Magnetic Order and Spin-Glass Behavior in the M[Mn(CN)₆] (M = V, Cr, Mn, Co, Ni) Prussian Blue Analogs, W. E. Buschmann, J. S. Miller, Inorg. Chem. 39, 2411-2421 (2000).

Magnetically ordered Prussian blue analogs with the general formulation of M[Mn(CN)₆] (M = V, Cr, Mn, Co, Ni) were made in aprotic media utilizing [Mn^IV(CN)₆]^2-. These analogues are, valence-ambiguous as they can be formulated as M^II[Mn^IV(CN)⁶] or M^III[Mn^III(CN)₆]. The X-ray powder diffraction of each member of this family can be indexed to the face-centered cubic fcc Prussian blue structure type, with atypically reduced unit cell parameters (a » 9.25 ± 0.25 Å) with respect to hydrated Prussian blue structured materials (a > 10.1 Å). The reduced a-values are attributed to a contraction of the lattice in the absence of water or coordinating solvent molecule (i. e., MeCN) that is necessary to help stabilize the structure during lattice formation. Based on u_CN IR absorptions, X-ray photoelectron spectra, and magnetic data, the following oxidation state assignments are made: M^II[Mn^IV(CN)₆] (M = Co, Ni) and M^III[Mn^III(CN)₆] (M = V, Cr, Mn). Formation of Mn^III[Mn^III(CN)₆] is in contrast to Mn^II[Mn^IV(CN)₆] prepared from aqueous media. Above 250 K, the magnetic susceptibilities of M[Mn(CN)₆] (M = V, Cr, Mn, Co, Ni) can be fit to the Curie-Weiss equation with q = -370, -140, -105, -55, and -120 K, respectively, suggesting strong antiferromagnetic coupling. The room temperature effective moments, respectively, are 3.71, 4.62, 5.66, 4.54, and 4.91 µ_B, consistent with the above oxidation state assignments. All compounds do not exhibit magnetic saturation at 50 kOe, and exhibit frequency dependent c'(T) and c"(T) responses characteristic of spin-glass-like behavior. M[Mn(CN)₆] order as ferrimagnets, with T_c's taken from the peak in the 10 Hz c'(T) data, of 19, 16, 27.1, <1.75, and 4.8 K for M = V, Cr, Mn, Co, and Ni, respectively. The structural and magnetic disorder prevents Ni^II[Mn^IV(CN)₆] from ordering as a ferromagnet as anticipated, and structural inhomogeneities allow Co^II[Mn^IV(CN)₆] and V^III[Mn^III(CN)₆] to unexpectedly order as ferrimagnets. Also, Mn^III[Mn^III(CN)₆] behaves as a reentrant spin glass showing two transitions at 20 and 27.1 K, and similar behavior is evident for Cr^III[Mn^III(CN)₆]. Hysteresis with coercive fields of 340, 130, 8, 9, and 220 Oe and remanent magnetizations of 40, 80, 1500, 4, and 250 emuOe/mol are observed for M = V, Cr, Mn, Co, and Ni, respectively.