Reorientational motion of ammonium ions in (NH<SUB>4</SUB>)<SUB>2</SUB>SO<SUB>4</SUB> and [(NH<SUB>4</SUB>)<SUB>0.16</SUB>K<SUB>0.84</SUB>]<SUB>2</SUB>SO<SUB>4</SUB>

We report measurements of quasielastic incoherent neutron scattering from paraelectric (NH4)2SO4 and [(NH4)0.16K0.84]2 SO4 in the temperature range of 227 to 420&#176;K and in the wave vector region of 0.8 to 2.38 &#197;-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 &#964;I and &#964;II for the two NH+4 ions at room temperature are found to be 12.0&#177;2.0 and 15.8&#177;1.1 psec, respectively. The temperature dependence of &#964; is very similar for the two salts and can be described by an average description of the type &#964;=t0expV/kBT with V=1.27&#177;0.17 kcal/mole and &#964;0=1.65&#177;0.7 psec. The present data are not accurate enough at all temperatures to give separate temperature dependences for &#964;I and &#964;II. The values of &#964;0 and V are compared with those derived by NMR