Determination of the Local Structure of Amorphous NaFeF sub 4 by Mossbauer Spectroscopy

The room-temperature paramagnetic and 4.2 K Zeeman sup 57 Fe Mossbauer spectra are reported and analysed for both crystalline (c-) and amorphous (a-) NaFeF sub 4. The results point strongly to the presence, in a-NaFeF sub 4, of distorted FeF sub 6 octahedra which are locally coordinated to (predominantly) four other octahedra via a linkage in which the two nonbridged, or terminal, fluorine bonds are in a trans-configuration (~180degrees apart). This locally planar magnetic structure is a distorted form of that which occurs in c-KFeF sub 4 and c-RbFeF sub 4, but is different from that in c-NaFeF sub 4 for which the smaller sized alkali-metal ions induce a puckering of the quadratic layer iron sublattice via a cis-terminal (~90degrees) coordination configuration of the FeF sub 6 octahedra.