  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.
