Abstract
The frequency distribution function g(ω) was measured by IINS in three series of experiments. The resolution Δω/ω in the ω region 15 to 120 cm−1 was about 6% in two of them and in the third one about 15%. The peaks obtained are interpreted as arising from quasi‐free rotation of ammonium ions in the crystal. Their positions and distances agree with those calculated assuming B = 6.30 cm−1 for the rotational constant of NH4. Their existence may be treated as a proof of a very low rotational barrier of the NH4 groups in NH4ClO4.