Hydrothermal synthesis, structure, stability and magnetism of Na₂Co₂(C₂O₄)₃(H₂O)₂ : a new metal oxalate ladder

Abstract

Purple crystals of the compound Na₂Co₂(C₂O₄)₃(H₂O)₂1 were synthesized in high yielding hydrothermal reactions. The room temperature crystal structure reveals an anionic cobalt oxalate network with a 2-leg ladder topology. The ladder frameworks run parallel to the a axis and are held together by both hydrogen bonding and electrostatic interactions. The magnetism of 1 was studied as a function of temperature and field. The susceptibility shows a broad maximum at about 21 K which is associated with short-range correlations, and a phase transition to a long range ordered Néel state at about 9 K. Above T_N the data are fitted by a range of models which suggests the intra-ladder couplings are of the order −4 to −5 cm⁻¹ and the inter-ladder coupling is approximately 0.2 cm⁻¹. Themogravimetric analysis shows 1 to be stable up to 350 °C, where water of crystallisation is lost. A second decomposition episode is seen at 400 °C.