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Synthesis and investigations on correlation between EPR and optical properties of Fe doped Li2SiO3
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Synthesis and investigations on correlation between EPR and optical properties of Fe doped Li2SiO3 ⁎
M.S. Pathaka, N.O. Gopalb, N. Singha, M. Mohapatrac, J.L. Raod, Jung-Kul Leea, , Vijay Singha,
⁎
a
Department of Chemical Engineering, Konkuk University, Seoul 05029, Republic of Korea Department of Physics, Vikrama Simhapuri University PG Center, Kavali 524201, India c Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India d Department of Physics, Sri Venkateswara University, Tirupati 517502, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Li2SiO3 EPR Optical absorption spectrum Combustion synthesis Fe
Iron doped lithium metasilicate sample was synthesized using a combustion technique and characterized by XRD (X ray diffraction), SEM (scanning electron microscopy), FTIR (Fourier transform infrared spectroscopy), optical, and EPR (electron paramagnetic resonance) analyses. The phase purity of the combustion synthesized products was confirmed by XRD analysis. SEM data suggested the formation of a porous compound by virtue of the entrapment of the gases that evolved during the sample synthesis. FTIR data confirmed the formation of SieO bonds in the system. Optical data confirmed the existence of both divalent and trivalent iron in the system. Characteristic absorption bands in the region 215–270 nm and 535–620 nm were observed due to the presence of Fe3+ in Oh and Td geometry respectively. On the other hand, the presence of bands at 967 and 1442 nm suggested the stabilisation of Fe2+ also in both Oh and Td geometries, respectively. The divalent iron being a non-Kramer ion, could not be observed by EPR. However, strong temperature-dependent EPR signals were observed in the sample owing to Fe3+. By analyzing the EPR data, super-paramagnetic type of behaviour was observed in the system. Furthermore, the relaxation times along with other EPR spectroscopic parameters were estimated for the system.
1. Introduction In recent years, alkali silicates have been widely studied due to their potential application as electrical, thermal, and optical materials [1–3]. Lithium metasilicate (Li2SiO3) is a promising material for the construction of solid tritium breeders due to its excellent properties such as tritium solubility; compatibility with other blanket and structural materials; thermo-physical, chemical, and mechanical stability at higher temperatures; and favorable irradiation behaviour [4–7]. Also, it is a technologically important ceramic system for applications in electronic devices such as the in-battery functionality and ceramics of low thermal expansion glass (that are used in ceramic bobs) [8–11]. A significant amount of research has been carried out in recent years on its application as a carbon dioxide (CO2) sorbent material as per the following reaction: Li2SiO3 + CO2 ↔ silicon dioxide (SiO2) + lithium carbonate (Li2CO3) [12]. Li2SiO3 is a member of the family of iso-structural compounds with the general formula A2BO3 [13]. Its polar orthorhombic symmetry is with the mm2 point group suggesting that the material is useful for piezoelectric, pyroelectric, and electro-optic
⁎
applications. Most of its properties such as the dielectric constant and conductivity depend on the composition and microstructure of the material. A number of techniques have been reported for the synthesis of lithium (Li) silicate compounds such as the solid-state reaction [14], microemulsion [15], sol–gel method [16], hydrothermal [17] and combustion synthesis [18]. Cruz et al. [19] studied the effects of temperature on the Li2SiO3 phase using a modified combustion method for which lithium hydroxide (LiOH) and silicic acid (H2SiO3) served as the precursors. They had prepared the Li2SiO3 phase with a few impurities (Li2Si2O5, SiO2) at 450 °C and pure Li2SiO3 at 650 °C. Zhang et al. [16] had prepared Li2SiO3 powder at 450 °C using a sol–gel method. However, it was thermally unstable and transformed completely to lithium disilicate (Li2Si2O5) at higher temperatures (≥900 °C). A lack of information exists regarding the exploration of the optical properties of Li2SiO3 as a phosphor material. In general, an inorganic host matrix that shows phosphorescence or fluorescence (photoluminescence) is known as a phosphor material. Only a number of reports have been published in the scientific literature to the best of our
Corresponding authors. E-mail addresses: [email protected] (J.-K. Lee), [email protected] (V. Singh).
https://doi.org/10.1016/j.jnoncrysol.2018.08.009 Received 19 June 2018; Received in revised form 2 August 2018; Accepted 7 August 2018 0022-3093/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Pathak, M.S., Journal of Non-Crystalline Solids (2018), https://doi.org/10.1016/j.jnoncrysol.2018.08.009
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knowledge regarding this aspect of the material. Naik et al. [20] reported the photoluminescence properties of a rare-earth-doped (Ce3+, Eu3+, Tb3+) Li silicate. Sabikoglu et al. [21] investigated the photoluminescence properties of the Li2SiO3:Ln (Ln = Er3+, Eu3+, Dy3+, and Sm3+) phosphor. Singh et al. [18] studied the radiation-induced defects in Li2SiO3:Sm phosphor. Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique that can provide valuable information regarding any paramagnetic species present in a matrix. EPR properties of Fe3+ doped host materials have been investigated widely. To the best of the authors' knowledge, however, the reports on the EPR and optical properties of Fe3+ doped Li2SiO3 have been hardly studied as compared to other hosts. In the present work, we discuss the synthesis of Fe3+ doped Li2SiO3 via a solution-combustion method using glycine as a fuel. The synthesized product was further characterized for their physical, optical and EPR using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, ultravioletvisible (UV–Vis) spectrometry, and EPR techniques. This was done in order to get a structure-property correlation of the Fe3+ ions in the Li2SiO3 matrix.
Fig. 1. Black lines at the top show the XRD pattern of the synthesized Fe doped lithium metasilicate (Li2SiO3) sample. Blue lines at the bottom show the standard XRD peaks taken from the International Centre for Diffraction Data (JCPDS Card No.-70-0330) database for the orthorhombic Li2SiO3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Material preparation and analysis Powder sample of Li2SiO3:Fe3+ phosphor was synthesized via a combustion route for which a fuel and an oxidizer are required. The basic condition for the solution combustion reaction was carried out as follows: The host to fuel ratio was calculated using the total oxidizing (O) and reducing valences (F) based on the concept of propellant chemistry. Akin to a typical synthesis, LiNO3, SiO2, Fe(NO3)3·9H2O and glycine (C2H5NO2) served as the precursor materials for a dissolution in minimum quantities of deionised water in a 150-ml glass beaker to obtain a homogeneous solution. The beaker was then transferred into a furnace that had been preheated to 550 °C. The combustion occurred with the introduction of the solution along with the evolution of the gases. The solution frothed and swelled, forming foam that ruptured with a flame that lasted for several seconds. The product-formation reaction was self-propagating that maintained a high temperature for 1–5 s. Then, the beaker was immediately removed from the muffle furnace. After the reaction, the materials were crushed with a mortar and pestle and then placed in 50-ml alumina crucibles for a 980-°C heat treatment that lasted for 7 h. The resultant powder was used for a further characterization. The crystal phase of the prepared phosphor was analyzed using an XRD pattern that had been measured using the X'Pert PRO-MRD device (PAN analytical, The Netherlands). The vibrational modes of the prepared phosphor were studied using FTIR spectrum that was measured using the Rx1 instrument (Perkin Elmer, U.S.A.) in the range from 4000 to 400 cm−1. The morphology was analyzed using the S-4300 SEM (Hitachi, Japan). The UV–Vis-near infrared (NIR) absorption of the samples was measured at room temperature (RT) using diffuse-reflectance spectroscopy for which the Cary Instruments 6000i UV–VisNIR spectrophotometer (Agilent Technologies, Inc., U.S.A.), equipped with an integrating sphere, was utilized. A powdered sample was taken in a quartz tube for the EPR measurements. The temperature dependence of the EPR spectra was studied using the FE1X EPR spectrometer (JEOL, Japan) operated in the X-band frequency with a field modulation of 100 kHz.
diffraction pattern (JCPDS) Card No.-70-0330 as shown in Fig. 1 (b). It was observed that the system crystallized in an orthorhombic phase with end centered lattice formation having space group Cmc21. From the observed pattern the approximate lattice parameters were evaluated as follows: a = 9.390, b = 5.40 and c = 4.66 Å. The XRD parameters including the peak positions and their assigned planes for the synthesized sample are listed in Table 1. There was no signature of impurity peaks (due to Fe3+ ion) at the current doping levels. The average crystallite size could be estimated from the 100% (most intense) peak using the following Scherrer equation:
D = 0.9 λ/β cosθ
(1)
Here λ is the wavelength of the incident X-ray, θ is the corresponding Bragg's diffraction angle, and β is the full width at half maximum (FWHM) of the (111) peak. Using the above equation the average crystallite size was calculated to be 48.92 nm. 3.2. SEM: morphological studies Fig. 2 (A) and (B) shows the SEM micrographs of the sample with two Table 1 XRD parameters of the Fe doped Li2SiO3 sample. 2θ values
Intensity (relative values)
hkl
26.98 18.88 18.95 33.05 33.17 38.61 38.52 38.41
100 81.3 80.9 49 42 41.9 23.2 19
3. Results and discussion
59.19 72.92
18.6 14.5
3.1. Powder XRD: structural studies
51.71
12.1
The XRD pattern of the Li2SiO3:Fe3+ synthesized using the solutioncombustion method is shown in Fig. 1 (a). All of the observed diffraction peaks for the sample were well matched with the standard powder
51.80 69.74
11 10.7
1 2 1 3 0 0 0 3 2 3 3 0 1 3 0 3 0
2
1 0 1 1 2 0 2 1 2 3 3 4 3 1 2 1 2
1 0 0 0 0 2 1 1 0 0 2 1 0 2 2 3 3
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Transmittance (%)
80
60
40
20
0
500
1000
1500
2000
2500
-1 Wavenumber(cm )
3000
3500
4000
Fig. 3. Fourier transform infrared (FTIR) spectrum of Fe doped Li2SiO3 sample.
transitions in the NIR region. In addition, these transitions are superimposed, either to the metal–metal charge-transfer bands (CTB) in the visible range or to the more intense metal–ligand CTB in the near-UV range [22–29]. The optical-absorption spectrum of the Fe-doped Li2SiO3 at RT (room temperature) is shown in Fig. 4. From this figure, it is clear that the sample exhibited the characteristic absorption bands of both the Fe2+ and Fe3+ ions in addition to the CTB. The intensity of these CTBs tend to be approximately two to three orders higher than that of the crystal-field bands as these are formed due to the transfer of electrons from the ligands to the metal ions [22–24]. As shown in Fig. 4, the strong absorption bands at 217 and 268 nm in the UV region are due to the metal (Fe3+/Fe2+)-to-ligand (O) CTB. Many of the earlier reports on the optical-absorption transitions of silicate-mineral-containing Fe and Fe-doped silicate glasses have shown the presence of ‘Fe’ ions in both Fe3+ and Fe2+ states in tetrahedral and octahedral geometries. Furthermore, these studies have also elucidated the effect of the Fe content on the structural and optical properties [24–29]. For instance, Ramesh Babu et al. [27] studied iron(III) oxide (Fe2O3)mixed Li2O–yttrium(III) oxide (Y2O3)–SiO2 glasses, and the observed optical-absorption bands were assigned to the Fe3+ and Fe2+ present in octahedral and tetrahedral geometries. Recently, Singh and Singh [28] studied the effects of Fe3+ reduction on the properties of silicate glasses and glass ceramics in situ, while Vercamer et al. [29] reported the effect of the Fe redox state on silicate glasses. In the present work, a small kink at ~394 nm and a weak absorption band at ~449 nm have been assigned to the 6A1g(S) → 4T2g(D) and 6A1g(S) → 4A1g(G), 4Eg(G) transitions of the Fe3+ in octahedral coordination, respectively [24–29]. The bands that were observed in the visible region around 535 and 620 nm have been assigned to 6A1g(S) → 4T2g(G) and 6A1g(S) → 4T1g(G), respectively, and are attributed to the Fe3+ ions situated in tetrahedral symmetry [24–29]. As shown in the inset of Fig. 4, the broad absorption band at around 976 nm is due to the 5T2g → 5Eg transition of the intra-octahedral transition of Fe2+ [27], and the band around 1442 nm is due to the 5Eg → 5T2g transition for the tetrahedrally coordinated Fe2+ ions [28,29]. All of the bands that are evident in the optical-absorption spectrum coincide well with the Fe-bearing silicate minerals and the Fe-doped silicate glasses [24–29].
Fig. 2. Scanning electron microscope (SEM) micrographs of the Fe doped Li2SiO3 sample. (A) Low magnification SEM micrograph of sample and (B) the enlarged section highlighted as (a) in (A).
different magnification levels. Fig. 2 (A) shows that the powder comprises of irregular lumps, with each showing a different morphology and wide size distribution. Fig. 2 (B) shows a magnified view of the “zone a” of Fig. 2 (A). The figure shows that the sample consists of small pores and voids among the aggregated lumps that are due to the gas evolution during the combustion process. In Fig. 2 (B), the presence of several small particles is evident, which is in agreement with the calculated crystallite size. 3.3. FTIR spectrum: vibrational properties Fig. 3 shows the FTIR spectrum of the sample. The peaks seen at 414 and 524 cm−1 are assigned to deformation vibrations, while the peaks at around 612, 733, 776, 849, 951, 986 and 1074 cm−1 are assigned to the SieOeSi and OeSieO stretching vibrations. The bands at 1441 cm−1 and 1508 cm−1 are assigned to C]O stretching vibration. The broad peak centered around 3447 cm−1 is attributed to the OeH vibration.
3.5. Electron paramagnetic resonance (EPR) studies EPR is an exceptional tool for the study of magnetic properties and spin dynamics of materials. The super-paramagnetic and ferrimagnetic type behaviour and the associated interparticle dipole and super-exchange interactions can be very well explained using the technique [30–35]. Fig. 5 shows the temperature dependent EPR spectra of the Fedoped Li2SiO3 sample. The EPR spectrum, recorded at RT, exhibited a resonance signal with a considerably large peak-to-peak linewidth
3.4. Optical studies Among the transition elements, the optical properties of iron (Fe) ions are of great interest. While Fe3+ exhibits low intensity spin forbidden optical transitions in the visible region, Fe2+ exhibits strong spin-allowed 3
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1442
0.6
535
800
1000
1200
1400
1600
Wavelength (nm)
620
449
0.3
394
Abs. (a.u.)
976
0.9
217 268
M.S. Pathak et al.
0.0 500
1000
1500
Wavelength (nm) Fig. 4. Absorption spectrum of Fe doped Li2SiO3 sample.
Fe ions in this sample. From the EPR spectra at various temperatures, the EPR parameters, such as the ΔHpp, g-factor, and number of spin (NS; that participate in the resonance) and the spin–spin relaxation time (T2) were estimated and presented in Table 2. Fig. 6 shows the variation of the ΔHpp as a function of the temperature. An appreciable increase of the EPR line width (from 2641 G at RT to 3121 G at 123 K) was observed with the decreasing of the temperature. This is attributed to the increased magnetic-dipole interactions at lower temperatures. The T2 values at various temperatures were obtained from the ΔHpp and the associated g values. The values of NS were obtained from the ΔHpp and the peak heights [40,41]. The T2 temperature dependence plot is shown in Fig. 7, wherein the T2 value continues to decrease with the decreasing of the temperature. This is also attributed to the increased dipolar interactions among the paramagnetic ions at lower temperatures.
First derivative of absorption (a.u.)
RT 263 K 243 K 213 K 183 K 153 K 123 K
4. Conclusion 0
200
400
600
800
Samples of Fe-doped Li2SiO3 were synthesized using a solution combustion technique and subsequently characterized using XRD, SEM, FTIR, optical and EPR spectroscopic techniques. It was observed that the solution combustion route provides a direct method for the preparation of homogenous samples and the method as a whole can be extended to synthesize a number of other oxide-based phosphor samples. The synthesized product showed a porous morphology in accordance with the
Magnetic Field (mT) Fig. 5. EPR spectra of Fe doped Li2SiO3 sample with varying temperature.
(ΔHpp) of approximately 2641 G and an effective ‘g’ value of 2.553, which may be due to the presence of very high Fe3+ concentration in the sample. Many EPR studies on Fe-doped and Fe-oxide complexed materials have revealed that the variations in the resonance ΔHpp and g-factor values are caused by strong microscopic magnetic interactions inside the materials. These interactions are primarily the result of inter-particle (magnetic) dipole interactions and the super-exchange interactions [36–39]. The magnetic dipole interactions between paramagnetic ions can increase the respective line widths and the g values. On the other hand, the superexchange interaction between the paramagnetic ions (via oxygen ions) can reduce the line widths and g values. Particularly, it is possible that the strong inter-particle magnetic-dipole interactions are the reason for the relatively broad lines with the associated higher g-values even at RT [39]. In the present work, the considerably large line width values and higher gvalues at RT indicated dominant dipole–dipole interactions between the Fe3+ ions, which may be due to the superparamagnetic behaviour of the
Table 2 EPR characteristics of the Fe doped Li2SiO3 sample as a function of temperature. Temp (K)
ΔHpp (G)
g value
Ns (×1010 spins g−1)
T2 (10−12 s)
RT 263 243 213 183 153 123
2641 2696 2799 2900 2983 3056 3121
2.553 2.564 2.576 2.583 2.591 2.597 2.623
5.35 5.57 6.35 7.87 10.22 11.81 12.32
9.73 9.49 9.13 8.75 8.49 8.26 8.01
ΔHpp = peak-to-peak line width. NS = number of spins. T2 = spin–spin relaxation time. 4
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EPR line-width (gauss)
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3000
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2600 120
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Temperature (K)
Fig. 6. Variation of peak-to-peak linewidth (ΔHpp) as a function of temperature (K).
9.8 9.6 9.4
T2 (10
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S)
9.2 9.0 8.8 8.6 8.4 8.2 8.0 7.8 120
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interpretation of the SEM data. This is perhaps due to the presence of the trapped gaseous substances evolved (and subsequently trapped in the matrix) during the synthesis. The infrared data for the bending and stretching bond vibrations confirmed the formation of Si-O bonds in the system. A correlational procedure was carried out between the optical and EPR data to obtain detailed information regarding the oxidation state and the local structure of the dopant ‘Fe’ atom in the phosphor. The existence of both the 2+ and 3+ Fe ions in the phosphor was confirmed from the optical data. The temperature-dependent EPR data showed that the trivalent-Fe ions exhibited a typical super-paramagnetic behaviour. In addition, the trend in the relaxation time values as a function of temperature suggested the existence of paramagnetic interaction among the Fe ions in the sample. Acknowledgements This paper was supported by Konkuk University in 2016.
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Synthesis and investigations on correlation between EPR and optical properties of Fe doped Li2SiO3 ⁎
M.S. Pathaka, N.O. Gopalb, N. Singha, M. Mohapatrac, J.L. Raod, Jung-Kul Leea, , Vijay Singha,
⁎
a
Department of Chemical Engineering, Konkuk University, Seoul 05029, Republic of Korea Department of Physics, Vikrama Simhapuri University PG Center, Kavali 524201, India c Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India d Department of Physics, Sri Venkateswara University, Tirupati 517502, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Li2SiO3 EPR Optical absorption spectrum Combustion synthesis Fe
Iron doped lithium metasilicate sample was synthesized using a combustion technique and characterized by XRD (X ray diffraction), SEM (scanning electron microscopy), FTIR (Fourier transform infrared spectroscopy), optical, and EPR (electron paramagnetic resonance) analyses. The phase purity of the combustion synthesized products was confirmed by XRD analysis. SEM data suggested the formation of a porous compound by virtue of the entrapment of the gases that evolved during the sample synthesis. FTIR data confirmed the formation of SieO bonds in the system. Optical data confirmed the existence of both divalent and trivalent iron in the system. Characteristic absorption bands in the region 215–270 nm and 535–620 nm were observed due to the presence of Fe3+ in Oh and Td geometry respectively. On the other hand, the presence of bands at 967 and 1442 nm suggested the stabilisation of Fe2+ also in both Oh and Td geometries, respectively. The divalent iron being a non-Kramer ion, could not be observed by EPR. However, strong temperature-dependent EPR signals were observed in the sample owing to Fe3+. By analyzing the EPR data, super-paramagnetic type of behaviour was observed in the system. Furthermore, the relaxation times along with other EPR spectroscopic parameters were estimated for the system.
1. Introduction In recent years, alkali silicates have been widely studied due to their potential application as electrical, thermal, and optical materials [1–3]. Lithium metasilicate (Li2SiO3) is a promising material for the construction of solid tritium breeders due to its excellent properties such as tritium solubility; compatibility with other blanket and structural materials; thermo-physical, chemical, and mechanical stability at higher temperatures; and favorable irradiation behaviour [4–7]. Also, it is a technologically important ceramic system for applications in electronic devices such as the in-battery functionality and ceramics of low thermal expansion glass (that are used in ceramic bobs) [8–11]. A significant amount of research has been carried out in recent years on its application as a carbon dioxide (CO2) sorbent material as per the following reaction: Li2SiO3 + CO2 ↔ silicon dioxide (SiO2) + lithium carbonate (Li2CO3) [12]. Li2SiO3 is a member of the family of iso-structural compounds with the general formula A2BO3 [13]. Its polar orthorhombic symmetry is with the mm2 point group suggesting that the material is useful for piezoelectric, pyroelectric, and electro-optic
⁎
applications. Most of its properties such as the dielectric constant and conductivity depend on the composition and microstructure of the material. A number of techniques have been reported for the synthesis of lithium (Li) silicate compounds such as the solid-state reaction [14], microemulsion [15], sol–gel method [16], hydrothermal [17] and combustion synthesis [18]. Cruz et al. [19] studied the effects of temperature on the Li2SiO3 phase using a modified combustion method for which lithium hydroxide (LiOH) and silicic acid (H2SiO3) served as the precursors. They had prepared the Li2SiO3 phase with a few impurities (Li2Si2O5, SiO2) at 450 °C and pure Li2SiO3 at 650 °C. Zhang et al. [16] had prepared Li2SiO3 powder at 450 °C using a sol–gel method. However, it was thermally unstable and transformed completely to lithium disilicate (Li2Si2O5) at higher temperatures (≥900 °C). A lack of information exists regarding the exploration of the optical properties of Li2SiO3 as a phosphor material. In general, an inorganic host matrix that shows phosphorescence or fluorescence (photoluminescence) is known as a phosphor material. Only a number of reports have been published in the scientific literature to the best of our
Corresponding authors. E-mail addresses: [email protected] (J.-K. Lee), [email protected] (V. Singh).
https://doi.org/10.1016/j.jnoncrysol.2018.08.009 Received 19 June 2018; Received in revised form 2 August 2018; Accepted 7 August 2018 0022-3093/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Pathak, M.S., Journal of Non-Crystalline Solids (2018), https://doi.org/10.1016/j.jnoncrysol.2018.08.009
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knowledge regarding this aspect of the material. Naik et al. [20] reported the photoluminescence properties of a rare-earth-doped (Ce3+, Eu3+, Tb3+) Li silicate. Sabikoglu et al. [21] investigated the photoluminescence properties of the Li2SiO3:Ln (Ln = Er3+, Eu3+, Dy3+, and Sm3+) phosphor. Singh et al. [18] studied the radiation-induced defects in Li2SiO3:Sm phosphor. Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique that can provide valuable information regarding any paramagnetic species present in a matrix. EPR properties of Fe3+ doped host materials have been investigated widely. To the best of the authors' knowledge, however, the reports on the EPR and optical properties of Fe3+ doped Li2SiO3 have been hardly studied as compared to other hosts. In the present work, we discuss the synthesis of Fe3+ doped Li2SiO3 via a solution-combustion method using glycine as a fuel. The synthesized product was further characterized for their physical, optical and EPR using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, ultravioletvisible (UV–Vis) spectrometry, and EPR techniques. This was done in order to get a structure-property correlation of the Fe3+ ions in the Li2SiO3 matrix.
Fig. 1. Black lines at the top show the XRD pattern of the synthesized Fe doped lithium metasilicate (Li2SiO3) sample. Blue lines at the bottom show the standard XRD peaks taken from the International Centre for Diffraction Data (JCPDS Card No.-70-0330) database for the orthorhombic Li2SiO3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Material preparation and analysis Powder sample of Li2SiO3:Fe3+ phosphor was synthesized via a combustion route for which a fuel and an oxidizer are required. The basic condition for the solution combustion reaction was carried out as follows: The host to fuel ratio was calculated using the total oxidizing (O) and reducing valences (F) based on the concept of propellant chemistry. Akin to a typical synthesis, LiNO3, SiO2, Fe(NO3)3·9H2O and glycine (C2H5NO2) served as the precursor materials for a dissolution in minimum quantities of deionised water in a 150-ml glass beaker to obtain a homogeneous solution. The beaker was then transferred into a furnace that had been preheated to 550 °C. The combustion occurred with the introduction of the solution along with the evolution of the gases. The solution frothed and swelled, forming foam that ruptured with a flame that lasted for several seconds. The product-formation reaction was self-propagating that maintained a high temperature for 1–5 s. Then, the beaker was immediately removed from the muffle furnace. After the reaction, the materials were crushed with a mortar and pestle and then placed in 50-ml alumina crucibles for a 980-°C heat treatment that lasted for 7 h. The resultant powder was used for a further characterization. The crystal phase of the prepared phosphor was analyzed using an XRD pattern that had been measured using the X'Pert PRO-MRD device (PAN analytical, The Netherlands). The vibrational modes of the prepared phosphor were studied using FTIR spectrum that was measured using the Rx1 instrument (Perkin Elmer, U.S.A.) in the range from 4000 to 400 cm−1. The morphology was analyzed using the S-4300 SEM (Hitachi, Japan). The UV–Vis-near infrared (NIR) absorption of the samples was measured at room temperature (RT) using diffuse-reflectance spectroscopy for which the Cary Instruments 6000i UV–VisNIR spectrophotometer (Agilent Technologies, Inc., U.S.A.), equipped with an integrating sphere, was utilized. A powdered sample was taken in a quartz tube for the EPR measurements. The temperature dependence of the EPR spectra was studied using the FE1X EPR spectrometer (JEOL, Japan) operated in the X-band frequency with a field modulation of 100 kHz.
diffraction pattern (JCPDS) Card No.-70-0330 as shown in Fig. 1 (b). It was observed that the system crystallized in an orthorhombic phase with end centered lattice formation having space group Cmc21. From the observed pattern the approximate lattice parameters were evaluated as follows: a = 9.390, b = 5.40 and c = 4.66 Å. The XRD parameters including the peak positions and their assigned planes for the synthesized sample are listed in Table 1. There was no signature of impurity peaks (due to Fe3+ ion) at the current doping levels. The average crystallite size could be estimated from the 100% (most intense) peak using the following Scherrer equation:
D = 0.9 λ/β cosθ
(1)
Here λ is the wavelength of the incident X-ray, θ is the corresponding Bragg's diffraction angle, and β is the full width at half maximum (FWHM) of the (111) peak. Using the above equation the average crystallite size was calculated to be 48.92 nm. 3.2. SEM: morphological studies Fig. 2 (A) and (B) shows the SEM micrographs of the sample with two Table 1 XRD parameters of the Fe doped Li2SiO3 sample. 2θ values
Intensity (relative values)
hkl
26.98 18.88 18.95 33.05 33.17 38.61 38.52 38.41
100 81.3 80.9 49 42 41.9 23.2 19
3. Results and discussion
59.19 72.92
18.6 14.5
3.1. Powder XRD: structural studies
51.71
12.1
The XRD pattern of the Li2SiO3:Fe3+ synthesized using the solutioncombustion method is shown in Fig. 1 (a). All of the observed diffraction peaks for the sample were well matched with the standard powder
51.80 69.74
11 10.7
1 2 1 3 0 0 0 3 2 3 3 0 1 3 0 3 0
2
1 0 1 1 2 0 2 1 2 3 3 4 3 1 2 1 2
1 0 0 0 0 2 1 1 0 0 2 1 0 2 2 3 3
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Transmittance (%)
80
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40
20
0
500
1000
1500
2000
2500
-1 Wavenumber(cm )
3000
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4000
Fig. 3. Fourier transform infrared (FTIR) spectrum of Fe doped Li2SiO3 sample.
transitions in the NIR region. In addition, these transitions are superimposed, either to the metal–metal charge-transfer bands (CTB) in the visible range or to the more intense metal–ligand CTB in the near-UV range [22–29]. The optical-absorption spectrum of the Fe-doped Li2SiO3 at RT (room temperature) is shown in Fig. 4. From this figure, it is clear that the sample exhibited the characteristic absorption bands of both the Fe2+ and Fe3+ ions in addition to the CTB. The intensity of these CTBs tend to be approximately two to three orders higher than that of the crystal-field bands as these are formed due to the transfer of electrons from the ligands to the metal ions [22–24]. As shown in Fig. 4, the strong absorption bands at 217 and 268 nm in the UV region are due to the metal (Fe3+/Fe2+)-to-ligand (O) CTB. Many of the earlier reports on the optical-absorption transitions of silicate-mineral-containing Fe and Fe-doped silicate glasses have shown the presence of ‘Fe’ ions in both Fe3+ and Fe2+ states in tetrahedral and octahedral geometries. Furthermore, these studies have also elucidated the effect of the Fe content on the structural and optical properties [24–29]. For instance, Ramesh Babu et al. [27] studied iron(III) oxide (Fe2O3)mixed Li2O–yttrium(III) oxide (Y2O3)–SiO2 glasses, and the observed optical-absorption bands were assigned to the Fe3+ and Fe2+ present in octahedral and tetrahedral geometries. Recently, Singh and Singh [28] studied the effects of Fe3+ reduction on the properties of silicate glasses and glass ceramics in situ, while Vercamer et al. [29] reported the effect of the Fe redox state on silicate glasses. In the present work, a small kink at ~394 nm and a weak absorption band at ~449 nm have been assigned to the 6A1g(S) → 4T2g(D) and 6A1g(S) → 4A1g(G), 4Eg(G) transitions of the Fe3+ in octahedral coordination, respectively [24–29]. The bands that were observed in the visible region around 535 and 620 nm have been assigned to 6A1g(S) → 4T2g(G) and 6A1g(S) → 4T1g(G), respectively, and are attributed to the Fe3+ ions situated in tetrahedral symmetry [24–29]. As shown in the inset of Fig. 4, the broad absorption band at around 976 nm is due to the 5T2g → 5Eg transition of the intra-octahedral transition of Fe2+ [27], and the band around 1442 nm is due to the 5Eg → 5T2g transition for the tetrahedrally coordinated Fe2+ ions [28,29]. All of the bands that are evident in the optical-absorption spectrum coincide well with the Fe-bearing silicate minerals and the Fe-doped silicate glasses [24–29].
Fig. 2. Scanning electron microscope (SEM) micrographs of the Fe doped Li2SiO3 sample. (A) Low magnification SEM micrograph of sample and (B) the enlarged section highlighted as (a) in (A).
different magnification levels. Fig. 2 (A) shows that the powder comprises of irregular lumps, with each showing a different morphology and wide size distribution. Fig. 2 (B) shows a magnified view of the “zone a” of Fig. 2 (A). The figure shows that the sample consists of small pores and voids among the aggregated lumps that are due to the gas evolution during the combustion process. In Fig. 2 (B), the presence of several small particles is evident, which is in agreement with the calculated crystallite size. 3.3. FTIR spectrum: vibrational properties Fig. 3 shows the FTIR spectrum of the sample. The peaks seen at 414 and 524 cm−1 are assigned to deformation vibrations, while the peaks at around 612, 733, 776, 849, 951, 986 and 1074 cm−1 are assigned to the SieOeSi and OeSieO stretching vibrations. The bands at 1441 cm−1 and 1508 cm−1 are assigned to C]O stretching vibration. The broad peak centered around 3447 cm−1 is attributed to the OeH vibration.
3.5. Electron paramagnetic resonance (EPR) studies EPR is an exceptional tool for the study of magnetic properties and spin dynamics of materials. The super-paramagnetic and ferrimagnetic type behaviour and the associated interparticle dipole and super-exchange interactions can be very well explained using the technique [30–35]. Fig. 5 shows the temperature dependent EPR spectra of the Fedoped Li2SiO3 sample. The EPR spectrum, recorded at RT, exhibited a resonance signal with a considerably large peak-to-peak linewidth
3.4. Optical studies Among the transition elements, the optical properties of iron (Fe) ions are of great interest. While Fe3+ exhibits low intensity spin forbidden optical transitions in the visible region, Fe2+ exhibits strong spin-allowed 3
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1442
0.6
535
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1200
1400
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Wavelength (nm)
620
449
0.3
394
Abs. (a.u.)
976
0.9
217 268
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0.0 500
1000
1500
Wavelength (nm) Fig. 4. Absorption spectrum of Fe doped Li2SiO3 sample.
Fe ions in this sample. From the EPR spectra at various temperatures, the EPR parameters, such as the ΔHpp, g-factor, and number of spin (NS; that participate in the resonance) and the spin–spin relaxation time (T2) were estimated and presented in Table 2. Fig. 6 shows the variation of the ΔHpp as a function of the temperature. An appreciable increase of the EPR line width (from 2641 G at RT to 3121 G at 123 K) was observed with the decreasing of the temperature. This is attributed to the increased magnetic-dipole interactions at lower temperatures. The T2 values at various temperatures were obtained from the ΔHpp and the associated g values. The values of NS were obtained from the ΔHpp and the peak heights [40,41]. The T2 temperature dependence plot is shown in Fig. 7, wherein the T2 value continues to decrease with the decreasing of the temperature. This is also attributed to the increased dipolar interactions among the paramagnetic ions at lower temperatures.
First derivative of absorption (a.u.)
RT 263 K 243 K 213 K 183 K 153 K 123 K
4. Conclusion 0
200
400
600
800
Samples of Fe-doped Li2SiO3 were synthesized using a solution combustion technique and subsequently characterized using XRD, SEM, FTIR, optical and EPR spectroscopic techniques. It was observed that the solution combustion route provides a direct method for the preparation of homogenous samples and the method as a whole can be extended to synthesize a number of other oxide-based phosphor samples. The synthesized product showed a porous morphology in accordance with the
Magnetic Field (mT) Fig. 5. EPR spectra of Fe doped Li2SiO3 sample with varying temperature.
(ΔHpp) of approximately 2641 G and an effective ‘g’ value of 2.553, which may be due to the presence of very high Fe3+ concentration in the sample. Many EPR studies on Fe-doped and Fe-oxide complexed materials have revealed that the variations in the resonance ΔHpp and g-factor values are caused by strong microscopic magnetic interactions inside the materials. These interactions are primarily the result of inter-particle (magnetic) dipole interactions and the super-exchange interactions [36–39]. The magnetic dipole interactions between paramagnetic ions can increase the respective line widths and the g values. On the other hand, the superexchange interaction between the paramagnetic ions (via oxygen ions) can reduce the line widths and g values. Particularly, it is possible that the strong inter-particle magnetic-dipole interactions are the reason for the relatively broad lines with the associated higher g-values even at RT [39]. In the present work, the considerably large line width values and higher gvalues at RT indicated dominant dipole–dipole interactions between the Fe3+ ions, which may be due to the superparamagnetic behaviour of the
Table 2 EPR characteristics of the Fe doped Li2SiO3 sample as a function of temperature. Temp (K)
ΔHpp (G)
g value
Ns (×1010 spins g−1)
T2 (10−12 s)
RT 263 243 213 183 153 123
2641 2696 2799 2900 2983 3056 3121
2.553 2.564 2.576 2.583 2.591 2.597 2.623
5.35 5.57 6.35 7.87 10.22 11.81 12.32
9.73 9.49 9.13 8.75 8.49 8.26 8.01
ΔHpp = peak-to-peak line width. NS = number of spins. T2 = spin–spin relaxation time. 4
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EPR line-width (gauss)
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Fig. 6. Variation of peak-to-peak linewidth (ΔHpp) as a function of temperature (K).
9.8 9.6 9.4
T2 (10
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9.2 9.0 8.8 8.6 8.4 8.2 8.0 7.8 120
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Temperature (K) Fig. 7. Variation of the spin–spin relaxation time (T2) as a function of the temperature (K).
interpretation of the SEM data. This is perhaps due to the presence of the trapped gaseous substances evolved (and subsequently trapped in the matrix) during the synthesis. The infrared data for the bending and stretching bond vibrations confirmed the formation of Si-O bonds in the system. A correlational procedure was carried out between the optical and EPR data to obtain detailed information regarding the oxidation state and the local structure of the dopant ‘Fe’ atom in the phosphor. The existence of both the 2+ and 3+ Fe ions in the phosphor was confirmed from the optical data. The temperature-dependent EPR data showed that the trivalent-Fe ions exhibited a typical super-paramagnetic behaviour. In addition, the trend in the relaxation time values as a function of temperature suggested the existence of paramagnetic interaction among the Fe ions in the sample. Acknowledgements This paper was supported by Konkuk University in 2016.
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