Preparation and properties of Sr_n+1Ti_nO _3n+1 ruddlesden-popper homologous series by metal-citric acid complex decomposition method

We investigated the Sr_n+1Ti_nO_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+1Ti_nO_3n+1 and the yttrium-doped Sr_n+1Ti_nO_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 Sr₂TiO₄ and Sr₃Ti₂O ₇ were produced without creation of SrCO₃ by heat-treatment at 1073 and 1473 K, respectively, for three hours. We were unable to produce single phase Sr₄Ti₃O₁₀. The samples consisted of Sr₄Ti₃O₁₀ and SrTiO₃ phases. Scanning electron microscopy showed that the grain shape of the Sr ₂TiO₄ was plate-like, while those of the Sr ₃Ti₂O₇ and Sr₄Ti₃O ₁₀ were not and the grains had large numbers of pores. High-density Sr_2-xY_xTiO₄ (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 _1.98Y_0.02TiO₄ was p=8.5×10^-5 ωcm). The Seebeck coefficient of Sr_1.98Y_0.02TiO ₄ was -160.7 μ V/K, and the thermal conductivity was 3.38 W/mK.