REVERSED (NEGATIVE) MAGNETIZATION FOR ELECTROCHEMICALLY DEPOSITED HIGH-T-C THIN-FILMS OF CHROMIUM HEXACYANIDE MAGNETS

Thin films of chromium cyanide Prussian blue-like materials, KnCrx[Cr( CN)(6)] (ca. 1.0 +/- 0.5 mu m thick) are electrodeposited on glassy ca rbon electrodes in amorphous (a) and crystalline (alpha) forms. The cr ystalline film shows a X-ray diffraction pattern typical of a face-cen tered cubic (fcc) Prussian blue-type cubic lattice [a = 10.41(2) Angst rom]. The films exhibit quasi-reversible cyclic voltammetric waves, di stinct morphologies in SEM images, and strong field-dependent magnetic behavior. The films undergo two one-electron processes, Cr-II[Cr-II(C N)(6)](2-) reversible arrow Cr-III[Cr-II(CN)(6)](-) + e(-), and Cr-III [CrII(CN)(6)](-) reversible arrow Cr-III[Cr-III(CN)(6)] + e(-), with s ome residual Cr-II being present. The composition is determined from a n analysis of the upsilon(CN) IR data {[Cr-II(CN)(6)](4-): upsilon(CN) = 2072s cm(-1); [Cr-III(CN)(6)](3-): upsilon(CN) = 2185m cm(-1)} and X-ray photoelectron (XPS) spectral data. The composition of the oxidiz ed and reduced films are Cr-III[Cr-III(CN)(6)](0.98)[Cr-II(CN)(6)](0.0 2) and K2.0CrII[Cr-II(CN)(6)], respectively, for both amorphous and cr ystalline forms. These materials exhibit broad intervalence charge-tra nsfer bands in their oxidized [lambda(max) = 18 800 cm(-1) (epsilon si milar to 1280 cm(-1) M-1); 35 500 cm(-1) (epsilon similar to 1180 cm(- 1) M-1)] and reduced [lambda(max) = 20 200 cm(-1) (epsilon similar to E 870 cm(-1) M-1); 36 700 cm(-1) (epsilon similar to 770 cm(-1) M-1)] forms. The crystal field splitting Delta(o)'s for Cr-II(NC)(6) and Cr- III(NC)(6) are 19 500 and 26 600 cm(-1), respectively. The magnetic or dering temperatures, T-c, are a function of Cr oxidation state and ran ge between 135 and 260 K for both film types. The T-c's are independen t of crystallinity, suggesting that the magnetic domains are smaller t han the short-range structural order for both a- and alpha-films, Hyst eresis is observed for the films, with coercive fields as high as 830 Oe at 20 K, confirming the existence of bulk ferrimagnetic behavior be low T-c. We determined that the coercivity of the amorphous films is l arger than that of the crystalline films and suggest that this due to the larger number of defects in the a-films.