M. E. Lines

Determination of the local structure of amorphous NaFeF4 by Mossbauer spectroscopy

The room-temperature paramagnetic and 4.2K Zeeman 57Fe Mossbauer spectra are reported and analysed for both crystalline (c-) and amorphous (a-) NaFeF4. The results point strongly to the presence, in a-NaFeF4, of distorted FeF6 octahedra which are locally coordinated to (predominantly) four other octahedra via a linkage in which the two non-bridged, or terminal, fluorine bonds are in a trans-configuration ( approximately=180 degrees apart). This locally planar magnetic structure is a distorted form of that which occurs in c-KFeF4 and c-RbFeF4, but is different from that in c-NaFeF4 for which the smaller sized alkali-metal ions induce a puckering of the quadratic layer iron sublattice via a cis-terminal ( approximately=90 degrees ) coordination configuration of the FeF4 octahedra. It is concluded that the crystalline constraint which produces this unusual cis-coordination in c-NaFeF4 is sufficiently relaxed in the amorphous state that the more common trans-coordination is restored. Amorphous NaFeF4 should therefore be pictured as a continuous random network of predominantly four-coordinated trans-terminal FeF6 octahedra with sodium ions filling the holes. Since 4.2K is more than one third of the spin-freezing temperature ( approximately=12K) of a-NaFeF4, significant effects relating to thermally induced spin fluctuations are observed in the amorphous spectrum at liquid helium temperature, and are interpreted self-consistently within the framework of the basic model.