Topotaxial formation of titanium-rich barium titanates during solid state reactions on (110) TiO 2 (rutile) and (001) BaTiO 3 single crystals

Abstract

The orientation relationships of Ti-rich barium titanate phases formed by solid state reactions at high temperatures were studied using (110) TiO 2 (rutile) and (001) BaTiO 3 single-crystal substrates. Well-oriented Ba 6Ti 17O 40 islands were observed after a vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 1 nm with the TiO 2 substrate, whereas a thin film consisting of well-oriented BaTiO 3 and Ba 6Ti 17O 40 grains was formed after vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 50 nm with the rutile substrate. A topotaxial orientation relationship between Ba 6Ti 17O 40 and TiO 2 was found. Topotaxy is facilitated by a certain similarity in the oxygen sublattices of TiO 2 and Ba 6Ti 17O 40. The mechanism of the reaction occurring between BaO vapor and the TiO 2 surface at high temperature is discussed. On the other hand, several well-oriented Ba 4Ti 13O 30, Ba 6Ti 17O 40 and Ba 2Ti 5O 12 phases were observed to be embedded in the mainly forming Ba 2TiSi 2O 8 phase after a solid–solid reaction of amorphous SiO 2 thin films with (001) BaTiO 3 substrates at temperatures above 1000 °C. They were formed by a topotaxial reaction involving the transformation of (111) planes of BaTiO 3 into (001) planes of the Ti-rich phases by removal of BaO and insertion of TiO 2. Cross-sections of the interfaces between the substrates and the various reaction products are studied by (high-resolution) transmission electron microscopy.