Structural and Spectroscopic Evidence for Hydrogen Bonding Induced NH+4Cation Ordering inβ-(NH4)2FeF5at Low Temperature

Abstract

An X-ray Rietveld structure determination ofβ-(NH4)2FeF5at 133 K showed that the transition, observed at 133 K on the parameter versus temperature curve, corresponds to the ordering of one of the two NH+4cations of the compound. The structure calculations could be carried out successfully thanks to the use of the rigid body approach, which considers collectively the four hydrogen atoms of the NH+4group, assumed to retain a tetrahedral symmetry. Very short H⋯F distances of 1.88 Å are observed that correspond to strong hydrogen bonding between the NH+4groups and the [FeF5]−chain. This results in the straightening of the corrugated fluoride chain. A Raman spectroscopic study confirmed the occurrence of the transition through high and low frequency spectrum recording. At low temperature, the Raman bands become narrower and/or split into several components. In the high frequency domain, an increase of a new component at about 3082 cm−1proves clearly the setting up of stronger hydrogen bonding around 133 K. The peaks at higher frequencies (3280, 3197, and 3119 cm−1) present a weak intensity, in agreement with the presence of the disordered NH+4cation, as found by the structure determination.