Rotational States of NH⁺₄in Dilute Solution in Alkali Halide Lattices. I. Inelastic Neutron Scattering Studies

Abstract

Rotational tunneling and torsional energy states of the NH + 4 ion in several alkali halide lattices (KCl, KBr, KI, RbI and CsI) have been characterized in the energy range 0< ε <42 meV. The most detailed spectra are obtained for the specimen KBr/NH 4 Br (0.5 mole%) at low energies ( ε <2 meV) and in the temperature region 1.5< T <30 K. The results are consistent with a model of energy states for a tetrahedron in an octahedral field. An investigation of the ratios of the intensities of neutron energy loss and energy gain peaks in the spectra for KBr/NH 4 Br (0.5%) leads to the conclusion that conversion between the nuclear spin symmetry species of NH + 4 occurs with a characteristic time of several hours. This is in general agreement with the findings of NMR measurements on the same specimen that are described in the following paper. Moreover, the heat capacity results display a non-lattice contribution which is much less than that which would correspond to rapid spin conversion. On the other hand, the intensity ratios for the neutron spectra of two specimens prepared by evaporation of aqueous solutions (KI/NH 4 I (2.3%) and RbI/NH 4 I (2.9%)) suggest that conversion is more facile in them.