The phase diagram and crystal chemistry of strontium-doped rare earth cobaltates: Ln2−xSrxCoO4+δ (Ln = La–Dy)

Single-phase tetragonal K2NiF4-type phases Ln2−xSrxCoO4+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.