Layered iron(III) arsenates: Synthesis and characterization of A(2)Fe(2)O(AsO4)(2) (A=K, Rb)

Two new iron(III) arsenates K2Fe2O(AsO4)(2) (1) and Rb2Fe2O(AsO4)(2) (2) have been synthesized and characterized by single-crystal X-ray diffraction, magnetic susceptibilities, and Mossbauer spectroscopy. The two compounds are isostructural and exhibit a sheet structure consisting of infinite slabs of edge-sharing FeO6 octahedra linked into sheets by AsO4 tetrahedra, with the alkali-metal cations in the interlayer region. Crystal data for 1: orthorhombic, space group Pnma (No. 62), a = 8.5219(3) Angstrom, b = 5.7612(2) Angstrom, c = 17.9452(6) Angstrom, Z = 4; Crystal data for 2: As above, except a = 8.5330(2) Angstrom, b = 5.7945(2) Angstrom, c = 18.6157(1) Angstrom. Room-temperature Mossbauer data confirm the presence of Fe-III. Magnetic susceptibility measurements on compound 2 show that it undergoes a transition to a weak ferromagnetic state near 25 K.