New Member of the “112” Family, LaBaCo₂O_5.5: Synthesis, Structure, and Magnetism

The outermost member of the 112 family LaBaCo 2 O 5.5 has been synthesized using a multistep method to stabilize its structure. Its structural study, combining neutron powder diffraction and electron microscopy, shows that its orthorhombic matrix consists of layers of corner-shared CoO 6 octahedra interconnected through CoO 5 pyramids like in the other 112 cobaltates but contains small LaBaMn 2 O 5.5 -type domains (8%) due to local oxygen displacement. Its magnetic properties and magnetic structure evidence the following features: G-type antiferromagnetic/G-type ferrimagnetic/paramagnetic with T N = 260 K and T C = 326 K. This cobaltate differs from other 112 cobaltates by its antiferromagnetic structure which keeps the same symmetry as its ferrimagnetic phase, probably due to the size effect of La 3+ upon crystal field. Trivalent cobalt is shown to keep the intermediate spin state in the whole temperature range from 10 to 326 K.