Electron microscopy study of intragranular nanoporosity and the occurrence of local structural disorder in cubic BaTiO3 nanopowders from alkoxide–hydroxide precipitation process

Abstract

The evolution of phase and defect structure in BaTiO3 nanopowders synthesized by an alkoxide–hydroxide process in benzyl alcohol under reflux conditions were investigated. As-prepared powders were heat-processed at temperatures ranging from 250 to 850°C. X-ray powder diffraction (XRD) showed that BaTiO3 remained cubic with crystallite size below 30nm after heat treatment up to 850°C. Internal pores of 1–2nm diameter inside the crystallites were well visible with transmission electron microscopy (TEM) at processing temperatures between 250 and 600°C. Their presence was confirmed by infrared spectroscopy (FT-IR) and is attributed to accumulation and curing of lattice hydroxyl defects. No tetragonal phase was detected by XRD up to 700°C. However, Raman spectroscopy (FT-RS) revealed Raman activity in all cubic materials. This was attributed to local tetragonal and orthorhombic distortions, resulting from the presence of internal nanopores. Considering that the nanopores constitute a relatively large fraction of the total volume of the nanocrystals, the internal strain may be substantial. Hence, a relatively high temperature is necessary not only to cure the hydroxyl defects, but also to release the internal strains accumulated in the crystals upon volume diffusion and sintering. Therefore, the cubic-to-tetragonal phase transition was observed only after heat treatment at 850°C, when the occurrence of a tetragonal phase was accompanied by substantial grain growth to ∼100nm size.