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This section gives a brief general description of the structure of glass, some structural phenomenon relevant to chalcogenide glasses and some structural properties of GLS glass relevant to the discussion in section 4.7.

2.8.1 General structure of glass

Glasses can be thought of as having a disordered structure since unlike crystals they do not have long range order. However, there is usually short range order present in that each constituent ion has a specific number of ligands. This description of glasses forms the basis of the continuous random network model, first proposed by Zachariasen in 1932.[63] Constituents of glass are generally divided into two broad categories: network

formers and network modifiers. Network formers can be thought of as the backbone of the glass structure through an interconnecting network of polyhedra. For example, SiO4,

ZrF4 and GaS4 are the network forming polyhedra for silicate fluorozirconate and GLS

glass, respectively. Network modifiers take up the interstitial space between the network forming polyhedra, breaking up the periodicity and preventing crystallisation. For example Al, Ba and La2S3 are network modifiers in silicate fluorozirconate and GLS

glass, respectively.[62]

2.8.2 Chalcogenide glass

Chalcogenide glasses can be either stoichiometric or non-stoichiometric. The structure of arsenic chalcogenides can be characterised in terms of the correlation between hetropolar (arsenic-chalcogen) and homopolar (arsenic-arsenic, chalcogen-chalcogen) chemical bonds. For example in the stoichiometric As2Se3 glass the concentration of

homopolar bonds is 10-35%.[64]

2.8.3 GLS glass

A study of the structure of bulk GLS glass using extended x-ray fine structure spectroscopy (EXAFS) has been presented by Benazeth et al.[65] The Ga-S distance was reported to be 2.26 Å which is characteristic of a covalent bond and therefore GaS4

units were identified as the glass forming units. Comparisons between the Raman and IR spectra of crystalline and amorphous GLS also indicate the presence of GaS4

structural units.[66, 67] In contrast, the crystal Ga2S3 presents many crystallographic

sites and dispersed Ga-S distances.[65] The structure of the crystal Ga2S3 is shown in

figure 2.8. Three sulphur atoms are bound to three gallium atoms. Two of these sulphur atoms (S1) are each engaged in two covalent bonds and one dative bond (S2) with three

surrounding gallium atoms. The remaining sulphur atom (S3) is the usual bridging atom

as it is bound to two gallium atoms.[34]

FIGURE 2.8 The covalent gallium environment of the crystalline Ga2S3, after [65]

Such an environment of sulphur atoms, where most of them present coordination numbers greater than two, is not usually known in glassy sulphides. And indeed, it is

impossible experimentally to obtain an amorphous structure from pure Ga2S3.[65]

Because of this a network modifying agent is required for glass formation. The main network modifier in GLS is La3+[68] which is 8 fold coordinated to sulphur with an undetermined symmetry.[69] GaS4 tetrahedra are formed from the reaction of Ga2S3

crystal with sulphur anions (S2-) brought by the addition of La2S3.[16, 65] These sulphur

ions break the Ga→S dative bonds, characteristic of the crystalline phase, which forms some GaS4 tetrahedra with a negative charge (figure 2.9 (a)). These negative ionic

cavities form some reception sites for La3+ ions, which act as charge compensators for these negative charges.[16, 68]. GLS also contains a small quantity of Ga-related tetrahedra, containing at least one threefold coordinated oxygen atom linked to three tetrahedra. Thus in GLS there is both an oxide and a sulphide environment for the La3+ ion.

“Sulphide” negative cavities

“Oxide” negative cavities

(a)

(b)

+

+

FIGURE 2.9 Formation of sulphide negative cavities (a) and oxide negative cavities (b), after[16].

When La2S3 is substituted by La2O3 to form GLSO it is expected that the reaction will

be that same as in figure 2.9 (a) but with O2- replacing S2- anions, as illustrated in figure 2.9 (b). Thus in GLSO reception sites for the La3+ ion are predominantly oxide in nature.[16] However, it is proposed that a small quantity of sulphide sites exist in GLSO in order to explain the two lifetime components observed in titanium doped GLSO, see section 5.2.4. This leads to a model the GLS system in which a covalent network of GaS4 tetrahedra are inter-dispersed by essentially ionic La-S channels.[65, 68, 69]

Chapter 3

Glass melting and spectroscopic techniques

3.1 Introduction

The first part of this chapter details the melting procedures for the fabrication of transition metal doped GLS, in particular, vanadium doped GLS. Other authors, notably Mairaj[34] and Brady,[70] have carried out a very exhaustive analysis of the raw material purification and glass melting procedures for GLS. The fabrication of transition metal doped samples examined in this work used many of the techniques developed by Mairaj and Brady without further enhancement; these techniques are therefore only briefly summarised. Detailed in this chapter are melting procedures developed for the fabrication of low doping concentration transition metal doped GLS glass. The second part of this chapter details all of the spectroscopic techniques used in the analysis of transition metal doped GLS in sufficient detail for the reader to understand, critique and repeat them. A brief introduction to each spectroscopic technique is given. Absorption, Raman, XPS and EPR measurements were taken on commercially manufactured equipment since they were available with the required specifications. The lack of commercially manufactured equipment with the required specifications for photoluminescence, excitation, lifetime and quantum efficiency measurements meant they were taken on in house equipment. The in house equipment may not have been able to compete with commercially manufactured versions, were they available, in repeatability but they did allow a degree of functionality and specialisation not possible with a manufactured system.