Abstract

The investigation of the Tl–Sr–Co–O system has allowed a new form of 1201-type cobaltite to be synthesized. The HREM study of this oxide, Tl_0.4Sr_2.5Co_1.1O_{5 − δ} (δ ≈ 0.20) shows that it consists of an association, in the same matrix, of two phasoïds, the tetragonal oxygen stoichiometric 1201-structure Tl_0.4Sr_2.5Co_1.1O₅ (a ≈ a₁₂₀₁, c ≈ c₁₂₀₁) and a new ordered oxygen deficient 1201 Tl_0.4Sr_2.5Co_1.1O_4.5 (a ≈ 2a₁₂₀₁, b ≈ a₁₂₀₁c ≈ 2c₁₂₀₁). The latter is described as the intergrowth of double rock salt layers [Sr_1.5Tl_0.4Co_0.1O₂]_∞ with single ordered oxygen deficient perovskite layers [SrCoO_2.5]_∞ similar to YBaFeCuO₅. The magnetic and transport properties of this Sr-rich oxide differ fundamentally from the 1201 cobaltite TlSr₂CoO₅, by the absence of a metal to insulator transition as a function of temperature. The as-synthesized sample exhibits a semiconducting and paramagnetic behavior down to 50 K, with θ_p = −160 K indicative of antiferromagnetic correlations. The oxygen pressure annealed sample shows a much lower resistivity, T independent down to 200 K, and weak ferromagnetism in agreement with its θ_p value (+50 K). These properties and thermopower measurements are interpreted on the basis of Co⁴⁺ carriers in an intermediate spins Co³⁺ matrix.