Ramsdellite

Ramsdellite is thermodynamically unstable toward a transformation into the stable ft -modification. Hence, it is rarely found in natural deposits. Natural ramsdellite has a stoichiometry close to the composition of Mn02 and can be considered another true modification of manganese dioxide. Attempts to synthesize ramsdellite in the laboratory usually lead to materials of questionable composition and structural classification. It is very likely that synthetic ramsdellite materials are more or less well-crystallized samples of the y-modification that will be described in more detail below. 

I Structural Chemistry of Manganese, Dioxide an Related Compounds 

The De Wolff disorder model has been extended to the cation vacancy model for /-Mn02 and -Mn02 by Ruetschi [42]. In this model the occurrence of manganese cation vacancies and the non stoichiometry of electrochemical Mn02 have been taken into account. Furthermore, the vacancy model deals with the explanation of the different water contents of manganese dioxide. Ruetschi makes some simple assumptions  

and vacant sites. The water in the structure is present in the form of OH anions and Mn4+ vacancies. 

Therefore, Ruetschi proposes a general formula for / - Mn02  

In the early literature this wide variety of different powder patterns led to the distinction between and -Mn02 

Further improvements on the previously discussed models were proposed in the latest model for y - and - MnOj by Chabre and Pannetier [12, 43, 44]. Starting from De Wolffs model they developed a structural description of manganese dioxides that accounts for the scattering function of all / — and - MnOj materials and provides a method of characterizing them quantitatively in terms of structural defects. All - and - MnOj samples can be described on the basis of an ideal ramsdellite lattice affected by two kinds of defects  

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A) A stacking disorder (De Wolff disorder intergrowth of ramsdellite- and pyrolusite- type units, as already described above). This kind of disorder can be quantified by two parameters ... 

Figure 6. Projection of the manganese atoms in the ramsdellite lattice onto the be-plane. The oxygen atoms are not shown. The twinning planes [02l (above) and [061] (below) are marked with arrows. The twins at these planes are generated by rotating the shaded ramsdellite cells by either 60° or 120° around the a-axis. (Adapted from Ref. [47].)...

Figure 1. The structures of (a) a - Mn02, (b) P MnOj, Mn02, and (d) ramsdellite -Mn02 Hatched regions represent MnOfi octahedra...

The structure of the lithiated ramsdellite- Mn02 product after reaction with LiOH at 300 °C is shown in Fig. 3(d) [6]. The overall ideal reaction, which is similar to that when LiOH is used as a lithiating agent, can be written as ... 

In this case, lithiation of ramsdellite occurs by incorporation of Li20 into the Mn02 structure, which is analogous to the situation for the stabilization of a - MnOz (see Sec. 1.2.2) the reaction... 

Pr,R probability that a ruitile-like layer is followed by a ramsdellite-like one in yls — Mn02 II. 1... 

Pr probability of occurrence of ramsdellite-like building blocks inll.l yts-y[n02 ... 

Fig. 7.4 Tunnel structures based on rutiles, (a) Rutile-like chains, showing alternately short and long metal-metal distances, as in MoOj. (h)-(

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