Synthesis, crystal structure, and magnetic susceptibilities of CsFeP₂O₇ and RbFeP₂O₇

Two new ternary iron(III) diphosphates, CsFeP₂O₇ and RbFeP₂O₇, have been prepared and the structure of the Cs compound was determined from single-crystal X-ray diffraction data. Base on powder X-ray diffraction RbFeP₂O₇ adopts the same structure type as that of the Cs compound. CsFeP₂O₇ crystallizes in the monoclinic space group P2₁/c with a = 7.684(3), b = 9.937(3), c = 8.378(3) Å, β = 104.84(3)°, V = 618.4(4) ų, Z = 4, and R = 0.048. The structure consists of intersecting tunnels running along the [001] and [110] directions, and the Cs⁺ cations are located at the intersection of these tunnels. The framework is built up from corner-sharing FeO₆ octahedra and P₂O₇ groups. CsFeP₂O₇ is isostructural with KFeP₂O₇. Variable-temperature powder magnetic susceptibility data exhibit an antiferromagnetic transition at T_N ∼ 25 K.