The Sr2.75Ce0.25Co2O7−δ oxide, n=2 member of the Ruddlesden–Popper series: Structural and magnetic evolution depending on oxygen stoichiometry

The second member of the Ruddlesden–Popper series, n=2 in Srn+1ConO3n+1, has been stabilized by substituting cerium for strontium leading to the pure compound Sr2.75Ce0.25Co2O7−δ. The oxygen vacancies of this phase can be partially filled by a post-annealing oxidizing treatment with δ decreasing from 1.1 to 0.3 for the as-prepared and oxidized phases, respectively. As the samples are oxidized from δ≈1.1 to 0.3, the a and b unit cell parameters decrease from 3.836 to 3.815 Å and from 20.453 to 20.047 Å, respectively. Despite the average value of the cobalt valence state, VCo≈+3.5, obtained in the oxidized Sr2.75Ce+40.25Co2O6.7 phase, a clear ferromagnetic state wit TC=175 K and MS=0.8 μB/Co is reached.