Two new iron(III) arsenates K2Fe2O(AsO4)(2) (1) and Rb2Fe2O(AsO4)(2) (
2) have been synthesized and characterized by single-crystal X-ray dif
fraction, magnetic susceptibilities, and Mossbauer spectroscopy. The t
wo compounds are isostructural and exhibit a sheet structure consistin
g of infinite slabs of edge-sharing FeO6 octahedra linked into sheets
by AsO4 tetrahedra, with the alkali-metal cations in the interlayer re
gion. 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 Mossba
uer data confirm the presence of Fe-III. Magnetic susceptibility measu
rements on compound 2 show that it undergoes a transition to a weak fe
rromagnetic state near 25 K.