Preparation and Properties of Sr n+1 Ti n O 3n+1 Ruddlesden-Popper Homologous Series by Metal-Citric Acid Complex Decomposition Method

We investigated the Sr n+1 Ti n O 3n+1 Ruddlesden-Popper homologous series. This material exhibits a wide range of electric behaviors, from a high-dielectric-constant tunable-paraelectric in its undoped form to a metallic superconductor when it is doped with a variety of elements. The Sr n+1 Ti n O 3n+1 and the yttrium-doped Sr n+1 Ti n O 3n+1 Ruddlesden-Popper homologous series were prepared through the thermal decomposition of a metal citric acid complex. The starting solution consisted of the raw materials strontium acetate and titanium alkoxide. Citric acid was used as the chelating agent, and ethanol and distilled water were mixed and used as a solvent. Single phase Sr 2 TiO 4 and Sr 3 Ti 2 O 7 were produced without creation of SrCO 3 by heat-treatment at 1073 and 1473 K, respectively, for three hours. We were unable to produce single phase Sr 4 Ti 3 O 10 . The samples consisted of Sr 4 Ti 3 O 10 and SrTiO 3 phases. Scanning electron microscopy showed that the grain shape of the Sr 2 TiO 4 was plate-like, while those of the Sr 3 Ti 2 O 7 and Sr 4 Ti 3 O 10 were not and the grains had large numbers of pores. High-density Sr 2-x Y x TiO 4 (x= 0 to 0.06) ceramic samples were produced under hot-press conditions (1823 K for one hour at a uniaxial pressure of 31.2 MPa). Although the undoped samples were insulators, the yttrium-doped samples showed high electrical conductivity (i.e. that of Sr 1.98 Y 0.02 TiO 4 was ρ= 8.5×10 −5 Ωcm). The Seebeck coefficient of Sr 1.98 Y 0.02 TiO 4 was –160.7 μV/K, and the thermal conductivity was 3.38 W/mK.