Chemometric analyses of XANES data collected on ⁹⁹Tc bearing silicate glasses

A challenge to the incorporation of ⁹⁹Tc into a glass matrix is that ⁹⁹Tc is volatile at vitrification temperatures. Understanding how this volatilization occurs requires knowledge of the multiple chemistries which Tc may take during vitrification. This paper presents an overview of how the localized chemistry of ⁹⁹Tc has been determined in a series of ⁹⁹Tc-bearing glasses by chemometric analyses of X-ray absorption near edge spectra (XANES). Linear combination fitting and principal component analysis of the glasses' XANES spectra suggested that the glasses contained 3-4 chemically distinct Tc environments. The identity of the detected chemistries were pertechnetates, (Tc(VII) as isolated oxyanions, or locally coordinated by Na or K), or isolated Tc(IV) species. The linear combination fitting distribution of local Tc sites agrees with reanalyzed Raman spectra, suggesting that as targeted KTcO₄ incorporation increases, a significant ion exchange takes place, and speciation in the glass changes to favor Tc(VII) formation, specifically NaTcO₄. Based on the statistical suggestion that not all Tc environments are accounted by the available standards, a new mechanism for the behavior of Tc during vitrification is proposed. (author)