Synthesis and characterization of novel barium iron phosphates: Insight into new structure types tailored by hydrogen atoms

A significant gap in our knowledge of phosphate crystal chemistry is the lack of understanding of what controls the depolymerization of [PO4] tetrahedra. A new route using phosphoric acid without any added water at 240 °C has been developed to synthesize two new compounds Ba2Fe2[H(PO3OH)2][H(P2O7)2] and Ba2Fe[H(P2O7)2], as well as a new polymorph β-BaFe2(P2O7)2 from annealing the former at ≥500 °C. Structural characterizations show that Ba2Fe2[H(PO3OH)2][H(P2O7)2] features a novel 2D 2∞([Fe2[H(PO3OH)2][H(P2O7)2]]4−) layer structure containing both hydrogen-diphosphate [H(P2O7)2] and hydrogen-monophosphate [H(PO3OH)2] groups, whereas Ba2Fe[H(P2O7)2] and β-BaFe2(P2O7)2 possess an 1D chain structure and a 3D framework structure, respectively. These structures support a control of hydrogen on the depolymerization of [PO4] groups and our work opens a new way for the synthesis of other novel phosphates using controlled water activities. - Graphical abstract: A new route of controlled water activities led to syntheses of three new compounds, indicating water plays a crucial role in the formation of novel phosphates. - Highlights: • A new mild thermal route for the synthesis of novel phosphates. • Water activities controlling the reaction: 2H3PO4↔H4P2O7+H2O. • Phosphate with mixed hydrogen-monophosphate and diphosphate groups. • H atoms in the structural backbone affect the depolymerization of PO4 groups