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

Abstract

BaM₂(PO₄)₂ (M²⁺ transition metals) compounds display a variety of polymorphs; among those, the well-known rhombohedral γ-form stands at the origin of a series of (quasi)2D-magnetic behaviors intensively studied in the last few decades. We have prepared original α-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 α-phases leads to a variety of structural rearrangements between α, α′ and β variants, according to unified 1st order reconstructive transitions. However, spontaneous long-time transformations under ambient conditions α → γ (Fe case) versus γ → α (Co case) denote a contrasted hierarchy of stable compounds verified by DFT calculations. Interestingly, in α-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 the most stable FeO₆ in the γ-type, as verified by Crystal Orbital Hamilton Population (COHP) analysis.