Knowledge Bank

A study has been made of the Raman spectrum of $SrTiO3$ both at room temperature and low temperatures. Both nominally pure and impure samples were examined. At room temperature the spectrum is entirely second order as expected from its cubic perovskite structure. Although the $SrTiO3$ and $KTaO3$ spectra appear to be quite different, this is largely due to superficial differences in the wave vector vs energy dispersion curves especially at the zone boundary. The predominant features in both spectra appear to originate from the same type optical modes. For this reason the more easily interpreted $KTaO3$ spectrum is a valuable aid in interpreting the second order $SrTiO3$ spectrum. The interpretation will be discussed and the derived single phonon energies compared with those obtained from neutron diffraction measurements. At lower temperatures the nominally pure and impure crystals behave quite differently. In both crystals three sharp lines appear at liquid nitrogen temperatures, but their energies are not the same in each crystal. In the pure crystal these lines appear to originate from local modes due to imperfections in the crystal, whereas in the impure crystal the energy shifts agree with the known values for the three polar transverse optic modes in $SrTiO3$. As the temperature is further reduced, no new lines are found in the pure crystal but three new lines are found in the impure crystal. These lines had energy shifts equal to those found in the pure crystal at liquid nitrogen temperature. The implications of these observations on the crystal structure of $SrTiO3$ at low temperatures will be discussed.