We have investigated the structural and dielectric properties of Sr2TiO4, the first member of the Srn+1TinO3n+1 Ruddlesden-Popper series, within density-functional theory. Motivated by recent work in which thin films of Sr2TiO4 were grown by molecular beam epitaxy on SrTiO3 substrates, the in-plane lattice parameter was fixed to the theoretically optimized lattice constant of cubic SrTiO3 (n=infinity), while the out-of-plane lattice parameter and the internal structural parameters were relaxed. The fully relaxed structure was also investigated. Density-functional perturbation theory was used to calculate the zone-center phonon frequencies, Born effective charges, and the electronic dielectric permittivity tensor. A detailed study of the contribution of individual infrared-active modes to the static dielectric permittivity tensor was performed. The calculated Raman and infrared phonon frequencies were found to be in agreement with experiment where available. Comparisons of the calculated static dielectric permittivity with experiments on both ceramic powders and epitaxial thin films are discussed.
