Reversed (Negative) Magnetization for Electrochemically Deposited High-T_c Thin Films of Chromium Hexacyanide Magnets

Thin films of chromium cyanide Prussian blue-like materials, K n Cr x [Cr(CN) 6 ] (ca. 1.0 ± 0.5 μm thick) are electrodeposited on glassy carbon electrodes in amorphous (a) and crystalline (α) forms. The crystalline film shows a X-ray diffraction pattern typical of a face-centered cubic (fcc) Prussian blue-type cubic lattice [ a = 10.41(2) Å]. The films exhibit quasi-reversible cyclic voltammetric waves, distinct morphologies in SEM images, and strong field-dependent magnetic behavior. The films undergo two one-electron processes, Cr II [Cr II (CN) 6 ] 2 - ⇌ Cr III [Cr II (CN) 6 ] - + e -, and Cr III [Cr II (CN) 6 ] - ⇌ Cr III [Cr III (CN) 6 ] + e -, with some residual Cr II being present. The composition is determined from an analysis of the υ CN IR data {[Cr II (CN) 6 ] 4 -: υ CN = 2072s cm - 1; [Cr III (CN) 6 ] 3 -: υ CN = 2185m cm - 1 } and X-ray photoelectron (XPS) spectral data. The composition of the oxidized and reduced films are Cr III [Cr III (CN) 6 ] 0.98 [Cr II (CN) 6 ] 0.02 and K 2.0 Cr II [Cr II (CN) 6 ], respectively, for both amorphous and crystalline forms. These materials exhibit broad intervalence charge-transfer bands in their oxidized [λ max = 18 800 cm - 1 (ε ∼ 1280 cm - 1 M - 1 ); 35 500 cm - 1 (ε ∼ 1180 cm - 1 M - 1 )] and reduced [λ max = 20 200 cm - 1 (ε ∼ 870 cm - 1 M - 1 ); 36 700 cm - 1 (ε ∼ 770 cm - 1 M - 1 )] forms. The crystal field splitting Δ o 's for Cr II (NC) 6 and Cr III (NC) 6 are 19 500 and 26 600 cm - 1, respectively. The magnetic ordering temperatures, T c, are a function of Cr oxidation state and range between 135 and 260 K for both film types. The T c 's are independent of crystallinity, suggesting that the magnetic domains are smaller than the short-range structural order for both a- and α-films. Hysteresis is observed for the films, with coercive fields as high as 830 Oe at 20 K, confirming the existence of bulk ferrimagnetic behavior below T c . We determined that the coercivity of the amorphous films is larger than that of the crystalline films and suggest that this due to the larger number of defects in the a-films.