Nonstoichiometric K ₂ NiF ₄ ‐Type Phases in the Lanthanum‐Cobalt‐Oxygen System

The K 2 NiF 4 ‐type phase in the La‐Co‐O system has been studied and vacancy formation on both the lanthanum and oxygen sublattices has been observed. The formula La 2 CoO 4 has generally been accepted for the K 2 NiF 4 ‐type phase in the La‐Co‐O system. However, a broad range of lattice constants are reported for this material, indicative of variations in composition. More recently, a range of stoichiometries has been suggested in the literature which is represented by the formula La 2 CoO 4+x . The variations in stoichiometry have previously been explained by the presence of excess oxygen or by inter‐growths of Ruddlesden‐Popper‐type phases, (AO(ABO 3 ) n ). In this work, X‐ray diffraction data confirmed the previous variations in lattice parameters but indicated that the material is lanthanum deficient, not rich in oxygen. Thermogravimetric analysis confirmed the existence of variable oxygen stoichiometry. These data were consistent with a range of compositions described by the general formula La 1.83 CoO 4‐x , where 0 ≤ x ≤ 0.13. These materials contain a maximum of 50% trivalent cobalt when x = 0. In addition, thermo‐gravimetric experiments in lower partial pressures of oxygen have enabled us to isolate a new, fully reduced form, La 1.83 CoO 3.75 , which contains only divalent cobalt. A two‐ phase region exists between La 1.83 CoO 3.75 and La 1.83 CoO 3.87 .