Reactivity in air of the Sr3Co2O7−δ RP = 2 phase: Formation of the hydrated Sr3Co2O5(OH)2⋠xH2O cobaltite

A new hydrated oxyhydroxide, Sr3Co2O5(OH)2⋅xH2O, has been prepared in air. Its structure, determined by combining transmission electron microscopy (TEM) and X-ray powder diffraction, is related to the n=2 member of the Ruddlesden-Popper series Srn+1ConO3n+1. This compound crystallizes in a monoclinic space group, I2/m, with the following parameters, a=c=3.7701(8)Å, b=28.59(1)Å and β=90.25(5)°. The introduction of OH− groups, to form (SrOH)+ layers, and the intercalation of additional H2O molecules are found to expand significantly the thickness of the block separating two successive perovskite blocks. As shown by the magnetic measurements, this phase is very unstable for T values beyond RT; this metastable character is also confirmed by the TEM study. Under both vacuum and electron beam, several structural forms have been observed, going from the oxygen deficient RP = 2 phase, Sr3Co2O7−δ, to the hydrated oxyhydroxide Sr3Co2O5(OH)2⋅xH2O, through the anhydrous oxyhydroxide Sr3Co2O5(OH)2. Finally, the topotactic behavior of the hydrolysis process for the Sr3Co2O7−δ, phase is also demonstrated.