Solvatomagnetism-induced Faraday effect in a cobalt hexacyanochromate-based magnet.

Solvent exchange caused reversible variations in color, magnetic properties, and the Faraday spectra of Co(II)(1.5)[Cr(III)(CN)(6)].7.5H2O (1) prepared in water. Compound 1 turned from peach to deep blue, which was due to a change in the coordination geometry on Co(II) ion from six-coordinate pseudo-octahedral (OhCo(II)) to four-coordinate pseudo-tetrahedral (TdCo(II)) geometries, when it was immersed in EtOH. The confirmed formula for the deep blue powder was Co(II)(1.5)[Cr(III)(CN)(6)].2.5H2O.2.0EtOH. The magnetic properties also changed; that is, the magnetic critical temperature, saturation magnetization, and coercive field went from 25 to 18 K, from 7.0 to 5.5 micro(B), and from 240 to 120 G, respectively. This solvatomagnetism is because the ferromagnetic magnetic coupling between OhCo(II) (S = 3/2) and Cr(III) (S = 3/2) is replaced by the antiferromagnetic coupling between TdCo(II) (S = 3/2) and Cr(III) (S = 3/2). Accompanying the solvatochromism and solvatomagnetism, the Faraday spectra drastically changed. The Faraday ellipticity (FE) spectrum of 1 had a distorted dispersive peak (A), which is due to the 4T1g --> 4T1g, 2T1g transitions of OhCo(II) ion, around 480 nm, but the FE spectra of 2 showed a new dispersive-shaped band (B) at 580 nm. The observed B band was assigned to the 4A2 --> 4T2 transition of the TdCo(II) ion. The Faraday spectra were well reproduced by a simulation that considers the ligand field splitting, spin-orbital coupling, and the ferromagnetic ordering. These solvatochromic effects were repeatedly observed.