δ‑Polymorph of Manganese Phosphate

A fourth polymorph of manganese phosphate, δ-Mn₃(PO₄)₂, crystallizes in the monoclinic space group <i>P</i>2₁<i>/c</i> with lattice parameters <i>a</i> = 8.9234(6) Å, <i>b</i> = 9.1526(6) Å, <i>c</i> = 8.6587(5) Å, β = 111.6670(10)°, <i>V</i> = 657.21(7) ų, and <i>Z</i> = 4. Its structure features planes of [MnO₅] pentahedra edge- and corner-shared with [PO₄] tetrahedra, resulting in a three-dimensional framework containing sub-nanometer channels. Both hydrothermal and solid-state synthesis routes can yield δ-Mn₃(PO₄)₂. In one method, the hydrothermal treatment of Mn₅(PO₄)₂­(PO₃OH)₂­·4H₂O (hureaulite) at 250 °C for 1–6 h yields δ-Mn₃(PO₄)₂ microcrystals; changing the precursor to Mn₃(PO₄)₂·3H₂O nanosheets leads to δ-Mn₃(PO₄)₂ nanoplates. A time study of δ-Mn₃(PO₄)₂ formation under hydrothermal conditions suggests a dissolution–nucleation mechanism. Alternatively, the solid state reaction of LiMnPO₄ and β′<i>-</i>Mn₃(PO₄)₂ in a 1:2 molar ratio under air-free conditions at 1000 °C produces δ-Mn₃(PO₄)₂. Differential scanning calorimetry and variable temperature X-ray diffraction measurements show that δ-Mn₃(PO₄)₂ is stable to 735 °C, beyond which it transforms into β′<i>-</i>Mn₃(PO₄)₂. The δ-Mn₃(PO₄)₂ → β′<i>-</i>Mn₃(PO₄)₂ conversion also occurs by hydrothermal treatment at 250 °C for 24 h