**2.2 Strontium titanate (SrTiO3)**

Another member of perovskite family is strontium ferrite STO (SrTiO3) is a complex oxide. It exhibits cubic structure at room temperature. O2− ions are bonded with six folded coordinate to Ti4+ ions and with twelve folded coordinate to Sr2+ ion. Each Sr2+ ion lies in between four TiO6 octahedral. To decrease in temperature, it undergoes a phase transition. The first transition below room occur at −168°C. The opposite rotation of adjacent oxygen octahedral at this term turns the cubic structure to tetragonal structure. To further decrease in temperature at −236°C the changeable phonon modes turns the tetragonal structure to orthorhombic structure. The orthorhombic structure is change to rhombohedra structure at −263°C [29]. As Curie–Weiss law suggests the dielectric constant increases with the phase transition below room temperature. At this temperature due to quantum fluctuations leads to the quantum PE [30]. The charge storage capacity is high, chemical stability, optical transparency in the visible region with good insulating properties makes it use in modern electronics applications such as phase shifters, high-voltage capacitors, delay lines, filters, tunable oscillators etc. [31–33]. It is used in cancer treatment and in thermo – electric generators due to they have insolubility and high melting point properties [34, 35]. It shows photoconductivity when exposed to light due to having an direct gap and indirect band gap of 3.75 eV and 3.25 eV respectively [36]. Its conductivity increases with the contact of light to the crystal. Its conductivity persists for several days, with small decay [37, 38].

The first oxide, which is superconducting below 0.35 K is the strontium oxide (STO) [39]. It can be used as a tremendous substrate for superconductors with a high operating temperature and for oxide-based thin films. It is used as single-crystal substrate due to the enhanced electrical conductivity of niobium doped STO for the growth of perovskite oxides. It is also used for other ferroelectric and magnetic oxide substrate due to its large lattice parameter. Variation of temperature and pressure parameter can lead to increase oxygen vacancies in both crystal and thin films of SrTiO3. So it becomes more conducting due to stimulate free electrons in the conduction band and also opaque. This used as gate-dielectric material due to the growth of high quality growth of epitaxial SrTiO3 layers on silicon without forming silicon oxide Furthermore, it allow the incorporation of other perovskite oxides based thin-film on top of silicon [40]. It is also used for its piezoelectricity, ferroelectricity and Pyro electricity application in nanoscale [41–44]. It is used for various technological applications such as in super capacitor, nonvolatile memory, tunable microwave capacitors, ultralow-temperature scanning microscopies, high-density dynamic random access memories, soft phonon devices, oxygen sensors [45, 46].
