Coordination and valence state of transition metal ions in alkali-borate glasses

Borate glasses of the 20R₂O·80B₂O₃ type, where R = Li, Na and K, were colored by doping with transition metal ions (Co, Ni, Cr and Mn). The glasses were obtained by melting at the temperature of 1150 °C. For these glasses optical absorption in UV–VIS–NIR range were recorded. Analysis of the spectra allows to be determined the coordination and oxidation states of the doping transition metal ions. Changes of their coordination or oxidation are presented as a function of the optical basicity Λ after Duffy. Cobalt and nickel are present in examined borate glasses as divalent ions (Co²⁺, Ni²⁺) in octahedral coordination mainly, but the tetrahedral coordination state of cobalt is also possible. Chromium and manganese are present in the borate glasses in various oxidation state, though Cr³⁺ and Mn³⁺ ions in the octahedral coordination are probably dominant. A decrease of the electronegativity of the modifiers (Li → Na → K) and an increase of the glass matrix basicity cause a shift of the oxidation/reduction equilibrium towards higher valences of the transition metals (Cr⁶⁺, Mn³⁺).