Electron transfer, linkage isomerization, bulk magnetic order, and spin-glass behavior in the iron hexacyanomanganate Prussian blue analogue

The reaction of [Fe-II(NCMe)(0)][B{C6H3(CF3)(2)}(4)](2) with [(Ph3P)(2)N](2 )[Mn-IV(CN)(6)] in THF/CH2Cl2 leads to the formation of dark green "Fe[Mn(C N)(6)]", which is isomorphous with face-centered cubic Prussian blue [Fm3m space group, a=9.6 (1)Angstrom]. Based on the IR nu(CN) absorption [2127cm( -1) (s), similar to 2170 cm(-1) (w,sh)], X-ray photoelectron spectroscopy ( XPS; Fe 2p(3/2)= 710.1 eV: Mn 2p(3/2) = 641.8 eV), and Fe-57 Mossbauer spec troscopy [23% Fe-II (high spin), 72% Fe-III (high spin), 3% Fe-II (low spin ), 2% Fe-III (low spin)] as well as charge neutrality, this material is for mulated as (Fe0.23Fe0.72Mn0.02Mn0.03IV)-Fe-II-Mn-III-Mn-III[Mn-IV(CN)(6)](0 .23)[Mn-III(CN)(6)](0.23)[Fe-III(CN)(6)](0.02)[Fe-II(CN)(6)](0.03) (1). Thi s arises from a not understood combination of election transfer and linkage isomerization. This solid is stable in its anhydrous form, but quickly bec omes deep blue (nu(CN)=2142 cm(-1)) upon hydration to form {(Fe0.15Fe0.80Mn 0.01Mn0.04IV)-Fe-II-Mn-III-Mn-III[Mn-IV(CN)(6)](0.15)[Mn-III(CN)(6)](0.80) [Fe-III(CN)(6)](0.01) [Fe-II(CN)(6)](0.04)}. xH(2)O (2) another complex com bination of site occupancies, election transfer, and linkage isomerization with the subsequent appearance of a new nu(CN) absorption (2065 cm(-1)) and an XPS peak consistent with [Fe-II(CN)(6)](4-) (Fe 2p(3/2) = 708.4 eV). Af ter the hydrated material has been mildly heated the composition becomes th e less complex {(Fe0.94Mn0.06IV)-Mn-III[Mn-III(CN)(6)](0.94)[Fe-II(CN)(6)]( 0.06)}.xH(2)O (3). The complex nature of the ion and charge distributions, with each metal ion existing in two different oxidation states in two diffe rent sites within the solid, is attributed to partial charge transfer and l inkage isomerization, which presumably occurs to differing degrees at diffe rent defect sites and emphasizes the elaborate nature of the details of the materials based on the seemingly simple Prussian blue structure type. Magn etic susceptibility measurements of 1 and 3 give room temperature moments, mu(eff) that are consistent with the formulations [mu(eff) = 6.51 mu(B) (6. 47 calcd) and = 6.59 mu(B) (6.40 calcd), respectively]. A Curie-Weiss const ant, theta, of -8K for 1 is consistent with the spin sites being antiferrom agnetically coupled, and bulk ferrimagnetic order is observed below its T-c of 15.0 K. Compounds 2 and 3 are also magnetically ordered with T-c's of 1 8.7 and 10.1K, respectively. Compounds 1, 2, and 3 each exhibit hysteresis with low coercive fields of 3, 1, and 4 Oe, respectively. These are surpris ing low values as 1, 2, and 3 possess significant crystallographic disorder as evidenced by X-ray diffraction studies and this results in spin-glass b ehavior in accord with the significant frequency dependence of the alternat ing current (ac) susceptibility data.