Puzzling polymorphism of layered Ba(CoPO4)2.

In this paper, we present the phase diagram and magnetic properties of the layered Co2+-based compounds of formula Ba(CoPO4)2 for which the rhombohedral γ form is well-known for its quasi-2D XY topology that is responsible for magnetization steps. The structural resolution of the new, room temperature-stable monoclinic α form shows similitude with the hydrated homologue Ba(CoPO4)2·H2O and consists of the stacking between [CoPO4]− sheets with chain subunits. We show by means of high temperature powder XRD and thermal analyses that the α form transforms into several polymorphs also exhibiting layered architectures upon heating. Three reversible transitions at 773, 893, and 993 K were observed from DTA which allowed us to define several forms as follows: α → α′ → α″ → β. The crystallographic relationships between the several polymorphs and hydrate analogue are discussed. The α′ and α″ cell parameters involve a direct relationship with the α form, whereas the trigonal β phase was fully solved and found isomor...