Journal of Non-Crystalline Solids 431 (2016) 135–139 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids j ourna l homepage: www.e lsev ie r .com/ locate / jnoncryso l Optical and structural properties of Mn2+ doped PbGeO3–SbPO4 glasses and glass–ceramics V. Volpi a, M. Montesso a,b, S.J.L. Ribeiro b, W.R. Viali a, C.J. Magon c, I.D.A. Silva c, J.P. Donoso c, M. Nalin a,b,⁎ a Chemistry Department, Federal University of São Carlos, São Carlos, SP, Brazil b Chemistry Institute, São Paulo State University, Araraquara, SP, Brazil c Physics Institute, University of São Paulo, São Carlos, SP, Brazil ⁎ Corresponding author at: Chemistry Institute, São Pau SP, Brazil. E-mail address: mnalin@iq.unesp.br (M. Nalin). http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.022 0022-3093/© 2015 Elsevier B.V. All rights reserved. a b s t r a c t a r t i c l e i n f o Article history: Received 19 December 2014 Received in revised form 9 April 2015 Accepted 11 April 2015 Available online 17 April 2015 Keywords: Glass; Glass–ceramic; Manganese; UV–Vis; EPR This paper shows the study of optical and structural properties of SbPO4–PbGeO3–MnCl2 glass and glass– ceramics. Glass was prepared bymelting–quenchingmethodologywhile the glass ceramic was obtained by con- trolled thermal treatment of the glass. The UV–Vis results show that the glass presents only Mn2+ species, but, the thermal treatment leads to oxidation of Mn2+ to Mn3+ species, what was evidenced by an absorption band in the visible range. High resolution transmission electron microscopy shows the presence of nano- crystalline phases ofMn2O3 and PbGeO3 confirming the formation of a glass–ceramic. Electron paramagnetic res- onance measurements suggest that manganese ions occupy only one environment in the glass. On the other hand, for glass–ceramic at least three different structural sites were identified by simulation of the EPR signal. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The advantage of glasses over other inorganic materials is the possi- bility to tune their compositions in order to obtain a suitable matrix where it is possible to dissolve, disperse and stabilize a large variety of ions and nanoparticles. Paramagnetic glasses and glass–ceramics con- taining transition metal oxides have been studied in the last years due to their interesting luminescent and magneto-optical properties [1–4]. In particular, manganese is one of themost studied species due to its in- teresting chemical properties, such as, the large variety of oxidation states allowing its uses in several technological fields. Visible lumines- cence [1,2,5–7], magnetic sensors [8] and magneto-optical recording [3] are among the possible applications of such element embedded in glasses. Optical andmagneto-optical properties depend on the environ- ments and valence states of the manganese ions. A large amount of pa- pers are found in the literature regarding both fundamental and application point of views,moreover, many doubts are still in discussion with respect towhat oxidation states and inwhat environments areMn ions into the glasses and glass–ceramics prepared by thermal treatment of the glasses. Usually, manganese is added into glassmatrixes asminor component and for this reasonMn2+ ions do not drive to strong colored glasses due to the low absorptivity [9]. On the other hand, more lo State University, Araraquara, concentrated samples may lead to formation of higher oxidation states (Mn3+ or Mn4+) leading to colored glasses [1,9,10]. Electron paramagnetic resonance, EPR, is a sensitive spectroscopic technique for the study of local coordination environment of paramag- netic centers, such as transition-metal and rare-earth ions, incorporated in a variety of materials. In particular, EPR has been extensively used to probe the electronic environment of paramagnetic manganese species. Common valence states of manganese are +2, +3 and +4. The three- fold state, although paramagnetic, is usually not observable by EPR at low frequency due to the large zero field splitting as well as shorter spin relaxation times [11]. Both two and fourfold states can, indeed, be detected even at room temperature. EPR of Mn4+ ions (3d3, S = 3/2) has been reported in manganese complex [12,13], layered oxides [14] and titanium oxide based systems [15,16]. Mn2+ is, by far, the manga- nese valence mostly studied by EPR in glasses [17–21]. Heavy metal oxide glasses, especially those based on antimony and germanium, are very interesting compositions for optical uses and have been studied due to their good transparency, high polarizability, high thermal expansion, low dispersion, high linear and non-linear re- fractive index and solubility for rare earths [22–26]. The symbiosis between the magneto-optical effects of manganese ions and the optical properties of heavy metal oxide glasses is interest- ing for photonic applications. In this sense, in the present study new optical glasses and glass–ceramics were obtained in Mn-containing PbGeO3–SbPO4 system and an extensive investigation on the optical and structural properties was undertaken. The studies were also ex- tended to the valence states and coordination of manganese in the http://crossmark.crossref.org/dialog/?doi=10.1016/j.jnoncrysol.2015.04.022&domain=pdf http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.022 mailto:mnalin@iq.unesp.br http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.022 http://www.sciencedirect.com/science/journal/00223093 www.elsevier.com/ locate/ jnoncrysol 300 350 400 450 500 550 -0,2 0,0 0,2 0,4 0,6 V1 H ea t F lo w ( W /g ) Temperature (oC) Tg = 392 oC Tx = 486 oC Tp = 504 oC 10 20 30 40 50 60 70 0 20 40 60 80 100 120 140 C ou nt s 2 (degree)θ Fig. 1.DSC curve of the antimony–lead germanate glass containingMn2+. Inset shows the amorphous X-ray diffraction pattern of the same sample. 136 V. Volpi et al. / Journal of Non-Crystalline Solids 431 (2016) 135–139 glass network by different techniques including UV–Vis and Raman spectroscopies, differential scanning calorimetry (DSC), X-ray diffrac- tion, electronic paramagnetic resonance, EPR and high resolution trans- mission electron microscopy, HRTEM. 2. Experimental procedure 2.1. Glass synthesis The synthesis of PbGeO3–SbPO4:MnCl2 glass was done in two steps: Step 1 consisted of the preparation of lead germanate glass, PbGeO3 following the study done earlier by L.A. Bueno [27]. Equimolar amounts of rawmaterials PbO and of GeO2 (PbO Synth grade purity 98,0% and GeO2 Aldrich grade purity 99,0%) were thoroughly mixed in agate mortar and melted in platinum crucible at 950 °C for about 30 min. The resultant melt was casted between two stainless steel plates at room temperature. In step 2, vitreous PbGeO3 wasmixed to SbPO4 andMnCl2 in the fol- lowing molar ratio 89.1PbGeO3–9.9SbPO4–1MnCl2 (SbPO4 was pre- pared as described earlier [28] and MnCl2, Synth grade purity 98,0%). The glass composition was named V1. The composition was then mixed in agate mortar and melted in platinum crucible at 1100 °C for 30 min. The melt was casted into a pre-heated stainless steel mold at 380 °C (value obtained from DSC measurements). The glasswas left at such temperature during 2 h before to be cooled down to room temperature. 2.2. Glass ceramic preparation The glass ceramic was prepared by treating the glass above the glass transition temperature. The temperature chosen was 450 °C. The glass piece, measuring 1 cm × 1 cm × 500 μm was heated in an adapted fur- nacewhichwas introduced into the spectrophotometer allowing to fol- low the evolution of the treatment, in situ, in the UV–Vis range. The sample obtained after thermal treatment was named V1-GC. 2.3. Glass characterization The characteristic temperatures of the glasses (Tg for glass transition temperature, Tx for onset of crystallization and Tp formaximumof crys- tallization peak) were determined by differential scanning calorimetry (DSC). Measurement was carried out using an equipment TA Instru- ments, model TA 2910. Powdered sample was heated at 10 °C min−1 in aluminum pans under N2 atmosphere in the range from 100 to 600 °C. The estimated errors were 2 °C for Tg and Tx temperatures and 1 °C for Tp. The amorphous structure of the glasses was confirmed by X-ray diffraction using a Siemens Crystalloflex Diffractometer with Cuκα radiation in the range of 2θ from 4 to 70°. UV–Vis absorption spectra of glasses were obtained “in situ” in the range from200 to 800 nmusing an adapted furnace coupled to a Varian, Cary 5000 scan spectrometer. X-band CW-EPR spectra were recorded at 15 K on a Bruker Elexsys E580 spectrometer operating at 9.485 GHz, equippedwith a continuous flow liquid helium Oxford cryogenic system. Solid state powder EPR spectra were simulated by the well-known software package named EasySpin [29], implemented in MATLAB (MathWorks, Inc.). Transmission electron microscopy (TEM) was performed to assess the structural information of the passivated nanoparticles. Low- magnification and high-resolution transmission electron microscopy images (HR-TEM) were obtained using the JEOL 3010 TEM-HR operat- ing at 300 kV. Fast Fourier Transforms (FFTs) of TEM images were ob- tained using the Digital Micrograph (Gatan) software to obtain the lattice d-spacing. For TEM analysis the sample dispersed in isopropyl alcohol was deposited on a copper grid coveredwith an amorphous car- bon film and left to dry in Ar-atmosphere. 3. Results and discussion Homogenous, transparent pale-yellow glass samples of composition 89.1PbGeO3–9.9SbPO4–1MnCl2 were obtained. Characteristic tempera- tures could be easily observed in DSC curves, Fig. 1, while the non- crystalline structure was confirmed by X-ray diffraction (inset in Fig. 1). From DSC results it is noticed that the stability parameter (i.e. the difference between the glass transition and onset of crystallization temperatures) is around 94 °C allowing the preparation of bulk samples 3mm thick. In order to study the effect of the temperature on the optical properties of the glass, a controlled heat treatment was performed as described earlier in the experimental section. The thermal treatment was monitored “in-situ” by using a UV–Vis spectrophotometer and Fig. 2 shows absorption spectra obtained each 10 min. d–d transitions due to the d5 configuration of Mn2+, occurring usually in the 300–450 nm for oxide glasses are not observed, probably due to low relativeMn2+ content and strong overlapwith the host glass band gap [3]. The glass color was observed to change to purple (inset of Fig. 2) and in fact, after 90 min of heat treatment it is possible to observe a rising band around 513 nm (red curve in Fig. 2). The intensity of such band in- creases while it shifts to lower wavelengths, up to 538 nm (blue curve), observed for 230 min of treatment. Fig. 2 also shows a second absorp- tion band appearing as a shoulder at around 615 nm. For longer heat treatments the broad absorption band is observed to shift to smaller wavelengths, being centered at 525 nm after 400 min (purple curve). The shoulder remains unchanged. Purple color in Mn doped glasses is usually attributed to the pres- ence of Mn3+ ions which in general present higher absorption coeffi- cients than Mn2+ [9]. Heat treatments lead to formation of Mn3+ species because treatments above the glass transition would allow the diffusion of oxygen through the glass leading to oxidation of divalent to trivalent Mn ions. The shoulder, or asymmetry, around 615 nm has been interpreted as been the result of a Jahn–Teller tetragonal distortion of the octahedral ligand field around MnO6 units and was observed in silicates, borates and sodium phosphate glasses containing manganese ions [30]. The TEM images of the heat treated sample, Fig. 3, revealed the pres- ence of crystalline nanoparticles, homogeneously dispersed through the glass (Fig. 3a). The particle size histogram for the glass sample, obtained 400 500 600 700 800 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 100 200 300 400 A bs or ba nc e Ti m e (m in ) Wavelength (nm) a Fig. 2. UV–Vis absorption curves obtained in-situ from the glass doped with manganese. Inset shows the photography of the glass before and after thermal treatment. 0 1000 2000 3000 4000 5000 6000 Mn(1) g 2 B V1a V1b V1-GCa Mn(2,3,4) g 4 V1-GCb ∼ ∼ Fig. 4. Experimental EPR spectra of 89.1PbGeO3–9.9SbPO4–MnCl2 of the non-treated glass (V1a) and the glass treated at 450 °C (V1-GCa). In (V1b) and (V1-GCb) are shown the nu- merical second derivative of both spectra, respectively. Spin Hamiltonian parameters are shown in Table 1. Structured spectra at g ≈ 2 and g ≈ 4 are attributed to Mn2+ site (1) and site (2,3,4) species, respectively, as explained in the text. 137V. Volpi et al. / Journal of Non-Crystalline Solids 431 (2016) 135–139 from the TEM micrographs, is shown as an inset, where vertical bars represent the experimental data, while the solid line results from the curve fitting of the data using the log-normal distribution function [31]. Values of the average particle diameter (DTEM) and distribution width standard diameter deviation (w) obtained from the fitting of the data presented in the histogram were 4.9 ± 0.1 nm and 0.2, respectively. Interplanar distances could be measured in HR-TEM micrographs (Fig. 3b). The shortest, measuring 2.50 Å corresponds to the (110) Bragg plane reflection of bulk rhombohedral Mn2O3 (JCPDS Card No. 33-900) and the longest one, measuring 3.28 Å was assigned to the (222) Bragg plane reflection of bulk monoclinic PbGeO3 (JCPDS Card No. 31-684). The inset in Fig. 3b shows the FFT patterns, used to stipu- late the interplanar distances. These findings indicate two different crystalline nanoparticles dispersed into the glass ceramic and also con- firm the presence of Mn3+ ions. In order to investigate the coordination environment of the manga- nese ions an EPR investigation was performed. Fig. 4 shows the X-band Fig. 3. a) Bright field of the glass–ceramics. Inset shows the histogram of the corresponding imag form (FFT) of the corresponding HR-TEM images. EPR spectra of Mn2+ doped glass sample in the vitreous phase, Fig. 4 (V1a), and after thermal treatment at 450 °C, Fig. 4 (V1-GCa). Two prominent features with effective g-values g ≈ 2 and g ≈ 4 appeared in the spectra. The former signal, which has been frequently reported in glasses [17,32–34] is attributed to magnetically isolated Mn2+ ions in symmetry close to octahedral or undistorted cubic sites. The low field signal at g ≈ 4 is associated with isolated Mn2+ ions in rhombic distorted sites subjected to high crystal field effects [33,35–37]. The Mn2+ ion has a 3d5 electronic configuration and spin S = 5/2. Manga- nese has a stable isotope with non-zero nuclear spin, 55Mn, with I = 5/2 (100% natural abundance). Therefore, the EPR spectrum is expected to show a hyperfine structure composed of six lines, resulting from the dipole–dipole interaction between the magnetic moment of the 55Mn nuclei and the electronic moment of the paramagnetic Mn2+ ion. Some overlap between the lines can be observed when their linewidth is comparable to the hyperfine splitting constant, A. For a better view of the Mn hyperfine structures, the second deriva- tive spectra were calculated and are displayed in Fig. 4 (V1b, V1-GCb). It is evident the resolution enhancement obtained by this simple signal processing technique. The partial spectrum centered at g≈ 2 is insensi- tive to the thermal treatment and exhibits a well resolved sextet e. b) High-resolution TEM images of the nanoparticles. Inset shows the Fast Fourier Trans- 1000 1500 2000 Magnetic field (G) (A) (B) (C) (D) (E) (F) Fig. 5. Second derivative EPR spectrum of 89.1PbGeO3–9.9SbPO4–MnCl2 glass treated at 450 °C in the range 1000–2500 G. (A) Simulated spectrum. (B) Experimental spectrum. (C) Broad component: S = 1/2, g = 4.269. (D) site (4) species. (E) Mn site (2) species. (F) Mn site (3) species. Spin Hamiltonian parameters are shown in Table 1. 138 V. Volpi et al. / Journal of Non-Crystalline Solids 431 (2016) 135–139 structure superimposed on a broader background. In this case, the hy- perfine splitting constant, A, was determined using the procedure given in the literature [36,38] and the result is listed in Table 1, under the denomination of site (1) species. For most of the reported EPR studies in manganese-doped glasses, the signal at g ≈ 4 is weak if compared to the main resonance at g≈ 2; besides, the hyperfine structure is not resolved at the former sig- nal [17,32–34,39]. For our experimental data, the structure observed at g ≈ 4 is not weak and is also insensitive to the thermal treatment. Therefore, we simulated the second derivative spectrum of the treated sample shown in Fig. 4 (V1-GCb), at g ≈ 4, taking into account a Mn ion with isotropic Zeeman and hyperfine interactions. Our conclusion was that the spectrum is too complex to be simulated by only one Mn environment and, indeed, at least three different sites were needed to reproduce the main features of the signal. The result of the simulation of the g≈ 4 structured spectrum of the glass treated sample is displayed in Fig. 5 and the best values for the spin Hamiltonian parameters are listed in Table 1. The line shapes were de- scribed by a combination of Gaussian and Lorentzian shapes. A broad spectral component, shown in Fig. 4, was necessary to be included in the simulation. One can notice that the left side of the spectrum, below 1600 G, could be well fitted by a single Mn site, but, for the re- maining right side two additional sites were needed. The different Mn environments were numbered by site (2), site (3) and site (4). They are related to magnetically isolated manganese ions. The analysis of the EPR parameters for these sites shows that species (2) and (3) differ from species (4) mainly in the value of the hyperfine constant, which are definitely lower for species (4). The fitting is capable to reproduce well the positions of all lines, although large errors in the amplitudes can be verified. These amplitude errors may result from the fact that the real broadeningmechanismswere not well described by the simple line shape assumption and that, a larger number of Mn environments coexist in the glass–ceramic matrix. In general, the hyperfine constants displayed in Table 1, are consistent with those previously reported in the literature for GeO2 based glasses [25]. The magnitude of the hyperfine constant, A, provides a qualitative measure of the bonding character between the Mn2+ ion and its li- gands. Van Wieringen determined empirically a correlation between A and the ionicity of the manganese ligand bond and noted that the strength of the hyperfine splitting depends on the glass matrix into which the ion is inserted and is mainly determined by the electronega- tivity of the neighbors [40,41]. In fluorite crystals, where the bonds are ionic, A ≈ 100 × 10−4 cm−1, while covalent semiconductors like Mn– Te have A values as low as 60 × 10−4 cm−1 [33,42]. The magnitude of the hyperfine splitting value obtained in the present article reveals that the bond between Mn2+ ions and the surrounding ligands, A = (80 − 97) × 10−4 cm−1, is moderately ionic. The results demonstrate the potential of such glasses to be used as hosts for magnetic nanoparticles since crystallineMn2O3 presents mag- netic behavior and may be studied for magneto-optical applications. In this sense, more studies are actually under consideration in our labora- tory concerning the increasing of the concentration of Mn into the glass host and the evaluation of the optical and magnetic properties of such new materials. Table 1 Spin Hamiltonian parameters obtained from numerical simulation of the second deriva- tive spectrum of 89.1PbGeO3–9.9SbPO4–MnCl2 glass–ceramic prepared at 450 °C. Mn species g (±0.002) A (±0.2 ∗ 10−4 cm−1) Mn (1) 2.005 84.2 × 10−4 cm−1 (232 MHz, 83 G) Mn (2) 4.640 94.2 × 10−4 cm−1 (260 MHz, 92.8 G) Mn (3) 3.932 97.7 × 10−4 cm−1 (270 MHz, 96.3 G) Mn (4) 3.960 80.1 × 10−4 cm−1 (221 MHz, 78.9 G) 4. Conclusions The influence of the thermal treatment in the glass composition 89.1PbGeO3–9.9SbPO4–MnCl2 was studied using several techniques. It was verified that thermal treatment leads to oxidation of Mn2+ to Mn3+ species and such statement was confirmed by in-situ UV–Vis and HRTEM. 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Introduction 2. Experimental procedure 2.1. Glass synthesis 2.2. Glass ceramic preparation 2.3. Glass characterization 3. Results and discussion 4. Conclusions Acknowledgments References