Synthesis and Structural Studies of Er3+ Containing Lead Cadmium Fluoroborate Glasses and Glass-Ceramics

Journal of the Brazilian Chemical Society

Messaddeq, Younes¹  Ribeiro, Sidney J. L.¹  Briois, Valerie¹  Poulain, Marcel¹  Silva, Maurício A.P.¹  Université de Rennes¹  Université Paris-Sud, Orsay-Cedex, France¹  Universidade Estadual Paulista, Araraquara, Brazil¹ 

关键词: oxifluoride glasses;    glass-ceramics;    x-ray absorption spectroscopy;   

DOI  :  10.1590/S0103-50532002000200011

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

In the last years a new class of oxifluoride glasses have been studied aiming at the development of new photonic devices. In mixed systems involving well known glass former oxides such as SiO2 and GeO2 and heavy metal fluorides, stable glasses have been prepared at ambient atmospheres by melting and casting techniques.1,2 Besides being stable and workable, e.g. as optical fibers and thin films, these glasses present interesting crystallization characteristics. It is possible to control the crystallization process in order to obtain transparent glass ceramics, where heavy metal nanocrystals are grown in the glassy oxide matrix. Moreover, when doped with optically active rare earth ions, these ions concentrate in the crystalline phase after the ceramization heat treatment. In parallel, similar oxyfluoride glasses display interesting ionic conduction properties. Haloborate and halosilicate glasses have been studied and show interesting anionic conductivity properties.3,4 Haloborate glasses in the B2O3-PbO-PbF2 and B2O3-PbO-PbF2-AlF 3 systems have been characterized by Raman scattering and X-ray photoelectron spectroscopy.4 Ionic conductivity increases with the PbF2 content. The structure of these glasses comprises discrete and polymerized structures displaying sp2-hybridized boron and fluorine atoms bonded to B, Al and also Pb atoms. In this work we have prepared and characterized new glasses in the B2O3-PbF2-CdF 2 system. Characterization techniques include thermal analysis, X-ray diffraction and Extended X-ray Absorption Fine Structure (EXAFS). Undoped and Er3+- doped samples of a selected composition were submitted to crystallization treatments and the samples were characterized by the above mentioned techniques and also by luminescence spectroscopy.  Experimental Glasses in the B2O3-PbF2-CdF 2 system were prepared by melting starting mixtures at 800 oC, in Pt crucibles, followed by quenching and casting the melts in brass molds (the cooling rate was estimated to be of the order of 103 K min-1). Short melting times (typically 5 min) were used in order to decrease fluorine losses. Due to the low viscosity of the melt, good homogenization was easily achieved by crucible agitation before casting. Starting reagents were CdF2 (Aldrich, 99%), orthorhombic a-PbF2 (Aldrich, 99.99%) and H3BO3. Glasses with composition (mol %) 40B2O3-30PbF2-30CdF 2 were also prepared containing 1mole% ErF3 prepared by fluorination of Er2O3 (Aldrich p.a.) The glasses were then submitted to heat treatments at temperatures near the onset of crystallization, identified form thermal analysis (Tx), in order to verify the crystallization behavior of the related glass ceramics. Table 1 shows the labels used to identify the samples studied, time and temperature employed for each heat treatment. Table 2 gives the chemical analyses results obtained by ICP-AES technique for undoped 40B2O3-30PbF2-30CdF 2 glass (B sample) and corresponding heat-treated material (BT2 sample).     Thermal analyses was performed in a Thermal Analyst 3100 calorimeter from TA Instruments, under N2 atmosphere. Characteristic temperatures Tg (glass transition temperature), Tx and Tp (onset and maximum of the crystallization peak, respectively) were obtained from DSC scans obtained with powdered samples heated at a constant heating rate q=10 K min-1. X-ray powder diffraction was performed in a D-5000 Siemens diffractometer with Cu Ka radiation at 0.02º s-1 scanning rate. Short range order in the samples identified in Table 1 was studied by means of Extended X-ray Absorption Fine Structure (EXAFS). Measurements were performed at the Cd K and Pb LIII edges, in the transmission mode, at the D44 beamline of the DCI storage ring (which works with a positron beam energy of 1.85 GeV) at LURE, France. During data collection the average current in the storage ring was 250 mA. For the Cd K-edge (26711 eV) measurements, a two-crystals Ge(400) monochromator and two krypton-filled ionization chambers were used. Spectra were obtained with an energy step of 4 eV and a counting time of 2 s. In the Pb LIII-edge (13035 eV) measurements, the ionization chambers were filled with argon and a two-crystals Si(111) monochromator was used. A step size of 3 eV per 2 s was employed for spectra recordings. CdF2 (cubic) and b-PbO (orthorhombic) were used as reference compounds in the X-ray absorption experiments. Measurements were performed at the liquid nitrogen temperature (77K), in order to reduce the thermal factor, which produces a broadening of EXAFS oscillations. Near infrared Er3+ emission spectra were obtained with a Spex Fluorolog spectrofluorimeter equipped with a North Coast Ge detector. Excitation was performed with a 450W Xe lamp.  Results Glasses were obtained in the form of yellowish transparent monoliths of approximately 20x10x2 mm. The samples transparency, the typical amorphous X-ray diffraction pattern and the detection of glass transition in DSC (Differential Scanning Calorimetry) measurements identified glass formation. The vitreous domain diagram for the ternary B2O3-PbF2-CdF 2 system is illustrated in Figure 1. Glass forming compositions (under the above-cited heating-quenching conditions) are represented in this figure by open circles, while the black ones represent compositions that crystallize under the same conditions. Binary composition B2O3-PbF2 was previously studied by Gressler et al.5 In the ternary diagram, the composition limits for glasses in the lead cadmium fluoroborate system, expressed in mole %, are 20-60% B2O3, 30-60% PbF2 and 0-50% CdF2.   Table 3 gives the characteristic temperatures obtained from DSC measurements for some typical compositions in this system. The value obtained for the difference (Tx-Tg) is usually taken as a stability parameter. With the increase in the heavy metal content a decrease is observed for (Tx-Tg). It must be noted that no crystallization could be identified for the sample with composition (mole%) 50B2O3-40PbF2-10CdF 2 in the temperature range scanned (50-600 ºC). ICP-AES technique results (Table 2) shows that, in spite of short melting time, fluorine and boron losses were not completely avoided.   Figure 2 shows the X-ray diffraction patterns of the undoped and Er-doped glassy and crystallized 40B2O3-30PbF2-30CdF 2 samples. Peaks marked with circles are those reported on the 6-0251 JCPDS file for the cubic lead fluoride b-PbF2. This cubic phase arises from heat treatment of the glassy samples at 470 °C. In the erbium-doped sample such crystallization occurs after a shorter time heat treatment (20 min, BErT1) compared to the undoped one (60 min, BT2). The diffraction peaks of Er-doped samples is shifted to higher angles, compared to the same peaks in undoped glass-ceramics, indicating that erbium atoms are enclosed in a solid solution of the type b-PbF2:Er3+, in which Pb2+ ions, with an ionic radius of 0.145 nm, are partially substituted by Er3+ ions with ionic radius of 0.114 nm. Despite of the weak intensity of the peaks, one can observe for the BT2 and BErT2 samples some less intense peaks, marked with circles in Figure 2. We assign these peaks to the occurrence of a non-random cationic arrangement in the cadmium glassy network. This interpretation is supported by EXAFS results at the Cd K-edge, as discussed below.   EXAFS formalism and data analysis are largely described in a number of books, reviews and papers. As an example, the reader will find in reference 6 a detailed description of EXAFS phenomena and data analysis. In this work, data analysis was performed on a Macintosh computer with the A. Michalowicz's "Exafs pour le Mac" software.7 The main steps of analysis procedure are: 1. Subtraction of absorption background by a linear function from the rough absorption data. 2. Calculation of EXAFS signal using the Lengeler-Eisenberger method8 and its conversion to a function of wave vector kc(k). 3. Data are then Fourier transformed, using k3 ponderation and a Kaiser window with t=2.5, leading to a spectrum scaled in distances (Å). 4. The peaks in the Fourier transforms corresponding to a coordination shell are filtered and back-transformed to k-space. 5. The resulting EXAFS-filtered signal is treated as a sum of sinusoidal wave functions using plane wave and single scattering approximations.6 where N is the number of atoms in the coordination shell, Ri is the average interatomic distance between the absorbing atom and the back scatterer atoms, si is the Debye-Waller factor, which takes into account the static and thermal structural disordering, l(k) is the photoelectron mean free path (b=k/G), and fij(p,k) and Fi(k) are the amplitude and the phase functions for this coordination shell. In the

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