Hydrothermal Synthesis, Structure and Magnetic Properties of a One‐Dimensional Iron Arsenate, 1 ∞ [NH 3 (CH 2 ) 2 NH(CH 2 ) 2 NH 3 ][Fe 2 F 4 (HAsO 4 ) 2 ]

A novel one‐dimensional iron arsenate, 1 ∞ [NH 3 (CH 2 ) 2 NH(CH 2 ) 2 NH 3 ][Fe 2 F 4 (HAsO 4 ) 2 ], built up exclusively from SBU‐4 units (two octahedral units sharing an edge and connected through two tetrahedra) with Fe 3+ ions in the high‐spin state has been synthesized by hydrothermal methods. Structure elucidation reveals that the compound is a two‐leg ladder with very weak antiferromagnetic exchanges along the chains. The magnetic susceptibility data show activated behaviour at T = 14 K, suggesting a finite spin gap between the nonmagnetic ground state and the excited triplet state. We have estimated the exchange parameters theoretically based on the S = 5/2 Majumdar−Ghosh model with frustrations and dimerisations. Although the magnetic gap is quite small, this is the first example, to the best of our knowledge, where the rungs with two S = 5/2 spins form strong singlet dimers. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)