Oxygen over stoichiometry in the 2H-perovskite related structure: the route to a large family of cation deficient Ising chain oxides Sr1+y[(Mn1−xCox)1−z□z]O3

Abstract

The realization of oxygen over stoichiometry through oxidation of Co²⁺ into Co³⁺ in the 2H-perovskite related oxides Sr_1+x(Mn_1−xCo_x)O₃ is demonstrated, leading to a large family of cation deficient Ising chain oxides Sr_1+y[(Mn_1−xCo_x)_1−z□_z]O₃. In these aperiodic hexagonal structures, also formulated as Sr_1+x(Mn_1−xCo_x)O_3+δ (a ≈ 9.6 Å, c₁ ≈ 2.6 Å, c₂ ≈ 3.9 Å), and built up of face sharing octahedra and trigonal prismatic polyhedra, the observed modulation vector γ = c₁/c₂ (ranging from ∼0.641 to ∼0.668) allows the proportion of octahedral (N_O) and prismatic sites (N_P) to be obtained according to the relationships: y = 2γ − 1 = N_P/(N_P + N_O). For these oxides, the content of cationic vacancies (z) in the chains and the oxygen over-stoichiometry (δ) can be deduced from the relationships: z = (x − y)/(1 + x) and δ = 3(x − y)/(1 + y). The analysis of these results shows that the structure of the chains, i.e., the proportion of tetrameric units “M₄O₁₂” coexisting with trimeric units “M₃O₉”, is highly sensitive to the oxygen over-stoichiometry for a fixed cationic composition. This is illustrated here by the structure of the oxide corresponding to x = 0.375 with δ = 0.09, which is commensurate with γ = 2/3, similar to Sr₄Mn₂CoO₉, but with a significant content of cationic vacancies in the trigonal prismatic sites according to the formula Sr₄(Mn^IV_1.818Co^III_0.544 Co^II_0.548□_0.09)O₉. Thus, the presence in those chains of cationic vacancies due to oxygen overstoichiometry appears as a very important concept which opens the route to the synthesis of fragmented chain structures and may have a huge impact upon the magnetic and magneto-electric behavior of Ising chain oxides.