Low-temperature synthesis of Sr_1-xCa_xCuO₂ from hydroxide precursor

The alkaline-earth copper oxides Sr_1-xCa_xCuO₂ (0 ≤ x ≤ 1) have been synthesized in flowing gas of N₂ or O₂ by thermal decomposition of hydroxide precursors Sr_1-xCa_xCu(OH)₄ · H₂O. In the first step, we have succeeded in obtaining hydroxide precursors Sr_1-xCa_xCu(OH)₄ · H₂O with 0 ≤ x ≤ 1 in the form of a precipitation in a highly concentrated NaOH solution. In the second step, namely, in the process of thermal decomposition, the hydroxide precursors have been found to be decomposed into an amorphous condition at a temperature as low as 200°C. The ambient-pressure phase of SrCuO₂ is obtained by heating the hydroxide precursors for 12 h at 500°C and 700°C in flowing gas of N₂ and O₂, respectively. The reaction temperature is much lower than the 1000°C in flowing gas of O₂ in the conventional solid-state reaction method. With increasing x up to 0.7, the temperature at which the ambient-pressure phase of Sr_1-xCa_xCuO₂ appears increases in flowing gas of both N₂ and O₂. The so-called infinite-layer compound of Sr_1-xCa_xCuO₂ is obtained in a narrow range of x around 0.9 at ∼ 1000°C in flowing gas of O₂, which is similar to the result in the conventional solid-state reaction method. For x ∼ 1, the ambient-pressure phase of Ca_1-xCuO₂ (y ∼ 0.15) appears in the temperature range between 600°C and 800°C only in flowing gas of O₂.