Mechanochemical reactions in Na2CO3–M2O5 (M = V, Nb, Ta) powder mixtures: Influence of transition-metal oxide on reaction rate

We have studied the mechanochemical reactions between Na2CO3 and the transition-metal oxides M2O5 (M = V, Nb, Ta), giving special attention to the course of the individual reaction and to the influence of the transition-metal oxide on the reaction rate. A common mechanism links the studied reactions and is characterized by the formation of an intermediate amorphous carbonato complex, in which the CO32– ions that are present initially in the Na2CO3, coordinate to the transition-metal cation. In a subsequent milling stage the carbonato complex decomposes, leading to the formation of the final binary compounds, i.e., NaVO3, NaNbO3 and NaTaO3. Quantitative experimental results, based on the Rietveld refinement method and thermogravimetric analyses, showed that the reaction rate follows the order Na2CO3 + V2O5 > Na2CO3 + Ta2O5 > Na2CO3 + Nb2O5. By analyzing the acid–base properties of the participating reagents, we found that the experimental observations agree with the acid–base interaction mechanism, which means that the larger the acidity of the transition-metal cation involved, the faster and more complete the mechanochemical reaction. Finally, it was found that only partial decomposition of the carbonato complex, even after a prolonged mechanochemical treatment, can be expected in reactions characterized by lower acid–base potential.