Layered Iron(III) Arsenates: Synthesis and Characterization of A(2)Fe(2)O(AsO(4))(2) (A = K, Rb)

Two new iron(III) arsenates K(2)Fe(2)O(AsO(4))(2) (1) and Rb(2)Fe(2)O(AsO(4))(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 FeO(6) octahedra linked into sheets by AsO(4) tetrahedra, with the alkali-metal cations in the interlayer region. Crystal data for 1: orthorhombic, space group Pnma (No. 62), a = 8.5219(3) Å, b = 5.7612(2) Å, c = 17.9452(6) Å, Z = 4. Crystal data for 2: As above, except a = 8.5330(2) Å, b = 5.7945(2) Å, c = 18.6157(1) Å. 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.