EPR Studies of Restricted Motion of NH 3 + Radicals in X‐Irradiated ADA and Glassy RADA

The electron paramagnetic resonance (EPR) spectra of NH 3 + radicals in NH 4 H 2 AsO 4 (ADA)and mixed Rb 1− x (NH 4 ) x H 2 AsO 4 (RADA) ( x = 0.35) in the temperature region 4.2 to 300 K are measured. It is shown that in ADA at room temperature (RT) NH 3 + molecules undergo fast rotation around the principal axes of the crystal field tensor, and make rapid jumps between them. These motions provide nearly isotropic averaging of the spectra. Below T N = 216 K rapid rotation only around the a‐ and b ‐axes is present. In contrast to ADA, in RADA the NH 3 + groups rotate rapidly only around the a ‐ and b ‐axes even at RT and inversion tunneling of nitrogen is suggested. The hyperfine structure of the ammonia proton is isotropically averaged down to 115 K in ADA and down to 60 K in RADA.