Reversed (Negative) Magnetization for Electrochemically Deposited High-<i>T</i>_c Thin Films of Chromium Hexacyanide Magnets

Abstract

Thin films of chromium cyanide Prussian blue-like materials, KnCrx[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, CrII[CrII(CN)6]2- ⇌ CrIII[CrII(CN)6]- + e-, and CrIII[CrII(CN)6]- ⇌ CrIII[CrIII(CN)6] + e-, with some residual CrII being present. The composition is determined from an analysis of the υCN IR data {[CrII(CN)6]4-: υCN = 2072s cm-1; [CrIII(CN)6]3-: υCN = 2185m cm-1} and X-ray photoelectron (XPS) spectral data. The composition of the oxidized and reduced films are CrIII[CrIII(CN)6]0.98[CrII(CN)6]0.02 and K2.0CrII[CrII(CN)6], respectively, for both amorphous and crystalline forms. These materials exhibit broad intervalence charge-transfer bands in their oxidized [λmax = 18 800 cm-1 (ε ∼ 1280 cm-1 M-1); 35 500 cm-1 (ε ∼ 1180 cm-1 M-1)] and reduced [λmax = 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 Δo's for CrII(NC)6 and CrIII(NC)6 are 19 500 and 26 600 cm-1, respectively. The magnetic ordering temperatures, Tc, are a function of Cr oxidation state and range between 135 and 260 K for both film types. The Tc'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 Tc. 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.