Microstructure characterization of ball milled prepared nanocrystalline perovskite CaTiO 3 by Rietveld method

Abstract

Perovskite CaTiO 3 has three polymorphic phases: cubic, tetragonal and orthorhombic. The present article deals with the first time preparation of nanocrystalline orthorhombic polymorphs of CaTiO 3 by high energy ball milling the equimolar mixture of CaO and anatase (a)-TiO 2 powders. It has been observed that a small amount of CaTiO 3 is formed just after 1 h of ball milling and the wt.% of the phase increases with increasing milling time. The starting mixture is transformed almost completely to nanocrystalline CaTiO 3 within 6 h of ball milling. To verify the stability of the phase at high temperature, the 6 h milled powder has been post-annealed at 773 and 1273 K for 2 h. Phase transition kinetic study, quantitative phase estimation and microstructure characterization of CaTiO 3 and other secondary phases evolved at different duration of ball milling in ball milled samples have been critically analyzed by Rietveld’s powder structure refinement method using X-ray powder diffraction data. Nanocrystalline (∼6 nm) CaTiO 3 powder has been obtained within 1 h of milling but in the course of milling up to 6 h of duration, crystallite size is seen to increase gradually with increasing milling time due to agglomeration of nanosized crystallites. It has been observed that the a-TiO 2 has been transformed to polymorphic srilankite(s)-TiO 2 phase and nanosized CaTiO 3 crystallites are formed by the solid state diffusion of nanosized s-TiO 2 and CaO crystallites. Post-annealing study reveals that the orthorhombic CaTiO 3 phase is stable up to 1273 K with larger and strain free crystallites of ∼125 nm.