INELASTIC NEUTRON-SCATTERING STUDY OF GAMMA-MNO2 - THE DYNAMICS OF (H+)4 ENTITIES IN MN4+ VACANCIES

The inelastic neutron-scattering spectra of gamma-MnO2 are reported at 20, 100, 200 and 300 K. The difference spectra (100-20 K and 200-20 K) show bands at 40, 90, 185 and 345 cm-1 which are not observed at 20 or 300 K. These bands correspond to the J = 0, 1, 2 and 3 levels of a freely rotating spherical top with a rotational constant B = 25 cm-1. This is assigned to (H+)4 entities in the lattice vacancies. The frequency shift of the J = 0 level is interpreted in terms of inversion, i.e., out-of-phase rotation and antitranslation of two H+...H+ pairs inside the (H+)4 entities. At 20 K the free rotation disappears. An equilibrium between two configurations for the four photons associated with the vacancy is proposed. At 300 K the spectrum indicates a gas of free particles.