Spectroscopic and magnetic studies of manganese ions in ZnO–Sb2O3–B2O3 glass system

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Abstract

ZnO–Sb2O3–B2O3 glasses containing different concentrations of MnO ranging from 0 to 1.0 mol% were prepared. A number of studies, viz. optical absorption, infrared and ESR spectra and magnetic susceptibility, were carried out as a function of manganese ion concentration. The analysis of the results indicate that manganese ions mostly exist in Mn2+ state in these glasses when the concentration of MnO≤0.6 mol% and above this concentration, these ions seem to exist in Mn3+ state in the glass network.

Introduction

Borate glasses containing antimony have special characteristics and are studied extensively in recent years. These glasses possess large non-linear optical susceptibility (χ3) coefficient [1] that makes them suitable for potential applications in non-linear optical devices (such as ultra fast optical switches and power limiters), broad band optical amplifiers operating around 1.5 μm [2], [3], [4] and in a number of solid state ionic devices. Addition of ZnO to these glasses enhances the applications suitably in plasma display panels, computer monitors and promising candidates for large area hang-on-wall TVs [5], [6], [7], [8]. ZnO can act both as a glass former and a glass modifier. As glass former ZnO enters the network with ZnO4 structural units and as network modifier, zinc ion is octahedrally coordinated and behaves like any other conventional alkali oxide modifier. Further, the frequency of vibrational band due to ZnO4 structural units lies, in the region of some of the fundamental modes of vibrations of SbO3 structural groups. As a result, the infrared transmittance of these glasses will be least affected due to the introduction of ZnO [9], [10], [11], [12].

Manganese ions have strong bearing on the optical, magnetic and electrical properties of glasses. A large number of interesting studies are available on the environment of manganese ion in various inorganic glass systems [13], [14], [15]. Ardelean et al. [16], [17], [18], [19] have carried out a commendable work in the recent years on the structural role of manganese ions in a variety of glass systems, viz. Bi2O3, phosphate, arsenate, etc. The lasing action from Mn2+ ions has also been observed in some inorganic glasses [20], [21]. A considerable number of investigations on the role of manganese ions on the physical properties of a variety of glass systems like tellurite, arsenate, phosphate, borate, etc. has also been reported from our laboratory in recent years [22], [23], [24], [25], [26]. Manganese ions exist in different valence states with different coordinations in glass matrices, for example as Mn3+ in borate glasses with octahedral coordination where as in silicate and germinate glasses as Mn2+ with both tetrahedral and octahedral environment [19]. The content of manganese in different coordinations in different valence states exist in the glass depends upon the quantitative properties of modifiers and glass formers, size of the ions in the glass structure, their field strength, mobility of the modifier cation, etc. Both Mn3+ and Mn2+ ions are well known paramagnetic ions. Mn3+ ion has a large magnetic anisotropy due to its strong spin–orbit interaction of the 3d orbital whereas such anisotropy energy of Mn2+ ion is small because its orbital angular momentum is zero. Further, it is also quite likely for manganese ions to have link with antimony/borate groups, thereby strengthen the glass structure and may raise the chemical resistance of the glass. The purpose of the present investigation is to understand the local environment of manganese ions in ZnO–Sb2O3–B2O3 glass network and their influence on the stability of glass network by means of the optical absorption, ESR and magnetic susceptibility studies.

Within the glass forming region of ZnO–Sb2O3–B2O3 glass system, we have chosen a composition of 20ZnO–40Sb2O3–40B2O3 for MnO doping. The details of the compositions selected in the present study are: (20−x)ZnO–40Sb2O3–40B2O3:xMnO (all in mol% with 0≤x≤1.0 in steps of 0.2) and the glasses are labeled as M0 (pure), M2 (0.2% of MnO), M4 (0.4% of MnO), M6 (0.6% of MnO), M8 (0.8% of MnO) and M10 (1.0% of MnO).

The method of preparation of these glasses and the details of the apparatus used for recording optical absorption, ESR, IR spectra and for measuring magnetic susceptibility are similar to those already reported in literature [27], [28], [29]. The amorphous nature of the samples is confirmed by X-ray diffraction and scanning electron microscopy.

Section snippets

Results

From the measured values of density d and calculated average molecular weight M¯, various physical parameters of ZnO–Sb2O3–B2O3:MnO glasses such as manganese ion concentration Ni, mean manganese ion separation Ri and polaron radius (Rp), are evaluated and are presented in Table 1.

The IR transmission spectra recorded for the pure glasses (Fig. 1) show two conventional bands due to the presence of borate groups and they are due to the stretching relaxation of the B–O bond of the trigonal BO3

Discussion

ZnO–Sb2O3–B2O3 glasses have a complex composition and are the admixtures of network formers and modifiers. It is well known that the chemical Sb2O3 as such does not form glass, However, in the presence of modifiers like ZnO and MnO, it forms glass with triangular SbO3 pyramids with the oxygen at three corners and the lone pair of electrons at the fourth corner. In the glass network, the Sb–O distances lie in between 2.0 and 2.6 Å with the coordination number of Sb as 3.0. The coordination

Conclusions

The conclusions drawn from the study of various spectroscopic and magnetic properties of ZnO–Sb2O3–B2O3 doped with different concentrations of MnO are summarized as follows:

  • (i) The optical absorption measurements in the range 350–650 nm indicate that manganese ions predominantly in Mn2+ state if MnO is present in lower concentrations (up to 0.6%); when MnO is present in higher concentrations, the bands due to Mn2+ ions are obscured with the presence of a new band due to Mn3+ ions, that take part

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