Rotational excitations of NH +4 ions in dilute solutions in alkali-metal halide lattices

Abstract

The rotational excitations of NH⁺₄ ions in (NH₄)_0.03 Rb_0.97 X (where X = I, Br, Cl) have been studied at 4 K by inelastic neutron scattering and dielectric measurements. The discrete tunnelling transition energies are found to be systematically lower in the rubidium salts than in the corresponding potassium salts. Results from the ternary mixed salt, (NH₄)_0.02Rb_0.06K_0.92l, strongly resemble those from the binary mixed salt, (NH₄)_0.09K_0.91l, suggesting that it is the total substitutional defect concentration which is important in the degradation of the tunnelling spectrum rather than the NH⁺₄ concentration itself. The NH⁺₄–NH⁺₄ interaction itself plays only a secondary role. The glass-like properties of these mixed salts probably stem from stress–strain interactions.