Modulated structures in the system TiO₂-SnO₂

Tetragonal crystalline solutions in the system TiO2–SnO2 were annealed at 850°C and 1000°C to study the mechanism of separation within the exsolution dome. X-ray diffraction results indicated a unidirectional composition variation along [001]. The formation of the modulated structure was verified by electron microscopy, which showed alternately dark and light lamellae composed of SnO2-rich and TiO2-rich plates perpendicular to [001]. The sum of the thicknesses of the dark and light lamellae (‘wavelength') decreased in the early stages of the separation and then increased at longer annealing times, the thickness varying between 180 and 400 Å. It is proposed that the precipitation proceeds as follows: crystalline solution→intermediate modulated phase → equilibrium phases. The first step shows all the characteritics of the spinodal type of mechanism. It is shown theoretically that a one-directional fluctuation of composition parallel to [001] can be expected in the system TiO2-SnO2.