Synthesis and Characterization of New Open-Framework Iron(II) Phosphate with 1D Tube and Double Layer Structure, NaFe₂(PO₄)(HPO₄)

A novel two-dimensional layered iron(II) phosphate, NaFe^II₂(PO₄)(HPO₄), has been synthesized under mild hydrothermal conditions. Its structure was determined by single-crystal X-ray diffraction and further characterized by FTIR, elemental analysis, inductively coupled plasma (ICP) analysis, powder X-ray diffraction, and magnetic susceptibility measurements. Its anionic network is based on strictly alternating Fe^IIO₄ tetrahedral units and P-centered units including PO₄ and PO₃(OH) tetrahedra linked through their vertices to form a double layered structure that contains columns of double six-membered rings and 1D tubes by four-membered rings sharing faces. The sodium atoms reside in the interlayer region. The title compound crystallizes in the triclinic system, space group P1̄, with the following cell parameters:  a = 5.1200(10) Å; b = 8.6232(17) Å; c = 8.7958(18) Å; α = 96.93(3)°; β = 100.45(3)°; γ = 105.75(3)°; V = 361.58(12) ų; Z = 2; D_c = 2.991 Mg/m³.