Structure of Cubic Ammonium Fluosilicate: Neutron-Diffraction and Neutron-Inelastic-Scattering Studies

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Abstract
The motion of the ammonium ion in the cubic phase of (NH4)2SiF6 has been investigated by the inelastic scattering of slow neutrons. The prominent feature of the neutron energy‐gain spectrum is a moderately broad band peaked at 168±8 cm−1 with a shoulder at 305±25 cm−1. These are assigned to the 1–0 and 2–0 transitions of a rotational motion of the ammonium ion. A precision refinement of single‐crystal neuron‐diffraction data has also been completed. There is a well‐defined disorder of the ammonium groups. Although a model with a static threefold disorder, with each ⅓ hydrogen atom undergoing harmonic vibration gives a very satisfactory fit to the data, such a model is probably unrealistic in view of the fact that the disordered positions are only 0.75 Å apart. A more realistic model is one involving an ordered hydrogen atom undergoing thermal motion in a very anharmonic potential well. We propose a model in which the hydrogen atom is relatively free to move over a region of about 1 Å2 by small rotations of the ammonium ion but with a very high barrier toward a reorientation which involves an interchange of two hydrogen atoms. Such a model is an agreement with both the neutron‐diffraction and the inelastic‐scattering data. The hydrogen‐atom positions found in the diffraction study do not agree with those reported by Vainshtein on the basis of an analysis of the electron‐diffraction data. The N–H bond length is not well defined because of the uncertainties regarding the thermal motion. An uncorrected mean atomic separation of 0.985 (σ=0.007) is obtained; the actual bond length is about 0.08 Å longer. The SiF62— ion is a regular octahedron with a corrected bond length Si–F=1.696 (σ=0.003).