H. WEN et al.: THERMAL, OPTICAL CHARACTERIZATION AND JUDD-OFELT ANALYSIS OF Nd 3+_ DOPED ... 305–309 THERMAL, OPTICAL CHARACTERIZATION AND JUDD-OFELT ANALYSIS OF Nd 3+_ DOPED BaO-TeO 2 -B 2 O 3 GLASSES TERMI^NO-OPTI^NA KARAKTERIZACIJA IN JUDD-OFELTOVA ANALIZA BaO-TeO 2 -B 2 O 3 STEKEL, DOPIRANIH Z Nd 3+ Hang Wen, Junying Zhang, Qian Yao, Lintao Liu, Weimin Dong, Jing Li * , Jiyang Wang State Key Laboratory of Crystal Materials, Shandong University, no. 27 Shandanan Road, Licheng District, Jinan 250 100, Shandong, China Prejem rokopisa – received: 2018-03-08; sprejem za objavo – accepted for publication: 2018-12-06 doi: 10.17222/mit.2018.037 Barium-tellurite-borate glasses doped with Nd 3+ ions were prepared by the melt-quench technique. The amorphous nature of the glasses was confirmed by XRD. The thermal stability of the borate glass was calculated from DTA profiles and TG. The absorption and the luminescence spectra of the glass were measured at room temperature. The spectrum properties of the glass were carried out by using Judd-Ofelt (J-O) theory, the intensity parameters k (k= 2, 4, 6) and radiative lifetimes were also calculated. The absorption band at 804 nm has a full-width at half-maximum of 18 nm. The fluorescence spectra revel three major bands. The strongest fluorescence spectrum-emission peak is located at 1062 nm, which can be attributed to the 4 F3/2– 4 I11/2 of the emission band of Nd 3+ , and the full-width at half-maximum is 29 nm. These properties show that the BaO-TeO2-B2O3 glass doped with Nd 3+ ions is expected to be a better laser glass. Keywords: barium-tellurite-borate glasses, Judd-Ofelt theory, thermal properties Avtorji so barij-teluritno-boratna stekla, dopirana s kationi Nd 3+ , pripravljali s talilno-kalilno tehniko. Amorfno naravo stekel so potrdili z rentgensko strukturno analizo (XRD). Termi~no stabilnost boratnega stekla so izra~unali iz profila, dobljenega z diferencialno termi~no analizo (DTA) in termo gravimetrijo (TG). Absorpcijske in luminiscen~ne spektre stekla so dolo~ili pri sobni temperaturi. Spektralne lastnosti stekla so dolo~ili z uporabo Judd-Ofeltove (J-O) teorije. Izra~unali so tudi intenzitetne parametre k (k= 2, 4, 6) in `ivljenjsko dobo radiacije. Absorpcijski pas pri 804 nm ima polno {irino pri polovi~nem maksi- mumu 18 nm. Fluorescen~ni spektri imajo tri glavne pasove. Najmo~nej{i vrh fluorescen~ne emisije je lociran pri 1062 nm, ki so ga pripisali 4 F3/2– 4 I11/2 emisijskemu pasu Nd 3+ , in polna {irina polovi~nega maksimuma je pri 29 nm. Te lastnosti pri~akovano ka`ejo, da so BaO-TeO2-B2O3 stekla, dopirana s kationi Nd 3+ , bolj{a laserska stekla. Klju~ne besede: barij-teluritno-boratna stekla, Judd-Ofeltova teorija, termi~ne lastnosti 1 INTRODUCTION Over the past several years, a great deal of work has been carried out by performing detailed analyses on rare-earth-ions-doped glassy materials, because of their wide range of applications in optoelectronic devices, lasers and sensors, and colour display devices. 1,2 Borate glass is an attractive host matrix for rare-earth ions because of its high transparency, high thermal stability, different coordination numbers and the good solubility of the rare-earth ions. 3 TeO 2 -doped oxide glass is a good-quality material owing to a high linear and non-linear refractive index, low photon energies, and the good solubility of rare earth ions. 4 Tellurium dioxide and borate are appropriate oxide glass hosts due to the advantages mentioned above, but the chemical stability of the glass host is relatively poor. 5 Also, a small amount of alkali metal oxide in the host can improve the thermal stability of the glass matrix and strengthen the physical properties. As tellurium dioxide and borate are suitable oxide glass hosts, they have been widely studied and have a large variety of applications. Extensive work has been carried out on rare-earth-ions-doped tellurite and borate glasses, such as Ho 3+ ions in the B 2 O 3 -TeO 2 -ZnO-Na 2 O glass system, 6 Dy 3+ ions in the B 2 O 3 -PbO 2 -MgF 2 -NaCl glass system and LiO 2 -B 2 O 3 glasses doped with Nd 3+ . 3,7 In the present work, the new glass system was prepared using the melt-quench technique. The abs- orption spectrum and fluorescence spectrum of the glass were measured at room temperature. J-O theory was applied to analyse the spectrum properties of the Nd 3+ in the BaO-TeO 2 -B 2 O 3 glass. 2 EXPERIMENTAL PART Glasses with varying compositions of (BaO-TeO 2 - B 2 O 3 ) 100–x (Nd 2 O 3 ) x where x= 0, 0.2 were prepared using H 3 BO 3 , BaCO 3 ,T e O 2 and Nd 2 O 3 as the starting ma- terials. Those powders were then collected in a platinum crucible and heated in a furnace at 850 °C. Then the melt was cast into a stainless-steel mould and annealed at 300 °C for 12 h to release the thermal strains. The phase identification was determined by a single- crystal X-ray diffraction on a Bruker SMART APEX-II diffractometer and the SAINT program. Thermogravi- Materiali in tehnologije / Materials and technology 53 (2019) 3, 305–309 305 UDK 620.1:666.112.6 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(3)305(2019) *Corresponding author e-mail: jingli@sdu.edu.cn metric (TG) and differential thermal analysis (DTA) curves were acquired with a heating rate of 10 °C min –1 in a N 2 atmosphere. The absorption spectra were recorded with a Hitachi-340-UV-VIS spectrophotometer (Hitachi.Co.Ltd, Japan) at 300–1000 nm. The fluore- scence spectra were recorded using a FLS920 spectro- photometer (Edinburgh Instruments Co. Ltd, UK). 3 RESULTS and DISCUSSION 3.1 Thermal stability The thermal stability of a glass is a very important factor for potential applications. The DTA was carried out in order to establish the thermal stability of the glass and the results are shown in Figure 1. Thermal charac- terization can provide valuable, indirect information about the structural changes that take place in a glass system. 8 From the DTA we obtained the glass-transition temperature (T g = 623 °C) and the crystallization tem- perature (T x = 685 °C). The quantity T=T x – T g was employed to estimate the thermal stability of the glass. The value of T is 62 °C, which suggests good thermal stability of the glass. The TG curve shows that the total mass of the glass powder is almost constant in the measured temperature range. 3.2 XRD The X-ray diffraction (XRD) patterns of the glass samples shown in Figure 2a exhibit a broad hump and there is the absence of discrete sharp peaks. This reveals the absence of long-range periodicity of the prepared samples. Figure 2b shows the XRD patterns of the samples a few months later, and there is also an absence of sharp crystallization peaks. The doped glass sample has good thermodynamic stability, from comparing the blue lines in Figures 2a and 2b. 3.3 FT-IR spectral studies Figure 3 shows the FT-IR spectra of the prepared glass system. IR spectroscopy can provide information about the structural units present in the amorphous materials. The band at 620 cm –1 is attributed to the B–O bending vibration of the BO 3 triangles and stretching vibrations of the Te–O bonds in TeO 4 units. 9,10 The band at 948 cm –1 is due to the stretching vibration of the BO 4 units in X various structural groups. The band at 1300 cm –1 is characteristic of the B–O stretching vibrations of the trigonal BO 3 units in various types of borate groups. The band at 2900 cm –1 is attributed to the OH bond group. The O–H absorption improves the optical attenuation and decreases the quantum efficiency H. WEN et al.: THERMAL, OPTICAL CHARACTERIZATION AND JUDD-OFELT ANALYSIS OF Nd 3+_ DOPED ... 306 Materiali in tehnologije / Materials and technology 53 (2019) 3, 305–309 Figure3: Room-temperature IR spectrum of the Nd 3+ doped glass Figure 1: DTA/TG analysis curves of Nd 3+ -doped glass Figure 2: XRD patterns of (BaO-TeO 2 -B 2 O 3 ) 100–x (Nd 2 O 3 ) x where x= 0, 0.2: (a) the samples with very short storage times and (b) the samples after a few months of the rare-earth-excited levels. 11 The absorption coeffi- cient of the OH in the doped sample is 3.60 mm –1 , while its absorption coefficient in the undoped sample is 4.76 mm –1 . The lower intensity of the OH band indicates that the present glass system is suitable for high-quality optical material with a lower OH content. 3.4 Spectroscopic characteristics and Judd-Ofelt anal- ysis Optical absorption spectra for the glass were measured in UV and near infra-red regions at room temperature in the wavelength range 300–1000 nm. As shown in Figure 4, there are a number of absorption bands that corresponds to the f–f electronic transition of Nd 3+ ions. The various spectroscopic transitions observed for Nd 2 O 3 -containing glasses are as follows: 2 P 1/2 (430 nm), 2 G 9/2 (476 nm), 4 G 9/2 (514 nm), 4 G 7/2 (526 nm), 4 G 5/2 (584 nm), 4 S 3/2 + 4 F 7/2 (748 nm), 4 F 5/2 (804 nm), 4 F 3/2 (878 nm). 12 From the absorption spectra, it is clear that the optical transition 4 I 9/2 4 G 5/2 is more intense than the other transitions. The absorption bands at 748 nm, 804 nm and 878 nm can be efficiently pumped using commercially available laser diodes. 13 The absorption band at 804 nm, the full-width at half-maximum (FWHM), is about 18 nm, which is close to the Nd:LaVO 4 crystal. The broad bandwidth is suitable for laser diode pumping because it is not temperature dependent. The absorption spectrum of the Nd 3+ ions in the BaO-TeO 2 -B 2 O 3 glass was analysed using the J-O theory. The spectral parameters of the rare-earth glass and the doped crystal are different. The Nd active ion, due to the screening effect of 5s 2 p 6 lamella, can only produce a very faint perturbation to the 4f electrons. The absorption cross-section abs is given by abs = 2.303 A/dN c Where A is the absorption, d is the sample thickness, and Nc is the concentration of Nd 3+ in the doped glass. The absorption cross-section at 804 nm is 4.93 × 10 –20 cm 2 , which is higher than a Nd:YVO 4 crystal. A com- parison of spectrum parameters is listed in Table 1. The experimental oscillator strength can be obtained using the relation: 18–20 f mc eNd OD exp . () = ∫ 2303 2 2 π c d where m and e are the mass and charge of the electron, c is the speed of the light, Nc is the Nd 3+ concentration, OD() ∫ d is the integrated absorption coefficient. f mcv hJ n n Sn S ed md cal = + + + ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ 8 321 2 9 222 π () () where v is the mean energy of the transition, n is the refractive index (roughly 1.88), 21 h is Planck’s constant, S ed is the electric dipole line strength, S md is the mag- netic dipole strength, which can be neglected in compa- rison with S ed . S ed can be calculated as follows: Se J J ed = = ∑ 2 246 ,, (' ' II II U where the term(' ' JJ II II U is the reduced matrix element of the tensor operator, and ( =2,4,6)are the J-O intensity parameters. Table 2 shows the calculation result. Among the three intensity parameters 2 , 4 and 6 , 2 is indica- tive of the crystal field symmetry of the rare-earth site and easily affected by the local structure changes of the host. The high value of 2 implies that the prepared glass has a highly covalent nature. The spectroscopic quality factor ( = 4 / 6 ) indicates the emission in- tensity of the 4 F 3/2 4 I 11/2 transition of Nd 3+ . 22 The small value of implies that the glass has a strong emission intensity of the 4 F 3/2 4 I 11/2 transition. H. WEN et al.: THERMAL, OPTICAL CHARACTERIZATION AND JUDD-OFELT ANALYSIS OF Nd 3+_ DOPED ... Materiali in tehnologije / Materials and technology 53 (2019) 3, 305–309 307 Table 1: Spectroscopic parameters of Nd 3+ -doped BaO-TeO 2 -B 2 O 3 and those of other Nd 3+ -doped crystals Materials Peak absorption wavelength / a (nm) FWHM(at 808) /nm abs (at 808) /×10 –20 cm 2 em (at 1062 nm) /×10 –19 cm 2 Ref 1%Nd:Lu 3 Al 5 O 12 808 5 1.86 0.967 14 1%Nd:YVO4 808.7 2 2.7 14.1 15,16 0.94%Nd:LaVO4 808 17 1.67 0.65 17 0.2%Nd:BaO-B 2 O 3 -TeO 2 804 18 4.93 8.72 This work Figure 4: Room-temperature optical absorption spectra of the Nd 3+ -doped glass Table 2: Optical parameters of the absorption spectrum of glass ( 2 , 4 and 6 are in units of 10 –20 cm 2 ) Transition 4 I 9/2 /nm f exp (×10 –6 ) f cal (×10 –6 ) 2 P1/2 430 0.28 0.20 2 G9/2 476 2.09 0.64 4 G9/2 514 0.74 1.87 4 G7/2 526 2.27 4.62 4 G5/2 584 33.03 27.80 4 F7/2, 4 S3/2 748 8.49 8.02 4 F5/2 804 10.57 9.60 4 F3/2 878 2.31 1.74 2 =9.79 4 =1.20 6 =9.19 Figure 5 shows the fluorescence spectrum of the glass under excitation of 358 nm at room temperature in the wavelength range 800–1600 nm. In the spectrum three fluorescence peaks can be seen: 903 nm, 1062 nm, 1335 nm. The main peak located at 1062 nm can be attributed to 4 F 3/2 4 I 11/2 and results from the emission of Nd 3+ ions. The FWHM of this emission band is 29 nm. The emission spectrum of Nd 3+ in the BaO-TeO 2 -B 2 O 3 glass is also investigated based on J-O theory. A en mc f JJ c a l ,' = 8 222 2 π where A J,J’ is the probability of the electric-dipole spon- taneous transition from the excited level J’ to the terminal level J. rad = − ∑ 1 AjJ (' ) where rad is the radiation lifetime of a given upper level. We obtained the radiation lifetime as 134 μs at the 1062 nm. The emission cross-section e can be derived using the formula: em r d = ∫ I cn I () () 8 2 π where I( ) is the intensity of the emission spectra and r is the radiative lifetime. The emission cross-section is calculated with the formula is 8.72 × 10 –19 cm 2 at the wavelength of 1062 nm. 4 CONCLUSIONS In this work, Nd 3+ -ions-doped barium-tellurite-borate glasses were prepared using the melt-quench technique. XRD confirmed the amorphous nature of the glass samples. DTA and TG were used to study the thermal properties of the samples and the results show that they have good thermal stability. The glass samples were characterized by using spectroscopic techniques such as optical absorption and fluorescence spectra. There are several main optical absorption peaks from 400 to 900 nm, whereas in the fluorescence spectrum three emission transition bands located at 903 nm, 1062 nm and 1335 nm were observed. The spectral properties were studied by applying the J-O theory. The intensity parameters 2 , 4 , and 6 and the radiative properties of the emission levels of the Nd 3+ : BaO-TeO 2 -B 2 O 3 glasses were cal- culated. 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