Elsevier

Solid State Sciences

Volume 11, Issue 2, February 2009, Pages 578-587
Solid State Sciences

Influence of redox behavior of copper ions on dielectric and spectroscopic properties of Li2O–MoO3–B2O3: CuO glass system

https://doi.org/10.1016/j.solidstatesciences.2008.06.022Get rights and content

Abstract

Li2O–MoO3–B2O3 glasses mixed with different concentrations of CuO (ranging from 0 to 1.2 mol%) were prepared. The samples were characterized by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Optical absorption, luminescence, ESR, IR and dielectric properties (viz., dielectric constant ɛ′, loss tan δ and a.c. conductivity σac, over a wide range of frequency and temperature) of these glass materials have been investigated. The results of differential scanning calorimetric studies suggest that the glass forming ability is higher for the glasses containing CuO beyond 0.6 mol%. The analysis of results of the dielectric properties has revealed that the glasses possess high insulating strength when the concentration of CuO is >0.6 mol%. The variation of a.c. conductivity with the concentration of CuO passes through a maximum at 0.6 mol%. In the high-temperature region, the a.c. conduction seems to be connected with the mixed conduction viz., electronic conduction and ionic conduction. The optical absorption spectra of these glasses exhibited bands due to Cu+ ions in the UV region in addition to the conventional band due to Cu2+ ions in the visible region. The ESR spectral studies have indicated that there is a gradual adoption of Cu2+ ions from ionic environment to covalent environment as the concentration of CuO increases beyond 0.6 mol% in the glass matrix. The luminescence spectra excited at 271 nm have exhibited an intense yellow emission band centered at about 550 nm and a relatively broad blue emission band at about 450 nm; these bands have been attributed to the 3D1  1S0 transition of isolated Cu+ ions and 3D1  1S0 transition of (Cu+)2 pairs, respectively. The quantitative analysis of the results of all these studies has indicated that as the concentration of CuO is increased beyond 0.6 mol% in the glass matrix, a part of Cu2+ ions have been reduced to Cu+ ions that have influenced the physical properties of these glasses to a substantial extent.

Graphical abstract

Optical absorption, luminescence, ESR, IR and dielectric properties of Li2O–MoO3–B2O3: CuO glasses have been investigated. The quantitative analysis of the results of all these studies has indicated that as the concentration of CuO is increased beyond 0.6 mol% in the glass matrix, a part of Cu2+ ions have been reduced to Cu+ ions that have influenced the physical properties of these glasses to a substantial extent.

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Introduction

Alkali borate glasses are well known due to their high transparency, low melting point, high thermal stability and high transition metal ion solubility. The presence of lithium oxide in higher quantities in glass matrices makes the glasses as ionic conducting materials; such glasses can be used as promising electrolyte materials in solid state batteries [1], [2], [3]. The molybdenum ions when dissolved in the glass systems like Li2O–B2O3, exist at least in two stable valence states viz., Mo(V) and Mo(VI) in the glass network depending upon their concentration and chemical composition of the host network. The Mo6+ ions participate in the network forming (with MoO42 structural units, alternating with BO4 structural units), whereas Mo5+ ions act as modifiers. Earlier ESR studies on the glass systems containing molybdenum ions, have predicted that Mo(V) ions are present in octahedral coordination along with distorted octahedrons approaching tetragons. Further, Mo–O bond in molybdenum hexavalent oxide is identified as significantly covalent [4], [5]. A considerable number of recent studies on various physical properties viz., spectroscopic, d.c. conductivity, dielectric properties, etc., of a variety of glass systems containing molybdenum ions are available in the literature [6], [7], [8], [9], [10], [11].

Copper is being extensively used in several commercial glasses, such as red glass hematite, aventurine and rubies. CuO containing glasses are also important in technological point of view, because of their semiconducting properties and due to other potential applications [12], [13]. In glasses, copper ions exist in two stable ionic states viz., monovalent Cu+ ions, divalent Cu2+ ions and may also exist as metallic copper. The electronic structure of the copper atom is [Ar] 3d10 4s1; the cuprous ion, having its five d orbitals occupied, does not produce coloring [14], while Cu2+ ions create color centers with an absorption band in the visible region [15], [16] and produce blue and green glasses. The color of the glass depends on the Cu2+ content, its specific coordination, composition and basicity of the glass. Colors produced by Cu2+ ions in various glasses have been interpreted [17] in terms of ligand field theory. Though, a reasonably good number of recent studies on the environment of copper ions in a variety of inorganic glass systems are available [18], [19], [20], [21] including molybdenum borate glasses [22], most of them are focused on structural investigations by means of Raman and IR spectroscopic studies. Virtually, no studies are available on dielectric properties of molybdenum borate glasses containing copper ions. The study of dielectric properties such as dielectric constant ɛ′, loss tan δ and a.c. conductivity σac over a wide range of frequency and temperature of the glass materials not only helps in accessing the insulating character and understanding the conduction phenomenon but also throws light on the structural aspects of the glasses to a large extent. Work along these lines has been carried out in the recent years on a variety of inorganic glass materials giving valuable structural information [23], [24], [25], [26]. The objective of this paper is to have a comprehensive understanding over the topology and valence states of copper ions and to throw some light on a.c. conduction phenomenon in Li2O–MoO3–B2O3 glass system, by a systematic study of dielectric properties coupled with spectroscopic investigations (optical absorption, luminescence emission, ESR, and IR spectra).

Section snippets

Experimental methods

Within the glass forming region of Li2O–MoO3–B2O3 glass system, a particular composition 40 Li2O–(4  x) MoO3–56 B2O3: x CuO (with x ranging from 0 to 1.2) is chosen for the present study. The details of the composition are as follows:

  • MC0: 40 Li2O–4.0 MoO3–56 B2O3

  • MC2: 40 Li2O–3.8 MoO3–56 B2O3: 0.2 CuO

  • MC4: 40 Li2O–3.6 MoO3–56 B2O3: 0.4 CuO

  • MC6: 40 Li2O–3.4 MoO3–56 B2O3: 0.6 CuO

  • MC8: 40 Li2O–3.2 MoO3–56 B2O3: 0.8 CuO

  • MC10: 40 Li2O–3.0 MoO3–56 B2O3: 1.0 CuO

  • MC12: 40 Li2O–2.8 MoO3–56 B2O3: 1.2 CuO

Results

Our usual examination of XRD, SEM and the appearance of glass transition temperature in DSC studies clearly indicates that the samples prepared are amorphous in nature. From the measured values of density d and calculated average molecular weight M¯, various physical parameters such as copper ion concentration Ni and mean copper ion separation ri of these glasses are evaluated (as Cu2+) using the conventional formulae and are presented in Table 1.

Fig. 1 shows the differential scanning

Discussion

Li2O–MoO3–B2O3 glass network is an admixture of network formers, intermediate glass formers and modifiers. The coordination geometry of boron atoms in the glass network is expected to be strongly dependent on the composition of the glasses and on the nature of the network modifiers. The addition of lithium oxide to the borate glass network is accompanied by a change in the boron coordination from three to four or vice versa depending up on its concentration and also other ingredients of the

Conclusions

The summary of the results on studies of various physical properties of Li2O–MoO3–B2O3 glasses doped with CuO is as follows: the analysis of DSC results suggests that the glass forming ability is higher for the glasses containing CuO beyond 0.6 mol%. The spectroscopic investigations (viz., optical absorption, ESR and luminescence) point out that the copper ions co-exist in Cu+ state with Cu2+ state. The dielectric parameters viz., ɛ′, tan δ and σac are found to increase and the activation energy

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