Topotaxial formation of titanium-rich barium titanates during solid state reactions on (110) TiO_2 (rutile) and (001) BaTiO_3 single crystals

摘要

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_(17)O_(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_(17)O_(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_(17)O_(40) and TiO_2 was found. Topotaxy is facilitated by a certain similarity in the oxygen sublattices of TiO_2 and Ba_6Ti_(17)O_(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_(13)O_(30), Ba_6Ti_(17)O_(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 deg 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.