Synthesis and characterization of La_1.5+xSr_0.5−xCo_0.5Ni_0.5O_4±δ (x = 0, 0.2)

The K₂NiF₄ phases La_1.5+xSr_0.5−xCo_0.5Ni_0.5O_4(+δ) (x = 0, 0.2) have been prepared by solid state reactions and structurally characterized by X-ray and neutron powder diffraction. The reduction behaviour, the magnetic properties and the electronic properties of these materials have also been examined. Oxygen hyperstoichiometry has been achieved in the lanthanum-rich phase La_1.7Sr_0.3Co_0.5Ni_0.5O_4.08 with retention of the tetragonal symmetry. The excess oxygen occupies the ideal interstitial (0, 0.5, 0.25) sites of the tetragonal structure. The materials withstand reducing conditions (10% H₂–N₂) up to 800 °C via reduction of the B-site oxidation state to the divalent state (Ni²⁺/Co²⁺) and formation of oxide-ion vacancies within the equatorial planes of the structure. Formation of Ni¹⁺ in these materials under reducing conditions is suggested to account for the oxygen stoichiometry and the magnetic behaviour of La_1.5Sr_0.5Co_0.5Ni_0.5O_3.70. The electrical conductivity of the oxygen-rich semiconductor materials reaches a SOFC applicable limit (>100 S cm⁻¹) at elevated temperatures.