 587. [16] M. Ignatovych, V. Holovey, T. Vidóczy, P. Baranyai, A. Keleman, V. Laguta, O. Chujko, Func. Mater. 12 (2005) 313. [17] B.T. Huy, V.X. Quang, H.T.B. Chau, J. Lumin. 128 (2008) 1601. [18] B.V. Padlyak, W. Wojtowicz, V.T. Adamiv, Ya.V. Burak, I.M. Teslyuk, in: Y. Zhydachevskii (Ed.), Book of Abstracts: “International Scientific Workshop Oxide Materials for Electronic Engineering—Fabrication, Properties and Application (ОМЕE-2009)”, June 22–26, 2009, Lviv, Ukraine, Lviv Polytechnic National University, Lviv, 2009, p. 90. [19] T. Castner Jr., G.S. Newell, W.C. Holton, C.P. Slichter, J. Chem. Phys. 32 (1960) 668. [20] B. Bleaney, K.W.H. Stevens, Rep. Prog. Phys. 16 (1953) 108. [21] D.L. Griscom, J. Non-Cryst. Solids 40 (1980) 211. [22] C.M. Brodbeck, R.R. Bukrey, Phys. Rev. B 25 (1981) 2334. [23] J.M. Dance, J.J. Videau, J. Portier, J. Non-Cryst. Solids 86 (1986) 88. [24] B. Sreedhar, Ch. Sumalatha, Kazuo Kojima, J. Non-Cryst. Solids 192 &193 (1995) 203. [25] B. Karthikeyan, S. Mohan, Mater. Lett. 57 (2003) 3789. [26] H. Smigielska, G. Lewandowicz, J. Goslar, S.K. Hoffmann, Acta Phys. Polon. A 108 (2005) 303. [27] H. Imagawa, Phys. Status Solidi (b) 30 (1968) 469. [28] C. Pedrini, Phys. Stat. Solidi B 87 (1978) 273. [29] G. Blasse, B.C. Grabmajerm, Luminescent Materials, Springer, Berlin, 1994. [30] J.W.M. Verwey, J.M. Coronado, G. Blasse, J. Solid State Chem. 92 (1991) 531. [31] A. Witkowska, B. Padlyak, J. Rybicki, Opt. Mater. 30 (2008) 699. [32] J. Krogh-Moe, Acta Cryst. B 24 (1968) 179.