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 whose the well-known rhombohedral -form stand at the origin of a series of (quasi)2D-magnetic behaviors intensively studied in the last decades. We have prepared original -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 -phases lead to a variety of structural rearrangements between , ’ and β variants, according to unified 1st order reconstructive transitions. However, spontaneous long-time transformations in ambient conditions (Fe case) versus (Co case) denote a contrasted hierarchy of stable compounds verified by DFT calculations. Interestingly, in -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 Å, destabilizing the edifice in favor of most stable FeO6 in the -type, as verified by Crystal Orbital Hamilton Population (COHP) analysis.