High entropy oxides: How many lanthanides can be introduced into Co-based double perovskite oxides?

The study shows the effects of introduction of multiple lanthanides into a single lattice site in a series of $\text{Ba}\left(\text{L}{\text{n}\prime }_{\frac{1}{\text{x}}}\text{Ln}{\prime \prime }_{\frac{1}{\text{x}}}\text{Ln}{\prime \prime \prime }_{\frac{1}{\text{x}}}\dots {\text{Ln}}^{\text{x}}{\prime }_{\frac{1}{\text{x}}}\right)\text{C}{\text{o}}_2{\text{O}}_{6-\text{δ}}$. The X-ray diffraction and transmission electron microscopy confirmed that up to 12 lanthanides can be successfully introduced into the material, and as a result, a tetragonal double perovskite structure is formed. Co mixed 3+/4+ valence state has been confirmed independently by iodometric titration and X-ray Absorption Spectroscopy. The effect of multiple elements' presence on the local environment of Co has been evaluated by employing Extended X-ray Absorption Fine Structure analysis. All materials are p-type conductors with total conductivity above 1000 $S\cdot {\mathrm{cm}}^{-1}$ at 300 °C and small positive Seebeck coefficients of about 1–23 $\mu V\cdot K^{-1}$. The electrical conductivity of multicomponent compounds is higher than for most double perovskite cobaltites.