The growth mechanism is oriented attachement (collisions of nanoparticles and adjustement in orientation, next nanoparticles attachement and self-recrystallization) of nanoparticles followed by the Ostwald ripening. Growth of nanoparticles is preffered beacuse it enables surface energy minimalisation. See my articles attached for BiFeO3 hydrothermal synthesis.
Best regards, Bartek
Article PEG-controlled thickness of BiFeO3 crystallites in microwave...