The Manganates

The holes may be localized either as small-polaron Mn(IV)06/2 complexes or in a molecular orbital of a two-manganese Mn(III)—O—Mn(IV) polaron. 

In the systems (Lai yLny)o.7Cao.3Mn03, the ratio Mn(IV)/Mn = 0.3 is held constant and the bandwidth is decreased by substituting smaller Ln ions for 

Two features of Fig. 24 are to be noted first, the extraordinary decrease in Tc with the isovalent substitution of Pr for La , from 250 K at y = 0 to about 80 K at y = 0.6, and second, the dramatic increase in the negative magnetoresistance just above Tc as Tc decreases. The resistivity pox shows a 

These dramatic changes are caused by a decrease in the (180° - p) Mn—0—Mn bond angle on substitution of the smaller Pr ion. According to Eq. (18) of Goodenough, this volume, the width of a narrow a band below Tc would be given by 

at high temperature, a(T) approaches a temperature-independent value of -20 pV/K in the O samples, a somewhat larger value in the O sample of Fig. 26. A temperature-independent a is indicative of polaronic conduction in which the statistical term 

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Oxidation states can be used to establish the stoichiometry for an equation. Consider the reaction between the manganate(VII) (permanganate) and ethanedioate (oxalate) ions in acidic solution. Under these conditions the MnO faq) ion acts as an oxidising agent and it is reduced to Mn (aq), i.e. 

It has been shown in reaction (3) that all the evolved oxygen comes from the hydrogen peroxide and none from the manganate(VII) or water, by using H2 02 and determining the isotope in the evolved gas. 

In acid solution, the manganate(VII) ion is reduced to the manganesefll) ion with decolorisation ... 

Note that here chlorine is oxidising the manganate(Vl) to man-ganate(VlI) under more acid conditions, the latter oxidises chloride to chlorine, p. 103). 

The manganate(VlI) ion slowly oxidises water, the essential reaction being... 

The manganate(VII) ion is one of the more useful oxidising agents in acid solution we have... 

The Cams Hquid-phase oxidation process is similar in principle however, it is operated continuously, its oxidation reaction vessels are of a much larger scale, and the separation of the manganate intermediate from the caustic melt is accompHshed without dilution by means of filtration (qv) (121—123). 

Aqueous solutions can be stabilized against viscosity loss by addition of 5—10 wt % anhydrous isopropyl alcohol, ethanol, ethylene glycol, or propylene glycol. The manganous ion (Mn " ) also is an effective stabilizer at concentrations of 10 -10 wt% of the solution. 

The manganous ion [16397-91 -4] in solution then reacts with higher valent manganese oxide and zinc ions in solution to form a new phase called... 

Early work of Dhar established that oxidation of oxalic acid by chromic acid occurs readily, but some of his kinetic data are unreliable as the substrate itself acted as the source of hydrogen ions. The reaction is first-order in oxidant and is subject to strong manganous ion catalysis (as opposed to the customary retardation), the catalysed reaction being zero-order in chromic acid. This observation is related to those found in the manganous-ion catalysed oxidations of several organic compounds discussed at the end of this section. 

Such a pre-equilibrium closely parallels that suggested by Dewar et for the manganic acetate oxidations of several aromatic ethers and amines (p. 405). Other features of the reaction are a p value of —0.7 and identical activation energies of 25.3 kcal.mole for oxidation of toluene, ethylbenzene, cumene, diphenylmethane and triphenylmethane. 

Finally, NMR has also be used to study other spinels materials that do not contain manganese. For example, the intercalation/deintercalation of lithium titanate spinels such as Li4/3+Ji5/304 and Lii.i-Tii.904+a have been investigated. These materials may be used as anode materials in combination with cathodes operating at 4 V (vs Li) to produce cells with potentials of ca. 2.5 V. These materials are either diamagnetic or metallic, and unlike the mangan-ates, only very small differences in shifts are seen for Li in the different sites of the spinel structure. Nonetheless, these shift differences are enough to allow the concentrations of the different sites to be quantified and monitored following insertion of Li or as a function sample preparation method. ... 

A classical case in the field of oxidation of organic ligands is the decomposition of the manganic oxalate complexes (31,84,85)... 

PreparaUm.—Powdered mangauio oxide is suspended in water and a current of sulphurous anh rdride passed through the liquid, when the manganic oxide gradoaUj dissolves. The solution ooutaius manganous dithionate or hyposulphate — JOaO, + 2SO, S, 0,Mno". 

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