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 isomorphic with the compounds CaZn2P2O8 and BaAl2Si2O8. The magnetic study of the α form shows an antiferromagnetic ordering at TN = 17 K, with spins canting below TN. Then, the analysis of the magnetic interactions paths occurring within the layers evidence superexchange paths and additional supersuperexchange paths between the chains. This scheme leads to a hexagonal frustrated topology responsible for the canted spin structure in a 2D-topology of anisotropic Co2+ magnetic ions.