The ammonium ion in the alkali halide lattice has the hindered rotational state. The rotational potential is expressed as
crystal field, which depends upon only one rotational motion. The tetrahedral ion receives an octahedral field in this system.
Four fundamental types of orientation appear due to the symmetry of ion and that of field. As the barrier height increases,
the rotational levels approach to the librational levels with tunnel splitting. In particular, the tunneling part in the ground
librational level is calculated using both free rotor bases and orientationally localized states. The level structure with
the degeneracy is elucidated, which is peculiar in each type of orientation. Thermal properties are shown as model calculations.