Synthesis and structures of carrier doped titanates with the Ruddlesden-Popper structure (Sr_0.95La_0.05)_n+1Ti_nO_3n+1 (n = 1, 2)

Carrier doping in Sr_n+1Ti_nO_3n+1 (n = 1, 2) was conducted by the substitution of La for Sr and the simultaneous introduction of oxygen vacancies. Single-phase products of (Sr_0.95La_0.05)_n+1Ti_nO _3n+1-δ (n = 1, δ = 0.05; n =2, δ = 0.125) were synthesized by the solid-state reaction of Sr₂TiO₄ (or Sr₃Ti₂O₇), La₂O₃ and Ti. When TiO₂ was used as one of the starting compounds to maintain oxygen stoichiometry, compounds with higher n were observed besides the object phases. Structural refinement by Rietveld analysis revealed that the a-axis expands while the c-axis contracts with La doping. The contraction of the c-axis is attributed to a shortened (Sr,La)-O distance, since the TiO₆ octahedra elongated in both a and c directions with doping. The change in the structure is discussed on the basis of the size of the substituting ion. The resistivity of the single-phase products was semiconducting with a very weak temperature dependence down to 80 K.