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Structure and EPR investigations on Gd3+ ions in magnesium-lead-borophosphate glasses
Journal Pre-proof Structure and EPR investigations on Gd3+ ions in magnesium-leadborophosphate glasses
Nallamal. Kiran, Venkata Krishnaiah. Kummara, Nirlakala Ravi, D. Lenin PII:
S0022-2860(20)30201-5
DOI:
https://doi.org/10.1016/j.molstruc.2020.127877
Reference:
MOLSTR 127877
To appear in:
Journal of Molecular Structure
Received Date:
12 July 2019
Accepted Date:
08 February 2020
Please cite this article as: Nallamal. Kiran, Venkata Krishnaiah. Kummara, Nirlakala Ravi, D. Lenin, Structure and EPR investigations on Gd3+ ions in magnesium-lead-borophosphate glasses, Journal of Molecular Structure (2020), https://doi.org/10.1016/j.molstruc.2020.127877
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Structure and EPR investigations on Gd3+ ions in magnesiumlead-borophosphate glasses Nallamal. Kiran1, Venkata Krishnaiah. Kummara2,3,*, Nirlakala Ravi4, D. Lenin5 1Department
of Materials Science and Engineering, Harbin Institute of Technology Shenzhen, Graduate School, Shenzhen, Guangdong 518055, PR China. 2Laser Applications Research Group, Ton Duc Thang University, Ho Chi Minh City, Vietnam. 3Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam. 4Department of Physics, Rajeev Gandhi Memorial College of Engineering and Technology, Nandyal - 518501, India. 5Department of EEE, Rajeev Gandhi Memorial College of Engineering and Technology, Nandyal - 518501, India. *Corresponding author: [email protected]; (Venkata Krishnaiah Kummara) Abstract: Gadolinium (Gd3+)-doped magnesium-lead-borophosphate glasses composed of MgHPO4−PbO−B2O3–Gd2O3 (PPbBMgGd) were fabricated by usual melt-quenching approach. The physical properties such as refractive index (n), density (ρ) and molar volume (Vm) were attained. Thermal analysis of the glass has been performed through differential scanning calorimetry (DSC). Raman, Electron paramagnetic resonance (EPR), Fourier transform infrared (FTIR) and steady-state luminescence of PPbBMgGd glasses were investigated. The Raman and FTIR spectra explore the vibrational groups of both phosphate and borate networks in the PPbBMgGd glasses. The EPR spectra display three characteristic signals at room temperature with definite g values of 6.2, 2.8 and 2.0. These character signals were ascribed perhaps the Gd3+ ions that are located respectively at feeble, moderate and sturdy cubic symmetry sites. Moreover, the EPR spectra were also obtained at various low temperatures (123-297 °K) for 5.0 mol% of Gd3+-doped PPbBMgGd5.0 glass. Difference in population among the Zeeman levels (N) and paramagnetic susceptibility (χ) were evaluated. Photoluminescence spectrum reveals a broad intense band of Gd3+ ions upon excitation at 278 nm, could be suitable for visible broadband applications.
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Journal Pre-proof Keywords: Gd3+ ion; Borophosphate glasses; Fourier transform infrared; Electron Paramagnetic Resonance; Photoluminescence. 1. Introduction Rare earth (RE3+)-doped glasses are fascinating materials due to their significant applications for lasers, optical amplification, solid state lighting, and stack the radioactive ions [1–3]. Among the RE3+ ions, the ground level of 8S7/2 of Gadolinium (Gd3+, 4f7) is regarded an S-state [4]. The wide energy gap (32000 cm–1) among the ground (8S7/2) and the foremost excited (6P7/2) states of Gd3+ ion [5–6] that allows one to treat this ion as a hypothetical mediator (sensitizer) between the matrix and other RE3+ ions. Gd3+-doped glasses are particularly fascinating materials owing to their optical and magnetic properties [7]. Moreover, nature of magnetic interactions among the Gd3+ ions and their distribution in the glass matrix could be revealed through Electron paramagnetic resonance (EPR) as well as magnetic susceptibility measurements [8–10]. The importance of Phosphate glasses are widely used in technological applications [11-13]. However, the applications of these glasses are limited beacuase of hygroscopic nature, very poor chemical durability and therefore not very stable [14]. The forementiond drawbacks could be minimized by adding B2O3 to the phosphate network that improves the thermal, mechanical, optical and chemical properties of phosphate glasses [1516]. It is reported that the addition of B2O3 increases Tg and hence the glass structure becomes more rigid [17]. Usually, B2O3 is an excellent glass former comprises a trigonal units of [BO3]3− in the glass network. This trigonal changes to tetragonal units of [BO4] progressively and reversibly [[BO3]−↔[BO4]−] (i.e. called as boron anomaly) when the addition of glass modifiers that includes alkaline (e.g., MgO) or heavy metal oxides (e.g., PbO) are added [18]. Heavy metal oxide, PbO acts as both glass former and modifier that depends on its content. The PbO improves the linear and non-linear refractive index and lowers the phonon energy of the glass matrix. The PbO addition to the borophosphate glasses expands the transparency in
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Journal Pre-proof the ultraviolet (UV) region, the ability of glass formation, resistance to moisture, low devitrification and chemical durability [19-20]. In addition, borophosphate glasses have been investigated for low-melting glass solders or glass seals applications [21]. In this work, Raman, FTIR, EPR, DSC and photoluminescence of Gd3+-doped magnesium-lead-borophosphate (PPbBMgGd) glasses were investigated. The structural modifications triggered by adding the Gd3+ ions into the PPbBMgGd glasses were examined. The environment around the Gd3+ ions is systematically inspected by EPR studies as a function of Gd3+ ions concentration. The effect of concentration of Gd3+ ions and temperature in the range of 123–297 K on the EPR resonance signals is discussed. Photoluminescence of PPbBMgGd1.0 glass is obtained upon 278 nm excitation. The feasibility of these glasses for visible broad band applications can be studied.
2. Experimental technique 2.1. Glass preparation The Gd3+-doped glasses composed of (20−x)MgHPO4−50PbO−30B2O3–xGd2O3 (x = 0.1, 0.3, 0.5, 1.0, 3.0 and 5.0 in mol%) were fabricated by the melt-quenching technique. The glasses are coded as PPbBMgGd0.1, PPbBMgGd0.3, PPbBMgGd0.5, PPbBMgGd1.0, PPbBMgGd3.0, PPbBMgGd5.0. The appropriate amounts of MgHPO4, PbO, B2O3 and Gd2O3 were mixed thoroughly in the agate mortar and then shifted to a ceramic crucible. The blends were fired in an electrical furnace in air at 1223 K for 30 minutes. The molted glass was quenched suddenly at room temperature and subsequently annealed at 600 K. The glasses were permitted to cool down to room temperature and then refined for further studies.
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Journal Pre-proof 2.2. Characerization techniques Density (ρ) of the PPbBMgGd1.0 glass was obtained as 5.323 g/cm3 by using the Archimedes principle with distilled water as an immersion liquid. Refractive index (n) of the PPbBMgGd1.0 glass was found to be 1.612 at sodium wavelength by using Abbe refractometer. Molar volume (Vm) was calculated as 29 cm3 mol-1 for the PPbBMgGd1.0 glass. Thermal analysis of the PPbBMgGd1.0 glass in the range of 200-800 °C was carried out using a differential scanning calorimetry (DSC, TA instruments with model: SDT Q600, USA). Raman modes of the PPbBMgGd glasses upon excitation of 514 nm line of Ar+ laser were obtained by Lab Ram HR800 Raman spectrometer in the back scattering geometry. Fourier Transform Infrared (FTIR) profiles of the powdered glasses mixed with KBr in the form of pellet was collected using a Shimadzu IR Prestige 21 spectrophotometer. JEOL-FE1X EPR spectrometer functioning at the X-band frequency of 9.205 GHz has been utilized to record the EPR spectra at ambient atmosphere. The EPR spectra were also attained at low temperatures in the range of 123-297 K by means of a flexible temperature controller (JES UCT 2AX). Polycrystalline DPPH having a g value of 2.0036 has been employed as a typical field indicator. YVON Fluorolog-3 fluorimeter was employed to explore the photoluminescence spectrum of glass upon excitation at 278 nm of Xe lamp. 3. Results and Discussion 3.1 Thermal studies The 1.0 mol% Gd3+ doped powdered PCfBfTiH1.0 glass was used for thermal studies using differential scanning calorimetry (DSC) which provides the information about the glass the glass transition (Tg), onset crystallization (Tx) and crystallization (Tc) temperatures. The DSC thermograph of PPbBMgGd1.0 glass is shown in Fig. 1. The themogrpah exhibits the glass transition temperature at 427 ºC, onset crystallization temperature at 507 ºC and crystallization temperature at 547 ºC. 4
Journal Pre-proof 3.2 Raman studies Raman vibrational modes of the PPbBMgGd glasses were revealed through Raman spectra are shown in Fig. 2. Raman spectra comprise of several typical bands at 400, 471, 574, 722, 982, 1141, 1251, 1325, 1481, 1598, 1731 cm-1 related to the phosphate and borate vibrational groups. Raman spectra show an intense band at 1251 cm-1 assigned as P-O-P vibrations and it signifies as a maximum phonon energy of these glasses [22]. Initial band at 400-574 cm−1 corresponds to the bending modes of the cation motion and chain structure bending of PO4 networks [23]. Besides, the band is observed at 722 cm−1 due to the existence of borophosphate rings that favours at low modifier contents near meta-stoichiometry [24]. The low intensity bands are observed at around 1325 and 1481 cm−1 due to vibrations of boron bonds [25]. The band at 1669 cm-1 is due to the B-O− stretching vibrations involving non-bridging oxygen (NBO) in various Borate groups [26]. 3.3 Fourier transform infrared (FTIR) studies The FTIR spectra of PPbBMgGd glasses in the energy range of 4000–400 cm−1 are shown in Fig. 3. The spectra exhibit several vibration bands at 3340, 2972, 2356, 1391, 1045, 878, 675 cm-1. These characteristic vibrations are related to the B-O-B and P-O-P networks. The vibrational band at 3340 cm-1 is assigned as stretching vibrations of O–H and another band at around
1658 cm-1 corresponds to the bending vibrations of H–O–H [27]. The band at around
2356 cm-1 is assigned as the asymmetric stretching of adsorbed water molecules [28]. The band around at 1045 cm-1 perhaps the asymmetric starching vibrations of P-O-P chins of Q2 units and a small contribution of B-O band stretching vibrations in BO4 units from diborate (B4O7-2) group [29]. A band around at 675 cm–1 is allotted to the symmetric vibrations of P–O–P bonds in Q1 units of P–O–B links [30]. The band at 513 cm–1 is ascribed to the bending modes of O–P–O in Q1 (P2O7)4 units [31]. The bands are resolved significantly with increase of Gd3+ ions concentration. The similar behaviour is also observed in the Raman spectra. Over all, from 5
Journal Pre-proof the IR spectra, the intensity of IR bands increases with the increase of Gd3+ ions, it indicates the stability of the main network units. 3.4 Electron Paramagnetic Resonance (EPR) studies EPR spectra of PPbBMgGd glasses at ambient atmosphere display the resonance signals for different Gd3+ concentration are shown in Fig. 4. The Gd3+ ions doped PPbBMg glasses reveals three eminent resonance signals at g ≈ 6.2, g ≈ 2.8 and g ≈ 2.0 are ascribed to Gd3+ ions positioned at feeble, moderate and sturdy cubic symmetry sites respectively. These resonance siganls are the chacteristic spectrum of Gd3+ ions in non-crystalline matrix [32], which are the unique regular signature of S-state of RE3+:glasses. On the other hand, EPR spectra of 5.0 mol% Gd3+-doped PPbBMgGd5.0 glass were also obtained at low temperatures in the range of 123-297 K are shown in Fig. 5. As we can see from Fig. 5, the intensity of resonance signal decreases with increase in temperature of the glass. The difference in population among the Zeeman levels (N) can be evaluated by relating the zone beneath the absorption band with that of established concentration of CuSO4.5H2O. Weil et al [33] have been proposed the following relation which comprises the tentative parameters of both the sample under study as well as the standard one. 1
N
A x ( S c a n x ) 2 G s td ( B m ) s td ( g s td ) 2 [ S ( S 1) ] s td ( Ps td ) 2 A x ( S c a n x ) 2 G x ( B m ) x ( g x ) 2 [ S ( S 1) ] x ( Px )
1 2
[ S td ]
(1)
The subscripts ‘x’ and ‘std’ denote the quantities of PPbBMgGd glass and the standard CuSO4.5H2O sample, respectively. It is observed that the intense EPR signal at g ≈ 6.2 which can be utilized to extract the difference in population among the Zeeman levels (N) for various temperatures. The variation of log N with respect to 1/T is shown in Fig. 6. A linear variation of difference in population was observed with increasing the temperature of the glass. Generally, EPR spectra of RE3+ ions in the non-crystalline materials are highly aeolotropic and 6
Journal Pre-proof delicate to disparities from site to site in the ligand field [34]. The name of the spectrum is characterized as ‘‘U’’ type spectrum with an effective g values of 6.0, 2.8 and 2.0 [35]. It is usually detected in the glass matrix perhaphs the presence of Gd3+ ions even at low concentration (as revealed in Fig. 6). The shapes from U-spectrum having g ≈ 5.99, g ≈ 2.85 and g ≈ 1.98 are allocated to Gd3+ ions in the intermediate crystal field sites whose coordination numbers higher than six [36]. Whereas the line at g = 4.7 is typical indication of Gd3+ ions in the strong crystal field sites having a low coordination number [37]. A very analogous spectral topographies of Gd3+ ions in dissimilar glassy hosts have been reported [38, 39]. Recently, Srinivasulu et al [40] have been adopted a analogous method and ascribed the U-spectrum to an isolated Gd3+ ions. These Gd3+ ions positioned in a diversity of locations with a coordination number which was evaluated to be 8 or 9 in the glass network of tetrahedral through a spreading of spinHamiltonian parameters. 3.5 Paramagnetic susceptibility (χ) evaluated from EPR spectrum The magnetic susceptibility (χ) of Gd3+ ions can be evaluated at diverse temperatures with the help of below relation [41]
𝜒=
𝑁g2𝛽2𝐽(𝐽 + 1) 3𝐾𝐵𝑇
(2)
where N represents the amount of spins per meter cube, g represents the spectroscopic splitting factor, , KB represents the Bolzmann constant, β represents the Bohr magneton and J = 3/2. The N value can be evaluated through Eq. (1) and g ≈ 6.2 (taken from EPR data). The value of is evaluated at ambient as well as low temperatures. The variation of susceptibility (1/) with respect to absolute temperature T is shown in Fig. 7. A linear variation of amount of spins (log N) that are contributed in the resonance with espect to the reciprocal of absolute temperature (1/T) is shown in Fig. 7. It is observed that the 7
Journal Pre-proof difference in population among the Zeeman levels increases with decrease in the temperature. A linear variation is detected amongst log N and 1/T that obeys the Boltzmann distribution law. The experimental data is fitted to a straight line log N = (19.80 + (65.61/T)) by a least square method. The activation energy is obtained as 2.86 x 10–21 J that equalent to 0.012 eV. The difference between the reciprocal of susceptibility and absolute temperature in Gd3+:PPbBMg glass is shown in Fig. 7. It is noteworthy that the susceptibility decreases with increase in temperature following the Curie–Weiss law. From the graph, it is detected that the PPbBMg: Gd3+ glass obeys Curie–Weiss type of behaviour ( = C / (T- p)). The paramagnetic Curie temperature (p) and Curie constant (C) were calculated and are found to be p =105 K and 0.803 emu/mol, respectively, which are in good agreement with the values reported elsewhere [42]. 3.6 Photoluminescence emission Photoluminescence spectrum of the Gd3+ doped PPbBMgGd1.0 glass upon excitation at 278 nm line of Xe lamp is shown in Fig. 8. The other glasses were also exhibit the similar behaviour. Broad and strong photoluminescence band in the spectral range of 270-450 nm is observed a peak around at 313 nm which was allocated to the f-f transitions of the Gd3+ ions. It is reported that the emission of Gd3+ ion has been reported in the range of 285-334 nm with a peak around 315 nm which perhaphs the 6P7/2→8S7/2 transition upon excitation of 8S7/2 → 6I9/2 transition at around 275 nm. The excitation band can be detected in the UV region at around 278 nm which perhaphs the 8S7/2 → 6I9/2 transition of Gd3+ ion. Upon excitation at 278 nm, the Gd3+ ion in the ground level is shifted to the excited levels 6IJ. Then it relaxes to the 6PJ levels non-radiatively. Moreover, the Gd3+ ion is excited through UV radiation at around 278 nm, the preliminary higher energy level is populated and downs to its lower energy levels till it reaches at the 6P7/2 level through phonons and delivers the emission at 313 nm [43, 44]. The broad and
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Journal Pre-proof intese peak at 313 of PPbBMgGd1.0 glass could be useful for broadband near UV emitting device applications.
5. Conclusion Gadolinium (Gd3+)-doped magnesium-lead-borophosphate (MgHPO4−PbO−B2O3– Gd2O3) glasses have been investigated through the Raman, FTIR, DSC, EPR and photoluminescence studies. Physical properties of PPbBMgGd1.0 glass such as refractive index (n), density (ρ) and molar volume (Vm) have been attained. The DSC thermogrsph has been revealed the glass transition (Tg = 427 ºC), onset crystallization (Tg = 507 ºC) and crystallization (Tg = 547 ºC) temperatures. Raman and FTIR spectra have been revealed the characteristic bands of stretching and bending groups of P–O–P, O–P–O and B-O-B of phosphate as well as borate chains. The resonance signals of EPR with g ≈ 6.2, g ≈ 2.8 and g ≈ 2.0 have been ascribed to Gd3+ions. The population density (N) among the Zeeman states involving in the resonance and their paramagnetic susceptibility (χ) for g ≈ 6.2 resonance streak have been evaluated. It is noticed that N and χ values enhanced significalty with increase of Gd3+ ion concentration. The reciprocal of susceptibility (1/χ) along with temperature has been obeyed the Curie-Weiss law. Intense broad emission of PPbBMgGd1.0 glass was observed at around 313 nm under 278 nm of Xe lamp excitayion perhaps the f–f transitions of Gd3+ ion. Based on the luminescence and EPR studies, the Gd3+-doped magnesium-lead-borophosphate glasses could be a potential candidate for the broadband emitting devices in the spectral range of 270-400 nm. References 1. K. H. Mahmoud, Physica B 405 (2010) 4746. 2. T.A.Lodi, M.Sandrini, A. N. Medina, M.J .Barboza, F.Pedrochi, A.Steimacher, Optical materials 76 (2018) 231-236. 9
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Credit Author Statement Title: Structure and EPR investigations on Gd3+ ions in magnesium-lead-borophosphate glasses Author(s): Nallamala Kiran, Venkata Krishnaiah Kummara, Nirlakalla Ravi, D. Lenine This work has neither been published previously, nor under consideration for publication elsewhere and approved by all the authors. I further inform you that this work will not be published elsewhere including electronically in the same form, in English or in any other language, without the written consent of the copyright-holder. N. Kiran and K.Venkata Krishnaiah were conducted the experiments and wrote the manuscript, conceived and analysed the results with the co-authors. N Ravi and D lenine were helped in the measurments as well as discussed the results and reviewed the manuscript. All the authors reviewed the manuscript. I am signing this on behalf of the all the authors.
Yours faithfully (K. VENKATA KRISHANIAH)
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Declaration of Interest Statement
24th January 2020 On behalf of all authors, I, the corresponding author hereby confirm that the manuscript submitted under the title “Structure and EPR investigations on Gd3+ ions in magnesium-leadborophosphate glasses” has no conflict of interest. The authors have no involvement in any organization or entity with any financial interest or non-financial in the subject matter or materials discussed in this manuscript. I further declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere.
Yours faithfully,
K. VENKATA KRISHNAIAH
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Fig. 1. DSC profile of PPbBMgGd1.0 glass
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Fig. 2. Raman spectra of PPbBMgGd glasses.
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Fig. 3. FTIR spectra of PPbBMgGd glasses in the energy range of 400-4000 cm-1.
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Fig. 4. EPR spectra of Gd3+-doped PPbBMgGd glasses for different Gd3+ ions concentration at room temperature.
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Fig. 5. EPR spectra of 5.0 mol% Gd3+-doped PPbBMgGd5.0 glass at different low temperatures in the range of 123 – 297 K.
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Fig. 6. The variation of log N with respect to 1/T for the PPbBMgGd5.0 glass.
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Fig. 7. The variation of reciprocal of susceptibility with respect to 1/T for the PPbBMgGd5.0 glass.
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Fig. 8. Photoluminescnce of Gd3+ ion in PPbBMgGd1.0 glass upon excitation at a wavelength of 278 nm
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Highlights Gadolinium (Gd3+) doped magnesium lead-boro-phosphate glasses were synthesized. Structure and photoluminescence of these glasses have been investigated. EPR spectra have been revealed the characteristic resonance signals of Gd3+ ions. Intense broad emission of Gd3+ ions has been obtained upon excitation of 278 nm light.
Nallamal. Kiran, Venkata Krishnaiah. Kummara, Nirlakala Ravi, D. Lenin PII:
S0022-2860(20)30201-5
DOI:
https://doi.org/10.1016/j.molstruc.2020.127877
Reference:
MOLSTR 127877
To appear in:
Journal of Molecular Structure
Received Date:
12 July 2019
Accepted Date:
08 February 2020
Please cite this article as: Nallamal. Kiran, Venkata Krishnaiah. Kummara, Nirlakala Ravi, D. Lenin, Structure and EPR investigations on Gd3+ ions in magnesium-lead-borophosphate glasses, Journal of Molecular Structure (2020), https://doi.org/10.1016/j.molstruc.2020.127877
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Structure and EPR investigations on Gd3+ ions in magnesiumlead-borophosphate glasses Nallamal. Kiran1, Venkata Krishnaiah. Kummara2,3,*, Nirlakala Ravi4, D. Lenin5 1Department
of Materials Science and Engineering, Harbin Institute of Technology Shenzhen, Graduate School, Shenzhen, Guangdong 518055, PR China. 2Laser Applications Research Group, Ton Duc Thang University, Ho Chi Minh City, Vietnam. 3Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam. 4Department of Physics, Rajeev Gandhi Memorial College of Engineering and Technology, Nandyal - 518501, India. 5Department of EEE, Rajeev Gandhi Memorial College of Engineering and Technology, Nandyal - 518501, India. *Corresponding author: [email protected]; (Venkata Krishnaiah Kummara) Abstract: Gadolinium (Gd3+)-doped magnesium-lead-borophosphate glasses composed of MgHPO4−PbO−B2O3–Gd2O3 (PPbBMgGd) were fabricated by usual melt-quenching approach. The physical properties such as refractive index (n), density (ρ) and molar volume (Vm) were attained. Thermal analysis of the glass has been performed through differential scanning calorimetry (DSC). Raman, Electron paramagnetic resonance (EPR), Fourier transform infrared (FTIR) and steady-state luminescence of PPbBMgGd glasses were investigated. The Raman and FTIR spectra explore the vibrational groups of both phosphate and borate networks in the PPbBMgGd glasses. The EPR spectra display three characteristic signals at room temperature with definite g values of 6.2, 2.8 and 2.0. These character signals were ascribed perhaps the Gd3+ ions that are located respectively at feeble, moderate and sturdy cubic symmetry sites. Moreover, the EPR spectra were also obtained at various low temperatures (123-297 °K) for 5.0 mol% of Gd3+-doped PPbBMgGd5.0 glass. Difference in population among the Zeeman levels (N) and paramagnetic susceptibility (χ) were evaluated. Photoluminescence spectrum reveals a broad intense band of Gd3+ ions upon excitation at 278 nm, could be suitable for visible broadband applications.
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Journal Pre-proof Keywords: Gd3+ ion; Borophosphate glasses; Fourier transform infrared; Electron Paramagnetic Resonance; Photoluminescence. 1. Introduction Rare earth (RE3+)-doped glasses are fascinating materials due to their significant applications for lasers, optical amplification, solid state lighting, and stack the radioactive ions [1–3]. Among the RE3+ ions, the ground level of 8S7/2 of Gadolinium (Gd3+, 4f7) is regarded an S-state [4]. The wide energy gap (32000 cm–1) among the ground (8S7/2) and the foremost excited (6P7/2) states of Gd3+ ion [5–6] that allows one to treat this ion as a hypothetical mediator (sensitizer) between the matrix and other RE3+ ions. Gd3+-doped glasses are particularly fascinating materials owing to their optical and magnetic properties [7]. Moreover, nature of magnetic interactions among the Gd3+ ions and their distribution in the glass matrix could be revealed through Electron paramagnetic resonance (EPR) as well as magnetic susceptibility measurements [8–10]. The importance of Phosphate glasses are widely used in technological applications [11-13]. However, the applications of these glasses are limited beacuase of hygroscopic nature, very poor chemical durability and therefore not very stable [14]. The forementiond drawbacks could be minimized by adding B2O3 to the phosphate network that improves the thermal, mechanical, optical and chemical properties of phosphate glasses [1516]. It is reported that the addition of B2O3 increases Tg and hence the glass structure becomes more rigid [17]. Usually, B2O3 is an excellent glass former comprises a trigonal units of [BO3]3− in the glass network. This trigonal changes to tetragonal units of [BO4] progressively and reversibly [[BO3]−↔[BO4]−] (i.e. called as boron anomaly) when the addition of glass modifiers that includes alkaline (e.g., MgO) or heavy metal oxides (e.g., PbO) are added [18]. Heavy metal oxide, PbO acts as both glass former and modifier that depends on its content. The PbO improves the linear and non-linear refractive index and lowers the phonon energy of the glass matrix. The PbO addition to the borophosphate glasses expands the transparency in
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Journal Pre-proof the ultraviolet (UV) region, the ability of glass formation, resistance to moisture, low devitrification and chemical durability [19-20]. In addition, borophosphate glasses have been investigated for low-melting glass solders or glass seals applications [21]. In this work, Raman, FTIR, EPR, DSC and photoluminescence of Gd3+-doped magnesium-lead-borophosphate (PPbBMgGd) glasses were investigated. The structural modifications triggered by adding the Gd3+ ions into the PPbBMgGd glasses were examined. The environment around the Gd3+ ions is systematically inspected by EPR studies as a function of Gd3+ ions concentration. The effect of concentration of Gd3+ ions and temperature in the range of 123–297 K on the EPR resonance signals is discussed. Photoluminescence of PPbBMgGd1.0 glass is obtained upon 278 nm excitation. The feasibility of these glasses for visible broad band applications can be studied.
2. Experimental technique 2.1. Glass preparation The Gd3+-doped glasses composed of (20−x)MgHPO4−50PbO−30B2O3–xGd2O3 (x = 0.1, 0.3, 0.5, 1.0, 3.0 and 5.0 in mol%) were fabricated by the melt-quenching technique. The glasses are coded as PPbBMgGd0.1, PPbBMgGd0.3, PPbBMgGd0.5, PPbBMgGd1.0, PPbBMgGd3.0, PPbBMgGd5.0. The appropriate amounts of MgHPO4, PbO, B2O3 and Gd2O3 were mixed thoroughly in the agate mortar and then shifted to a ceramic crucible. The blends were fired in an electrical furnace in air at 1223 K for 30 minutes. The molted glass was quenched suddenly at room temperature and subsequently annealed at 600 K. The glasses were permitted to cool down to room temperature and then refined for further studies.
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Journal Pre-proof 2.2. Characerization techniques Density (ρ) of the PPbBMgGd1.0 glass was obtained as 5.323 g/cm3 by using the Archimedes principle with distilled water as an immersion liquid. Refractive index (n) of the PPbBMgGd1.0 glass was found to be 1.612 at sodium wavelength by using Abbe refractometer. Molar volume (Vm) was calculated as 29 cm3 mol-1 for the PPbBMgGd1.0 glass. Thermal analysis of the PPbBMgGd1.0 glass in the range of 200-800 °C was carried out using a differential scanning calorimetry (DSC, TA instruments with model: SDT Q600, USA). Raman modes of the PPbBMgGd glasses upon excitation of 514 nm line of Ar+ laser were obtained by Lab Ram HR800 Raman spectrometer in the back scattering geometry. Fourier Transform Infrared (FTIR) profiles of the powdered glasses mixed with KBr in the form of pellet was collected using a Shimadzu IR Prestige 21 spectrophotometer. JEOL-FE1X EPR spectrometer functioning at the X-band frequency of 9.205 GHz has been utilized to record the EPR spectra at ambient atmosphere. The EPR spectra were also attained at low temperatures in the range of 123-297 K by means of a flexible temperature controller (JES UCT 2AX). Polycrystalline DPPH having a g value of 2.0036 has been employed as a typical field indicator. YVON Fluorolog-3 fluorimeter was employed to explore the photoluminescence spectrum of glass upon excitation at 278 nm of Xe lamp. 3. Results and Discussion 3.1 Thermal studies The 1.0 mol% Gd3+ doped powdered PCfBfTiH1.0 glass was used for thermal studies using differential scanning calorimetry (DSC) which provides the information about the glass the glass transition (Tg), onset crystallization (Tx) and crystallization (Tc) temperatures. The DSC thermograph of PPbBMgGd1.0 glass is shown in Fig. 1. The themogrpah exhibits the glass transition temperature at 427 ºC, onset crystallization temperature at 507 ºC and crystallization temperature at 547 ºC. 4
Journal Pre-proof 3.2 Raman studies Raman vibrational modes of the PPbBMgGd glasses were revealed through Raman spectra are shown in Fig. 2. Raman spectra comprise of several typical bands at 400, 471, 574, 722, 982, 1141, 1251, 1325, 1481, 1598, 1731 cm-1 related to the phosphate and borate vibrational groups. Raman spectra show an intense band at 1251 cm-1 assigned as P-O-P vibrations and it signifies as a maximum phonon energy of these glasses [22]. Initial band at 400-574 cm−1 corresponds to the bending modes of the cation motion and chain structure bending of PO4 networks [23]. Besides, the band is observed at 722 cm−1 due to the existence of borophosphate rings that favours at low modifier contents near meta-stoichiometry [24]. The low intensity bands are observed at around 1325 and 1481 cm−1 due to vibrations of boron bonds [25]. The band at 1669 cm-1 is due to the B-O− stretching vibrations involving non-bridging oxygen (NBO) in various Borate groups [26]. 3.3 Fourier transform infrared (FTIR) studies The FTIR spectra of PPbBMgGd glasses in the energy range of 4000–400 cm−1 are shown in Fig. 3. The spectra exhibit several vibration bands at 3340, 2972, 2356, 1391, 1045, 878, 675 cm-1. These characteristic vibrations are related to the B-O-B and P-O-P networks. The vibrational band at 3340 cm-1 is assigned as stretching vibrations of O–H and another band at around
1658 cm-1 corresponds to the bending vibrations of H–O–H [27]. The band at around
2356 cm-1 is assigned as the asymmetric stretching of adsorbed water molecules [28]. The band around at 1045 cm-1 perhaps the asymmetric starching vibrations of P-O-P chins of Q2 units and a small contribution of B-O band stretching vibrations in BO4 units from diborate (B4O7-2) group [29]. A band around at 675 cm–1 is allotted to the symmetric vibrations of P–O–P bonds in Q1 units of P–O–B links [30]. The band at 513 cm–1 is ascribed to the bending modes of O–P–O in Q1 (P2O7)4 units [31]. The bands are resolved significantly with increase of Gd3+ ions concentration. The similar behaviour is also observed in the Raman spectra. Over all, from 5
Journal Pre-proof the IR spectra, the intensity of IR bands increases with the increase of Gd3+ ions, it indicates the stability of the main network units. 3.4 Electron Paramagnetic Resonance (EPR) studies EPR spectra of PPbBMgGd glasses at ambient atmosphere display the resonance signals for different Gd3+ concentration are shown in Fig. 4. The Gd3+ ions doped PPbBMg glasses reveals three eminent resonance signals at g ≈ 6.2, g ≈ 2.8 and g ≈ 2.0 are ascribed to Gd3+ ions positioned at feeble, moderate and sturdy cubic symmetry sites respectively. These resonance siganls are the chacteristic spectrum of Gd3+ ions in non-crystalline matrix [32], which are the unique regular signature of S-state of RE3+:glasses. On the other hand, EPR spectra of 5.0 mol% Gd3+-doped PPbBMgGd5.0 glass were also obtained at low temperatures in the range of 123-297 K are shown in Fig. 5. As we can see from Fig. 5, the intensity of resonance signal decreases with increase in temperature of the glass. The difference in population among the Zeeman levels (N) can be evaluated by relating the zone beneath the absorption band with that of established concentration of CuSO4.5H2O. Weil et al [33] have been proposed the following relation which comprises the tentative parameters of both the sample under study as well as the standard one. 1
N
A x ( S c a n x ) 2 G s td ( B m ) s td ( g s td ) 2 [ S ( S 1) ] s td ( Ps td ) 2 A x ( S c a n x ) 2 G x ( B m ) x ( g x ) 2 [ S ( S 1) ] x ( Px )
1 2
[ S td ]
(1)
The subscripts ‘x’ and ‘std’ denote the quantities of PPbBMgGd glass and the standard CuSO4.5H2O sample, respectively. It is observed that the intense EPR signal at g ≈ 6.2 which can be utilized to extract the difference in population among the Zeeman levels (N) for various temperatures. The variation of log N with respect to 1/T is shown in Fig. 6. A linear variation of difference in population was observed with increasing the temperature of the glass. Generally, EPR spectra of RE3+ ions in the non-crystalline materials are highly aeolotropic and 6
Journal Pre-proof delicate to disparities from site to site in the ligand field [34]. The name of the spectrum is characterized as ‘‘U’’ type spectrum with an effective g values of 6.0, 2.8 and 2.0 [35]. It is usually detected in the glass matrix perhaphs the presence of Gd3+ ions even at low concentration (as revealed in Fig. 6). The shapes from U-spectrum having g ≈ 5.99, g ≈ 2.85 and g ≈ 1.98 are allocated to Gd3+ ions in the intermediate crystal field sites whose coordination numbers higher than six [36]. Whereas the line at g = 4.7 is typical indication of Gd3+ ions in the strong crystal field sites having a low coordination number [37]. A very analogous spectral topographies of Gd3+ ions in dissimilar glassy hosts have been reported [38, 39]. Recently, Srinivasulu et al [40] have been adopted a analogous method and ascribed the U-spectrum to an isolated Gd3+ ions. These Gd3+ ions positioned in a diversity of locations with a coordination number which was evaluated to be 8 or 9 in the glass network of tetrahedral through a spreading of spinHamiltonian parameters. 3.5 Paramagnetic susceptibility (χ) evaluated from EPR spectrum The magnetic susceptibility (χ) of Gd3+ ions can be evaluated at diverse temperatures with the help of below relation [41]
𝜒=
𝑁g2𝛽2𝐽(𝐽 + 1) 3𝐾𝐵𝑇
(2)
where N represents the amount of spins per meter cube, g represents the spectroscopic splitting factor, , KB represents the Bolzmann constant, β represents the Bohr magneton and J = 3/2. The N value can be evaluated through Eq. (1) and g ≈ 6.2 (taken from EPR data). The value of is evaluated at ambient as well as low temperatures. The variation of susceptibility (1/) with respect to absolute temperature T is shown in Fig. 7. A linear variation of amount of spins (log N) that are contributed in the resonance with espect to the reciprocal of absolute temperature (1/T) is shown in Fig. 7. It is observed that the 7
Journal Pre-proof difference in population among the Zeeman levels increases with decrease in the temperature. A linear variation is detected amongst log N and 1/T that obeys the Boltzmann distribution law. The experimental data is fitted to a straight line log N = (19.80 + (65.61/T)) by a least square method. The activation energy is obtained as 2.86 x 10–21 J that equalent to 0.012 eV. The difference between the reciprocal of susceptibility and absolute temperature in Gd3+:PPbBMg glass is shown in Fig. 7. It is noteworthy that the susceptibility decreases with increase in temperature following the Curie–Weiss law. From the graph, it is detected that the PPbBMg: Gd3+ glass obeys Curie–Weiss type of behaviour ( = C / (T- p)). The paramagnetic Curie temperature (p) and Curie constant (C) were calculated and are found to be p =105 K and 0.803 emu/mol, respectively, which are in good agreement with the values reported elsewhere [42]. 3.6 Photoluminescence emission Photoluminescence spectrum of the Gd3+ doped PPbBMgGd1.0 glass upon excitation at 278 nm line of Xe lamp is shown in Fig. 8. The other glasses were also exhibit the similar behaviour. Broad and strong photoluminescence band in the spectral range of 270-450 nm is observed a peak around at 313 nm which was allocated to the f-f transitions of the Gd3+ ions. It is reported that the emission of Gd3+ ion has been reported in the range of 285-334 nm with a peak around 315 nm which perhaphs the 6P7/2→8S7/2 transition upon excitation of 8S7/2 → 6I9/2 transition at around 275 nm. The excitation band can be detected in the UV region at around 278 nm which perhaphs the 8S7/2 → 6I9/2 transition of Gd3+ ion. Upon excitation at 278 nm, the Gd3+ ion in the ground level is shifted to the excited levels 6IJ. Then it relaxes to the 6PJ levels non-radiatively. Moreover, the Gd3+ ion is excited through UV radiation at around 278 nm, the preliminary higher energy level is populated and downs to its lower energy levels till it reaches at the 6P7/2 level through phonons and delivers the emission at 313 nm [43, 44]. The broad and
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Journal Pre-proof intese peak at 313 of PPbBMgGd1.0 glass could be useful for broadband near UV emitting device applications.
5. Conclusion Gadolinium (Gd3+)-doped magnesium-lead-borophosphate (MgHPO4−PbO−B2O3– Gd2O3) glasses have been investigated through the Raman, FTIR, DSC, EPR and photoluminescence studies. Physical properties of PPbBMgGd1.0 glass such as refractive index (n), density (ρ) and molar volume (Vm) have been attained. The DSC thermogrsph has been revealed the glass transition (Tg = 427 ºC), onset crystallization (Tg = 507 ºC) and crystallization (Tg = 547 ºC) temperatures. Raman and FTIR spectra have been revealed the characteristic bands of stretching and bending groups of P–O–P, O–P–O and B-O-B of phosphate as well as borate chains. The resonance signals of EPR with g ≈ 6.2, g ≈ 2.8 and g ≈ 2.0 have been ascribed to Gd3+ions. The population density (N) among the Zeeman states involving in the resonance and their paramagnetic susceptibility (χ) for g ≈ 6.2 resonance streak have been evaluated. It is noticed that N and χ values enhanced significalty with increase of Gd3+ ion concentration. The reciprocal of susceptibility (1/χ) along with temperature has been obeyed the Curie-Weiss law. Intense broad emission of PPbBMgGd1.0 glass was observed at around 313 nm under 278 nm of Xe lamp excitayion perhaps the f–f transitions of Gd3+ ion. Based on the luminescence and EPR studies, the Gd3+-doped magnesium-lead-borophosphate glasses could be a potential candidate for the broadband emitting devices in the spectral range of 270-400 nm. References 1. K. H. Mahmoud, Physica B 405 (2010) 4746. 2. T.A.Lodi, M.Sandrini, A. N. Medina, M.J .Barboza, F.Pedrochi, A.Steimacher, Optical materials 76 (2018) 231-236. 9
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Credit Author Statement Title: Structure and EPR investigations on Gd3+ ions in magnesium-lead-borophosphate glasses Author(s): Nallamala Kiran, Venkata Krishnaiah Kummara, Nirlakalla Ravi, D. Lenine This work has neither been published previously, nor under consideration for publication elsewhere and approved by all the authors. I further inform you that this work will not be published elsewhere including electronically in the same form, in English or in any other language, without the written consent of the copyright-holder. N. Kiran and K.Venkata Krishnaiah were conducted the experiments and wrote the manuscript, conceived and analysed the results with the co-authors. N Ravi and D lenine were helped in the measurments as well as discussed the results and reviewed the manuscript. All the authors reviewed the manuscript. I am signing this on behalf of the all the authors.
Yours faithfully (K. VENKATA KRISHANIAH)
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Declaration of Interest Statement
24th January 2020 On behalf of all authors, I, the corresponding author hereby confirm that the manuscript submitted under the title “Structure and EPR investigations on Gd3+ ions in magnesium-leadborophosphate glasses” has no conflict of interest. The authors have no involvement in any organization or entity with any financial interest or non-financial in the subject matter or materials discussed in this manuscript. I further declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere.
Yours faithfully,
K. VENKATA KRISHNAIAH
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Fig. 1. DSC profile of PPbBMgGd1.0 glass
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Fig. 2. Raman spectra of PPbBMgGd glasses.
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Fig. 3. FTIR spectra of PPbBMgGd glasses in the energy range of 400-4000 cm-1.
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Fig. 4. EPR spectra of Gd3+-doped PPbBMgGd glasses for different Gd3+ ions concentration at room temperature.
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Fig. 5. EPR spectra of 5.0 mol% Gd3+-doped PPbBMgGd5.0 glass at different low temperatures in the range of 123 – 297 K.
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Fig. 6. The variation of log N with respect to 1/T for the PPbBMgGd5.0 glass.
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Fig. 7. The variation of reciprocal of susceptibility with respect to 1/T for the PPbBMgGd5.0 glass.
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Fig. 8. Photoluminescnce of Gd3+ ion in PPbBMgGd1.0 glass upon excitation at a wavelength of 278 nm
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Highlights Gadolinium (Gd3+) doped magnesium lead-boro-phosphate glasses were synthesized. Structure and photoluminescence of these glasses have been investigated. EPR spectra have been revealed the characteristic resonance signals of Gd3+ ions. Intense broad emission of Gd3+ ions has been obtained upon excitation of 278 nm light.