Ln₂BaMO₅ oxides (Ln = trivalent lanthanide; M = Co, Ni, Cu, Zn) present very interesting structural, magnetic and spectroscopic properties and, when MII= Cu, they are generated as byproducts or impurities (the so-called "green phases") during the syntheses of the LnBa₂Cu₃0₇_δ-type superconductors. Despite their identical stoichiometries, the Ln₂BaMO₅ materials belong to four different structural types, depending on the LN(III) and/or M(II) cations present in the lattices. Besides, during the past few years, it has also been shown that some of them present temperature dependent dimorphism.Facultad de Ciencias Exactas