NMR study of molecular reorientations in NH4H2AsO4

Zeeman (T1Z) and dipolar (T1D) spin-lattice relaxation times of protons in NH4H2AsO4 were measured as a function of temperature. The existence of a slow motion (τ ≈ 10−3 see) is established, which is most probably a low frequency hindered reorientation of H2AsO4 groups. This motion is slowed down below the Curie point Tc. A sharp increase of the dipolar relaxation rate above T = 314°K indicates the possibility of a new high temperature phase transition in this compound