Cooling-rate Dependent Ferromagnetism in a Two-dimensional Cyano-bridged Sm(III)-W(V) Complex

We report on cooling-rate dependent ferromagnetism in samarium octacyanotungstate, Sm(H<sub>2</sub>O)<sub>5</sub>[W(CN)<sub>8</sub>]. X-ray single-crystal structural analysis showed that this compound consists of a two-dimensional cyano-bridged network, in which Sm(III) and W(V) ions are linked in alternating fashion. When the sample was slowly cooled from 300 to 10 K at a cooling rate of −1 K min<sup>-1</sup>, the magnetization vs temperature curve showed antiferromagnetism with a Néel temperature (<i>T</i><sub>N</sub>) of 3.0 K. Heat capacity measurement and IR spectral variations of the slowly cooled sample suggest that a structural phase transition occurs at 166 K with a change in the coordination geometry of W(CN)<sub>8</sub>. Conversely, when the sample was placed directly into a sample chamber at <i>T</i> = 10 K, ferromagnetism with a Curie temperature (<i>T</i><sub>C</sub>) of 2.8 K was observed. This phenomenon is due to the overcooling effect of the structural phase transition. On the basis of calculations considering interlayer superexchange interactions and interlayer dipole−dipole interactions, the observed cooling-rate dependent ferromagnetism can be understood by a change of the interlayer superexchange interactions.