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(H₂O)₅[W(CN)₈]. 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^-1, the magnetization vs temperature curve showed antiferromagnetism with a Néel temperature (T_N) 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)₈. Conversely, when the sample was placed directly into a sample chamber at T = 10 K, ferromagnetism with a Curie temperature (T_C) 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.