Characterization, optical and luminescence features of cobalt ions in multi-component PbOAl2O3TeO2GeO2SiO2 glass ceramics

doi:10.1016/j.optmat.2018.11.050

Optical Materials

Volume 88, February 2019, Pages 289-298

https://doi.org/10.1016/j.optmat.2018.11.050 Get rights and content

Highlights

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PbOAl₂O₃TeO₂GeO₂SiO₂: CoO glass ceramics were prepared by melt quenching and heat treatment.
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Prepared samples were characterized by XRD, SEM, EDS and DTA.
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Absorption bands of the samples indicated the cobalt ions exist in Co²⁺ and Co³⁺ state.
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Photoluminescence spectra of the samples exhibited emission bands at 628 and 870 nm.
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Samples doped with 1.0 mol% of CoO are suitable for insulating and photonic devices.

Abstract

Glass ceramics of the composition PbO single bond Al₂O₃TeO₂GeO₂SiO₂ doped with CoO (ranging between 0 and 1.0 mol %) were prepared by melt quenching technique followed by heat treatment. The techniques such as XRD, SEM, EDS and DTA were employed for the characterization of the prepared samples. Conventional spectroscopic studies viz., optical absorption, photoluminescence, FTIR and Raman were also performed on these samples. The optical absorption spectra of the titled glass ceramics exhibited two principal absorption bands, one at about 594 nm and the second one consisting of multiplets with the meta centre at about 1455 nm ascribed to the ⁴A₂ (⁴F) → ⁴T₁ (⁴P) and ⁴A₂ (⁴F) → ⁴T₁ (⁴F) transitions of tetrahedrally coordinated Co²⁺ ions, respectively. Besides these, the spectra also exhibited two feeble bands at about 532 and 643 nm which are the characteristics of octahedrally coordinated Co²⁺ and Co³⁺ ions, respectively. Using Tanabe Sugano diagram the crystal field splitting parameter Dq, Racah parameters B, C and nephelauxetic ratio, β were evaluated for Co²⁺ ions. The photoluminescence spectra of the titled glass ceramics have exhibited two broad emission bands at 628 and 870 nm ascribed to ⁴T₁ (⁴P) → ⁴A₂ (⁴F) and ⁴T₁ (⁴P) → ⁴T₂ (⁴F) tetrahedral transitions of Co²⁺ ions, respectively. FTIR and Raman spectra of the titled samples exhibited various asymmetrical and symmetrical vibrational bands due to viz., silicate, germanate, PbO₄, PbO₆, AlO₆, TeO_4, TeO₃ and Co^III single bond O vibrations in the glass ceramic matrix. The spectroscopic investigations on the titled glass ceramics have revealed that if CoO is present in small quantities the cobalt ions predominantly occupy the octahedral sites in the glass network, whereas the tetrahedrally coordinated Co²⁺ ions are found to increase if the concentration of nucleating agent CoO is gradually increased up to 1.0 mol%. As a result the degree of depolymerization of the glass network decreases which in turn enhances its rigidity with increasing the concentration of nucleating agent CoO. From these investigations we report that the glass ceramics doped with 1.0 mol% of CoO are suitable for insulating devices as well as for photonic devices.

Graphical abstract

Optical absorption spectra of PbO single bond Al₂O₃TeO₂GeO₂SiO₂: CoO glass ceramics and the pre-heated samples viz., Co2, Co10.

Introduction

Glass-ceramic materials have a fine-grained homogeneous structure consisting of very small and strain free intertwined crystals that obstruct the crack growth. Generally, these materials possess outstanding properties like, high mechanical strength and ionic conductivity, good optical transparency, high chemical durability and very low coefficient of thermal expansion. While the other are opaque, isolated and they exhibit very high coefficient of thermal expansion. They are widely used in the field of vacuum sealing, electronics, biomedical etc.,. However, the characteristics of glass ceramics depend on the type and number of the crystal phases present as well as on the properties of residual glass phase [1,2].

Silicate glasses have been studied extensively and used mostly as the basic constituents in assortment of glasses or glass ceramics as they have domestic, optical, mechanical and dental applications [[3], [4], [5]]. Germanium oxide (GeO₂) is also a good glass former which exhibits superior mechanical strength, chemical durability, temperature stability and optical properties like high refractive index and high transmittance in the NIR region. Hence, these glasses are being widely used in telecommunication, optics based industries as well as in the design of laser devices, IR technology and also for the light guiding core of optical fibers [6,7]. The addition of GeO₂ to the silicate glasses or glass ceramics is predicted to enhance the expansion coefficient at high temperatures and useful for sealant [8]. Germanosilicate glasses, gained increasing interest due to their potential applications covering diversified fields such as civil, mechanical, electron irradiation, biomedical and aerospace engineering. These glasses are also being used for constructing CW Raman fiber lasers [9,10]. Germanosilicate glasses or glass ceramics have been recognized as efficient luminescent material due to their high phonon energy and are very different from those observed with low phonon glasses such as fluorozirconate glasses [11]. In general it is well known that the integration of heavy metal oxides can improve the physical properties and chemical durability of these glasses. In particular, PbO has been shown to increase the stability and reduce the melting temperature because of its dual role such as network former and modifier [12] depending upon nature of bond between lead and oxygen ions.

Further, the addition of Al₂O₃ to these glasses also increases its chemical durability, and mechanical strength [13,14]. The addition of metal elements such as Pb and Al to the germanosilicate glasses, it was found that a great increase of radiation-induced attenuation (RIA) which is suitable for radiation dosimetry application [15]. TeO₂ mixed glasses are promising candidates for photonic applications as they exhibit good thermal stability, resistance to humidity and corrosion, wide transparency window, high refractive indices [16] and high non-linear susceptibility when compared to the pure traditional glasses viz., SiO₂, B₂O₃ and P₂O₅ [17]. With this view small concentration of TeO₂ is added to the main glass composition. TeO₂ participate in the glass network with TeO₄ disphenoid.

Doping of transition metal ions Ti, Cr, Mn, Ce, V, Fe, Co, Ni, and Zr, etc., to the transparent glass ceramics endow the glass materials several interesting properties that make them useful for active and passive optical applications, such as up-conversion devices. Among these transitions metal ions, cobalt ions are expected to influence the optical properties of these glasses, as they exist in two stable valence states viz., Co²⁺ and Co³⁺ and they participate in the glass network with different structural units. Generally Co²⁺ ions participate in the glass network in octahedral (six-coordinated) and tetrahedral (four-coordinated) structural units where as Co³⁺ ions occupy only octahedral (six-coordinated) structural units [18,19]. The glasses and glass ceramics containing cobalt ions are good candidates for nonlinear optical absorbers, Q-switching devices, visible and near infrared laser materials, ferromagnetic sensors, saturable absorbers etc., [20,21]. Further, CoO doped lasing materials also exhibit strong luminescence in the visible and near-infrared regions.

The above mentioned potential applications of germanosilicate glasses doped with cobalt ions have motivated us to synthesize the PbO single bond Al₂O₃TeO₂GeO₂SiO₂ glass ceramics doped with different concentrations of CoO ranging from 0 to 1.0 mol%. The clear objective of the present investigation is to evaluate the influence of valence state of cobalt ions on structural and spectroscopic features of these glass ceramics by means of XRD, SEM, EDS, DTA and conventional spectroscopic studies viz., optical absorption, photoluminescence, FTIR and Raman.

Section snippets

Materials and methods

Glass ceramics of the chemical composition 29PbO − 5Al₂O₃ single bond 1TeO₂ − 10GeO₂ – (55–x) SiO₂: x CoO (where x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol %) were prepared by conventional melt quenching and heat treatment method. The prepared samples were labelled as Co0, Co2, Co4, Co6, Co8 and Co10 respectively. The analytical raw materials of high purity glass constituents viz., Al₂O₃, TeO₂, GeO₂, SiO₂ (Sigma Aldrich, 99.99% pure), PbO and CoO (LOBA, 99.99% pure) were properly weighed using standard digital

Physical parameters

Density ρ and molar volume, V_m of the synthesized samples were measured. Using these values the other physical parameters such as cobalt ion concentration, N_i, mean inter–ionic separation, R_i and polaron radius, R_p were calculated using the conventional formulae [23] and presented in Table 1. A slight increase in the density of the prepared glass ceramics is observed with increase of CoO content and it is understood due to slight structural modification of the glass network [[24], [25], [26]].

Conclusions

Multi-component 29PbO single bond 5Al₂O₃1TeO₂ − 10GeO₂–(55-x)SiO₂: xCoO (0 ≤ x ≤ 1.0) glass ceramics were synthesized through melt quenching technique followed by heat treatment. The observed increase in the density of the samples with the integration of cobalt ions was explained based on the change in the molecular weight and structural modifications taking place in these glass ceramics. The XRD patterns of the prepared glass ceramics have confirmed the presence of different crystalline phases. SEM and DTA

Acknowledgements

The authors are grateful to the University Grants Commission (UGC) and the Department of Science and Technology (DST), New Delhi, India for sanctioning DSA (Grant No. F. 5301/11/DSA-1/2015 (SAP-1)) and FIST level-I (Grant No. File. No. SR/FIST/PSI-163/2011 (C)) projects respectively to the Physics department of the University.

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Citation Excerpt :
Materials with crystal phases containing tetrahedral cobalt ions also have absorption bands in the IR region and can be used in compact laser devices [7–10]. Cobalt-containing single crystals [5] and glass ceramics [6,7,9] are used as saturable absorbers in eye-safe Er lasers with nanosecond pulse duration. Materials in which cobalt ions are in a four-coordinated state are of particular interest.
New promising optical glasses containing cobalt ions in tetrahedral coordination have been synthesized, their mechanical and optical properties, as well as their characteristic temperatures, have been studied. High-quality glasses of the composition (70-x)ZnO–10SiO₂-(20 + x)B₂O₃ (x = 0; 5; 10 wt%) doped with 0.1, 0.3, and 0.6 wt% CoO have been synthesized for the first time. The selected glass compositions were close to the eutectic one in the ZnO–SiO–B₂O₃ system. This made it possible to reduce the glass synthesis temperature from the 1350–1500 °C, described in the literature for similar zinc borosilicate glasses, to the 1170 °C. The presence of tetrahedral cobalt ions (CoO₄) was identified by the absorption bands at 17000 cm⁻¹ (⁴A₂(⁴F) → ⁴T₁(⁴P)) and at 6954 cm⁻¹ (⁴A₂(⁴F) → ⁴T₁(⁴F)). Then the intense absorption bands in the visible region have been attributed to the ⁴A₂(⁴F) → ²A₁(²G), ⁴A₂(⁴F) → ⁴T₁(⁴P), ⁴A₂(⁴F) → ²T₂(²G), ⁴A₂(⁴F) → ²T₁(²G) and ⁴A₂(⁴F) → ²E(₂G) transitions. The most intense absorption band of octahedral cobalt ions (CoO₆) corresponds to the ⁴T₁(F) → ⁴T₁(P) transition and is located in the region of 18000–20000 cm⁻¹. According to analysis of Co²⁺ absorption spectra, the tetrahedral Co²⁺ ions concentration increased with increasing the CoO concentration in glasses. To study the nature of the bond between cobalt ion and ligands, the values of the splitting force of the ligand field (10Dq) and Racah parameter B were determined.

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Characterization, optical and luminescence features of cobalt ions in multi-component PbOAl2O3TeO2GeO2SiO2 glass ceramics

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and methods

Physical parameters

Conclusions

Acknowledgements

Ceram. Int.

Ceram. Int.

J. Phys. Chem. Solid.

Physica B

Corrosion Sci.

Measurement

Physica B

Ceram. Int.

Nucl. Instrum. Methods Phys. Res., Sect. B

J. Non-Cryst. Solids

J. Lumin.

Physica B

J. Phys. Chem. Solid.

Opt. Mater.

J. Non-Cryst. Solids

Solid State Ionics

Acta

Thin Solid Films

J. Alloy. Comp.

J. Mol. Struct.

J. Alloy. Comp.

J. Non-Cryst. Solids

J. Mol. Struct.

Physica B

Characterization, optical and luminescence features of cobalt ions in multi-component PbOAl₂O₃TeO₂GeO₂SiO₂ glass ceramics