Nuclear Magnetic Resonance and X-Ray Studies of Potassium Ferrocyanide Trihydrate Crystal

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Abstract
The atomic coordinates of one previously unknown water oxygen in potassium ferrocyanide trihydrate are determined by X-ray structure analysis in the paraelectric phase at room temperature. In K 4 Fe(CN) 6 ·3H 2 O and its isomorphous K 4 Ru(CN) 6 ·3H 2 O proton magnetic resonance patterns start to narrow at about -140°C, and definite change in both spectra occurs in the same temperature region around -40°C, about twenty degrees below the respective Curie points. Angular dependence of peak separations has been measured at liquid nitrogen temperature where all water molecules are effectively rigid. Direction angles of three proton-proton vectors with respect to the crystallographic a , b and c axes are, respectively, α 0 =134°, β 0 =49°, γ 0 =73°; α 0 =90°, β 0 =90°, γ 0 =0° and α 0 =0°, β 0 =90°, γ 0 =90°. A possible molecular mechanism for the ferroelectric origin is discussed.