Stability and Decomposition of Perovskite-Type Titanates upon High-Temperature Reduction

Abstract

Controlling the properties of the surface region of ternary perovskite‐type titanates is of high importance in the field of future energy and information technologies. A common method to modify the surface of perovskite oxides such as SrTiO3 is annealing under vacuum conditions. Here we show that the local oxygen partial pressure near to the surface plays a crucial role during annealing. While the oxide is found to be macroscopically stable during high‐temperature reduction under standard vacuum conditions and only segregation effects on the nanoscale seem to take place, surface decomposition due to incongruent sublimation occurs as soon as an oxygen getter such as Ti is positioned close to the oxide. In consequence, owing to the high volatility of Sr‐containing species at very low oxygen pressures, a Sr‐depleted TiOx surface layer is formed. This effect might reveal an alternative possibility to tailor composition and structure of perovskite‐type titanate surfaces.

Electrically controlled transformation of memristive titanates into mesoporous titanium oxides via incongruent sublimation

“…10 −8 mbar due to oxygen release from the anode; see Supplementary Information Ia). As we have calculated and demonstrated recently by performing annealing experiments on SrTiO 3 while the oxygen partial pressure was reduced by placing an oxygen getter such as Ti in close vicinity to the surface 14 , under such extremely low oxygen activities ( p O2 < 10 −17 mbar at 1000 °C), incongruent sublimation of the alkaline earth metal can be expected. Despite the high thermodynamic stability of SrTiO 3 , a decomposition via incongruent sublimation can take place via the vapour phase establishing above the surface of the oxide.…”

“…In general, two channels of surface decomposition of a solid compound exist, namely segregation and incongruent sublimation 11 . While segregation effects are known to occur upon thermal annealing only to limited extent 12 , 13 , we have shown recently 14 that macroscopic decomposition of titanates via incongruent sublimation of the alkaline earth metal can occur at extremely low oxygen activity even at relatively moderate temperatures (>650 °C). In the present study, we now give evidence that phase decomposition in the surface region related to incongruent sublimation can be controlled upon the application of an electric field causing a movement of oxygen from the cathode region towards the anode region.…”

Kelvin probe force microscopy work function characterization of transition metal oxide crystals under ongoing reduction and oxidation

“…1. Heavily reducing conditions enable not only the removal of oxygen from the crystal but additionally trigger an incongruent sublimation of strontium, resulting in the titanium enrichment of the surface [26]. This general process has also been observed for the broader class of transition metal oxides, such as CaTiO 3 or BaTiO 3 .…”

“…The crystals were mounted onto Omicron direct heating holders and introduced into the UHV chamber, with a base pressure of 5 × 10 −10 mbar, maintained by the use of turbo-molecular, ion and sublimation pumps. In order to produce a TiO nanowire network on the surface of STO(100), the extremely low oxygen partial pressure (ELOP) method of perovskite decomposition was adapted (for more information see [12,26]). The SrTiO 3 (100) single crystal (10 × 3 × 0.5 mm 3 ) sample was mounted on an n-doped Si(111) single crystal, which acted as an oxygen getter, allowing extremely low effective oxygen partial pressures to be reached on the STO surface (below 10 −20 mbar).…”

Interactions of Ruddlesden-Popper Phases and Migration-Induced Field-Stabilized Polar Phase in Strontium Titanate

“…The picture of an ideal crystal interacting with the gas phase has to be seen as a model approach. The presence of oxygen getters as electrode materials may sensitively affect the local oxygen partial pressure in the surface region and even lead to decomposition [51]. V O in strontium titanate changes the valence state of the titanium ion from 4+ to 3+ and the coordination from octahedral to pyramidal.…”