optical and electronic properties of the aluminophosphate glasses

Optical and electronic properties of the aluminophosphate glasses doped with 3D-transition... Rev.Adv.Mater.Sci. 10 (2005) 367-374

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OPTICAL AND ELECTRONIC PROPERTIES OF THE ALUMINOPHOSPHATE GLASSES DOPED WITH 3D-TRANSITION METAL IONS M.Elisa1, Cristiana E.A.Grigorescu1, Cristina Vasiliu1, M.Bulinski2, V.Kuncser3, Daniela Predoi3, G. Filoti3, Aurelia Meghea4, Nicoleta Iftimie4 and Maria Giurginca4 1

Department for Advanced Materials, National Institute of Optoelectronics-INOE 2000 1, Atomistilor Str., P.O.Box MG - 5, RO-77125, Bucuresti-Magurele, Romania 2 Department of Optics, Faculty of Physics, University of Bucharest, P.O.Box MG 11, RO-77125, BucharestMagurele, Romania 3 Magnetic Spectroscopy Laboratory, National Institute for Materials Physics, P.O. Box MG-7, RO-77125, Bucuresti-Magurele, Romania 4 Applied Spectroscopy Laboratory, Faculty of Industrial Chemistry, University POLITEHNICA of Bucharest, Applied Spectroscopy Laboratory, 1, Polizu Str., 1 Sector, Bucharest, Romania

Received: April 28, 2005 Abstract. Aluminophosphate glasses doped with Fe, Mn, and Cr have been obtained by a wet non-conventional method. Structural information was provided by IR absorption spectra in the range 2000-500 cm-1. The optical behaviour (transmission and refractive index) of the samples has been studied by UV-VIS-NIR spectroscopy. The Fe valence state and the local coordination were also analysed via 57Fe Mossbauer spectroscopy, whose data revealed the redox equilibrium in the Fe-doped glasses according to the redox potentials of the transition ions.

1. INTRODUCTION Recently, the aluminophosphate glasses gained a special scientific interest due to their applications in optics, optoelectronics and medicine [1]. It is worth to be mentioned optical filters, laser active media based on rare-earth-doped glasses [2], implant materials as prosthesis based on invert phosphate glasses [3], protection glass against nuclear radiation [4], optical switches, wave-guide, screens for luminescent lamps, composite materials made of phosphate glass and organic polymers and fullerens [5], etc. The investigation of the redox processes in the phosphate glasses doped with transition ions has an important role in the obtaining of the coloured and homogeneous optical glasses [6].

The oxidation state of the redox ions depends on the chemical composition of the vitreous melt, temperature and oxygen partial pressure [7]. Iron oxides used as dopants (FeO and Fe2O3) provide interesting phenomena over the structure and the properties of the phosphate glasses [8]. Small amounts of Fe2O3 (2-5 wt.%) induce an increase of 104-fold of the chemical strength against water. Thus, the iron-doped phosphate glasses are used to embed and to make inert the nuclear wastes (Cs, Sr, Fe, Co, Ni, Mn, U, Po, rare-earth, sulphides, chlorides, etc.) from the atomo-electrical plants [9,10]. The nuclear wastes, on their turn, increase the glass chemical stability and diminish the crystallization tendency of the iron-doped phosphate glass. These doped-vitreous materials together with policaprolac-

Corresponding author: M. Elisa, e-mail: [email protected] © 2005 Advanced Study Center Co. Ltd.

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M.Elisa, Cristiana E.A.Grigorescu, Cristina Vasiliu, M.Bulinski, V.Kuncser, et al.

tone and pollylactic acid are used for bone implants [11]. On the other hand, the iron-doped glasses exhibit interesting electrical and magnetic properties that depend on the iron redox species and their coordination symmetry [12]. It is known that the undoped phosphate glass has diamagnetic properties; the iron-doped phosphate glass is a paramagnetic material in contrast with the crystallized phosphate glass, which is ferromagnetic. Based on the IR absorption of the Fe2+ ions (1050 nm), these doped phosphate glasses are used to retain the heat radiation in different devices [13]. The manganese-doped phosphate glasses are interesting due to their magnetic properties. The dependence of the magnetic susceptibility on temperature and the correlation between the Curie temperature and the manganese ion amount were studied in [14,15]. In this paper we present the optical and electronic properties of the aluminophosphate glasses doped with iron as well as with additional manganese and chromium ions by a wet method [16]. The optical, structural and electronic properties of the doped aluminophosphate glasses were investigated by ultra-violet-visible-near-infra-red (UV-VIS-NIR), infrared (IR) and Mössbauer spectroscopy (MS). The Mössbauer data provide also information about the oxidation states and the coordination symmetry of the transition doping ions. The dependence of the refractive index vs. wavelength and the redox equilibrium of the transition metal ions with multiple oxidation states (iron, manganese and chromium) were also investigated.

2.EXPERIMENTAL In the present paper, by the wet method, we have obtained undoped and 3d transition ion-doped aluminophosphate glasses, belonging to the following oxide systems: - S1: 58.6LiPO3 29.3Al(PO3)3 10Ba(PO3)3 2La2O3 - S2: 58LiPO3 29Al(PO3)3 10Ba(PO3)3 2La2O3 1FeO - S3: 57,33LiPO3 28.66Al(PO 3) 3 10Ba(PO 3) 2 2La2O31FeO 1MnO2; - S4: 57.33LiPO3 28.66Al(PO3)3 10Ba(PO3)2 2La2O3 1FeO 1CrO3. The chemical reagents used to obtain the undoped and doped aluminophosphate glasses are shown in the Table 1. The wet-method stages for obtaining the undoped/doped aluminophosphate glasses were: (a) the homogenisation and the evaporation of the reagents up to 100-120 °C, (b) the drying process at 180-200 °C, (c) the preliminary heat treatment at

Table 1. The chemical reagents used for the glass preparation. Li2CO3 a.g. or LiOH a.g. Al2O3 a.g. BaCO3 a.g. H3PO4 a.g., sol. conc.85% La2O3 a.g.

Doping reagents CrO3 a.g MnO2 a.g. (Merck) FeSO4 7H2O a.g.

200-800 °C, (d) the glass melting and the refining processes at 1000-1200 °C, (e) the casting process and (f) the annealing stage. The sample glass S1, S2, S3, S4 (Table 2) were melted at 1200 °C for 4 hours and subsequently annealed at 400 °C for 2 hours. The UV-VIS-NIR transmission spectra were obtained with an UV-VIS-NIR-JASCO 570 V - 1999 spectrophotometer, in the range 250-2300 nm, band width 2 nm, band width (NIR) 8 nm, data pitch 2 nm, scanning speed 400 nm/min. The IR absorption spectra were taken with a FTIR620-JASCO spectrophotometer, in the range 2500-400 cm-1, accumulation 16, resolution 4 cm-1, gain 2, aperture 7.1 mm, scanning speed 2 mm/ sec. The refractive indexes of the analysed samples were measured with a PR2 Pulfrich refractometer (Carl-Zeiss Jena) and the data for the whole spectral range were fitted using Cauchy type series. The 57Fe Mössbauer spectra were acquired at room temperature (RT) by using a constant acceleration spectrometer and a 57Co(Rh) source. The isomer shifts were reported to RT α-Fe.

3. RESULTS AND DISCUSSION The transmission spectra of the analysed samples in the UV-VIS-NIR range are presented in Fig.1. As observed, the undoped aluminophosphate glass (S1) exhibits a relative high optical transmission from about 250 nm to more than 1500 nm. The UV-VIS-NIR spectra of S2 and S3 samples, doped with Fe and with Fe+Mn, respectively, show mainly the same shape, but they are characterized by a higher absorption in comparison with the undoped sample, especially above 500 nm. However, a new small absorption peak appears in these samples at around 1050 nm, suggesting the presence of Fe2+ ions [13,14]. On the contrary, the absorption spectrum of the S4 sample (doped with Fe and Cr) shows a very different behaviour, with two

Optical and electronic properties of the aluminophosphate glasses doped with 3D-transition...

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Table 2. The chemical reagents (a.g.) and the oxide composition (wt.%.) of the glass samples S1, S2, S3, and S4. Glass sample

The chemical reagents

The reagent amount (g)/(cm3)

The oxide brought by the reagent

Oxide composition (wt.%)

The undoped glass (S1)

Li2CO3 Al2O3 BaCO3 La2O3 H3PO4 Li2CO3 Al2O3 BaCO3 La2O3 H3PO4 FeSO4 7H2O Li2CO3 Al2O3 BaCO3 La2O3 H3PO4 FeSO4 7H2O MnO2 Li2CO3 Al2O3 BaCO3 La2O3 H3PO4 FeSO4 7H2O CrO3

9,97 5,97 7,88 2,6 56,12 (cm3) 8,58 6 7,88 2,6 55 1,112 8,48 5,81 7,88 2,6 54,86 1,112 0,348 8,48 5,81 7,88 2,6 54,86 1,112 0,4

Li2O Al2O3 BaO La2O3 P2O5 Li2O Al2O3 BaO La2O3 P2O5 FeO Li2O Al2O3 BaO La2O3 P2O5 FeO MnO2 Li2O Al2O3 BaO La2O3 P2O5 FeO CrO3

5,36 9,12 9,34 3,98 72,2 4,5 7,85 8 3,4 75,88 0,37 4,37 7,4 7,8 3,31 76,31 0,36 0,44 4,49 7,6 8 3,4 75,63 0,37 0,52

Fe-doped glass (S2)

Fe+Mndoped glass (S3)

Fe+Crdoped glass (S4)

absorption bands, specific to the Cr3+ ions at about 460 nm and 660 nm, respectively [14]. The absorption above 1000 nm is lower for S4 sample than that of the S2 and S3 samples and slowly higher than that of the undoped glass. Based on the above observations, we may assess that the transition metal ions are the ones responsible for the peculiar behaviour of the absorption spectra in the visible range of the spectrum. For the S3 sample, taking into account the high UV absorption due to Fe3+ ions and the high optical transmission in the VIS range due to Mn2+ and Fe2+ ions, we may assume the following redox equilibrium: Mn4+ + Fe2+ ↔ Mn2+ + Fe3+. Thus, manganese has a positive reduction potential and iron has a positive oxidation potential. The S4 glass sample has UV absorption peaks specific to Fe3+ (430-440 cm-1) and Cr6+ (UV absorp-

tion) ions and absorption features characteristic to Cr3+ ions, mentioned above. Thus, we may suppose the following redox equilibrium: Fe2+ + Cr6+ ↔ Fe3+ + Cr3+. Accordingly, chromium has a positive reduction potential and iron has a positive oxidation potential. Information on the local structure of the undoped (S1) and respectively of the Fe (S2), Fe+Mn (S3), and Fe+Cr (S4) -doped aluminophosphate glasses is provided in a first step by the absorption spectra in the IR range. The IR absorption spectra of the doped samples are shown in Fig. 2. It is worth mentioning that the IR spectrum of the undoped sample resembles quite well with the IR spectrum of pure P2O5 glasses. The frequencies of the optical phonons observed in the doped samples, as compared with those of P2O5 glasses [13,15] are given in Table 3.

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Fig. 1. The transmission spectrum of the undoped aluminophosphate glass (S1) as compared with Fe (S2), Fe+Mn (S3), Fe+Cr (S4)-doped aluminophosphate glass in the UV-VIS-NIR range.

Fig. 2. The IR absorption spectra of the undoped (S1), Fe (S2), Fe+Mn (S3) and Fe+Cr (S4)-doped aluminophosphate glass.

Typical peaks for optical phonons specific for aluminophosphate units are evidenced in all the samples proving that phosphorous atoms are suitable vitreous network formers.

The S2, S3, S4 samples exhibit absorption bands in the IR range (2500-400 cm-1), that are also specific to the P2O5-based glass (Table 3). We assigned the 1266 cm-1 peak to PO2 asymmetrical


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Table 3. The IR absorption peaks (cm-1) for the typical P2O5 glasses and for the analyzed glasses (undoped S1, and transition ion-doped glasses: S2, S3, S4, respectively). Phosphate units

P2O5 glass [13,15]

Undoped glass (S1)

Fe-doped glass (S2)

Fe+Mndoped glass (S3)

Fe+Crdoped glass (S4)

P-O-H P=O stretch P-O-P asymmetrical stretch P-O-P symmetrical stretch P-O-P bend [P2O7]4pyrophosphate unit PO43- symmetrical PO2 symmetrical stretch PO2 asymmetrical stretch PO32- symmetrical PO32- asymmetrical

1380 1240-1270 840-950

1317 894

1266 896

1266 896

1266 896

670-800

740 790 480

742 792 488

742 792 510

742 792 476

1092

1089

1089

1089

1200-1300

1317

1266

1266

1266

980-1050 1110-1190

950 1092

946 1089

946 1089

946 1089

420-620 1027 1179 1015 1100-1170

stretch and/or to P=O stretch and the 1089 cm-1 peak to PO32- asymmetrical stretch and/or to PO2 symmetrical stretch. The 480, 488, 510, 476 cm-1 peaks were assigned to P-O-P bend. The 792 cm-1 and 742 cm-1 peaks attributed to P-O-P symmetrical stretch were obtained by the decomposing of a large IR absorption band. The 1089 cm-1, 946 cm-1 (assigned to PO32- symmetrical stretch) and 896 cm-1 (P-O-P asymmetrical stretch) peaks are also obtained by IR spectra decomposing. The dependencies of the refractive index (n) versus the wavelength are presented in the Fig. 3. The dispersion, measured by the standard parameter Abbes number, ν, shows crown type glasses, with a low dispersion and a visible refractive index in the range 1.54 ÷ 1.57. The refractive index decreases vs. the wavelength via a Cauchy type dependence. At low wavelengths, where the variation of the refractive index is the sharpest, the behaviour of Fe+Mn (S3)-doped glass is more similar to the variation of the refractive index in the only Fe (S2) doped glass, whereas the variation of the Fe+Cr (S4)-doped glass is much similar to the Fe (S2)-doped glass, at longer wavelengths. Both manganese and chromium ions seem to reduce the decrement of the refractive index, but they have different effects on the absolute values of the refractive index.

The dispersion curves of booth S3 and S4 samples are reduced as compared with the glass doped only with iron (sample S2). At the same time, it is worth mentioning that at any wavelength, the refractive index of Fe+Mn (S3)-doped glass is higher than the refractive index of the Fe (S2) doped glass, which in turn is higher than for the Fe+Cr (S4)-doped glass. At this point, we should stress again that the overall optical behaviour of the analysed glasses is very sensitive to the electronic configuration of the 3d transition metal ions. In this respect, the 57Fe Mössbauer spectroscopy can give a direct information about the oxidation state of iron as well as on the local symmetry and coordination around the iron sites. Indirectly, through such information can be evidenced the redox mechanisms involving doping transition elements in the S2-S4 samples. The 57Fe Mössbauer spectra of these samples collected at room temperature (RT) are shown in Fig. 4. All spectra evidence the presence of the iron paramagnetic ions with different valence and coordination states. The best convergence test required a fit with three paramagnetic doublets in the S2 and S3 samples and only two central doublets in the S4 sample. The fitted Mössbauer parameters (isomer shift, IS, quadrupolle splitting, QS, and the popula-

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Fig. 3. Refractive index obtained at different wavelengths corresponding to the undoped (S1), Fe (S2), Fe+Mn (S3) Fe+Cr (S4)-doped aluminophosphate glass. The dispersion curves respect Cauchy type dependence.

tion of each iron position, obtained via the relative area of the corresponding Mossbauer component, R.A.) are presented in the Table 4. The oxidation state and the oxygen coordination around the central iron, as suggested by the values of the hyperfine parameters, are shown in the same table. The Mössbauer spectra of the S2 and S3 samples evidence the presence of two significantly split doublets with high quadrupolle splitting (3.17 mm/s and 2.24 mm/s in sample S2 and 2.66 mm/s and 1.98 mm/s in sample S3) and isomer shift (1.17 mm/s and 1.01 mm/s in sample S2 and 1.18 mm/s and 1.14 mm/s in sample S3). The most shifted doublets, with isomer shifts ranging between 1.14 mm/s and 1.18 mm/s were assigned to Fe2+ ions with octahedral oxygen coordination whereas the ones with isomer shift of about 1.00 mm/s were assigned to Fe2+ ions in tetrahedral coordination [17,18,19]. Finally, the third iron component with much lower IS (0.21 mm/s in sample S2 and 0.34 mm/s in sample S3) and QS (0.54 mm/s in sample S2 and almost negligible in sample S3) were assigned to Fe3+ ions with tetrahedral coordination in the S2 sample and octahedral coordination in S3 sample [17,18]. The Mössbauer data involve for both S2 and S3 samples, a mixture of Fe2+ (42%) and Fe3+ (58%) states. The S4 sample contains only Fe3+ ions. Tacking into account the behaviour of the absorption spectra presented in Fig.1, one can conclude that the absorption coefficient has to be dependent mainly on the Fe oxida-

Fig. 4. Mössbauer spectra of the Fe (S1), Fe+Mn (S3) and Fe+Cr (S4) containing aluminophosphate glasses, respectively, obtained at room temperature.

tion state, which in turn has to influence the oxidation state of the pair-doping ion. In the present case, Mn and Fe ions should accommodate a bivalent oxidation state and Cr a hexavalent one. The fact that Fe3+ ions (in a proportion of 58%) are evidenced in both S2 and S3 samples stands for a relatively higher oxidation potential of the aluminophosphate matrix, inducing electron transfer process from Fe (or Mn) ions to the matrix. Nevertheless, the matrix presents the same oxidation potential also in the S4 sample. The presence of only Fe3+ ions (100%) in S4 sample, can be explained only via an electron transfer from the Fe2+ ions (still 42% in sample S2 and S3) to the unstable hexavalent Cr ions, resulting in an additional valence states of Cr (e.g. Cr3+) in the S4 sample, with direct influence of the UV-


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Table 4. Mossbauer components of the analyzed samples and their corresponding hyperfine parameters. The oxidation state and the coordination of the different iron sites are also presented. Sample

Mossbauer component

IS (mm/s)

QS (mm/s)

R. A. (%)

Oxidation state

Oxygen coordination

S2 1%Fe

Doublet 1 Doublet 2 Doublet 3 Doublet 1 Doublet 2 Doublet 3 Doublet 1 Doublet 2

1.17(1) 1.01(2) 0.21(2) 1.18(1) 1.14(2) 0.34(2) 0.16(2) 0.03(1)

3.17(1) 2.24(3) 0.54(2) 2.66(1) 1.98(4) 0.01(3) 0.96(5) 0.40(2)

19 23 58 32 10 58 34 66

Fe2+ Fe2+ Fe3+ Fe2+ Fe2+ Fe3+ Fe3+ Fe3+

6 4 4 6 6 6 4 Lower than 4

S3 1%Fe +1%Mn S4 1%Fe +1%Cr

VIS-NIR. Therefore, the different absorption spectrum of the S4 sample (in comparison with the spectra of the S2 and S3 samples) has to be due to the Cr ions in this sample. On the other site, starting from the Mössbauer data related to the iron coordination for different iron sites and the relative population of those sites, could be estimated an average Fe coordination for each sample. Average coordination of 4.84, 4.38 and lower than 4.00 were obtained for the glasses doped with Fe+Mn (S3), Fe (S2) and Fe+Cr (S4). At a glance of Fig. 3, can be observed that whatever the wavelength is, the refractive index are the highest in glasses doped with Fe+Mn (S3), intermediate in glasses doped with Fe (S2) and the lowest in Fe+Cr (S4) doping glasses, suggesting so a correlation between the absolute values of the refractive index and the Fe coordination.

4. CONCLUSIONS Undoped and transition metal-doped aluminophosphate glasses prepared by wet non-conventional method were investigated by optical methods and Mössbauer spectroscopy. The UV-VIS-NIR absorption spectra are strongly dependent on the doping ions. The Fe and Fe+Mn doped samples evidence a 1050 nm peak typical for the Fe2+ ions, in agreement with Mössbauer results proving a 58% relative amount of Fe2+ ions in the both samples. The Fe+Cr doped glass exhibit absorption peaks at 460 and 660 nm, specific for Cr3+ ions. The Mössbauer results suggest indirectly a valence state in the above sample, through the complete oxidation of the Fe2+ ions. The IR spectra

prove the role of vitreous network former for phosphorous whereas the tetrahedral/octahedral configurations of the Fe doped glasses, evidenced by Mössbauer spectroscopy, suggests the former/ modifier role of the doping ions. A correlation between the values of the refractive index and the Fe mean coordination might be suggested via the analysis of the dispersion curves and the Mössbauer data.

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