The evolution of structure induced by intensive milling in the system 2Bi2O3 · 3TiO2

Concurrent mechanochemical treatment of either a stoichiometric 2Bi2O3 · 3TiO2 powder mixture or (pulverized) Bi4Ti3O12 compound was performed in a planetary ball mill. Relevant structural parameters: crystallite size, amount of amorphous phase and the transformed fraction (as a result of chemical reaction between Bi2O3 and TiO2) of the powders milled for various milling times and intensities were derived from X-ray powder diffraction data. The obtained structural parameters were used to follow the kinetics of the reduction of crystallite size, amorphous phase formation and chemical reaction. In the powder mixture, during the early stage of mechanochemical treatment the Bi2(CO3)O2 phase was found as an intermediate product which transformed into the highly amorphized Bi4Ti3O12 phase as the milling progressed. On the other hand, mechanochemical treatment of Bi4Ti3O12 powder induced a gradual deformation of the crystal lattice and destruction of the perovskite-type structure. However, in both cases, after a certain milling time, a very disordered, amorphous/nanocrystalline structure was obtained. It was demonstrated that in the steady state, the amorphous/nanocrystalline phase ratio depends on the milling conditions. Higher milling intensities induce ‘nanocrystallization’ of the amorphous phase, i.e., precipitation of crystallites with an average size below 20 nm. A kinetic model involving a nanocrystalline ↔ amorphous reaction, in which the forward and reverse reaction were first-order was postulated and used to analyze the formation of an amorphous phase.