Synthesis of the Tl0.64Sr1.5La0.5CoO5-ε phase with a 1201 structure

The procedure for preparing complex Tl-La-Sr cobaltites by reacting the La2-xSrxCoO4-y precursors with Tl2O3 in a sealed gold ampule at 890 degrees C was elaborated. The phase Tl0.64Sr1.5La0.5CoO5-epsilon Was prepared, and its structure and bulk and surface compositions were studied. The phase is tetragonal with the unit cell parameters a = 3.781 Angstrom and c = 8.697 Angstrom. Some specific features of the mechanism of the cobaltite synthesis (the reaction occurs at the three-phase interface, low stability of the ultimate and intermediate products, the tendency of thallium oxides toward surface segregation) were found to be responsible for kinetic hindrances at the final stages of the synthesis and for the occurrence of small impurities of extraneous phases. Possible modes of accommodation of the complex cobaltite structure at a considerable deviation from the thallium stoichiometry are discussed.