Temperature dependence of microwave and THz dielectric response in   Srn+1TinO3n+1 (n=1-4)
  The microwave, near-millimetre and infrared (IR) dielectric response of Srn+1TinO3n+1 (n=1-4) Ruddlesden-Popper homologous series was studied in the temperature range 10 to 300 K. Remarkable softening of the polar optical mode was observed in Sr4Ti3O10 and Sr5Ti4O13 which explains the increase in microwave permittivity and dielectric loss upon cooling. However, both samples have a distinct content of SrTiO3 dispersed between SrO layers. It is proposed therefore that the observed soft mode originates from the SrTiO3 microscopic inclusions. 
D. Noujni, S. Kamba, A. Pashkin, V. Bovtun, J. Petzelt, A-K. Axelsson,   N. McN Alford, P.L. Wise, I.M. Reaney