Thermal solid—solid interaction between sodium and manganese oxides

Abstract

The solid—solid interaction between sodium and manganese oxides was studied using DTA, TG, DTG, X-ray diffraction and chemical determination of the amount of sodium ions contributing in solid solutions and in solid—solid interactions. The results obtained revealed that sodium-doping of MnCO 3 enhanced its thermal decomposition and also favoured the thermal decomposition of MnO 2 giving Mn 2O 3 which decomposed less readily at 900°C yielding Mn 3O 4. In other words, sodium treatment decreased the thermal stability of both MnCO 3 and MnO 2 but increased that of Mn 2O 3. Such effects were directly proportional to the amount of dopant ions added. The amount of sodium ions dissolved in Mn 2O 3 was not greatly changed by increasing the dissolution temperature from 400 to 500°C and decreased on increasing the temperature to 750°C. A highly crystalline sodium manganate phase, Na 4Mn 9O 18, was formed by heating the sodium-treated manganese oxides in air at 900°C. Such a newly formed compound was found to decompose readily at 1050°C giving Mn 3O 4 and Na 2O which was effectively dissolved in manganic oxide forming an Na 2OMn 3O 4 solid solution. The observed changes in the thermal stability of manganese oxides due to sodium-treatment were attributed to alterations in the oxidation state of the doped solids.