Spectroscopic properties of the Ce-doped borate glasses

Spectroscopic properties of the Ce-doped borate glasses

Optical Materials 59 (2016) 20 e27 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Spe...

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Optical Materials 59 (2016) 20 e27

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Spectroscopic properties of the Ce-doped borate glasses  ski c, Y.O. Kulyk d I.I. Kindrat a, B.V. Padlyak a, b, *, S. Mahlik c, B. Kuklin ra, Institute of Physics, Division of Spectroscopy of Functional Materials, 4a Szafrana Str., 65-516 Zielona Go ra, Poland University of Zielona Go Vlokh Institute of Physical Optics, Sector of Spectroscopy, 23 Dragomanov Str., 79-005 Lviv, Ukraine c  sk, Institute of Experimental Physics, Condensed Matter Spectroscopy Division, 57 Wita Stwosza Str., 80-952 Gdan  sk, Poland University of Gdan d Ivan Franko National University of Lviv, Faculty of Physics, 8 Kyrylo and Mefodiy Str., 79-005 Lviv, Ukraine a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 October 2015 Received in revised form 30 January 2016 Accepted 29 March 2016 Available online 6 June 2016

The EPR, optical absorption and photoluminescence (emission and excitation) spectra as well as decay kinetics of a series of the Ce-doped glasses with Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 compositions have been investigated and analysed. The borate glasses were obtained from the corresponding polycrystalline compounds in the air atmosphere, using standard glass technology. The EPR signals of the isolated Ce3þ and pair Ce3þeCe3þ centres, coupled by magnetic dipolar and exchange interactions were registered at liquid helium temperatures. The characteristic for glass host broad bands corresponding to the 4f / 5d transitions of the Ce3þcentres have been observed in the optical absorption and photoluminescence (emission and excitation) spectra. The obtained luminescence decay curves can be satisfactory described by exponential function with lifetimes in the 19.8e26.1 ns range, which depend on the basic glass composition. The local structure of Ce3þ centres in the investigated glasses has been considered and discussed. © 2016 Elsevier B.V. All rights reserved.

Keywords: Borate glasses Ce3þ centres EPR Optical absorption Luminescence spectra Decay kinetics

1. Introduction In recent years, the study of borate crystals and glasses represents considerable interest due to their attractive optical and physical properties and wide 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 [1e3], scintillators and thermoluminescent dosimeters [4,5], detectors and transformers of the ionising radiation [6,7], and many other applications [8]. From the technological point of view the glassy (or vitreous) borate compounds are more promising materials than their crystalline analogues because the growth of borate single crystals is a difficult, long-term and, consequently, very expensive process. Hence, borate glasses represent potential materials due to low cost production, high chemical and physical stability, and good solubility of doping rare-earth and transition elements. Spectroscopic properties of Ce3þ-doped crystals and glasses

ra, Institute of Physics, Division * Corresponding author. University of Zielona Go ra, of Spectroscopy of Functional Materials, 4a Szafrana Str., 65-516 Zielona Go Poland. E-mail addresses: [email protected], [email protected] (B.V. Padlyak). http://dx.doi.org/10.1016/j.optmat.2016.03.053 0925-3467/© 2016 Elsevier B.V. All rights reserved.

have been very promising due to its relatively high light output and short luminescence decay time. Thus, Ce3þ-doped glasses are used as efficient phosphors in the near UV, violet, and blue spectral regions and fast scintillators for detection of the X- and g-rays as well as neutrons [9,10]. Beside wide practical applications the Ce3þ ions can be used as a probe of the local crystal field, ion-lattice, and ioneion interactions [11]. It should be noted that cerium can be incorporated into oxide glasses in two valence states, Ce3þ and Ce4þ, and their ionic equilibrium depends on the conditions of glass formation and the type and composition of glass system [12]. Tetravalent (Ce4þ) ions not show luminescence due to closed electronic shell, whereas a broad charge transfer (Ce4þ þ e / Ce3þ) band appears in the UVeVis region of the optical absorption spectra [13]. The Ce3þ ions (4f1 electron configuration) show efficient luminescence in the UVeVis spectral region due to allowed 5de4f transitions. The emission occurs from the lowest crystal field component of the 5d configuration to the 2F5/2 and 2F7/2 levels of the 4f configuration, separated around 2000 cm1 due to spineorbit coupling [14]. The wavelength position of the 5de4f transitions depends strongly on the nature and structure of host through the crystal-field splitting of the 5d configuration and widely varies from near UV to red regions [15]. The decay time of the Ce3þ emission is very short (107e108 s) due to parity and

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spin-allowed 5de4f transitions [14]. Optical properties of the Ce-doped lithium tetraborate (Li2B4O7) crystal and glass were investigated in Ref. [16]. The Ce-doped Li2B4O7 single crystal show emission peaked at 365 nm and two excitation bands at 260 and 330 nm, whereas in the Li2B4O7:Ce glass emission band peaked at 350 nm are efficiently excited around 290 nm. Similar luminescence band around 400 nm with decay time 36 ns appears under both 275 and 345 nm excitation in the Ca3(BO3)2 crystals [17]. The emission of Ce-doped PbB4O7 glassceramics excited at 350 nm shows one broad band with maximum at 480 nm [18]. Optical properties of the Ce3þ- doped phosphate, borate, silicate and germanate glasses were investigated and analysed in Ref. [19]. It was found that highest intensity of the excitation and emission bands were observed in borate glasses in comparison with other three glass systems. Fluorescence spectra of Ce3þ in silicate, borate, and phosphate glasses excited at 266 nm show asymmetric broad band with maximum between 280 and 500 nm that depend on glass composition [20]. According to [21], the luminescence properties of Ce3þ in silica-based (borosilicate, phosphosilicate, borophosphosilicate) glasses are strongly affected by presence of B and P former elements. Decay times equal 54 ns and 58 ns for Ce-doped silicate and borosilicate glasses. For borophosphosilicate and phosphosilicate glasses the lifetime values lowering from 32 to 29 ns and from 29 to 25 ns, respectively, while Ce molar ratio increases from 104 to 102 [21]. In the xCeO2e20PbOe(80x)B2O3 glass was observed broad emission bands peaked at 482 and 514 nm under excitation at 380 nm [22]. Photoluminescence spectrum of the Ce-doped lithium-alumino-borate glass upon 290 nm excitation exhibit a broad band at 360 nm with lifetimes 25 ± 2 ns [23]. The fluorescence spectrum of barium-sodium borate glass shows weaklyresolved band at 367 nm upon 306 nm excitation [24]. Also one broad emission band at 338 nm with lifetime t ¼ 28.5 ns was observed upon 270 nm excitation in the Ce-doped 57.5Li2Oe5B2O3e37.5P2O5 glass [25]. Under excitation at 347 nm, the photoluminescence spectrum of the NaCaBO3:0.01Ce3þ powder sample shows an asymmetric emission band that extends from 350 to 550 nm with a maximum at 427 nm [26]. Similar results were obtained in Ref. [27] for LiCaBO3:Ce3þ polycrystalline compounds, in which the emission spectrum upon 365 nm excitation shows one asymmetric band at 428 nm. Analysis of the available referenced data shows that spectroscopic and luminescence properties of the lithium and calcium borate glass are studied insufficiently. In particular, very limited studies have been done for Ce-doped Li2B4O7 glass, whereas LiKB4O7:Ce, CaB4O7:Ce, and LiCaBO3:Ce glasses have not been investigated yet. Therefore, the main aim of this work is an investigation of the features of the spectroscopic and luminescence properties of a series borate glasses with Li2B4O7:Ce, LiKB4O7:Ce, CaB4O7:Ce, and LiCaBO3:Ce compositions and different Ce concentrations as well as determination of the electron and local structures of the Ce luminescence centres in those glasses using EPR and optical spectroscopy methods, supported by corresponding X-ray diffraction (XRD) structural data. 2. Experimental details 2.1. The glass synthesis and samples preparation The Ce-doped borate glasses with Li2B4O7 (Li2Oe2B2O3), LiKB4O7 (0.5Li2Oe0.5K2Oe2B2O3), CaB4O7 (CaOe2B2O3), and LiCaBO3 (0.5Li2OeCaOe0.5B2O3) were obtained in the air atmosphere from corresponding polycrystalline compounds according to the standard glass synthesis method and technological

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conditions according to [28]. For solid-state synthesis of the polycrystalline compounds were used carbonates (Li2CO3, K2CO3, CaCO3) and boric acid (H3BO3) of high chemical purity (99.999%, Aldrich). The cerium impurity was added to the raw materials as Ce2O3 oxide of chemical purity (99.99%). Solid-state synthesis of the polycrystalline borate compounds was performed using multi-step heating reactions, presented elsewhere [28,29]. Large samples of borate glasses with Li2B4O7:Ce, LiKB4O7:Ce, CaB4O7:Ce, and LiCaBO3:Ce compositions were obtained by fast cooling of the corresponding melts, heated more than 100 K above the melting points (Tmelt ¼ 917  C (1190 K), 807  C (1080 K), 980  C (1253 K), and 777  C (1050 K) for Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 compounds, respectively) to make the crystallisation process impossible [28]. The glass samples for optical measurements were cut and polished to the approximate size of 8  5  2 mm3. For EPR investigation the glass samples were cut to the approximate size of 3  2  2 mm3. 2.2. Experimental equipment and samples characterisation The paramagnetic impurities in the Li2B4O7:Ce, LiKB4O7:Ce, CaB4O7:Ce, and LiCaBO3:Ce glasses were detected in the 4.2e50 K temperature range using the X-band EPR spectrometer Bruker (model ELEXSYS E-500) completed with helium-flow cryostat (Oxford Instruments). The optical absorption spectra of the investigated glasses were recorded with usage Shimadzu UVeVis spectrophotometer (model UV-2450). The luminescence excitation and emission spectra were registered in the UV and visible spectral ranges using a Horiba spectrofluorimeter (model FluoroMax-4). The luminescence kinetics was investigated using the special equipment for time-resolved luminescence [30]. The experimental setup consists of laser system as the excitation source, which includes the PL 2143 A/SS laser and the parametric optical generator PG 401/SH that generates 30 ps laser pulses with frequency 10 Hz. The detection part consists of the spectrograph 2501S (Bruker Optics) and the Hamamatsu Streak Camera (model C4334-01). The investigated Ce-doped borate glasses are almost uncoloured and characterised by a high optical quality. The nominal cerium concentrations in the obtained glasses have not been proved analytically, but our previous investigation [29] has shown that the incorporation coefficient of rare-earth impurities into the borate glass network is close to unity. The X-ray diffraction studies were carried out using computer controlled X-ray diffractometer (model DRON-3) with monochromatic Cu Ka line. The XRD patterns of the Li2B4O7:Ce LiKB4O7:Ce, CaB4O7:Ce, and LiCaBO3:Ce borate glasses are shown in Fig. 1. The obtained XRD patterns confirm disorder glass structure of the investigated samples. 3. Results and discussion 3.1. EPR spectroscopy of the Ce-doped borate glasses The Ce impurity can be incorporated in the structure of oxide crystals and glasses as Ce3þ (4f1, 2F5/2) paramagnetic Kramers ions and Ce4þ ([Xe]4f0, 1S0) non-paramagnetic ions. So, only Ce3þ ions can be observed by EPR spectroscopy. Registration and identification of the Ce3þ centres in the EPR spectra of glasses is not easy due to the following reasons. First, the spin-lattice relaxation times of the rare-earth ions (with exception of the S-state Kramers 4f7 (8S7/ 2þ 3þ 4þ 2) ions e Eu , Gd , Tb ) are very short for observation EPR lines at room and liquid nitrogen temperatures. Second, the experimental g-factor values for Ce3þ ions in crystal fields of different symmetry vary in a wide range, as reported for single crystal

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belongs to the isolated Fe3þ (3d5, 6S5/2) non-controlled impurity ions, localised in the octahedral and/or tetrahedral sites of the glass network with a strong rhombic distortion [34,35], whereas weak EPR signal with geff y 2.00 belongs to the Fe3þ isolated ions in the glass network sites with nearly cubic local symmetry [34]. Based on the referenced data for other rare-earth ions for example Sm3þ [29] in borate glasses one can supposed that weak EPR signal with geff y 2.01, observed in all investigated Ce-doped glasses, belongs to the Ce3þeCe3þ pair centres, coupled by the magnetic dipolar and exchange interactions. 3.2. Optical absorption and photoluminescence spectra of the Cedoped borate glasses

Fig. 1. The XRD patterns of the investigated Ce-doped borate glasses.

studies [31,32]. Third, when the rare-earth ions are incorporated in the glass network, the EPR spectra becomes more complicated because the paramagnetic centres are exposed by several distorted local crystal fields due to both vitreous matrix and structural defects (vacancies or substitutional ions), especially when the charge of the rare-earth and substitutional ions are different. All these reasons lead to homogeneous and inhomogeneous broadening of the Ce3þ lines with wide distribution of g-values in the EPR spectra of glasses [21] and other disordered solids. According to [33], the Xband EPR spectra of the non-S-state Kramers rare-earth ions (Ce3þ, Nd3þ, Dy3þ, Er3þ, Yb3þ) in zeolites consists of extremely broad asymmetric signals, which can be observed only at liquid helium temperatures. The EPR spectra of all investigated Ce-doped borate glasses at liquid helium temperatures are fairly similar and consist of the resonance signal with geff y 7.9, geff y 4.26, geff y 2.01, and geff y 2.00 (Fig. 2). The asymmetric broad EPR signal with geff y 7.9 can be related to the Ce3þ (4f1, 2F5/2) isolated ions [29]. The EPR signal of Ce3þ isolated centres disappear at approximately T ¼ 20 K due to shortening of their spin-lattice relaxation time (T2) that leads to line broadening according to relation DH f T1 2 . Sharp EPR signal with geff y 4.26 is characteristic for glassy compounds and

Fig. 2. The X-band (n y 9.46 GHz) EPR spectra of the Li2B4O7:Ce (1.0 mol. %) glass, recorded at different temperatures in the 4e50 K range.

The optical absorption spectra of the Li2B4O7:Ce glass containing 1.0 mol. % Ce2O3 and un-doped nominally-pure Li2B4O7 glass are presented in Fig. 3. As it was shown in Fig. 3, the Ce-doped Li2B4O7 glass reveals strong absorption in the UV region that leads to the shift of absorption edge to longer wavelength. According to [36], the Ce3þ and Ce4þ ions in borate glasses show broad overlapped absorption band in the UVevisible spectral region. We suggest that the weak absorption band peaked at 321 nm can be assigned to the 4f / 5d (2F5/2 / 5d1) transition of the Ce3þ ions that overlaps with fundamental absorption edge of the glass host as well as charge transfer (Ce4þ þ e / Ce3þ) absorption band of the Ce4þ ions. The similar optical absorption spectra have been observed also for other investigated borate glasses. The photoluminescence spectra exhibit broad unresolved emission bands in the UVeVis spectral region, which belong to the 5d / 4f (5d1 / 2F5/2, 2F7/2) transitions of the Ce3þ ions (Fig. 4). Appropriate excitation spectra show a broad band in the UV range that corresponds to the 4f / 5d (2F5/2 / 5d1) transition. The full width at half maximum (FWHM) values of the observed luminescence excitation and emission bands are equal about 3500 cm1. The 5d / (2F5/2, 2F7/2) characteristic double emission band was not observed in the investigated glasses due to inhomogeneous broadening of the spectral lines that is characteristic for a glass host. The observed luminescence excitation and emission bands of the Ce3þ centres in the investigated glasses reveal maxima at different wavelengths (see Fig. 4), caused by the influence of the basic glass composition on the photoluminescence spectra. The position of the Ce3þ 5d / 4f transitions in luminescence spectra of the samples with the same Ce impurity concentration varies from 317 to 341 nm for excitation bands and from 368 to 417 nm for emission bands (Fig. 4). This dependence can be interpreted on the basis the crystal field splitting of the 5d level (see Section 3.4). The observed 5d / 4f transitions of Ce3þ centres in the borate glasses is characterised by large Stokes shift. The Stokes shift of the Ce3þ emission band increases from 4372 to 5345 cm1 for glass matrices in the following order: LiKB4O7, Li2B4O7, CaB4O7, LiCaBO3. Parameters of the Ce3þ luminescence centres in the borate glasses containing 0.5 mol. % Ce2O3 are summarised in Table 1. The influence of the Ce impurity concentration is also revealed in the luminescence spectra. The peak of the emission and excitation bands is shifted toward longer wavelengths as the Ce concentration increased (Fig. 5). For example, the Ce3þ emission bands are shifted from 362 to 383 nm with increasing Ce content from 0.01 to 1.0 mol. % in the Li2B4O7:Ce glasses (see Fig. 5). The observed red shift in the luminescence spectra can be explained by increasing of the crystal field splitting with increasing of the Ce3þ impurity concentration. Beside this, according to [37] the energy transfer between the pair of paramagnetic centres causes the red shift of the luminescence band. Therefore, additional red shift in the Li2B4O7:Ce glasses can be caused by exchange-coupled Ce3þeCe3þ

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Fig. 3. Optical absorption spectra of the un-doped Li2B4O7:Ce glass (a) and Li2B4O7:Ce glass containing 1.0 mol. % Ce2O3 (b), recorded T ¼ 300 K.

pair centres, which are registered by EPR spectroscopy (see Fig. 2 and Section 3.1). It should be noted that weak absorption band at 321 nm (see Fig. 3) well agrees with corresponding excitation band for the Li2B4O7:Ce glass, containing 1.0 mol. % Ce2O3 (see Fig. 5, curve a) and belongs to the Ce3þ centres. One can notice also that all investigated borate glasses additionally exhibit a broadband intrinsic luminescence emission band with maximum in the 460e480 nm range (see Figs. 4e6) that will be a subject of our future work. 3.3. The luminescence kinetics of the Ce3þ centres in the borate glasses The luminescence decay curves of Ce3þ centres in the Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 glasses are presented in Fig. 7 using a semi-logarithmic scale. The obtained lifetimes for borate glasses containing 0.5 mol. % Ce2O3 are: t ¼ 26.1 ns (LiKB4O7:Ce), t ¼ 25.5 ns (Li2B4O7:Ce), t ¼ 23.7 ns (CaB4O7:Ce), and t ¼ 19.8 ns (LiCaBO3:Ce) (Fig. 7). Concentration quenching of lifetime values in the investigated borate glasses is negligible. In particular, the lifetime value decreases from 25.5 to 25.3 ns with increasing Ce concentration from 0.5 to 1.0 mol. % in the Li2B4O7:Ce glass (see insert of Fig. 7). It should be noted that the decay curve for LiCaBO3:Ce glass containing 0.5 mol. % Ce2O3 can't be satisfactory described by a single exponent fit due to strong overlapping of the Ce3þ emission with a broadband intrinsic luminescence that characterised by very short (t ¼ 1.5 ns) lifetime (see Fig. 6). Weak broadband intrinsic luminescence with similar lifetimes had been observed also for other investigated Ce-doped glasses using the time-resolved luminescence technique. The obtained results show that the luminescence lifetimes strongly depend on the basic glass composition. Various lifetime values for identical Ce2O3 content in different borate glasses are caused by some differences in the local structure of Ce3þ luminescence centres in the glass network. The relations between local

structure parameters and lifetime values of the Ce3þ centres in the Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 glasses are considered in Section 3.4. 3.4. The local structure of Ce3þ centres in the borate glasses The investigation of local structure of the Ce3þ centres in borate glasses allows explaining the strong dependence of the luminescence properties on the basic glass composition. Investigation of the local structure of Ce3þ luminescence centres in the network of borate glasses is based on the obtained XRD data. Presented above XRD data (Fig. 1) were Fourier-transformed to obtain the corresponding pair correlation functions and to derive the average interatomic distances in the glass network. Let us consider below the structural features of the investigated glasses. The boron atoms in the Li2B4O7:Ce glass, are localised in the oxygen-coordinated tetrahedral BO4 units with the average interatomic BeO distance equal to 0.159 nm. A certain proportion of triangular BO3 units also are presented in the Li2B4O7:Ce glass network. The local environment of Li atoms consists of tetrahedral oxygen units with the average LieO distance equals to 0.254 nm. In the LiKB4O7:Ce glass the boron atoms are localised in the oxygen-coordinated tetrahedral and triangular sites with the average BeO distance equals to 0.157 nm, whereas Li (K) and O atoms form a distorted tetrahedral units with the average distance Li (K)eO equals to 0.257 nm. The structure of the CaB4O7:Ce glass consists of BO4 tetrahedra, with the average BeO distance equals to about 0.160 nm. The Ca atoms are localised at the sites with the coordination number N ¼ 4, where the average distance CaeO equals to 0.248 nm. In the LiCaBO3:Ce glass network the boron atoms form triangular BO3 units with the average interatomic BeO distance 0.161 nm. The Ca atoms are located at the sites with the coordination number N ¼ 6e7 and average CaeO distance 0.246 nm. The Li atoms are located at the sites with the coordination number N ¼ 4e5 (the average LieO distance should be less than 0.246 nm).

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Fig. 4. Luminescence excitation (blue curves) and emission spectra (red curves) of the Ce3þ centres in the LiKB4O7:Ce (a), Li2B4O7:Ce (b), CaB4O7:Ce (c), and LiCaBO3:Ce (d) glasses containing 0.5 mol. % Ce2O3 at T ¼ 300 K. Positions of the observed bands are given in Table 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1 Parameters of luminescence and local structure of the Ce3þ centres in the investigated borate glasses containing 0.5 mol. % Ce2O3. Glasses

LiKB4O7:Ce Li2B4O7:Ce CaB4O7:Ce LiCaBO3:Ce

Excitation band

Emission band

lmax ~nmax

FWHM

lmax ~nmax

FWHM

317 nm 31546 cm1 318 nm 31447 cm1 325 nm 30769 cm1 341 nm 29326 cm1

33 nm 3324 cm1 36 nm 3605 cm1 40 nm 3861 cm1 43 nm 3701 cm1

368 nm 27174 cm1 373 nm 26809 cm1 390 nm 25641 cm1 417 nm 23981 cm1

48 nm 3446 cm1 50 nm 3477 cm1 54 nm 3477 cm1 62 nm 3418 cm1

Presented above XRD structural data show good agreement with the results of the 11B MAS NMR spectroscopy [38] confirming the existence of triangular (BO3) and tetrahedral (BO4) atomic groups in the borate glasses and allows determine their proportions in the structure of borate compounds. The analysis of the 7Li MAS NMR spectra [38] shows that the LiO4 structural units dominate in the glassy and crystalline Li2B4O7, LiB3O5, LiCaBO3, and LiKB4O7 compounds. Based on the presented above structural data for borate glasses we can state that the Ce3þ ions are incorporated into the Liþ (Kþ, Ca2þ) sites of the Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 glasses. This statement well agrees with [39], in which it was shown by EXAFS

Stokes shift, ~nexc e~nem

Lifetime value, t

Average interatomic distance, dCeeO

4372 cm1

26.1 ns

0.257 nm

4638 cm

1

25.5 ns

0.254 nm

5128 cm

1

23.7 ns

0.248 nm

5345 cm1

19.8 ns

0.246 nm

(Extended X-ray Absorption Fine Structure) spectroscopy that trivalent rare-earth dopants (Nd, Gd, Dy, and Er) occupy the Liþ sites in the lithium tetraborate (Li2B4O7) glasses. The local environment of the Ce3þ centres consists of O2 anions with statistically-distributed structural parameters (CeeO interatomic distances and coordination numbers to oxygen) in the first coordination shell (so-called “positional disorder”) that reveals as inhomogeneous broadening of the spectral lines. The proposed local structure of the Ce3þ centres shows good correlation with lifetime values and position of luminescence bands (see Fig. 8 and Table 1). In particular, the observed luminescence band shifts to longer wavelength with decreasing the CeeO average

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CeeO average interatomic distance leads to increasing of the local crystal field strength and increasing of crystal field splitting of the 5d level. So, the lowest crystal-field component from which the emission originates will be lowered and luminescence bands will be observed at longer wavelength in the glasses with shorter CeeO average interatomic distance.

4. Conclusions The borate glasses of Li2B4O7:Ce, LiKB4O7:Ce, CaB4O7:Ce, and LiCaBO3:Ce compositions have been detailed investigated by EPR, optical spectroscopy (absorption, photoluminescence, decay kinetics), and XRD techniques. Based on the obtained experimental results one can conclude the following:

Fig. 5. Luminescence excitation (blue curves) and emission spectra (red curves) of the Ce3þ centres in the Li2B4O7:Ce glasses containing 1.0 (a), 0.5 (b) and 0.01 (c) mol. % Ce2O3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

interatomic distance. This effect is related to the increasing crystal field splitting of the 5d level and lowering of the lowest crystal-field component from which the emission originates. Shortening of the

 The EPR spectroscopy shows that the Ce impurity in the investigated glass samples is incorporated as isolated Ce3þ (4f1, 2F5/2) paramagnetic Kramers ions (geff y 7.9) and Ce3þeCe3þ pair centres (geff y 2.01), coupled by magnetic dipolar and exchange interaction. The EPR signals of Ce3þ isolated centres with effective g e factor geff y 7.9 are clearly observed in the 4 ÷ 20 K temperature ranges and disappear at higher temperatures due to homogeneous broadening, caused by shortening of the Ce3þ paramagnetic relaxation time.  In the optical absorption and photoluminescence (excitation and emission) spectra of all investigated Ce-doped glasses with Li2B4O7:Ce, LiKB4O7:Ce, CaB4O7:Ce, and LiCaBO3:Ce compositions were observed characteristic for disordered glass host

Fig. 6. Time-resolved luminescence of the Li2B4O7:Ce (a), CaB4O7:Ce (b), LiCaBO3:Ce (c) glasses containing 0.5 mol. % Ce2O3 and un-doped LiCaBO3 glass (d).

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Fig. 7. Luminescence decay curves of the LiKB4O7:Ce (black curve), Li2B4O7:Ce (red curve), CaB4O7:Ce (green curve), and LiCaBO3:Ce (blue curve) compounds containing 0.5 mol. % Ce2O3. Inset presents the luminescence decay curves of the Li2B4O7:Ce glasses containing 0.5 (black curve) and 1.0 (red curve) mol. % Ce2O3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Correlation between luminescent properties and local structure of the Ce3þ centres in the borate glasses containing 0.5 mol. % Ce2O3. The maximum of the luminescence excitation and emission bands are indicated by blue circles and red triangles, respectively. The luminescence lifetimes are denoted by gray quadrates. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

inhomogeneously broadened bands corresponding to the 4f / 5d1 and 5d1 / 4f transitions of the Ce3þcentres.  The position of the Ce3þ 5d / 4f transitions in the luminescence spectra varies at the same impurity concentration in the samples from 317 to 341 nm for the excitation bands and from 368 to 417 nm for the emission bands. The luminescence lifetimes lie in the 19.8e26.1 ns range and also depend on the basic glass composition.

 The multisite character of the Ce3þ luminescence centres in the Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 glasses is related to the presence of Liþ, Kþ, and Ca2þ cationic sites in their structure, which are occupied by Ce3þ ions that leads to different their coordination numbers (N ¼ 4e7) and statistical distribution of the Ce3þeO2 distances (positional disorder).  The proposed local structure of the Ce3þ centres shows good correlation with lifetime values and position of luminescence bands. In particular, the observed luminescence band shifts to

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longer wavelength with decreasing the CeeO average interatomic distance due to increasing crystal field splitting of the 5d level and lowering of the lowest crystal-field component from which the emission originates. Acknowledgement The authors would like to thank Dr. Sci. V. T. Adamiv, Dr. Sci. 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. Especially authors thank Prof. N. Guskos and Dr. G. _ Zołnierkiewicz from West Pomeranian University of Technology (Szczecin, Poland) for EPR spectra registration at liquid helium temperatures. References

[18]

[19]

[20]

[21]

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