Enhancing spin state phase separation by strontium addition in the Y (Ba1−xSrx)Co2O5.5 layered cobaltite

In this paper we present a neutron diffraction study of the effects of replacing Ba with smaller Sr cations in the layered cobaltite YBaCo2O5+d for d0.5. Neutron thermodiffractograms are reported in the temperature range 20 K < =T < =300 K, as well as high-resolution neutron diffraction experiments at selected temperatures. The systematic refinement of the series of samples with Sr substituting at the Ba site permits the understanding of the complex magnetic behaviour of this system, and reinforces a two-phase scenario for the parent compound. We have found that Sr addition destabilizes the ferrimagnetic spin state ordered phase, as well as the '122' order characteristic of cobaltites with d = 0.5, favouring instead a tetragonal and less ordered structure.