Reorientational motion of ammonium ions in (NH₄)₂SO₄ and [(NH₄)_0.16K_0.84]₂SO₄

Abstract

We report measurements of quasielastic incoherent neutron scattering from paraelectric (NH₄)₂SO₄ and [(NH₄)_0.16K_0.84]₂ SO₄ in the temperature range of 227 to 420°K and in the wave vector region of 0.8 to 2.38 Å^-1. (NH₄)₂SO₄ has two inequivalent NH⁺₄ ions in a unit cell, labeled I and II, whereas the mixed salt has only one type of NH⁺₄ ion, that is, type II. It is shown that it is possible to get information about the two NH⁺₄ ions separately by combining the data from the two salts. The form factors for the two NH⁺₄ ions and the temperature dependence of the characteristic reorientation time are derived. From the form factors it is shown that for NH⁺₄ (II) ion there is a distribution of characteristic relaxation times for reorientation about the four N-H bonds. On the other hand, for NH⁺₄ (I) ion the reorientation rate about all the four N-H bonds is nearly same. The average characteristic times τ_I and τ_II for the two NH⁺₄ ions at room temperature are found to be 12.0±2.0 and 15.8±1.1 psec, respectively. The temperature dependence of τ is very similar for the two salts and can be described by an average description of the type τ=t0expV/kBT with V=1.27±0.17 kcal/mole and τ₀=1.65±0.7 psec. The present data are not accurate enough at all temperatures to give separate temperature dependences for τ_I and τ_II. The values of τ₀ and V are compared with those derived by NMR.