Motion of ammonium ions in non-cubic crystal sites

Abstract

This paper is concerned with the motion of the ammonium ion in crystal sites of different symmetry. The symmetry of k= 0 modes of vibration of the NH⁺₄ ion have been enumerated using group theoretical methods. The application of laser Raman scattering and infra-red spectroscopy to the study of molecular motion is discussed. The spectra of several salts containing the ammonium ion have been studied using the techniques in the temperature range 100–500 K. Confirmation as to whether the motion is translatory or rotatory (librational) has been obtained by deuteration.

Two forms of ammonium fluorosilicate have been studied. An external mode at 180 cm^–1 has been observed and identified to be translatory. No torsional modes have been found and the motion is therefore interpreted as resembling free rotation or to have a torsional frequency below 30 cm^–1. It has been confirmed that the NH⁺₄ ions in ammonium perchlorate are rotating freely. The ferroelectric transition in (NH₄)₂SO₄ has been studied and a preliminary survey of the results is presented.