EPR and optical studies of Mn2+ ions in Li2O–Na2O–B2O3 glasses – An evidence of mixed alkali effect

https://doi.org/10.1016/j.jnoncrysol.2007.04.002Get rights and content

Abstract

EPR and optical absorption spectra of 0.5 mol% MnO2 doped xLi2O–(30  x)Na2O–69.5B2O3 (5  x  25) glasses have been studied. The EPR spectra exhibit resonance signals characteristic of Mn2+ ions. The resonance signal at g  2.0 is due to Mn2+ ions in an environment close to octahedral symmetry, whereas the resonances at g  4.3 and g  3.3 are attributed to the rhombic surroundings of the Mn2+ ions. The ionic character (A), the number of spins participating in resonance (N), optical band gap energies (Eopt) and Urbach energies (ΔE) show the mixed alkali effect (MAE) with composition. The present study gives an indication that the size of alkalis we choose, is also an important contributing factor in showing the MAE. The variation of N with temperature obeys the Boltzmann law. The optical absorption spectra show a single broad band at ∼21 000 cm−1 corresponding to the transition 6A1g(S)  4T1g(G) which exhibits a blue shift with x. The theoretical values of optical basicity (Λth) have also been evaluated.

Introduction

The mixed alkali effect (MAE) is one of the most outstanding and poorly understood phenomenon in glass science [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. It has been observed quite generally in alkali modified network oxide glasses, that the substitution of one alkali by another at a total fixed concentration of the alkalies gives rise to non-linear variation of properties. This is most notable in properties related to alkali ion transport. The other dynamical properties also exhibit a more or less pronounced deviation, viz. internal friction, dielectric loss, viscosity and glass transition etc. The phenomenon is described as mixed alkali effect (MAE) and the field has attracted recent attention [11], [12], [13], [14].

Although a number of models and theories have been proposed to account for MAE, none of them give satisfactory explanation for this phenomenon. Approaches made to understand MAE in a particular property often have their own implications on other properties and have therefore been critically examined in the literature. Also, the MAE has a bearing on many technological applications of glasses (toughening of glass, control of dielectric properties, etc.) [3], [4], [7] it is necessary to examine this phenomenon in greater detail.

As a part of our programme on the MAE [15], [16], [17], in the present study, we have carried out electron paramagnetic resonance (EPR) and optical studies in mixed alkali borate (Li2O–Na2O–B2O3) glasses doped with a small quantity of paramagnetic impurity MnO2. On the otherhand, the spectroscopic investigations such as EPR and optical absorption techniques are valuable to gain insight into the microscopic origin of the MAE. Our main interest is to know whether, the MAE is limited to dynamical properties or the MAE is dependent on the nature of the glass itself. We are also interested to know the effect of alkali ions on the spin-Hamiltonian parameters and also to know the site symmetry around Mn2+ ions in these glasses. The effect of temperature (123–300 K) on EPR signals has also been studied.

Section snippets

Glass preparation

The glass samples studied in the present work (Table 1) were obtained by the classical melt quenching technique. They were prepared by mixing and grinding together appropriate amounts of Li2CO3, Na2CO3, H3BO3 and MnO2 in an agate mortar before transferring to a porcelain crucible. The mixtures were heated in an electric furnace in air at 1223 K for 20 min. The melt was then quenched to room temperature in air by pouring it onto a polished porcelain plate and pressing it with another porcelain

EPR studies

The EPR spectra (Fig. 2, Fig. 3) of all the investigated samples (Table 1) exhibit resonance signals due to the Mn2+ (3d5:6S5/2) ions entering the matrix as paramagnetic species. The spectrum of Mn2+ ions exhibits a broad resonance at g  2.0, shows a six line hyperfine structure (hfs). In addition to this, shoulder around g  3.3 and sharp signal at g  4.3 were also observed (see Fig. 2). The resonance signal centered at g  3.3 is broad, unresolved giving a shoulder like signal. The absorption

EPR studies

Fig. 2 shows the EPR spectra of 0.5 mol% of Mn2+ ions in xLi2O–(30  x)Na2O–69.5B2O3 (5  x  25) glasses, observed at room temperature. All the glass samples doped with manganese ions show a broad resonance at g  2.0 with six line hyperfine pattern, which is a characteristic of Mn2+ ions with a nuclear spin I = 5/2. Besides the resonance at g  2.0, all the glass samples show resonance signals at g  3.3 and g  4.3 in their EPR spectra. The EPR signal from g  2.0 is given by two covered signals: one signal

Conclusions

We have shown for the first time that electron paramagnetic resonance and optical absorption techniques are useful tools to study the mixed alkali effect. The EPR spectra of all the investigated samples exhibit resonance signals characteristic of Mn2+ ions. The ionic character (A) decreases with x and reaches a minimum around x = 15 and thereafter it increases showing the mixed alkali effect. It is interesting to note that the number of Mn2+ ions participating in resonance (N) exhibits the mixed

Acknowledgement

Mrs B. Yasoda is grateful to University Grants Commission (UGC), New Delhi for providing F.I.P. Teacher fellowship. Dr RPSC is grateful to Dr H.S. Maiti, Director, CGCRI and Dr K.K. Phani, Head, Glass Technology Lab, CGCRI for their constant encouragement.

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