L.T. Yu

Inelastic Neutron Scattering Study of the Proton Dynamics in Manganese Oxides I . and Manganite

The inelastic neutron scattering spectra of two untreated γ-MnO 2 samples (chemical manganese dioxide or CMD, and electrodeposited manganese dioxide or EMD) at 20 K reveal that the observed intensities are proportional to the amount of Mn 4+ vacancies in the lattice. Localized protons give broad bands between 500 and 1100 cm -1 . In addition, the broad scattering continua extending over the whole energy transfer range are interpreted in terms of proton recoil (Compton effect). These almost free protons experience a very shallow potential barrier (∼30 cm -1 )

Rechargeability of MnO2 in KOH Media Produced by Decomposition of Dissolved KMnO4 and Bi ( NO 3 ) 3 Mixtures I. Mn‐Bi Complexes

Bi-doped MnO 2 samples produced by the slow decomposition of KMnO 4 and Bi(NO 3 ) 3 mixtures at room temperature in 0.5 M nitric acid solution have stable voltammograms in 1 M KOH solution even for an extended sweep range from -1.1 to 0.7 V vs. a normal hydrogen electrode. Analysis of the voltammograms shows the existence of complexes between Mn(X) (X = IV, III, and II) and Bi(III), which implies the existence of formation reactions. These reactions take place during the first stage of the slow decomposition of KMnO 4 (only X = III, IV) and during the discharge or recharge when the dissolved Mn(X) species are present in the intergranular electrolyte. X-ray diffraction studies show the absence of Mn 3 O 4 when Bi(III) is present. This indicates that the reactions leading to the formation of Bi-Mn complexes prevent the hausmannite spinel lattice forming from dissolved Mn(II) and Mn(III).

An inelastic neutron scattering study of the proton dynamics in γ-MnO2

Abstract The inelastic neutron scattering (INS) spectra between ≈0 and 4000 cm −1 of electrodeposited manganese dioxide (γ-MnO 1.96 ) 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°C during 16 h in presence of air. Three different types of bands are distinguished in the spectra. (i) A broad continuum rising at ≈ 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 covalently 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°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.

Rechargeability of MnO{sub 2} in KOH media produced by decomposition of dissolved KMnO{sub 4} and Bi(NO{sub 3}){sub 3} mixtures. 1: Mn-Bi complexes

Bi-doped MnO 2 samples produced by the slow decomposition of KMnO 4 and Bi(NO 3 ) 3 mixtures at room temperature in 0.5 M nitric acid solution have stable voltammograms in 1 M KOH solution even for an extended sweep range from -1.1 to 0.7 V vs. a normal hydrogen electrode. Analysis of the voltammograms shows the existence of complexes between Mn(X) (X = IV, III, and II) and Bi(III), which implies the existence of formation reactions. These reactions take place during the first stage of the slow decomposition of KMnO 4 (only X = III, IV) and during the discharge or recharge when the dissolved Mn(X) species are present in the intergranular electrolyte. X-ray diffraction studies show the absence of Mn 3 O 4 when Bi(III) is present. This indicates that the reactions leading to the formation of Bi-Mn complexes prevent the hausmannite spinel lattice forming from dissolved Mn(II) and Mn(III).

Inelastic neutron scattering study of γ-MnO2: the dynamics of (H+)4 entities in Mn4+ vacancies

Abstract The inelastic neutron-scattering spectra of γ-MnO 2 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 assignment 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 disppears. 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.

Inelastic neutron scattering studies of polyanilines and partially deuterated analogues

Abstract Inelastic neutron scattering spectra have been measured from 16 to 4000 cm−1 for various polyaniline samples at 30 K: the emeraldine-base and the emeraldine-salt, the ring-deuterated analogues and their hydrated forms. The spectra of the totally hydrogenated samples are dominated by bands due to protons bound to the aromatic rings. The spectrum of the ring-deuterated base reveals that most of the remaining protons are not bound to nitrogen atoms. There is a continuum of intensity due to the recoil of free particles with mass 1 amu. In the emeraldine-salt additional protons (H+) are trapped in very shallow potential-wells with dissociation threshold ≈ 300 cm−1. At energy transfer greater than this threshold, these protons are free to recoil. The spectra of the hydrated samples reveal that free entities are not trapped by water molecules. These new dynamics are tentatively related to the electronic structure and conductivity of these polymers. These are supposed to be determined by the position of the electronic state of “H0” entities relatively to the half-filled π-band of the metal-like form of the pemigraniline-base. In the emeraldine-base electrons transferred from “H0” entities to the conduction band give an insulator with a totally-filled band and a gas of H+ entities. In the emeraldine-salt the electronic state of “H0” entities is lowered. The structure is that of a metal-like half-filled band with a gas of “H0” entities and weakly bound protons.

Inelastic Neutron Scattering Study of the Proton Dynamics in Manganese Oxides II . Proton Insertion in Electrodeposited

The inelastic-neutron scattering spectra of partially reduced electrodeposited MnO 2 samples (MnO 2 H y with y=0.04, 0.34, 0.42, 0.56, 0.72, and 0.96) at 20 K are reported. Band decomposition in the 600 to 1200 cm -1 region reveals four components at almost fixed frequencies for all the samples

Inelastic neutron-scattering study of free proton dynamics in γ-MnO2

Abstract Inelastic neutron scattering measurements of γ-MnO 2 at 30, 100 and 200 K give S( Q , ω) maps of intensity over large energy and momentum transfer ranges (from 0 to 1 eV and from 0 to 40 A −1 , respectively). They reveal a ridge of intensity due to recoil of free particles with effective mass of ∼ 1 amu. The width of the recoil line is one order of magnitude greater than that anticipated from the kinetic momentum distribution of an isolated gas of protons at the temperature of the sample. Several models are considered: ideal gas obeying Maxwell-Boltzmann or Fermi statistics, multiple scattering, phonon wings, protons trapped in shallow potentials and quantum correlations between free and bound protons. None of them is satisfactory. Finally, broadening by zero-point motions of the lattice provides a satisfactory theoretical framework to represent the shape and intensity of the recoil spectra. New aspects of proton dynamics in γ-MnO 2 are highlighted.

Structural changes induced by chemical reduction of various MnO2 species

Several kinds of MnO2 were progressively reduced by cinnamic alcohol (CA) and aqueous hydrazine solutions (AHS) to compare changes in their structure. With α-MnO2 stabilized by NH+4, the maximum homogeneous degree of H-insertion (MHID) is only 0.62H per Mn, which involves the filling of each NH+4-free tunnel by four protons. This MHID value is consistent with the discharge capacity during the electrochemical reduction in 1m KOH solution and in nonaqueous media (~0.65 and ~0.63 faradays per Mn, respectively). This result shows that Li+ and H+ ions occupy the same sites. The lowest degrees of oxidation are obtained when AHS are used, resulting in progressive appearance of a spinel structure which replaces the original lattice. For degrees of reduction x lower than MnO1.33, pyrochroite exists in a poorly crystallized form since it is not observed in the XRD patterns. The XRD patterns of γ-MnO2 reduced to MnO1.12 usually show the spinel structure while the patterns of the Bi-doped MnO2 reduced to MnO1.14 exhibit peaks corresponding to pyrochroite and bismite (Bi2O3). Thus, the presence of Bi3+ hinders the formation of the nonelectroactive compound Mn3O4 or γ-Mn2O3, but the mechanism to explain this cannot be determined by XRD data alone.

Proton dynamics in manganese dioxide

Abstract The inelastic neutron scattering spectra of chemically reduced γ − MnO 2 ( MnO 2 H 0.96 ) show three main peaks at 1120, 2200 and 3100 cm -1 which are assigned to the 0→1, 0→2 and 0→3 transitions of a degenerate isotropic oscillator. The protons are located at the centre of the oxygen octahedra.