A vast diversity of electronic phenomena in oxides, amongst others piezo-, pyro-, and ferroelectricity, multiferroicity, magnetism and superconductivity, as well as catalytic and redox behavior, originates from details in the electronic structure. Such details are highly sensitive to broken structural symmetries commonly

accompanied with atomic displacements, e.g. due to compositional changes, non-stoichiometry and defects, or in response to external fields. Depending on the boundary conditions in terms of explicit atomic confinement on the one hand or ambiance interactions on the other hand, in particular the physical properties of the surface region can be sensitively manipulated.

This chapter of the book presents examples of switching physical properties in the model system SrO–TiO2, with a focus on strontium titanate SrTiO3 as central perovskite, by means of compositional changes within the surface region induced by high external electric fields at ambient temperatures. A crucial role is attributed to the additional oxygen deficiency in the oxide crystal structure and its redistribution in an electric field throughout the bulk crystal. The chapter is subdivided into four parts:

Section 10.1 introduces the stable phases within the quasi-binary system SrO–TiO2, in particular the homologous series of SrO(SrTiO3)n Ruddlesden-Popper phases (quasi-binary and ternary phase diagrams) and briefly presents means for chemical modifications.

Section 10.2 covers the effect of external electric fields on the chemical composition in bulk strontium titanate and in comparison to other perovskites, including in particular the dynamic processes of ionic transport in dependence on temperature and crystallographic orientation and related electrocoloration phenomena.

Section 10.3 focuses on stoichiometry changes in the surface region of strontium titanate. It gives experimental evidence of structural and compositional changes and introduces the models of Ruddlesden-Popper phase and migration-induced fieldstabilized polar (MFP) phase formation due to the ionic transport initiated by the electric field.

Section 10.4 presents changes of the physical and chemical properties induced by these phase transformations in the surface region. This includes electric properties, such as conductivity and band gap, mechanical properties, such as hardness and elasticity, as well as polar properties, such as pyro- and piezoelectricity. Exploiting the switching phenomena of these properties, the section further describes related applications of the oxide e.g. as solid-state battery or switchable pyroelectric.