Energetics of lanthanide cobalt perovskites: LnCoO_3-δ (Ln = La, Nd, Sm, Gd)

Lanthanide cobalt perovskites LnCoO_3-δ (Ln = La, Nd, Sm, and Gd) are important materials for electroceramics, catalysts, and electrodes in solid oxide fuel cells. Formation enthalpies of LnCoO_3-δ compounds were measured using high temperature oxide melt solution calorimetry. The formation enthalpies of LaCoO_2.992, NdCoO_2.985, SmCoO_2.982 and GdCoO_2.968 from constituent binary oxides (Ln₂O₃, CoO) and O₂ gas are -111.87 ± 1.36, -98.49 ± 1.33, -91.56 ± 1.46 and -88.16 ± 1.45 kJ mol^-1, respectively. Thus these perovskites become energetically less stable with decrease in ionic radius of the lanthanide (from La to Gd), which corresponds to a decreasing tolerance factor and increasing oxygen deficiency. The thermodynamic stability of LaCoO_2.992, NdCoO_2.985, SmCoO_2.982 and GdCoO_2.968 was also assessed considering their oxygen partial pressures for decomposition, with good agreement between thermochemical and equilibrium data.