The phase diagram and crystal chemistry of strontium-doped rare earth cobaltates: Ln_2-xSr_xCoO_4+δ (Ln = La-Dy)

Single-phase tetragonal K₂NiF₄-type phases Ln_2-xSr_xCoO_4+δ (Ln = La³⁺-Dy³⁺) have been prepared for (x = 0.75-1.50). For the first time, the phase field for these compounds has been defined using a combination of electron microscopy and powder X-ray diffraction. An extensive solid solution was observed for the largest rare earth ions, while only a limited range was found for Ln = Sm³⁺, Gd³⁺, and Dy³⁺. The K₂NiF₄ structure could not be formed for Y³⁺ and smaller rare earths ions under atmospheric conditions. Thermogravimetric analysis indicates a range of average cobalt oxidation states increases with Sr-doping and varies from 2.96 to 3.72. Rietveld refinement using X-ray powder diffraction data confirm that Ln³⁺ and Sr²⁺ ions are disordered over the same lattice sites.