The electrochemical series of the 3d transition metal ions in alkali borate glasses

Abstract

The optical and electron spin resonance (ESR) spectra of binary sodium borate glasses doped with oxides of the 3d transition metal (TM) ions (from TiO2 to CuO) have been studied in order to establish their electrochemical series. Glasses containing two pairs of redox oxides were brought to equilibrium with respect to the furnace atmosphere (air) at a fixed temperature (1000 °C). After cooling down, their optical and ESR spectra and chemical analyses show that considerable mutual interactions of the two different TM ions have taken place, compared with those reference glasses which contain only one type of TM ions. The order of increasing redox potential power of the 3d TM ions in a sodium borate glass with 10 mol % Na2O can be given as: Ti4+/Ti3+ > Cu2+/Cu1+ > Fe3+/Fe2+ > V5+/V4+ > Cr6+/Cr3+ > Mn3+ /Mn2+. This order changes in 20 and 30 mol % Na2O containing borate glasses as follows: Ti4+/Ti3+ > Fe3+/Fe2+ > V5+/V4+ > Cu2+/Cu1+ > Cr6+/Cr3+ > Mn3+ /Mn2+. In these series, a TM ion pair on the left will be oxidized and will reduce the TM ion on the right. The redox behaviour of the cobalt ions with other TM ions could be obtained only in glasses with high sodium oxide content (50 to 70 mol % Na2O), and their electrochemical order is Cu2+/Cu1+ > Cr6+/Cr3+ > Mn3+/Mn2+ > Co3+/Co2+.