Spectroscopy of Nd3+ luminescence centres in Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses

Author’s Accepted Manuscript Spectroscopy of Nd3+ luminescence centres in Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses B.V. Padlyak, R. Lisiecki, T.B. Padlyak, V.T. Adamiv www.elsevier.com/locate/jlumin

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S0022-2313(17)31880-X https://doi.org/10.1016/j.jlumin.2018.02.046 LUMIN15393

To appear in: Journal of Luminescence Received date: 7 November 2017 Revised date: 14 January 2018 Accepted date: 14 February 2018 Cite this article as: B.V. Padlyak, R. Lisiecki, T.B. Padlyak and V.T. Adamiv, Spectroscopy of Nd3+ luminescence centres in Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.02.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Spectroscopy of Nd3+ luminescence centres in Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses B.V. Padlyaka,b*, R. Lisieckic, T.B. Padlyakb, V.T. Adamivb a

University of Zielona Góra, Institute of Physics, Division of Spectroscopy of Functional Materials, 4a Szafrana Str., 65-516 Zielona Góra, Poland

b

Vlokh Institute of Physical Optics, Sector of Spectroscopy, 23 Dragomanov Str., 79-005, Lviv, Ukraine c

Institute of Low Temperatures and Structure Research of the Polish Academy of Sciences, Department of Spectroscopy of Laser Materials, 2 Okólna Str., 50-422 Wrocław, Poland

Abstract. The electron paramagnetic resonance (EPR), optical absorption, luminescence (emission and excitation) spectra as well as luminescence kinetics of the Nd3+ centres in a series of borate glasses with Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd compositions containing 0.5 and 1.0 mol. % Nd2O3 have been investigated and analysed. Using EPR and optical spectroscopy data it was shown that the Nd impurity is incorporated into the network of investigated glasses as Nd3+ (4f3, 4I9/2) ions, exclusively. All observed f – f electronic transitions of the Nd3+ centres in optical absorption and luminescence spectra of the investigated glasses were identified. Local structure of the Nd3+ luminescence centres in the borate glasses network is proposed. Theoretical oscillator strengths (ftheor) and phenomenological intensity parameters (2, 4, 6) for observed Nd3+ absorption transitions containing 1.0 mol. % Nd2O3 have been determined using standard Judd–Ofelt theory. The spectroscopic parameters of relevance to laser applications such as the radiative transitions rate (Wr), luminescence branching ratio (β), radiative lifetime (τrad), and emission cross-section (σem) for Nd3+ centres in the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses have been calculated. Luminescence decay curves of the Nd3+ centres from the 4F3/2 emitting level for all investigated glasses were satisfactory described by single exponential decay. The obtained experimental lifetimes were compared with those calculated and quantum efficiency () of the 4

F3/2  4I11/2 transition for Nd3+ centres in the investigated glasses were estimated. Based on the

obtained results it can be concluded that the further efforts should be made to improve the Nd-doped borate glasses parameters relevant for neodymium solid state lasers with LED pumping. Keywords: Borate glasses; Nd3+ ions; Optical absorption; Luminescence; Judd–Ofelt analysis; Decay kinetics. *Corresponding author. E-mail: [email protected]; [email protected], Fax: +48 68 328 2920.

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1.

Introduction The Nd-doped oxide glasses and crystals, especially activated with Nd3+ ions glasses [1,2] and

single garnet crystals [3-8], are still attractive as an active media for powerful solid-state lasers, operating in the near infrared (NIR) spectral range (max  1064 nm, 4F3/2  4I11/2 emission channel). That leads to extensively search and spectroscopic researches of novel oxide glasses and crystals activated with Nd3+ ions as perspective materials for solid-state lasers. The study of spectroscopic and luminescence properties as well as local structure of the Nd3+ centres in disordered oxide compounds, particularly in glassy hosts, represents current topic of the physics of lasers materials, because effective laser generation can be obtained from Nd3+ centres, which are located in a special structural sites [5-8]. Practical results of such investigations were obtaining new active elements for Nd3+- lasers based on the single garnet crystals [5-8] as well as gallogermanate disordered crystals [9] and glasses [10,11], which can be also successfully used for active elements of the solid-state Nd3+- lasers with LED (light emitting diode) pumping [12]. During last decades the borate glasses doped with rare-earth and transition elements have been widely studied [13-31] as a very promising luminescent materials for different branches of modern technique, especially for quantum electronics and optoelectronics due to their attractive spectroscopic and luminescence properties. The borate glasses with compositions, which are similar to well-known crystalline analogies (Li2B4O7, LiCaBO3, LiKB4O7, CaB4O7 etc.), activated with trivalent rare-earth ions in a wide concentration range [15,16,18,19,21-25,28] including Nd3+ ions [29-31], can be easily obtained by standard technology of borate glasses. From the technological point of view, the borate glasses are more perspective in comparison with corresponding single crystals, which are produced using very expensive and complicated techniques of crystals growth. Moreover, a very low velocity of the borate crystal growth and a high viscosity of the melt leads to problems with doping of the borate

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single crystals by rare-earth elements, in particular by Nd. Therefore, up to now only in [32] it was reported about successfully growth of the Nd-doped lithium tetraborate (Li2B4O7:Nd) single crystals using Bridgman-Stockbarger technique and investigation their optical transmittance as well as crystallographic density and structure by X-ray diffraction (XRD) technique. Optical and luminescence properties of the Nd-doped borate glasses and crystals with different chemical compositions have been extensively studied during last decades by spectroscopic methods and published in a number papers [33-46]. The effect of glass host on the optical absorption of the Nd3+ and other rare-earth (Sm3+, Dy3+) ions in the lead borate glasses was studied in [33]. In [34] were presented the results of structural (FTIR spectral analysis), optical (Judd–Ofelt (J–O) analysis) and glass transition (differential scanning calorimetry) studies of the Nd3+-doped lead bismuth borate (PbO–Bi2O3–B2O3) glasses. In particular, the structural studies in [34] confirm presence [BiO3], BO4, BO3 and PbO4 units as the local structures in the lead bismuth borate glasses. In [35] were presented spectroscopic researches of the 67B2O3·xLi2O·(32-x)Na2O and 67B2O3·xLi2O·(32-x)K2O (where x = 8, 12, 16, 20, 24) glasses containing 1 mol. % Nd2O3. The (J–O) intensity parameters (Ω2, Ω4, Ω6), total probabilities of radiative transitions, radiative lifetimes, branching ratios as well as integrated absorption and emission crosssections have been calculated in [35] for certain excited states of the Nd3+ in mixed alkali borate glasses. Particularly, in [35] it was shown that lithium sodium glass with x = 12 and lithium potassium glass with x = 20 show high absorption and emission cross-sections. Spectroscopic and physical properties of the Nd3+-doped alkali lead borate glasses with R2O + 30PbO + 49.5B2O3 + 0.5Nd2O3 (R = Li, K) and alkaline-earth lead borate glasses with 20RO + 30PbO + 49.5B2O3 + 0.5Nd2O3 (R = Ca, Ba, Pb) compositions were described in [36]. In [37] results of optical spectroscopy of the Nd-doped alkali borogermanate glasses were presented. The spectroscopic properties of a series glasses with (90-x)B2O3 + (x-2)PbO + 5Bi2O3 + 5Al2O3 + 2Nd2O3 (x = 10, 15, 20, 25, 30) molar compositions as a potential laser

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materials were detailed described in [38]. In [39] synthesis and spectroscopic properties of the 20B2O3·(79.5-x)Bi2O3·xSiO2 (10 ≤ x ≤ 40, mol. %) glasses doped with 0.5 mol. % of Nd3+ ions as a function of Bi2O3 content in the glass host were presented. Particularly, the J–O intensity parameters and radiative properties of the Nd3+ centres in bismuth boro-silicate glasses were calculated and analysed. In [40-42] were presented behaviours of spectroscopic, optical, and luminescence properties of Nd3+-doped zinc bismuth and zinc alumino bismuth borate glasses. Optical properties and spectroscopic parameters of relevance to laser applications of Sr6NdSc(BO3)6:Nd3+ and Sr4B14O25:Nd3+ single crystals were published in [43] and [44], respectively. In [45] were described growth process and laser properties of the Sr6YSc(BO3)6:Nd3+ single crystal. In [46] were reported features of growth and spectroscopic properties of new laser crystals La2CaB10O19:Nd3+ with self-doubling generation frequency. At present time large samples of high optical quality of a series of borate glasses with Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd compositions containing 0.5 and 1.0 mol. % Nd2O3 were obtained by us [29]. Preliminary results of optical and electron paramagnetic resonance (EPR) spectroscopy of the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses were presented in [29-31], but up to now all obtained parameters of EPR, optical absorption, and luminescence spectra as well as local structure of the Nd 3+ centres in these glasses were not detailed described, analysed, and discussed. This article presents the results of detailed spectroscopic investigations of the Nd3+ centres in the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses including calculations and analysis of the parameters of relevance to application in laser technique. Peculiarities of incorporation and local structure of the Nd3+ impurity ions in the network of borate glasses also are detailed considered and discussed based on the obtained spectroscopic results and published structural data for investigated glasses and their crystalline analogues.

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2.

Experimental details

2.1. Preparation and short characterisation of the Nd-doped borate glasses The glasses with Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd compositions of high optical quality and chemical purity were obtained in the air atmosphere from corresponding polycrystalline compounds according to standard glass synthesis using technological conditions, reported in [29,30,47]. For synthesis of the Li2B4O7, LiCaBO3, and CaB4O7 polycrystalline compounds were used carbonates (Li2CO3, CaCO3) and boric acid (H3BO3) of high chemical purity (99.999 %, Aldrich). The Nd impurity was added to the raw materials as Nd2O3 compound of chemical purity in amounts 0.5 and 1.0 mol. %. Solid-state synthesis of the corresponding polycrystalline borate compounds were carried out using multi-step chemical reactions and heating processes [47], which can be described for Li2B4O7, LiCaBO3, and CaB4O7 compounds by the following equations: H3BO3 = -НВО2 + H2O (170oC),

(1)

2-НВО2 = B2O3 + H2O (250oC),

(2)

Li2CO3 + 2B2O3 = Li2B4O7 + CO2 (800oC),

(3)

Li2CO3 + 2CaCO3 + В2О3 = 2LiCaBO3 + СО2 (700С)

(4)

CaCO3 + 2B2O3 = CaB4O7 + CO2 (800oC),

(5)

The Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses were obtained by rapid cooling of the corresponding melted compounds, which were heated on 100 K higher than their melting temperatures (Tmelt = 1190, 1050, and 1253 K for Li2B4O7, LiCaBO3, and CaB4O7 compounds, respectively) [47] for blocking of crystallisation process. Two types of crucibles, graphite (C) and corundum ceramic (Al2O3), were used for obtaining the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses. Optical quality of the obtained glasses practically is independent of the type of used crucibles. It should be noted that the glass samples obtained in both, graphite and corundum, crucibles were characterised by significant 5

mechanical stresses. The mechanical stresses in the obtained borate glasses were eliminated by thermal annealing of the samples in air in the temperature range of 680  730 K [47]. The obtained Nd-doped glass samples show typical glassy-like XRD patterns similar as for Smdoped [22] and Ce-doped [25] borate glasses with the same chemical compositions without any discrete sharp peaks that confirms their disordered structure. Colour of the obtained Nd-doped glass samples varies depending on the Nd concentration from almost uncoloured (Nd2O3 content – 0.5 mol. %) to lightly-blue (Nd2O3 content – 1.0 mol. %). The nominal Nd dopant concentration were not analytically proved in the Nd-doped glasses, but our EDS investigation of the Sm-doped borate glasses [22] show that the coefficient of incorporation of the rare-earth impurities into the glass network is close to unity. The samples for optical spectroscopy were cut and polished to the approximate size of 532 mm3. For EPR investigations the Nd-doped glass samples were cut to the approximate size of 322 mm3.

2.2. Experimental methods and equipment The paramagnetic impurities in the obtained Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses at room temperature (RT) were controlled by electron paramagnetic resonance technique using computer controlled X-band (ν  9.4 GHz) RADIOPAN (Poznań, Poland) radiospectrometers of the SE/X-2013 and SE/X-2544 types, operating in the high-frequency (100 kHz) modulation mode of magnetic field. The work microwave frequency of the EPR spectrometers was measured using the Hewlett Packard frequency counter (model 5350 B) and DPPH g - marker (g = 2.0036 ± 0.0001). The EPR spectra of Nd3+ centres in the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses were registered in the 4  50 K temperature range using commercial Bruker (model ELEXSYS E-500) X-band EPR spectrometer completed with helium-flow cryostat (Oxford Instruments). The scanned magnetic field in all used EPR spectrometers was measured by digital NMR (Nuclear Magnetic Resonance) magnetometers. 6

Optical absorption spectra were recorded at RT with usage a Varian (model 5E UV–Vis–NIR) spectrophotometer. The luminescence excitation and emission spectra as well as luminescence kinetics decay were registered at RT (T = 295 K) in the UV–Vis–IR spectral range. The emission and luminescence excitation spectra were acquired with a Dongwoo (model DM711) scanning system consisting of an excitation monochromator with 150 mm focal length and emission monochromator having 750 mm focal length equipped with a photomultiplier and an InGaAs detector. The resulting signal was analysed by a Stanford (model SRS 250) boxcar integrator and stored in a personal computer. The luminescence decay curves were recorded with a Tektronix (model TDS 3052) digital oscilloscope at RT. Excitation of the glass samples was carried out by a Continuum Surelite I Optical Parametric Oscillator (OPO) pumped by a third harmonic of a YAG:Nd3+ laser ( = 355 nm) and the emitted light was filtered using a GDM grating monochromator (focal length – 1000 mm).

3.

Results and discussion

3.1. EPR spectra of the Nd-doped borate glasses The Nd impurity can be incorporated into the structure of oxide crystals and glasses as paramagnetic Nd3+ (4f3, 4I9/2) Kramers and Nd2+ (4f4, 5I4) non-Kramers ions with characteristic EPR and optical spectra. In glasses with Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd compositions at room and liquid helium temperatures were observed virtually identical EPR spectra. Typical X-band EPR spectra of the CaB4O7:Nd glasses recorded at different temperatures in the 4 ÷ 50 K range (scan range of magnetic field was 200  2800 G) and at T = 4 K (scan range of magnetic field was 0  4000 G) are presented in Fig. 1a and Fig. 1b, respectively. Very broad asymmetric EPR signal with geff  6.00 that is observed in the Nd-doped glasses in the 4  15 K temperature range (Fig. 1a) according to [48-50] can be assigned to the single (isolated) Nd3+ centres in a strongly-distorted sites of the glass network.

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Particularly, in [48] it was concluded that the Nd3+ ions in the phosphate and borate glasses “dictate their own environment and impose sites with more or less defined symmetry and coordination”. According to this conclusion can be explained very wide distribution and differences of the effective g-factor values in the investigated by us borate glasses and borate glasses of the CaO–Li2O–B2O3 and Na2O–Al2O3– B2O3 systems, which were studied in [48]. Increasing of the Nd2O3 content in the investigated glasses leads to increasing of integral intensity of the Nd3+ EPR signal with geff  6.00, whereas the position and lineshape of this signal practically are independent (within the experimental errors) of the basic glass composition and Nd concentration. The EPR signal of single Nd3+ ions disappears at T  15 K (Fig. 1, a) due to homogeneous line broadening caused by shortening of the Nd3+ spin-lattice relaxation time (T2), because the linewidth ΔHpp ~ 1/T2. As a result, in the EPR spectra of all investigated glasses at temperatures T > 15 K are observed only intense sharp signal with geff  4.26 and relatively weak asymmetric complex signal near g = 2.0, which also are clearly observed at RT (see Fig. 2). According to [13,14,17-19,51-53] the characteristic EPR signal in oxide glasses with geff  4.26 belongs to the isolated Fe3+ (3d5, 6S5/2) ions of non-controlled iron impurity, localised in the octahedral and / or tetrahedral sites of glass network with a strong rhombic distortion. Presence of the Fe3+ EPR signal with geff  4.26 clearly demonstrates classical glass structure of the investigated materials. Asymmetric EPR signal near g = 2.0 is a superposition of two relatively weak lines with geff  2.00 and geff  2.01 (see Fig. 1b and Fig. 2). Most intense EPR signals with geff  2.00 and geff  2.01 have been observed in the Li2B4O7:Nd glass containing 1.0 mol. % Nd2O3 even at RT (Fig. 2). The EPR signal with geff  2.00 belongs to the Fe3+ isolated centres in glass network sites with nearly cubic local symmetry [51,53] and / or Fe3+ – Fe3+ pair centres, which are coupled by magnetic dipolar and exchange interactions [17,18]. 8

Based on the published data on EPR spectroscopy of the Sm-doped [22], Er-doped [24], and Cedoped [25] borate glasses one can supposed that the EPR signal with geff  2.01 most probably is related to the Nd3+ – Nd3+ pair centres and their small clusters, which are coupled by magnetic dipolar and exchange interactions in the glass network. It should be noted that the clustering of Nd3+ ions in the Nd2O3-doped SiO2 glasses was observed by pulsed echo-detected EPR techniques even at the lowest Nd2O3 doping levels [54]. So, the obtained results of EPR spectroscopy at low and room temperatures clearly show presence of the Nd3+ and Fe3+ single centres as well as Nd3+ – Nd3+ and Fe3+ – Fe3+ pair centres and their small clusters in the investigated Nd-doped borate glasses with low (0.5 mol.%) and high (1.0 mol.%) content of the Nd2O3 impurity.

3.2. Optical absorption and Judd–Ofelt analysis of Nd3+ centres in the borate glasses Optical absorption spectra of the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses are quite similar and characteristic for glassy compounds, activated with Nd3+ ions. Therefore, optical absorption spectrum of the Li2B4O7:Nd glass only will be detailed considered below. Typical absorption spectrum of the Li2B4O7:Nd glass in the UV-Vis-NIR spectral range at RT consists of nine unresolved and weakly-resolved absorption bands (Fig. 3), which according to [29-31] belong to the Nd3+ centres. The observed optical absorption spectra of Nd3+ ions in the Li2B4O7:Nd, LiCaBO3:Nd, and Ca2B4O7:Nd glasses are closely similar to the corresponding spectra of other Nd-doped borate glasses [33-39] as well as disordered oxide crystals and glasses with different compositions [5-12]. The linewidth and resolution of the Nd3+ absorption bands in the borate glasses practically were not changed at lowering temperature up to liquid nitrogen that is an evidence of the inhomogeneous broadening of spectral lines related to disordering of the glass structure. Observed absorption bands in the studied glasses were identified in accordance with the Nd3+ energy levels diagram [55,56] and were ascribed to the appropriate f – f

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transitions from the 4I9/2 ground state to the following excited states: (2P1/2, 2D(1)5/2), (2K15/2, 2G(1)9/2, 2

D(1)3/2, 4G11/2), (2K13/2, 4G7/2, 4G9/2), (4G5/2, 2G(1)7/2), 2H11/2, 4F9/2, (4F7/2, 4S3/2), (4F5/2, 2H(2)9/2), and 4F3/2,

which are denoted in Fig. 3. It should be noted that some complex and weakly-resolved Nd3+ bands in the Li2B4O7:Nd (see Fig. 3) as well as in the LiCaBO3:Nd and CaB4O7:Nd glasses were ascribed to groups of the corresponding f – f transitions. Characteristic absorption bands of the Nd2+ (4f4, 5I4) ions were not observed in optical absorption spectra of the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses. So, the optical absorption spectroscopy confirms incorporation of the Nd impurity into the network of investigated borate glasses in the Nd3+ valence state, exclusively. Transition energies and relative intensities of the Nd3+ absorption bands in all investigated glasses were calculated and analysed using the Judd–Ofelt theory [57,58]. The J–O calculations were carried out for all main absorption bands of the Nd3+ centres in glasses with Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd compositions containing 1.0 mol. % Nd2O3. Obtained from experimental spectra average values of maxima ( E ) of the observed absorption bands are presented in Tabs. 1 – 3. Using special computer program for J–O calculations, the oscillator strengths (ftheor) for all observed absorption transitions and phenomenological intensity parameters (2, 4, 6) for Nd3+ centres in the investigated glasses were obtained (see Tabs. 1 – 3). For comparison in Tabs. 1 – 3 are presented also the experimental oscillator strengths (fexp), which were calculated from average energies of maxima ( E ) of the experimental absorption bands using the following relation:

f exp ( J  J ) 

3ch2 J  1 , 8 3  e 2  ed

(6)

where J and J represent the total angular momentum of the initial and final states, respectively, c is the velocity of light; h is the Planck constant, e is the electron charge;  is the integrated absorption coefficient as a function of  (integral absorption band intensity),  is the concentration of Nd3+ ions in

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the sample,  – average wavelength value of the specific absorption band that corresponds to the J  J absorption transition, and

(n( ) 2  2) 2  ed  , 9n ( )

(7)

where n( ) is the dispersion of refractive index of the investigated glasses. As one can see from Tabs. 1 – 3, differences between calculated theoretical (ftheor) and experimental (fexp) oscillator strengths for all observed Nd3+ absorption bands in the Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd glasses are very small that shows good correlation of the experimental results with theoretical J–O calculations. The calculated J–O intensity parameters t (t = 2, 4, 6) for Nd3+ centres in the investigated glasses and some borate glasses with close chemical compositions, obtained by other authors are presented in Tab. 4. As one can see from Tab. 4, the corresponding t parameters for our glasses with Li2B4O7 (or 33.33Li2O + 66.66B2O3) and CaB4O7 (or 33.33CaO + 66.66B2O3) basic compositions and glasses with close compositions (30Li2O + 70B2O3 and 30CaO + 70B2O3, respectively) [59] show some differences. The corresponding t parameters, obtained in this work for Nd3+ centres in the Li2B4O7, CaB4O7, and LiCaBO3 glasses also show some differences (see Tab. 4). The J–O intensity parameters, obtained in [59] for Nd3+ centres in borate glasses with different modifiers (30Na2O + 70B2O3 and 30K2O + 70B2O3) are close to the t parameters for Nd3+ centres in glasses with 30Li2O + 70B2O3 and 30CaO + 70B2O3 compositions [35], but adding the Li2O modifier oxide to these glasses leads to essential (in 2 – 4 times) increasing of the corresponding t parameters (see Tab. 4). The observed differences in J–O intensity parameters are related to different local environment of the Nd3+ centres in the studied by us borate glasses as well as in other borate glasses, in particular glasses studied in [35,59].

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The Ω2 parameter characterised distortion and asymmetry of the local structure (coordination polyhedron) and degree of covalency of bonds with ligands for RE3+ impurity centres, whereas the Ω4 is related to the rigidity of host [39,60,61]. The Ω6 parameter according to [39,60,61] is related to covalency of RE3+ – O2 bonds and decreases with increasing covalency (or reciprocal value (1/Ω6) indicates the ionicity of bonds between RE3+ ions and ligands). As one can see from Tab. 4, the Ω4 in the Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd glasses with the same Nd content practically is independent of the basic glass composition. Relatively large differences between 2 and 6 parameters in the investigated glasses (see Tab. 4) show essential different distortion of the local environment for Nd3+ centres in the structural network of these glasses. Among the investigated borate glasses the largest 2 parameter was observed in the Li2B4O7:Nd glass (Tab. 4) that indicates the lowest symmetry of structural polyhedra coordinated by oxygen and the highest covalency of the ion-ligand bonds for Nd3+ centres. The highest covalency of bonds of the Nd3+ ions with ligands in the Li2B4O7:Nd glass also shows lowest value of 1/6 for this glass. Large values of the 2 parameter for Nd3+ centres in the Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd glasses show strong distortion their local environment that correlates with the EPR spectroscopy data for Nd3+ single centres in these glasses, which are described in the Subsection 3.1.

3.3. The local structure of Nd3+ centres in the investigated borate glasses Let us to consider the local structure of the Nd3+ centres in the network of glasses with Li2B4O7, CaB4O7, and LiCaBO3 compositions. By direct EXAFS (Extended X-ray Absorption Fine Structure) investigation of the L3-edge of rare-earth impurity ions in [62] it was shown that the local structure (first coordination shell) of rare-earth impurities in oxide crystals and glasses with the same composition, particularly in the crystal and glass with Ca3Ga2Ge3O12 (3CaO–Ga2O3–3GeO2) composition, is closely

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similar. Therefore, the local structure of the Nd3+ luminescence centres in the network of investigated borate glasses is considered based on the analysis of structural data for Li 2B4O7, CaB4O7, and LiCaBO3 glasses [22,23] and their crystalline analogues [63-65]. In structure of the Li2B4O7 single crystal the B3+ cations occupy threefold- and fourfold-coordinated (B1 and B2) sites with average B3+ – O2 distances equal 1.373 and 1.477 Å, respectively, whereas the Li+ cations are located in distorted fourfold-coordinated tetrahedral L1 sites with Li+ – O2 distances, which lie in the range of 1.97 ÷ 2.14 Å [61] (see Fig. 4). The number of nearest oxygen (O2) anions (coordination number to oxygen N) equals 5, 6, and 7 for Li+ – O2 distances 2.63, 2.85, and 2.88 Å, respectively [63]. Statistical distribution of the Li+ – O2 distances for different coordination numbers (N = 4  7) leads to so-called “positional disorder” in the lattice of Li2B4O7 crystal. According to structural data for Li2B4O7 crystal [63], the Nd3+ impurity ions are expected to incorporation into the Li1 sites (Fig. 4) due to extremely small ionic radius of the B3+ ions (0.23 Å) and close Li+ and Nd3+ ionic radii, which equal approximately 0.76 Å and 0.995 Å, respectively. The average Li+ – O2 distance, obtained by XRD in the Li2B4O7 glass equals 2.79 Å [47]. Based on the structural XRD data for Li2B4O7 glass [47] and crystal [63], supported by MAS NMR (Magic Angle Spinning Nuclear Magnetic Resonance) spectroscopy [66,67] was suggested that the Nd3+ ions can be located in the Li1 sites of the Li2B4O7 glass network (see Fig. 4). This suggestion was directly confirmed by EXAFS spectroscopy [31]. Particularly, by EXAFS spectroscopy in the fluorescence mode in [31] it was shown that impurity trivalent rare-earth (RE3+) ions (RE3+ = Nd3+, Gd3+, Dy3+, and Er3+) are located in the Li+ sites of the Li2B4O7 glass network. In [31] were obtained the following structural parameters of the Nd3+ impurity centres in the Li2B4O7 glass network: Nd3+– O2 distance, R = (2.47 ± 0.03) Å, coordination numbers to oxygen, N = (8.32 ± 0.58). It should be noted that the structural parameters for Li2B4O7:Nd glass,

13

obtained in [31] by EXAFS spectroscopy, show good correlation with corresponding XRD data for undoped Li2B4O7 glass [47] and its crystalline analogue [63]. For analysis of the local structure of cationic sites in the CaB4O7 glass were used structural data for isomorphic SrB4O7 crystal [64] glass [47], as well as for Sm-doped CaB4O7 glass [22], because the structure of un-doped CaB4O7 glass were not investigated up to now. Based on the structural data for undoped SrB4O7 glass [47] and crystal [64], we can state that in the structure of CaB4O7 glass exist BO4 tetrahedral groups with an average distance B3+– O2 equals about 1.40 Å. Two types of the BO4 tetrahedra with close B3+– O2 distances, which are characteristic for the SrB4O7 crystal lattice [64], are unresolved in the intensity curve and pair correlation function of the SrB4O7 glass [47]. The Sr2+ – O2 distances in SrB4O7 crystal lie in the range of (2.73 ÷ 3.39) Å [64]. The coordination number to oxygen for Sr2+ cations depends on the Sr2+– O2 distances and N = 4 when the radius of the first coordination shell is less than 2.90 Å [64]. So, according to [47,64] one can conclude that the Sr2+(Ca2+) cations are stabilised in the SrB4O7 (CaB4O7) glass network in sites with a coordination number to oxygen N = 4 and average distance Sr2+– O2 (Ca2+– O2) that equals to about 2.62 Å. Presented above results of the local structure analysis for isomorphic SrB4O7 and CaB4O7 glasses show good correlation with corresponding XRD data for Sm-doped CaB4O7 glass [22]. According to [22] the CaB4O7:Sm glass contains BO4 tetrahedra with average distance B3+ – O2 that equals to about 1.44 Å. The Ca2+ modifier cations are stabilised in the CaB4O7:Sm glass network at structural sites with coordination number to oxygen N = 4 and average distance Ca2+– O2 that equals to 2.54 Å. Based on the analysis of XRD [22,47,64] and MAS NMR [66,67] data, we can assumed that the Nd3+ and other trivalent rare-earth ions are located in the Ca2+ tetrahedral sites of the CaB4O7 glass network. In structure of the LiCaBO3 crystal the B3+ cations are coordinated by 3 oxygen, O2, anions with distances B3+ – O2, which are equal to 1.3699, 1.3844, and 1.3835 Å [65]. The Ca2+ cations are 14

coordinated by 7 O2 anions with distances Ca2+ – O2, which lie between 2.3606 and 2.5122 Å, whereas the Li+ cations are coordinated by 5 O2 anions with distances Li+ – O2, which lie between 2.004 and 2.226 Å [65]. On the basis of the XRD data for LiCaBO3 glass [47] and LiCaBO3 crystal [65] we can inferred that boron in the LiCaBO3 glass network form triangular BO3 units with average distance B3+ – O2 that equals 1.49 Å. The Ca2+ cations are located in sites with coordination numbers N = 6  7 and average distance Ca2+ – O2 that equals 2.58 Å, whereas the Li+ cations are located in sites with coordination numbers N = 4  5 and average distance Li+ – O2 that is less than 2.58 Å. The obtained average distances B3+ – O2, Ca2+ – O2, and Li+ – O2 and coordination numbers to oxygen in the LiCaBO3 glass structure show good correlation with corresponding distances and coordination numbers in its crystalline analogue [65] and Sm-doped LiCaBO3 glass [22]. Based on the analysis of XRD data [22,47,65], which are supported by MAS NMR [66,67] spectroscopy, we can assumed that the Nd3+ and other trivalent rare-earth ions are located in sites of the glass modifier cations (Li+ and Ca2+) of the LiCaBO3 glass network. Based on the presented above XRD data for investigated borate glasses [22,47] and their crystalline analogues [63-65] as well as results of MAS NMR [66,67] and EXAFS [31] spectroscopy we can state that the Nd3+ impurity ions are located in sites of the Li+ (Ca2+) modifiers of the Li2B4O7, CaB4O7, and LiCaBO3 glasses. This result correlates with ionic radii of the Nd3+ ( 0.995 Å), Li+ ( 0.76 Å), and Ca2+ ( 0.99 Å) modifiers in the investigated glasses. Compensation of the excess positive charge at heterovalence substitutions Nd3+ → Li+ in the Li2B4O7 glass and Nd3+ → Ca2+ (Ca2+, Li+) in the CaB4O7 (LiCaBO3) glasses can be realised by cationic vacancies, (VLi) and (VCa)2, which are presented in the structure of borate compounds [68].

15

3.4. Luminescence spectra, decay kinetics, and spectroscopic parameters of relevance to laser applications of the Li2B4O7:Nd3+, LiCaBO3:Nd3+, and CaB4O7:Nd3+ glasses

The luminescence (emission and excitation) spectra and luminescence decay curves of the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses are similar. Therefore, below are detailed considered luminescence spectra and decay curves for Li2B4O7:Nd glasses, only. In luminescence emission spectra of the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses, recorded at RT in the 800  1450 nm spectral range are clearly observed three characteristic bands of the Nd3+ centres (Fig. 5). The emission spectra of the Li2B4O7:Nd glasses containing 0.5 and 1.0 mol. % Nd2O3, which are presented in Fig. 5 (curves a, b), were registered under excitation at exc = 585 nm that corresponds to the 4I9/2  (2K13/2, 4G7/2, 4G9/2) transitions of the Nd3+ centres in their optical absorption spectrum (Fig. 3). The Nd3+ emission spectra in the Li2B4O7:Nd glasses (Fig. 5) consist of three bands, which correspond to the following f – f transitions: 4F3/2  4I9/2 (max  905 nm), 4F3/2  4I11/2 (max  1060 nm), and 4F3/2  4I13/2 (max  1330 nm). In the relatively broad emission bands of the Nd3+ centres in the investigated borate glasses has been observed weakly-resolved Stark structure (see Fig. 5). The measured emission spectra of the investigated borate glasses were used to estimate the emission cross-sections, according to FüchtbauerLadenburg formula:

 em ( ) 

  5 I ( ) 8 n 2 c rad   I ( ) d

(8)

where I() represents the experimental emission intensity at the wavelength , c is the light velocity, n,

, and rad are the refractive index of material, branching ratio, and radiative lifetime of the 4F3/2 multiplet, respectively. Calculated peak values of the emission cross-section (em) for Li2B4O7:Nd3+, CaB4O7:Nd3+, and LiCaBO3:Nd3+ glasses are presented in Tab. 5. The maximal emission cross-section 16

em= 2.71×10-20 cm2 is obtained for Li2B4O7:Nd3+glass. The obtained emission cross-sections for investigated glasses (see Tab. 5) are slightly lower than the emission cross-section em= 3.25×10-20 cm2 reported for Nd-doped APG-1 laser phosphate glass in [69]. Laser peak wavelengths (P) and effective linewidths (eff) have been found for all investigated glasses and are presented in Tab. 5. For the comparison eff = 29 nm was estimated for Nd-doped NAP-4 laser phosphate glass [69]. The luminescence excitation spectrum of the Li2B4O7:Nd3+ at RT consists of nine weakly-resolved and unresolved bands, located in the 340  900 nm spectral range (Fig. 6), which show good correlation with corresponding optical absorption bands (Fig. 3). It should be noted that bands, which correspond to the 4I9/2  2P3/2 and 4I9/2  2H11/2 transitions of the Nd3+ centres, only weakly reveal in the optical absorption (Fig. 3) and luminescence excitation (Fig. 6) spectra of the Li2B4O7:Nd glass containing 1.0 mol. % Nd2O3 as well as in the corresponding spectra of LiCaBO3:Nd and CaB4O7:Nd glasses. Weak bands, which correspond to the 4I9/2  2P3/2 and 4I9/2  (2I11/2, 4D5/2, 4D3/2) transitions of the Nd3+ centres, were not observed in the absorption (Fig. 3) and luminescence excitation (Fig. 6) spectra of the Li2B4O7:Nd glass and other investigated borate glasses. The measured luminescence decay curves of the Nd3+ centres in the Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses for the same emission bands are characterised by different lifetimes. In Fig. 7 (a, b) are presented the results of luminescence kinetics measurements and analysis using semilogarithmic scale for 4F3/2  4I11/2 transition of the Nd3+ centres in the Li2B4O7:Nd glasses. The experimental Nd3+ luminescence decay curves were satisfactory described in the framework of a single exponential approximation with lifetime values at RT exp = (78 ± 3) s and exp = (72 ± 3) s for Li2B4O7:Nd glasses containing 0.5 and 1.0 mol. % Nd2O3, respectively (Fig. 7 a, b). The luminescence decay curves for 4F3/2  4I11/2 transition of the Nd3+ centres in the LiCaBO3:Nd and CaB4O7:Nd glasses also were described by a single exponential function with the following lifetimes at RT: exp = (110 ± 3) s and 17

exp = (115 ± 3) s for LiCaBO3:Nd glasses containing 0.5 and 1.0 mol. % Nd2O3; exp = (52 ± 3) s and exp = (55 ± 3) s for CaB4O7:Nd glasses containing 0.5 and 1.0 mol. % Nd2O3, respectively. It is worth to noticing that 4F3/2 experimental lifetime (exp) of neodymium is frequently shortened in the borate glasses owing to specific structural and vibrational features of these optical materials. For instance, the 4F3/2 experimental lifetime equals to 62 µs was measured for BZB:1%Nd borate glass [70] and exp = 45 µs was found for bismuth borate glass [71]. It should be noted that the lowering of Nd3+ lifetime value in the Li2B4O7:Nd glass containing 1.0 mol. % Nd2O3 in comparison with Nd3+ lifetime, obtained for the Li2B4O7:Nd glass containing 0.5 mol. % Nd2O3 can be related to influence of the Nd3+ – Nd3+ exchange interaction [72] that takes place at higher concentration of the Nd3+ centres. The lowering of lifetime values for Nd3+ centres in the Li2B4O7:Nd glass with increasing of the Nd2O3 content (Fig. 7 a, b) shows good correlation with highest intensity of EPR signal of the Nd3+ – Nd3+ pair centres in the Li2B4O7:Nd glass that contains 1.0 mol. % Nd2O3 (see Section 3.1 and Fig. 2). So, the luminescence kinetics confirms presence of the Nd3+ – Nd3+ pair centres in the Li2B4O7:Nd glasses. The Nd3+ lifetime values in the LiCaBO3:Nd and CaB4O7:Nd glasses practically are independent (in the framework of experimental errors) of Nd2O3 content in the 0.5 ÷ 1.0 mol. % range that indicates negligible exchange interaction between Nd3+ centres in these glasses. Obtained results of luminescence kinetics in the Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd glasses show that the lifetime of Nd3+ centres strongly depends on the basic glass composition, but weakly depends on the Nd3+ concentrations. Various lifetimes in the investigated glasses with different basic composition, which contain the same amount of Nd2O3, are caused by differences in the local structure of Nd3+ luminescence centres in the borate glass network, because the luminescence lifetime is very sensitive to local environments.

18

Using obtained 2, 4, and 6 intensity parameters, the radiative transition rates (Wr), branching ratios () for 4F3/2  4I9/2, 4F3/2  4I11/2, 4F3/2  4I13/2, and 4F3/2  4I15/2 electric dipole transitions and radiative lifetime (rad) for 4F3/2 metastable level of the Nd3+ centres in Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd glasses have been calculated by standard formulas according to [24]. All calculated spectroscopic parameters of relevance to laser application for Nd3+ centres in the investigated glasses are presented in Tab. 5. The experimental lifetimes (exp) for Nd3+ centres in all investigated borate glasses, which were obtained from the corresponding luminescence decay curves, are considerably shorter than the calculated radiative lifetimes (τrad), presented in Tab. 5. The quantum efficiency of the emitting level that is defined as ratio of the experimental lifetime to the predicted radiative lifetime (QE = τexp/τrad) describes local (internal efficiency) and shows the emission probability of luminescence centres. The calculated quantum efficiencies of the 4F3/2 emitting level of Nd3+ centres for Li2B4O7:Nd, LiCaBO3:Nd, and CaB4O7:Nd glasses are presented in Tab. 5. As one can see from Tab. 5 the maximal quantum efficiency (  31 %) was obtained for LiCaBO3:Nd glass, containing 1.0 mol. % Nd2O3. For the comparison the quite low quantum efficiency 26 % was found for ZnBiB:Nd borate glass [73] and   30 % was estimated for BBT:Nd borate glass [74]. Actually, estimated quantum efficiency of Nd3+ emission in investigated glasses is not high but it should be keeping in mind that relaxation dynamic of 4F3/2 excited state in borate materials may depends on nonradiative processes and energy transfer phenomena. The 4F3/2 – 4I15/2 energy gap is relatively large but highest phonon energy in borate materials is close to 1400 cm-1 and consequently multiphonon emission not may be negligible. Moreover, systematic studies of rare-earth ions in different borate hosts reveal that other effects like the electron-phonon coupling phenomenon and the feature of phonon spectrum significantly affect the rate of multiphonon relaxation. On the other hand the estimated 4F3/2 radiative lifetimes depend on incertitude of the Judd–Ofelt calculations. 19

4.

Conclusions In the article are presented and analysed detailed results of spectroscopic investigations of the Nd3+

luminescence centres in borate glasses with Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd compositions. Optical absorption, luminescence (emission and excitation) spectra as well as luminescence kinetics and basic spectroscopic parameters of the Nd3+ centres of relevance to laser application in the investigated glasses are discussed in comparison with corresponding data for other borate glasses with close chemical compositions. The local structure of Nd3+ luminescence centres in the Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd glasses and its influence on the Nd3+ spectroscopic properties also has been considered and discussed. Based on the obtained results and their analysis it were concluded the following: 

The neodymium impurity is incorporated into the network of investigated borate glasses as Nd3+ (4f3, 4I9/2) ions, exclusively, because the characteristic optical spectra of the Nd2+ (4f4, 4I5) ions were not registered in all investigated glass samples.

 Optical absorption and luminescence (excitation and emission) spectra of the Nd3+ centres in glasses with Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd compositions are quite similar to corresponding Nd3+ optical spectra, which are observed in other borate glasses and disordered crystals and characterised by inhomogeneous broadening of spectral lines. All observed UV, visible, and IR transitions of the Nd3+ centres in optical absorption, luminescence (excitation and emission) spectra were identified.  The Nd3+ optical absorption spectra were analysed in the framework of standard Judd–Ofelt theory. The oscillator strengths (ftheor) and phenomenological J–O intensity parameters t (2, 4, 6) of the Nd3+ centres in for Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd glasses were determined and

20

compared with corresponding t parameters, obtained for other borate glasses and crystals with close chemical compositions.  Using analysis of the X-ray diffraction data for Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd glasses and corresponding their crystalline analogues it was shown that the Nd3+ impurity ions are located in the Li (Ca) - sites, which are coordinated by positionally-disordered O2 anions in the glass network. The local structure of Nd3+ centres in the borate glasses is confirmed by EXAFS spectroscopy. Charge compensation mechanism for Nd3+  Li+ (Ca2+) heterovalence substitutions is related to presence of cationic vacancies, (VLi) and (VCa)2, in the borate glasses.  The multisite character of Nd3+ luminescence in the Li2B4O7:Nd, CaB4O7:Nd, and LiCaBO3:Nd glasses is related to presence a number of the Nd3+ centres in the Li (Ca) - sites with different statistically-distributed Nd3+ – O2 distances. Such positional disorder leads to distribution of the Nd3+ spectroscopic parameters and is reveals as inhomogeneous broadening of their spectral lines.  The experimental luminescence decay curves of the Nd3+ centres for 4F3/2  4I11/2 transition, registered at T = 300 K, were satisfactory described by single exponential function with corresponding lifetime values, which were determined for all investigated glasses containing 0.5 and 1.0 mol. % Nd2O3. The spectroscopic parameters of relevance to laser application (radiative transition rates (Wr), branching ratios (β), laser peak wavelengths (P), effective linewidths (eff), emission cross-section (em), radiative lifetime (τrad), and quantum efficiency (η)) for Nd3+ centres in the investigated glasses have been obtained and analysed in comparison with corresponding published data for other borate glasses and crystals, activated with Nd3+ ions.  The obtained results show that the investigated borate glasses can be considered as promising materials for solid state Nd3+-lasers with LED pumping, but further researches of the Nd-doped borate glasses are needed for improvement their parameters relevant to laser application. 21

Acknowledgements. Authors would like to thank Prof. N. Guskos and Dr. G. Żołnierkiewicz from Institute of Physics of the West Pomeranian University of Technology (Szczecin, Poland) for EPR spectra registration at low temperatures. This work was financially supported by Vlokh Institute of Physical Optics (Lviv, Ukraine) in the framework of scientific research project No. 0116U002578 of the Ministry of Education and Science of Ukraine.

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29

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30

Table 1. Calculated experimental (fexp) and theoretical (ftheor) oscillator strengths as well as their differences for Nd3+ centres in the Li2B4O7:Nd glass containing 1.0 mol. % Nd2O3. Absorption

Average energy

Experimental

Theoretical

Difference,

transitions

of transitions,

oscillator strength,

oscillator strength,

Δf = |fexp – ftheor|

from 4I9/2 to:

E [cm–1]

fexp (10–6)

ftheor (10–6)

(10–6)

4

F3/2

11295

2.65

2.69

0.04

4

F5/2+2H(2)9/2

12406

8.06

8.82

0.76

4

F7/2+4S3/2

13379

9.09

9.09

0.00

4

F9/2

14624

0.62

0.63

0.01

2

H11/2

15902

0.24

0.20

0.04

4

G5/2+2G(1)7/2

17029

27.00

27.00

0.00

2

K13/2+4G7/2+4G9/2

19229

8.06

7.07

0.99

Table 2. Calculated experimental (fexp) and theoretical (ftheor) oscillator strengths as well as their differences for Nd3+ centres in the LiCaBO3:Nd glass containing 1.0 mol. % Nd2O3. Average energy Absorption transitions from 4I9/2 to levels:

Experimental

of transitions, oscillator strength,

E [cm–1]

fexp (10–6)

Theoretical oscillator strength, –6

ftheor (10 )

Difference, Δf = |fexp – ftheor| (10–6)

4

F3/2

11414

2.35

2.47

0.12

4

F5/2+2H(2)9/2

12426

7.62

7.13

0.49

4

F7/2+4S3/2

13400

7.04

7.04

0.00

4

F9/2

14648

0.54

0.56

0.02

2

H11/2

15913

0.26

0.18

0.08

4

G5/2+2G(1)7/2

17136

16.68

16.70

0.02

2

K13/2+4G7/2+4G9/2

19087

5.72

5.58

0.14

2

K15/2+2G(1)9/2+2D(1)3/2+4G11/2

21333

1.53

1.31

0.22

2

P1/2

23184

0.40

0.52

0.12

31

Table 3. Calculated experimental (fexp) and theoretical (ftheor) oscillator strengths as well as their differences for Nd3+ centres in the CaB4O7:Nd glass containing 1.0 mol. % Nd2O3. Average energy Absorption transitions from 4I9/2 to:

of transitions,

Theoretical

Experimental

oscillator

oscillator strength,

E [cm–1]

fexp (10–6)

strength,

Difference, Δf = |fexp – ftheor|

ftheor (10–6)

(10–6)

4

F3/2

11452

2.64

2.64

0.00

4

F5/2+2H(2)9/2

12476

7.29

7.67

0.16

4

F7/2+4S3/2

13455

7.56

7.56

0.38

4

F9/2

14709

0.69

0.61

0.08

2

H11/2

15972

0.28

0.22

0.06

4

G5/2+2G(1)7/2

17204

21.90

21.90

0.00

2

K13/2+4G7/2+4G9/2

19134

6.55

6.27

0.28

2

K15/2+2G(1)9/2+2D(1)3/2+4G11/2

21351

1.29

1.40

0.11

2

P1/2

23268

7.08

6.98

0.10

Table 4. The Judd-Ofelt intensity parameters (t [10–20 cm2]) for Nd3+ centres in the investigated glasses containing 1.0 mol. % Nd2O3 and some other borate glasses with close chemical compositions. Glass basic composition

2

4

6

RMS

References

Li2B4O7 (33.33Li2O+66.66B2O3) 30Li2O+70B2O3

6.92

5.42

6.27

6.2610–7

This work

4.20

3.89

4.74

−

[59]

CaB4O7 (33.33CaO+66.66B2O3) 30CaO+70B2O3

4.87

5.46

5.31

2.0610–7

This work

4.4

3.7

4.6

−

[59]

LiCaBO3 (25Li2O+50CaO+25B2O3) 30Na2O+70B2O3

3.08

5.17

5.01

2.3910–7

This work

4.91

3.28

4.51

−

[59]

12Li2O+20Na2O+67B2O3

9.59±0.92

7.87±0.72

9.84±0.98

−

[35]

30K2O+70B2O3

4.94

3.10

3.42

−

[59]

24Li2O+8K2O+67B2O3

11.52±1.06

9.55±0.91

13.62±1.25

−

[35]

32

Table 5. Spectroscopic parameters of relevance to laser application for Nd3+ centres in the investigated borate glasses containing 1.0 mol. % Nd2O3.

Glass

Transition

Wr

composition

from 4F3/2 to:

[s–1]

4

Li2B4O7:Nd3

λP

eff

em

rad



[nm]

[nm]

[10-20 cm2]

[s]



1061

39

2.71

322

24

1063

37

2.42

366

31

1065

36

2.34

335

16



I9/2

1347

0.43

4

I11/2

1455

0.47

4

I13/2

288

0.09

4

I15/2

15

0.01

I9/2

1253

0.46

4

I11/2

1228

0.45

4

I13/2

237

0.09

4

I15/2

12

0.00

I9/2

1365

0.46

4

I11/2

1344

0.45

4

I13/2

260

0.09

4

I15/2

13

0.00

+

4

LiCaBO3:Nd 3+

4

CaB4O7:Nd3+

33

CAPTIONS FOR FIGURES Fig. 1. The X-band EPR spectra of the CaB4O7:Nd glass containing 1.0 mol. % Nd2O3. (a) EPR spectra recorded at different temperatures in the 4  50 K range and scan of magnetic field in the 200  2800 G range; (b) EPR spectrum recorded at T = 4 K and scan of magnetic field in the 0  7000 G range. Fig. 2. The X-band EPR spectrum of the Li2B4O7:Nd glass containing 1.0 mol. % Nd2O3 recorded at room temperature (T = 295 K). Fig. 3. Optical absorption spectrum of the Nd3+ centres in Li2B4O7:Nd glass containing 1.0 mol. % Nd2O3, recorded at room temperature. Fig. 4. Proposed model of the local environment for Nd3+ impurity centres in the Li2B4O7 glass network. For Li1 sites, which occupies by Nd3+ ions are shown only 4 oxygen atoms (O1 – O4) of the first coordination sphere, limited by r ≤ 2.14 Å [63]. Fig. 5. Emission spectra of the Nd3+ centres in Li2B4O7:Nd glass containing 0.5 mol. % (a) and 1.0 mol. % (b) Nd2O3, recorded under excitation with exc = 585 nm (4I9/2  4G5/2, 2G(1)7/2 absorption transition) at room temperature. Fig. 6. Luminescence excitation spectrum of the Nd3+ centres in Li2B4O7:Nd glass containing 1.0 mol. % Nd2O3, monitored at mon = 1065 nm (4F3/2  4I11/2 emission transition) and recorded at room temperature. Fig. 7. Luminescence decay curves, presented in semilogarithmic scale for Nd3+ centres (4F3/2  4I11/2 emission transition, max = 1069 nm), registered at room temperature in the Li2B4O7:Nd glasses containing 0.5 mol. % (a) and 1.0 mol. % (b) Nd2O3. Empty circles show the experimental data, black solid lines present the results of single exponential fitting.

34

Fig. 1, a

EPR intensity, d"/dH [arb. unis]

40000

CaB4O7:Nd glass

4K 8.1K 15.1K 25.9K 40.3K 49.6K

1.0 mol. % Nd2O3

30000

X-band (= 9.459 GHz)

20000

Nd

10000

3+

Fe

3+

(geff 4.26)

(geff .00)

0 -10000 -20000 0

500

1000

1500

2000

2500

Magnetic field, H [G] Fig. 1, b

EPR intensity, d"/dH [arb. units]

40000

CaB4O7:Nd glass 1.0 mol. % Nd2O3

30000

X-band (= 9.459 GHz) T=4K

20000

Nd

10000

3+

(geff 6.00)

3+

3+

3+

3+

3+

Nd - Nd (geff 4.26) (geff 2.01) Fe

0

-10000

Fe - Fe (geff 2.00)

-20000 0

1000

2000

3000

4000

5000

Magnetic field, H [G] 35

6000

7000

Fig. 2 Li2B4O7:Nd glass, 1.0 mol. % Nd2O3 X-band, T = 295 K

EPR intensity, d"/dB [arb. units]

600

400

Fe

200

3+

3+

3+

Fe - Fe (geff 2.00)

(geff 4.26) 0 3+

3+

Nd - Nd (geff 2.01)

-200

-400 100

150

200

250

300

350

400

450

Magnetic field, B [mT]

Fig. 3 

3

-1

Wavenumber,  [10 cm ]

4

H9/2, F5/2

20

-1

4

4

4

2

S3/2, F7/2

H9/2, F5/2

4 2

2

G9/2, G7/2, K13/2

2 4

2 2 2

2

D5/2, P1/2

1,5

4

G11/2, D3/2, G9/2, K15/2

2,0

G7/2, G5/2

-1

2,5

2

1,0

H11/2

4

4

F9/2

F3/2

0,5

4

400

500

600

700

800

900

Wavelength,  [nm] 4

1,0

4

F9/2

F3/2

0,5

2

2

D5/2, P1/2

3

15

3,0

2

4

S3/2, F7/2



Wavenumber,  [10 cm ] 25

4

4 2 2

G9/2, G7/2, K13/2

2

5

4

4

1,5

4

2,0

2

2

2,5

G7/2, G5/2

3,0

10

Li2B4O7:Nd glass 1.0 mol. % Nd2O3 T = 295 K

G11/2, D3/2, G9/2, K15/2

-1

Absorption coefficient,  [cm ]

3,5

15

Absorption coefficient,  [cm ]

25 20

500

2

H11/2

750

1000

1250

1500

Wavelength,  [nm]

36

1750

2000

Fig. 4

37

Fig. 5

Luminescence intensity [arb. units]

800

4

4

F3/2

I11/2

Li2B4O7:Nd glass, emission (a) 0.5 mol. % Nd2O3 (b) 1.0 mol. % Nd2O3 exc = 585 nm, T = 295 K

700 600 500 400 300

4

200 4

100

4

F3/2

F3/2

4

I13/2

I9/2 (b) (a)

0 900

1000

1100

1200

1300

1400

Wavelength,  [nm] Fig. 6 

3

-1

Wavenumber,  [10 cm ] 20

15

Li2B4O7:Nd glass, 1.0 mol. % Nd2O3 Excitation, mon = 1059 nm, T = 295 K

4

25

G7/2, G5/2

30

2

400

35

4

H11/2

4

F9/2

2

4

2

2

4

2

D5/2, P1/2

100

H9/2, F5/2

4

S3/2, F7/2

4

2

4

2

200

G11/2, D3/2, G9/2, K15/2

2

G9/2, G7/2, K13/2

300

2

Luminescence intensity [arb. units]

40

0 300

400

500

600

Wavelength,  [nm]

38

700

800

4

F3/2

Luminescence intensity [arb. units]

Fig. 7, a

Li2B4O7:Nd glass, 0.5 mol. % Nd2O3

2

e

4

4

F3/2 I11/2 transition (em = 1059 nm) exc = 585 nm, T = 295 K

1

e

0

e

-1

e

 = 78 s

-2

e

-3

e

-4

e

0

50

100

150

200

250

300

350

Time, t [s]

Luminescence intensity [arb. units]

Fig. 7, b

Li2B4O7:Nd glass, 1.0 mol. % Nd2O3

2

e

4

4

F3/2 I11/2 transition (em = 1059 nm) exc = 585 nm, T = 295 K

1

e

0

e

-1

e

 = 72 s -2

e

-3

e

-4

e

0

50

100

150

200

Time, t [s]

39

250

300

350