AN INELASTIC NEUTRON-SCATTERING STUDY OF THE PROTON DYNAMICS IN GAMMA-MNO2

The inelastic neutron scattering (INS) spectra between-almost-equal-to-0 and 4000 cm-1 of electrodeposited manganese dioxide (gamma-MnO1.96) at 4.5 K are reported. The spectrum of the commercial product is compared to those obtained from various samples heated at 150, 250, 350 and 450-degrees-C during 16 h in presence of air. Three different types of bands are distinguished in the spectra. (i) A broad continuum rising at almost-equal-to-30 cm-1 and extending over the whole frequency range is interpreted in terms of an ideal gas of protons moving almost freely in the crystal. (ii) Bands between 500 and 1100 cm-1 are assigned to protons trapped in crystal defects. (iii) Sharp and relatively intense bands between 30 and 90 cm-1 are assigned to covalenty bonded protons strongly coupled to the libration modes of the crystal. They are likely to correspond to oxyhydroxide MnO(OH). The concentration in MnO(OH) increases substantially after ambiant atmosphere rehydratation of the sample previously treated at 450-degrees-C. The dynamics of these different protons and their stabilities with respect to thermal treatments are discussed. It appears that most of the protons are not covalently bonded to the oxygen atoms in the crystal but are rather mobile. Moreover, INS spectra provide direct evidence that some of the protons are localized by crystal defects which may thus play a key role in the electroactivity of the material.