Polymorphs, phase transitions and stability in BaM2(PO4)2 M = Mn, Fe, Co systems

BaM2(PO4)(2) (M2+ transition metals) compounds display a variety of polymorphs; among those, the well-known rhombohedral gamma-form stands at the origin of a series of (quasi)2D-magnetic behaviors intensively studied in the last few decades. We have prepared original alpha-variants for M = Fe2+ and Mn2+ and investigated their stabilities and sequence of phase transitions by analogy to the already reported M = Co2+ case. Heating the alpha-phases leads to a variety of structural rearrangements between alpha, alpha ' and beta variants, according to unified 1st order reconstructive transitions. However, spontaneous long-time transformations under ambient conditions alpha -> gamma (Fe case) versus gamma -> alpha (Co case) denote a contrasted hierarchy of stable compounds verified by DFT calculations. Interestingly, in alpha-BaFe2(PO4)(2) the quasi triangular Fe2+O3+2 coordination is rarely observed and relies on one very short Fe-O distance of ca. 1.78 angstrom, destabilizing the edifice in favor of the most stable FeO6 in the gamma-type, as verified by Crystal Orbital Hamilton Population (COHP) analysis.