**5. Ferroelectric SrTiO3**

SrTiO3 is a typical crystal with perovskite structure (Fig. 8(a)) and it is easy to get a pure transparent single crystal. As a well-known incipient ferroelectric, it is one of the most widely studied dielectric crystals [24]. In 1999, Itoh et al. found the ferroelectricity in SrTiO3 by the isotope substitution of O16 by O18 [25]. Since then, there has been considerable renewed interest in the low-temperature properties of SrTiO3. Hereafter, we refer to SrTiO3 16 and SrTiO3 <sup>18</sup> as STO16 and STO18, respectively. First, we will review the soft mode behavior in the normal STO16 and in the next subsection the soft mode in STO18 will be discussed.
