Oxidic glasses as hosts for migrating metal ions

Abstract

The results of experiments where Tl⁺ and Pb²⁺ ions are electrolysed into a sodium borate glass (35 mol% Na₂O and 50 °C) are brought up to date in order to take into account recent developments in the chemistry of borate glasses. It is first necessary to consider the unique chemistry of the oxide(-II) species in terms of its electronegativity, electronic polarisability and acid–base properties, and the significant relationship between these is discussed. It is described how the Lewis basicity of oxidic materials such as glasses can be expressed quantitatively on the optical basicity scale and how determinations are made by various experimental methods. These methods include optical spectroscopy of ‘probe’ ions such as Tl⁺ or Pb²⁺, measurement of electronic polarisability and far-infrared spectroscopic ‘rattling’ frequencies of constituent metal ions. When Pb²⁺ ions are electrolysed into the sodium borate glass, it is found that there is migration of Na⁺ ions away from and of O²⁻ ions towards the (lead) anode with formation of PbO. There is almost complete depletion of Na₂O in the anode region so that the composition of the glass approximates to B₂O₃. A similar process occurs to a limited degree in the case of thallium, but the Tl⁺ ions are able to penetrate more deeply into the glass. Their ultraviolet ¹S₀ → ³P₁ frequency indicates that the sites they occupy have much greater basicity than the bulk glass. The two-site model of Kamitsos proposes that in borate glasses, there are higher and lower basicity sites, and the measured optical basicity of Tl⁺ indicates occupation of the higher sites. Since it has been shown that BO₄ groups in the glass are weakly basic, it is unlikely that they are involved in the higher sites. It is discussed how the higher site basicity implies greater covalency in the interaction of the Tl⁺ ion with the oxide(-II)s that constitute the pathway, and it is suggested that this is an important factor in the electromigration process.