Probing phase coexistence and stabilization of the spin-ordered ferrimagnetic state by calcium addition in the Y(Ba_1−xCa_x)Co₂O_5.5 layered cobaltites using neutron diffraction

We study the effects of a partial substitution of Ba with the smaller cation Ca in the layered cobaltites YBaCo₂O_5+δ for δ≈0.5. Neutron thermodiffractograms are reported for the compounds YBa_0.95Ca_0.05Co₂O_5.5 (x_Ca=0.05) and YBa_0.90Ca_0.10Co₂O_5.5 (x_Ca=0.10) in the temperature range 20 K⩽T⩽300 K, as well as high-resolution neutron diffraction experiments at selected temperatures for the samples x_Ca=0.05, x_Ca=0.10, and the parent compound x_Ca=0. We have found the magnetic properties to be strongly affected by the cationic substitution. Although the “122” perovskite structure seems unaffected by Ca addition, the magnetic arrangements of Co ions are drastically modified: the antiferromagnetic (AFM) long-range order is destroyed, and a ferrimagnetic phase with spin state order is stabilized below T∼290 K. For the sample with x_Ca=0.05 a fraction of AFM phase coexists with the ferrimagnetic one below T∼190 K, whereas for x_Ca=0.10 the AFM order is completely lost. The systematic refinement of the whole series has allowed for a better understanding of the observed low-temperature diffraction patterns of the parent compound YBaCo₂O_5.5, which had not yet been clarified. A two-phase scenario is proposed for the x_Ca=0 compound which is compatible with the phase coexistence observed in the x_Ca=0.05 sample.